area per molecule isotherms

The Effect of Added Electrolyte on Surface Pressure/Area per Molecule Isotherms MARK

S. A S T O N

AND T H E L M A

M. H E R R I N G T O N

1

Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 2AD, UnitedKingdom Received December 7, 1989; accepted May 18, 1990 Surface pressure-area isotherms of two nonionie surfactants, Synperonic A2 and B246, a block copolymer, were determined at the water-air interface. The effects of high concentrations of electrolyte in the subphase were assessed. It is shown that comparison of the pressure-area isotherms leads to a misinterpretation of the effect of the electrolyte on the surfactant monolayer. It is suggested that the surface tension-area isotherms give a better indication of the true effect of the electrolyte. A qualitative discussion of this point is put on a quantitative basis using thermodynamic equations based on the Gibbs surface phase. Using this new approach, the close-packed molecular areas of the surfactants were shown to be independent of electrolyte concentration. This result contrasts with an apparent increase in the surfactant molecular area in the presence of added electrolyte, indicated by the surface pressure-area isotherms. © 1991 Academic Press, Inc.

INTRODUCTION

m o n o l a y e r . I f electrolyte is present in the subphase, t h e surface t e n s i o n o f the clean surface will be different (this is n o r m a l l y raised) so t h a t Eq. [ 1] b e c o m e s

F o r sparingly soluble surfactants, surface p r e s s u r e - a r e a ( ~ r - A ) m e a s u r e m e n t s m a y be u s e d as a m e t h o d for investigating s u r f a c t a n t electrolyte interactions. H o w e v e r , t h e p r e s e n t s t u d y has s h o w n t h a t the i n t e r p r e t a t i o n o f 7rA d a t a in t h e presence o f electrolyte is n o t straightforward. This has i m p o r t a n t i m p l i c a tions, for example, in u n d e r s t a n d i n g e m u l s i o n stability a n d in the study o f b i o l o g i c a l m e m b r a n e systems. I n a c o n v e n t i o n a l 7r-A m e a s u r e m e n t , the surface pressure is d e t e r m i n e d as a f u n c t i o n o f t h e a r e a available p e r s u r f a c t a n t m o l e c u l e at t h e surface. T h e c a l c u l a t i o n o f A is straightf o r w a r d f r o m t h e m a s s o f s u r f a c t a n t at the surface, the total a r e a o f surface available, a n d t h e s u r f a c t a n t m o l a r mass. T h e surface pressure is d e f i n e d b y a- = r ~ - 3q,

~re = 3,~'° - y~,

[2]

where 3"~,o is t h e surface tension o f the p u r e electrolyte s o l u t i o n a n d 3'] is the surface tension with the s u r f a c t a n t m o n o l a y e r present. T h e effect o f electrolyte is n o r m a l l y assessed b y c o m p a r i n g 7r-A a n d 7re-A isotherms. T y p ical b e h a v i o r for a c o n c e n t r a t e d electrolyte subphase is shown in Fig. 1 (a) where the effect o f electrolyte is to e x p a n d the 7r-A isotherm. T h e i n t e r p r e t a t i o n o f this w o u l d be that the electrolyte interacts w i t h the surfactant, effectively i n c r e a s i n g its m o l e c u l a r area at the surface. E x p e r i m e n t a l observations in the present w o r k h a v e r e v e a l e d s h o r t c o m i n g s in this app r o a c h . T h e 7r-A i s o t h e r m s o b t a i n e d i n d e e d h a d t h e f o r m s h o w n in Fig. 1 ( a ) , b u t a direct plot o f surface tension versus area per molecule gave i s o t h e r m s w h i c h converged as the area p e r m o l e c u l e was r e d u c e d , as depicted in Fig. 1 ( b ) . ( N o t e t h a t t h e 3" axis is inverted for ease

[1]

w h e r e -~~ is t h e surface t e n s i o n o f t h e " c l e a n " w a t e r surface a n d 3"~ is t h e m e a s u r e d surface t e n s i o n in t h e presence o f t h e s u r f a c t a n t To whom correspondence should be addressed. 50 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and lnterJace Science, Vol. 141, No. 1, January 1991

SURFACE PRESSURE

51

Also, from the first and second laws for the surface phase dU ~ = -TdS ~ -pdV

~ + yda + ~ #~dN[. i

"'-.. A

)

A~

Then at constant temperature and pressure for a surface phase in equilibrium with the bulk phase and setting the surface concentration o f solvent, Pl, as zero,

FIG. 1. Typical effect of concentrated electrolyte on ~rA and 7 - A isotherms o f a n o n i o n i c surfactant; ( - - ) without electrolyte; (. • • ) with electrolyte.

of comparison.) To plot ~- versus A may be partly historical, since 7r was measured directly using a floating barrier in early measurements ( 1 ). A plot of 7 against A seems an equally reasonable approach, bearing in mind that for soluble monolayers, when adsorption from solution is studied, the usual method of displaying the data is to plot 7 against log (concentration). Clearly the interpretation of Figs. 1 (a) and l ( b ) will be different. In the case of the 7 - A plot (Fig. 1 (b)), electrolyte has no significant effect on the area per molecule for the closelypacked monolayer of surfactant. This contrasts with the apparent increase in the close-packed surfactant molecular area in the presence of electrolyte, as judged from the ~--A isotherms (Fig. 1(a)). The resolution of this dilemma is presented in this paper. First the effects will be described using a simple thermodynamic argument. This will be followed by a detailed thermodynamic treatment which shows that the method of ~--A comparison is fundamentally unsound when surfactant monolayers are considered at the aqueous electrolyte-air interface. THEORY

-d%

d [ ~ 2 -~ 7 1 / ~ 2 ]

= 71d(l/F2)

- (~3/I~2)d~3

(071/0~3)1/P2 = --[O(P3/~2)/O(1/P2)]#3

and, since 1 / r2 is A, the area per molecule o f surfactant, then

(O'Y'/OU3)A= --(~AA) --tz3

= -[P3 + A(OP3/dA),3]

[4]

Thus, if the surface tension is independent of the chemical potential, and hence concentration, of added electrolyte, at constant area per molecule of surfactant, then P3, the surface excess o f salt, is zero. In other words, for the compact monolayer part of the isotherm where (03"1/OC3)A = 0, there is no negative adsorption of electrolyte from the subphase, consistent with the observation that the electrolyte is not affecting the surfactant molecular area. Detailed Thermodynamic Model

Let us define a Gibbs energy for the surface phase by G °= U ~-

TS ,-PV

~.

[5]

Then dG ~ = -S'dT

+ V~dP + 7da + ~ #idN[

Am = U ~ -

[3]

where component 2 is the surfactant and component 3 is a soluble solute (electrolyte):

Qualitative T h e r m o d y n a m i c A r g u m e n t

Consider two bulk phases a and/3 and a surface phase a of finite thickness and volume. Then the Helmholtz energy for this phase may be defined by

= I~2d/d,2 -]- I~3d~3,

T S ~.

[6]

i Journal of Colloid and Interface Science,

Vol. 141,No. 1, January 1991

52

ASTON

AND

and

(0G&

solvent is the same in the surface phase as in the bulk, so that

_- [ 0ao /

ON7 ]T,P,N~,a

HERRINGTON

iz~° - t~, °

\ ON7 ]T,P,N~j,'y

= kTln[x~f~/x~f]]

[71 \ Oa ]T,P, N7

a

"

[8]

71"-ff

~i = (OG / O N i )T,P, Nj,,y

and ai = ( Oa/ ON'[)T,p,N~,v;

is an intrinsic surface chemical potential and ai is the partial molar surface area of i. For the surface region the relationship between the intrinsic chemical potential, (, and the surface composition is defined by ~i = ~

+ kTln x~fT,

[9]

where ( ~ is the standard intrinsic surface chemical potential, xg is the mole fraction of i in the surface phase, and f 7 is the activity coefficient of i in the surface phase. For pure solvent

#~,o = #~,o.

[14]

kT ln[x~f~/x{f~].

[15]

Therefore

where tr

[13]

For pure solvent at the same temperature and overall pressure

Thus #~ = ~ i - "Yai,

+ ~ral.

a~

Consider a binary system of water and surfactant denoted by subscripts 1 and 2 respectively. For a surfactant barely soluble in the bulk phase Xllf~ = 1 and kT

7r = - - - ln(xTfT).

[16]

al

From the definition of the partial molar area ai, integration by Euler's theorem gives for the total surface area, a, [17] In a monolayer experiment, the area per molecule, A, of surfactant is measured and A = a / N ~ . Therefore a = N ~ a l + N~a2.

N~ = N ~ a l / ( A - a2)

ul '°=(~-71a1° o

and

x~ = N ~ / ( N ~ + N~)

and for a solution

[18]

and

#~ = ~ , o + k T l n x ~ f ~

kT 7r

+7~aT-7~a1.

[101

al

{ln[1 + a l / ( A - a2)] - l n f ~ } , [191

For dilute solutions aj - a?

and

u ~ = # 7,0 + k T In x~f~ + ~ral.

[ 11 ]

(Note that an equation o f this form is only applicable to solvent.) For the water-air interface, the bulk phase is aqueous solution and fl the vapor phase. In the bulk liquid phase U~ = #l~o + k T l n x ~ f ~ .

[12]

At equilibrium the chemical potential of the Journalof ColloidandInterfaceScience,Vol. 141, No.

1, January 1991

which is the equation of state obtained by Gaines (2) for an insoluble monolayer at the water-air interface. Consider now the binary system of water plus a soluble solute, component 3. If the presence of soluble solute is denoted by superscript e and the absence of surfactant by superscript o, then . y loa l o - - , y lea l e,o

= kTln[x~'e'°f~'°'°/x}e'°f~e'°].

[20]

SURFACE

In the presence o f surfactant (three-component system) o

o

~

e

71al - 7~al

[211

= -kTln[x~'efT'e/x{'ef)e],

where xlt e and fife are equal, respectively, to x/f ~'° and flfe,o as the surfactant is sparingly soluble in the bulk liquid phase. Also in dilute solution a] ~ a~ '° a n d then C,o-

7~ kT

:

7r"e - -

a7

ln[xl

o-,e ~,e ff,e o a,e,o fl /X1 ' fl ].

[221

PRESSURE

53

N o w [26] contains terms in a, e, o as well as terms in o, e and o, whereas [23] only contains terms in a, e and o. T h u s the surface t e n s i o n area per molecule isotherms ( 3 , ] / A ~ a n d % / A ) m a y be used to c o m p a r e the properties o f the m o n o l a y e r at the same area per m o l e c u l e (A e = A ) , whereas the 7r¢/A e a n d ~r/A isot h e r m s cannot. It m u s t be e m p h a s i z e d that this can be no m o r e than a qualitative c o m parison as, for a quantitative model, a statistical mechanical treatment is necessary. This can be exemplified as follows. F o r the three c o m p o n e n t system a e

F r o m [15] and [21] 7el -- 71

kT

_

a,e

e

~,e

e

~e

a

~re

f l ], fie

[23] ~re ~- ka---f~ T { l n [ N ? e ' ° / ( N ~ .... + N~,e,o)]

al

+ ln[1 + ( a leN 1 ~,e + a ~ N ; ' e ) / ( A e _ a 2~) N /~e + N ; ' e / N g 'e] oe

o- e,o

-ln[fl'/f~'

7~,° = -

[28]

a n d A e = a ~ / N ~ "e as before. Therefore a

ln[xlf l/xl

where it has been assumed that the electrolyte has little effect on the partial molar area o f solvent, i.e., a~ -~ a l , and that x~ ~ x ) e and f { ~ f{* as the surfactant is sparingly soluble in the bulk phase. F r o m [20] 7~-

e

= N I al + N 2 a2 + N3' a3

[291

1}.

Subtraction o f [19] from [ 2 9 ] , with a~ _~ al,

k T l n { ~ * e ° r ~ ' e ' ° / t*ocl#o~ - a~ :~" a~ [xf' al' ~

gives

[24] 71- e - - 7r =

and taking (as f o r [ 1 6 ] ) X l l#,of l l,e,ofor the bulk phase as ~-1, then 7 ~ - 7 ~ '° -

kT

in ,rx ,.... j¢, l ..... j1

'e'° +

N~#,°)]

+ a~NS'~)/

e o-,e (A e - a2)N2 + N'~'e/N~ "e]

- ln[1 + a l / ( A 2 - ae)] - ln[fT'~/f~f~#'°])

[30]

a n d f r o m [ 23 ], 7~

71-e - - 7r :

{ ln[N~'e'°/(N~

+ ln[1 + ( a i. N . . i.

[25]

which is the alteration in the surface tension o f pure solvent by the addition o f electrolyte. Note that this is a significant effect (see Table I) and x ~r,e,o f ~~r e o -~ 1. F r o m [22] and [16]

kT

-al

-

71

kT = -

{ln[l +

( a l Nel "

~

+

a~N~#)/

al

kT - - - ln{x~'~f~'e/(x~f~x~'e'°J'~#'°)},

[261

( A . . . a. 2. ) N . l e

+ N~'~/N~ #]

al

where, as for [ 2 3 ] , the assumption is that a~ ~a~.

-- ln[1 + a l / ( A 2 - a2)] -ln[f~#/f~]}.

F r o m [26] and [23] it can be seen that ~re - ~r = - ( 7 ~ - % ) only ifT~ = 7~ '°, which is not valid as discussed above. N o t e that "Y~ - - 7 l

=

71" - - 71" e - - ( 7 7

-- 7~'°)

-

[27]

[31]

F r o m [ 30 ] a n d [ 31 ] it can be seen that even i f A = A e, the area per molecule o f surfactant is the same, the difference in surface pressures a n d in surface tensions contains the partial Journal of Colloid and Interface Science, Vol. 14 l, No. 1, January 1991

54

ASTON AND H E R R I N G T O N

molar areas of all three components and the activity coefficient of the solvent in the presence and absence of electrolyte, and the behavior of these quantities is unknown. If the soluble solute is a fully dissociated 1:1 electrolyte the number of moles of comp o n e n t 3 in Eq. [28 ] would be doubled. (This would assume that the anions and cations are equally surface active, which is certainly not the case (3).) The above analysis is equally valid at the electrolyte-oil interface provided that the surfactant remains at the interface and does not dissolve in the oil phase. In any event the comparison of 7r/ (Area per molecule) isotherms in the presence and absence of electrolyte does not enable statements about the effect o f electrolyte on the surfactant monolayer to be made. EXPERIMENTAL

Materials The surfactants used were B246, a polymeric surfactant (ICI Speciality Chemicals), and "Synperonic A2" (ICI Chemicals and Polymers). B246 is a polyester-ether-polyester " X YX" block copolymer; the headgroup (Y) is a polyethylene oxide chain ( P E O ) and the tails (X) are poly ( 12-hydroxystearic acid ) ( P H S ) . It is made by a single-step polymerization process starting from 12-hydroxysteatic acid and a PEO polymer of molar mass 1500 g mo1-1 . The n u m b e r average molar mass, by vapor-pressure measurement, is 3543 + 30 g mol -~ and the polydispersity, by GPC, is 1.94. Synperonic A2 has the formula C14Hz90(CH2CH20)zH; it contains only C13 and C~5 alkyl chains and consists of approximately 50 wt% linear alcohols, the remainder being monobranched and predominantly the 2-methyl isomer. The water used was deionized and then double-distilled; surface tension was 72.0 m N m -l at 25°C and conductivity was less than 10 -6 S cm -1 . The hexane and ethanol used for the mixed spreading solvent were analytical grade; both were checked for surface active impurities by adding to the water surface and Journal of Colloid and Interface Science, Vol. 141, No. l, January 1991

compressing to m i n i m u m area on the Langmuir trough. Calcium nitrate, sodium nitrate, and a m m o n i u m sulfate were AR grade. Ammonium nitrate was obtained as pure recrystallized material from Fredrick Allen and Sons, Ltd., London. No surface active impurities from solutions of the electrolytes in water were detected using the Langmuir trough.

The Langmuir Trough A semi-automatic Langmuir trough was used, based on the design of Doroszkowski and Monk (4). The glass trough assembly was housed in a thermostatically controlled cabinet and the subphase temperature was controlled by water circulating through a glass coil. The surface pressure was determined by the Wilhelmy plate method. A 9:1 v / v mixture of nhexane and ethanol was used as the spreading solvent in all the water-air monolayer experiments. Stock solutions contained a total surfaetant concentration of 300 mg dm -3. The method of successive addition (constant trough area) was used at surface pressures from 0-15 m N m -1 , with 10- or 20-ul aliquots of the surfactant stock solution being added. Sufficient time was always allowed between surfactant additions for evaporation of the spreading solvent from the monolayers, and for the surface pressure to reach a steady value. To obtain surface pressures above 15 mN m - 1, the monolayer was compressed using a moving barrier (rate of area change: 1.0 cm 2 s-~). RESULTS AND DISCUSSION

The ~r-A and 3'-A isotherms for Synperonic A2 in the presence of increasing concentrations of electrolyte in the subphase are shown in Figs. 2 and 3, respectively. The ~r-A isotherms suggest a strong interaction of the headgroup with the electrolyte, leading to expansion of the monolayer. However, the 7 - A isotherms reveal that there is in fact a negligible interaction since the latter curves converge. Note that convergence occurs at an area per molecule of about 70 ~2, and that this corresponds with the extrapolated minimum

SURFACE

50

40

30 "i

\ 20-

10-

I

0

510

Ac 100t 02 A/ A rnolecu[e -1

1-50

PRESSURE

55

sion at which collapse occurs is relatively independent of the electrolyte concentration: Collapse also occurs at approximately a constant area per molecule ( 100 + 20 A2), irrespective of the electrolyte concentration. These facts demonstrate the "mechanical nature" of the collapse process, and confirm that the electrolyte does not significantly influence monolayer behavior. A number of experiments were carried out to investigate the effect of different electrolytes on the B246 monolayer. The electrolytes used were a m m o n i u m nitrate, calcium nitrate, sodium nitrate, and a m m o n i u m sulfate. These particular electrolytes were chosen so that anionic and cationic effects could be distinguished. The electrolytes are all highly watersoluble so that substrates containing up to 4 mol dm-3 could be studied. The 7r-A and 3'A results are shown in Figs. 6 and 7, respectively. The 7r-A results misleadingly suggest

FIG. 2. Surface pressure-area isotherms at 2 5 ° C f o r

Synperonic A2 at the electrolyte-air interface. (•) pure water; (©) 1.25 tool dm 3NH4NO3;(11) 8.75 mol dm -3 NH4NO 3. Ordinate: 7r/mN m-l; abscissa: A / A 2 molecule-~.

30.

40'

close-packed area (Ac) for the 7r-A water-air isotherm. This means that 3, becomes independent of the electrolyte concentration once the monolayer is close-packed. The 7r-A isotherms for B246 at the ammonium nitrate solution-air interface are shown in Fig. 4. The monolayer apparently becomes more expanded as the electrolyte concentration is increased; the collapse pressure is also raised. The 3,-A isotherms are shown in Fig. 5. Because the curves converge, the electrolyte causes no change in surfactant molecular area. Convergence occurs at an area per molecule of 300 _+ 50 A2, corresponding with the close-packed area, Ac, obtained by conventional extrapolation of the original water-air 7r-A isotherm. Thus, like Synperonic A2 above, convergence appears to take place at the onset of close packing of the monolayer. Considering the monolayer collapse process, it can be seen from Fig. 5 that the Surface ten-

50S E

60.

70 ¸

5'0

Q

16o

~go

A / ~ 2 m o l e c u l e -1

FIG. 3. Surfacetension-area isothermsat 25 °C for Synperonic A2 at the electrolyte-air interface. (•) pure water; (O) 1.25 tool dm -3 NH4NO3; (11) 8.75 mol dm -3 NH4NO3. Ordinate: ~,/mN m-l; abscissa: A/• 2 molecule-l. Journal of Colloid and Interface Science, Vol. 141, No. 1, January 1991

56

ASTON

AND

HERRINGTON

5O

the surface tension of water and is therefore negatively adsorbed according to the Gibbs notation (the electrolyte effectively increases 4O the degree of structuring in water). However, the interpretation of Table I is not straightforward because the data concern both monovalent and divalent ions, making the separation of specific ion effects difficult. Consider the role of the cations, at a constant anion (NO~) concentration of 4 tool dm -3 20 (solutions a, b, and c). Changing the nature of the cation clearly has no significant effect on the surface tension. This is the case, despite lo the relatively low Ca 2+ concentration in solution c. It might be inferred that Ca 2+ has strong structure-making properties in water. However, no anomalous effects were observed oo A/,,~ 2 rnolecule -1 with Ca 2+ in the present work, so a more likely FIG. 4. Surface pressure-area isotherms at 25°C for interpretation is that the surface tension beB246 at the electrolyte-air interface. (•) pure water; (©) havior is dominated by the NO ~ anion, which 1.25 mol dm -3 NH4NO3;(•) 4.00 tool dm -3 NH4NO3; is at constant concentration. In comparing so(&) 6.25 tool dm -3 NH4NO3; (11) 8.75 mol dm -3 lutions a and e, changing the anion from NH4NO3. Ordinate: 7r/mN m-~; abscissa: A/A 2 mole- NO~ to SO24- at constant cation ( N H ~ ) concule-~. centration has no significant effect on the suran interaction of each electrolyte with B246. However, it is clear from Fig. 7 that a significant interaction in fact only occurs with amm o n i u m sulfate. Since a m m o n i u m nitrate produces no interaction, the effect must be due to the sulfate and not to the a m m o n i u m ion. The sulfate ion apparently causes condensation of the monolayer, the degree of condensation increasing with increasing concentration. The monolayer collapses at higher 3' in the presence of a m m o n i u m sulfate and so is less stable. Note that the 7r-A curves of Fig. 6 wrongly suggest that a m m o n i u m sulfate causes an expansion of the monolayer. The collapse pressure is also raised suggesting, again wrongly, that the stability of the film is also increased. The above condensation effects observed with a m m o n i u m sulfate can be explained by the effect of this electrolyte on the surface tension of water. Table I shows the surface tensions of the various electrolyte solutions in the absence of surfactant. Each electrolyte raises Journal of Colloid and Interface Science, Vol. 141, No. 1, January 1991

30.

I

40.

S E Z E \

50,

60-

70-

,

t

~oo

~ooo

~'oo

A/,~ 2 molecule -1

FIG. 5. Surfacetension-area isothermsat 25°C for B246 at the electrolyte-air interface. (•) pure water; (•) 4.00 tool dm -3 NH4NO3; (11) 8.75 mol d m -3 NH4NO3. Ordinate: -y/mN m-l; abscissa:A/A. 2 molecule-1 .

SURFACE PRESSURE 5£

I1•

E z E N

30 @[2 ° A

20,

O • 10

•

o

0

• @

@

o

560

10'00

15'00

57

low concentrations o f electrolyte have been used so that only slight expansions o f the 7rA isotherms have been observed. However, a recent study (5) has d e t e r m i n e d the effect o f high concentrations o f electrolyte ( N H a N O 3 ) on the interfacial area per molecule (A) o f an oil-soluble surfactant at the electrolyte-oil interface; the surfactant used was a polymeric surfaetant prepared by condensing a P E G 1500 ester with 12-hydroxystearie acid so that it is similar chemically to B246. T h e area per molecule at 2 5 ° C calculated f r o m the Gibbs adsorption e q u a t i o n was f o u n d to be independent o f electrolyte concentration, within experimental error, up to 8.5 m o l d m -3 a m m o n i u m nitrate, the highest c o n c e n t r a t i o n studied.

02 -1 A/A molecule

FIG. 6. The effect of different electrolytes on the B246 electrolyte-air surface pressure-area isotherms at 25°C. ( • ) pure water; (©) 2 tool dm -3 (NHD2SO4; (~') 4 mol dm -s (NH4)2504; (A) 4 mol dm -3 NH4NO3 ; (m) 4 mol dm -3 NaNOs; (D) 2 tool dm -3 Ca(NOs)2; (0) 4 mol dm -3 Ca(NOs)z. Ordinate: 7r/raN m-l; abscissa: A/.A 2 molecule -~. face tension. This is so, despite the relatively low SO42- concentration in solution e. The interpretation o f this, which would explain the current observations, is that SO42- has strong structure-making properties in water and shows strong negative adsorption. T h e relatively high surface tension o f solution f is in keeping with a particularly strong SO 2- negative adsorption. It can be inferred from the m o n o l a y e r experiments with B246, that a m m o n i u m sulfate gives an increased surface tension by virtue o f its strong structure-making properties, even when a closely-packed surfactant is present at the surface. This would a c c o u n t for the observed condensation effects, a n d the increased surface tension at collapse. C o n d e n s a t i o n is unlikely to be caused by a reduction in the molecular area o f B 2 4 6 because the molecular area at collapse is unaffected by the presence o f the a m m o n i u m sulfate. In most studies in the literature, relatively

CONCLUSIONS In interpreting the effect o f electrolyte on the area per molecule o f a spread surfactant m o n o l a y e r it is m o r e instructive to plot the 30-

40-

T E

50-

\ 60-

70.

560

10'00

1,'500

02 -1 A/A molecule

FIG. 7. The effect of different electrolytes on the B246 electrolyte-air surface tension-area isotherms at 25°C. (•) pure water; (O) 2 mol dm -3 (NH4)2504; (Ak)4 mol dm -3 (NH4)2SO4; (A) 4 mol dm -3 NH4NO3; (111)4 mol dm s NaNO3; (D) 2 mol dm -s Ca(NOH)z; (0) 4 mol dm -s Ca(NO3h. Ordinate: 3,/mN m-'; abscissa: A / A 2 molecule-'. Journal of Colloid and Interface Science, Vol. 141, No. 1, January 1991

58

ASTON AND HERRINGTON TABLE I Surface Tensions of the Electrolyte Solutions Determined in the Present Study at 25°C Concentrations/rnol dm -3 Solution

Electrolyte

Electrolyte

Cation

Anion

3,/raN m-1

a b c d

NH4NO3 NaNO3 Ca(NO3)2 Ca(NO3)2

e

(NH4)2SO4

f

(NH4)2SO4

4.0 4.0 2.0 4.0 2.0 4.0

4.0 4.0 2.0 4.0 4.0 8.0

4.0 4.0 4.0 8.0 2.0 4.0

76.6 77.4 77.2 81.9 77.4 84.9

interracial tension against the area per molecule than to plot the surface pressure. For close-packed monolayers of the nonionic surfactants Synperonic A2 and B246 at the waterair interface there was no significant change in surfactant molecular area in the presence of a m m o n i u m nitrate, up to the m a x i m u m electrolyte concentration studied of 8.75 dm-3. When the effect of Ca 2+, N H +, N a +, S O ] - , and N O 5 ions on the B246 water-air monolayer are compared, an interaction is produced by the sulfate ion. This takes the form of an increase in surface tension of the monolayer covered surface due to the particularly strong negative adsorption of the SO 2- ion. It m u s t be borne in m i n d that the surfactants used are polydisperse samples. Monodisperse systems should be examined in further studies to confirm that the results presented here are not produced by the preferential adsorption or dissolution of particular molecular components. In addition ultrapurification of the inorganic electrolytes from all traces of organic impurities would make the present findings beyond any question. Nevertheless, polydisperse surfactants and unpurified electrolytes are often used in studying practical systems and uncritical judgments presented. In any case, the thermodynamic arguments presented here are valid. APPENDIX:

a ai

NOMENCLATURE

T h e r m o d y n a m i c area Partial molar surface area of i

Journal of Colloid and Interface Science, Vol. 141, No. 1, January 1991

A Ac

Area per molecule Area per molecule obtained by conventional extrapolation of the ~r-A isotherm at close-packing AH Helmholtz energy f~ Activity coefficient of component i in the bulk phase f~,e Activity coefficient of c o m p o n e n t i in the bulk phase in the presence of a soluble solute Activity coefficient of component i in f~ .... the bulk phase in the presence of a soluble solute, but in the absence of surfactant f7 Activity coefficient of component i in the surface phase f 7 "~ Activity coefficient of component i in the surface phase in the presence of a soluble solute f~,~,o Activity coefficient of component i in the surface phase in the presence of a soluble solute, but in the absence of surfactant N7 N u m b e r of molecules of component i in the surface phase N7 "~ N u m b e r of molecules of component i in the surface phase in the presence of a soluble solute N7 '~'° N u m b e r of molecules of component i in the surface phase in the presence of a soluble solute but in the absence of surfactant Mole fraction of component i Xi Mole fraction of component i in the x7 surface phase

SURFACE

"7

~o ~,~,o

Surface or interfacial tension Surface t e n s i o n o f p u r e c o m p o n e n t i Surface t e n s i o n o f s o l u t i o n o f solvent i c o n t a i n i n g soluble solute (electrolyte) b u t no surfactant Surface t e n s i o n o f s o l u t i o n o f solvent i c o n t a i n i n g a soluble solute in the presence o f surfactant Intrinsic surface c h e m i c a l potential o f i defined b y ~i = ( O G ° / N ~ ) T,P,N ~::

71" 71-e

u o

where subscript Ny m e a n s c o n s t a n t n u m b e r o f m o l e c u l e s o f all c o m p o n e n t s except i Surface pressure = 3: ~ - 3,1 Surface pressure in the presence o f electrolyte = q,~,o _ 3'~. Chemical potential of component i Chemical potential of pure component i

59

PRESSURE

Chemical potential of component i in t h e surface phase C h e m i c a l p o t e n t i a l o f c o m p o n e n t i in t h e b u l k liquid

ACKNOWLEDGMENTS

We thank SERC for the provision of a support gram and ICI for additional financial support. REFERENCES 1. Adam, N. K., "Physics and Chemistry of Surfaces." Oxford Univ. Press, London/New York, 1941. 2. Gaines, G. L., Jr., J. Chem. Phys. 69, 924 (1978). 3. Aveyard, R., and Haydon, D. A., "An Introduction to the Principles of Surface Chemistry," p. 111. Cambridge Univ. Press, London/New York, 1973. 4. Doroszkowski, A., and Monk, C. J. H., J. Sci. Instrum. Ser. 2, 2, 536 (1969). 5. Battaeharya, D. N., Kelkar, R. Y., and Chikhale, S. V., Tenside Surf Deterg. 25, 298 (1988).

Journal of Cblloidand InterfaceScience, Vol.

141, No. 1, January 1991