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Wetting films
109
Advances in Colbid and Interface Science, 40 (1992) 109-146 Elsevier Science Publishers B.V.. Amsterdam 00092 A
WETTING FlLMS N.v.CHURAEV and Z.M.ZORIN Institute of Physical Chemistry, Academy of Sciences of USSR, 117915 Moscow (U S S R)
ABSTRACT Wetting films of nonpolar liquids are stabilized due to action of the repulsion dispersion forces. For aqueous films, it is necessary to takes additionally into account action of electrostatical and structural forces. Disjoining pressure isotherms of a thick methastable p -films of electrolyte and surfactant solutions can be quantitavely described on the basis of theory of long-range electrostatical forces. Thicknesses of thinner d-films
of
water formed as a result of vapour adsorption depend on the surface hydrophilicity and are controlled by the action of structural repulsion forces. CONTENTS Abstract
109
1. Introduction
110
2. Methods for films investigation
111
3. Onecomponent liquids
115
4. Aqueous electrolyte solutions
122
5. Anionic surfactants
128
6. Cationic surfac+ants
132
7. Nonionic polymers
141
8. Summary
142
References OOOl-6636/92/$15.00
144
0
1992 -
Elsevier Science Publishers B.V. All rights reserved.
110
1. INTRODUCTION Wetting films play an important role is such phenomena as polymolecular adsorption wetting end masstransfer in the porous bodies not saturated by liquids. Mobility and thickness of wetting films depends on the relative pressure of surrounded vapour or on the capillary pressure of a meniscus or droplet, ajoining the film. Wetting films, along with free foam films p-51,
represent
also suitable model systems for investigations of long-range surface forces [6,7]. Disjoining pressure II of a fla$etting film on a solid substrate can be easily determined by the capillary pressure Pee n film thickness h
of equ%librated meniscus, while the
may be measured by means of methods of
interferometry or ellipsometry. This allows one to derive the isotherms of disjoining pressure
a( h)
simplier than in the
case of thin liquid interlayers between two solid surfaces. In the latter case directly measured forces of interaction need to by recalculated into disjoining pressure taking into account the local curvature of solid surfaces at the contact point, In distinction from foam films and liquid interlayers, wetting films represent an asymmetric system, bounded up with two different phases: a solid and a gaseous one. This changes in a considerable way the calculations of surface forces. Molecular forces in wetting films are not the forces of attraction (as in the case of symmetric systems), but of the repulsion [6] . Due to a difference in the values of electrical potentials of the wetting film surfaces (y,*p%>
111
the forces of electrostatical repulsion are changed by the forces of attraction at
kc
h,
, where h,
is some critical
thickness. The different states of boundary layers of liquid near solid substate and air will change also the effect of structural forces. It is known, that oscillating density profile of a liquid near to a solid surface is changed by a smooth monotoneous density profile near to liquid-vapour
inter-
face. Mean density of a liquid near to a solid substrate is increased whereas density near to the liquid-gas interface is
gradually decreased, Solid surface is usually lyophilic in relation to the liquid forming a wetting film, while the contacting gas phase may rather be considered as a lyophobic one* Therefore, calculation of surface forces in wetting films is more complicated as compared with symmetrical interlayers. At the same time, investigations of wetting films offer some new possibilities for considering complex phenomena in asymmetrical systems, such as, for instance, heterocoagulation[7]. 2. METHODS FOR FILMS INVESTIGATION In Fig. 1 are shown different types of isotherms of disjoining pressure. The monotonous one (curve I), disposed in the range of
flz 0
, corresponds to the case of complete
wetting. Such situation takes place for nonpolar liquids on the more polar substrates. Film stability is here determined by the action of dispersion forces of repulsion, n,7
0 .
Its value depends on the difference in the polarity of the substrate and of the liquid, and can be expressed in terms of Hamaker constant A C 0.
112
h
-(
8 f
1
L-3 .-3
I:
2
+n
2 4
i
-n
"C O
Fig. 1. Schematic representation of different types of isotherms of disjoining pressure of wetting films. Fig. 2. Schematic diagram of experimental set up. An opposite case, that is the case of poor wetting, represents isotherm 2. The negative values of n
may be
caused by low polarity of substrate (alkane films on PTFE, when
A 7 0
183 ), or by the forces of electrostatical
attraction,
"s4
0,
of oppositevely charged surfaces of
an aqueous film. For these isotherms (curve 2) stability condition ( on/ah<
0)
is fulfilled only for the lower
part of the isotherm related to very thin
CL -films.
Usually their thickness is equal to not more than a few molecular layers. Isotherm 3 reflects some intermediate situations when both thick ( h .h,)
and thin
(h&M
wetting films
are stable. In this case in the range of low disjoining pressure
l-l 4 II, two different states of the film are
113
possible: the metastable one, corresponding to the upper branch of the isotherm (
p-films), and the thermodynamically
stable, corresponding to thin
d-films. An equilibrium state
of flat films having thicknesses in the range between
12,
and h, can not be realized, the films are unstable. The and d -states is regulated by the transition between Psurface forces, acting in the films [9] . Isotherms of type
3 relate, as a rule, to aqueous solu-
tions. The aqueous films can lose their stability usually due to a change in the sign of electrostatical forces at
Metastable
p -films are formed when bulk liquid is
thinned out. In Fig. 2 is presented the schematic diagram of the setup. The capillary meniscus of liquid is form8d in a silver or titanium tube '1(r c 0.5-2 mm) in Teflon cell Lowering the level
3.
H of the.liquid in the vessel 4 and
sucking out the liquid from the tube, it is possible to form a wetting film on a polished quartz plate 2. Disjoining pressure of the equilibrium film is equal to a
is the density of liquid, and %
n
, where =P%H is the gravity acceleration.
However, far wetting films of small radii ( r, <
700 pm)
the values of disjoining pressure coincide practically with the capillary pressure of the spherical meniscus in the tube: n "
P, =2xpz
, where v
is the surface tension, Such de-
termination of fl give better results when compared with measurements of the level difference H
e
In this case, the n values are changed from 103 to 3 X: q03 dyn/cm' using different radii %
of the tubes (from
114
2 to 0.5 mm). Film thickness was measured using microinterference method [5,103 . The method consists in measuring the intensity of monochromatic light, reflected from the film, Knowing the refractive index of the liquid and of the substrate, it is possible to determine the /L values at an accuracy of
2-3 nm.
Measurements of the wetting film thickness at much higher values of disjoining pressure are performed by means of He-Ne laser ellipsometry. As the size of the light spot on the substrate was equal to about 0.5 mm, the tube radius must be not smaller than 2 mm. This allows one to obtain the films having radius
r,b~~.
In this case, the highest disjoining
pressure, which could be reached in the cell, is determined by the width of a gap between the end of the tube 1 and the quartz plate 2. The cell shown in Fig. 2 enables one to obtain the disjoining pressure isotherms
in the range from
W>
IO3 to IO4 dyne/cm2. Thermodynamically stable thin
4 -films were usually
formed as a result of vapour polymolecular adsorption on a flat substrate in an evaquated chamber [6,11] . The isotherms n(h)
were obtained by ellipsometric measurements of film thick-
V,/RT). P/P,=exP(-n v‘m is the molar volume of liquid; R is the gas
ness at different relative vapour pressure Here
constant, TS is the saturated vapour pressure and
T
iS
the temperature. This method is suitable only for onecomponent liquids (or for liquid mixtures containing volatile components 1123 ), and can not be applied, for instance, for electrolyte or surfactant
115
In the last two cases one need to investigate the
solutions.
d-films which remain on the solid substrate after rupturing the metastable p-films. Unfortunately, the arising hindrances are for the present hardly to overcome. First of all, it is difficult (using current cells) to reach
fl
values
higher than nc (Fig. 1). Moreover, to establish the equilibrium thickness and composition of thin
d-film
(formed after
rupturing) a very long time is required. At present we known d-film
only some evaluations of about IO nm b3]
thickness - that is of
o
3. ONBZOMIWENT LIQUIDS Isotherms of disjoining pressure
of thin wetting n(h) films of nonpolar onecomponent liquids are well described in the theory of dispersion forces [6,7] . For many alkanes and inert gases (in liquid state) experimental data are in agreement with the known equation of dispersion forces: l-I =-A/6dL3, where
A ( 0
(11
is the Hamaker constant.
In the Fig. 3, as an example, are shown (by points) experimental data for nitrogen adsorption (at 78 K) on porous silica 1143
l
Solid curve was calculated according to Eq. (1). The
best fit of this curve with experimental data gives the value of
A L -2.8 x lo-l9 2
value of
A
, which coincides with the theoretical
calculated on the basis of spectral data of nitro-
gen and silica. The deviations at
h 4 0.5 nm reflect dis-
cretness of molecular structure of liquid. At
p/p,70.7
influence of capillary condensation begins to show up.
the
116
W-)
_
2-
l-
0.2
0
0.4
0.6
0.8ma
1
Fig. 3. Isotherm of polymolecular adsorption of nitrogen on silica. For wetting films on flat substrates i3q.('I)works well for b
values up to 20-30 nm. Recently, a good agreement of
Lifshitz theory with experimental data was obtained for wetting films of alkanes (from
n-pentane to n-octane) on polished
quartz surface in the range of the thicknesses from 0.5 to 20 nm [I53 . No difference in Hamaker constant was found for all the liquids. This was supposed to be connected with a similar orientation of alkane molecules parallel to the quartz surface. Earlier, similar results were obtained for wetting films of n-tetradecane on mica cleavage 116) . In Fig. 4,a
are
shown in log-log scale the dependences of ellipsometrically measured thicknesses on disjoining pressure set by a level difference H
(see
Fig.2). Statictical treatment of experi-
mental data gives the power IIn, /x-~ ),
close to
n c 2.95 A 0.07
n L 3
in Eq, (I).
(assuming
117
Fig. 4,b
shows the same experimental data rebuilt in
coordinates which correspond to linearized form of Eq. (1); . The slope of the graph I gives the value of -21 t Hamaker constant A o -5.5 x IO , coinciding with theoreh ../~413
tical one 161 . The same slope has also graph 2 (Fig. 4,b) expressing the data of polymolecular vapour adsorption of n-hexane on mica. Therefore, the experimental data for n-tetradscan ne and n-hexene are also in agreement with theory of dispersion forces, at least in the range of thicknesses from 1 to 20 run. Some deviations take place only at
h&l
nm (graph 2). Similar
results were obtained for wetting films of n-tetxadecane on the molecularly smooth surface of quartz capillaries 163 . In some cases experimental isotherms follow power-law dependence (1) but with
n c 2.4 + 2.6
lower than theoretical value
n c 3
16,171 , that is
for dispersion forces in
homogenious films. This deviation may be connected both with substrate roughnesses [6] , end with nonhomogeneity of the wetting film. In very first approximation a film can be considered as composed from a boundary layer and a bulk part. The structure of boundary layer is modified as a result of interaction with solid surface, Its thickness 6
can be in the
order of correlation length in bulk liquid. Isotherm of molecular forces in such two-layered film has the following form [63 : n
A =-- -m 6abt’
c
6a(h-S
I3
(2)
In the case when a boundary layer is more polar than bulk liquid, coefficient C
differs in sign from Hamaker constant A .
118
UD)
15
10
5
Fig. 4. Isotherms of disjoining pressure of tetradecane wetting films on mica surface.
2 n*fo-‘[email protected]
1
u
Fig. 5. Isotherms of disjoining pressure of water d -films on silica surfaces.
119
As a result, the film thickness will decrease (as the fl values increases) less sharply, and this may be interpreted as a lowering of the power
n
in Bq. (1).
Lq. (I) with a Hamaker constant A IO
corresponds to
the condition of complete wetting (curve 1, Fig. I). However, in the case OP lowenergetic substrates wetting films may become unstable
(A 7 0)
, which corresponds to isotherm 3
on PigO 1. This situation takes place, for example, for some
alkanes. The experimental values of contact angles of alkanes on P'TTFl3 (up to 46. for hexadecane) coincide with calculated ones according to theory of dispersion forces, assuming that stable d-films
(see curve 3, Fig. 1) have the thickness of
about 0.16-0.2 nm, that is in the order of monolayer of molecules 181 D Plenty of isotherms of polymolecular adsorption of water vapour on silica surfaces were obtained using ellipsometric methods. For water films not only dispersion forces, but also electrostatical and structural ones, must be taken into account. A difference between the isotherms (Fig. 5) is related to different treatment and different degree of purification of glass or quartz surface. At higher hydrophilicity of the surface the thickness of d-films can
is higher. The same effect
be caused also by surface heterogeinity, presence of area
of different hydrophilicity. Surface roughnesses can influence the mean d-film thickness. Small-angle X-ray data p8] shows that at a mean height of the roughmesses of about 0.75 run,thin film (with mean thickness of about 1.4 run)may include some dry spots.
120
The same effect may be caused also by surface heterogeindty, by the presence of areas of different hydrophilicity, by a different ability to forms hydrogen bonds with water molecules. Mean thickness of such films, measured ellipsometrically, could hardly be interpreted in the framework of disjoining pressure concept, nonuniformity of the film thickness give also rise to enormous high viscous resistance of thin, partly "bracken" water films, In distinction to water, thin films of nonpolar liquids ( h2/3[nm)
preserve their bulk viscosity [I91
. In this
case, due to the action of dispersion forces only, the effect of surface beterogeinity is markedly damped. Near to the vapour saturation ( P/P,-4 d-film thickness does not tend to infinity (as in the case of -films), P and is usually equal to 5-8 nm. The isotherm crosses the h-axis and continues in the region of negative disjoining pressures. In the region of supersaturation
d-films
can be covered by
small water drops, forming finite contact angle with thin film (II] . Stability of d-films
(hN,5+.10 nm)
and a very high
positive disjoining pressure acting on it, is possible to be explained on the basis of structural forces of repulsion (20-J. Isotherm of structural forces has an exponential form [7]
II,=Kexp(-h/h)
,
where x is a correlation length, and parameter K
:
(3)
depends
on hydxophilicity of the surfaces. Jq. (3) is applicable to the experimental isotherms of d-films
(at
fl > 0) with
s\ 2 2 run.Combining structural,
121
molecular and electrostatical forces it was possible theoretically to describe the whole S-shaped isothermclike curve 3 on Fig. I), including
p-
and d. -parts, and the region of
instability f203 . l'heeffect of structural forces in d-films is confirmed by its thermal sensibility. It was shown that the thickness of A-films on quartz, measured ellipsometrically at
p/p,+ I
decreases down to monolayer when temperature increases up to t,
65X,
On the same substrate at t c ?O*C d-film
thickness
was equal to 9 run.In this range of the temperature dispersion and electrostatical forces are practically not influenced. An exception to the above-mentioned isotherms of adsorption films of water are the experiments performed on quartz surfaces after long and careful purification 121, 223 . It may be assumed that in these experiments quartz surface of highest degree of hydrophilicity was prepared. The experimental data shown in Pig. 6 give the hyperbolic form of isotherm: fi=Vh, where
C = 1.2 x IO3
dyn/cm. No one of the known components of
disjoining pressure can explain such n ( h > dependence. The isotherm in Fig. 6 is an equilibrium one and corresponds to the case of complete wetting. Since isotherm was obtained as a result of vapour condensation, Debay radius
d/X
of condensate may be very high, in the order of 1
pm. There-
fore, it is not excluded that the stability of thick water films may be caused by a very strong overlapping of diffuse electrical layers. Only the range of small thicknesses may be influenced by the action of structural forces.
122
Rig, 6. Thicknesses of adsorption water films on quartz surface st
P/P, near to 1 121, 223 .
4. AQUtiOUSELIKZTR0LYT-E SOLUTlONS For measurements of film thickness
the cell shown in
Fig. 2 was used. The equilibrium film thicknesses
h
at dif-
ferent values of disjoining pressure were measured by means of the microinterference method. The profile of surrounding meniscus ce.nbe determined from photograph Newtonian rings. In the case of partial wetting the values of contact angle were determined by using the nondisturbed by surface forces part of the meniscus, In the case of complete wetting, the extrspolated profile of nondisturbed meniscus do not intersect the substrate plane. Then, th& shortest distance II, (see Fig.2), at which such a profile approaches the substrate, can be
123
determined into of
from
account
the
the
equilibrium.
wetting the
change
will
Laplace
curvature of
. The higher
231
from
Negative
constant
of
The relation
[IO,
better
solution
values
of
3
of
the
Ho to
are
be the wetting.
5 r 0 (this
equation,
the
surface
reflect
the
in
the
characterize
h
values
of
The positive
corresponds
taking
to
the
s = Ho/h
values
8
of
of
,
5 S
r: 0 > to
condition
state
pE2.
partial
wetting. In the case on the
basis
energy
of
of
of
meniscus
I-I, values
the film
when complete G (
of
wetting
thicknesses
also
(h )=
takes
place
the
from
pore,
it
to ca,lculate
G
h > characterizes
the
in a slit
P,(
part
of
equilibrium
h
/% -film,
are
isotherm
one,
free
>. nor
fl values
the
possible
excess
II, -
and
is
positive,
jn
h , to
the region
inffnity:
(4) Special surface.
attention
After
washed
the
by ethanol,
was paid
optical
polishing,
acetone,
solution
of
was placed
several
hours
upon,
the
20 set these
plate
into
operations
distillate the
the wetted with
the
(like
and water
solution
being plate
hydrogen into
was thoroughly
20 % aqueous in
washed
[21]
rapidly This
quartz
Then it
a hot
alcaline
investigated.
same solution.
the
peroxide
),
bidistillate,
fas
preparation
and water,
by 5 5 aqueous for
to the
quartz
plate
was
was treated
and after
that
chrome
mixture.
by water
and put
Therefor
solution.
On performing
the
was rinsed
plate
and then Just
transferred allov*s
of
before into
immersed
all
by into
the experiments the
one to obtain
cell
filled
well
re-
124
producible results. The solutions were prepared by using water tridistillate having electrical conductivity of about IO4 and pII c 6 +6.5
Fig. 7.
Ohm-' cm",
.
Isotherms of wetting films of aqueous of different concentration:
KC1
solutions
C c 10-4 PA, \y, = -15OmV(l);
C c 5 x 10m4 I:, \y, e -150 mV (2); C e 10-3 hl, -125 mV (3); C L: 5 x 10B3
Ll,
%=
- 100
y,
IE
mV (4).
In Pig. 7 are shown by points the results of measurements of film thickness
h of
joining pressure,
II
KC1 solutions at two values of dis-
c 1.5 x IO3 ( r = 1 mm) and
dyn/cn2 ( r = 0.5 mm), where
II E 3 x 103
is the radius of the tube 1
(Fig0 2) [21+-jD /The solid curves on Fig. 7 represent the results of calculation of electrostatical "s( h >
disjoining pressure
on the basis of assumed values of
y,
and YL
125
of film surfaces* The
potentials
quartz
surface
in contact
conccntrat~an kinetics
are
of
values
Calculations lated
are IIe( k)
of
data
the values
were performed
of
v+ and
\y% do not
in all
series
the
results
gives of
6’
condition
I const,
experimental value
is
water
films
points
close (
experimentsl. especially effect
disjoining
r -34
is
case
of
pressure
differ
the
If,
( h >
of
tabu-
the
condition
much from
the
charge.
mV. This
the experiments
with
. Yhc agreement
between
of
of
so2utions.
at
molecular
k7
The
30 nm is
component
shown by a dashed
was adopted
free
as satisfactory,
thickness
is
well
5.5 mV.
yp p -45
may be considered
isotherm
-
surface
assuming
on film
are not
f: c t?.&:‘tl!- 1-5t1
Low concentration
forces
the
on the fSLm thickness,
experiments
mV ) [2?]
and theory
, Since
of
line
to be A r - 7.2
on x
J [201j m Fig*
of
strength II
molecular
e
electro-
That means that
const.
do not
known one from
7. The Wsmaker constant
10n2’
pfi
the
that
to
c:-? i ‘1 (- .;Is<>3Ct:jcrl
* ‘Ihe calculated
negligible
Pig.
y2
[25]
depend
our
was obtained the
data in
of
to
of
the
on the ba.sis
\y*
where
TIM :i..j3 :d Iri;l-!.r*l;i;l’
from
-25
of
different
interface
from
using
ly L const condition
film-air
f2i1
Uowever , nearly
of
capillaries
for
varied
potential
determined
quartz
potential
yfl, ya
?QlOWl-i,
in
of
KC3 solutions
independently
measurements
values
with
values
the
8 repxttsents
solutions
the
dependence
s of
film
thrickness
EC4, + KCI + KCW at a constant
i r 10 -3 mol/am3 , and a constant
= 1 Or x ‘IO3 dyn/cm2.
disjoining
on the
ionic pressure
126
60
-+f (mv
h(m)
120
40 80
40 0 2
4
6
8
1OpH
Fig. 8. Dependences of aqueous film thickness (curve 1) and of potential
y,
of quartz surface (curve 2) on the
pI1 of the solutions,
The experimental data, shown by points, coincide with solid curve 1, calculated [261 by using
\y, values measured
in quartz capillaries (curve 2), and assuming to
\y2 to be equal
-45 mV. At low
pII,absolute values of y, potential decrease,
and this leads to some decrease in film thickness. Near to isoelectric point of quartz surface (
Y; = 0
wetting films lose their stability. Rupture of
at
pI1
2.7 )
k-film and
formation of thin d-film was observed by microscope; p+& transition is accompanied by txansition from complete to partial wetting. More information about the II(h) isotherms of wetting films gives the ellipsometrical measurement [28, 297 . Tn this
127
ca.se , it was possible to change the disjoining pressure gxa-
dually from 103 to IO4
dyn/cm2. Since the film thickness of
electrolyte solutions under investigation was higher than 30 nm, the effect of surface roughness can be neglected and simple ellipsometrical model of homogenious films was used. In Big, 9 are shown by points film thicknesses measured for fiaClsolutions. Theoretical
"a( h >
isotherms (solid
curves) are calculated 126-Jusing the following values of ‘y, and u(*potentials : C
IO-3 M, and
5 x 10W3 11. The
W? = -150
v, I:- 100
and
and
ye = -30 mV -25
Y2=
mV
for
for C
E
v, values coincide well with the ones measured
by capiJ.laryelectrokinetics, and
v2 values are near to the
known ones for water-air interface [27J
.
Measurements of a number of film thicknesses for every concentration of the solution allow one to compare more strictly the experimental and theoretical data. Fig. 9 shows that depondences of film thickness on the disjoining pressure may be really described using reasonabLe values of
‘y, and y2
potentials. This means that the thickness and stability of thick StatiCal
p-films are determined by the action of electrofomX?S
only. Isotherms of
p-films can be predicted
when the electrical potentials of film surfaces are known,
Fig, 9. Isotherms of disjoining NaCl solutions of different concentration: 5 x '10 -' M (2); IO-' M (31,
and 5 x 10e3 M
Fig.10. Bllipsom~tric measurements of thickness of films of aqueous N&l
+ NaDS solutions
1, C r 70-4 Kj;Cs t5xlO
+
2. C e:5 x 10-' M; C, c5XlO 3. c = I?-: IL.:c, I: 5 x If4
Pi -Q M; IL;
4, C Lt: 5 x 10-3 XI;Cs r:5 x lo+ Mo 5. AKLONIC SUIG'ACTANTS As shovvnabove, e-film thickness depends on electriCELLcharge of the film interfaces, In this connection, *he stability of ~-films must be very sensitive to additkons of ionic surfactants, which may be adsorbed on the both interfaces. Experiments were performed with aqueous solutions of
129 anionic sodium dodecylsulfate ( MRDoS)
, Merck, '7 99 5%
active substance. The background electrolyte concentration was changed from 10-4 to 10-3 M NaCl. In neutral aqueous solutions small addition of NaDos
do not change the \y4
potential of the quartz surface. Therefore, in this case it was possible to use the same values of y, when no h’c~DoS was present. The u(& potentials of film-air interface are known from the experiments with free foam films [30-j. In the concentration range of
NaDoS
from C
L ION5 L1 up to CMC
(and a background electrolyte concentration C e:4 x 10-4 F? . NaCl ) the values of
Y2
potential are constant and equal to
Y2= - (75 c 00) mV. This allows one to calculate also in this case the theoretical "s( h ) isotherms and to compare them with experimental data. Thickness of
(s-films
was measured using both microinterferometry and ellipsometry. The data obtained by means of the microinterference method are represented in Table I. The accuracy of measurement of the thiclalessesamounts to d 3 nm. As one can see from 'TableI, the agreement between h
and
12, values
is satisfactory. An addition of N&DOS
fluences the film thickness due to a change in caused by adsorbtion. At the same NaCl C E 1o-4 M, an addition of Na,T)o'j
y2
invalues,
concentration,
increases the film
thickness by about 10 nm. In the case of more concentrated solutions, addition of
NaDoS
due to increasing ionic strength.
decrease the film thickness
130 TABLE 1
caecueated
Comparison of experimental film thicknesses (h) and theYones forces theory
(ht) on the basis of electrostatical for
NaCl
+ NaDoS
solutions
__^_________________~~-~~~~--~~~~~~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~ C NaCl
h
's
ht
n
-3,
-Y,
Ho
dyn/cm* nmnm M M mV mV nm ___________-________-~~_~_~--~-~-~-~_-~-_~-~~~~~~~-~~~~~----~-~~ 10-4
a0
86
3000
125
45
110
103
97
3000
150
75
115
0
10'4
5 x 10-5
--_______--_---_-__-~~~~-~~~~-~~~~~~~~~~-~~~~~~~-~--~~--~--~~___ 5x10-4 5x1o-4
0 5 x 10-5
72
62
3000
100
45
84
67
70
3000
150
75
80
______-______-___-__~~--~~~~~~~--~~~~~~--~--~-~-~-~_~~__~_______ 5x1o-4
5 x 1o-4
74
71
2500
150
75
90
10-3
5 x IO -4
56
52
2500
150
75
78
_____-_______-__-___~~-~~-~~--~~~~~~~~~~-~~~~~~~_~--~--~~-____~__
In all the cases under consideration, the condition of complete wetting was fulfilled. The values of
Ho, which de-
termine the position of an undisturbed meniscus (Fig.2) are always higher than h
. Between
the bulk meniscus and equi-
librium film there are formed a transition zones having gradually changing thicknesses [313 . The higher are the values of relation Ho/h
the better is the wetting [23] 0
In Fig. 10 are shown the results of ellipsometrical determination of
fl (h) isotherms for aqueous solutions of NaDoS
of different concentration. The points characterize the me-
131
asured
thicknesses,
using
tabulated
the
potentials
the
obtained
single
The thickness
at
of the
tion
y4
and
Yz
leads (see
10)
of
coincide
solutions
The effect
of
by about
of
of Rigs. of
with
5 x 10-T
9 end 10)
M
mofe
that
not
some parts
with
increases pronounced
the
dimensionless
as in Fig.
coordinates.
10,
film
at lower
at C E IO -4 the
film
nm.
The same isotherms
But
an addi-
543210
11.
the
the concentra-
?I1 Fig.
of
predictions.
electrolyte,
increases
of
only
in Debay length.
For instance, NaDoS
of
1. An analysis
theoretical
decreases
background
theoretically same pair
conclusion
1) but even
NaEoS is
a solution.
20-30
and the
as in Table
well
films
calculated
due to a decrease
NaDoS (compare
strength
h > [26]
Table
wetting
are
to the
same concentration
an addition ness
“s(
of
the
thickness. ionic
for
results
(Pig,
cuxves
data
measurements
isotherms
tion
solid
rebuilt
in
M BaC1,
thick-
132
zh
In Fig.
11 isotherms
instead
of
N is
the
Boltzman
k
are
constant.
of
pairs
of
and
y,
of
the potential
in Fig.
data
relating
to
bility
of
that
by the In the
ionic
[32-34 1 , only dictions
isotherms
qualitative
potentials
suBfaces.
6.
film
of
L - 75
of
the
ca,ses
two curves. shown the
pressure,
between
in Fig.
A possi11,
isotherms
is
leads mainly
wetting
films
agreement
with
performed theoretical
agreement
more reliable
earlier pre-
in our
data
for
experiy,
and v)z
CATIOMIC SURFACTANTS In distinction
adsorb
from
their
aqueous
negative strongly
wetting
phenomena.
cationic
solutions
charges.
fluence
Aronson
to anionic
the
film
and Erincen
surfactants
surfactants, on quartz
That is
leads
the cationic surface,
why cationic
stability
[33]
to
a. solution.
Quantitative
by using
are
on
two
yr
these
the
results
for
in all
scale,
a.11 the
of
the
150,
between
disjoining
ments wa.s obtained of
E -
of
with
was obtained.
give
pressures
in magnifying
strength
experiments
curves
situated
a difference
is
H
ionic
= 75 mV. Almost
of
and
any more on the
v,
values
, where
only
Two solid
are
small
ions,
li
depend
disjoining
11,
combination
conclusion
caused
but not
y: points
of
coordinates:
of
isotherms
values:
=-loo,
left
the
a case
electrostatic
the experimental On the
one kind
In such
values, VL YY, and strength of the solution. calculation
in another
instead ll/r\l~T
, and
concentration
plotted
have
ones
decreasing
surfactants
in-
and correspondingly
shown that
to the rupturing
of
the
an addition wetting
of
films.
133
The critical concentration for rupturing decreases as the hydrocarbon chain increases in length. However, because of and y]% potentials, it was imY, possible to compare the experimental results with the theorethe lack of data about
tical isotherms of disjoining pressure. Y&ehave examined earlier mechanism of charge regulation by adsorption.of cationic surfactant using electxokinetic measuremen-tsin thin quartz capillaries [29] o Only the results of
Yt
and yy, measurements would be discussed here, which
is necessary for the calculations of electrostatic forces in wetting films. In Fig. 12 are shown the dependences of
y,
potentials of quartz on the concentxation of aqueous cetyl~rimethyla~onium bromide (CTAB) solutions at constant background concentration of
5 x IO-4 I\nNaCl 0 CTAB, 99 % purity,
was obtained from Merck Company.
Fig. '12.Results of electrokinetic measurements of ~,potentia~ of quartz surface in aqueous CTAB solutions at a background concentration C = 5 x 10 -4 M NaC1.
134
Adsorption negative
of
values
quax tz
of
concentration
of
Cs
10-5
of
quartz
of
measurtiments calculated
surface
fox
film-a.ir
with
free
films
ones.
were
from
negligible
$ -films.
Table
in
of
are
unstable,
angle
pattern.
potentials
\yz = + 100 mV) . Thin action
of
molecular
forces
y,
con-
poten-
the experiments
near
of of
of
5 x IO4
3
d, -films
and structural
be measured
sign
remain
repulsion
with
measured
by using
(an/ah7
c;)
of
( “6 4 0)
an inverse
x
formed
forms
is
p -films
attraction
cs p 23
are
meniscus
of
50 to
forces.
The bulk
, which
and
to experi-
from
nm> cannot
50.
y,
components
(hL10
about
of
electrostatical.
and immediate-&y
Lability
of
another
thicknesses
CTAB concentration
by electrostatical hwing
of
thicknesses
with
method,
surfaces
the
the background
values
the range
by the microinterference
caused
l
with
adopted
influence
thicknesses
is
from
the
theoretical
Their
an interference
*c/cm2
electrostatical
o The values
2,
as compared
a contact
The surface
microinterferential
made at
axe taken
means that
pressure
p -films
val.ues
CTAB solutions of
than
positive
of
of
,
give
In the range
&-films
of
, on the basis
[35]
This
disjoining
thin
h
measurements
As can be seen
x -lo -7 M
lower
= * 3.5
the results
interface
potentials
&
the
to CTAB
order
constant
to
C t 5 x IO -4 I,! NaCJ
tial
70 nm is
amounts
thickness
b The
,
centra.tion
of
corresponds
IL!, on the
CMC, high
compared
film
one,
[26]
Ye mental
-4
in
an overcharge
\yl e + 150 mV, axe stabilized.
In #Table 2 are
theory
point
to 10
Cs 3
a decrease
and then
potential,
near
&I). At
potential,
charge
v,
at first
The isoelectric
surf ace.
CMC ( c
CTAB caused
( \y, = stable forces.
film 75 mv ,
due to
the
135
TASLE 2 Comparison of the experimental
(h) and theoretical
thicknesses of v;etting films of background
MaCl
cs
h
CTAB,
concentration,
ht
l-l
CTAB
( ht)
solutions at constant
C r 5 x 10BL' L?
y, mV
yy, mV
Ho
State
nm
of the films '/
nm
hm
d&cm2
72
62
3000
-100
-45
84
S
52
60
3000
-75
-45
71
s
mole/l 0
5 x 1O-8
2.5
low7
-
-
2900
-75
+I00
-
1
5 x 10-7
-
-
2800
-75
+I00
-
1
10~~
-
-
2800
-70
+I00
-
1
1o-6
-
-
2800
-70
+I00
-
10-5
-
-
2700
-30
+I00
-
m
5 x lO-5
60
60
2600
+25
+I00
65
m
1.2.5x 10-4. 59
62
2400
+55
+I00
74
m
2.5 x lO-4
69
66
2000
+75
+I00
83
s
5 x IO-'
65
64
1500
+I00
+I00
80
S
1.25x 1O-3
56
52
1500
+I00
+I00
65
S
2.5
x
x
5x
2.5
x
‘1
s_
stable films;
1 - labile films.
m - metastable films;
1
136
Microphoto (Fig. 13,a) illustrate the coexistence of iand ~-films after rupturing the latter. Formation of d-films starts usually in the middle of p-films, after which gradually expands, and Of
b-ir d
CL-film
transition takes place. Shape
boundary reflects the real nonhomogeneous state
A/e
of the quartz surface, As nas shown above, structure ofd-films is
very
sensitive to local hydrop~~i~icityof the substrate. In
the absence of a solid substrate, for instance, in the case of free films [q-5] , black
d-films
have the form of a
circle. After realisation of the state shown in Fig. 13,a, the capiU.ary pressure has increased and the advancing meniscus approaches
the
d -film boundary (Fig. 13,b). Interference
pattern draws together, refloctine;transition from complete to partial wetting.
Fig. 13. Microphoto of ruptured p -films: a) formation of thin d -L'ilmin the middle of
p-film; b) formation
3.Cadvancing coni;actangle ( B,r: 50*) with
&.-film.
137
At 2.5 x IO-5 b
cs +1.25x
IO-4 M ,
p-films
are
time, from several seconds (at
formed but only for a short
M) to several minutes. In the latter case cs = 2.5 x IO-' it was possible to measure their thickness (Table 2). Rupturing of p -films in this range of CTAB
concentration shows that
disjoining pressure (2400-2700 dyn/cm2) was near to the critical one. Lxistence of some critical pressure l7,
on the
isotherm (see Fig. 1) indicate that in this case the
n(h)
condition of
\y =:const
Finally, at
was fulfilled.
Cs B 2.5 x IO-4 P,[stable
e -films are
formed again. The potentials of the film surfaces acquire the same sign, and their values draw together. It seems that these
p-films may also rupture but at more higher values of
disjoining pressure, approaching the critical one, which cannot be reached in our experiments.
As in the case of anionic surfactant, the values of Ho are higher then h
(Tabl. 2) indicating complete wetting of
p _'. films with bulk solution. In Fig. 14 are shown the results of ellipsometrical measurements of the thicknesses of stable
P -films of CTAB solutions at a constant background concentration of N&l (5 x 10-4MI.
An increase in C,
leads to some decrease in the thic'knessof
e -films, mainly due to en in&rease in the ionic s-Lrength of a solution (curve 3 and 4). Some decrease in the film thickness at
Cs = 10-7 M, as compared with
C, e 0,
is
connected (see Tab1.2) with the lowering of the values of y, potential of quartz surface, Solid curves in Pig. 14 are calculated theoretically. The
138
best agreement with experimental points was reached using
followingpairs of the surface potentials: = -55 mV (curve I);
y, I +125,
y,
= -150,
Y; = -150, y2=:
yX = -45 mV (curve 2);
\Ya rs-1.75 mV (curve 3), and
Yr" + 75 mV (curve 4).
The
y, = +I25 >
adopted values of
y
are
not very different from the ones used in fabl.2.
- 80
I - 60 : 3
- 40
8
--T--T-’
4 n*ro-3~dyn.cm-q
".lo-3(qgn.ce-2)
Fig. 14, Sl.lipsometricallymeasured thicknesses of wetting films of aqueous C!PAB solution at a background concentrat)on C = 5 x 10m4 !;I)$aCl, C* L 10-7 IdI(2); cs
r
C
s=
0 (I);
5 x 1o-4 TJ(3), and
Cs E 10-5 ISI(4). Fig. 15. Eeversibifity of eU.ipsometric measurements of wetting film thickness of aqueous solution (C = 10m3 El NaGI_,C, = 5 x 10m3 1!CTAAB)at rising (white points) and lowering (black points) disjoining pressure.
139
Fig.
1% demonstra‘tes
ellipsometric
the
by consistent
capillary
menta.1 point adopted Film
reversibility
Black
measurements.
correpondingly, of
the
pressure
satisfies
surface
concentration
are of
In Pig.
theoretical
thicknesses
in the
electrolyte
of
experimenta,
fl,(
M
Y=
the const
of
the
was used.
constancy
would
be realized, va.lues
is
is
dition
(but
have
cases
condition
IIC
mechanism
v’
and
6’
of
condition charge
of
formation,
well of
values
of
curve
1
when the condition that
of
condition
the of
for
a very
surface y
=const
at disjoining
z 3 x ?03 dyn/cm2. vr
are
very
different
as was shown above,
films
is
= const
the
solution
Solid
fulfilled.
the more concentrated
known 1363 , in
films
starting
condition
and
wetting
the
to note
the
same sign),
and in
‘-y = const
As is and free
for
interest
film
calculation
a.nd the
the
more higher
2 described
must be suptured
to
the
curve
theoretical
If
p-films
y-’ = const
intermediate
of
fulfilled.
Fhen the potentials magnitude
of
= - 35 mV were used.
a solution
near
with
strength
Eotted
r const
= + 75 mv.
yt
measured
same calcula.tion
It
of
charge
s’
YYr
low concentration
pressure
of
150, results
and
having
(10m3 Ei E?‘a.Cl).
low ionic
when in
, condition
Y, = shows
data,
obtained,
increase
isotherm
shown ellipsometrically case
are
of
The experi-
in connection
(5 x low5 LI NaCl + 10W7 E CTAB). the
(FiC.2).
“a( h )
lower
background
16 are
points
Y% = + 140
here
the results
and following
the cell
potentials
thicknesses
and white
docrea.sc
in
of
ca.se of
s’ Ic const
cannot
in
con-
In some solutions
the
be distinguished.
symmetrical
interlayers
corresponds
to adsorption
and condition
of
y
e: const
-
140
aa0 .60 -40
-20
10
4
8
8
6
4
‘0
2
n*10~(m*cm-2)
n.10-%lyn.cnl-2)
Fig. 16. Comparison of clliosometrically measured thicknesses of wetting films of aqueous solution (C = 5 x NaCl, Cs = low7 M CTAB)
Fig. 17.
M
with theoretical isotherm
calculated using condition and
10 -5
y r const (curve 1)
6' = const (curve 2).
Isotherms of disjoining pressure of wetting films of aqueous solutions of PEO at a background concentration of KC1
C E lO-4 bl (curves 1-S) and
C
r
lo-3
iv
(curves 4 and 5). Concentration of 1330;C
0 (curves 1 and 4),; P= C r lo-4 g/dm3 (curve 2), and C z IO-' g/dm3 P P (curves 3 and 5).
141
-
to
the
the dissociation cases
of
of
iongenic
asymmetrical
consideration,
since
different
surfaces
film
surface
wetting
groups.
films
the mechanisms
need
of
Rowever,
some special
charge
formation
on
ca,n be different.
7. ~ONIONIC FJLYEERS Adsorption
of
stabilization kinetic
of
nonionic
layer
studied
the
Film e llipsome
of
of
silica
of
thicknesses trically
17 are
centration relate
to
lower
increase
in
film
ones
forces,
Carbide),
This
.:inco
( 6 = I72 nm) was much smaller
clear
of
a change
when experimental
isotherms
of
(curve
the values
fit
1)
with
correspondingly. a.bsolute
g/dm3. of
data
of
va,lues
Addition of
y,
(Big.2)
PZO with
molecular
z1.1
, for
PXO con-
upper
of
action
are
are of
UP to
film
compared of
equal
Cp z ?O-” -80
of
in
an
the
.WO layer thickness.
thickness
becomes
theoretical
repulsion.
-110 g/dm3
mV (curve
cases,
film
with
to
(I-3)
sterical
a.dsorption
than the netting
curves
C = 10v4 f;:,
lowering
and yX corresponding
data,
ad-
wet~tine; films,
KC1 solution
due to the
forces y,
electro-
same cell
Three
to the
in wetting
electrostatical
experimental
of
the results
the maximum thickness
The cause
of
MW/$I,,,
leads
~:as not
sterical
with
to C = 10m3 Ai;. In a.11 the
P&O concentra,tion
thickness.
in the
shown by points
(4,5)
covered
solutions
C E IO -4 and IO-1 P oack;;round concentration
and two
for
to the
sta.bility
measured
aqueous
mass FIW z G x IO5 (Union In Pig.
used
(PtiO) (383 , we have
Pi30 on the
were
for
often
particles
polyethylenoxide
influence
is
(371 , In connection
colloids
investiga.tion
sorption
polymers
2),
At Cp = 0 to and
the best -25
mV,
PiD decrease and addition
142 of
Cp = 10"
g/&n3
to
-50
ml’
(curve
3).
At
much
higher
background concentration of electrolyte (curves 4 and 5) mlues
of
y(,
also
change from
yl. = -100 mV (curve 4)
Y, = -50 mV (curve 5).
to
Pherefore,
adsorption of PBO decreases the
of the quartz surface, while the
Yt
v,
potential
potential of film-air
interface remain constant and equal to
ye = -25 mV. This confirms the earlier expressed supposition that an adsorption PEO Of.‘ decreases the degree of dissociation of OH-group of quartz surface [j8] . It is also not excluded that adsorption layer of PRO influence the double electrical layer. Iiowever, taking into account a low volume fraction of polymer in adsorbed layer of PRO, the latter effect cannot be responsible for such a large change in
y, values.
Wetting films were used in the present case as a model system, which allowed one to trace the change in
\v, values
that determine the electrical potential of quartz surface under the adsorption layer of polymer.
I,
The thicknesses of wetting films of nonpolar liquids
can be calculated on the basis of the theory of dispersion forces. For aqueous films, it is necessary to take additionally into account the electrostatical and structural forces. 2. Ioothe$ms of disjoining pressure of thick p-films of aqueous solutions are in quonti-dativeagreement with the calculated ones using electrical potentials of film surfaces y: and
yll
, determined in the course of independent experi-
ments. Concentration of electrolytes, ionic surfactants and
143
polymers influence wetting film thickness (at h>30 its action on
‘y, and
nm) due to
\Yr potentials of film surfaces and
due to a change in Debay length, 3. For stable
p -films of low concentrated
solutions
the condition of constant charge on both surfaces (by film thinning out) is better fulfilled. In all other cases, the condition of constant potentials is realized. Eowever, for wetting films these conditions need some reconsiderations, since the mechanism of charge formation on solid and on film-air interfaces, can be different. 4. Thin aqueous thick metastable
P
d -films formed after rupturing of -films remain to be investigated. The
attainment of the equilibrium
state of such films requires a
very long time. Furthermore much higher values of disjoining pressure must be used. This can be realized only with a modified cells for ellipsometric measurements. The thickness of
d -films of water formed as a result of
vapour adsorption on flat substrates depends on the surface hydrophilicity, at P
near P,
The thickness usually increases up to 5-7 . Hydrophobization
decrease the thickness of i-later
run
and raising of temperature d -films,
144
I. A.Sheludko, Adv. Colloid Interface Sci., 1 (1967) 391. 2. J.S.Clunie, J.F.Goodman, B.'i'.Ingram, In: "Surface and Colloid Science", Vol D 3, &I. ~.IYlatijeViC, Filey Press, N.Y., 1971, P. 167. 3. R.Buscall, R.II.Ottewill,In: "Colloid science", Vol. 2,
Ed. D.H.Yverett, Chem. Sot. London, 1975, p. 191. 4. D.hxerowa, D.Kashchiev, Contemp. Physics, 2 5. P.KruSlyakov, D.&crowa,
Chimiya,
~YOSCOW,
(1986) 429.
Foam and Foam Films (in Russian),
1990.
6. B.V.Derjaguin, N.V.Churaev, Vetting Films (in Russian), r:auka,I!~OSCOW, 1984. V.;i;.F,'!uller, Surface Ibrces, 7. &.V.Derja,uin, :;.V.Clluraev, Plenum Press, I!cwYork, 1987. 8, D.B.Iioulj;h, L.R.VMte,
Aav.Colloia Interface Sci.,s
(1980) 3.
9. D.Kashchiev, Surface Sci., 225 (1990) 107. 10. Z.Zorin, D.Platikanov, T.Kolarov, Colloids and Surfaces,
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R.VN.Cranston,P.A.Inkley, Adv.Catalysis, 2 (1957) 143.
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145
16, B.V.Derjaguin, Z.I:I.Zorin, N.V.Churaev, V.A.Shishin, In: Wetting, Soreading and Adhesion" Acad. Press, London, '1977,P. 201. 17. G.R,Pindenegg, R.L&ing,
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(IYSY) 7505.
19. R,V.Churaev, Pure Appl. Chem., a 20.
(1989) 1959.
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(1979) 491.
22. L.R.Fisher, R.A.Gamble, J.Middlehurst, Nature, s
(1981)
575. 23.
1J.V.Churaev,Rev. Phys. Appl., 2
(I988) 975.
24. Z.i:!,Sorin, T.Kolarov, i‘J.ti.%sipova, D.Platikanov, I.P. Ser~;o_va,!;ol_i:,id.1?1. US";R,-52 (1990) 666. 25. I,P,Sergueva, V.D.Sobol*v, l$.V,Churaev,Kolloid,ZEL,,. USSR g
(1981) 918.
26. O.F.Devereux, P.L. de Bruyn, "Interaction of Plane-parallel Double Layers", fi1IT Press, 1963. 27, D.Exerowa, Kolloid Zt., 232 (1969) 703. 28. Z.E!,i;orin , :;.K.Gasanov,N.P&siPova, I.N.Kornilfev, Kolloidn. Lh. USSR (in press). 29, I.P.Ser@eeva, V.D.Sobolev, N.V.Churaev, Kolloid.%h. USSR, 52 (1990) 1129; '&.M.Zorin,B.V.Churaev, N.K.Zsipova, I,P,Sergeeva, V.D.Sobolev, R,K.Gasanov, J.Colloid Interface Sci. (in press).
146
30. D.&erova, II.Zacharieva,R.Cohen, D .L ?atikenov, Colloid Polymer Sci,, a
(1979) 1089.
31. B.V.Churaev, V.BZ.Sterov,B.V.Derja.gin, J.Co1loi.dInterface
Sci., 3
(1982) 16,
32. H.J.SC~U~Z~,
c.Cichos,
Z.Phys.Chem. (DDR) 251 11972) 145.
33. BI.P.Aronson,R.B?.Princen,J.Colloid Interface Sci., 52
(1975) 345; CoLloid Polym.Sci,, a
(1978) 140.
34. D.A.Callagan, K.f":'.Baldy, In "Cjetting,Spreading and
Adhesion", Acad. ,Press,London, 1978, p. 161. 35. D,Zxerowa, Proc. 5th Intern. ConSr. Surf. Activity,
Barcelona.,SA2, 1968, p. 153. 36. G.A.rlartynov, V.M.NI.ler, In "Surface Forces in min
Films
and Disperse Systems / (in Russian), Rauka, Moscow, 1972, P. 7. 37. D.H.Napper, Polymeric Stabilization of Colloidal Dis-
persions, Acad. Press., L., 1983. 38. Z.A.Nikologorskaja, F,V.Churaev, Rolloidn. Ih. USSR, -52
(1990) 1086; Colloids and Surface::(in press).
Advances in Colbid and Interface Science, 40 (1992) 109-146 Elsevier Science Publishers B.V.. Amsterdam 00092 A
WETTING FlLMS N.v.CHURAEV and Z.M.ZORIN Institute of Physical Chemistry, Academy of Sciences of USSR, 117915 Moscow (U S S R)
ABSTRACT Wetting films of nonpolar liquids are stabilized due to action of the repulsion dispersion forces. For aqueous films, it is necessary to takes additionally into account action of electrostatical and structural forces. Disjoining pressure isotherms of a thick methastable p -films of electrolyte and surfactant solutions can be quantitavely described on the basis of theory of long-range electrostatical forces. Thicknesses of thinner d-films
of
water formed as a result of vapour adsorption depend on the surface hydrophilicity and are controlled by the action of structural repulsion forces. CONTENTS Abstract
109
1. Introduction
110
2. Methods for films investigation
111
3. Onecomponent liquids
115
4. Aqueous electrolyte solutions
122
5. Anionic surfactants
128
6. Cationic surfac+ants
132
7. Nonionic polymers
141
8. Summary
142
References OOOl-6636/92/$15.00
144
0
1992 -
Elsevier Science Publishers B.V. All rights reserved.
110
1. INTRODUCTION Wetting films play an important role is such phenomena as polymolecular adsorption wetting end masstransfer in the porous bodies not saturated by liquids. Mobility and thickness of wetting films depends on the relative pressure of surrounded vapour or on the capillary pressure of a meniscus or droplet, ajoining the film. Wetting films, along with free foam films p-51,
represent
also suitable model systems for investigations of long-range surface forces [6,7]. Disjoining pressure II of a fla$etting film on a solid substrate can be easily determined by the capillary pressure Pee n film thickness h
of equ%librated meniscus, while the
may be measured by means of methods of
interferometry or ellipsometry. This allows one to derive the isotherms of disjoining pressure
a( h)
simplier than in the
case of thin liquid interlayers between two solid surfaces. In the latter case directly measured forces of interaction need to by recalculated into disjoining pressure taking into account the local curvature of solid surfaces at the contact point, In distinction from foam films and liquid interlayers, wetting films represent an asymmetric system, bounded up with two different phases: a solid and a gaseous one. This changes in a considerable way the calculations of surface forces. Molecular forces in wetting films are not the forces of attraction (as in the case of symmetric systems), but of the repulsion [6] . Due to a difference in the values of electrical potentials of the wetting film surfaces (y,*p%>
111
the forces of electrostatical repulsion are changed by the forces of attraction at
kc
h,
, where h,
is some critical
thickness. The different states of boundary layers of liquid near solid substate and air will change also the effect of structural forces. It is known, that oscillating density profile of a liquid near to a solid surface is changed by a smooth monotoneous density profile near to liquid-vapour
inter-
face. Mean density of a liquid near to a solid substrate is increased whereas density near to the liquid-gas interface is
gradually decreased, Solid surface is usually lyophilic in relation to the liquid forming a wetting film, while the contacting gas phase may rather be considered as a lyophobic one* Therefore, calculation of surface forces in wetting films is more complicated as compared with symmetrical interlayers. At the same time, investigations of wetting films offer some new possibilities for considering complex phenomena in asymmetrical systems, such as, for instance, heterocoagulation[7]. 2. METHODS FOR FILMS INVESTIGATION In Fig. 1 are shown different types of isotherms of disjoining pressure. The monotonous one (curve I), disposed in the range of
flz 0
, corresponds to the case of complete
wetting. Such situation takes place for nonpolar liquids on the more polar substrates. Film stability is here determined by the action of dispersion forces of repulsion, n,7
0 .
Its value depends on the difference in the polarity of the substrate and of the liquid, and can be expressed in terms of Hamaker constant A C 0.
112
h
-(
8 f
1
L-3 .-3
I:
2
+n
2 4
i
-n
"C O
Fig. 1. Schematic representation of different types of isotherms of disjoining pressure of wetting films. Fig. 2. Schematic diagram of experimental set up. An opposite case, that is the case of poor wetting, represents isotherm 2. The negative values of n
may be
caused by low polarity of substrate (alkane films on PTFE, when
A 7 0
183 ), or by the forces of electrostatical
attraction,
"s4
0,
of oppositevely charged surfaces of
an aqueous film. For these isotherms (curve 2) stability condition ( on/ah<
0)
is fulfilled only for the lower
part of the isotherm related to very thin
CL -films.
Usually their thickness is equal to not more than a few molecular layers. Isotherm 3 reflects some intermediate situations when both thick ( h .h,)
and thin
(h&M
wetting films
are stable. In this case in the range of low disjoining pressure
l-l 4 II, two different states of the film are
113
possible: the metastable one, corresponding to the upper branch of the isotherm (
p-films), and the thermodynamically
stable, corresponding to thin
d-films. An equilibrium state
of flat films having thicknesses in the range between
12,
and h, can not be realized, the films are unstable. The and d -states is regulated by the transition between Psurface forces, acting in the films [9] . Isotherms of type
3 relate, as a rule, to aqueous solu-
tions. The aqueous films can lose their stability usually due to a change in the sign of electrostatical forces at
Metastable
p -films are formed when bulk liquid is
thinned out. In Fig. 2 is presented the schematic diagram of the setup. The capillary meniscus of liquid is form8d in a silver or titanium tube '1(r c 0.5-2 mm) in Teflon cell Lowering the level
3.
H of the.liquid in the vessel 4 and
sucking out the liquid from the tube, it is possible to form a wetting film on a polished quartz plate 2. Disjoining pressure of the equilibrium film is equal to a
is the density of liquid, and %
n
, where =P%H is the gravity acceleration.
However, far wetting films of small radii ( r, <
700 pm)
the values of disjoining pressure coincide practically with the capillary pressure of the spherical meniscus in the tube: n "
P, =2xpz
, where v
is the surface tension, Such de-
termination of fl give better results when compared with measurements of the level difference H
e
In this case, the n values are changed from 103 to 3 X: q03 dyn/cm' using different radii %
of the tubes (from
114
2 to 0.5 mm). Film thickness was measured using microinterference method [5,103 . The method consists in measuring the intensity of monochromatic light, reflected from the film, Knowing the refractive index of the liquid and of the substrate, it is possible to determine the /L values at an accuracy of
2-3 nm.
Measurements of the wetting film thickness at much higher values of disjoining pressure are performed by means of He-Ne laser ellipsometry. As the size of the light spot on the substrate was equal to about 0.5 mm, the tube radius must be not smaller than 2 mm. This allows one to obtain the films having radius
r,b~~.
In this case, the highest disjoining
pressure, which could be reached in the cell, is determined by the width of a gap between the end of the tube 1 and the quartz plate 2. The cell shown in Fig. 2 enables one to obtain the disjoining pressure isotherms
in the range from
W>
IO3 to IO4 dyne/cm2. Thermodynamically stable thin
4 -films were usually
formed as a result of vapour polymolecular adsorption on a flat substrate in an evaquated chamber [6,11] . The isotherms n(h)
were obtained by ellipsometric measurements of film thick-
V,/RT). P/P,=exP(-n v‘m is the molar volume of liquid; R is the gas
ness at different relative vapour pressure Here
constant, TS is the saturated vapour pressure and
T
iS
the temperature. This method is suitable only for onecomponent liquids (or for liquid mixtures containing volatile components 1123 ), and can not be applied, for instance, for electrolyte or surfactant
115
In the last two cases one need to investigate the
solutions.
d-films which remain on the solid substrate after rupturing the metastable p-films. Unfortunately, the arising hindrances are for the present hardly to overcome. First of all, it is difficult (using current cells) to reach
fl
values
higher than nc (Fig. 1). Moreover, to establish the equilibrium thickness and composition of thin
d-film
(formed after
rupturing) a very long time is required. At present we known d-film
only some evaluations of about IO nm b3]
thickness - that is of
o
3. ONBZOMIWENT LIQUIDS Isotherms of disjoining pressure
of thin wetting n(h) films of nonpolar onecomponent liquids are well described in the theory of dispersion forces [6,7] . For many alkanes and inert gases (in liquid state) experimental data are in agreement with the known equation of dispersion forces: l-I =-A/6dL3, where
A ( 0
(11
is the Hamaker constant.
In the Fig. 3, as an example, are shown (by points) experimental data for nitrogen adsorption (at 78 K) on porous silica 1143
l
Solid curve was calculated according to Eq. (1). The
best fit of this curve with experimental data gives the value of
A L -2.8 x lo-l9 2
value of
A
, which coincides with the theoretical
calculated on the basis of spectral data of nitro-
gen and silica. The deviations at
h 4 0.5 nm reflect dis-
cretness of molecular structure of liquid. At
p/p,70.7
influence of capillary condensation begins to show up.
the
116
W-)
_
2-
l-
0.2
0
0.4
0.6
0.8ma
1
Fig. 3. Isotherm of polymolecular adsorption of nitrogen on silica. For wetting films on flat substrates i3q.('I)works well for b
values up to 20-30 nm. Recently, a good agreement of
Lifshitz theory with experimental data was obtained for wetting films of alkanes (from
n-pentane to n-octane) on polished
quartz surface in the range of the thicknesses from 0.5 to 20 nm [I53 . No difference in Hamaker constant was found for all the liquids. This was supposed to be connected with a similar orientation of alkane molecules parallel to the quartz surface. Earlier, similar results were obtained for wetting films of n-tetradecane on mica cleavage 116) . In Fig. 4,a
are
shown in log-log scale the dependences of ellipsometrically measured thicknesses on disjoining pressure set by a level difference H
(see
Fig.2). Statictical treatment of experi-
mental data gives the power IIn, /x-~ ),
close to
n c 2.95 A 0.07
n L 3
in Eq, (I).
(assuming
117
Fig. 4,b
shows the same experimental data rebuilt in
coordinates which correspond to linearized form of Eq. (1); . The slope of the graph I gives the value of -21 t Hamaker constant A o -5.5 x IO , coinciding with theoreh ../~413
tical one 161 . The same slope has also graph 2 (Fig. 4,b) expressing the data of polymolecular vapour adsorption of n-hexane on mica. Therefore, the experimental data for n-tetradscan ne and n-hexene are also in agreement with theory of dispersion forces, at least in the range of thicknesses from 1 to 20 run. Some deviations take place only at
h&l
nm (graph 2). Similar
results were obtained for wetting films of n-tetxadecane on the molecularly smooth surface of quartz capillaries 163 . In some cases experimental isotherms follow power-law dependence (1) but with
n c 2.4 + 2.6
lower than theoretical value
n c 3
16,171 , that is
for dispersion forces in
homogenious films. This deviation may be connected both with substrate roughnesses [6] , end with nonhomogeneity of the wetting film. In very first approximation a film can be considered as composed from a boundary layer and a bulk part. The structure of boundary layer is modified as a result of interaction with solid surface, Its thickness 6
can be in the
order of correlation length in bulk liquid. Isotherm of molecular forces in such two-layered film has the following form [63 : n
A =-- -m 6abt’
c
6a(h-S
I3
(2)
In the case when a boundary layer is more polar than bulk liquid, coefficient C
differs in sign from Hamaker constant A .
118
UD)
15
10
5
Fig. 4. Isotherms of disjoining pressure of tetradecane wetting films on mica surface.
2 n*fo-‘[email protected]
1
u
Fig. 5. Isotherms of disjoining pressure of water d -films on silica surfaces.
119
As a result, the film thickness will decrease (as the fl values increases) less sharply, and this may be interpreted as a lowering of the power
n
in Bq. (1).
Lq. (I) with a Hamaker constant A IO
corresponds to
the condition of complete wetting (curve 1, Fig. I). However, in the case OP lowenergetic substrates wetting films may become unstable
(A 7 0)
, which corresponds to isotherm 3
on PigO 1. This situation takes place, for example, for some
alkanes. The experimental values of contact angles of alkanes on P'TTFl3 (up to 46. for hexadecane) coincide with calculated ones according to theory of dispersion forces, assuming that stable d-films
(see curve 3, Fig. 1) have the thickness of
about 0.16-0.2 nm, that is in the order of monolayer of molecules 181 D Plenty of isotherms of polymolecular adsorption of water vapour on silica surfaces were obtained using ellipsometric methods. For water films not only dispersion forces, but also electrostatical and structural ones, must be taken into account. A difference between the isotherms (Fig. 5) is related to different treatment and different degree of purification of glass or quartz surface. At higher hydrophilicity of the surface the thickness of d-films can
is higher. The same effect
be caused also by surface heterogeinity, presence of area
of different hydrophilicity. Surface roughnesses can influence the mean d-film thickness. Small-angle X-ray data p8] shows that at a mean height of the roughmesses of about 0.75 run,thin film (with mean thickness of about 1.4 run)may include some dry spots.
120
The same effect may be caused also by surface heterogeindty, by the presence of areas of different hydrophilicity, by a different ability to forms hydrogen bonds with water molecules. Mean thickness of such films, measured ellipsometrically, could hardly be interpreted in the framework of disjoining pressure concept, nonuniformity of the film thickness give also rise to enormous high viscous resistance of thin, partly "bracken" water films, In distinction to water, thin films of nonpolar liquids ( h2/3[nm)
preserve their bulk viscosity [I91
. In this
case, due to the action of dispersion forces only, the effect of surface beterogeinity is markedly damped. Near to the vapour saturation ( P/P,-4 d-film thickness does not tend to infinity (as in the case of -films), P and is usually equal to 5-8 nm. The isotherm crosses the h-axis and continues in the region of negative disjoining pressures. In the region of supersaturation
d-films
can be covered by
small water drops, forming finite contact angle with thin film (II] . Stability of d-films
(hN,5+.10 nm)
and a very high
positive disjoining pressure acting on it, is possible to be explained on the basis of structural forces of repulsion (20-J. Isotherm of structural forces has an exponential form [7]
II,=Kexp(-h/h)
,
where x is a correlation length, and parameter K
:
(3)
depends
on hydxophilicity of the surfaces. Jq. (3) is applicable to the experimental isotherms of d-films
(at
fl > 0) with
s\ 2 2 run.Combining structural,
121
molecular and electrostatical forces it was possible theoretically to describe the whole S-shaped isothermclike curve 3 on Fig. I), including
p-
and d. -parts, and the region of
instability f203 . l'heeffect of structural forces in d-films is confirmed by its thermal sensibility. It was shown that the thickness of A-films on quartz, measured ellipsometrically at
p/p,+ I
decreases down to monolayer when temperature increases up to t,
65X,
On the same substrate at t c ?O*C d-film
thickness
was equal to 9 run.In this range of the temperature dispersion and electrostatical forces are practically not influenced. An exception to the above-mentioned isotherms of adsorption films of water are the experiments performed on quartz surfaces after long and careful purification 121, 223 . It may be assumed that in these experiments quartz surface of highest degree of hydrophilicity was prepared. The experimental data shown in Pig. 6 give the hyperbolic form of isotherm: fi=Vh, where
C = 1.2 x IO3
dyn/cm. No one of the known components of
disjoining pressure can explain such n ( h > dependence. The isotherm in Fig. 6 is an equilibrium one and corresponds to the case of complete wetting. Since isotherm was obtained as a result of vapour condensation, Debay radius
d/X
of condensate may be very high, in the order of 1
pm. There-
fore, it is not excluded that the stability of thick water films may be caused by a very strong overlapping of diffuse electrical layers. Only the range of small thicknesses may be influenced by the action of structural forces.
122
Rig, 6. Thicknesses of adsorption water films on quartz surface st
P/P, near to 1 121, 223 .
4. AQUtiOUSELIKZTR0LYT-E SOLUTlONS For measurements of film thickness
the cell shown in
Fig. 2 was used. The equilibrium film thicknesses
h
at dif-
ferent values of disjoining pressure were measured by means of the microinterference method. The profile of surrounding meniscus ce.nbe determined from photograph Newtonian rings. In the case of partial wetting the values of contact angle were determined by using the nondisturbed by surface forces part of the meniscus, In the case of complete wetting, the extrspolated profile of nondisturbed meniscus do not intersect the substrate plane. Then, th& shortest distance II, (see Fig.2), at which such a profile approaches the substrate, can be
123
determined into of
from
account
the
the
equilibrium.
wetting the
change
will
Laplace
curvature of
. The higher
231
from
Negative
constant
of
The relation
[IO,
better
solution
values
of
3
of
the
Ho to
are
be the wetting.
5 r 0 (this
equation,
the
surface
reflect
the
in
the
characterize
h
values
of
The positive
corresponds
taking
to
the
s = Ho/h
values
8
of
of
,
5 S
r: 0 > to
condition
state
pE2.
partial
wetting. In the case on the
basis
energy
of
of
of
meniscus
I-I, values
the film
when complete G (
of
wetting
thicknesses
also
(h )=
takes
place
the
from
pore,
it
to ca,lculate
G
h > characterizes
the
in a slit
P,(
part
of
equilibrium
h
/% -film,
are
isotherm
one,
free
>. nor
fl values
the
possible
excess
II, -
and
is
positive,
jn
h , to
the region
inffnity:
(4) Special surface.
attention
After
washed
the
by ethanol,
was paid
optical
polishing,
acetone,
solution
of
was placed
several
hours
upon,
the
20 set these
plate
into
operations
distillate the
the wetted with
the
(like
and water
solution
being plate
hydrogen into
was thoroughly
20 % aqueous in
washed
[21]
rapidly This
quartz
Then it
a hot
alcaline
investigated.
same solution.
the
peroxide
),
bidistillate,
fas
preparation
and water,
by 5 5 aqueous for
to the
quartz
plate
was
was treated
and after
that
chrome
mixture.
by water
and put
Therefor
solution.
On performing
the
was rinsed
plate
and then Just
transferred allov*s
of
before into
immersed
all
by into
the experiments the
one to obtain
cell
filled
well
re-
124
producible results. The solutions were prepared by using water tridistillate having electrical conductivity of about IO4 and pII c 6 +6.5
Fig. 7.
Ohm-' cm",
.
Isotherms of wetting films of aqueous of different concentration:
KC1
solutions
C c 10-4 PA, \y, = -15OmV(l);
C c 5 x 10m4 I:, \y, e -150 mV (2); C e 10-3 hl, -125 mV (3); C L: 5 x 10B3
Ll,
%=
- 100
y,
IE
mV (4).
In Pig. 7 are shown by points the results of measurements of film thickness
h of
joining pressure,
II
KC1 solutions at two values of dis-
c 1.5 x IO3 ( r = 1 mm) and
dyn/cn2 ( r = 0.5 mm), where
II E 3 x 103
is the radius of the tube 1
(Fig0 2) [21+-jD /The solid curves on Fig. 7 represent the results of calculation of electrostatical "s( h >
disjoining pressure
on the basis of assumed values of
y,
and YL
125
of film surfaces* The
potentials
quartz
surface
in contact
conccntrat~an kinetics
are
of
values
Calculations lated
are IIe( k)
of
data
the values
were performed
of
v+ and
\y% do not
in all
series
the
results
gives of
6’
condition
I const,
experimental value
is
water
films
points
close (
experimentsl. especially effect
disjoining
r -34
is
case
of
pressure
differ
the
If,
( h >
of
tabu-
the
condition
much from
the
charge.
mV. This
the experiments
with
. Yhc agreement
between
of
of
so2utions.
at
molecular
k7
The
30 nm is
component
shown by a dashed
was adopted
free
as satisfactory,
thickness
is
well
5.5 mV.
yp p -45
may be considered
isotherm
-
surface
assuming
on film
are not
f: c t?.&:‘tl!- 1-5t1
Low concentration
forces
the
on the fSLm thickness,
experiments
mV ) [2?]
and theory
, Since
of
line
to be A r - 7.2
on x
J [201j m Fig*
of
strength II
molecular
e
electro-
That means that
const.
do not
known one from
7. The Wsmaker constant
10n2’
pfi
the
that
to
c:-? i ‘1 (- .;Is<>3Ct:jcrl
* ‘Ihe calculated
negligible
Pig.
y2
[25]
depend
our
was obtained the
data in
of
to
of
the
on the ba.sis
\y*
where
TIM :i..j3 :d Iri;l-!.r*l;i;l’
from
-25
of
different
interface
from
using
ly L const condition
film-air
f2i1
Uowever , nearly
of
capillaries
for
varied
potential
determined
quartz
potential
yfl, ya
?QlOWl-i,
in
of
KC3 solutions
independently
measurements
values
with
values
the
8 repxttsents
solutions
the
dependence
s of
film
thrickness
EC4, + KCI + KCW at a constant
i r 10 -3 mol/am3 , and a constant
= 1 Or x ‘IO3 dyn/cm2.
disjoining
on the
ionic pressure
126
60
-+f (mv
h(m)
120
40 80
40 0 2
4
6
8
1OpH
Fig. 8. Dependences of aqueous film thickness (curve 1) and of potential
y,
of quartz surface (curve 2) on the
pI1 of the solutions,
The experimental data, shown by points, coincide with solid curve 1, calculated [261 by using
\y, values measured
in quartz capillaries (curve 2), and assuming to
\y2 to be equal
-45 mV. At low
pII,absolute values of y, potential decrease,
and this leads to some decrease in film thickness. Near to isoelectric point of quartz surface (
Y; = 0
wetting films lose their stability. Rupture of
at
pI1
2.7 )
k-film and
formation of thin d-film was observed by microscope; p+& transition is accompanied by txansition from complete to partial wetting. More information about the II(h) isotherms of wetting films gives the ellipsometrical measurement [28, 297 . Tn this
127
ca.se , it was possible to change the disjoining pressure gxa-
dually from 103 to IO4
dyn/cm2. Since the film thickness of
electrolyte solutions under investigation was higher than 30 nm, the effect of surface roughness can be neglected and simple ellipsometrical model of homogenious films was used. In Big, 9 are shown by points film thicknesses measured for fiaClsolutions. Theoretical
"a( h >
isotherms (solid
curves) are calculated 126-Jusing the following values of ‘y, and u(*potentials : C
IO-3 M, and
5 x 10W3 11. The
W? = -150
v, I:- 100
and
and
ye = -30 mV -25
Y2=
mV
for
for C
E
v, values coincide well with the ones measured
by capiJ.laryelectrokinetics, and
v2 values are near to the
known ones for water-air interface [27J
.
Measurements of a number of film thicknesses for every concentration of the solution allow one to compare more strictly the experimental and theoretical data. Fig. 9 shows that depondences of film thickness on the disjoining pressure may be really described using reasonabLe values of
‘y, and y2
potentials. This means that the thickness and stability of thick StatiCal
p-films are determined by the action of electrofomX?S
only. Isotherms of
p-films can be predicted
when the electrical potentials of film surfaces are known,
Fig, 9. Isotherms of disjoining NaCl solutions of different concentration: 5 x '10 -' M (2); IO-' M (31,
and 5 x 10e3 M
Fig.10. Bllipsom~tric measurements of thickness of films of aqueous N&l
+ NaDS solutions
1, C r 70-4 Kj;Cs t5xlO
+
2. C e:5 x 10-' M; C, c5XlO 3. c = I?-: IL.:c, I: 5 x If4
Pi -Q M; IL;
4, C Lt: 5 x 10-3 XI;Cs r:5 x lo+ Mo 5. AKLONIC SUIG'ACTANTS As shovvnabove, e-film thickness depends on electriCELLcharge of the film interfaces, In this connection, *he stability of ~-films must be very sensitive to additkons of ionic surfactants, which may be adsorbed on the both interfaces. Experiments were performed with aqueous solutions of
129 anionic sodium dodecylsulfate ( MRDoS)
, Merck, '7 99 5%
active substance. The background electrolyte concentration was changed from 10-4 to 10-3 M NaCl. In neutral aqueous solutions small addition of NaDos
do not change the \y4
potential of the quartz surface. Therefore, in this case it was possible to use the same values of y, when no h’c~DoS was present. The u(& potentials of film-air interface are known from the experiments with free foam films [30-j. In the concentration range of
NaDoS
from C
L ION5 L1 up to CMC
(and a background electrolyte concentration C e:4 x 10-4 F? . NaCl ) the values of
Y2
potential are constant and equal to
Y2= - (75 c 00) mV. This allows one to calculate also in this case the theoretical "s( h ) isotherms and to compare them with experimental data. Thickness of
(s-films
was measured using both microinterferometry and ellipsometry. The data obtained by means of the microinterference method are represented in Table I. The accuracy of measurement of the thiclalessesamounts to d 3 nm. As one can see from 'TableI, the agreement between h
and
12, values
is satisfactory. An addition of N&DOS
fluences the film thickness due to a change in caused by adsorbtion. At the same NaCl C E 1o-4 M, an addition of Na,T)o'j
y2
invalues,
concentration,
increases the film
thickness by about 10 nm. In the case of more concentrated solutions, addition of
NaDoS
due to increasing ionic strength.
decrease the film thickness
130 TABLE 1
caecueated
Comparison of experimental film thicknesses (h) and theYones forces theory
(ht) on the basis of electrostatical for
NaCl
+ NaDoS
solutions
__^_________________~~-~~~~--~~~~~~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~ C NaCl
h
's
ht
n
-3,
-Y,
Ho
dyn/cm* nmnm M M mV mV nm ___________-________-~~_~_~--~-~-~-~_-~-_~-~~~~~~~-~~~~~----~-~~ 10-4
a0
86
3000
125
45
110
103
97
3000
150
75
115
0
10'4
5 x 10-5
--_______--_---_-__-~~~~-~~~~-~~~~~~~~~~-~~~~~~~-~--~~--~--~~___ 5x10-4 5x1o-4
0 5 x 10-5
72
62
3000
100
45
84
67
70
3000
150
75
80
______-______-___-__~~--~~~~~~~--~~~~~~--~--~-~-~-~_~~__~_______ 5x1o-4
5 x 1o-4
74
71
2500
150
75
90
10-3
5 x IO -4
56
52
2500
150
75
78
_____-_______-__-___~~-~~-~~--~~~~~~~~~~-~~~~~~~_~--~--~~-____~__
In all the cases under consideration, the condition of complete wetting was fulfilled. The values of
Ho, which de-
termine the position of an undisturbed meniscus (Fig.2) are always higher than h
. Between
the bulk meniscus and equi-
librium film there are formed a transition zones having gradually changing thicknesses [313 . The higher are the values of relation Ho/h
the better is the wetting [23] 0
In Fig. 10 are shown the results of ellipsometrical determination of
fl (h) isotherms for aqueous solutions of NaDoS
of different concentration. The points characterize the me-
131
asured
thicknesses,
using
tabulated
the
potentials
the
obtained
single
The thickness
at
of the
tion
y4
and
Yz
leads (see
10)
of
coincide
solutions
The effect
of
by about
of
of Rigs. of
with
5 x 10-T
9 end 10)
M
mofe
that
not
some parts
with
increases pronounced
the
dimensionless
as in Fig.
coordinates.
10,
film
at lower
at C E IO -4 the
film
nm.
The same isotherms
But
an addi-
543210
11.
the
the concentra-
?I1 Fig.
of
predictions.
electrolyte,
increases
of
only
in Debay length.
For instance, NaDoS
of
1. An analysis
theoretical
decreases
background
theoretically same pair
conclusion
1) but even
NaEoS is
a solution.
20-30
and the
as in Table
well
films
calculated
due to a decrease
NaDoS (compare
strength
h > [26]
Table
wetting
are
to the
same concentration
an addition ness
“s(
of
the
thickness. ionic
for
results
(Pig,
cuxves
data
measurements
isotherms
tion
solid
rebuilt
in
M BaC1,
thick-
132
zh
In Fig.
11 isotherms
instead
of
N is
the
Boltzman
k
are
constant.
of
pairs
of
and
y,
of
the potential
in Fig.
data
relating
to
bility
of
that
by the In the
ionic
[32-34 1 , only dictions
isotherms
qualitative
potentials
suBfaces.
6.
film
of
L - 75
of
the
ca,ses
two curves. shown the
pressure,
between
in Fig.
A possi11,
isotherms
is
leads mainly
wetting
films
agreement
with
performed theoretical
agreement
more reliable
earlier pre-
in our
data
for
experiy,
and v)z
CATIOMIC SURFACTANTS In distinction
adsorb
from
their
aqueous
negative strongly
wetting
phenomena.
cationic
solutions
charges.
fluence
Aronson
to anionic
the
film
and Erincen
surfactants
surfactants, on quartz
That is
leads
the cationic surface,
why cationic
stability
[33]
to
a. solution.
Quantitative
by using
are
on
two
yr
these
the
results
for
in all
scale,
a.11 the
of
the
150,
between
disjoining
ments wa.s obtained of
E -
of
with
was obtained.
give
pressures
in magnifying
strength
experiments
curves
situated
a difference
is
H
ionic
= 75 mV. Almost
of
and
any more on the
v,
values
, where
only
Two solid
are
small
ions,
li
depend
disjoining
11,
combination
conclusion
caused
but not
y: points
of
coordinates:
of
isotherms
values:
=-loo,
left
the
a case
electrostatic
the experimental On the
one kind
In such
values, VL YY, and strength of the solution. calculation
in another
instead ll/r\l~T
, and
concentration
plotted
have
ones
decreasing
surfactants
in-
and correspondingly
shown that
to the rupturing
of
the
an addition wetting
of
films.
133
The critical concentration for rupturing decreases as the hydrocarbon chain increases in length. However, because of and y]% potentials, it was imY, possible to compare the experimental results with the theorethe lack of data about
tical isotherms of disjoining pressure. Y&ehave examined earlier mechanism of charge regulation by adsorption.of cationic surfactant using electxokinetic measuremen-tsin thin quartz capillaries [29] o Only the results of
Yt
and yy, measurements would be discussed here, which
is necessary for the calculations of electrostatic forces in wetting films. In Fig. 12 are shown the dependences of
y,
potentials of quartz on the concentxation of aqueous cetyl~rimethyla~onium bromide (CTAB) solutions at constant background concentration of
5 x IO-4 I\nNaCl 0 CTAB, 99 % purity,
was obtained from Merck Company.
Fig. '12.Results of electrokinetic measurements of ~,potentia~ of quartz surface in aqueous CTAB solutions at a background concentration C = 5 x 10 -4 M NaC1.
134
Adsorption negative
of
values
quax tz
of
concentration
of
Cs
10-5
of
quartz
of
measurtiments calculated
surface
fox
film-a.ir
with
free
films
ones.
were
from
negligible
$ -films.
Table
in
of
are
unstable,
angle
pattern.
potentials
\yz = + 100 mV) . Thin action
of
molecular
forces
y,
con-
poten-
the experiments
near
of of
of
5 x IO4
3
d, -films
and structural
be measured
sign
remain
repulsion
with
measured
by using
(an/ah7
c;)
of
( “6 4 0)
an inverse
x
formed
forms
is
p -films
attraction
cs p 23
are
meniscus
of
50 to
forces.
The bulk
, which
and
to experi-
from
nm> cannot
50.
y,
components
(hL10
about
of
electrostatical.
and immediate-&y
Lability
of
another
thicknesses
CTAB concentration
by electrostatical hwing
of
thicknesses
with
method,
surfaces
the
the background
values
the range
by the microinterference
caused
l
with
adopted
influence
thicknesses
is
from
the
theoretical
Their
an interference
*c/cm2
electrostatical
o The values
2,
as compared
a contact
The surface
microinterferential
made at
axe taken
means that
pressure
p -films
val.ues
CTAB solutions of
than
positive
of
of
,
give
In the range
&-films
of
, on the basis
[35]
This
disjoining
thin
h
measurements
As can be seen
x -lo -7 M
lower
= * 3.5
the results
interface
potentials
&
the
to CTAB
order
constant
to
C t 5 x IO -4 I,! NaCJ
tial
70 nm is
amounts
thickness
b The
,
centra.tion
of
corresponds
IL!, on the
CMC, high
compared
film
one,
[26]
Ye mental
-4
in
an overcharge
\yl e + 150 mV, axe stabilized.
In #Table 2 are
theory
point
to 10
Cs 3
a decrease
and then
potential,
near
&I). At
potential,
charge
v,
at first
The isoelectric
surf ace.
CMC ( c
CTAB caused
( \y, = stable forces.
film 75 mv ,
due to
the
135
TASLE 2 Comparison of the experimental
(h) and theoretical
thicknesses of v;etting films of background
MaCl
cs
h
CTAB,
concentration,
ht
l-l
CTAB
( ht)
solutions at constant
C r 5 x 10BL' L?
y, mV
yy, mV
Ho
State
nm
of the films '/
nm
hm
d&cm2
72
62
3000
-100
-45
84
S
52
60
3000
-75
-45
71
s
mole/l 0
5 x 1O-8
2.5
low7
-
-
2900
-75
+I00
-
1
5 x 10-7
-
-
2800
-75
+I00
-
1
10~~
-
-
2800
-70
+I00
-
1
1o-6
-
-
2800
-70
+I00
-
10-5
-
-
2700
-30
+I00
-
m
5 x lO-5
60
60
2600
+25
+I00
65
m
1.2.5x 10-4. 59
62
2400
+55
+I00
74
m
2.5 x lO-4
69
66
2000
+75
+I00
83
s
5 x IO-'
65
64
1500
+I00
+I00
80
S
1.25x 1O-3
56
52
1500
+I00
+I00
65
S
2.5
x
x
5x
2.5
x
‘1
s_
stable films;
1 - labile films.
m - metastable films;
1
136
Microphoto (Fig. 13,a) illustrate the coexistence of iand ~-films after rupturing the latter. Formation of d-films starts usually in the middle of p-films, after which gradually expands, and Of
b-ir d
CL-film
transition takes place. Shape
boundary reflects the real nonhomogeneous state
A/e
of the quartz surface, As nas shown above, structure ofd-films is
very
sensitive to local hydrop~~i~icityof the substrate. In
the absence of a solid substrate, for instance, in the case of free films [q-5] , black
d-films
have the form of a
circle. After realisation of the state shown in Fig. 13,a, the capiU.ary pressure has increased and the advancing meniscus approaches
the
d -film boundary (Fig. 13,b). Interference
pattern draws together, refloctine;transition from complete to partial wetting.
Fig. 13. Microphoto of ruptured p -films: a) formation of thin d -L'ilmin the middle of
p-film; b) formation
3.Cadvancing coni;actangle ( B,r: 50*) with
&.-film.
137
At 2.5 x IO-5 b
cs +1.25x
IO-4 M ,
p-films
are
time, from several seconds (at
formed but only for a short
M) to several minutes. In the latter case cs = 2.5 x IO-' it was possible to measure their thickness (Table 2). Rupturing of p -films in this range of CTAB
concentration shows that
disjoining pressure (2400-2700 dyn/cm2) was near to the critical one. Lxistence of some critical pressure l7,
on the
isotherm (see Fig. 1) indicate that in this case the
n(h)
condition of
\y =:const
Finally, at
was fulfilled.
Cs B 2.5 x IO-4 P,[stable
e -films are
formed again. The potentials of the film surfaces acquire the same sign, and their values draw together. It seems that these
p-films may also rupture but at more higher values of
disjoining pressure, approaching the critical one, which cannot be reached in our experiments.
As in the case of anionic surfactant, the values of Ho are higher then h
(Tabl. 2) indicating complete wetting of
p _'. films with bulk solution. In Fig. 14 are shown the results of ellipsometrical measurements of the thicknesses of stable
P -films of CTAB solutions at a constant background concentration of N&l (5 x 10-4MI.
An increase in C,
leads to some decrease in the thic'knessof
e -films, mainly due to en in&rease in the ionic s-Lrength of a solution (curve 3 and 4). Some decrease in the film thickness at
Cs = 10-7 M, as compared with
C, e 0,
is
connected (see Tab1.2) with the lowering of the values of y, potential of quartz surface, Solid curves in Pig. 14 are calculated theoretically. The
138
best agreement with experimental points was reached using
followingpairs of the surface potentials: = -55 mV (curve I);
y, I +125,
y,
= -150,
Y; = -150, y2=:
yX = -45 mV (curve 2);
\Ya rs-1.75 mV (curve 3), and
Yr" + 75 mV (curve 4).
The
y, = +I25 >
adopted values of
y
are
not very different from the ones used in fabl.2.
- 80
I - 60 : 3
- 40
8
--T--T-’
4 n*ro-3~dyn.cm-q
".lo-3(qgn.ce-2)
Fig. 14, Sl.lipsometricallymeasured thicknesses of wetting films of aqueous C!PAB solution at a background concentrat)on C = 5 x 10m4 !;I)$aCl, C* L 10-7 IdI(2); cs
r
C
s=
0 (I);
5 x 1o-4 TJ(3), and
Cs E 10-5 ISI(4). Fig. 15. Eeversibifity of eU.ipsometric measurements of wetting film thickness of aqueous solution (C = 10m3 El NaGI_,C, = 5 x 10m3 1!CTAAB)at rising (white points) and lowering (black points) disjoining pressure.
139
Fig.
1% demonstra‘tes
ellipsometric
the
by consistent
capillary
menta.1 point adopted Film
reversibility
Black
measurements.
correpondingly, of
the
pressure
satisfies
surface
concentration
are of
In Pig.
theoretical
thicknesses
in the
electrolyte
of
experimenta,
fl,(
M
Y=
the const
of
the
was used.
constancy
would
be realized, va.lues
is
is
dition
(but
have
cases
condition
IIC
mechanism
v’
and
6’
of
condition charge
of
formation,
well of
values
of
curve
1
when the condition that
of
condition
the of
for
a very
surface y
=const
at disjoining
z 3 x ?03 dyn/cm2. vr
are
very
different
as was shown above,
films
is
= const
the
solution
Solid
fulfilled.
the more concentrated
known 1363 , in
films
starting
condition
and
wetting
the
to note
the
same sign),
and in
‘-y = const
As is and free
for
interest
film
calculation
a.nd the
the
more higher
2 described
must be suptured
to
the
curve
theoretical
If
p-films
y-’ = const
intermediate
of
fulfilled.
Fhen the potentials magnitude
of
= - 35 mV were used.
a solution
near
with
strength
Eotted
r const
= + 75 mv.
yt
measured
same calcula.tion
It
of
charge
s’
YYr
low concentration
pressure
of
150, results
and
having
(10m3 Ei E?‘a.Cl).
low ionic
when in
, condition
Y, = shows
data,
obtained,
increase
isotherm
shown ellipsometrically case
are
of
The experi-
in connection
(5 x low5 LI NaCl + 10W7 E CTAB). the
(FiC.2).
“a( h )
lower
background
16 are
points
Y% = + 140
here
the results
and following
the cell
potentials
thicknesses
and white
docrea.sc
in
of
ca.se of
s’ Ic const
cannot
in
con-
In some solutions
the
be distinguished.
symmetrical
interlayers
corresponds
to adsorption
and condition
of
y
e: const
-
140
aa0 .60 -40
-20
10
4
8
8
6
4
‘0
2
n*10~(m*cm-2)
n.10-%lyn.cnl-2)
Fig. 16. Comparison of clliosometrically measured thicknesses of wetting films of aqueous solution (C = 5 x NaCl, Cs = low7 M CTAB)
Fig. 17.
M
with theoretical isotherm
calculated using condition and
10 -5
y r const (curve 1)
6' = const (curve 2).
Isotherms of disjoining pressure of wetting films of aqueous solutions of PEO at a background concentration of KC1
C E lO-4 bl (curves 1-S) and
C
r
lo-3
iv
(curves 4 and 5). Concentration of 1330;C
0 (curves 1 and 4),; P= C r lo-4 g/dm3 (curve 2), and C z IO-' g/dm3 P P (curves 3 and 5).
141
-
to
the
the dissociation cases
of
of
iongenic
asymmetrical
consideration,
since
different
surfaces
film
surface
wetting
groups.
films
the mechanisms
need
of
Rowever,
some special
charge
formation
on
ca,n be different.
7. ~ONIONIC FJLYEERS Adsorption
of
stabilization kinetic
of
nonionic
layer
studied
the
Film e llipsome
of
of
silica
of
thicknesses trically
17 are
centration relate
to
lower
increase
in
film
ones
forces,
Carbide),
This
.:inco
( 6 = I72 nm) was much smaller
clear
of
a change
when experimental
isotherms
of
(curve
the values
fit
1)
with
correspondingly. a.bsolute
g/dm3. of
data
of
va,lues
Addition of
y,
(Big.2)
PZO with
molecular
z1.1
, for
PXO con-
upper
of
action
are
are of
UP to
film
compared of
equal
Cp z ?O-” -80
of
in
an
the
.WO layer thickness.
thickness
becomes
theoretical
repulsion.
-110 g/dm3
mV (curve
cases,
film
with
to
(I-3)
sterical
a.dsorption
than the netting
curves
C = 10v4 f;:,
lowering
and yX corresponding
data,
ad-
wet~tine; films,
KC1 solution
due to the
forces y,
electro-
same cell
Three
to the
in wetting
electrostatical
experimental
of
the results
the maximum thickness
The cause
of
MW/$I,,,
leads
~:as not
sterical
with
to C = 10m3 Ai;. In a.11 the
P&O concentra,tion
thickness.
in the
shown by points
(4,5)
covered
solutions
C E IO -4 and IO-1 P oack;;round concentration
and two
for
to the
sta.bility
measured
aqueous
mass FIW z G x IO5 (Union In Pig.
used
(PtiO) (383 , we have
Pi30 on the
were
for
often
particles
polyethylenoxide
influence
is
(371 , In connection
colloids
investiga.tion
sorption
polymers
2),
At Cp = 0 to and
the best -25
mV,
PiD decrease and addition
142 of
Cp = 10"
g/&n3
to
-50
ml’
(curve
3).
At
much
higher
background concentration of electrolyte (curves 4 and 5) mlues
of
y(,
also
change from
yl. = -100 mV (curve 4)
Y, = -50 mV (curve 5).
to
Pherefore,
adsorption of PBO decreases the
of the quartz surface, while the
Yt
v,
potential
potential of film-air
interface remain constant and equal to
ye = -25 mV. This confirms the earlier expressed supposition that an adsorption PEO Of.‘ decreases the degree of dissociation of OH-group of quartz surface [j8] . It is also not excluded that adsorption layer of PRO influence the double electrical layer. Iiowever, taking into account a low volume fraction of polymer in adsorbed layer of PRO, the latter effect cannot be responsible for such a large change in
y, values.
Wetting films were used in the present case as a model system, which allowed one to trace the change in
\v, values
that determine the electrical potential of quartz surface under the adsorption layer of polymer.
I,
The thicknesses of wetting films of nonpolar liquids
can be calculated on the basis of the theory of dispersion forces. For aqueous films, it is necessary to take additionally into account the electrostatical and structural forces. 2. Ioothe$ms of disjoining pressure of thick p-films of aqueous solutions are in quonti-dativeagreement with the calculated ones using electrical potentials of film surfaces y: and
yll
, determined in the course of independent experi-
ments. Concentration of electrolytes, ionic surfactants and
143
polymers influence wetting film thickness (at h>30 its action on
‘y, and
nm) due to
\Yr potentials of film surfaces and
due to a change in Debay length, 3. For stable
p -films of low concentrated
solutions
the condition of constant charge on both surfaces (by film thinning out) is better fulfilled. In all other cases, the condition of constant potentials is realized. Eowever, for wetting films these conditions need some reconsiderations, since the mechanism of charge formation on solid and on film-air interfaces, can be different. 4. Thin aqueous thick metastable
P
d -films formed after rupturing of -films remain to be investigated. The
attainment of the equilibrium
state of such films requires a
very long time. Furthermore much higher values of disjoining pressure must be used. This can be realized only with a modified cells for ellipsometric measurements. The thickness of
d -films of water formed as a result of
vapour adsorption on flat substrates depends on the surface hydrophilicity, at P
near P,
The thickness usually increases up to 5-7 . Hydrophobization
decrease the thickness of i-later
run
and raising of temperature d -films,
144
I. A.Sheludko, Adv. Colloid Interface Sci., 1 (1967) 391. 2. J.S.Clunie, J.F.Goodman, B.'i'.Ingram, In: "Surface and Colloid Science", Vol D 3, &I. ~.IYlatijeViC, Filey Press, N.Y., 1971, P. 167. 3. R.Buscall, R.II.Ottewill,In: "Colloid science", Vol. 2,
Ed. D.H.Yverett, Chem. Sot. London, 1975, p. 191. 4. D.hxerowa, D.Kashchiev, Contemp. Physics, 2 5. P.KruSlyakov, D.&crowa,
Chimiya,
~YOSCOW,
(1986) 429.
Foam and Foam Films (in Russian),
1990.
6. B.V.Derjaguin, N.V.Churaev, Vetting Films (in Russian), r:auka,I!~OSCOW, 1984. V.;i;.F,'!uller, Surface Ibrces, 7. &.V.Derja,uin, :;.V.Clluraev, Plenum Press, I!cwYork, 1987. 8, D.B.Iioulj;h, L.R.VMte,
Aav.Colloia Interface Sci.,s
(1980) 3.
9. D.Kashchiev, Surface Sci., 225 (1990) 107. 10. Z.Zorin, D.Platikanov, T.Kolarov, Colloids and Surfaces,
-22 (1987) 147; T.Kolarov, Z.Zorin, D.Platikanov, Colloids and Surfaces (in press). 11. B.V.Derjaguin, Z.i.?.Zorin, In: "Proc. Second. Intern. Con@. Surface Activity", V. 2, London, 1957, p. 145. 12. L.B.Boinovich, B.V.Derjaguin, Kolloid.Zh., U&R, -49 (1987) 627, 631. 13. %.M.Zorin, I.N.Kornil'ev, N.V.Churaev, Kolloid.Zh., USSR,
-46 (1984) 437. 14.
R.VN.Cranston,P.A.Inkley, Adv.Catalysis, 2 (1957) 143.
15. EI.L.Gee,T.W.Bealy, L.R.Vhite, J.Colloid Interface Sci., m
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