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Surfactant-polymer interactions in soap films
Physica A 200 (1993) 743-750 North-Holland SDI: 037%4371(93)E0165-B
Surfactant-polymer
interactions
in soap films
Oleg Krichevsky and Joel Stavans Department
of Electronics,
Weizmann
Institute of Science, Rehovot
76100, Israel
We present a preliminary account of an experimental study of the influence of nonionic polymers on the draining process and thickness of freely suspended vertical soap films. We studied films of both cationic and anionic surfactants. While the presence of a polymer leads to a monotonous reduction in thickness with concentration in the former case, the behavior in the latter is very different: an increase in polymer concentration leads to an increase in thickness of black films and to their eventual loss of stability through disruption of micellar stratification.
1. Introduction
It has been increasingly recognized in recent years that polymers may alter significantly the properties of self-assembled structures of surfactants in solution and the interactions between them [1,2]. For example, polymers disrupt lamellar order [3,4], induce spontaneous curvature in vesicles [5] and form micelle-decorated polyelectrolyte-like structures [6]. These and other experimental studies [7,8] have spurred considerable theoretical interest on polymer-surfactant interactions at interfaces and many effects have been predicted, among others an increase in the rigidity and surface viscosity of monolayers and bilayers when polymers adsorb on them [l], an enhancement of polymer adsorption on monolayers [9] and a decrease (increase) in the mean curvature rigidity (Gaussian rigidity) of bilayers [lo]. When adding a polymer to a surfactant solution, one may envision two possible scenarios: one of repulsion between polymer and surfactant molecules and one of attraction. In the former case one expects an attractive interaction between self-assembled structures due to depletion interactions [ll] while in the latter polymers are expected to induce a repulsive interaction as in colloid stabilization, besides altering the self-assembled structures themselves [l]. In order to test some of these ideas we have carried out an experimental study of the effects of nonionic polymers on the draining properties and thickness of free-standing soap films. A soap film is an attractive system in that its simple architecture allows not only for a detailed study of the modification 0378-4371/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
744
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
of interfacial properties due to the addition of polymer, but also for studies of the modification of the interactions between the monolayers comprising the film. In order to study both cases of depletion and adsorption, we have used in our experiments films stabilized by either cationic or anionic surfactants for which interactions with nonionic polymers are known to be weak and strong respectively [12]. The dependence of the draining process on the velocity at which films are initially drawn out of solution was studied by Lionti-Addad and di Meglio [13] in the case of sodium dodecyl sulfate-polyethylene oxide (SDS/PEO) films. They found that while low molecular weight polymers have essentially no effect on the draining process, high molecular weight polymers strongly affect it. The changes in the draining behavior could be ascribed neither to the increase in the bulk viscosity nor to non-Newtonian effects induced by the polymer and more complicated interactions between the surfactant and the polymer had to be invoked. These authors did not report data on the final states attained by the films.
2. Experimental
methods
and materials
Films supported by a U-shaped teflon frame were drawn out of surfactantpolymer solutions contained in a teflon container within a temperature-controlled sealed cell. The temperature of the cell was kept constant at 21°C with a precision of +O.O2”C. The -1 cm* films were drawn out twelve hours after the solutions were introduced into the sealed cell. The humidity inside the cell achieved a constant value before the beginning of each run. The film thickness was extracted from light reflectivity measurements using a 10 mW HeNe laser. In a typical measurement the reflected beam subtended an angle of 90” with the incident beam. For each film a complete record of the draining process due to gravity and capillary forces was obtained. The thickness was then obtained from the ratio of the instantaneous reflected intensity Z and the maximum reflected intensity I, by the formula Z
%=
(1 + R*)* sin*( /3) (l-R2)*+4R2sin2(P)
’
where p = 21rnd cos(8 ‘)/A, n being the index of refraction of the solution, d the equivalent water thickness of the film and the angle 8 ’ is determined from the condition n sin(0’) = sin(@), where 8 is the incidence angle. Moreover R = sin(f3 - 8’)lsin(8 + 0). The materials used are PEO (Mw = 100 000 and M, = 900 000 purchased from Aldrich) and polyvinylpyrrolidone (PVP) (M, = 1000 000 from Polysciences) as nonionic polymers. Hexadecyltrimethylam-
0.
Krichevsky,
.I. Stavans I Surfactant-polymer
interactions in soap films
745
monium bromide (HDTAB) and dodecyltrimethylammonium bromide (DTAB) were used as cationic surfactants and SDS as anionic surfactant, all compounds purchased from Sigma with a purity of 99%. All materials were used as supplied. We chose these materials since soap films stabilized by these surfactants have been amply studied and interactions with PEO and PVP at interfaces have been previously investigated. Solutions out of which films were drawn were prepared according to the following concentrations. HDTAB: and glycerol at 9 wt%; DTAB: 2.3c,,, and 2 wt%; SDS: SC,,, and 96c,nK 4 wt% glycerol. The concentrations were chosen so as to obtain films of long enough lifetime.
3. Results Draining SDS films have been shown to pass through a succession of metastable states of progressively longer lifetimes, during which the thickness of the films remains constant. These metastable states have been attributed recently to the successive removal of layers of micelles [14]. This interpretation has been supported by recent measurements of forces between two mica plates immersed in a micellar solution using the surface force apparatus technique [15]. In our experiments we observed two long-lived states of thickness -270 8, and -170 A. We label these thick and thin films. Typically the lifetime of thick films in the absence of polymers had great variability: sometimes they lasted for 2-3 hours after which they decayed into thin films, and sometimes they lasted for days. We plot in fig. 1 draining curves (thickness versus time) of SDS films approaching thick film states for different concentrations c of PEO. Beyond increasing slightly the time to reach this state, the shape of the draining curve is barely affected by the polymer. A more marked effect is observed on the thickness and stability of the metastable states themselves. In fig. 2 we plot the thickness of thick (circles) and thin (squares) films versus c for two polymer molecular weights. Error bars are one standard error from typically four or five measurements. The salient features in the figure are the following: (a) thick films become progressively thicker with increasing c, the polymers inducing a repulsive interaction between the monolayers comprising the film; (b) above c - 0.7 no thick films are obtained, films draining directly into thin states; note too that as c approaches the value 0.7 from below, the lifetime of thick films decreases, decaying more rapidly into thin films; (c) the behavior is dependent on the monomer concentration and not on polymer molecular weight. The effects of PVP on SDS films are qualitatively the same though stronger. PVP interacts more strongly with anionic surfactants than PEO [12].
746
0.
Krichevsky,
J. Stavans
I Surfactant-polymer
03 3 I
1
in soap films
:r--_ 0
0
x3
interactions
10
5
15
20
u:k ~ol?f c=o.7
0
5
10
15
10
5
20
t
15
20
x 1O-2 (set)
Fig. 1. Thickness d as function of time t of films drawn from a solution of SDS and PEO for different values of PEO monomer concentration c. Only the approach to thick films is shown.
3 -0
250
-
200
Q i
150
100
@
-
P
.
’
0
0.4
0.6
1.2 c
1.6
2
(W.P.1
Fig. 2. Thickness d of thick (circles) and thin (squares) SDS films versus PEO monomer concentration c for two molecular weights: MW = 100 000 (full symbols) and M, = 900 000 (empty symbols).
Films of cationic surfactants behave quite differently in the presence of polymers. Note first that for c = 0 HDTAB films attained only one final state -640 8, thick. This thickness can be decreased with a suitable addition of counterions. In fig. 3 we show draining curves for HDTAB/PEO films with 0.06 M of NaCl reaching the final state. Note the following features: (a) the thickness of the film before reaching the asymptotic state increases with c and
0.
Krichevsky,
J. Stavans
I Surfactant-polymer
147
interactions in soap films
4
0s 3 0
:K 0
Ii” -03
2 1 0
10
c=l.O
3
1 20
30
20 ELI 0 4
,
10
20
,
30
I
c=o.5
b-7 l#Ili!
0
10
20
30
0
10
t
20
30
x 1O-2 (set)
Fig. 3. Thickness d as function of time t of films drawn from a solution of HDTAB NaCl and PEO (M, = 100 000) for different values of PEO monomer concentration
with 0.06 M c.
at the same time (b) the transition to the asymptotic state becomes considerably sharper with increasing polymer concentration. Note that although the time necessary to reach the asymptotic state appears to be shorter with increasing c as seen in fig. 3, this behavior is not always observed. Features (a) and (b) are robust (we observe the same effect with DTAB films). Finally in fig. 4 we plot the thickness of HDTAB/PEO films as a function of c for both M, = 100 000 (full circles) and M, = 900 000 (empty circles). Films become monotonically
c (W.P.) Fig. 4. Thickness d of HDTAB films versus PEO monomer weights: M, = 100 000 (full circles) and M,,, = 900 000 (empty
concentration circles).
c for two molecular
748
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
thinner as the polymer concentration is increased. Experiments with DTAB films, which go through a sequence of metastable states similar to that of SDS show the same behavior as those with HDTAB.
4. Discussion The experimental evidence for the strong interaction between anionic surfactants and nonionic polymers in bulk solution and at interfaces is substantial [3,6,7,12,13]. Cabane and Duplessix [6] found that in bulk solution micelles adsorb onto polymer chains forming necklace-type structures. For small polymer concentrations, the structure of the micellar solution and the intermicellar spacing are barely affected, since polymers saturate stoichiometrically with adsorbed micelles. As c is increased, more micelles adsorb onto chains and the intermicellar structure is progressively altered. Chari and Hossain [7] found in their experimental studies of the air-water interface that both the surface tension and polymer adsorption at the interface increase simultaneously with c. To explain their results they suggested that polymer and surfactant molecules form two-dimensional aggregates at the air-water interface similar in structure to those in the bulk. Based on these facts and the idea of micelle stratification due to the confining geometry of a soap film [14,15], we propose the following picture to account for our findings. Polymers adsorb onto both surfactant monolayers and on micelles within the film. For small values of c, the polymers do not affect significantly the micellar organization into layers, and the adsorbed polymer layers at each interface and on micelles induce repulsive interactions similar to those found on systems of polymers adsorbed on hard walls [16,17]. This explains the thickening of our thick films As c is increased, there is a gradual for small polymer concentrations. disruption of the micellar layers and therefore a destruction of the metastable states of the film. This accounts for the fact that thick films are not observed for large polymer concentrations. The idea of adsorbed layers is consistent with the independence of the phenomena we observe experimentally on the molecular weight of the polymers. We also point out that an attractive interaction due to bridging is unlikely due to the collapse of the chains on the monolayer at the air-water interface before a film is drawn out of the solution. In contrast to anionic-surfactants-nonionic-polymer systems, cationic surfactants interact weakly with both PEO and PVP [12]. This suggests a picture in which polymers do not adsorb onto the lamellae and induce an effective attraction due to osmotic pressure and depletion interactions. During the draining process, polymers are excluded in great part from the thin parts of the film, causing a buildup in concentration and therefore the considerable
0.
Krichevsky,
.I. Stavans I Surfactant-polymer
interactions in soap films
749
thickening with increasing c we observe before films attain their asymptotic state. For small monomer concentrations c in which both long and short chains are in the dilute regime, one expects a stronger depletion-induced attractive interaction from long chains than from short ones. As c is increased, long chains reach the onset cc of their semidilute regime while short chains are still below their overlap concentration c&, . In this regime of c the relevant scale in the case of long chains is the mesh size 5 (which decreases with increasing c) and not the typical dimension of a single polymer coil. A weaker dependence of the attractive force on c is then expected for long chains and therefore a bend in the thickness vs. c curve. When c approaches C& a similar effect is expected for the short chains. This picture is consistent with the results shown in fig. 4. Note that the values of cc-O.4 and c&, ~2.3 (extracted from viscosity measurements) roughly coincide with the above expectations. Attractive forces similar to the ones reported here have also been observed between membranes immersed in a solution of non-adsorbing polymer [8]. Studies of the effects of polymers on the interaction potential between the monolayers comprising the film by light scattering methods are under way. We expect our findings to be of significance to the understanding of the stability of emulsions in the presence of polymer additives.
Acknowledgements
We acknowledge useful conversations with S. Safran, N. Dan, D. Roux and F. Nallet and thank P. Richetti for bringing refs. [14] and [15] to our attention. This study was supported by the Asher and Jeannette and Alhadeff Research Award (J.S.) and the Minerva Foundation.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
P.G. de Gennes, J. Chem. Phys. 94 (1990) 8407. R. Nagarajan, .I. Chem. Phys. 90 (1989) 1980. M.R. Kuzma, W. Wedler, A. Saupe, S. Shin and S. Kumar, Phys. Rev. Lett. 68 (1992) 3436. P. Kekicheff, B. Cabane and M. Rawiso, J. Colloid Interface Sci. 102 (1984) 51. G. Decher, E. Kuchinra, H. Ringsdorf, J. Venzmer, D. Bitter-Suermann and C. Weissberger, Angew. Makromol. Chem. 166/167 (1989) 71. B. Cabane and R. Duplessix, J. Phys. (Paris) 43 (1982) 1529; Colloids Surf. 13 (1985) 19; J. Phys. (Paris) 48 (1987) 651. K. Chari and T.Z. Hossain, J. Phys. Chem. 95 (1991) 3302. E. Evans and D. Needham, Macromolecules 21 (1988) 1822. D. Andelman and J.F. Joanny, J. Phys. II (France) 3 (1992) 121.
750 [lo] [ll] [12] [13] [14]
[15] [16] [17]
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
J.T. Brooks, CM. Marques and M.E. Cates, J. Phys. II (France) (1991) 673. S. Asakura and F. Oosawa, J. Chem. Phys. 22 (1954) 1255; J.F. Joanny, L. Leibler and P.G. de Gennes, J. Polym. Sci. Polym. Phys. 17 (1979) 1073 E.D. Goddard, Colloids Surfaces 19 (1986) 255, and references therein; M.J. Schwuger, J. Colloid Interface Sci. 43 (1973) 491. S. Lionti-Addad and J.M. di Meglio, Langmuir 8 (1992) 324. V. Bergeron and C.J. Radke, Langmuir 8 (1992) 3020; A.D. Nikolov and D.T. Wasan, J. Colloid Interface Sci. 133 (1989) 1; M.N. Jones, K.J. Mysels and P.C. Scholten, Trans. Faraday Sot. 62 (1966) 1336. J.L. Parker, P. Richetti, P. Kekicheff and S. Sarman, Phys. Rev. Lett. 68 (1992) 1955. J. Klein and P. Luckham, Nature 300 (1982) 429. P.G. de Gennes, Macromolecules 15 (1982) 492; J.M.H.M. Scheutjens and G.J. Fleer, Macromolecules 18 (1985) 1882.
Surfactant-polymer
interactions
in soap films
Oleg Krichevsky and Joel Stavans Department
of Electronics,
Weizmann
Institute of Science, Rehovot
76100, Israel
We present a preliminary account of an experimental study of the influence of nonionic polymers on the draining process and thickness of freely suspended vertical soap films. We studied films of both cationic and anionic surfactants. While the presence of a polymer leads to a monotonous reduction in thickness with concentration in the former case, the behavior in the latter is very different: an increase in polymer concentration leads to an increase in thickness of black films and to their eventual loss of stability through disruption of micellar stratification.
1. Introduction
It has been increasingly recognized in recent years that polymers may alter significantly the properties of self-assembled structures of surfactants in solution and the interactions between them [1,2]. For example, polymers disrupt lamellar order [3,4], induce spontaneous curvature in vesicles [5] and form micelle-decorated polyelectrolyte-like structures [6]. These and other experimental studies [7,8] have spurred considerable theoretical interest on polymer-surfactant interactions at interfaces and many effects have been predicted, among others an increase in the rigidity and surface viscosity of monolayers and bilayers when polymers adsorb on them [l], an enhancement of polymer adsorption on monolayers [9] and a decrease (increase) in the mean curvature rigidity (Gaussian rigidity) of bilayers [lo]. When adding a polymer to a surfactant solution, one may envision two possible scenarios: one of repulsion between polymer and surfactant molecules and one of attraction. In the former case one expects an attractive interaction between self-assembled structures due to depletion interactions [ll] while in the latter polymers are expected to induce a repulsive interaction as in colloid stabilization, besides altering the self-assembled structures themselves [l]. In order to test some of these ideas we have carried out an experimental study of the effects of nonionic polymers on the draining properties and thickness of free-standing soap films. A soap film is an attractive system in that its simple architecture allows not only for a detailed study of the modification 0378-4371/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
744
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
of interfacial properties due to the addition of polymer, but also for studies of the modification of the interactions between the monolayers comprising the film. In order to study both cases of depletion and adsorption, we have used in our experiments films stabilized by either cationic or anionic surfactants for which interactions with nonionic polymers are known to be weak and strong respectively [12]. The dependence of the draining process on the velocity at which films are initially drawn out of solution was studied by Lionti-Addad and di Meglio [13] in the case of sodium dodecyl sulfate-polyethylene oxide (SDS/PEO) films. They found that while low molecular weight polymers have essentially no effect on the draining process, high molecular weight polymers strongly affect it. The changes in the draining behavior could be ascribed neither to the increase in the bulk viscosity nor to non-Newtonian effects induced by the polymer and more complicated interactions between the surfactant and the polymer had to be invoked. These authors did not report data on the final states attained by the films.
2. Experimental
methods
and materials
Films supported by a U-shaped teflon frame were drawn out of surfactantpolymer solutions contained in a teflon container within a temperature-controlled sealed cell. The temperature of the cell was kept constant at 21°C with a precision of +O.O2”C. The -1 cm* films were drawn out twelve hours after the solutions were introduced into the sealed cell. The humidity inside the cell achieved a constant value before the beginning of each run. The film thickness was extracted from light reflectivity measurements using a 10 mW HeNe laser. In a typical measurement the reflected beam subtended an angle of 90” with the incident beam. For each film a complete record of the draining process due to gravity and capillary forces was obtained. The thickness was then obtained from the ratio of the instantaneous reflected intensity Z and the maximum reflected intensity I, by the formula Z
%=
(1 + R*)* sin*( /3) (l-R2)*+4R2sin2(P)
’
where p = 21rnd cos(8 ‘)/A, n being the index of refraction of the solution, d the equivalent water thickness of the film and the angle 8 ’ is determined from the condition n sin(0’) = sin(@), where 8 is the incidence angle. Moreover R = sin(f3 - 8’)lsin(8 + 0). The materials used are PEO (Mw = 100 000 and M, = 900 000 purchased from Aldrich) and polyvinylpyrrolidone (PVP) (M, = 1000 000 from Polysciences) as nonionic polymers. Hexadecyltrimethylam-
0.
Krichevsky,
.I. Stavans I Surfactant-polymer
interactions in soap films
745
monium bromide (HDTAB) and dodecyltrimethylammonium bromide (DTAB) were used as cationic surfactants and SDS as anionic surfactant, all compounds purchased from Sigma with a purity of 99%. All materials were used as supplied. We chose these materials since soap films stabilized by these surfactants have been amply studied and interactions with PEO and PVP at interfaces have been previously investigated. Solutions out of which films were drawn were prepared according to the following concentrations. HDTAB: and glycerol at 9 wt%; DTAB: 2.3c,,, and 2 wt%; SDS: SC,,, and 96c,nK 4 wt% glycerol. The concentrations were chosen so as to obtain films of long enough lifetime.
3. Results Draining SDS films have been shown to pass through a succession of metastable states of progressively longer lifetimes, during which the thickness of the films remains constant. These metastable states have been attributed recently to the successive removal of layers of micelles [14]. This interpretation has been supported by recent measurements of forces between two mica plates immersed in a micellar solution using the surface force apparatus technique [15]. In our experiments we observed two long-lived states of thickness -270 8, and -170 A. We label these thick and thin films. Typically the lifetime of thick films in the absence of polymers had great variability: sometimes they lasted for 2-3 hours after which they decayed into thin films, and sometimes they lasted for days. We plot in fig. 1 draining curves (thickness versus time) of SDS films approaching thick film states for different concentrations c of PEO. Beyond increasing slightly the time to reach this state, the shape of the draining curve is barely affected by the polymer. A more marked effect is observed on the thickness and stability of the metastable states themselves. In fig. 2 we plot the thickness of thick (circles) and thin (squares) films versus c for two polymer molecular weights. Error bars are one standard error from typically four or five measurements. The salient features in the figure are the following: (a) thick films become progressively thicker with increasing c, the polymers inducing a repulsive interaction between the monolayers comprising the film; (b) above c - 0.7 no thick films are obtained, films draining directly into thin states; note too that as c approaches the value 0.7 from below, the lifetime of thick films decreases, decaying more rapidly into thin films; (c) the behavior is dependent on the monomer concentration and not on polymer molecular weight. The effects of PVP on SDS films are qualitatively the same though stronger. PVP interacts more strongly with anionic surfactants than PEO [12].
746
0.
Krichevsky,
J. Stavans
I Surfactant-polymer
03 3 I
1
in soap films
:r--_ 0
0
x3
interactions
10
5
15
20
u:k ~ol?f c=o.7
0
5
10
15
10
5
20
t
15
20
x 1O-2 (set)
Fig. 1. Thickness d as function of time t of films drawn from a solution of SDS and PEO for different values of PEO monomer concentration c. Only the approach to thick films is shown.
3 -0
250
-
200
Q i
150
100
@
-
P
.
’
0
0.4
0.6
1.2 c
1.6
2
(W.P.1
Fig. 2. Thickness d of thick (circles) and thin (squares) SDS films versus PEO monomer concentration c for two molecular weights: MW = 100 000 (full symbols) and M, = 900 000 (empty symbols).
Films of cationic surfactants behave quite differently in the presence of polymers. Note first that for c = 0 HDTAB films attained only one final state -640 8, thick. This thickness can be decreased with a suitable addition of counterions. In fig. 3 we show draining curves for HDTAB/PEO films with 0.06 M of NaCl reaching the final state. Note the following features: (a) the thickness of the film before reaching the asymptotic state increases with c and
0.
Krichevsky,
J. Stavans
I Surfactant-polymer
147
interactions in soap films
4
0s 3 0
:K 0
Ii” -03
2 1 0
10
c=l.O
3
1 20
30
20 ELI 0 4
,
10
20
,
30
I
c=o.5
b-7 l#Ili!
0
10
20
30
0
10
t
20
30
x 1O-2 (set)
Fig. 3. Thickness d as function of time t of films drawn from a solution of HDTAB NaCl and PEO (M, = 100 000) for different values of PEO monomer concentration
with 0.06 M c.
at the same time (b) the transition to the asymptotic state becomes considerably sharper with increasing polymer concentration. Note that although the time necessary to reach the asymptotic state appears to be shorter with increasing c as seen in fig. 3, this behavior is not always observed. Features (a) and (b) are robust (we observe the same effect with DTAB films). Finally in fig. 4 we plot the thickness of HDTAB/PEO films as a function of c for both M, = 100 000 (full circles) and M, = 900 000 (empty circles). Films become monotonically
c (W.P.) Fig. 4. Thickness d of HDTAB films versus PEO monomer weights: M, = 100 000 (full circles) and M,,, = 900 000 (empty
concentration circles).
c for two molecular
748
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
thinner as the polymer concentration is increased. Experiments with DTAB films, which go through a sequence of metastable states similar to that of SDS show the same behavior as those with HDTAB.
4. Discussion The experimental evidence for the strong interaction between anionic surfactants and nonionic polymers in bulk solution and at interfaces is substantial [3,6,7,12,13]. Cabane and Duplessix [6] found that in bulk solution micelles adsorb onto polymer chains forming necklace-type structures. For small polymer concentrations, the structure of the micellar solution and the intermicellar spacing are barely affected, since polymers saturate stoichiometrically with adsorbed micelles. As c is increased, more micelles adsorb onto chains and the intermicellar structure is progressively altered. Chari and Hossain [7] found in their experimental studies of the air-water interface that both the surface tension and polymer adsorption at the interface increase simultaneously with c. To explain their results they suggested that polymer and surfactant molecules form two-dimensional aggregates at the air-water interface similar in structure to those in the bulk. Based on these facts and the idea of micelle stratification due to the confining geometry of a soap film [14,15], we propose the following picture to account for our findings. Polymers adsorb onto both surfactant monolayers and on micelles within the film. For small values of c, the polymers do not affect significantly the micellar organization into layers, and the adsorbed polymer layers at each interface and on micelles induce repulsive interactions similar to those found on systems of polymers adsorbed on hard walls [16,17]. This explains the thickening of our thick films As c is increased, there is a gradual for small polymer concentrations. disruption of the micellar layers and therefore a destruction of the metastable states of the film. This accounts for the fact that thick films are not observed for large polymer concentrations. The idea of adsorbed layers is consistent with the independence of the phenomena we observe experimentally on the molecular weight of the polymers. We also point out that an attractive interaction due to bridging is unlikely due to the collapse of the chains on the monolayer at the air-water interface before a film is drawn out of the solution. In contrast to anionic-surfactants-nonionic-polymer systems, cationic surfactants interact weakly with both PEO and PVP [12]. This suggests a picture in which polymers do not adsorb onto the lamellae and induce an effective attraction due to osmotic pressure and depletion interactions. During the draining process, polymers are excluded in great part from the thin parts of the film, causing a buildup in concentration and therefore the considerable
0.
Krichevsky,
.I. Stavans I Surfactant-polymer
interactions in soap films
749
thickening with increasing c we observe before films attain their asymptotic state. For small monomer concentrations c in which both long and short chains are in the dilute regime, one expects a stronger depletion-induced attractive interaction from long chains than from short ones. As c is increased, long chains reach the onset cc of their semidilute regime while short chains are still below their overlap concentration c&, . In this regime of c the relevant scale in the case of long chains is the mesh size 5 (which decreases with increasing c) and not the typical dimension of a single polymer coil. A weaker dependence of the attractive force on c is then expected for long chains and therefore a bend in the thickness vs. c curve. When c approaches C& a similar effect is expected for the short chains. This picture is consistent with the results shown in fig. 4. Note that the values of cc-O.4 and c&, ~2.3 (extracted from viscosity measurements) roughly coincide with the above expectations. Attractive forces similar to the ones reported here have also been observed between membranes immersed in a solution of non-adsorbing polymer [8]. Studies of the effects of polymers on the interaction potential between the monolayers comprising the film by light scattering methods are under way. We expect our findings to be of significance to the understanding of the stability of emulsions in the presence of polymer additives.
Acknowledgements
We acknowledge useful conversations with S. Safran, N. Dan, D. Roux and F. Nallet and thank P. Richetti for bringing refs. [14] and [15] to our attention. This study was supported by the Asher and Jeannette and Alhadeff Research Award (J.S.) and the Minerva Foundation.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
P.G. de Gennes, J. Chem. Phys. 94 (1990) 8407. R. Nagarajan, .I. Chem. Phys. 90 (1989) 1980. M.R. Kuzma, W. Wedler, A. Saupe, S. Shin and S. Kumar, Phys. Rev. Lett. 68 (1992) 3436. P. Kekicheff, B. Cabane and M. Rawiso, J. Colloid Interface Sci. 102 (1984) 51. G. Decher, E. Kuchinra, H. Ringsdorf, J. Venzmer, D. Bitter-Suermann and C. Weissberger, Angew. Makromol. Chem. 166/167 (1989) 71. B. Cabane and R. Duplessix, J. Phys. (Paris) 43 (1982) 1529; Colloids Surf. 13 (1985) 19; J. Phys. (Paris) 48 (1987) 651. K. Chari and T.Z. Hossain, J. Phys. Chem. 95 (1991) 3302. E. Evans and D. Needham, Macromolecules 21 (1988) 1822. D. Andelman and J.F. Joanny, J. Phys. II (France) 3 (1992) 121.
750 [lo] [ll] [12] [13] [14]
[15] [16] [17]
0.
Krichevsky,
J. Stavans I Surfactant-polymer
interactions in soap films
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