Langmuir 1992,8, 1889-1892
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The Langmuir Lectures Amphiphilic Association Structures and Thin Films Stig E. Friberg Center for Advanced Materials Processing and Department of Chemistry, Clarkson University, Potsdam, New York 13699-5814 Received November 12, 1991.I n Final Form: April 6, 1992 The article discusses two generally accepted phenomena in colloid chemistry: the variation of van der Waals' forces during flocculation and coalescence in emulsions and the importance of a monomolecular surface layer of surfactant for the stability of foams. It is shown that the opinion that the van der Waals force is monotonously increasing during flocculation and coalescence of emulsion droplets is not generally true. The force may be a discontinuous function of film thickness. It is also demonstrated that stable foams may be obtained in solvents in which the added surfactant lacks surface activity. The common stabilizer, a surfactant monolayer, is replaced by a separate phase. Introduction The systematic approach to colloidal stability has developed in a regular manner since the introduction of the DLVO theory some 40years ago.s2 Its basic hypothesis of the distance dependence of two independent interparticle potentials has been applied to polymeric stabil i ~ a t i o n in ~ -a~fruitful manner. The influence of specific chemical species was demonstrated in a large number of cases by Matijevid and collaborators.610 However, in all these investigations the expression for the van der Waals potential delivered the same message. When a thin film becomes thinner, the negative van der Waals potential becomes more negative and the function is monotonous. In this article, the aim is to demonstrate that a whole class of emulsions shows a more complicated distance dependence of the van der Waals potential, in fact the van der Waals force may be a discontinuous function of interdroplet distance. Another generally accepted phenomenon is the relation between foam stability and adsorption of surfactants to the liquid/air interface in the foam thin films. A pure liquid, i.e. a liquid without a layer of surfactant, cannot form a stable f0am.11-13 In fact the stability of the thin liquid film has been smcessfully related to a combination of colloidal forces and surface characteristic^.'^ ~
(1) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the stability of lyophobic colloids; Elsevier: Amsterdam, 1948. (2) Derjaguin, B. Acta Physicochem. USSR 1941, 14, 633. (3) Sato,T.;Ruch,R. Stabilizationof C o l l o i d a l ~ i s p e r s iby o ~Polymer Adsorption; Schick, M. J., Fowkes, F. M., Eds.; Marcel Dekker: New York, 1980. (4) Vincent, B.; Whittington, S. G. In Surface and Colloid Science; MatijeviC, E., Ed.; Plenum Press: New York, 1982; Vol. 12, Chapter 1. (5) Hesselink, F. Th. J. Colloid Interface Sci. 1977, 60, 448. (6) MatijeviC, E. J. Colloid Interface Sci. 1977, 58, 2. (7) MatijeviC, E.; Kratohvil, S.; Stickels, J. J. Phys. Chem. 1969, 73, 3.
(8) MatijeviC, E.; Bleier, A. Croat. Chem. Acta 1977, 50, 1. (9) Eisenlauer, J.; MatijeviC, E. J . Colloid Interface Sci. 1980, 75, 1. (10) MatijeviC, E. Pure Appl. Chem. 1981,53. (11) Clunie, J. S.; Goodman, J. F.; Ingram, B. T. Surf. Colloid Sci. 1971, 3, 167. (12) Bikerman, J. J. Foams; Springer-Verlag: Berlin and New York, 1973. (13) Akers, R. J., Ed. Foams; Academic Press: New York, 1976. (14) Vrij, A.; Hesselink, F. Th.;Lucassen, J.; van der Tempel, M. Proc. K . Ned. Akad. Wet., Ser. B 1970, 73,109.
0743-7463/92/2408-1889$03.00/0
Such an approach leads to problems when the stability of a hydrocarbon foam is explained. A traditional hydrocarbon chain based surfactant cannot lower the surface tension of the hydrocarbon further and will, hence, not be enriched a t the surface. Without a surfactant layer a stable foam cannot be formed. However, extremely stable foams are formed in surfactant/hydrocarbon systems without water15and also in hydrocarbon/surfactant solutions with solubilized water16J7 without a surfactant layer on the hydrocarbon surface to air. Both the discontinuous van der Waals force dependence on film thickness in coalescing emulsion drops and the stable hydrocarbon foams are exception from traditional wisdom and deserve a discussion. van der Waals Potential in Emulsion Flocculation and Coalescence In an emulsion the primary destabilization mechanism is the flocculation followed by coalescence, Figure 1. In the first process the van der Waals force between the flocculating droplets at short distances is a simple function
in which A is the Hamaker constant, a the droplet radius, and d the distance between droplet surfaces. After the flocculation is completed the van der Waals potential is better described by two half-spheres. In a zeroth-order approximation one obtains
Assuming the initial flocculation to happen without distortion for two droplets of radius a = 1 pm and distance between surfaces d = 0.1 pm, a primary estimation gives
F = -1.3 X lo6 A F = -2.6 X lo6 A The essential information from these two extremely approximate numbers is that the van der Waals force is-as (15)Friberg, S. E.; Wohn, C. S.; Greene, G.;Van Gilder, R. J . Colloid Interface Sci. 1984, 101, 2. (16) Friberg, S. E.; Ahmad, S. I. J . Colloid Interface Sci. 1975,35, 1. (17) Friberg, S. E.; Blute, I.; Kunieda, H. Langmuir 1986, 2, 659.
0 1992 American Chemical Society
1890 Langmuir, Vol. 8, No.8, 1992
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A
B
n W
Figure 3. A liquid crystal covering an emulsion droplet and appearing radiantwhen viewed in an optical microscope between crossed polarizers.
'
Figure 1. Primary steps in the destabilizationof an emulsion: flocculation (A); coalescence (B).
Distance between outer &aces
in p m
Figure 2. Two-phase emulsions showing the negative van der Waals force increasing monotonously during film thinning. W
expected-enhanced during the flattening process, Figure 1,and that it will continue to increase faster during the coalescence process according to Figure 2. This is a trend that would be confirmed by more accurate calculations and illustrates the essential message of this article. The van der Waalsforce is monotonously increased during the flocculation-coalescence process. However, this is true only for a two-phase emulsions, and a large number of emulsions in foods,18 cosmetics, and pharmaceutics contain another phase in addition to the traditional oil and aqueous phases. This third phase is often in the form of a lamellar liquid crystal as found by several authors.19,20 This phase is easily detected in the emulsion, because it is birefringent, Figure 3. The van der Waals potential for such a system, Figure (18) Friberg, S. E.; Goubran, R; Kayali, I. Emulsion Stability; Larseon, K.,Friberg, S.E., Eds.; Marcel Dekker: New York, 1990. (19) Friberg, S. E.; Mandell, L.; Lareson, M. J . Colloid Interface Sci. 1%9,29,155. (20) Barry, B.W.Adu. Colloid Interface Sci. 1975,5,37. (21) Groves, M.J.; Yalabik, H. S. Pharm. Technol. 1977,2,21.
W
Figure4. Coalescencebetween twodropletacovered by a lameller liquid crystal modeled by sequential removal of the layers.
4, during flocculation and coalescence will be different from the simple expressions used for a two-phase system. The van der Waals potential during the flocculation process is similar to that in a two-phase system, Figure 5, with the modification caused by the enhanced distance between the droplets. However, the coalescence process is fundamentally different. Instead of a gradual thinning of a liquid film,as is the case in the two-phase emulsion, the coalescence process now consists of the removal of the intervening layers, Figure 4. This change in the mechanism has a profound influence on distance dependence of the van der Waals potential, Figure 5. The distance dependence becomes a function of discrete points instead of a continuous one. In addition, the van der Waals force during the process cannot be expressed as a continuous function, because the coalescence process takes place through stepwise removal of layers. The force
Langmuir, Vol. 8, No. 8, 1992 1891
Amphiphilic Association Structures Distance between outer 8urfaces
4
Two LlquidaJ
.......
fI
7
HYDROCARBON
Two Liquids +
Llquid Crystal
I
Figure 5. Distance dependence of van der Waals potential of a three-phase emulsion with the third phase a liquid crystal (two liquids + liquid crystal) compared with the simple two-phase emulsion (two liquids).
WATER
SURFACTANT
Figure 7. Foam stability found in and only in the two-phase region of inverse micellar solution (4) and liquid crystal (5) in a system of water, oil-soluble surfactant, and hydrocarbon.
I
Flocculation
\
Distance Figure 6. van der Waals force (from the potential in Figure 5 ) is discontinuousfor the three-phase emulsionwhen the flocculated state has been reached and the coalescenceprocess should begin.
Figure 8. Lamellar liquid crystal found at the hydrocarbon solution/gas interface in a foam from the system in Figure 7.
driving this removal is, hence, calculated as the potential difference between two states divided by the distance difference. The expression for multilayered structures with spherical symmetry was built on the approach by Vold.22 It becomes rather cumbersome when developed for a multilayer structure.23 The essential result is that after flocculation the van der Waals force is strongly reduced at the initiation of coalescence. In fact, Figure 6, the force function versus distance becomes discontinuous going from a very high value at the end of the flocculation to an extremely low one for the coalescence. Hence, these emulsions, which are ubiquitous in many applications, do not show a continuous increase of the van der Waals force during the primary destabilization process. It is obvious that a van der Waals force distance dependence with this general shape has a pronounced influence on the colloidal stability of emulsions and that continued experimental and theoretical examinations of the phenomena are justified.
resultsgave simple rules for the tendency to form transient foams of organic solutions or emulsions of two immiscible solvents. The results may be summarized according to the following. For solutions, foaming increases when the composition approaches the limit of solubility. The foaming of the two-phase system takes place only when the phase with the higher surface free energy is the continuous one. For that case, the foamability increases with reduced amounts of the dispersed phase present, so that the foamability is a continuous function of composition across the solubility limit. The part of the compositions, which give the low surface tension phase as the continuous one in the emulsion, no foamabilitywas found. These are foams of transient stability. Long-term stability has so far been found only for systems in which the hydrocarbon solution is in equilibrium with a liquid crystalline ~ h a s e , l Figure ~ * ~ ~7. These results are valid both for water-free systems15and for those in which water is solubilized into inverse micelles in the 0i1.l~ The operative stabilizationmechanism is obvious from microscopy photos of the foams, Figure 8. The liquid crystal is localized at the surface of the hydrocarbon solution toward the air, where it forms a macroscopically thick viscous layer stabilizing the thin film. The stabilization by such a layer is expected and obvious; the essential problem to be examined is the reason for the liquid crystal to intervene between the hydrocarbon
Hydrocarbon Foam Stability The foamability of organic solvents was investigated by Ross et al. in a series of pioneering article^.^^.^ His (22) Vold, M. J. J. Colloid Interface Sci. 1961, 16, 1. (23) Janaeon, P.0.;Friberg, S. E.Mol. Cryst. Liq. Cryst. 1976,34,75. (24) Ross, S.; Niehioka, G. J . Phys. Chem. 1975, 79,1561. (25) R m ,S.; Townsend, D. F. Chem. Eng. Commun. 1981,ll.
(26) Ahmad, S. I.; Friberg, S. E.Acta Polytech. S c a d . , Chem. Technol. Metall. Ser. 1971, No. 102.
1892 Langmuir, Vol. 8,No. 8,1992 solution and the air. The location per se demonstrates that the surface free energy of the lamellar liquid crystal is lower than that of the hydrocarbon solution, but the determination of the surface free energy of the lamellar liquid crystal with precision is not a trivial endeavor. With this problem in mind an indirect method was found to demonstrate the lower surface tension of the liquid crystal. The method17used two hydrocarbons for the foam stability measurement. One hydrocarbon, benzene, has a high surface tension (-30 mN/m) while the other, 2,2,4trimethylpentane, has a lower value (=21 mM/m). The rationale behind this choice is that if the lamellar liquid crystal shows a surface tension below 30 nM/m but in excess of 21 nM/m, foam stability will be observed in the system with benzene but not in the trimethylpentane. This was the case,17which is an indirect proof that the surface tension of the liquid crystal should be substantially lower than 30 mN/m but in excess of 21 mN/m. Accepting this conclusion, it is necessary to find the structural reason for the lowering of the surface tension, when the inverse micellar solution is transformed to a lamellar liquid crystal by the addition of water, Figure 6. The solution to this problem is found in the difference between the two association structures. The inverse micellar is a small “droplet”, the hydrocarbon chains of which are mixed with the hydrocarbon molecules. Hence, the presence of the inverse micelles do not cause a reduction of surface tension. Simplifying the conditions it may be stated that liquid crystalline phase is formed because the addition of water causes ita fraction to be too large to be accommodated in
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the form of inverse micelles; the spatial arrangement of the liquid crystal certainly allows more water per surfactant molecule than that of the inverse micelle. Hence, the function of the added water is to reorganize the association structure of the surfactants. The consequence of this reorganization is that the surface of the new phase is predominantly occupied by methyl groups.n@ Methyl groups in close-packed form expose a surface with a free energy of 23 mN/m, while a surface of methylene groups displays an energy of 28 mN/m.29 The reorganization of the amphiphilic association structure, hence, lowers the surface tension, but not in the traditional manner by a surface-active substance being enriched at the surface but by an orientational change of the direction of a molecule to preferentially expose a low energy part of it at the surface. The phenomenon may aptly be called induced surface activity. It is noteworthy that this change is caused by the addition of water, a substance that by itself posesses a high surface tension.
Acknowledgment. This research was supported in part by the New York State Commissions for Science and Technology through its CAMP program at Clarkson University. ~
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(27) Gruen, D. W. R. Chem. Phys. Lipids 1982,30, 105. (28) Gruen, D. W. R.; de Lacey, E. H. B., In Proceedings of the International Symposium on Surfaces of Solutions; Lindman, B., Mittal, K., Eds.; Plenum Press: New York, 1983. (29) Zisman, W. In Contact Angle Wettability and Adhesions; Gould, R., Ed.; Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964.
