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Ionizing monolayers and pH effects
Advances in Colloid and Interface Science. 4 ( 1 9 7 4 ) 4 5 - 7 8 © Elsevier S c i e n t i f i c Publishing C o m p a n y . A m s t e r d a m - lhrinted in T h e N e t h e r l a n d s
IONIZING M O N O L A Y E R S A N D pH E F F E C T S E.D. GODDARD
Lever Brothers Company° E d g e ~ t e r . N e w Jersey (U.S.A.)
CONTENTS a. Introduction. B. S o l u t i o n s o f p o l y e l e c t r o l y t e s
C. I n s o l u b l e m o n o l a y e r s (i) F a t t y acids (ii) F a t t y a l c o h o l s a n d e t h e r s (iii) F a t t y a m i n e s (iv) A l k y l p h o s p h a t e s and p h o s p h o n a t e s (v) A l k y l sulfates a n d alkar~e s u l f o n a t e s (vi) P h o s p h o l i p i d s a n d o t h e r z w i t t e r i o n i c m a t e r i a l s . (vii) P o l y m e r s . . . D. A d s o r b e d m o n o l a y e r s (i) F a t t y acid s o a p s a n d a m i n e s (ii) A l k y l sulfates a n d a l k a n e s u f f o n a t c s . E. Micellar s y s t e m s . F. C o n c l u s i o n s . Acknowledgement References
45 47 48 52 57 57 58 59 60 64 68 69 70 74 76 77 77
ABSTRACT A p p l i c a t i o n o f the electrical d o u b l e layer t r e a t m e n t to charged m o n o l a y e l s is o u t l i n e d . P r o p e r t i e s . as I n f l u e n c e d b y solution p i l . o f a n u m b e r o f t y p e s o f Ionizing m o n o l a y e r s , (i) spread, (11) a d s o r b e d , a n d ( 8 0 at the m i c e l l e / w a t e r I n t e r f a c e , are r e v i e w e d . A n a l y s e s o f the first a n d ~hird c a t e g o r i e s b y electrical d o u b l e layer m e t h o d s , a n d o f the s e c o n d b y the G i b b s a d s o r p t i o n e q u a t i o n . 0xe p r e s e n t e d a n d discussed.
A. I N T R O D U C T I O N
In the early 1900's there was considerable interest in the tlistribution o f ions near charged interfaces or bodies In contact with electrolyte solutions. This interest led to the d e v e l o p m e n t o f the theories o f G o u y , Chapman, Stern, and others. For the subject o f m o n o l a y e r "titrations'" we will be concerned directly with Ionic distribution as it affects hydrogen (or h y d r o x y l ) tons near interfaces containing lontzable materials. In this respect the most widely In-
46
E.D. GODDARD
vestlgated matet'ials have been f a t t y acids, the properties o f w h i c h were k n o w n to be h i g h l y i n f l u e n c e d by the pH o f the ir a d j a c e n t s o l u t i o n s f r o m the e a r l y w o r k on i n t e r r a c ial tension b y R e i n d c r s ) and on m o n o l a y e r s by A d a m 2.3 a n d others. T h e first s y s t e m a t i c investigation o f the p h e n o m e n o n s e e m s to have been u n d e r t a k e n by Peters 4 . w h o e x a m i n e d the i n f l u e n c e o f pH and. to s o m e e x t e n t , ionic s t r e n g t h o f a q u e o u s s o l u t i o n s on the ir i n t e r f a c i a l te nsion against b e n z e n e s o l u t i o n s o f various f a t t y acids a nd a l i p h a t i c amines. It was n o t i c e d t h at the resulting curves o f interracial te nsion versus pH r e s e m b l e d bulk phase ti t rat i o n curves b u t w i t h an a p p a r e n t shif t o f a b o u t 3 pH units in tile direction o f increased weakness. T his s u b j e c t was e x t e n d e d by Daniellis , w h o . by a s s u m i n g t h a t t h e degree o f lowering o f interracial te nsion was d i r e c t l y prop o r t i o n a l to the f r a c t i o n o f f a t t y acid ionized in the inte r f a c e , c o n f i r m e d that the p K o f the f a t t y acid could be s h i f t e d in the direction o f w e a k n e s s b y as m u c h as 4 u n i t s a n d t h a t this shift was greater the l o w e r the ionic s t r e n g t h o f the a q u e o u s solution. D a n i e l l i ' s results closely paralleled those o f Schulm a n et al. 6. 7 on the surface p o t e n t i a l versus pH curves o f spread m o n o l a y c r ~ o f f a t t y acids at the a l r / a q u e o u s s o l u t i o n interface. O t h e r a p p r o a c h e s for s t u d y i n g the i o n i z a t i o n o f spread f i l m s o f f a t t y acids, involving direct analysis, e i t h e r cl~emical6 or infra r e d " , o f s a m p l e s " ' s k i m m e d " f r o m the surface o f s u b s o l u t i o n s o f varying pH. have led to results in h a r m o n y w i t h the p r e c e d i n g studies. It is n o w g e n e r a l l y a c c e p t e d that i o n i z a b l e m a t e r i a l s at i n t e r f a c e s d i s p l a y an a p p a r e n t w e a k n e s s ascribable to, and d e t e r m i n e d in m a g n i t u d e b y , the electrical p o t e n t i a l ~ generated at the i n t e r f a c e b y i o n i z a t i o n . T h i s lowering o f t e n d e n c y t o w ards i o n i z a t i o n has led to general a c c e p t a n c e o f the c o n c e p t o f an effective pH in the surface phase, pH s, w h i c h d i f f e r s f r o m that In the bulk, p H b a s given b y the B o l t z m a n n e x p r e s s i o n pH, = pH b + ~xP/2.3 k T
(1)
w i t h a resulting shift in the d i s s o c i a t i o n c o n s t a n t K given by pK s '~ p K b - egt/2.3 k T
(2)
As a t y p i c a l e x a m p l e , i f the area per charge In a m o n o l a y e r is 100 A ~ a n d the s u b l y l n g a q u e o u s s o l u t i o n has c o n c e n t r a t i o n s 0.01 and 0. I M, respectively, o f u n l - u n i v a l e n t e l e c t r o l y t e , the qPo p o t e n t i a l ( G o u y ) w o u l d be 170 a n d 110 mV, s u f f i c i e n t to cause shifts in pH and, c o n s e q u e n t l y , In pK In the surface o f 2.85 a n d 1.85 units. T h e p o t e n t i a l q~ e m p l o y e d in the c a l c u l a t i o n s d e p e n d s on the m o d e l used, In m ost cases, % , the p o t e n t i a l in the plane o f the generated charges, appears to b© the most approF"late choice. It Is generaUy c a lc ula te d via the Gouy-Ch~.i,,ig;; or S:ern models. F o r positions s h i f t e d slightly Into the
I O N I Z I N G M O N O L A Y E R S A N D pll E F F E C T S
47
a q u e o u s p h a s e o t h e r ' [ ' values are e m p l o y e d : q ' D , t h e D o n n a n m e m b r a n e pot e n t i a l , is t h e p o t e n t i a l a v e r a g e d o v e r t h e D e b y e - H t l c k e l d i s t a n c e 1]~; ~', tile e l e c t r o k i n e t i c p o t e n t i a l , is t h e p o t e n t i a l at t h e " s u r f a c e o f s h e a r " . T h e a b o v e c o n c e p t s h a v e b e e n a p p l i e d in m a n y a r e a s o f colloid a n d s u r f a c e c h e m i s t r y . It is t h e p u r p o s e o f this p a p e r to p r e s e n t a review,,, b y n o m e a n s c o m p l e t e , d e a l i n g w i t h t h e i r r e c e n t a p p l i c a t i o n to i o n i z i n g m o n o l a y e r s s p r e a d , a d s o r b e d , a n d p r e s e n t at t h e w a t e r / m i c e l l e i n t e r f a c e . F o r s i m p l i c i t y a n d c o n v e n i e n c e , in all cases t h e a q u e o u s s o l u t i o n s c o n s i d e r e d will be r e s t r i c t e d to t h o s e c o n t.'lining u n i v a l e n t c o u n t e r ions o n l y .
B. S O L U T I O N S O F P O L Y E L E C T R O L Y T E S
It is p e r h a p s a p p r o p r i a t e at t h e o u t s e t t o p o i n t o u t the f o r m a l s i m i l a r i t y o f a c i d - b a s e t i t r a t i o n o f m o n o l a y e r s to t h a t o f p o l y e | e c t r o l y t e s in s o l u t i o n . T h e l a t t e r , b o t h n a t u r a l a n d s y n t h e t i c , have b e e n v e r y w i d e l y i n v e s t i g a t e d . A t y p i c a l t i t r a t i o n curve s u g g e s t s t h e r e is a c o n t i n u o u s r a n g e o f p K values o f a p o l y acid, f o r e x a m p l e , w h i c h b e c o m e s a p p a r e n t l y w e a k e r as t h e d e g r e e o f n e u t r a l i z a t i o n progresses. L i n d e r s t r o m - L a n g to e a r l y r e c o g n i z e d t h a t t h e c o n c e n t r a t i o n o f ions, i n c l u d i n g | l * a n d O t l - . n e a r s u c h a c h a r g e d m a c r o n ~ o l e cule in s o l u t i o n s , d i f f e r s f r o m t h a t in b u l k to a n e x t e n t d e t e r m i n e d b y the e l e c t r i c a l p o t e n t i a l at the s u r f a c e o f the colloid particle. T h i s f i n d i n g h a s b e e n t h e basis o f the a p p r o a c h e s s u b s e q u e n t l y e m p l o y e d to e x p l a i n the observed titration c h a r a c t e r i s t i c s o f p o l y e l e c t r o l y t e s . F o r e x a m p l e , e s t i m a t e s o f t h e d i f f e r e n c e , ( p H s - - p H b ), for t h e o v a l b u m i n m o l e c u l e in s o l u t i o n w e r e m a d e b y D a n i e l l P t using: (a) t h e D o n n a n a p p r o a c h a n d ( b ) p u b l i s h e d values of the zeta potential. Excellent agreement was o b t a i n e d b e t w e e n the two m e t h o d s o v e r a c o n s i d e r a b l e r a n g e o f pH, a n d c o n s e q u e n t l y , o f d e g r e e o f ionization. A n e m p i r i c a l e q u a t i o n ~ : , t 3 f o r t h e d i s s o c i a t i o n o f a p o l y m e r i c acid pH=pK e -n
log l - ~
(3)
O~
w h e r e p K , a n d n are e m p i r i c a l c o n s t a n t s , has b e e n s u p e r s e d e d b y t h e t h e o r e t i c a l l y d e r i v e d e q u a t i o n t ~, t s pH = p K -
log
I--a t~
eqso 2.3kT
in w h i c h tx is the d e g r e e o f d i s s o c i a t i o n a n d " ~ P o " is f r e q u e n t l y e s t i m a t e d (for polyolectrolytoa) from electrophoretle measurements.
(4)
48
E.D. G O D D A R D
(2. I N S O L U B L E M O N O L A Y E R S
In de a l i n g w i t h the t i t r a t i o n o f an i o n i z a b l e m a t e r i a l at an i n t e r f a c e we n o w treat the t y p i c a l case o f a weak acid, HA, i o n i z i n g a c c o r d i n g to HA~
--'H ÷ + A -
(1 - a )
(a)
P a y e n s t 6 has p o i n t e d o u t the f o r m a l s i m i l a r i t y o f the Stern r e l a t i o n to eqn. (4), w h i c h can be w r i t t e n 1 -tita _
[ H + l b e x p [--(eq, o - - A G O ) / k T l
(5)
w h ere A Go is the s t a n d a r d free e n e r g y o f i o n i z a t i o n o f the acid, e q t m t a b l e , in the p r e s e n t c o n t e x t , to the specific a d s o r p t i o n p o t e n t i a l o f h y d r o g e n ions on i o n i z e d sites in the m o n o l a y e r . F o r i n s o l u b l e m o n o l a y e r s , q'o is usua lly o b t a i n e d f r o m the G o u y - - C h a p m a n c a p a c i t y e q u a t i o n s 7 for flat plates, viz. .tit° _ 2 k T e
sinh-
'
02 500FI~ 1/2 m
(6)
DRT]
w h e r e o is the charge per cm z , m is the c o n c e n t r a t i o n o f u n i - u n i v a l e n t electrol y t e in s o l u t i o n , D the d i e l e c t r i c c o n s t a n t a n d R the gas c o n s t a n t . T h e e q u a t i o n b e c o m e s ~ 7 at 200C g,,
_ 2kT e
sinh_ I (134t~) \Am
(7)
! 121
for a m o n o l a y e r o f a r e a / m o l e c u l e A (A 2 [ m o l . ) a nd degree o f d i s s o c i a t i o n ¢t. T h e G o u y d e r i v a t i o n involves the usual a s s u m p t i o n o f u n i f o r m , i m p e n e trable charge in the m o n o l a y e r a n d p o i n t charges f o r the c o u n t e r i o n s . Strictly s p ea k i n g , it s h o u l d a p p l y o n l y at low sur f a c e charge d e n s i t i e s a n d low e l e c t r o l y t e c o n c e n t r a t i o n . H ow ever, by f o r t u i t o u s c a n c e l l a t i o n o f errors, as discussed b y Davies a n d R i d e a P 7, it has been f o u n d to a p p l y well o u t s i d e its e x p e c t e d w o r k i n g range. It is b y far the m o s t w i d e l y used m o d e l for c o m p u t a t i o n o f g, o p o t e n t i a l s o f charged m o n o l a y e r s . O t h e r m o d e l s , develo p e d f o r s p h e r i c a l s y s t e m s , are m e n t i o n e d in the s e c t i o n o n micelles. T h e G i b b s - D o n n a n " m e m b r a n e " e q u i l i b r i u m has also b e e n a p p l i e d ~ 7 to haonolayers. I n th~s ease there is o f course no f o r m a l " m e m b r a n e " b u t , m a t h e m a t i c a l l y , the r e s t r i c t i o n o f m o n o l a y e r m a t e r i a l to the surface la ye r can be c o n s i d e r e d e q u i v a l e n t to c o n f i n e m e n t b y a m e m b r a n e . I f the thickness of the Doflnan surface phase is taken as the D e b y e - H t l c k e l term 1/~, the c o n v e n t i o n a l D o n n a n e x p r e s s i o n s
IONIZING MONOLAYERS AND ptl I.FI'E(~I'S
49
k T in f b m + ) = k T in ( m + ) + eOJt~
(8)
k T In ( b m - ) = k T In (sin--) --e~Pt~
(9)
c a n be s h o w n ~ s t o lead t o t h e e x p r e s s i o n
•I'D
si h_t (2--/ _134
e
rio)
Am~l= ]
w h i c h resembles that o f G o u y . Here m+ and m - refer to the c o n c e n t r a t i o n s o f u n i v a l e n t ions. T i l e t w o e x p r e s s i o n s lead t o s i m i l a r v a l u e s o f ~P f o r c o n d i t i o n s o f low p o t e n t i a l ; at h i g h c h a r g e d e n s i t y a n d i o n i c s t r e n g t h s t h e D o n n a n a p p r o a c h gives l o w e r values o f q, a n d is t h e p r e f e r r e d m o d e l . It s h o u l d , h o w e v e r , be r e m e m b e r e d t h a t xl, D a n d q'o are n o t s t r i c t l y i n t e r c h a n g e a b l e in t h e S c h u i m a n - - H u g h e s e q u a t i o n ( 1 4 ) g i v e n b e l o w , e s p e c i a l l y f o r c o n d i t i o n s o f l o w m , i.e. 1[~: large. C o r r e c t i o n s t o t h e D o n n a n t r e a t m e n t have been discussed by O v e r b e e k t s A c o n c o m i t a n t e f f e c t o f c h a r g i n g o f t h e m o n o l a y e r is t h e d e v e l o p m e n t o f a c h a n g e , A F , in t h e f r e e e n e r g y o f t h e d o u b l e l a y e r , w h i c h a c c o r d i n g t o t h e D L V O t r e a t m e n t 20 is g i v e n b y
A F = --
~
0
(11)
o . d ff,,
0
~cDk 2 T 2 Ile 2
c o s h [ ( e X P o / 2 k T ) -- 1 ]
(12)
T h i s r e s u l t s in a p o s i t i v e c o n t r i b u t i o n A I I t o t h e s u r f a c e p r e s s u r e , s h o w n I ~ b y D a v i e s t o be a t 2 0 ° C
AII=--
AF=6"I
mtl2
{c°shsinh-I
( 134a
)
(13)
In addition to direct analysis o f the c o m p o s i t i o n o f film material, there are thus t w o m e t h o d s to f o l l o w the course o f i o n i z a t i o n in the m o n o l a y e r in cons e q u e n c e o f the developing'~, potential. T h e surface potential m e t h o d was pioneered by S c h u l m a n and Hughes 7 , w h o first revealed that surface p o t e n t i a l is a particularly sensitive measure o f ionization. T h e y f o u n d that the A V - p H relationship o f a fatty acid r e s e m b l e d a titration curve, again w i t h a substantial shift o f apparent p K in the direction o f weakness. T h e S c h u l m a n - H u g h e s e x pression for the surface potential o f an ionized m o n o l a y e r , w h i c h c o m b i n e ~ a dipole term arid the ~d term
I~,1:1,Cl01~gAgD
~lO AV,,, I ~.Ti~u/A + %
(14)
booomoM f o r . p . r t l . l l y ionized monol.yer
~V=(12II/A)
l i t - ~)/~, +~#~ l + q ~ .
(15)
whcrcs/~= and/J= arc the surface moments of the unionized and Ionized forms, • respectively.
To utilize surface potential measurements as a function of pH for estimation of K, the intrinsic dissociation constant. Betts and Pethlca =9 proceeded as follows. Equation (15) was dffl'ercntlated to give i~A
_ 1211 A , m , r - ---M--
and by
means
( ) aa
(/l= -- JUj )
~
a.m.r
+ k,~p---~
(16) A..,.T
or e(ins. (4) and (7), tile following relations were obtained
(a*vx
_
,2n
/i) A0 +
\O--~I)A.,,,.
fl7)
/-
and 2k T
1 -¢=
J A.,.. T Betts and Pethtca p o i n t e d out that u n d e r c o n d i t i o n s o f low ¢, where the G o u y e q u a t i o n s are most applicable, the first term on the right-hand side o r eqn. (17) a p p r o a c h e s zero and hence
r
\a---~ ).i,.,. T
"/'his result allows calculation e r a and hence pK as a function of'a, by u ~ o f experimental (0 WOpH) values and a "calibration curve" {¢q,, (18)) o f (0#o / 0pH) vs. a, constructed from ¢qng, (4) and (7). Extrapolation o f the pK valu=s to a = 0 yields the Intrinsic pK value. As a refinement, i f pK shows appreciable variation with a, different values of the term . ~ =, (/~= =/J= ) are tested to see which leads to the least variation of p£=¢= with =. A related, yet simpler, method of obtaining pK was used by Goddard and AcklIlP s . in rids method values o£ ~0 are calculated For arbitrary values of 0. Then. a plot of' ~o vs. [iol;(a/! = a) == ( e ~ o / 2 . 3 k T ) ] , which Is equal to
IONIZINft 14ONOLAYI~IeEAND pll I'-'I.FF-L3-1~
sI
( p l I - p K ) . 1, m=l¢10. The t]rtlinkltu QI thi~ pint i~ ~t=p~rl,np¢,~¢d o,= Itlu ~xpurhllullt~=l A V - p H plot. ~nd shll'ted laterally t=) l, ring tile two curve~, Into c'()inciduneu at low pH to ,. 01. It can h¢ shown that ht the region oi" initial di~sochltio, the c h a n g e In A V Is o c c a s i o n e d largely I~y the ~P. t e r m tff uqn. (15), a n d the ~:xtent o f the r e q u i r e d lateral shift is a direct m e a s u r e =)I" pK for c o n d i t i o n s u n d e r which the G o u y - C h a p m a n equations =ire most al~Idlcablu (low a). The second method of following monohiyer Ionization tJlilizes surface pressure measurements. Various interrelated apl~roa~:Itcs have been p r t s t n l u d by Paytns=6 for an:tlyzlng the course of ionizallon of a monolayer front the pressure changt:s, A l l , a c c o m p a n y i n g w~riation o f the s t i b s o h l t i o n pi I. O n e Is to c o m p a r e the o b s e r v e d c h a n g e o r pressure as a l ' u n c t i o n o1" pit w i t h t h a t calc u l a t e d using various values o f the d i s s o c i a t i o n c o n s t a n t a n d the d o u b l e l a y e r equ:ttions. A n o t h e r is to c o m p a r e o b s e r v e d a n d c a l c u l a t e d values o f A([, at c o n s t a n t values o f pH a n d area. as a f u n c t i o n o f the ionic s t r e n g t h o f the s u b s o l u t i o n . F o r a p a r t i c u l a r set o f c o n d i t i o n s , an initial increase o f A l l can be o b s e r v e d o n i n c r e a s i n g the i o n i c s t r e n g t h gillt2e the ¢onst:qtmnt l o w e r i n g o f ~o p o t e n t i a l is a c c o m p a n i e d b y qn increase in i o n i z a t i o n p o t e n t i a l ( a n d an increase in i o n i z a t i o n ) a c c o r d i n g to cqn. (2). At h i g h e r salt concentratiot~s, w h e n dlssociatio,~ is m o r e or,replete, a r c t h t c t i o n o1" All can be e x p e c t e d because o1" tit,: l o w e r e d ~P,, p o t e n t i a l , a n d I~cnce, electro,ltatic repttlsive t e r m . T h e s e ¢l'l'ects are d e s c r i b e d q u a n t i t a t i v e l y ilt equations t l e v c l o p c d I,y B e l t s a n d P e t h l c a ' ~. viz. 6.1
= -2- .1112
cash {''q'" \ 2k-T)
sinh (eq'oi 2k'l') -- 2( t --o) ÷cash (('~./2/~T) "- I
}
(19) and
aIl) ap--p-H m.,4.T
6.1 ml/= ( I - a ) sinh(edSo/2k7")
2(I L-a)+coth(eqso/2kT")
(20)
One can also compare observed 11 - A characteristics of a weak electrolyte monolayer with a theoretically derived equation of state. For example, at the oil/water lnt¢rfnc¢, where |ttterchaln forces are assumed to b~, ~¢ttlil;lble, the generally ~sumed Amullut-typ¢ equation of state, n o (A --AB !- " kT, for uncharged monolayers, can be combined w | t h ¢qn. (13) to yield theoretical values of II ~ (A = A 0 ) for comparison with ¢xpertmenta! values t ~. Payens' approaches are less certain at the air/water Interface, except p~rhaps at high areas/molecule, in view of the likelihood of [nlermol¢cular intoraetion, both from th© chains and head groupL Equations of state at tills interface are less well developed, and in any cas¢, as will b¢ d~monstrated, II-A characteristics are generally less sensitive to Ionization titan A V~A
.52
ILD, GODDARD --
11
I
•
I
!
| 40
0
~:~ z
6
PH
8
tO
Fig. I . Extent o f lonLzation o f steaJric acid (e) and behenic acid (,,) monolayers " s k i m m e d " from the aureate o f 0ol Af sodium bicaJrbonate buffer subsolutlons o f different ptl.
characteristics. This conclusion certainly holds for long-chain amphipatldc materials, if not always for polymers. However, for polymers, it must be remembered that Incorporation of electrostatic terms represents a further level of complexity in existing equations of state which are at best approximate for unionized forms. We pass now to examples o f difl'erent monolayer types.
(U Fatty aclda These ate by fat the most widely examined materlals~ yet prior to tl~¢ work of Bags ~ et el,, no direct determinations had been reported on the composition of spread fatty acid monolayers on subsolutlons containing only monovalent eounterions, The above workers carried out Infrared analysis on pre-spread films skimmed from the surface of subsolutlons of different pH, The solutions were 0,1 =0,;1 M In sodium Ion and Incorporated different buffers or were unbuffered. Composition vs. pH data are IIlven In Fig, 1 for the bicarbonate system, and pK data for all the systems are given In Table 1. Although there Is some dependenoe on the buffer system, probably reflecting dlfferen¢e. In buffering efficiency, all reveal the ¢har= aoterlstle apparent weakness of the sold In monolayer form. It Is of Interest that the bicarbonate buffer led to derived value~ of pK, based on u ~ o f eqn. (4) and the Gouy--Chapmun expression (7), In ltood aBreement with t h o u obtained by the surface potential method Illven below, despite the n e e e u l t y o f employing high valuea of ¢= for the derivation.
53
IONIZING MONOLAYERS AND pll I~FFECT$ T,bl0 1 V~lu©~ of I ~ from infrlir©d antilysl~ (ref, 9)
PNt, Substrata
Acid
App~©nt
Cah:ulated
0,1 M N a C I - N , O l l 0.2 jtf N~OII-NuCI 0.1 M N a C I - N a O I t 0.2 M N a O i [ - N a C l Phosphate buffer Phosphate buffer Bicarbonate buffer Bicaxbonate buffer
Stea~le Stearic rlehenlc Behenic Steartc Behenic Ste~ric
8,9 8.5 tl.9 8.$ 8.9 8,9 8.2 8.2
6.3 6.2 6.3 6,2 6.6 6.6
Behenic
5.6 5.6
4°°
300
\
20(:
\
""
b
\ 0
"\ c. %
I00
~ 6 A II aalMNetf:lla.e31Nt4e~jpt.~|l
0
~I b II I t ~.8e|N Nd~&Oj eil I I /41 tJd,~lS~l t~[mFi (MIIliIIj d61d)
4
6
pVl
il
%%
"%
\
%
~,
%
% %
tO
Pill, I, Vlirlililon wllh pll of lh# ,iitfiee li~i.nihil (,i Vm~i,iI i . ~llii~ pli~k#d molloliiyi~,J of 4I~Mi~ (arid myrlsiie) aid, ,l!po, tt~l by vttiou! wolk~il,
54
E.D. GODDARD
T h e original A V--pH titration data o f S c h u l m a n and Hughes 7 have be e n c o n s i d e r a b l y m o d i f i e d b y more recent work. largely as a result o f ,:loser att e n t i o n paid to e x p e r i m e n t a l details (see Fig. 2). Glazer and Dogan 2z , using HCO_ - / C Oan~d2 - bui'fers, o b t a i n e d s u b s t a n t i a l l y higher A V values t h a n did Schu['man Hughes over most o f the pH range. Recognizing the high sensitivity o f ionized fatty acids to foreign lens, s u b s e q u e n t wor ke r s t e n d e d to avoid c o m p l e x b u f f e r systems. T h u s Sanders a nd S p l n k 2 :j utilized a m m o n i a systems, a n d Betts a n d Pethlca used c o m p l e t e l y u n b u f f e r e d s o d i u m chloride s u b s o l u t t o n s ud]ustc¢l in pH w i t h small a m o u n t s of HCI or NaOH. The restilts o b t a i n e d b y these latter workers again led t o s u b s t a n t i a l l y Idgher A V values, w i t h A V vs. ptt being r e p r e s e n t e d by t h e m as e sse ntia lly linear from pH 3 to 9. By i n t r o d u c i n g a c o r r e c t i o n dipole term (p~ -/.t I ) o f - 2 6 4 roD, Betts a n d Pettlica f o t m d an essentially c o n s t a n t value o f 5.5 for the derived p K over a range o f a o f 0 to 0.01. A similar value ( pK ~ 5) was o b t a i n e d by the m e t h o d o f G o d d a r d a n d Acklllt ~l 0 whose results essentially c o n f h ' m e d the data o f Bctta and Pethlca, but d e m o n s t r a t e d the e x i s t e n c e o f a previously u n d e t e c t e d critical point at ca. pH 9 above w h i c h there was a drastic r e d u c t i o n In A V values. This f i n d i n g led these uuthor s to c onside r the possibility that c o m p l e x , 1.¢. a c l d ~ s o a p , f o r m a t i o n was occurring in these close-packed m o n o layer systems. However, an e l e c t r o s t a t i c m o d e l analysis in the range o f f or ma l a p p l i c a b i l i t y o f the G o u y e q u a t i o n s did not allow a choice b e t w e e n the simple attd ucld~soup dissociation modes. T h e I1--A characteristics ~ ~ of ih~ stcaric acid m o n o l a y e r s were very revealing. Despite increasing Ionization. the coJldensud n a t u r e of the film was almost u n c h a n g e d tip tO pH 9. A c c o r d i n g to eqn. (13), c o n s i d e r a b l e increases in the fihn pressure from tlt~ electrical contribution, A r l , o f the ionized component were to be 0ntlcipated at rids pH. It was found that the pH had to be increased well beyond 9 before this increase occurred =4 . These two observations concerning A~, and II behavior In the vicinity o f pH 9 seemed to be linked and thle conclusion led to further Investigation o f the phenomenon in th~ light o f th~ w~ll known stron[~ association =t= which occurs bctw¢¢n Ionized and unionized forms o f f a t t y acids In bulk phase. The p o t e n t i a l w o r k =~ was r e p e a t e d o n u n b u f f e r e d s u b s o l u t l o n s in a CO2-free envlronm©nt, a n d on purified b o r a x s u b s o l u t l o n s ; it was d e m o n s t r a t e d that the critical p h e n o m e n a occurring a r o u n d pH 9 wer0 in part, but not w h o l l y , the c o n s e q u e n c e o f b u f f e r i n g by a t m o s p h e r i c C O a , since a slope change, albeit smaller, in the A V--pH characteristics still oc c ur r e d at the same pH (see Fig. 3). T h e hazards o f changing even the t y p e o f m o n o v a l e n t c o u n t e r l o n on the critical p h e n o m e n a o c c u r r i n g a r o u n d pH 9 are e m p h a s i z e d in w o r k r e c e n t l y r e p o r t e d b y S h a h ~ ~. To avoid the possibility o f d i s s o l u t i o n of Ionized acid at high pH, the work was c o n t i n u e d w i t h the longer c h a i n b e h c n l c acid =6. The A V--pH r e l a t i o n s h i p s for this C~= acid agreed ve r y w©ll w i t h those ob-
IONIZING MONOLAYI:RS AND pI! I:FI'I:CTS
57
(iD FatO' ah'oholx and etiter.~ It is well k n o w n that m o n o l a y e r s o f these materials are little a f f e c t e d by s u b s o h t t i o n pH. The A V _ p H me:tsuretnents o f S c h u h u a n and [lttghes t on o c t a d e c a n o l attd o c t a d e c y l l l l e t l l y l ether, h o w ~ ' v c r , r e v e a l t w o i n t e r e s t i n g features: (at a fall-off in A V vahtes o f the a l c o h o l at very high ptl (ca. 12), indicative o f the behavior o f an e x t r e m e l y weak acid; {b) an upswing o f A V at very low pH (ca. It. o f b o t h the a l c o h o l and the ether, and, in fact, o f fatty acids*, alld ascribed in all cases to o x o n i u m ion form.'ltion.
(tit) F a t t y atnine~ F a t t y a l k y l p r i m a r y a m i n e s have b e e n e x a m i n e d , f o l l o w i n g the early work o f Peters, b o t h by Glazer anti Dogan 2~ (Cj s ) and by Butts anti Petition t*' ( C t g ) over a wide pH range. T h e A V - p t t c h a r a c t e r i s t i c s o b t a i n e d again r e s e m b l e a t i t r a t i o n curve, o n c e more wttl~ I ndic a tions o f a p p a r e n t weakness. Boll1 sets o f d a t a i n d i c a t e high positive values ttrottnd 7 0 0 mV (Ionized f o r m ) at low pH, w h i c h fall o f f at high pH { u n i o n i z e d forttt), althougit It is clear that the two sets o f data d i f f e r c o n s k l e r n b l y in detail. Moreover. the Butts and Putltica data reveal a cotlsidcrable tlepeitdence ot~ the i]tc)t|ovtilultt cottnterion present {Nit* vs. NII~ ) L l h e i r e~tiltmled pK e,~trapoklt,,,d to it value t)f |O. I in the limit o f ~ m O, T his s o m e w h a t Itivth value COtlld be lowered i)y ¢tl. 0.5 traits if a ~uitabh; dipole c o r r e c t i o n wcrc i n t r o d u c e d but there wa,, t~o i)Id l e a t l o n o f c o n s t a n c y o f oK w i t h co. Mar0 work on tile p|[ d e p e n d e n c e (If tire p r o p e r t i e s o f alllJne Inonolityers is rt~quircd. E x u m h m t i o n o f tile I I ~ A / p l l da ta o f Bctt~ and Pcthi¢~) t~u~ge~t~t tile degree o f e x p a n s i o n o f the m o n o l a y c r s at low pH i, i)ot c o n s i s t e n t (~ee c q n (I 3)) w i t h that o f a fully charged monolttyer. Tills conch)slon i~ r e i n f o r c e d by data 3j on the C~0 a m i n e at pfl 3, with th~ p o s s i b i l i t y ~l~ain being r e f le c te d o f t o n / d i p o l e association. R¢~;¢ntly the pH dependence o f the m o n o l a y e r c h a r a c t e r i s t i c s o f d o c o s y l d i m c t h y l a m i n o o x i d e was r e p o r t e d 29 . A m i n e o x i d e s are very weak bases Ionizing a c c o r d i n g to -I-
R-N-(CHa
)~ + H , O
--
--" R - - N - - ( C H 3), + O H -
pK'~ 9
I O H Strong evidence was adduced for association in the m o n o l a y e r s at low pH at w h i c h the m o n o l a y e r is largely in the p r o t o n a t e d form, i n a s m u c h as the latter yielded less e x p a n d e d films over m u c h o f the area range than did the uncharged dipolar form. Analysis o f the data in terms o f pH and o K shifts was not a t t e m p t e d . 0
58 6 0 0
E.D. G O D D A R D ~
1
I~
t
=
f
"
~
;
i
T"'~
'~w
•
q
\ \
~00
\
\ ",\
401
\ 3O0
"0
,i
\
\ §
oH
Fi~. 4. ~ V - p l l curves ~° [or alkyl phosphoric acid monol~ycrs on 0.1 Ionic ~trcnl[tll ~ubsolut|ons at 18°C. (o) Mono, octadecyl acid at u n a r e a o f 30 A2/~noi.; (4) dioctadecyl acid at an ~ ¢ u of' 45 A2/mol.
(iv) A Ikyl l~hosphates arid phosphonates S t u d i e s o f m o n o - a n d d i - o c t a d e c y l p h o s p h a t e m o n o l a y e r s as a f u n c t i o n o f pH have b e e n u n d e r t a k e n b y Parreira 3°. A l t h o u g h an e x p a n s i o n o f these monolayers w i t h i n c r e a s e d pH did occur, the m o n o l a y e r s r e m a i n e d r e l a t i v e l y condensed. In the first m a t e r i a l , this b e h a v i o r is very likely the result o f the singly charged species" possessing a h y d r o g e n b o n d i n g O H g r o u p a nd the l i m i t e d pH range u t i l i z e d a n d , in the s e c o n d m a t e r i a l , o f the pr e se nc e o f two oct a d e c y l chains. O n the o t h e r h a n d , a p lot o f sur f a c e p o t e n t i a l versus pH again had
the form
of a titration
curve
with
an inflection
around
pH 4, i.e, con-
s i d e r a b l y h i g h e r t h a n the b u l k phase p K value o f p h o s p h o r i c acids (see Fig. 4). S i m i l a r d a t a f o r t h e d i a l k y l p h o s p h a t e have b e e n o b t a i n e d b y G o d d a r d a n d A l e x a n d e r 3t , a n d at close p a c k i n g b o t h the l a t t e r a n d those o f Parreira reveal a s h a r p u p s w i n g in A I,r at very l o w pH. A n a n a l y s i s o f his d a t a using the Betts-P e t h i e a a p p r o a c h was u n d e r t a k e n b y Parreira, w h o arrived at p K values for
IONIZING MONOLAYERS AND pll EFFECTS
$9
the mono- and d i a l k y l phosphates o f 2,8 and 1.7, w i t h o u t a dipole correction. and 3.0 and 2. I w i t h one. Payens t6 demonstrated qualitatively that the stirface pressure o f partially ionized alkyl phosphonate monolayers spread at the air/water interface is increased by addition o f salt, but devoted his main :ttt e n t i o n to spread monolayers o f stcaryl phosplmnute at the o i l / w a t e r interface so as to lessen the possibility o f association between the monolayer molecules. Measured and calculated salt effe~;ts on surface pres.~ure were compared as shown In Fig. 5. Agreement was moderately good at intermediate concentrations provided the (somewhat high) value o f pK = 3.8 was assumed. Payens offered tin explanation o1" thi.,, value, which exceetl~ bulk phase v.'dues by over ! unit, in terms o f changed h y d r a t i o n ch:tracteristtc~ o f the lens at the interface. A t low and. especially, at high salt concentrations agreement was unsatisfactory, it is o f interest that In most cases the monolayers did |lot c o n f o r m to the Davies equation o f state for monolayers at the o i l / w a t e r interface: measured surface pressur©s were generally appreciably lower. Payens a s c r i b e d t h e d i s c r e p a n c y t o r c s l d u a l c o h e s i o n in t h e m o n o l a y e r , not accounted f o r in t h e e q u a t i o n o f s t a t e ,
(v) A / k y l sulfates and alkane sul[onates Surface pressure :=nd surface potential/area curves for nonadccylbenzcne sulfonate are presented Jz in Fig. 6. It is o f interest th=,t in thi.,i ca~u tlw l l - t l data reveal considerable changes occurring at p l l "-" 4. and that at pll 2 and 3 the m o n o l a y c r is noticeably more expanded than at the higher pl l's. A similar effect has been reported by Dreher and Wilson 3:~ and by other workers =4.=s l b r monolayers o f alky] sulfates. Pethica anti Few 3= ascribe the effect to an H-bonded structure involving h y d r o n i u m ions, while Dreher ascribes it to dcionization o f the film. A n interesting feature w h i c h the data o f Fig. 6 displ=y is relative insensitivity o f A V in the pH range where the H - A characteristics change, but a marked elevation o f potential at very low pH. Despite the fact that the latter conditions necessitated an increase in subsolution ionic strensth, it is likely that the same phenomenon is involved as that i n v o k e d e a r l i e r b y S e h u l m a n a n d H u g h e s f o r f a t t y et~;ids, a l c o h o l s a n d ether,:. i.e. o x o n i u m i o n f o r m a t i o n , a n d w h i c h w e also o b s e r v e d f o r t h e a l k y l p h o s phates. T h e p H e f f e c t s d e s c r i b e d a b o v e raise t h e q u e s t i o n o f t h e a c t u a l a c i d s t r e n g t h o f a l k y l s u l f o n a t e s a n d s u l f a t e s , e s p e c i a l l y in t h e ~urfaee. T h k quextion of s u r f a c e h y d r o l y s i s , i.e. p r e f e r e n t i a l a d s o r p t i o n o f H + o r its c h e m i c a l c o m bination w i t h the head group, has been u n d e r discussion for some years; we shall r e t u r n t o this t o p i c in t h e s e c t i o n o n a d s o r b e d m o n o l a y e r s .
60
EoD. G O D D A R D
O tel
14 3T IO
6
t,t ~ D E
¢
0
2
p NaCI
4
6
Fig. 5. Th eo r etical (curves) and e x p e r i m e n t a l (points) changes in surface pressure o f s t e a w l p h o s p h o n i c acid m o n o l a y e r s at the oil/water interface 16 on adding neutral fair to the aqueous phase, pH. 5 . 8 . 2 5 ° C . (c,) 50 AX/mol; (A) 100 A2/mol.; (o) 150 A 2/ m ol .
(vi) Phospholipids a n d o t h e r z w i t t e r i o n i e materials It is i m p o r t a n t to p o i n t o u t i m m e d i a t e l y t h a t , in d e i o n i z i n g o n e o f the g r o u p s o f a z w i t t e r i o n , " s u r f a c e w e a k n e s s " is n o t e x p e c t e d in view o f the p r o x i m i t y o f the charge o f the o t h e r gr oup. F u r t h e r m o r e , while the e x a c t c o n f i g u r a t i o n o f the i o n i c d i p o l e o f m o n o l a y e r z w i t t e r i o n s is n o t a b s o l u t e l y e s t a b l i s h e d , it s e e m s r e a s o n a b l e t h a t t h e r e e xists a c o - p l a n a r h o r i z o n t a l arr a n g e m e n t o f t h e positive and negative charges, for w h i c h a r r a n g e m e n t the fit p o t e n t i a l w o u l d be e f f e c t i v e l y zero. A vertical a r r a y o f d i p o l e s w o u l d have an e x t r e m e l y high repulsive e n e r g y a n d p o t e n t i a l , w h i c h have n o t b e e n observed. S t u d i e s o f t h e i n f l u e n c e o f pH o n p h o s p h o l i p i d s have b e e n u n d e r t a k e n b y P c t h i c a a n d his c o - w o r k e r s ~ , ~ ' . W o r k i n g w i t h d i s t e a r o y l p h o s p h a t i d y l c h o l i n e , PC, a z w i t t e r i o n c o m p r i s i n g a m e d i u m s t r e n g t h a c id g r o u p a n d a
61
IONIZING MONOLAYERS AND pH EFFECTS 400 E 200
o
?,
\,,
::2:!o .
.
,o
30
50
TO A
90 ~ l tooL
Fig. 6. E f f e c t o f p H o n I~--A a n d ~ I,'--,4 i s o t h e r m s a2 o f n o n a d e c y i b e n z e n e s u l f o n a t ¢ m o n o l a y e t s at 2 5 ° C . S u b $ o l u t i o n s ate o f ionic s t r e n g t h 0.01 ( N a O H o r HCI]NaCI), e x c e p t f o r the 0.1 N a n d I N H2 S 0 4 .
s t r o n g b a s e g r o u p , A n d e r s o n a n d P e t h i c a 37 o b s e r v e d a p l a t e a u r e g i o n f r o m p H --- 2 . 5 t o 8 in t h e A V - - p H p l o t (see Fig. 7). T h e s h a r p u p s w i n g in A V b e l o w p H 2.5 is c o n s i s t e n t w i t h d e i o n i z a t i o n o f t h e p h o s p h a t e g r o u p ; o n t h e o t h e r h a n d , t h e s h a r p d o w n s w i n g in Lt V, e x p e c t e d f o r d e i o n i z a t i o n o f t h e b a s i c g r o u p s , o c c u r s a t p H 8 a n d is d i f f i c u l t t o e x p l a i n in v i e w o f t h e s t r o n g l y b a s i c n a t u r e o f t h e c h o l i n e g r o u p . W h a t s e e m s likely is t h a t a r e o r i e n t a t i o n o f t h e h e a d g r o u p o c c u r s in t h e p H r a n g e a b o v e 8. O t l l e r e v i d e n c e f o r thi~ c o m e s f r o m t h e c h a n g e in I I - - A c h a r a c t e r i s t i c s a t p H 10, a t w h i c h p H t h e m o n o l a y e r d e v e l o p s f e e b l e s t a b i l i t y . T h e r e is n o i n d i c a t i o n o f m o n o l a y e r e x p a n s i o n a t l o w p H , w h i c h is e x p e c t e d t o a c c o m p a n y d e i o n i z a t i o n o f t h e phosphate group. R e s u l t s m u c h m o r e in k e e p i n g w i t h a n t i c i p a t e d b e h a v i o r o f a z w i t t e r i o n o f d o u b l e - s i d e d s t r e n g t h w e r e o b s e r v e d a2 w i t h m o n o l a y e r s o f a syntbc*.ic s u l f o b e t a i n e , d o c o s y l a m m o n i o p r o p a n e s u l f o n a t e , C 2 o H4 ! N+ ( C H a ) = ( C H 2 ) a SO3. T h i s m a t e r i a l is a n a n a l o g o f a l e c i t h i n b u t w i t h t h e t e r m i n a l g r o u p of the zwitterion negative rather than positive. For this material only minor c h a n g e s w e r e o b s e r v e d in b o t h I I - - A a n d A V - - A c h a r a c t e r i s t i c s o v e r a p H
62
E.D. GODDARD O
~'
"
-
'.
'
w'
\
600
\~,~__
410
540
'\',
~
zgo
o
!.
"\t!
480
*
0
4
|70
A t
_u
l,,in
8
t2
Fig. 7. ~ IF- pH r e l a t i o n s h i p s o f p h o s p h o l i p i d m o n o l a y e t s 36"37. (~). (o) D i p a l m i t o y l p h o s p h a t i d y l e t h a n o l . a m i n e , at a n a.-ea o f 4 4 A 2 / m o l . . s p r e a d o n 0 . 0 1 . 0 . 1 N NaCI s u b s o l u t i o n s at 2 3 ° C ; (o) d i s t e a r o y l p h o s p h a t i d y l c h o l i n e , a t 5 0 A 2 / m o l . . s p r e a d o n 0.1 2~t NaC! at 2 0 " C .
r a n g e o f 1 t o 12; i.e. t h e r e w e r e n o i n d i c a t i o n s o f a n y s u r f a c e w e a k n e s s o f either head group constituent. T h e a d d i t i o n a l A V vs. p H d a t a s h o w n i n Fig. 7 w e r e o b t a i n e d b y S t a n d i s h a n d P e t h i c a s6 f o r d i p a l m i t o y l p h o s p h a t i d y l e t h a n o l a m i n e , P E , a s y n t h e t i c c e p h a l i n , w h i c h possesses a m e d i u m s t r e n g t h a c i d g r o u p a n d a w e a k base group. The plateau regions, consistent with an effectively uncharged monol a y e r , are s e e n a t i o n i c s t r e n g t h s o f b o t h 0.1 a n d 0 . 0 1 , as are t h e a n t i c i p a t e d u p s w i n g o f p o t e n t i a l at l o w p H a n d d o w n s w i n g at h i g h p H , c o r r e s p o n d i n g to deionization of the phosphate group and primary amine group, respectively T h e o b s e r v e d salt e f f e c t is q u a l i t a t i v e l y c o n s i s t e n t w i t h e l e c t r i c a l p r e d i c t i o n s . Calculations were made, from the data, o f pK values o f the c o n s t i t u e n t acidic a n d basic g r o u p s , u s i n g t h e m e t h o d o f e x t r a p o l a t i o n t o z e r o d e g r e e o f i o n i z a t i o n . T h e d e r i v e d v a l u e s 36 so o b t a i n e d w e r e p K a = 0 . 3 2 , m u c h l o w e r t h a n t h e b u l k p h a s e v a l u e of--~2, a n d PKbase = 3 . 5 5 , c o m p a r a b l e t o t h e v a l u e i n b u l k . It is n o t c l e a r w h i c h o f t h e salt c o n c e n t r a t i o n s was e m p l o y e d f o r t h e computations, or if the above values were averaged for the two. The un-
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Fig. 8. Effect of pH on rl-A and A V - A isotherms of the sult'oamine, 2-LAr-methyl-N-2-hydroxydocosyl) a m i n o e t h a n e s u l f o n a t e , o n i o n i c s t r e n g t h 0 . 0 1 ( N a C I ÷ H C I o r N a O i l ) s u b s o l u t t o n s 32 a t 2 5 = C . ltzset: Curve - plot of ,~V values at an area of 40 A2/mol. vs. pH; (X) - calculated values of the q'o potential ( 4 0 A 2 / m o l . ) vs. p H .
usually low value o f the derived p K a indicates that the simple electrical analysis is n o t a p p l i c a b l e u n d e r a c i d c o n d i t i o n s . It is t o be n o t e d t h a t a s l i g h t reorientation within the monolayer would necessitate a substantial dipole c o r r e c t i o n to A V for this kind o f m o l e c u l e . A m a r k e d e x p a n s i o n o f the II--A c u r v e s o c c u r r e d b e t w e e n p H 9 . 8 a n d 1 1.9, c o r r e s p o n d i n g t o " ' i o n i z a t i o n " o f a n i n c r e a s i n g l y n e g a t i v e l y c h a r g e d m o n o l a y e r . N o e x p a n s i o n was, h o w e v e r , n o t e d a t l o w p H . F o r c o m p a r i s o n w i t h t h e a b o v e , we use m o n o l a y e r d a t a r e p o r t e d 32 f o r t h e " a n a l o g " o f a c e p h a l i n , n a m e l y t h e st, l f o a m i n e , 2-(Nm e t h y l - N - 2 - h y d r o x y d o c o s y l ) a m i n o e t h a n e s u l f o n a t e , Czo H 4 ~ - C H ( O H ) CH2 N ( C H 3 ) ( C H 2 )2 SO3 N a , w h i c h p o s s e s s e s a s t r o n g t e r m i n a l a c i d i c g r o u p a n d a n i o n i z a b l e t e r t i a r y n i t r o g e n g r o u p . It s h o w s s e v e r a l s i m i l a r i t i e s t o t h e p h o s p h o U p i d s , as s e e n in Fig. 8. T h u s its m o n l a y e r s a r o u n d n e u t r a l p H , a n d p r e s u m a b l y in z w i t t e r i o n i c f o r m , are i n s e n s i t i v e t o c h a n g e s in i o n i c c o m p o s i t i o n o f t h e s u b s o l u t i o n a n d t h e r e is a p l a t e a u r e g i o n in t h e A V - - p H c u r v e .
64
E.D. G O D D A R D
At high pH, when the amine group becomes progressively unionized and the monolayer develops a net negative charge, a marked expansion of the monol a y e r o c c u r s , a n d t h e s u r f a c e p o t e n t i a l b e c o m e s p r o g r e s s i v e l y less p o s i t i v e . F o l l o w i n g S t a n d i s h a n d P e t h i c a 3a , t h e a u t h o r s o f t h i s w o r k a s s u m e d t h e xPo p o t e n t i a l o f t h e m o n o l a y e r t o be z e r o in t h e p l a t e a u r e g i o n f o r c a l c u l a t i o n p u r p o s e s , a n d t h e y u t i l i z e d t h e G o u y e q u a t i o n s t o e s t i m a t e t h e p K value o f t h e a m i n e . C a l c u l a t i o n s w e r e r e s t r i c t e d t o ~ v a l u e s b e l o w 0 . 0 5 in o r d e r : (a) t o m e e t m o r e c l o s e l y t h e c o n d i t i o n s u n d e r w h i c h t h e t h e o r y is a p p l i c a b l e , a n d (b) t o s t a y w i t h i n t h e r a n g e w h e r e t h e d i p o l e c o n t r i b u t i o n w o u l d e f f e c t i v e l y be c o n s t a n t . In t h i s . w a y , t h e p K a value w a s e s t i m a t e d to be 10, w h i c h is r e a s o n a b l y close t o t h e b u l k value f o r t r i m e t h y l a m i n e . O n t h e o t h e r h a n d , t h e u p s w i n g in A V as t h e p H is l o w e r e d a p p e a r s t o s t a r t at a p H w h i c h is t o o h i g h t o be c o n s i s t e n t w i t h t h e h i g h acid s t r e n g t h o f t h e s u l f o n a t e g r o u p s , a n d m a y a g a i n r e f l e c t a r e o r g a n i z a t i o n in t h e m o n o l a y e r . L a c k o f a n y s i g n i f i c a n t e x p a n s i o n o f t h e m o n o l a y e r at t h e l o w e s t p H also i n d i c a t e s t h a t t h e m o l e c u l e s are n o t cor, a p l e t e l y in t h e p o s i t i v e l y c h a r g e d from and again suggests appreciable molecular association.
(vii) P o l y m e r s In 1 9 5 2 , G l a z e r a n d D o g a n ~n p u b l i s h e d A V - - p H d a t a o n s p r e a d m o n o l a y e r s o f b o v i n e s e r u m a l b u m i n , a n d i n t e r p r e t e d t h e r e s u l t s in t e r m s o f t i t r a t i o n o f t h e side c h a i n c a r b o x y l a n d a m i n o g r o u p s , t h e p K ' s o f b o t h o f w h i c h appear to shift to higher values t h a n the bulk phase values o f proteins. H o w e v e r , u s i n g t h e i r a p p r o a c h o f c o m p a r i s o n at c o n s t a n t p r e s s u r e , t h e y w e r e u n a b l e t o d i s c e r n i n f l e c t i o n s c o r r e s p o n d i n g to n e u t r a l i z a t i o n o f t h e s i d e c h a i n i m i d a z o l e g r o u p s , H a r r a p 39 , w o r k i n g w i t h i n s u l i n , d e m o n s t r a t e d t h a t if A V - - p H c u r v e s are c o n s t r u c t e d at c o n s t a n t a r e a r a t h e r t h a n c o n s t a n t pressure t h i s w a s n o t t h e case. C o m p a r e d w i t h b u l k p h a s e v a l u e s , t h e p K ' s o f t h e acid a n d basic g r o u p s were again shifted u p w a r d . A l t h o u g h the p K values h a v e n o t b e e n a n a l y z e d in d e t a i l , it s h o u l d b e n o t e d t h a t , c o m p a r e d t o p K v a l u e s o f i s o l a t e d C O O H a n d N H 2 g r o u p s in b u l k p h a s e , s u b s t a n t i a l s h i f t s in t h e d i r e c t i o n o f w e a k n e s s o f t h e s e g r o u p s in p r o t e i n m o n o l a y e r s are n o t e x p e c t e d . T h e s i t u a t i o n is a n a l o g o u s t o t h e case o f z w i t t e r i o n s a l r e a d y disc u s s e d : w h e n t h e - - C O O H g r o u p s are b e i n g i o n i z e d t h e m o l e c u l e c a r r i e s a net positive charge which would promote ionization; when the NH 2 groups are b e i n g i o n i z e d , t h e m o l e c u l e s c a r r y a n e t n e g a t i v e c h a r g e w h i c h w o u l d d o t h e s a m e . T h i s s i t u a t i o n e x i s t s , o f c o u r s e , also f o r b u l k p h a s e t i t r a t i o n s o f p r o t e i n s . A s e x p e c t e d o n e l e c t r i c a l g r o u n d ~ , H a r r a p s h o w e d t h a t s u r f a c e pressures for insulin m o n o l a y e r s were at a m i n i m u m at the isoelectrie point o f 5.7, as s e e n in Fig. 9.
IONIZING MONOLAYERS AND pH EFFECTS |
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Fig. 9. r l - p H (A) and n V--pH (o) curves for insulin t-rims spread on ionic strength 0.01 subsolutions 39.
A great deal of work on monolayers of synthetic protein analogs, together w i t h a s m a l l e r a m o u n t o n p o l y a c i d s , w a s c a r r i e d o u t d u r i n g t h e 1950"s b y l s e m u r a a n d his c o l l e a g u e s 4 ° - 4~. A p p r e c i a b l e e f f e c t s w e r e f o u n d o n t h e m e a s u r e d I I - - A . A V - - A a n d r/--A c h a r a c t e r i s t i c s as t h e p H w a s c h a n g e d . T h e s e c h a n g e s w e r e in a s e n s e g e n e r a l l y p r e d i c t a b l e f r o m t h e i o n i z a t i o n / d e i o n i z a t i o n o f t h e p o l a r g r o u p s p r e s e n t , s u c h as a m a x i m u m in s u r f a c e v i s c o s i t y (r/), a n d a m i n i m u m in s u r f a c e p r e s s u r e , a r o u n d t h e i s o e l e c t r i c p o i n t . I o n i z a t i o n o f C O O H g r o u p s in p o l y a c i d s led t o p r e d i c t a b l e d e c r e a s e s in m e a s u r e d A V's; h o w e v e r , FI v a l u e s g e n e r a l l y d e c r e a s e d as t h e a c i d i t y w a s d e c r e a s e d m a i n l y because of solubility of the ionized form which severely restricted the pH r a n g e i n v e s t i g a t z b l e . C o m p a r a b l e d a t a w e r e o b t a i n e d b y Miller 44 f o r m o n o l a y e r s o f p o l y ( v i n y l b e n z o i e a c i d ) . In o n e p a p e r b y l s e p a u r a a n d H a m a g u c h i * t , qz 0 values o f a m o n o l a y e r o f s y n t h e t i c p o l y p e p t i d e (ly~sine, p h e n y l a l a n i n e , g l u t a m i c acid c o p o l y m e r ) w e r e e s t i m a t e d f r o m ( a ) II a n d ( b ) A V, u s i n g t h e Davies equations. Good agreement was obtained. M o r e r e c e n t l y , J a f f 6 a n d R u y s s c h a e r t 45 h a v e i n v e s t i g a t e d t h e p r o p e r t i e s of a variety of ionizable polyeleetrolytes, both addition type (polyvinyl) and
66
E.D. G O D D A R D
Table 2 Derived values o f pK for m o n o l a y e r s o f poly (methacrylic acid) (ref. 46) Interfac©
Stereoregularity
PKGouy
PKDonnan
AErlwater
Atactic Isotactic Atactic lsotactic
4.6 4.4 4.9 4.8
4.6 4.6 5.0 4.9
Cyclohexane/water
condensation type (polypeptides). Working with a copolymer of 2-vinylpyrid i n e a n d a c r y l i c acid t h e y d e m o n s t r a t e d t h a t t h e G o u y e q u a t i o n s s a t i s f a c t o r i l y p r e d i c t values o f t h e e l e c t r i c a l c o n t r i b u t i o n A H to t h e s u r f a c e p r e s s u r e f o r low n e t c h a r g e d e n s i t i e s a r o u n d t h e i s o e l e c t r i c p o i n t . T h e y s h o w e d t o o t h a t identical v a l u e s o f t h e i n t r i n s i c d i s s o c i a t i o n c o n s t a n t K o f p o l y a c i d s are o b t a i n e d b y t h e s u r f a c e t i t r a t i o n m e t h o d as b y t h e b u l k t i t r a t i o n m e t h o d , using t h e Katchalsky expression (3). In t h e i r m o s t r e c e n t w o r k t h e s e a u t h o r s 46 have t u r n e d t h e i r a t t e n t i o n t o a study of the effects of stereoregularity of polyelectrolytes on their surface p r o p e r t i e s at b o t h t h e a i r / w a t e r a n d o i l / w a t e r i n t e r f a c e . R e c o g n i z i n g t h e diff i c u l t y o f i n t e r p r e t i n g s u r f a c e p r e s s u r e m e a s u r e m e n t s t h e y c h o s e to m e a s u r e and analyze surface potential measurements, employing both the Gouy-Chapman and Donnan approaches. The essential part of the analysis was the u t i l i z a t i o n o f t h e B e t t s - - P e t h i c a m e t h o d o f d e t e r m i n i n g t h e p g values b y e x t r a p o l a t i o n o f ( p H s 4- log (1 --e,/0t) values o b t a i n e d a t l o w values o f a to yield PKrr_, 0" T h e v a l u e s o b t a i n e d f o r p o l y ( m e t h a c r y l i c a c i d ) a r e g i v e n in T a b l e 2 a n d reveal t h a t , u n l i k e t h e case o f t h e s e p o l y e l e e t r o l y t e s in s o l u t i o n . the isotactic c o n f i g u r a t i o n has a slightly stronger acidity. This difference was ascribed to greater accessibility of carboxyl groups allowed by this configurat i o n a t t h e i n t e r f a c e . A l t h o u g h t h e d i f f e r e n c e s in b e h a v i o r e v i d e n t f r o m t h e d a t a o f T a b l e 2 a p p e a r small, this w a y o f p r e s e n t i n g t h e clata belies t h e c o n siderable actual differences occasioned both by the stereoregularity and by t h e t y p e o f i n t e r f a c e . T h i s v a r i a t i o n is e v i d e n t in t h e c o m p i l a t i o n o f e x p e r i m e n t a l data given in Fig. 10. A n a l y s i s o f s u c h data, as y e t u n a t t e m p t e d , would appear fruitful to provide information on configurational changes a c c o m p a n y i n g t h e p r o g r e s s o f i o n i z a t i o n . It is o f i n t e r e s t t h a t t h e G o u y - C h a p m a n a n d D o n n a n a p p r o a c h e s led to v i r t u a l l y i d e n t i c a l xp a n d d e r i v e d p K values f o r t h e c o n t r i t i o n s e m p l o y e d . In a s e c o n d p a p e r a p p l y i n g t h e s u r f a c e p o t e n t i a l m e t h o d , C a s p e r s et al. 4~ e x a m i n e d t h e i n f l u e n c e o f i n t e r f a c e ( a i r / w a t e r vs. c y c l o h e x a n e / w a t e r ) o n t h e i o n i z a t i o n o f m o n o l a y e r s o f p o l y - L - g l u t a m i e acid. It is s t a t e d t h a t t h i s m a t e r i a l a s s u m e s a helical c o n f i g u r a t i o n in t h e i n t e r f a c e . In t h i s case, t h e A V - - p H c h a r a c t e r i s t i c s did n o t s h o w t h e g r o s s d i f f e r e n c e s at t h e t w o in-
IONIZING MONOLAYERS AND pH EFFECTS
!
Fig. tO. I n f l u e n c e o f p H o n s u r f a c e - p o t e n t i a l , - c o n c e n t r a t i o n curves o f a t a c t i c (a) a n d iso~actic (i) poty(m©thacryfic acid) m o n o l a y e r s at the air/water and oil/water interface 46.
67
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68
E.D. GODDARD
terraces evidenced by the p o l y ( m e t h a c r y l i c acid). T h e a u t h o r s d e d u c e d f r o m a s t u d y o f A V vs. p H t h a t t h e o i b t w a t e r i n t e r f a c e a l l o w s less r e a d y a c c e s s i b i l i t y of the COOH groups for ionization than does the air/water interface, although t h i s is n o t e v i d e n t in t h e r e s p e c t i v e pKa_, a v a l u e s o f 4 . 4 ( G o u y , D o n n a n ) a n d 4.5 ( G o u y , D o n n a n ) w h i c h w e r e d e r i v e d . T h e d a t a s h o w a r e s i d u a l d e p e n d e n c e o f p K o n i o n i c s t r e n g t h , d e s p i t e t h i s f a c t o r h a v i n g b e e n a l l o w e d f o r in t h e G o u y - C h a p m a n a n d D o n n a n d e r i v a t i o n s . It was a s c r i b e d e i t h e r t o s t a b i l i z a t i o n o f t h e h e l i c a l s t r u c t u r e a t low i o n i c s t r e n g t h o r a n i n t r i n s i c d i f f i c u l t y o f t h e h y d r o g e n i o n in leaving t h e m a c r o m o l e c u l e , i.e. o v e r a n d a b o v e t h a t a l l o w e d for by simple electrical t h e o r y . D. A D S O R B E D M O N O L A Y E R S
W h i l e it is well k n o w n t h a t a d s o r b e d f i l m s at a q u e o u s i n t e r f a c e s will res p o n d t o c h a n g e s in pH o f t h e b u l k a q u e o u s p h a s e i f t h e film m a t e r i a l is ionizable, c o m p a r a t i v e l y little w o r k has been d o n e to o b t a i n q u a n t i t a t i v e i n f o r m a t i o n o n t h e c o m p o s i t i o n o f t h e i n t e r f a c e . D a n i e l l i ' s s a p p r o a c h involved t h e u n p r o v e n a s s u m p t i o n t h a t t h e l o w e r i n g o f i n t e r r a c i a l (c, i l / w a t e r ) t e n s i o n b y f a t t y a c i d s was d i r e c t l y p r o p o r t i o n a l t o t h e a m o u n t o f i ¢ n i z e d acid at t h e i n t e r f a c e . More r e c e n t a p p r o a c h e s have g e n e r a l l y i n v o l v e d t h e a p p l i c a tion of the Gibbs adsorption equation to surface and interracial tension data, although direct measurements of the composition of the surface by f o a m i n g a n d r a d i o t r a c e r t e c h n i q u e s h a v e also b e e n m a d e . A t c o n s t a n t pressure a n d t e m p e r a t u r e t h e G i b b s e q u a t i o n , r e l a t i n g i n t e r f a c i a l t e n s i o n , % t o s u r f a c e excess, r , has t h e f o r m --d3, = d l l = ~
rtd/a i
(25)
w h i c h , a p p l i e d t o a salt o f a w e a k l o n g c h a i n acid HZ, b e c o m e s --d~ = ruz
d/~mz + r z d/z z + ~ /
r i d/~l
(26)
T h e / t e r m s r e p r e s e n t c o m p o n e n t s w h i c h are n o n s u r f a c e - a c t i v e , p e r se. P a y e n s 16 • b y s p l i t t i n g t h e c h e m i c a l p o t e n t i o n a l t e r m t~z i n t o e l e c t r i c a l a n d n o n - e l e c t r i c a l t e r m s , e ~ a n d / a ~ , has s h o w n t h a t e q n (1 1) a p p l i e s t o a d s o r b e d f i l m s , b o t h i o n i z e d a n d p a r t i a l l y i o n i z e d , as well as i n s o l u b l e f i l m s , a n d has used t h i s e q u a t i o n f o r a n a l y s i s o f his a d s o r p t i o n results. We s h a l l n o w d e a l w i t h t w o b r o a d c a t e g o r i e s o f s o l u b l e s u r f a c e a c t i v e m a t e r i a l , viz. a m p h i p a t h i c w e a k e l e c t r o l y t e s a n d a m p h i p a t h i c s t r o n g e l e c t r o lytes.
IONIZING MONOLAYERS
E IJI
AND pH EFFECTS
--O -o A
60 o f O / °
~" 5 o
69
-. _.......---o-- 8
// / o
o /
O
3c
?
/
/ o |
"7
|
~
I,
pH
I
II
F i g . 11. E q u i l i b r i u m s u r f a c e t e n s i o n v a l u e s s2 o f ( A ) 0 . 0 0 1 N a n d (B) 0 . 0 0 5 N s o d i u m l a u r a t e s o l u t i o n s a t
25"C.
(i) F a t t y a c i d s o a p s a n d a m i n e s F o l l o w i n g R a i s o n 4a a n d L e v i a l d i 49 , E a g l a n d a n d F r a n c k s so c a r r i e d o u t f o a m i n g s t u d i e s o n s o d i u m o l e a t e s o l u t i o n s . C h e m i c a l a n a l y s i s o f t h e coll a p s e d f o a m r e v e a l e d t h a t t h e h y d r o l y s i s c o n s t a n t o f o l e a t e in t h e s u r f a c e w a s a b o u t o n e t h o u s a n d t i m e s h i g h e r t h a n in b u l k p h a s e , d u e in p a r t t o a l o w e r i n g o f t h e e f f e c t i v e p H in t h e s u r f a c e r e g i o n . E a r l y w o r k b y L o n g et al. s z .s2 s e r v e d t o i l l u s t r a t e t h e i n f l u e n c e o f p H o n the surface t e n s i o n o f soap solutions. A l t h o u g h n o q u a n t i t a t i v e analysis was a t t e m p t e d , the results s h o w e d that, on r e d u c i n g t h e pH, a d s o r p t i o n o f uni o n i z e d a c i d w a s e v i d e n t e v e n at p H 1 1 (see Fig. 1 I). Q u a n t i t a t i v e a n a l y s i s o f t h e a d s o r p t i o n o f p o t a s s i u m l a u r a t e ( K Z ) / l a u r i c a c i d ( H Z ) at t h e a q u e o u s i n t e r f a c e , b o t h w i t h air a n d n - d o d c c a n e , w a s u n d e r t a k e n b y v a n V o o r s t V a d e r ss . B o u n d a r y t e n s i o n m e a s u r e m e n t s w e r e m a d e o v e r t h e p H r a n g e 7.5 t o 9 . 3 , a n d t h e G i b b s e q u a t i o n w a s a p p l i e d in t h e f o r m --dT/RT= P K d Ina K +(Pz+PHz
) d In CT + PHZ d In aHZ + ( r z + l " H z ) d i n f
z
(27) in w h i c h C T r e p r e s e n t s t h e t o t a l s o a p c o n c e n t r a t i o n , a t h e a c t i v i t i e s a n d f the activity coefficients.
T o calculate ( r z + l ~ n z ) , surface t e n s i o n data o n s o l u t i o n s o f c o n s t a n t [K + ] and [H + ] w e r e m e a s u r e d . T h e activity c o e f f i c i e n t s , f r and f z , w e r e a s s u m e d to r e m a i n c o n s t a n t and e q n . ( 2 7 ) b e c o m e s
70
E.D. G O D D A R D
l
(~~T)
all. a K
=I"Z +rllZ
(28}
F o r e s t i m a t i n g the f a t t y acid a d s o r p t i o n , l-*ll Z , o n l y one o f the t w o m e t h o d s o f vail V o o r s t V a d e r is given as mt illustration. F o r this tile variation o f surface tension with pH was m e a s u r e d at c o n s t a n t [K + ] and total soap conc e n t r a t i o n , if changes t a f t : and f z are nel~lected, eCln (27) becontes
=
dT_)
a l n a,
cwcT
,,RT r. z
(201
Tabl~ 3, e x t r a c t e d from van V o o r s t V a d c r ' s publication, presents tile main fh:dlngs, An I m p o r t a n t concltmion d r a w n was that FIi z remains higher than e x p e c t e d on incrcasin$ t h , pH, Wc illtmtrotc tills point by ealculnting "'pK'" from tile t o b u l - t e d d~ta with no allowance for s0rfl|ce p l l effects, A s t e a d y upward trend, i.e. Increasing weakn0~s, Is evident. Van Voorst Vadcr ascribes tills trend to specific Interaction forces between tile laurate Ion and h;uric acid at the interface (see section on insoluble monolayers) and stresses the point that these forces act between the hydrophilic parts o1" the molecules s{nce the same trend of' Pttz values with ptl was found at the oH/water interface. It is clear, however, that p!! shifts c a n n o t be neglected in such analyses. Payens '6 used t h e r m o d y n a m i c reasoning analogotJs to van V o o r s t V a d e r ' s to derive the a d s o r p t i o n values and c o n s t r u c t " ' f o r c e - a r e a " curves for caprylic acid and o c t y l a m i n e . T h e n , by m e a n s o l ' e q n . (I !) a n d the G o u y double layer t r e a t m e n t , he c o m p a r e d t h e o r y and e x p e r i m e n t in t e r m s o f ,~II (eqn. (1 I)) values p l o t t e d against salt c o n c e n t r a t i o n at selected pH values. Q u i t e good a g r e e m e n t was o b t a i n e d for the a m i n e b u t n o t for the caprylic acid when bulk phase values o f the association c o n s t a n t s were e m p l o y e d . T h e d i s c r e p a n c y was again a t t r i b u t e d to cohesion forces.
(It) A Ik),l sulfates and alkane stdfonates it is a p p r o p r i a t e to treat first t h e q u e s t i o n o f surface " h y d r o l y s i s " , i.e. preferential a d s o r p t i o n o f H+ or its chemical c o m b i n a t i o n w i t h tile head group, for the strong acid a m p h i p a t h i c materials, alkyl sulfates and alkan¢ sulfonates. Preferential a d s o r p t i o n for these cases has been u n d e r discussion for m a n y years. As for the weak a m p h i p a t h i e e l e c t r o l y t e s , the p r o b l e m has been a p p r o a c h e d b y s t u d y i n g t h e d e p e n d e n c e o f the surface p r o p e r t i e s o f a d s o r b e d m o n o l a y e r s u p o n pH. F o r a I : 1 s u r f a c t a n t electrolyte Na*S - , the G i b b s a d s o r p t i o n e q u a t i o n can be w r i t t e n --d3' = xkTT'¢ d In mNa~ Ikls,.S"
(30)
IONIZING MONOLAYERS AND pit EFFECTS
71
Table 3 Values of PKap p for laurlc acid from data of tel', 53
]+Z
l+llZ (molelcm + X IO l °
PKap p
lntelface
pll b
(mole/era a x IO i °
Ablwalut
9.33 8.72 g.0? 1.4 ?
3.55 3,35 3.15 2.N
2.2 2.65 2,95 3,9
9.11 8.62 8.04 7.6 I
tt+Dodet:ano/water 9.~$
3.1S 2,75 2.7 2,3
1,45
%00 S.$7
R.72
N.07 7.47
1,95
2.2~ ;L I
a.O0
7.60
wllcr¢ 7' Is t|lU surl'ilC¢ IenMon, I',= tile surl'uc¢ excess and lhu rest of Ih¢ terms have their usual slgnificuI1C¢, lu tlw cqm~tion, x equals I or 2 for coInpI¢[¢ or z~ro 11ydrolysls s4 . respectively. A t t e m p t s to Incasur¢ I11¢ i l n l o i l n l o f adsorptlon directly, and thus e.qablish I|I~ correct f o r m of' t|1~ equation, h a w ilwolvcd in the nlain t w o n1¢t|1ods, viz. the radiotraccr tcci1111que, and a substantial extension of the i n t e r f a c e b y f o a m i n g f o l l o w e d by collal,si=lg tilt: f o a m and d e t e r m i n i n g the s u r f a c a n t c o n c e n t r a t i o n s . F o a m i n g experiment+,, s4 o n s o d i u m d o d e c y l sulfate ( N a D S ) and d o d e c a n e sull'onate ( N a D S O j ) s h o w e d good a g r e e m e n t w i t h the " 2 " f o r m o f the e q u a t i o n , and are in e x c e l l e n t a g r e e m e n t ( N a D S ) w i t h rec e n t direct d e t e r m i n a t i o n s by the r a d i o t r a c e r m e t h o d ss , w h i c h ind i c a t e s little hydrolysis. A s e c o n d , p e r h a p s m o r e direct, m e t h o d of s t u d y i n g " h y d r o l y s i s " or the e x t e n t o f H + u p t a k e , has b e e n to m e a s u r e the pH o f the collapsed f o a m o f f o a m e d s o l u t i o n s o f ionic s u r f a c t a n t s s4.s6 ,sT. MeasurementsS4 revealed, for b o t h NaDs and N a D S O a , that the pH o f the collapsed f o a m was in fact slightly lower t h a n that o f the p a r e n t s o l u t i o n . Bujake a nd G o d d a r d s4 , however, stressed t h e p o i n t t h a t if an a n i o n i c d e t e r g e n t is locally c o n c e n t r a t e d , e.g. b y a d s o r p t i o n , the c o n c e n t r a t i o n of all c o u n t e r i o n s , lttchtdlng hydrogen Ion, w o u l d also be locally c o n c e n t r a t e d a n d t h e r e f o r e a decrease in pH o f the collapsed f o a m was to be e x p e c t e d and did not necessarily i m p l y h y d r o l y s i s . In fact, w h e n the ratios o f t[ + and D S - ion were c o m p a r e d in the p a r e n t s o l u t i o n ( R ~ ) and collapsed f o a m ( R e ) , the d e p a r t u r e s o f R t / P , : f r o m u n i t y were f o u n d to be w i t h i n e x p e r i m e n t a l error, w h i c h again i m p l i e d no h y d r o l y s i s u n d e r the n e a r - n e u t r a l c o n d i t i o n s e m p l o y e d . In o r d e r to o b t a i n i n f o r m a t i o n on the h y d r o l y s i s , we now deal w i t h dir e c t surface t e n s i o n - p H " t i t r a t i o n " o f a s o l u t i o n o f the s u r f a c t a n t s4.s+-sa m a i n t a i n e d at c o n s t a n t ionic s t r e n g t h but o f va r ying pH. Figur e i 2 p r e s e n t s da t a for NaDSO3 w h i c h s h o w the r e v e r s i b i l i t y o f its s o l u t i o n s to a c i d i f i c a t i o n . .mat
"/2
E.D. G O D D A R D
60
?f
E 0
/ "0
e~e
~ O ~ O
A
o
o
~0
(~.0--0-0
0
'
•
*--B
4O
6 pH
IO
2F51g- I ~ ?urfae~i" t e n , ! o n vL p H f a r I 0 - s M i o d i u m d o d c c a n e sulfonat© s'* in H c c u o l y t e s o l - d o n s at ¢-. A o o e a e . c c t r m y t e ( N a C l + HCI) c o n c e n t r a t i o n is 1 0 - * ~ f iJ~ curve A , and 1 0 - * M i n c u r v e B.
(*) S o l u t i o n s first t a k e n to p H ~ 2 prior to adjusting to final pH b y a d d i t i o n o f N a O H , as a c h e c k f o r chemical hydrolysis.
I t w a s o b s e r v e d s4 t h a t a d e f i n i t e r e d u c t i o n o f t h e s u r f a c e t e n s i o n o f t h e s o l u t i o n t a k e s p l a c e s a t a c e r t a i n p H , w h i c h d e p e n d s o n the ionic s t r e n g t h o f the solution. This result can only m e a n that some preferential a d s o r p t i o n o f H +, o r h y d r o l y s i s ) o f t h e s u r f a c t a n t t a k e s p l a c e at these a n d l o w e r p H values, as w a s e v i d e n t f r o m the H - A r e s u l t s o b t a i n e d f o r t h e long c h a i n sulfonate monolayers. The surface tension data allow an estimate of the d e g r e e o f h y d r o l y s i s b y use o f t h e G i b b s e q u a t i o n ~4 A c t i v i t y c o e f f i c i e n t s are a s s u m e d to be u n i t y f o r t h e c o n d i t i o n s o f s t u d y , a n d t h e e q u a t i o n is w r i t t e n
--dT/kT=F n. dinC a++l'Na +dlnCNa+
+Ps_
dlnC s_ +PHs dlnC.ns
(31) We c o n s i d e r a c h a n g e in pH at c o n s t a n t i o n i c s t r e n g t h a n d a t c o n s t a n t surf a c t a n t c o n c e n t r a t i o n . N o f o r m a t i o n o f H S is a s s u m e d in t h e first case, a n d we set d C n + = --dCNa +. T h e last t w o t e r m s o f e q n . ( 3 1 ) d i s a p p e a r l e a d i n g to the relation s 4 d q T ' ( 2 . 3 0 3 k T d p H ) = l"lt + (1 + CtI+ICNa. ) -- l " y ( C , . / C N a . )
(32)
C a l c u l a t i o n s w e r e m a d e f o r t h e c o n d i t i o n s o f c u r v e A, Fig. 12) i.e. 1 0 - a M NaDSO3, 10-2 M added electrolyte, and an assumed ( c o n s t a n t ) value o f l / r s - o f 5 0 A 2 / m o l e c u l e . T h e r e s u l t s i n d i c a t e t h a t w h e n t h e p H is c h a n g e d f r o m 4 to 2, the PH + / P s - r a t i o i n c r e a s e s f r o m 0 . 0 8 to n e a r u n i t y . In t h e s e c o n d case) f o r m a t i o n o f u n i o n i z e d acid is a s s u m e d a c c o r d i n g t o the equilibrium
IONIZING MONOLAYERS AND pil EFFECTS
73
[H* ! [ S - 1 = K [ H S ! B e c a u s e o f t h e h i g h acid s t r e n g t h o f t h e s e l o n g - c h a i n acids, t h e r e l a t i o n dCn+ = --dCNa+ still h o l d s to a g o o d a p p r o x i m a t i o n , anti e q n . ( 3 1 ) can be written
- d " t l k T = Fll+ d I n e l l + -- r s : t q , ÷ / c N . + ) [-'llS (d In CII+ + d In C s _ )
d In % + + r s _ d In % _
+
(33)
W i t h t h e f u r t h e r a s s u m p t i o n o f d C s _ = 0. Fn+ = rN,,+ ( C I I + ] C N a + ) , a n d tile same value o f P s - • calculations based on the final e q u a t i o n
- - d ' y / k T = rtlS d In Ctl +
(34)
i n d i c a t e t h a t I"Its]P s - i n c r e a s e s f r o m a v a l u e o f 0 . 0 8 at pH 4 t o 0 . 4 at pH 2 f o r t h e c o n d i t i o n s o f c u r v e A. Such calculations allow a rough estimate of the dissociation constant K of HS ( i f p r e s e n t in t h e s u r f a c e ) t o be m a d e . F r o m t h e G o u y t h e o r y , tile q% p o t e n t i a l u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s will be a r o u n d 1 8 0 m V a n d h e n c e t h e " ' s u r f a c e " pH a b o u t t h r e e u n i t s l o w e r t h a n tile b u l k DH. F r o m t h e c o m p u t e d v a h l e o f Vii s / ! " s - at a b u l k pH o f 2, a v a l u e o f K a r o u n d 2 0 is t h e n o b t a i n e d . T h i s r e s u l t c a n be c o m p a r e d w i t h a v a l u e o f 22 r e p o r t e d f o r p - t o l u e n e s u l f o n i c acid s 9. T o s u m u p , t h e r e s u l t s h a v e i n d i c a t e d t h a t , u n d e r n e a r - n e u t r a l c o n d i t i o n s , t h e e x t e n t o f s u r f a c e h y d r o l y s i s in s i m p l e s o l u t i o n s o f s o d i u m l a u r y l s u l f a t e s a n d s u l f o n a t e s is v i r t u a l l y n e g l i g i b l e ; tile " 2 " f o r m o f t h e G i b b s e q u a t i o n is a~)plicable in t h e c o n c e n t r a t i o n r a n g e I 0 - : t o 1 0 - a M, a n d p r o b a b l y d o w n t o a b o u t 1 0 - 4 M in n e a r - n e u t r a l s o l u t i o n s . O n l y in very dilute solutions where the hydrogen and sodium concentrations become c o m p a r a b l e , s u c h as w e r e u s e d in s o m e o f t h e e a r l y r a d i o t r a c e r w o r k , o r in s o l u t i o n s a r t i f i c i a l l y a d j u s t e d in pH t o a c h i e v e this c o n d i t i o n , w o u l d t h i s situation change. Surface tension studies, however, indicate that under cond i t i o n s o f l o w pH, m a n i f e s t a t i o n s o f p r e f e r e n t i a l a d s o r p t i o n o f I-I* o v e r Na + b e c o m e e v i d e n t in a l k y l s u l f o n a t e s a n d a l k y l s u l f a t e s . T h i s c o n c l u s i o n l e n d s general support to the results obtained by the insoluble monolayer technique T h e r e s u l t s d o n o t a l l o w a n u n e q u i v o c a l c h o i c e b e t w e e n t h e v a r i o u s explanations of surface weakness of these electrolytes, which comprise prefere n t i a l a d s o r p t i o n o f h y d r o g e n i o n s , f o r m a t i o n o f u n i o n i z e d acid in t h e surface, i o n p a i r f o r m a t i o n , o r o t h e r a l t e r n a t i v e s , a l t h o u g h t h e d a t a p r e s e n t e d do seem to provide fairly strong evidence for deionization.
74
E.D. G O D D A R D
E. M I C E L L A R S Y S T E M S
T h e r e has b e e n r e n e w e d i n t e r e s t in r e c e n t y e a r s in k i n e t i c s t u d i e s o f rea c t i o n s involving m a t e r i a l s w h i c h a g g r e g a t e , o r are .solubilized, i n t o c h a r g e d s u r f a c t a n t micelles. A s t h e p H in t h e v i c i n i t y o f t h e micelle c a n be s h i f t e d b y several u n i t s f r o m its value in t h e s i m p l e s o l u t i o n p h a s e , a p p r e c i a b l y diff e r e n t r a t e c o n s t a n t s o f acid- o r b a s e - c a t a l y z e d r e a c t i o n s are to be a n t i c i p a t e d a n d have, in f a c t , b e e n c o n f i r m e d 6° - 62 H a r t l e y a n d R o e 6a d e v e l o p e d a- e l e g a n t a n d s i m p l e t e c h n i q u e t o i l l u s t r a t e this s h i f t in p H b y use o f a c i d - - b a s e i n d i c a t o r d y e s w h i c h w e r e s o l u b i l i z e d b y t h e micelle. T h e y s u g g e s t e d , f u r t h e r m o r e , t h a t t h e m a g n i t u d e o f the shift c o u l d in c e r t a i n i n s t a n c e s be c a l c u l a t e d f r o m t h e m e a s u r e d q' p o t e n t i a l o f t h e micell¢ a c c o r d i n g t o t h e s t a n d a r d B o l t z m a n n e x p r e s s i o n ( e q m ( 1 ) ) w i t h qs p l a c e d e q u a l t o ~'. It a p p e a r s likely t h a t a c h o i c e o f ~Po, i.e. t h e p o t e n t i a l in t h e p l a n e o f t h e h e a d g r o u p s , f o r "Is w o u l d be m o r e a p p r o p r i a t e . T h i s s u b j e c t h a s b e e n e x p l o r e d f u r t h e r b y M u k e r j e e a n d B a n e r j e e 64 . While confirming the essential validity of H a r t l e y ' s a p p r o a c h , these a u t h o r s were a b l e to d e m o n s t r a t e t h a t t h e m o s t a p p r o p r i a t e v a l u e o f q ' t o be e m p l o y e d lay b e t w e e n t h o s e o f ~" a n d g'0 • i.e. ~" < ~I, < ~ o - I n a d d i t i o n , to o b t a i n m o s t s a t i s f a c t o r y a g r e e m e n t b e t w e e n t h e o r y a n d t h e s p e c t r a l d y e d a t a , a s h i f t in p K ( t o h i g h e r values) in t h e s u r f a c e h a d to be i n v o k e d . T h i s i n c r e a s e w a s a s c r i b e d t o a l o w e r i n g o f t h e d i e l e c t r i c c o n s t a n t in t h e s u r f a c e region. W o r k o n t h e t i t r a t i o n o f i o n i z a b l e m a t e r i a l s in the m i c e l l a r s t a t e h a s b e e n r e l a t i v e l y s c a n t y . R o s a n o e t al. ~5 have r e p o r t e d d a t a o n t h e acid t i t r a t i o n characteristics of aqueous potassium laurate solutions of different concentrations, b o t h submicellar and micellar. A f t e r neutralization of K O H f o r m e d b y h y d r o l y s i s , a p l a t e a u r e g i o n o f c o n s t a n t p H w a s o b s e r v e d , w h i c h in t h e case o f m i e e l l a r K L is several u n i t s o f p H ,higher t h a n f o r n o n - m i c e l l a r K L . While it is t e m p t i n g t o a s c r i b e t h e s h i f t t o a " s u r f a c e " p H e f f e c t in t h e v i c i n i t y o f t h e micelles, e x a c t i n t e r p r e t a t i o n o f t h e d a t a is h a m p e r e d b y t h e a p p e a r a n c e o f solid c r y s t a l l i n e m a t e r i a l (acid s o a p ) a l o n g t h e p l a t e a u r e g i o n 66 . A system much more amenable to electrical analysis has been provided by Y a l k o w s k y a n d Z o g r a f i 6~. T h e s e a u t h o r s s t u d i e d t h e t i t r a t i o n c h a r a c t e r i s t i c s o f v a r i o u s a c y l c a r n i t i n e s w h i c h h a v e t h e f o r m u l a Cn Hon+ l ) C O O C H ( C H 2 C O 0 - ) " C I - I 2 - N + ( C H 3 ) 3 , a n d w h i c h , f o r n = 7, 9, 1 1, 13 a n d 15, w e r e d e m o n s t r a t e d t o f o r m micelles. I n z w i t t e r i o n i c f o r m t h e m i c e l l e s b e a r z e r o n e t c h a r g e a n d n o s h i f t in " s u r f a c e " p H f r o m b u l k is a n t i c i p a t e d . A d d i t i o n o f a c i d p r o g r e s sively n e u t r a l i z e s t h e C O O - g r o u p , l e a d i n g t o a n i n c r e a s i n g positive c h a r g e o n t h e micelle e x p r e s s e d in t e r m s o f a n i n c r e a s i n g d e g r e e o f i o n i z a t i o n , ~, and positive surface potential, g'. This positive potential increases pHs, thus ~ a p e d i n g deionization o f the C O O - g r o u p and increasing the a p p a r e n t s t r e n g t h o f t h e l a t t e r g r o u p f r o m its i n t r i n s i c v a l u e o f K to a n e w v a l u e G.
IONIZING MONOLAYERS AND ptI EFFECTS
75
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P Fig. 13. Aggregation numbe~ (N), degree o f ionization (fl) relations, for mylristylcarnitine ralcelles67. based on potentiometric measurements and the use o f (A) the Loeb-Overtmek-Wiersema equation; (B) the first D e b y e - t l t l c k e l equation; (C) the second D e b y e - i l t i c k e l equation and (D) the G o u y Chapman equation. Open circles are results obtained by light scattering.
T h e r e l a t i o n m a y be w r i t t e n ~It = 2 . 3 0 3 ~ -
(pK--pG)
(35)
T h e d a t a w h i c h are given in ref. 67 d e m o n s t r a t e in a v e r y e f f e c t i v e w a y the e s s e n t i a l p r i n c i p l e d e v e l o p e d in this review. T h e d a t a w e r e t h e n a n a l y z e d in the f o l l o w i n g w a y . A s m o o t h a n d imp e n e t r a b l e s p h e r e , r a d i u s b, w a s c h o s e n f o r t h e micelle, a n d t h e u s u a l ass u m p t i o n s o f a u n i f o r m l y s p r e a d c h a r g e o n t h e s p h e r e a n d a B o l t z r n a n n dist r i b u t i o n o f u n i v a l v n t i o n s in its v i c i n i t y w e r e m a d e . It w a s s h o w n f r o m p u r e l y g e o m e t r i c a l c o n s i d e r a t i o n s t h a t t h e r a d i u s o f t h e micelle b c a n be e x p r e s s e d as b = 3re/eft
(36)
w h e r ¢ o is t h e ( c o n s t a n t ) v o l u m e o f a n a c y l c a m i t i n e m o n o m e r in the micelle. F o u r m o d e l s w e r e t h e n e x a m i n e d , w h i c h r e l a t e o, t h e s u r f a c e c h a r g e d e n s i t y , b a n d ~ , in o r d e r t o c a l c u l a t e b. T h e s e w e r e (i) t h e first D e b y e - - H U c k e l approximation (point charges); the second Debye--Htickel approximation ( f i n i t e size o f e l e c t r o l y t e ions); ( i / 0 t h e e m p i r i c a l e q u a t i o n o f L o e b , O v e r b e e k a n d W i e r s e m a 6s f o r s p h e r e s e x p r e s s e d as
(ii)
o -
DkTr" [2sinh ~ e"t' + -~ 4II-------e-
tanh
~--~]
(37)
76
E.D. G O D D A R D
and (iv) the G o u y - C h a p m a n e q u a t i o n (6) f or fiat plates. T h e values so obt a i n e d w e r e t h e n used to calculate aggregation n u m b e r s a nd c o m p a r e d w i t h t h o s e o b t a i n e d f r o m light scattering m e a s u r e m e n t s . E x c e l l e n t c o n c o r d a n c e was o b t a i n e d over the w h o l e range o f i o n i z a t i o n i f the L o e b - - O v e r b e e k - W i e r s e m a e q u a t i o n was e m p l o y e d . Most serious d e p a r t u r e s were o b t a i n e d for these h i g h c urvature, m i c e l l a r particles w h e n the G o u y - - C h a p m a n (flat plate) e q u a t i o n was e m p l o y e d , the D e b y e - - H t i c k e l e q u a t i o n s le a ding generally, to i n t e r m e d i a t e values. A general discussion on the c o n d i t i o n s governing the a p p l i c a b i l i t y o f such e q u a t i o n s can be f o u n d in ref. 20.
F. CONCLUSIONS
( I ) F o r spread m o n o l a y e r s o f acids a n d bases, the i n f l u e n c e o f pH on siarface p o t e n t i a l can g e n e r a l l y be describe d b y a B o l t z m a n n d i s t r i b u t i o n o f h y d r o g e n or h y d r o x y l ions b e t w e e n the surface a nd b u l k phases a n d b y the G o u y - - C h a p m a n d o u b l e l a y e r m o d e l , at least f o r low degrees o f i o n i z a t i o n . R e s i d u a l c o h e s i o n a n d i n t e r a c t i o n , b e t w e e n the i o n i z e d a n d u n i o n i z e d f o r m s o f t h e m o n o l a y e r m o l e c u l e s , r e n d e r surface pressure v a r i a t i o n w i t h pH less sensitive t h a n surface p o t e n t i a l a n d m a k e s s i m p l e electrical d o u b l e l a y e r analysis via surface pressure less g e n e r a l l y a p p l i c a b l e , e s p e c i a l l y f o r m o n o layers at the a l r / w a t e r interface. (2) W h e r e a s t r e n d s w i t h pH o f the p r o p e r t i e s o f m o n o l a y e r s c o n t a i n i n g b o t h acidic a n d basic i o n i z i n g groups c o n f o r m q u a l i t a t i v e l y to t h e s i m p l e electrical d o u b l e l a y e r m o d e l , q u a n t i t a t i v e c o n f o r m i t y is o f t e n less s a t i s f a c t o r y T h e d i s c r e p a n c i e s are m o s t l i k e l y caused b y c ha nge s in o r i e n t a t i o n o f i o n i c d i p o l e s as the pH varies, w h i c h are n o t r e a d i l y a c c o u n t e d f o r in the s i m p l e modeL (3) S u r f a c e t e n s i o n m e a s u r e m e n t s reveal t h a t p r e f e r e n t i a l a d s o r p t i o n o f h y d r o g e n i o n s o n a l k y l sulfate a n d a l k a n e s u l f o n a t e m o n o l a y e r s can be d e t e c t e d at r e l a t i v e l y high p H values ( f o r e x a m p l e , at pH --- 4 f o r an i o n i c s t r e n g t h o f 0.0 I). T h i s s i t u a t i o n , w h i c h reflects w e a k n e s s o f these m a t e r i a l s in m o n o l a y e r f o r m , results f r o m the high q¢ p o t e n t i a l w h i c h t h e y g e n e r a t e a n d is c o n f i r m e d b y results o n spread m o n o l a y e r s . S h i f t s in the d i r e c t i o n o f increased w e a k n e s s are also e v i d e n t f o r a d s o r b e d a nd spread f a t t y acid monolayers. (4) E v i d e n c e f o r o x o n i u m ion f o r m a t i o n at v e r y low pH ('~0) is p r e s e n t e d fo r a l k y l p h o s p h a t e and sulfate a n d a l k y l b e n z e n e s u l f o n a t e m o n o l a y e r s . T h e s e results e x t e n d t he e a r l i e r f i n d i n g s o f ~ c h u l m a n a n d H u g h e s f o r s u c h f o r m a t i o n in f a t t y acid, a l c o h o l a n d e t h e r m o n o l a y e r s at low pH.
I O N I Z I N G b I O N O L A Y E R S A N D pH EFFCTS
77
(5) T h e change in p o t e n t i a l a r o u n d a c h a r g e d s u r f a c t a n t micelle, ,as the pH is varied, is n o t a d e q u a t e l y d e s c r i b e d by the G o u y - C h a p m a n m o d e l f o r flat plates, in the o n e case r e p o r t e d . M u c h b e t t e r a g r e e m e n t is o b t a i n e d by use o f the e m p i r i c a l e q u a t i o n o f L o e b , O v e r b e e k a n d W i e r s e m a f o r charged, spherical particles. I m p r o v e m e n t s in e l e c t r i c a l a n a l y s i s o f the d i s s o c i a t i o n o f i o n i z a b l e m o n o layers at i n t e r f a c e s can be e x p e c t e d t o a c c o m p a n y r e f i n e m e n t s in the d o u b l e layer t r e a t m e n t e m p l o y e d . O n e s u c h r e f i n e m e n t w o u l d be to allow f o r high localized ionic s t r e n g t h e f f e c t s u p o n the e f f e c t i v e a q u e o u s dielectric c o n s t a n t . R e c e n t w o r k by M u k e r j e e and Desai i n d i c a t e s , h o w e v e r , for the short range i n t e r a c t i o n s i n v o l v e d , that t h e s e e f f e c t s will p r o b a b l y be o u t w e i g h e d by d i e l e c t r i c s a t u r a t i o n and l o w d i e l e c t r i c c o n s t a n t p r o x i m i t y e f f e c t s 69 . ACKNOWLEDGEMENT
T h e a u t h o r e x p r e s s e s his t h a n k s to L e v e r B r o t h e r s C o m p a n y for pel m i s s i o n t o p u b l i s h this paper.
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26. E.D. G o d d a t d , S.R. S m i t h a n d O. Kao, J. Colloid I n t e r f a c e Sci.. 21 ( 1 9 6 6 ) 320. 27. D.O. Shah, J. Colloid I n t e r f a c e Sci.. 32 ( 1 9 7 0 ) 5 7 0 . 28. E.D. G o d d a r d , O . K a o a n d H.C. K u n g , J. Colloid I n t e r f a c e Sci.. 27 ( 1 9 6 8 ) 616. 29. E.D. G o d d a r d a n d H.C. K u n g , J. C o U o i d l n t e r f a c e S c L , 43 ( 1 9 7 3 ) 5 1 1 . 30. H.C. Parrelxa, J. Colloid I n t e r f a c e Sc£, 2 0 ( 1 9 6 5 ) 742. 31. E.D. G o d d a r d , Croat. Chem. A c t a , 45 (19"/3) 13. 32. E.D. G o d d a r d and H.C. Kung, J. Colloid I n t e r f a c e Sci., 37 ( 1 9 7 1 ) 585. 33. K.D. D r e h e r and J.E. Wilson, J. Colloid I n t e r f a c e Sc£, 32 ( 1 9 7 0 ) 248. 34. B.A. Pethica a n d A.V. F e w , Discuss. Faraday Soc., 18 ( 1 9 5 4 ) 258. 35. J.N. Phillips and E.K. Rideal, Proc. R o y . Soc. Set. A , 232 ( 1 9 5 5 ) 159. 36. M.M. Standish and B.A. Pethica, Trans. Faraday ,~oc., 64 ( 1 9 6 8 ) 1113. 37. P.J. A n d e r s o n ~v,d B.A. Pethica, Int. Conf. B i o c h e m . Prob. LllMds. G h e n t 1955, p. 24. 38. J. Glazer and M.Z. Dogan, N a t u r e ( L o n d o n ) , 170 ( 1 9 5 2 ) 4 1 7 . 39. B.S. Harrap. Proc. Inf. Wool Textile Rex. Conf., Australia, 1955, p. B98. 40. T. Isemura, H. H o t t a and S. O t s u k a , Bull. Chen~ Soc. Jap.. 27 ( 1 9 5 4 ) 93. 41. T. l s e m u r a and K. Hamaguchi, Bull. Chem. Soc. Jap., 27 ( 1 9 5 4 ) 339. 42. H. Hotta, J . C o l l o i d S c L , 9 ( 1 9 5 4 ) 504. 43. K, H a m a g u c h i a n d T. Isemwra, Bull. Chem. Soc. Jap., 28 ( 1 9 5 5 ) 9. 44. I.R. Miller, J. C o l l o i d S e £ , 9 ( 1 9 5 4 ) 5 7 9 . 45. J. Jaff~ and J.M. R u y s s c h a e r t , J. P o l y r ~ Sc£. P a r t A . 3 ( I 9 6 5 ) 4 0 4 7 . 46. J. Jaffa, J.M. R u y s s e h a e r t and G. B r i t m a n , J. P o l y m . ScL. Part A - 2 , 8 ( 1 9 7 0 ) 8 1 7 . 47. J. Caspers, J.M. R u y s s e h a e r t and J. J a f f e . J. Polyn~ SoL. Part C. N o . 34 ( 1 9 7 1 ) 73. 48. M. Raison. C o m p t . R e n d . A e a d . SeL. 2 3 2 ( 1 9 5 1 ) 1660. 49. A. Levialdi, C o m p t . R e n d . Acad. Sc£. 234 ( 1 9 5 2 ) 1287. 50. D. Eagland and F. F r a n c k s , Int. Congr. Surface A c t i v i t y , 3 r d , Vol. 4, Butterworths0 L o n d o n , ( 1 9 6 1 ) , p. 539. 51. F.A. Long, G.C. Nutting and W.D. Harkins, J. A m e r . Chem. Soc., 59 (19-~7) 2 1 9 7 . 52. G.C. N u t t i n g and F.A. Long. 3". A m e r . Che1~L Soc.. 63 ( 1 9 4 1 ) 8 4 . 53. F. van V o o r s t Vader, Trans. Faraday S o t . , 57 ( 1 9 6 1 ) 2 2 6 3 . 54. J.E. Bujake a n d E.D. G o d d a r d , Trans. Faraday Soc.. 61 ( 1 9 6 5 ) 190. 55. M. Mttramatsu, Private c o m m u n i c a t i o n . 56. J.W. J a m e s and B.A. Pethica, Int. Congr. Surface A c t i v i t y , 3rd. Vol. 2, B u t t e r w o r t h s . L o n d o n , 1961, p. 227. 57. M.A. C o o k and E.L. T a l b o t . J. P h y ~ Chem.. 56 ( 1 9 5 2 ) 4 1 2 . 58. W. Kling a n d H. Lange, Int. Congr. S u r f a c e A c t i v i t y . 2nd. Vol. 1. B u t t e r w o r t h s , L o n d o n , 1957, p. 310. 59. R.H. Dinius and G.R. C h o p i n , J . lY~y~ Chem., 6 6 ( 1 9 6 2 ) 268. 60. E.F.J. D u y n s t e e a n d E. G r u n w a l d , J. A m e r . C h e m . Soc.. 81 ( 1 9 5 9 ) 4 5 4 0 , 4 5 4 2 . 61. J.L. K m , J. Phys. Chem.. 66 ( 1 9 6 2 ) 2 2 3 9 . 62. R.B. D u n l a p and E.H. Cordes, J. A m c r . Chem. S o t . , 9 0 ( 1 9 6 8 ) 4 3 9 5 . 63. G.S. Haxtley a n d J.W. R o e , Trans. Faraday Soc., 36 ( 1 9 4 0 ) 101. 64. P. Mukerjee and K. Banerjee, J. Phys. Chem., 68 ( 1 9 6 4 ) 3567. 65. H . L Rosario, K. Breindel, J.H. S c h u l m a n a n d A J . E y d t , J. Colloid Interface Sci.. 22 ( 1 9 6 6 ) 5 8 . 66. J. Lucassen, J. Phys. Chem., 70 ( 1 9 6 6 ) 1824. 67. S.IL Y a l k o w s k y and G. Zografi, J. Colloid I n t e r f a c e Sci., 34 ( 1 9 7 0 ) 525. 68. A.L. L o e b , J. T h . G . O v e r b e e k a n d P.tl. Wiexseman, T h e Electrical D o u b l e L a y e r A r o u n d a Spherical Colloid Particle, M I T Press, C a m b r i d g e , Mass., 1961. 6 9 . P. Mukerjee and N.R. Desai, N a t u r e ( L o n d o n ) . 223 ( 1 9 6 9 ) 1056.
IONIZING M O N O L A Y E R S A N D pH E F F E C T S E.D. GODDARD
Lever Brothers Company° E d g e ~ t e r . N e w Jersey (U.S.A.)
CONTENTS a. Introduction. B. S o l u t i o n s o f p o l y e l e c t r o l y t e s
C. I n s o l u b l e m o n o l a y e r s (i) F a t t y acids (ii) F a t t y a l c o h o l s a n d e t h e r s (iii) F a t t y a m i n e s (iv) A l k y l p h o s p h a t e s and p h o s p h o n a t e s (v) A l k y l sulfates a n d alkar~e s u l f o n a t e s (vi) P h o s p h o l i p i d s a n d o t h e r z w i t t e r i o n i c m a t e r i a l s . (vii) P o l y m e r s . . . D. A d s o r b e d m o n o l a y e r s (i) F a t t y acid s o a p s a n d a m i n e s (ii) A l k y l sulfates a n d a l k a n e s u f f o n a t c s . E. Micellar s y s t e m s . F. C o n c l u s i o n s . Acknowledgement References
45 47 48 52 57 57 58 59 60 64 68 69 70 74 76 77 77
ABSTRACT A p p l i c a t i o n o f the electrical d o u b l e layer t r e a t m e n t to charged m o n o l a y e l s is o u t l i n e d . P r o p e r t i e s . as I n f l u e n c e d b y solution p i l . o f a n u m b e r o f t y p e s o f Ionizing m o n o l a y e r s , (i) spread, (11) a d s o r b e d , a n d ( 8 0 at the m i c e l l e / w a t e r I n t e r f a c e , are r e v i e w e d . A n a l y s e s o f the first a n d ~hird c a t e g o r i e s b y electrical d o u b l e layer m e t h o d s , a n d o f the s e c o n d b y the G i b b s a d s o r p t i o n e q u a t i o n . 0xe p r e s e n t e d a n d discussed.
A. I N T R O D U C T I O N
In the early 1900's there was considerable interest in the tlistribution o f ions near charged interfaces or bodies In contact with electrolyte solutions. This interest led to the d e v e l o p m e n t o f the theories o f G o u y , Chapman, Stern, and others. For the subject o f m o n o l a y e r "titrations'" we will be concerned directly with Ionic distribution as it affects hydrogen (or h y d r o x y l ) tons near interfaces containing lontzable materials. In this respect the most widely In-
46
E.D. GODDARD
vestlgated matet'ials have been f a t t y acids, the properties o f w h i c h were k n o w n to be h i g h l y i n f l u e n c e d by the pH o f the ir a d j a c e n t s o l u t i o n s f r o m the e a r l y w o r k on i n t e r r a c ial tension b y R e i n d c r s ) and on m o n o l a y e r s by A d a m 2.3 a n d others. T h e first s y s t e m a t i c investigation o f the p h e n o m e n o n s e e m s to have been u n d e r t a k e n by Peters 4 . w h o e x a m i n e d the i n f l u e n c e o f pH and. to s o m e e x t e n t , ionic s t r e n g t h o f a q u e o u s s o l u t i o n s on the ir i n t e r f a c i a l te nsion against b e n z e n e s o l u t i o n s o f various f a t t y acids a nd a l i p h a t i c amines. It was n o t i c e d t h at the resulting curves o f interracial te nsion versus pH r e s e m b l e d bulk phase ti t rat i o n curves b u t w i t h an a p p a r e n t shif t o f a b o u t 3 pH units in tile direction o f increased weakness. T his s u b j e c t was e x t e n d e d by Daniellis , w h o . by a s s u m i n g t h a t t h e degree o f lowering o f interracial te nsion was d i r e c t l y prop o r t i o n a l to the f r a c t i o n o f f a t t y acid ionized in the inte r f a c e , c o n f i r m e d that the p K o f the f a t t y acid could be s h i f t e d in the direction o f w e a k n e s s b y as m u c h as 4 u n i t s a n d t h a t this shift was greater the l o w e r the ionic s t r e n g t h o f the a q u e o u s solution. D a n i e l l i ' s results closely paralleled those o f Schulm a n et al. 6. 7 on the surface p o t e n t i a l versus pH curves o f spread m o n o l a y c r ~ o f f a t t y acids at the a l r / a q u e o u s s o l u t i o n interface. O t h e r a p p r o a c h e s for s t u d y i n g the i o n i z a t i o n o f spread f i l m s o f f a t t y acids, involving direct analysis, e i t h e r cl~emical6 or infra r e d " , o f s a m p l e s " ' s k i m m e d " f r o m the surface o f s u b s o l u t i o n s o f varying pH. have led to results in h a r m o n y w i t h the p r e c e d i n g studies. It is n o w g e n e r a l l y a c c e p t e d that i o n i z a b l e m a t e r i a l s at i n t e r f a c e s d i s p l a y an a p p a r e n t w e a k n e s s ascribable to, and d e t e r m i n e d in m a g n i t u d e b y , the electrical p o t e n t i a l ~ generated at the i n t e r f a c e b y i o n i z a t i o n . T h i s lowering o f t e n d e n c y t o w ards i o n i z a t i o n has led to general a c c e p t a n c e o f the c o n c e p t o f an effective pH in the surface phase, pH s, w h i c h d i f f e r s f r o m that In the bulk, p H b a s given b y the B o l t z m a n n e x p r e s s i o n pH, = pH b + ~xP/2.3 k T
(1)
w i t h a resulting shift in the d i s s o c i a t i o n c o n s t a n t K given by pK s '~ p K b - egt/2.3 k T
(2)
As a t y p i c a l e x a m p l e , i f the area per charge In a m o n o l a y e r is 100 A ~ a n d the s u b l y l n g a q u e o u s s o l u t i o n has c o n c e n t r a t i o n s 0.01 and 0. I M, respectively, o f u n l - u n i v a l e n t e l e c t r o l y t e , the qPo p o t e n t i a l ( G o u y ) w o u l d be 170 a n d 110 mV, s u f f i c i e n t to cause shifts in pH and, c o n s e q u e n t l y , In pK In the surface o f 2.85 a n d 1.85 units. T h e p o t e n t i a l q~ e m p l o y e d in the c a l c u l a t i o n s d e p e n d s on the m o d e l used, In m ost cases, % , the p o t e n t i a l in the plane o f the generated charges, appears to b© the most approF"late choice. It Is generaUy c a lc ula te d via the Gouy-Ch~.i,,ig;; or S:ern models. F o r positions s h i f t e d slightly Into the
I O N I Z I N G M O N O L A Y E R S A N D pll E F F E C T S
47
a q u e o u s p h a s e o t h e r ' [ ' values are e m p l o y e d : q ' D , t h e D o n n a n m e m b r a n e pot e n t i a l , is t h e p o t e n t i a l a v e r a g e d o v e r t h e D e b y e - H t l c k e l d i s t a n c e 1]~; ~', tile e l e c t r o k i n e t i c p o t e n t i a l , is t h e p o t e n t i a l at t h e " s u r f a c e o f s h e a r " . T h e a b o v e c o n c e p t s h a v e b e e n a p p l i e d in m a n y a r e a s o f colloid a n d s u r f a c e c h e m i s t r y . It is t h e p u r p o s e o f this p a p e r to p r e s e n t a review,,, b y n o m e a n s c o m p l e t e , d e a l i n g w i t h t h e i r r e c e n t a p p l i c a t i o n to i o n i z i n g m o n o l a y e r s s p r e a d , a d s o r b e d , a n d p r e s e n t at t h e w a t e r / m i c e l l e i n t e r f a c e . F o r s i m p l i c i t y a n d c o n v e n i e n c e , in all cases t h e a q u e o u s s o l u t i o n s c o n s i d e r e d will be r e s t r i c t e d to t h o s e c o n t.'lining u n i v a l e n t c o u n t e r ions o n l y .
B. S O L U T I O N S O F P O L Y E L E C T R O L Y T E S
It is p e r h a p s a p p r o p r i a t e at t h e o u t s e t t o p o i n t o u t the f o r m a l s i m i l a r i t y o f a c i d - b a s e t i t r a t i o n o f m o n o l a y e r s to t h a t o f p o l y e | e c t r o l y t e s in s o l u t i o n . T h e l a t t e r , b o t h n a t u r a l a n d s y n t h e t i c , have b e e n v e r y w i d e l y i n v e s t i g a t e d . A t y p i c a l t i t r a t i o n curve s u g g e s t s t h e r e is a c o n t i n u o u s r a n g e o f p K values o f a p o l y acid, f o r e x a m p l e , w h i c h b e c o m e s a p p a r e n t l y w e a k e r as t h e d e g r e e o f n e u t r a l i z a t i o n progresses. L i n d e r s t r o m - L a n g to e a r l y r e c o g n i z e d t h a t t h e c o n c e n t r a t i o n o f ions, i n c l u d i n g | l * a n d O t l - . n e a r s u c h a c h a r g e d m a c r o n ~ o l e cule in s o l u t i o n s , d i f f e r s f r o m t h a t in b u l k to a n e x t e n t d e t e r m i n e d b y the e l e c t r i c a l p o t e n t i a l at the s u r f a c e o f the colloid particle. T h i s f i n d i n g h a s b e e n t h e basis o f the a p p r o a c h e s s u b s e q u e n t l y e m p l o y e d to e x p l a i n the observed titration c h a r a c t e r i s t i c s o f p o l y e l e c t r o l y t e s . F o r e x a m p l e , e s t i m a t e s o f t h e d i f f e r e n c e , ( p H s - - p H b ), for t h e o v a l b u m i n m o l e c u l e in s o l u t i o n w e r e m a d e b y D a n i e l l P t using: (a) t h e D o n n a n a p p r o a c h a n d ( b ) p u b l i s h e d values of the zeta potential. Excellent agreement was o b t a i n e d b e t w e e n the two m e t h o d s o v e r a c o n s i d e r a b l e r a n g e o f pH, a n d c o n s e q u e n t l y , o f d e g r e e o f ionization. A n e m p i r i c a l e q u a t i o n ~ : , t 3 f o r t h e d i s s o c i a t i o n o f a p o l y m e r i c acid pH=pK e -n
log l - ~
(3)
O~
w h e r e p K , a n d n are e m p i r i c a l c o n s t a n t s , has b e e n s u p e r s e d e d b y t h e t h e o r e t i c a l l y d e r i v e d e q u a t i o n t ~, t s pH = p K -
log
I--a t~
eqso 2.3kT
in w h i c h tx is the d e g r e e o f d i s s o c i a t i o n a n d " ~ P o " is f r e q u e n t l y e s t i m a t e d (for polyolectrolytoa) from electrophoretle measurements.
(4)
48
E.D. G O D D A R D
(2. I N S O L U B L E M O N O L A Y E R S
In de a l i n g w i t h the t i t r a t i o n o f an i o n i z a b l e m a t e r i a l at an i n t e r f a c e we n o w treat the t y p i c a l case o f a weak acid, HA, i o n i z i n g a c c o r d i n g to HA~
--'H ÷ + A -
(1 - a )
(a)
P a y e n s t 6 has p o i n t e d o u t the f o r m a l s i m i l a r i t y o f the Stern r e l a t i o n to eqn. (4), w h i c h can be w r i t t e n 1 -tita _
[ H + l b e x p [--(eq, o - - A G O ) / k T l
(5)
w h ere A Go is the s t a n d a r d free e n e r g y o f i o n i z a t i o n o f the acid, e q t m t a b l e , in the p r e s e n t c o n t e x t , to the specific a d s o r p t i o n p o t e n t i a l o f h y d r o g e n ions on i o n i z e d sites in the m o n o l a y e r . F o r i n s o l u b l e m o n o l a y e r s , q'o is usua lly o b t a i n e d f r o m the G o u y - - C h a p m a n c a p a c i t y e q u a t i o n s 7 for flat plates, viz. .tit° _ 2 k T e
sinh-
'
02 500FI~ 1/2 m
(6)
DRT]
w h e r e o is the charge per cm z , m is the c o n c e n t r a t i o n o f u n i - u n i v a l e n t electrol y t e in s o l u t i o n , D the d i e l e c t r i c c o n s t a n t a n d R the gas c o n s t a n t . T h e e q u a t i o n b e c o m e s ~ 7 at 200C g,,
_ 2kT e
sinh_ I (134t~) \Am
(7)
! 121
for a m o n o l a y e r o f a r e a / m o l e c u l e A (A 2 [ m o l . ) a nd degree o f d i s s o c i a t i o n ¢t. T h e G o u y d e r i v a t i o n involves the usual a s s u m p t i o n o f u n i f o r m , i m p e n e trable charge in the m o n o l a y e r a n d p o i n t charges f o r the c o u n t e r i o n s . Strictly s p ea k i n g , it s h o u l d a p p l y o n l y at low sur f a c e charge d e n s i t i e s a n d low e l e c t r o l y t e c o n c e n t r a t i o n . H ow ever, by f o r t u i t o u s c a n c e l l a t i o n o f errors, as discussed b y Davies a n d R i d e a P 7, it has been f o u n d to a p p l y well o u t s i d e its e x p e c t e d w o r k i n g range. It is b y far the m o s t w i d e l y used m o d e l for c o m p u t a t i o n o f g, o p o t e n t i a l s o f charged m o n o l a y e r s . O t h e r m o d e l s , develo p e d f o r s p h e r i c a l s y s t e m s , are m e n t i o n e d in the s e c t i o n o n micelles. T h e G i b b s - D o n n a n " m e m b r a n e " e q u i l i b r i u m has also b e e n a p p l i e d ~ 7 to haonolayers. I n th~s ease there is o f course no f o r m a l " m e m b r a n e " b u t , m a t h e m a t i c a l l y , the r e s t r i c t i o n o f m o n o l a y e r m a t e r i a l to the surface la ye r can be c o n s i d e r e d e q u i v a l e n t to c o n f i n e m e n t b y a m e m b r a n e . I f the thickness of the Doflnan surface phase is taken as the D e b y e - H t l c k e l term 1/~, the c o n v e n t i o n a l D o n n a n e x p r e s s i o n s
IONIZING MONOLAYERS AND ptl I.FI'E(~I'S
49
k T in f b m + ) = k T in ( m + ) + eOJt~
(8)
k T In ( b m - ) = k T In (sin--) --e~Pt~
(9)
c a n be s h o w n ~ s t o lead t o t h e e x p r e s s i o n
•I'D
si h_t (2--/ _134
e
rio)
Am~l= ]
w h i c h resembles that o f G o u y . Here m+ and m - refer to the c o n c e n t r a t i o n s o f u n i v a l e n t ions. T i l e t w o e x p r e s s i o n s lead t o s i m i l a r v a l u e s o f ~P f o r c o n d i t i o n s o f low p o t e n t i a l ; at h i g h c h a r g e d e n s i t y a n d i o n i c s t r e n g t h s t h e D o n n a n a p p r o a c h gives l o w e r values o f q, a n d is t h e p r e f e r r e d m o d e l . It s h o u l d , h o w e v e r , be r e m e m b e r e d t h a t xl, D a n d q'o are n o t s t r i c t l y i n t e r c h a n g e a b l e in t h e S c h u i m a n - - H u g h e s e q u a t i o n ( 1 4 ) g i v e n b e l o w , e s p e c i a l l y f o r c o n d i t i o n s o f l o w m , i.e. 1[~: large. C o r r e c t i o n s t o t h e D o n n a n t r e a t m e n t have been discussed by O v e r b e e k t s A c o n c o m i t a n t e f f e c t o f c h a r g i n g o f t h e m o n o l a y e r is t h e d e v e l o p m e n t o f a c h a n g e , A F , in t h e f r e e e n e r g y o f t h e d o u b l e l a y e r , w h i c h a c c o r d i n g t o t h e D L V O t r e a t m e n t 20 is g i v e n b y
A F = --
~
0
(11)
o . d ff,,
0
~cDk 2 T 2 Ile 2
c o s h [ ( e X P o / 2 k T ) -- 1 ]
(12)
T h i s r e s u l t s in a p o s i t i v e c o n t r i b u t i o n A I I t o t h e s u r f a c e p r e s s u r e , s h o w n I ~ b y D a v i e s t o be a t 2 0 ° C
AII=--
AF=6"I
mtl2
{c°shsinh-I
( 134a
)
(13)
In addition to direct analysis o f the c o m p o s i t i o n o f film material, there are thus t w o m e t h o d s to f o l l o w the course o f i o n i z a t i o n in the m o n o l a y e r in cons e q u e n c e o f the developing'~, potential. T h e surface potential m e t h o d was pioneered by S c h u l m a n and Hughes 7 , w h o first revealed that surface p o t e n t i a l is a particularly sensitive measure o f ionization. T h e y f o u n d that the A V - p H relationship o f a fatty acid r e s e m b l e d a titration curve, again w i t h a substantial shift o f apparent p K in the direction o f weakness. T h e S c h u l m a n - H u g h e s e x pression for the surface potential o f an ionized m o n o l a y e r , w h i c h c o m b i n e ~ a dipole term arid the ~d term
I~,1:1,Cl01~gAgD
~lO AV,,, I ~.Ti~u/A + %
(14)
booomoM f o r . p . r t l . l l y ionized monol.yer
~V=(12II/A)
l i t - ~)/~, +~#~ l + q ~ .
(15)
whcrcs/~= and/J= arc the surface moments of the unionized and Ionized forms, • respectively.
To utilize surface potential measurements as a function of pH for estimation of K, the intrinsic dissociation constant. Betts and Pethlca =9 proceeded as follows. Equation (15) was dffl'ercntlated to give i~A
_ 1211 A , m , r - ---M--
and by
means
( ) aa
(/l= -- JUj )
~
a.m.r
+ k,~p---~
(16) A..,.T
or e(ins. (4) and (7), tile following relations were obtained
(a*vx
_
,2n
/i) A0 +
\O--~I)A.,,,.
fl7)
/-
and 2k T
1 -¢=
J A.,.. T Betts and Pethtca p o i n t e d out that u n d e r c o n d i t i o n s o f low ¢, where the G o u y e q u a t i o n s are most applicable, the first term on the right-hand side o r eqn. (17) a p p r o a c h e s zero and hence
r
\a---~ ).i,.,. T
"/'his result allows calculation e r a and hence pK as a function of'a, by u ~ o f experimental (0 WOpH) values and a "calibration curve" {¢q,, (18)) o f (0#o / 0pH) vs. a, constructed from ¢qng, (4) and (7). Extrapolation o f the pK valu=s to a = 0 yields the Intrinsic pK value. As a refinement, i f pK shows appreciable variation with a, different values of the term . ~ =, (/~= =/J= ) are tested to see which leads to the least variation of p£=¢= with =. A related, yet simpler, method of obtaining pK was used by Goddard and AcklIlP s . in rids method values o£ ~0 are calculated For arbitrary values of 0. Then. a plot of' ~o vs. [iol;(a/! = a) == ( e ~ o / 2 . 3 k T ) ] , which Is equal to
IONIZINft 14ONOLAYI~IeEAND pll I'-'I.FF-L3-1~
sI
( p l I - p K ) . 1, m=l¢10. The t]rtlinkltu QI thi~ pint i~ ~t=p~rl,np¢,~¢d o,= Itlu ~xpurhllullt~=l A V - p H plot. ~nd shll'ted laterally t=) l, ring tile two curve~, Into c'()inciduneu at low pH to ,. 01. It can h¢ shown that ht the region oi" initial di~sochltio, the c h a n g e In A V Is o c c a s i o n e d largely I~y the ~P. t e r m tff uqn. (15), a n d the ~:xtent o f the r e q u i r e d lateral shift is a direct m e a s u r e =)I" pK for c o n d i t i o n s u n d e r which the G o u y - C h a p m a n equations =ire most al~Idlcablu (low a). The second method of following monohiyer Ionization tJlilizes surface pressure measurements. Various interrelated apl~roa~:Itcs have been p r t s t n l u d by Paytns=6 for an:tlyzlng the course of ionizallon of a monolayer front the pressure changt:s, A l l , a c c o m p a n y i n g w~riation o f the s t i b s o h l t i o n pi I. O n e Is to c o m p a r e the o b s e r v e d c h a n g e o r pressure as a l ' u n c t i o n o1" pit w i t h t h a t calc u l a t e d using various values o f the d i s s o c i a t i o n c o n s t a n t a n d the d o u b l e l a y e r equ:ttions. A n o t h e r is to c o m p a r e o b s e r v e d a n d c a l c u l a t e d values o f A([, at c o n s t a n t values o f pH a n d area. as a f u n c t i o n o f the ionic s t r e n g t h o f the s u b s o l u t i o n . F o r a p a r t i c u l a r set o f c o n d i t i o n s , an initial increase o f A l l can be o b s e r v e d o n i n c r e a s i n g the i o n i c s t r e n g t h gillt2e the ¢onst:qtmnt l o w e r i n g o f ~o p o t e n t i a l is a c c o m p a n i e d b y qn increase in i o n i z a t i o n p o t e n t i a l ( a n d an increase in i o n i z a t i o n ) a c c o r d i n g to cqn. (2). At h i g h e r salt concentratiot~s, w h e n dlssociatio,~ is m o r e or,replete, a r c t h t c t i o n o1" All can be e x p e c t e d because o1" tit,: l o w e r e d ~P,, p o t e n t i a l , a n d I~cnce, electro,ltatic repttlsive t e r m . T h e s e ¢l'l'ects are d e s c r i b e d q u a n t i t a t i v e l y ilt equations t l e v c l o p c d I,y B e l t s a n d P e t h l c a ' ~. viz. 6.1
= -2- .1112
cash {''q'" \ 2k-T)
sinh (eq'oi 2k'l') -- 2( t --o) ÷cash (('~./2/~T) "- I
}
(19) and
aIl) ap--p-H m.,4.T
6.1 ml/= ( I - a ) sinh(edSo/2k7")
2(I L-a)+coth(eqso/2kT")
(20)
One can also compare observed 11 - A characteristics of a weak electrolyte monolayer with a theoretically derived equation of state. For example, at the oil/water lnt¢rfnc¢, where |ttterchaln forces are assumed to b~, ~¢ttlil;lble, the generally ~sumed Amullut-typ¢ equation of state, n o (A --AB !- " kT, for uncharged monolayers, can be combined w | t h ¢qn. (13) to yield theoretical values of II ~ (A = A 0 ) for comparison with ¢xpertmenta! values t ~. Payens' approaches are less certain at the air/water Interface, except p~rhaps at high areas/molecule, in view of the likelihood of [nlermol¢cular intoraetion, both from th© chains and head groupL Equations of state at tills interface are less well developed, and in any cas¢, as will b¢ d~monstrated, II-A characteristics are generally less sensitive to Ionization titan A V~A
.52
ILD, GODDARD --
11
I
•
I
!
| 40
0
~:~ z
6
PH
8
tO
Fig. I . Extent o f lonLzation o f steaJric acid (e) and behenic acid (,,) monolayers " s k i m m e d " from the aureate o f 0ol Af sodium bicaJrbonate buffer subsolutlons o f different ptl.
characteristics. This conclusion certainly holds for long-chain amphipatldc materials, if not always for polymers. However, for polymers, it must be remembered that Incorporation of electrostatic terms represents a further level of complexity in existing equations of state which are at best approximate for unionized forms. We pass now to examples o f difl'erent monolayer types.
(U Fatty aclda These ate by fat the most widely examined materlals~ yet prior to tl~¢ work of Bags ~ et el,, no direct determinations had been reported on the composition of spread fatty acid monolayers on subsolutlons containing only monovalent eounterions, The above workers carried out Infrared analysis on pre-spread films skimmed from the surface of subsolutlons of different pH, The solutions were 0,1 =0,;1 M In sodium Ion and Incorporated different buffers or were unbuffered. Composition vs. pH data are IIlven In Fig, 1 for the bicarbonate system, and pK data for all the systems are given In Table 1. Although there Is some dependenoe on the buffer system, probably reflecting dlfferen¢e. In buffering efficiency, all reveal the ¢har= aoterlstle apparent weakness of the sold In monolayer form. It Is of Interest that the bicarbonate buffer led to derived value~ of pK, based on u ~ o f eqn. (4) and the Gouy--Chapmun expression (7), In ltood aBreement with t h o u obtained by the surface potential method Illven below, despite the n e e e u l t y o f employing high valuea of ¢= for the derivation.
53
IONIZING MONOLAYERS AND pll I~FFECT$ T,bl0 1 V~lu©~ of I ~ from infrlir©d antilysl~ (ref, 9)
PNt, Substrata
Acid
App~©nt
Cah:ulated
0,1 M N a C I - N , O l l 0.2 jtf N~OII-NuCI 0.1 M N a C I - N a O I t 0.2 M N a O i [ - N a C l Phosphate buffer Phosphate buffer Bicarbonate buffer Bicaxbonate buffer
Stea~le Stearic rlehenlc Behenic Steartc Behenic Ste~ric
8,9 8.5 tl.9 8.$ 8.9 8,9 8.2 8.2
6.3 6.2 6.3 6,2 6.6 6.6
Behenic
5.6 5.6
4°°
300
\
20(:
\
""
b
\ 0
"\ c. %
I00
~ 6 A II aalMNetf:lla.e31Nt4e~jpt.~|l
0
~I b II I t ~.8e|N Nd~&Oj eil I I /41 tJd,~lS~l t~[mFi (MIIliIIj d61d)
4
6
pVl
il
%%
"%
\
%
~,
%
% %
tO
Pill, I, Vlirlililon wllh pll of lh# ,iitfiee li~i.nihil (,i Vm~i,iI i . ~llii~ pli~k#d molloliiyi~,J of 4I~Mi~ (arid myrlsiie) aid, ,l!po, tt~l by vttiou! wolk~il,
54
E.D. GODDARD
T h e original A V--pH titration data o f S c h u l m a n and Hughes 7 have be e n c o n s i d e r a b l y m o d i f i e d b y more recent work. largely as a result o f ,:loser att e n t i o n paid to e x p e r i m e n t a l details (see Fig. 2). Glazer and Dogan 2z , using HCO_ - / C Oan~d2 - bui'fers, o b t a i n e d s u b s t a n t i a l l y higher A V values t h a n did Schu['man Hughes over most o f the pH range. Recognizing the high sensitivity o f ionized fatty acids to foreign lens, s u b s e q u e n t wor ke r s t e n d e d to avoid c o m p l e x b u f f e r systems. T h u s Sanders a nd S p l n k 2 :j utilized a m m o n i a systems, a n d Betts a n d Pethlca used c o m p l e t e l y u n b u f f e r e d s o d i u m chloride s u b s o l u t t o n s ud]ustc¢l in pH w i t h small a m o u n t s of HCI or NaOH. The restilts o b t a i n e d b y these latter workers again led t o s u b s t a n t i a l l y Idgher A V values, w i t h A V vs. ptt being r e p r e s e n t e d by t h e m as e sse ntia lly linear from pH 3 to 9. By i n t r o d u c i n g a c o r r e c t i o n dipole term (p~ -/.t I ) o f - 2 6 4 roD, Betts a n d Pettlica f o t m d an essentially c o n s t a n t value o f 5.5 for the derived p K over a range o f a o f 0 to 0.01. A similar value ( pK ~ 5) was o b t a i n e d by the m e t h o d o f G o d d a r d a n d Acklllt ~l 0 whose results essentially c o n f h ' m e d the data o f Bctta and Pethlca, but d e m o n s t r a t e d the e x i s t e n c e o f a previously u n d e t e c t e d critical point at ca. pH 9 above w h i c h there was a drastic r e d u c t i o n In A V values. This f i n d i n g led these uuthor s to c onside r the possibility that c o m p l e x , 1.¢. a c l d ~ s o a p , f o r m a t i o n was occurring in these close-packed m o n o layer systems. However, an e l e c t r o s t a t i c m o d e l analysis in the range o f f or ma l a p p l i c a b i l i t y o f the G o u y e q u a t i o n s did not allow a choice b e t w e e n the simple attd ucld~soup dissociation modes. T h e I1--A characteristics ~ ~ of ih~ stcaric acid m o n o l a y e r s were very revealing. Despite increasing Ionization. the coJldensud n a t u r e of the film was almost u n c h a n g e d tip tO pH 9. A c c o r d i n g to eqn. (13), c o n s i d e r a b l e increases in the fihn pressure from tlt~ electrical contribution, A r l , o f the ionized component were to be 0ntlcipated at rids pH. It was found that the pH had to be increased well beyond 9 before this increase occurred =4 . These two observations concerning A~, and II behavior In the vicinity o f pH 9 seemed to be linked and thle conclusion led to further Investigation o f the phenomenon in th~ light o f th~ w~ll known stron[~ association =t= which occurs bctw¢¢n Ionized and unionized forms o f f a t t y acids In bulk phase. The p o t e n t i a l w o r k =~ was r e p e a t e d o n u n b u f f e r e d s u b s o l u t l o n s in a CO2-free envlronm©nt, a n d on purified b o r a x s u b s o l u t l o n s ; it was d e m o n s t r a t e d that the critical p h e n o m e n a occurring a r o u n d pH 9 wer0 in part, but not w h o l l y , the c o n s e q u e n c e o f b u f f e r i n g by a t m o s p h e r i c C O a , since a slope change, albeit smaller, in the A V--pH characteristics still oc c ur r e d at the same pH (see Fig. 3). T h e hazards o f changing even the t y p e o f m o n o v a l e n t c o u n t e r l o n on the critical p h e n o m e n a o c c u r r i n g a r o u n d pH 9 are e m p h a s i z e d in w o r k r e c e n t l y r e p o r t e d b y S h a h ~ ~. To avoid the possibility o f d i s s o l u t i o n of Ionized acid at high pH, the work was c o n t i n u e d w i t h the longer c h a i n b e h c n l c acid =6. The A V--pH r e l a t i o n s h i p s for this C~= acid agreed ve r y w©ll w i t h those ob-
IONIZING MONOLAYI:RS AND pI! I:FI'I:CTS
57
(iD FatO' ah'oholx and etiter.~ It is well k n o w n that m o n o l a y e r s o f these materials are little a f f e c t e d by s u b s o h t t i o n pH. The A V _ p H me:tsuretnents o f S c h u h u a n and [lttghes t on o c t a d e c a n o l attd o c t a d e c y l l l l e t l l y l ether, h o w ~ ' v c r , r e v e a l t w o i n t e r e s t i n g features: (at a fall-off in A V vahtes o f the a l c o h o l at very high ptl (ca. 12), indicative o f the behavior o f an e x t r e m e l y weak acid; {b) an upswing o f A V at very low pH (ca. It. o f b o t h the a l c o h o l and the ether, and, in fact, o f fatty acids*, alld ascribed in all cases to o x o n i u m ion form.'ltion.
(tit) F a t t y atnine~ F a t t y a l k y l p r i m a r y a m i n e s have b e e n e x a m i n e d , f o l l o w i n g the early work o f Peters, b o t h by Glazer anti Dogan 2~ (Cj s ) and by Butts anti Petition t*' ( C t g ) over a wide pH range. T h e A V - p t t c h a r a c t e r i s t i c s o b t a i n e d again r e s e m b l e a t i t r a t i o n curve, o n c e more wttl~ I ndic a tions o f a p p a r e n t weakness. Boll1 sets o f d a t a i n d i c a t e high positive values ttrottnd 7 0 0 mV (Ionized f o r m ) at low pH, w h i c h fall o f f at high pH { u n i o n i z e d forttt), althougit It is clear that the two sets o f data d i f f e r c o n s k l e r n b l y in detail. Moreover. the Butts and Putltica data reveal a cotlsidcrable tlepeitdence ot~ the i]tc)t|ovtilultt cottnterion present {Nit* vs. NII~ ) L l h e i r e~tiltmled pK e,~trapoklt,,,d to it value t)f |O. I in the limit o f ~ m O, T his s o m e w h a t Itivth value COtlld be lowered i)y ¢tl. 0.5 traits if a ~uitabh; dipole c o r r e c t i o n wcrc i n t r o d u c e d but there wa,, t~o i)Id l e a t l o n o f c o n s t a n c y o f oK w i t h co. Mar0 work on tile p|[ d e p e n d e n c e (If tire p r o p e r t i e s o f alllJne Inonolityers is rt~quircd. E x u m h m t i o n o f tile I I ~ A / p l l da ta o f Bctt~ and Pcthi¢~) t~u~ge~t~t tile degree o f e x p a n s i o n o f the m o n o l a y c r s at low pH i, i)ot c o n s i s t e n t (~ee c q n (I 3)) w i t h that o f a fully charged monolttyer. Tills conch)slon i~ r e i n f o r c e d by data 3j on the C~0 a m i n e at pfl 3, with th~ p o s s i b i l i t y ~l~ain being r e f le c te d o f t o n / d i p o l e association. R¢~;¢ntly the pH dependence o f the m o n o l a y e r c h a r a c t e r i s t i c s o f d o c o s y l d i m c t h y l a m i n o o x i d e was r e p o r t e d 29 . A m i n e o x i d e s are very weak bases Ionizing a c c o r d i n g to -I-
R-N-(CHa
)~ + H , O
--
--" R - - N - - ( C H 3), + O H -
pK'~ 9
I O H Strong evidence was adduced for association in the m o n o l a y e r s at low pH at w h i c h the m o n o l a y e r is largely in the p r o t o n a t e d form, i n a s m u c h as the latter yielded less e x p a n d e d films over m u c h o f the area range than did the uncharged dipolar form. Analysis o f the data in terms o f pH and o K shifts was not a t t e m p t e d . 0
58 6 0 0
E.D. G O D D A R D ~
1
I~
t
=
f
"
~
;
i
T"'~
'~w
•
q
\ \
~00
\
\ ",\
401
\ 3O0
"0
,i
\
\ §
oH
Fi~. 4. ~ V - p l l curves ~° [or alkyl phosphoric acid monol~ycrs on 0.1 Ionic ~trcnl[tll ~ubsolut|ons at 18°C. (o) Mono, octadecyl acid at u n a r e a o f 30 A2/~noi.; (4) dioctadecyl acid at an ~ ¢ u of' 45 A2/mol.
(iv) A Ikyl l~hosphates arid phosphonates S t u d i e s o f m o n o - a n d d i - o c t a d e c y l p h o s p h a t e m o n o l a y e r s as a f u n c t i o n o f pH have b e e n u n d e r t a k e n b y Parreira 3°. A l t h o u g h an e x p a n s i o n o f these monolayers w i t h i n c r e a s e d pH did occur, the m o n o l a y e r s r e m a i n e d r e l a t i v e l y condensed. In the first m a t e r i a l , this b e h a v i o r is very likely the result o f the singly charged species" possessing a h y d r o g e n b o n d i n g O H g r o u p a nd the l i m i t e d pH range u t i l i z e d a n d , in the s e c o n d m a t e r i a l , o f the pr e se nc e o f two oct a d e c y l chains. O n the o t h e r h a n d , a p lot o f sur f a c e p o t e n t i a l versus pH again had
the form
of a titration
curve
with
an inflection
around
pH 4, i.e, con-
s i d e r a b l y h i g h e r t h a n the b u l k phase p K value o f p h o s p h o r i c acids (see Fig. 4). S i m i l a r d a t a f o r t h e d i a l k y l p h o s p h a t e have b e e n o b t a i n e d b y G o d d a r d a n d A l e x a n d e r 3t , a n d at close p a c k i n g b o t h the l a t t e r a n d those o f Parreira reveal a s h a r p u p s w i n g in A I,r at very l o w pH. A n a n a l y s i s o f his d a t a using the Betts-P e t h i e a a p p r o a c h was u n d e r t a k e n b y Parreira, w h o arrived at p K values for
IONIZING MONOLAYERS AND pll EFFECTS
$9
the mono- and d i a l k y l phosphates o f 2,8 and 1.7, w i t h o u t a dipole correction. and 3.0 and 2. I w i t h one. Payens t6 demonstrated qualitatively that the stirface pressure o f partially ionized alkyl phosphonate monolayers spread at the air/water interface is increased by addition o f salt, but devoted his main :ttt e n t i o n to spread monolayers o f stcaryl phosplmnute at the o i l / w a t e r interface so as to lessen the possibility o f association between the monolayer molecules. Measured and calculated salt effe~;ts on surface pres.~ure were compared as shown In Fig. 5. Agreement was moderately good at intermediate concentrations provided the (somewhat high) value o f pK = 3.8 was assumed. Payens offered tin explanation o1" thi.,, value, which exceetl~ bulk phase v.'dues by over ! unit, in terms o f changed h y d r a t i o n ch:tracteristtc~ o f the lens at the interface. A t low and. especially, at high salt concentrations agreement was unsatisfactory, it is o f interest that In most cases the monolayers did |lot c o n f o r m to the Davies equation o f state for monolayers at the o i l / w a t e r interface: measured surface pressur©s were generally appreciably lower. Payens a s c r i b e d t h e d i s c r e p a n c y t o r c s l d u a l c o h e s i o n in t h e m o n o l a y e r , not accounted f o r in t h e e q u a t i o n o f s t a t e ,
(v) A / k y l sulfates and alkane sul[onates Surface pressure :=nd surface potential/area curves for nonadccylbenzcne sulfonate are presented Jz in Fig. 6. It is o f interest th=,t in thi.,i ca~u tlw l l - t l data reveal considerable changes occurring at p l l "-" 4. and that at pll 2 and 3 the m o n o l a y c r is noticeably more expanded than at the higher pl l's. A similar effect has been reported by Dreher and Wilson 3:~ and by other workers =4.=s l b r monolayers o f alky] sulfates. Pethica anti Few 3= ascribe the effect to an H-bonded structure involving h y d r o n i u m ions, while Dreher ascribes it to dcionization o f the film. A n interesting feature w h i c h the data o f Fig. 6 displ=y is relative insensitivity o f A V in the pH range where the H - A characteristics change, but a marked elevation o f potential at very low pH. Despite the fact that the latter conditions necessitated an increase in subsolution ionic strensth, it is likely that the same phenomenon is involved as that i n v o k e d e a r l i e r b y S e h u l m a n a n d H u g h e s f o r f a t t y et~;ids, a l c o h o l s a n d ether,:. i.e. o x o n i u m i o n f o r m a t i o n , a n d w h i c h w e also o b s e r v e d f o r t h e a l k y l p h o s phates. T h e p H e f f e c t s d e s c r i b e d a b o v e raise t h e q u e s t i o n o f t h e a c t u a l a c i d s t r e n g t h o f a l k y l s u l f o n a t e s a n d s u l f a t e s , e s p e c i a l l y in t h e ~urfaee. T h k quextion of s u r f a c e h y d r o l y s i s , i.e. p r e f e r e n t i a l a d s o r p t i o n o f H + o r its c h e m i c a l c o m bination w i t h the head group, has been u n d e r discussion for some years; we shall r e t u r n t o this t o p i c in t h e s e c t i o n o n a d s o r b e d m o n o l a y e r s .
60
EoD. G O D D A R D
O tel
14 3T IO
6
t,t ~ D E
¢
0
2
p NaCI
4
6
Fig. 5. Th eo r etical (curves) and e x p e r i m e n t a l (points) changes in surface pressure o f s t e a w l p h o s p h o n i c acid m o n o l a y e r s at the oil/water interface 16 on adding neutral fair to the aqueous phase, pH. 5 . 8 . 2 5 ° C . (c,) 50 AX/mol; (A) 100 A2/mol.; (o) 150 A 2/ m ol .
(vi) Phospholipids a n d o t h e r z w i t t e r i o n i e materials It is i m p o r t a n t to p o i n t o u t i m m e d i a t e l y t h a t , in d e i o n i z i n g o n e o f the g r o u p s o f a z w i t t e r i o n , " s u r f a c e w e a k n e s s " is n o t e x p e c t e d in view o f the p r o x i m i t y o f the charge o f the o t h e r gr oup. F u r t h e r m o r e , while the e x a c t c o n f i g u r a t i o n o f the i o n i c d i p o l e o f m o n o l a y e r z w i t t e r i o n s is n o t a b s o l u t e l y e s t a b l i s h e d , it s e e m s r e a s o n a b l e t h a t t h e r e e xists a c o - p l a n a r h o r i z o n t a l arr a n g e m e n t o f t h e positive and negative charges, for w h i c h a r r a n g e m e n t the fit p o t e n t i a l w o u l d be e f f e c t i v e l y zero. A vertical a r r a y o f d i p o l e s w o u l d have an e x t r e m e l y high repulsive e n e r g y a n d p o t e n t i a l , w h i c h have n o t b e e n observed. S t u d i e s o f t h e i n f l u e n c e o f pH o n p h o s p h o l i p i d s have b e e n u n d e r t a k e n b y P c t h i c a a n d his c o - w o r k e r s ~ , ~ ' . W o r k i n g w i t h d i s t e a r o y l p h o s p h a t i d y l c h o l i n e , PC, a z w i t t e r i o n c o m p r i s i n g a m e d i u m s t r e n g t h a c id g r o u p a n d a
61
IONIZING MONOLAYERS AND pH EFFECTS 400 E 200
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Fig. 6. E f f e c t o f p H o n I~--A a n d ~ I,'--,4 i s o t h e r m s a2 o f n o n a d e c y i b e n z e n e s u l f o n a t ¢ m o n o l a y e t s at 2 5 ° C . S u b $ o l u t i o n s ate o f ionic s t r e n g t h 0.01 ( N a O H o r HCI]NaCI), e x c e p t f o r the 0.1 N a n d I N H2 S 0 4 .
s t r o n g b a s e g r o u p , A n d e r s o n a n d P e t h i c a 37 o b s e r v e d a p l a t e a u r e g i o n f r o m p H --- 2 . 5 t o 8 in t h e A V - - p H p l o t (see Fig. 7). T h e s h a r p u p s w i n g in A V b e l o w p H 2.5 is c o n s i s t e n t w i t h d e i o n i z a t i o n o f t h e p h o s p h a t e g r o u p ; o n t h e o t h e r h a n d , t h e s h a r p d o w n s w i n g in Lt V, e x p e c t e d f o r d e i o n i z a t i o n o f t h e b a s i c g r o u p s , o c c u r s a t p H 8 a n d is d i f f i c u l t t o e x p l a i n in v i e w o f t h e s t r o n g l y b a s i c n a t u r e o f t h e c h o l i n e g r o u p . W h a t s e e m s likely is t h a t a r e o r i e n t a t i o n o f t h e h e a d g r o u p o c c u r s in t h e p H r a n g e a b o v e 8. O t l l e r e v i d e n c e f o r thi~ c o m e s f r o m t h e c h a n g e in I I - - A c h a r a c t e r i s t i c s a t p H 10, a t w h i c h p H t h e m o n o l a y e r d e v e l o p s f e e b l e s t a b i l i t y . T h e r e is n o i n d i c a t i o n o f m o n o l a y e r e x p a n s i o n a t l o w p H , w h i c h is e x p e c t e d t o a c c o m p a n y d e i o n i z a t i o n o f t h e phosphate group. R e s u l t s m u c h m o r e in k e e p i n g w i t h a n t i c i p a t e d b e h a v i o r o f a z w i t t e r i o n o f d o u b l e - s i d e d s t r e n g t h w e r e o b s e r v e d a2 w i t h m o n o l a y e r s o f a syntbc*.ic s u l f o b e t a i n e , d o c o s y l a m m o n i o p r o p a n e s u l f o n a t e , C 2 o H4 ! N+ ( C H a ) = ( C H 2 ) a SO3. T h i s m a t e r i a l is a n a n a l o g o f a l e c i t h i n b u t w i t h t h e t e r m i n a l g r o u p of the zwitterion negative rather than positive. For this material only minor c h a n g e s w e r e o b s e r v e d in b o t h I I - - A a n d A V - - A c h a r a c t e r i s t i c s o v e r a p H
62
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r a n g e o f 1 t o 12; i.e. t h e r e w e r e n o i n d i c a t i o n s o f a n y s u r f a c e w e a k n e s s o f either head group constituent. T h e a d d i t i o n a l A V vs. p H d a t a s h o w n i n Fig. 7 w e r e o b t a i n e d b y S t a n d i s h a n d P e t h i c a s6 f o r d i p a l m i t o y l p h o s p h a t i d y l e t h a n o l a m i n e , P E , a s y n t h e t i c c e p h a l i n , w h i c h possesses a m e d i u m s t r e n g t h a c i d g r o u p a n d a w e a k base group. The plateau regions, consistent with an effectively uncharged monol a y e r , are s e e n a t i o n i c s t r e n g t h s o f b o t h 0.1 a n d 0 . 0 1 , as are t h e a n t i c i p a t e d u p s w i n g o f p o t e n t i a l at l o w p H a n d d o w n s w i n g at h i g h p H , c o r r e s p o n d i n g to deionization of the phosphate group and primary amine group, respectively T h e o b s e r v e d salt e f f e c t is q u a l i t a t i v e l y c o n s i s t e n t w i t h e l e c t r i c a l p r e d i c t i o n s . Calculations were made, from the data, o f pK values o f the c o n s t i t u e n t acidic a n d basic g r o u p s , u s i n g t h e m e t h o d o f e x t r a p o l a t i o n t o z e r o d e g r e e o f i o n i z a t i o n . T h e d e r i v e d v a l u e s 36 so o b t a i n e d w e r e p K a = 0 . 3 2 , m u c h l o w e r t h a n t h e b u l k p h a s e v a l u e of--~2, a n d PKbase = 3 . 5 5 , c o m p a r a b l e t o t h e v a l u e i n b u l k . It is n o t c l e a r w h i c h o f t h e salt c o n c e n t r a t i o n s was e m p l o y e d f o r t h e computations, or if the above values were averaged for the two. The un-
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Fig. 8. Effect of pH on rl-A and A V - A isotherms of the sult'oamine, 2-LAr-methyl-N-2-hydroxydocosyl) a m i n o e t h a n e s u l f o n a t e , o n i o n i c s t r e n g t h 0 . 0 1 ( N a C I ÷ H C I o r N a O i l ) s u b s o l u t t o n s 32 a t 2 5 = C . ltzset: Curve - plot of ,~V values at an area of 40 A2/mol. vs. pH; (X) - calculated values of the q'o potential ( 4 0 A 2 / m o l . ) vs. p H .
usually low value o f the derived p K a indicates that the simple electrical analysis is n o t a p p l i c a b l e u n d e r a c i d c o n d i t i o n s . It is t o be n o t e d t h a t a s l i g h t reorientation within the monolayer would necessitate a substantial dipole c o r r e c t i o n to A V for this kind o f m o l e c u l e . A m a r k e d e x p a n s i o n o f the II--A c u r v e s o c c u r r e d b e t w e e n p H 9 . 8 a n d 1 1.9, c o r r e s p o n d i n g t o " ' i o n i z a t i o n " o f a n i n c r e a s i n g l y n e g a t i v e l y c h a r g e d m o n o l a y e r . N o e x p a n s i o n was, h o w e v e r , n o t e d a t l o w p H . F o r c o m p a r i s o n w i t h t h e a b o v e , we use m o n o l a y e r d a t a r e p o r t e d 32 f o r t h e " a n a l o g " o f a c e p h a l i n , n a m e l y t h e st, l f o a m i n e , 2-(Nm e t h y l - N - 2 - h y d r o x y d o c o s y l ) a m i n o e t h a n e s u l f o n a t e , Czo H 4 ~ - C H ( O H ) CH2 N ( C H 3 ) ( C H 2 )2 SO3 N a , w h i c h p o s s e s s e s a s t r o n g t e r m i n a l a c i d i c g r o u p a n d a n i o n i z a b l e t e r t i a r y n i t r o g e n g r o u p . It s h o w s s e v e r a l s i m i l a r i t i e s t o t h e p h o s p h o U p i d s , as s e e n in Fig. 8. T h u s its m o n l a y e r s a r o u n d n e u t r a l p H , a n d p r e s u m a b l y in z w i t t e r i o n i c f o r m , are i n s e n s i t i v e t o c h a n g e s in i o n i c c o m p o s i t i o n o f t h e s u b s o l u t i o n a n d t h e r e is a p l a t e a u r e g i o n in t h e A V - - p H c u r v e .
64
E.D. G O D D A R D
At high pH, when the amine group becomes progressively unionized and the monolayer develops a net negative charge, a marked expansion of the monol a y e r o c c u r s , a n d t h e s u r f a c e p o t e n t i a l b e c o m e s p r o g r e s s i v e l y less p o s i t i v e . F o l l o w i n g S t a n d i s h a n d P e t h i c a 3a , t h e a u t h o r s o f t h i s w o r k a s s u m e d t h e xPo p o t e n t i a l o f t h e m o n o l a y e r t o be z e r o in t h e p l a t e a u r e g i o n f o r c a l c u l a t i o n p u r p o s e s , a n d t h e y u t i l i z e d t h e G o u y e q u a t i o n s t o e s t i m a t e t h e p K value o f t h e a m i n e . C a l c u l a t i o n s w e r e r e s t r i c t e d t o ~ v a l u e s b e l o w 0 . 0 5 in o r d e r : (a) t o m e e t m o r e c l o s e l y t h e c o n d i t i o n s u n d e r w h i c h t h e t h e o r y is a p p l i c a b l e , a n d (b) t o s t a y w i t h i n t h e r a n g e w h e r e t h e d i p o l e c o n t r i b u t i o n w o u l d e f f e c t i v e l y be c o n s t a n t . In t h i s . w a y , t h e p K a value w a s e s t i m a t e d to be 10, w h i c h is r e a s o n a b l y close t o t h e b u l k value f o r t r i m e t h y l a m i n e . O n t h e o t h e r h a n d , t h e u p s w i n g in A V as t h e p H is l o w e r e d a p p e a r s t o s t a r t at a p H w h i c h is t o o h i g h t o be c o n s i s t e n t w i t h t h e h i g h acid s t r e n g t h o f t h e s u l f o n a t e g r o u p s , a n d m a y a g a i n r e f l e c t a r e o r g a n i z a t i o n in t h e m o n o l a y e r . L a c k o f a n y s i g n i f i c a n t e x p a n s i o n o f t h e m o n o l a y e r at t h e l o w e s t p H also i n d i c a t e s t h a t t h e m o l e c u l e s are n o t cor, a p l e t e l y in t h e p o s i t i v e l y c h a r g e d from and again suggests appreciable molecular association.
(vii) P o l y m e r s In 1 9 5 2 , G l a z e r a n d D o g a n ~n p u b l i s h e d A V - - p H d a t a o n s p r e a d m o n o l a y e r s o f b o v i n e s e r u m a l b u m i n , a n d i n t e r p r e t e d t h e r e s u l t s in t e r m s o f t i t r a t i o n o f t h e side c h a i n c a r b o x y l a n d a m i n o g r o u p s , t h e p K ' s o f b o t h o f w h i c h appear to shift to higher values t h a n the bulk phase values o f proteins. H o w e v e r , u s i n g t h e i r a p p r o a c h o f c o m p a r i s o n at c o n s t a n t p r e s s u r e , t h e y w e r e u n a b l e t o d i s c e r n i n f l e c t i o n s c o r r e s p o n d i n g to n e u t r a l i z a t i o n o f t h e s i d e c h a i n i m i d a z o l e g r o u p s , H a r r a p 39 , w o r k i n g w i t h i n s u l i n , d e m o n s t r a t e d t h a t if A V - - p H c u r v e s are c o n s t r u c t e d at c o n s t a n t a r e a r a t h e r t h a n c o n s t a n t pressure t h i s w a s n o t t h e case. C o m p a r e d w i t h b u l k p h a s e v a l u e s , t h e p K ' s o f t h e acid a n d basic g r o u p s were again shifted u p w a r d . A l t h o u g h the p K values h a v e n o t b e e n a n a l y z e d in d e t a i l , it s h o u l d b e n o t e d t h a t , c o m p a r e d t o p K v a l u e s o f i s o l a t e d C O O H a n d N H 2 g r o u p s in b u l k p h a s e , s u b s t a n t i a l s h i f t s in t h e d i r e c t i o n o f w e a k n e s s o f t h e s e g r o u p s in p r o t e i n m o n o l a y e r s are n o t e x p e c t e d . T h e s i t u a t i o n is a n a l o g o u s t o t h e case o f z w i t t e r i o n s a l r e a d y disc u s s e d : w h e n t h e - - C O O H g r o u p s are b e i n g i o n i z e d t h e m o l e c u l e c a r r i e s a net positive charge which would promote ionization; when the NH 2 groups are b e i n g i o n i z e d , t h e m o l e c u l e s c a r r y a n e t n e g a t i v e c h a r g e w h i c h w o u l d d o t h e s a m e . T h i s s i t u a t i o n e x i s t s , o f c o u r s e , also f o r b u l k p h a s e t i t r a t i o n s o f p r o t e i n s . A s e x p e c t e d o n e l e c t r i c a l g r o u n d ~ , H a r r a p s h o w e d t h a t s u r f a c e pressures for insulin m o n o l a y e r s were at a m i n i m u m at the isoelectrie point o f 5.7, as s e e n in Fig. 9.
IONIZING MONOLAYERS AND pH EFFECTS |
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Fig. 9. r l - p H (A) and n V--pH (o) curves for insulin t-rims spread on ionic strength 0.01 subsolutions 39.
A great deal of work on monolayers of synthetic protein analogs, together w i t h a s m a l l e r a m o u n t o n p o l y a c i d s , w a s c a r r i e d o u t d u r i n g t h e 1950"s b y l s e m u r a a n d his c o l l e a g u e s 4 ° - 4~. A p p r e c i a b l e e f f e c t s w e r e f o u n d o n t h e m e a s u r e d I I - - A . A V - - A a n d r/--A c h a r a c t e r i s t i c s as t h e p H w a s c h a n g e d . T h e s e c h a n g e s w e r e in a s e n s e g e n e r a l l y p r e d i c t a b l e f r o m t h e i o n i z a t i o n / d e i o n i z a t i o n o f t h e p o l a r g r o u p s p r e s e n t , s u c h as a m a x i m u m in s u r f a c e v i s c o s i t y (r/), a n d a m i n i m u m in s u r f a c e p r e s s u r e , a r o u n d t h e i s o e l e c t r i c p o i n t . I o n i z a t i o n o f C O O H g r o u p s in p o l y a c i d s led t o p r e d i c t a b l e d e c r e a s e s in m e a s u r e d A V's; h o w e v e r , FI v a l u e s g e n e r a l l y d e c r e a s e d as t h e a c i d i t y w a s d e c r e a s e d m a i n l y because of solubility of the ionized form which severely restricted the pH r a n g e i n v e s t i g a t z b l e . C o m p a r a b l e d a t a w e r e o b t a i n e d b y Miller 44 f o r m o n o l a y e r s o f p o l y ( v i n y l b e n z o i e a c i d ) . In o n e p a p e r b y l s e p a u r a a n d H a m a g u c h i * t , qz 0 values o f a m o n o l a y e r o f s y n t h e t i c p o l y p e p t i d e (ly~sine, p h e n y l a l a n i n e , g l u t a m i c acid c o p o l y m e r ) w e r e e s t i m a t e d f r o m ( a ) II a n d ( b ) A V, u s i n g t h e Davies equations. Good agreement was obtained. M o r e r e c e n t l y , J a f f 6 a n d R u y s s c h a e r t 45 h a v e i n v e s t i g a t e d t h e p r o p e r t i e s of a variety of ionizable polyeleetrolytes, both addition type (polyvinyl) and
66
E.D. G O D D A R D
Table 2 Derived values o f pK for m o n o l a y e r s o f poly (methacrylic acid) (ref. 46) Interfac©
Stereoregularity
PKGouy
PKDonnan
AErlwater
Atactic Isotactic Atactic lsotactic
4.6 4.4 4.9 4.8
4.6 4.6 5.0 4.9
Cyclohexane/water
condensation type (polypeptides). Working with a copolymer of 2-vinylpyrid i n e a n d a c r y l i c acid t h e y d e m o n s t r a t e d t h a t t h e G o u y e q u a t i o n s s a t i s f a c t o r i l y p r e d i c t values o f t h e e l e c t r i c a l c o n t r i b u t i o n A H to t h e s u r f a c e p r e s s u r e f o r low n e t c h a r g e d e n s i t i e s a r o u n d t h e i s o e l e c t r i c p o i n t . T h e y s h o w e d t o o t h a t identical v a l u e s o f t h e i n t r i n s i c d i s s o c i a t i o n c o n s t a n t K o f p o l y a c i d s are o b t a i n e d b y t h e s u r f a c e t i t r a t i o n m e t h o d as b y t h e b u l k t i t r a t i o n m e t h o d , using t h e Katchalsky expression (3). In t h e i r m o s t r e c e n t w o r k t h e s e a u t h o r s 46 have t u r n e d t h e i r a t t e n t i o n t o a study of the effects of stereoregularity of polyelectrolytes on their surface p r o p e r t i e s at b o t h t h e a i r / w a t e r a n d o i l / w a t e r i n t e r f a c e . R e c o g n i z i n g t h e diff i c u l t y o f i n t e r p r e t i n g s u r f a c e p r e s s u r e m e a s u r e m e n t s t h e y c h o s e to m e a s u r e and analyze surface potential measurements, employing both the Gouy-Chapman and Donnan approaches. The essential part of the analysis was the u t i l i z a t i o n o f t h e B e t t s - - P e t h i c a m e t h o d o f d e t e r m i n i n g t h e p g values b y e x t r a p o l a t i o n o f ( p H s 4- log (1 --e,/0t) values o b t a i n e d a t l o w values o f a to yield PKrr_, 0" T h e v a l u e s o b t a i n e d f o r p o l y ( m e t h a c r y l i c a c i d ) a r e g i v e n in T a b l e 2 a n d reveal t h a t , u n l i k e t h e case o f t h e s e p o l y e l e e t r o l y t e s in s o l u t i o n . the isotactic c o n f i g u r a t i o n has a slightly stronger acidity. This difference was ascribed to greater accessibility of carboxyl groups allowed by this configurat i o n a t t h e i n t e r f a c e . A l t h o u g h t h e d i f f e r e n c e s in b e h a v i o r e v i d e n t f r o m t h e d a t a o f T a b l e 2 a p p e a r small, this w a y o f p r e s e n t i n g t h e clata belies t h e c o n siderable actual differences occasioned both by the stereoregularity and by t h e t y p e o f i n t e r f a c e . T h i s v a r i a t i o n is e v i d e n t in t h e c o m p i l a t i o n o f e x p e r i m e n t a l data given in Fig. 10. A n a l y s i s o f s u c h data, as y e t u n a t t e m p t e d , would appear fruitful to provide information on configurational changes a c c o m p a n y i n g t h e p r o g r e s s o f i o n i z a t i o n . It is o f i n t e r e s t t h a t t h e G o u y - C h a p m a n a n d D o n n a n a p p r o a c h e s led to v i r t u a l l y i d e n t i c a l xp a n d d e r i v e d p K values f o r t h e c o n t r i t i o n s e m p l o y e d . In a s e c o n d p a p e r a p p l y i n g t h e s u r f a c e p o t e n t i a l m e t h o d , C a s p e r s et al. 4~ e x a m i n e d t h e i n f l u e n c e o f i n t e r f a c e ( a i r / w a t e r vs. c y c l o h e x a n e / w a t e r ) o n t h e i o n i z a t i o n o f m o n o l a y e r s o f p o l y - L - g l u t a m i e acid. It is s t a t e d t h a t t h i s m a t e r i a l a s s u m e s a helical c o n f i g u r a t i o n in t h e i n t e r f a c e . In t h i s case, t h e A V - - p H c h a r a c t e r i s t i c s did n o t s h o w t h e g r o s s d i f f e r e n c e s at t h e t w o in-
IONIZING MONOLAYERS AND pH EFFECTS
!
Fig. tO. I n f l u e n c e o f p H o n s u r f a c e - p o t e n t i a l , - c o n c e n t r a t i o n curves o f a t a c t i c (a) a n d iso~actic (i) poty(m©thacryfic acid) m o n o l a y e r s at the air/water and oil/water interface 46.
67
-i
68
E.D. GODDARD
terraces evidenced by the p o l y ( m e t h a c r y l i c acid). T h e a u t h o r s d e d u c e d f r o m a s t u d y o f A V vs. p H t h a t t h e o i b t w a t e r i n t e r f a c e a l l o w s less r e a d y a c c e s s i b i l i t y of the COOH groups for ionization than does the air/water interface, although t h i s is n o t e v i d e n t in t h e r e s p e c t i v e pKa_, a v a l u e s o f 4 . 4 ( G o u y , D o n n a n ) a n d 4.5 ( G o u y , D o n n a n ) w h i c h w e r e d e r i v e d . T h e d a t a s h o w a r e s i d u a l d e p e n d e n c e o f p K o n i o n i c s t r e n g t h , d e s p i t e t h i s f a c t o r h a v i n g b e e n a l l o w e d f o r in t h e G o u y - C h a p m a n a n d D o n n a n d e r i v a t i o n s . It was a s c r i b e d e i t h e r t o s t a b i l i z a t i o n o f t h e h e l i c a l s t r u c t u r e a t low i o n i c s t r e n g t h o r a n i n t r i n s i c d i f f i c u l t y o f t h e h y d r o g e n i o n in leaving t h e m a c r o m o l e c u l e , i.e. o v e r a n d a b o v e t h a t a l l o w e d for by simple electrical t h e o r y . D. A D S O R B E D M O N O L A Y E R S
W h i l e it is well k n o w n t h a t a d s o r b e d f i l m s at a q u e o u s i n t e r f a c e s will res p o n d t o c h a n g e s in pH o f t h e b u l k a q u e o u s p h a s e i f t h e film m a t e r i a l is ionizable, c o m p a r a t i v e l y little w o r k has been d o n e to o b t a i n q u a n t i t a t i v e i n f o r m a t i o n o n t h e c o m p o s i t i o n o f t h e i n t e r f a c e . D a n i e l l i ' s s a p p r o a c h involved t h e u n p r o v e n a s s u m p t i o n t h a t t h e l o w e r i n g o f i n t e r r a c i a l (c, i l / w a t e r ) t e n s i o n b y f a t t y a c i d s was d i r e c t l y p r o p o r t i o n a l t o t h e a m o u n t o f i ¢ n i z e d acid at t h e i n t e r f a c e . More r e c e n t a p p r o a c h e s have g e n e r a l l y i n v o l v e d t h e a p p l i c a tion of the Gibbs adsorption equation to surface and interracial tension data, although direct measurements of the composition of the surface by f o a m i n g a n d r a d i o t r a c e r t e c h n i q u e s h a v e also b e e n m a d e . A t c o n s t a n t pressure a n d t e m p e r a t u r e t h e G i b b s e q u a t i o n , r e l a t i n g i n t e r f a c i a l t e n s i o n , % t o s u r f a c e excess, r , has t h e f o r m --d3, = d l l = ~
rtd/a i
(25)
w h i c h , a p p l i e d t o a salt o f a w e a k l o n g c h a i n acid HZ, b e c o m e s --d~ = ruz
d/~mz + r z d/z z + ~ /
r i d/~l
(26)
T h e / t e r m s r e p r e s e n t c o m p o n e n t s w h i c h are n o n s u r f a c e - a c t i v e , p e r se. P a y e n s 16 • b y s p l i t t i n g t h e c h e m i c a l p o t e n t i o n a l t e r m t~z i n t o e l e c t r i c a l a n d n o n - e l e c t r i c a l t e r m s , e ~ a n d / a ~ , has s h o w n t h a t e q n (1 1) a p p l i e s t o a d s o r b e d f i l m s , b o t h i o n i z e d a n d p a r t i a l l y i o n i z e d , as well as i n s o l u b l e f i l m s , a n d has used t h i s e q u a t i o n f o r a n a l y s i s o f his a d s o r p t i o n results. We s h a l l n o w d e a l w i t h t w o b r o a d c a t e g o r i e s o f s o l u b l e s u r f a c e a c t i v e m a t e r i a l , viz. a m p h i p a t h i c w e a k e l e c t r o l y t e s a n d a m p h i p a t h i c s t r o n g e l e c t r o lytes.
IONIZING MONOLAYERS
E IJI
AND pH EFFECTS
--O -o A
60 o f O / °
~" 5 o
69
-. _.......---o-- 8
// / o
o /
O
3c
?
/
/ o |
"7
|
~
I,
pH
I
II
F i g . 11. E q u i l i b r i u m s u r f a c e t e n s i o n v a l u e s s2 o f ( A ) 0 . 0 0 1 N a n d (B) 0 . 0 0 5 N s o d i u m l a u r a t e s o l u t i o n s a t
25"C.
(i) F a t t y a c i d s o a p s a n d a m i n e s F o l l o w i n g R a i s o n 4a a n d L e v i a l d i 49 , E a g l a n d a n d F r a n c k s so c a r r i e d o u t f o a m i n g s t u d i e s o n s o d i u m o l e a t e s o l u t i o n s . C h e m i c a l a n a l y s i s o f t h e coll a p s e d f o a m r e v e a l e d t h a t t h e h y d r o l y s i s c o n s t a n t o f o l e a t e in t h e s u r f a c e w a s a b o u t o n e t h o u s a n d t i m e s h i g h e r t h a n in b u l k p h a s e , d u e in p a r t t o a l o w e r i n g o f t h e e f f e c t i v e p H in t h e s u r f a c e r e g i o n . E a r l y w o r k b y L o n g et al. s z .s2 s e r v e d t o i l l u s t r a t e t h e i n f l u e n c e o f p H o n the surface t e n s i o n o f soap solutions. A l t h o u g h n o q u a n t i t a t i v e analysis was a t t e m p t e d , the results s h o w e d that, on r e d u c i n g t h e pH, a d s o r p t i o n o f uni o n i z e d a c i d w a s e v i d e n t e v e n at p H 1 1 (see Fig. 1 I). Q u a n t i t a t i v e a n a l y s i s o f t h e a d s o r p t i o n o f p o t a s s i u m l a u r a t e ( K Z ) / l a u r i c a c i d ( H Z ) at t h e a q u e o u s i n t e r f a c e , b o t h w i t h air a n d n - d o d c c a n e , w a s u n d e r t a k e n b y v a n V o o r s t V a d e r ss . B o u n d a r y t e n s i o n m e a s u r e m e n t s w e r e m a d e o v e r t h e p H r a n g e 7.5 t o 9 . 3 , a n d t h e G i b b s e q u a t i o n w a s a p p l i e d in t h e f o r m --dT/RT= P K d Ina K +(Pz+PHz
) d In CT + PHZ d In aHZ + ( r z + l " H z ) d i n f
z
(27) in w h i c h C T r e p r e s e n t s t h e t o t a l s o a p c o n c e n t r a t i o n , a t h e a c t i v i t i e s a n d f the activity coefficients.
T o calculate ( r z + l ~ n z ) , surface t e n s i o n data o n s o l u t i o n s o f c o n s t a n t [K + ] and [H + ] w e r e m e a s u r e d . T h e activity c o e f f i c i e n t s , f r and f z , w e r e a s s u m e d to r e m a i n c o n s t a n t and e q n . ( 2 7 ) b e c o m e s
70
E.D. G O D D A R D
l
(~~T)
all. a K
=I"Z +rllZ
(28}
F o r e s t i m a t i n g the f a t t y acid a d s o r p t i o n , l-*ll Z , o n l y one o f the t w o m e t h o d s o f vail V o o r s t V a d e r is given as mt illustration. F o r this tile variation o f surface tension with pH was m e a s u r e d at c o n s t a n t [K + ] and total soap conc e n t r a t i o n , if changes t a f t : and f z are nel~lected, eCln (27) becontes
=
dT_)
a l n a,
cwcT
,,RT r. z
(201
Tabl~ 3, e x t r a c t e d from van V o o r s t V a d c r ' s publication, presents tile main fh:dlngs, An I m p o r t a n t concltmion d r a w n was that FIi z remains higher than e x p e c t e d on incrcasin$ t h , pH, Wc illtmtrotc tills point by ealculnting "'pK'" from tile t o b u l - t e d d~ta with no allowance for s0rfl|ce p l l effects, A s t e a d y upward trend, i.e. Increasing weakn0~s, Is evident. Van Voorst Vadcr ascribes tills trend to specific Interaction forces between tile laurate Ion and h;uric acid at the interface (see section on insoluble monolayers) and stresses the point that these forces act between the hydrophilic parts o1" the molecules s{nce the same trend of' Pttz values with ptl was found at the oH/water interface. It is clear, however, that p!! shifts c a n n o t be neglected in such analyses. Payens '6 used t h e r m o d y n a m i c reasoning analogotJs to van V o o r s t V a d e r ' s to derive the a d s o r p t i o n values and c o n s t r u c t " ' f o r c e - a r e a " curves for caprylic acid and o c t y l a m i n e . T h e n , by m e a n s o l ' e q n . (I !) a n d the G o u y double layer t r e a t m e n t , he c o m p a r e d t h e o r y and e x p e r i m e n t in t e r m s o f ,~II (eqn. (1 I)) values p l o t t e d against salt c o n c e n t r a t i o n at selected pH values. Q u i t e good a g r e e m e n t was o b t a i n e d for the a m i n e b u t n o t for the caprylic acid when bulk phase values o f the association c o n s t a n t s were e m p l o y e d . T h e d i s c r e p a n c y was again a t t r i b u t e d to cohesion forces.
(It) A Ik),l sulfates and alkane stdfonates it is a p p r o p r i a t e to treat first t h e q u e s t i o n o f surface " h y d r o l y s i s " , i.e. preferential a d s o r p t i o n o f H+ or its chemical c o m b i n a t i o n w i t h tile head group, for the strong acid a m p h i p a t h i c materials, alkyl sulfates and alkan¢ sulfonates. Preferential a d s o r p t i o n for these cases has been u n d e r discussion for m a n y years. As for the weak a m p h i p a t h i e e l e c t r o l y t e s , the p r o b l e m has been a p p r o a c h e d b y s t u d y i n g t h e d e p e n d e n c e o f the surface p r o p e r t i e s o f a d s o r b e d m o n o l a y e r s u p o n pH. F o r a I : 1 s u r f a c t a n t electrolyte Na*S - , the G i b b s a d s o r p t i o n e q u a t i o n can be w r i t t e n --d3' = xkTT'¢ d In mNa~ Ikls,.S"
(30)
IONIZING MONOLAYERS AND pit EFFECTS
71
Table 3 Values of PKap p for laurlc acid from data of tel', 53
]+Z
l+llZ (molelcm + X IO l °
PKap p
lntelface
pll b
(mole/era a x IO i °
Ablwalut
9.33 8.72 g.0? 1.4 ?
3.55 3,35 3.15 2.N
2.2 2.65 2,95 3,9
9.11 8.62 8.04 7.6 I
tt+Dodet:ano/water 9.~$
3.1S 2,75 2.7 2,3
1,45
%00 S.$7
R.72
N.07 7.47
1,95
2.2~ ;L I
a.O0
7.60
wllcr¢ 7' Is t|lU surl'ilC¢ IenMon, I',= tile surl'uc¢ excess and lhu rest of Ih¢ terms have their usual slgnificuI1C¢, lu tlw cqm~tion, x equals I or 2 for coInpI¢[¢ or z~ro 11ydrolysls s4 . respectively. A t t e m p t s to Incasur¢ I11¢ i l n l o i l n l o f adsorptlon directly, and thus e.qablish I|I~ correct f o r m of' t|1~ equation, h a w ilwolvcd in the nlain t w o n1¢t|1ods, viz. the radiotraccr tcci1111que, and a substantial extension of the i n t e r f a c e b y f o a m i n g f o l l o w e d by collal,si=lg tilt: f o a m and d e t e r m i n i n g the s u r f a c a n t c o n c e n t r a t i o n s . F o a m i n g experiment+,, s4 o n s o d i u m d o d e c y l sulfate ( N a D S ) and d o d e c a n e sull'onate ( N a D S O j ) s h o w e d good a g r e e m e n t w i t h the " 2 " f o r m o f the e q u a t i o n , and are in e x c e l l e n t a g r e e m e n t ( N a D S ) w i t h rec e n t direct d e t e r m i n a t i o n s by the r a d i o t r a c e r m e t h o d ss , w h i c h ind i c a t e s little hydrolysis. A s e c o n d , p e r h a p s m o r e direct, m e t h o d of s t u d y i n g " h y d r o l y s i s " or the e x t e n t o f H + u p t a k e , has b e e n to m e a s u r e the pH o f the collapsed f o a m o f f o a m e d s o l u t i o n s o f ionic s u r f a c t a n t s s4.s6 ,sT. MeasurementsS4 revealed, for b o t h NaDs and N a D S O a , that the pH o f the collapsed f o a m was in fact slightly lower t h a n that o f the p a r e n t s o l u t i o n . Bujake a nd G o d d a r d s4 , however, stressed t h e p o i n t t h a t if an a n i o n i c d e t e r g e n t is locally c o n c e n t r a t e d , e.g. b y a d s o r p t i o n , the c o n c e n t r a t i o n of all c o u n t e r i o n s , lttchtdlng hydrogen Ion, w o u l d also be locally c o n c e n t r a t e d a n d t h e r e f o r e a decrease in pH o f the collapsed f o a m was to be e x p e c t e d and did not necessarily i m p l y h y d r o l y s i s . In fact, w h e n the ratios o f t[ + and D S - ion were c o m p a r e d in the p a r e n t s o l u t i o n ( R ~ ) and collapsed f o a m ( R e ) , the d e p a r t u r e s o f R t / P , : f r o m u n i t y were f o u n d to be w i t h i n e x p e r i m e n t a l error, w h i c h again i m p l i e d no h y d r o l y s i s u n d e r the n e a r - n e u t r a l c o n d i t i o n s e m p l o y e d . In o r d e r to o b t a i n i n f o r m a t i o n on the h y d r o l y s i s , we now deal w i t h dir e c t surface t e n s i o n - p H " t i t r a t i o n " o f a s o l u t i o n o f the s u r f a c t a n t s4.s+-sa m a i n t a i n e d at c o n s t a n t ionic s t r e n g t h but o f va r ying pH. Figur e i 2 p r e s e n t s da t a for NaDSO3 w h i c h s h o w the r e v e r s i b i l i t y o f its s o l u t i o n s to a c i d i f i c a t i o n . .mat
"/2
E.D. G O D D A R D
60
?f
E 0
/ "0
e~e
~ O ~ O
A
o
o
~0
(~.0--0-0
0
'
•
*--B
4O
6 pH
IO
2F51g- I ~ ?urfae~i" t e n , ! o n vL p H f a r I 0 - s M i o d i u m d o d c c a n e sulfonat© s'* in H c c u o l y t e s o l - d o n s at ¢-. A o o e a e . c c t r m y t e ( N a C l + HCI) c o n c e n t r a t i o n is 1 0 - * ~ f iJ~ curve A , and 1 0 - * M i n c u r v e B.
(*) S o l u t i o n s first t a k e n to p H ~ 2 prior to adjusting to final pH b y a d d i t i o n o f N a O H , as a c h e c k f o r chemical hydrolysis.
I t w a s o b s e r v e d s4 t h a t a d e f i n i t e r e d u c t i o n o f t h e s u r f a c e t e n s i o n o f t h e s o l u t i o n t a k e s p l a c e s a t a c e r t a i n p H , w h i c h d e p e n d s o n the ionic s t r e n g t h o f the solution. This result can only m e a n that some preferential a d s o r p t i o n o f H +, o r h y d r o l y s i s ) o f t h e s u r f a c t a n t t a k e s p l a c e at these a n d l o w e r p H values, as w a s e v i d e n t f r o m the H - A r e s u l t s o b t a i n e d f o r t h e long c h a i n sulfonate monolayers. The surface tension data allow an estimate of the d e g r e e o f h y d r o l y s i s b y use o f t h e G i b b s e q u a t i o n ~4 A c t i v i t y c o e f f i c i e n t s are a s s u m e d to be u n i t y f o r t h e c o n d i t i o n s o f s t u d y , a n d t h e e q u a t i o n is w r i t t e n
--dT/kT=F n. dinC a++l'Na +dlnCNa+
+Ps_
dlnC s_ +PHs dlnC.ns
(31) We c o n s i d e r a c h a n g e in pH at c o n s t a n t i o n i c s t r e n g t h a n d a t c o n s t a n t surf a c t a n t c o n c e n t r a t i o n . N o f o r m a t i o n o f H S is a s s u m e d in t h e first case, a n d we set d C n + = --dCNa +. T h e last t w o t e r m s o f e q n . ( 3 1 ) d i s a p p e a r l e a d i n g to the relation s 4 d q T ' ( 2 . 3 0 3 k T d p H ) = l"lt + (1 + CtI+ICNa. ) -- l " y ( C , . / C N a . )
(32)
C a l c u l a t i o n s w e r e m a d e f o r t h e c o n d i t i o n s o f c u r v e A, Fig. 12) i.e. 1 0 - a M NaDSO3, 10-2 M added electrolyte, and an assumed ( c o n s t a n t ) value o f l / r s - o f 5 0 A 2 / m o l e c u l e . T h e r e s u l t s i n d i c a t e t h a t w h e n t h e p H is c h a n g e d f r o m 4 to 2, the PH + / P s - r a t i o i n c r e a s e s f r o m 0 . 0 8 to n e a r u n i t y . In t h e s e c o n d case) f o r m a t i o n o f u n i o n i z e d acid is a s s u m e d a c c o r d i n g t o the equilibrium
IONIZING MONOLAYERS AND pil EFFECTS
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[H* ! [ S - 1 = K [ H S ! B e c a u s e o f t h e h i g h acid s t r e n g t h o f t h e s e l o n g - c h a i n acids, t h e r e l a t i o n dCn+ = --dCNa+ still h o l d s to a g o o d a p p r o x i m a t i o n , anti e q n . ( 3 1 ) can be written
- d " t l k T = Fll+ d I n e l l + -- r s : t q , ÷ / c N . + ) [-'llS (d In CII+ + d In C s _ )
d In % + + r s _ d In % _
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W i t h t h e f u r t h e r a s s u m p t i o n o f d C s _ = 0. Fn+ = rN,,+ ( C I I + ] C N a + ) , a n d tile same value o f P s - • calculations based on the final e q u a t i o n
- - d ' y / k T = rtlS d In Ctl +
(34)
i n d i c a t e t h a t I"Its]P s - i n c r e a s e s f r o m a v a l u e o f 0 . 0 8 at pH 4 t o 0 . 4 at pH 2 f o r t h e c o n d i t i o n s o f c u r v e A. Such calculations allow a rough estimate of the dissociation constant K of HS ( i f p r e s e n t in t h e s u r f a c e ) t o be m a d e . F r o m t h e G o u y t h e o r y , tile q% p o t e n t i a l u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s will be a r o u n d 1 8 0 m V a n d h e n c e t h e " ' s u r f a c e " pH a b o u t t h r e e u n i t s l o w e r t h a n tile b u l k DH. F r o m t h e c o m p u t e d v a h l e o f Vii s / ! " s - at a b u l k pH o f 2, a v a l u e o f K a r o u n d 2 0 is t h e n o b t a i n e d . T h i s r e s u l t c a n be c o m p a r e d w i t h a v a l u e o f 22 r e p o r t e d f o r p - t o l u e n e s u l f o n i c acid s 9. T o s u m u p , t h e r e s u l t s h a v e i n d i c a t e d t h a t , u n d e r n e a r - n e u t r a l c o n d i t i o n s , t h e e x t e n t o f s u r f a c e h y d r o l y s i s in s i m p l e s o l u t i o n s o f s o d i u m l a u r y l s u l f a t e s a n d s u l f o n a t e s is v i r t u a l l y n e g l i g i b l e ; tile " 2 " f o r m o f t h e G i b b s e q u a t i o n is a~)plicable in t h e c o n c e n t r a t i o n r a n g e I 0 - : t o 1 0 - a M, a n d p r o b a b l y d o w n t o a b o u t 1 0 - 4 M in n e a r - n e u t r a l s o l u t i o n s . O n l y in very dilute solutions where the hydrogen and sodium concentrations become c o m p a r a b l e , s u c h as w e r e u s e d in s o m e o f t h e e a r l y r a d i o t r a c e r w o r k , o r in s o l u t i o n s a r t i f i c i a l l y a d j u s t e d in pH t o a c h i e v e this c o n d i t i o n , w o u l d t h i s situation change. Surface tension studies, however, indicate that under cond i t i o n s o f l o w pH, m a n i f e s t a t i o n s o f p r e f e r e n t i a l a d s o r p t i o n o f I-I* o v e r Na + b e c o m e e v i d e n t in a l k y l s u l f o n a t e s a n d a l k y l s u l f a t e s . T h i s c o n c l u s i o n l e n d s general support to the results obtained by the insoluble monolayer technique T h e r e s u l t s d o n o t a l l o w a n u n e q u i v o c a l c h o i c e b e t w e e n t h e v a r i o u s explanations of surface weakness of these electrolytes, which comprise prefere n t i a l a d s o r p t i o n o f h y d r o g e n i o n s , f o r m a t i o n o f u n i o n i z e d acid in t h e surface, i o n p a i r f o r m a t i o n , o r o t h e r a l t e r n a t i v e s , a l t h o u g h t h e d a t a p r e s e n t e d do seem to provide fairly strong evidence for deionization.
74
E.D. G O D D A R D
E. M I C E L L A R S Y S T E M S
T h e r e has b e e n r e n e w e d i n t e r e s t in r e c e n t y e a r s in k i n e t i c s t u d i e s o f rea c t i o n s involving m a t e r i a l s w h i c h a g g r e g a t e , o r are .solubilized, i n t o c h a r g e d s u r f a c t a n t micelles. A s t h e p H in t h e v i c i n i t y o f t h e micelle c a n be s h i f t e d b y several u n i t s f r o m its value in t h e s i m p l e s o l u t i o n p h a s e , a p p r e c i a b l y diff e r e n t r a t e c o n s t a n t s o f acid- o r b a s e - c a t a l y z e d r e a c t i o n s are to be a n t i c i p a t e d a n d have, in f a c t , b e e n c o n f i r m e d 6° - 62 H a r t l e y a n d R o e 6a d e v e l o p e d a- e l e g a n t a n d s i m p l e t e c h n i q u e t o i l l u s t r a t e this s h i f t in p H b y use o f a c i d - - b a s e i n d i c a t o r d y e s w h i c h w e r e s o l u b i l i z e d b y t h e micelle. T h e y s u g g e s t e d , f u r t h e r m o r e , t h a t t h e m a g n i t u d e o f the shift c o u l d in c e r t a i n i n s t a n c e s be c a l c u l a t e d f r o m t h e m e a s u r e d q' p o t e n t i a l o f t h e micell¢ a c c o r d i n g t o t h e s t a n d a r d B o l t z m a n n e x p r e s s i o n ( e q m ( 1 ) ) w i t h qs p l a c e d e q u a l t o ~'. It a p p e a r s likely t h a t a c h o i c e o f ~Po, i.e. t h e p o t e n t i a l in t h e p l a n e o f t h e h e a d g r o u p s , f o r "Is w o u l d be m o r e a p p r o p r i a t e . T h i s s u b j e c t h a s b e e n e x p l o r e d f u r t h e r b y M u k e r j e e a n d B a n e r j e e 64 . While confirming the essential validity of H a r t l e y ' s a p p r o a c h , these a u t h o r s were a b l e to d e m o n s t r a t e t h a t t h e m o s t a p p r o p r i a t e v a l u e o f q ' t o be e m p l o y e d lay b e t w e e n t h o s e o f ~" a n d g'0 • i.e. ~" < ~I, < ~ o - I n a d d i t i o n , to o b t a i n m o s t s a t i s f a c t o r y a g r e e m e n t b e t w e e n t h e o r y a n d t h e s p e c t r a l d y e d a t a , a s h i f t in p K ( t o h i g h e r values) in t h e s u r f a c e h a d to be i n v o k e d . T h i s i n c r e a s e w a s a s c r i b e d t o a l o w e r i n g o f t h e d i e l e c t r i c c o n s t a n t in t h e s u r f a c e region. W o r k o n t h e t i t r a t i o n o f i o n i z a b l e m a t e r i a l s in the m i c e l l a r s t a t e h a s b e e n r e l a t i v e l y s c a n t y . R o s a n o e t al. ~5 have r e p o r t e d d a t a o n t h e acid t i t r a t i o n characteristics of aqueous potassium laurate solutions of different concentrations, b o t h submicellar and micellar. A f t e r neutralization of K O H f o r m e d b y h y d r o l y s i s , a p l a t e a u r e g i o n o f c o n s t a n t p H w a s o b s e r v e d , w h i c h in t h e case o f m i e e l l a r K L is several u n i t s o f p H ,higher t h a n f o r n o n - m i c e l l a r K L . While it is t e m p t i n g t o a s c r i b e t h e s h i f t t o a " s u r f a c e " p H e f f e c t in t h e v i c i n i t y o f t h e micelles, e x a c t i n t e r p r e t a t i o n o f t h e d a t a is h a m p e r e d b y t h e a p p e a r a n c e o f solid c r y s t a l l i n e m a t e r i a l (acid s o a p ) a l o n g t h e p l a t e a u r e g i o n 66 . A system much more amenable to electrical analysis has been provided by Y a l k o w s k y a n d Z o g r a f i 6~. T h e s e a u t h o r s s t u d i e d t h e t i t r a t i o n c h a r a c t e r i s t i c s o f v a r i o u s a c y l c a r n i t i n e s w h i c h h a v e t h e f o r m u l a Cn Hon+ l ) C O O C H ( C H 2 C O 0 - ) " C I - I 2 - N + ( C H 3 ) 3 , a n d w h i c h , f o r n = 7, 9, 1 1, 13 a n d 15, w e r e d e m o n s t r a t e d t o f o r m micelles. I n z w i t t e r i o n i c f o r m t h e m i c e l l e s b e a r z e r o n e t c h a r g e a n d n o s h i f t in " s u r f a c e " p H f r o m b u l k is a n t i c i p a t e d . A d d i t i o n o f a c i d p r o g r e s sively n e u t r a l i z e s t h e C O O - g r o u p , l e a d i n g t o a n i n c r e a s i n g positive c h a r g e o n t h e micelle e x p r e s s e d in t e r m s o f a n i n c r e a s i n g d e g r e e o f i o n i z a t i o n , ~, and positive surface potential, g'. This positive potential increases pHs, thus ~ a p e d i n g deionization o f the C O O - g r o u p and increasing the a p p a r e n t s t r e n g t h o f t h e l a t t e r g r o u p f r o m its i n t r i n s i c v a l u e o f K to a n e w v a l u e G.
IONIZING MONOLAYERS AND ptI EFFECTS
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P Fig. 13. Aggregation numbe~ (N), degree o f ionization (fl) relations, for mylristylcarnitine ralcelles67. based on potentiometric measurements and the use o f (A) the Loeb-Overtmek-Wiersema equation; (B) the first D e b y e - t l t l c k e l equation; (C) the second D e b y e - i l t i c k e l equation and (D) the G o u y Chapman equation. Open circles are results obtained by light scattering.
T h e r e l a t i o n m a y be w r i t t e n ~It = 2 . 3 0 3 ~ -
(pK--pG)
(35)
T h e d a t a w h i c h are given in ref. 67 d e m o n s t r a t e in a v e r y e f f e c t i v e w a y the e s s e n t i a l p r i n c i p l e d e v e l o p e d in this review. T h e d a t a w e r e t h e n a n a l y z e d in the f o l l o w i n g w a y . A s m o o t h a n d imp e n e t r a b l e s p h e r e , r a d i u s b, w a s c h o s e n f o r t h e micelle, a n d t h e u s u a l ass u m p t i o n s o f a u n i f o r m l y s p r e a d c h a r g e o n t h e s p h e r e a n d a B o l t z r n a n n dist r i b u t i o n o f u n i v a l v n t i o n s in its v i c i n i t y w e r e m a d e . It w a s s h o w n f r o m p u r e l y g e o m e t r i c a l c o n s i d e r a t i o n s t h a t t h e r a d i u s o f t h e micelle b c a n be e x p r e s s e d as b = 3re/eft
(36)
w h e r ¢ o is t h e ( c o n s t a n t ) v o l u m e o f a n a c y l c a m i t i n e m o n o m e r in the micelle. F o u r m o d e l s w e r e t h e n e x a m i n e d , w h i c h r e l a t e o, t h e s u r f a c e c h a r g e d e n s i t y , b a n d ~ , in o r d e r t o c a l c u l a t e b. T h e s e w e r e (i) t h e first D e b y e - - H U c k e l approximation (point charges); the second Debye--Htickel approximation ( f i n i t e size o f e l e c t r o l y t e ions); ( i / 0 t h e e m p i r i c a l e q u a t i o n o f L o e b , O v e r b e e k a n d W i e r s e m a 6s f o r s p h e r e s e x p r e s s e d as
(ii)
o -
DkTr" [2sinh ~ e"t' + -~ 4II-------e-
tanh
~--~]
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76
E.D. G O D D A R D
and (iv) the G o u y - C h a p m a n e q u a t i o n (6) f or fiat plates. T h e values so obt a i n e d w e r e t h e n used to calculate aggregation n u m b e r s a nd c o m p a r e d w i t h t h o s e o b t a i n e d f r o m light scattering m e a s u r e m e n t s . E x c e l l e n t c o n c o r d a n c e was o b t a i n e d over the w h o l e range o f i o n i z a t i o n i f the L o e b - - O v e r b e e k - W i e r s e m a e q u a t i o n was e m p l o y e d . Most serious d e p a r t u r e s were o b t a i n e d for these h i g h c urvature, m i c e l l a r particles w h e n the G o u y - - C h a p m a n (flat plate) e q u a t i o n was e m p l o y e d , the D e b y e - - H t i c k e l e q u a t i o n s le a ding generally, to i n t e r m e d i a t e values. A general discussion on the c o n d i t i o n s governing the a p p l i c a b i l i t y o f such e q u a t i o n s can be f o u n d in ref. 20.
F. CONCLUSIONS
( I ) F o r spread m o n o l a y e r s o f acids a n d bases, the i n f l u e n c e o f pH on siarface p o t e n t i a l can g e n e r a l l y be describe d b y a B o l t z m a n n d i s t r i b u t i o n o f h y d r o g e n or h y d r o x y l ions b e t w e e n the surface a nd b u l k phases a n d b y the G o u y - - C h a p m a n d o u b l e l a y e r m o d e l , at least f o r low degrees o f i o n i z a t i o n . R e s i d u a l c o h e s i o n a n d i n t e r a c t i o n , b e t w e e n the i o n i z e d a n d u n i o n i z e d f o r m s o f t h e m o n o l a y e r m o l e c u l e s , r e n d e r surface pressure v a r i a t i o n w i t h pH less sensitive t h a n surface p o t e n t i a l a n d m a k e s s i m p l e electrical d o u b l e l a y e r analysis via surface pressure less g e n e r a l l y a p p l i c a b l e , e s p e c i a l l y f o r m o n o layers at the a l r / w a t e r interface. (2) W h e r e a s t r e n d s w i t h pH o f the p r o p e r t i e s o f m o n o l a y e r s c o n t a i n i n g b o t h acidic a n d basic i o n i z i n g groups c o n f o r m q u a l i t a t i v e l y to t h e s i m p l e electrical d o u b l e l a y e r m o d e l , q u a n t i t a t i v e c o n f o r m i t y is o f t e n less s a t i s f a c t o r y T h e d i s c r e p a n c i e s are m o s t l i k e l y caused b y c ha nge s in o r i e n t a t i o n o f i o n i c d i p o l e s as the pH varies, w h i c h are n o t r e a d i l y a c c o u n t e d f o r in the s i m p l e modeL (3) S u r f a c e t e n s i o n m e a s u r e m e n t s reveal t h a t p r e f e r e n t i a l a d s o r p t i o n o f h y d r o g e n i o n s o n a l k y l sulfate a n d a l k a n e s u l f o n a t e m o n o l a y e r s can be d e t e c t e d at r e l a t i v e l y high p H values ( f o r e x a m p l e , at pH --- 4 f o r an i o n i c s t r e n g t h o f 0.0 I). T h i s s i t u a t i o n , w h i c h reflects w e a k n e s s o f these m a t e r i a l s in m o n o l a y e r f o r m , results f r o m the high q¢ p o t e n t i a l w h i c h t h e y g e n e r a t e a n d is c o n f i r m e d b y results o n spread m o n o l a y e r s . S h i f t s in the d i r e c t i o n o f increased w e a k n e s s are also e v i d e n t f o r a d s o r b e d a nd spread f a t t y acid monolayers. (4) E v i d e n c e f o r o x o n i u m ion f o r m a t i o n at v e r y low pH ('~0) is p r e s e n t e d fo r a l k y l p h o s p h a t e and sulfate a n d a l k y l b e n z e n e s u l f o n a t e m o n o l a y e r s . T h e s e results e x t e n d t he e a r l i e r f i n d i n g s o f ~ c h u l m a n a n d H u g h e s f o r s u c h f o r m a t i o n in f a t t y acid, a l c o h o l a n d e t h e r m o n o l a y e r s at low pH.
I O N I Z I N G b I O N O L A Y E R S A N D pH EFFCTS
77
(5) T h e change in p o t e n t i a l a r o u n d a c h a r g e d s u r f a c t a n t micelle, ,as the pH is varied, is n o t a d e q u a t e l y d e s c r i b e d by the G o u y - C h a p m a n m o d e l f o r flat plates, in the o n e case r e p o r t e d . M u c h b e t t e r a g r e e m e n t is o b t a i n e d by use o f the e m p i r i c a l e q u a t i o n o f L o e b , O v e r b e e k a n d W i e r s e m a f o r charged, spherical particles. I m p r o v e m e n t s in e l e c t r i c a l a n a l y s i s o f the d i s s o c i a t i o n o f i o n i z a b l e m o n o layers at i n t e r f a c e s can be e x p e c t e d t o a c c o m p a n y r e f i n e m e n t s in the d o u b l e layer t r e a t m e n t e m p l o y e d . O n e s u c h r e f i n e m e n t w o u l d be to allow f o r high localized ionic s t r e n g t h e f f e c t s u p o n the e f f e c t i v e a q u e o u s dielectric c o n s t a n t . R e c e n t w o r k by M u k e r j e e and Desai i n d i c a t e s , h o w e v e r , for the short range i n t e r a c t i o n s i n v o l v e d , that t h e s e e f f e c t s will p r o b a b l y be o u t w e i g h e d by d i e l e c t r i c s a t u r a t i o n and l o w d i e l e c t r i c c o n s t a n t p r o x i m i t y e f f e c t s 69 . ACKNOWLEDGEMENT
T h e a u t h o r e x p r e s s e s his t h a n k s to L e v e r B r o t h e r s C o m p a n y for pel m i s s i o n t o p u b l i s h this paper.
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