Adhesion of Water Droplets in Organic Solvent - Langmuir (ACS

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Langmuir 1998, 14, 6341-6343

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Adhesion of Water Droplets in Organic Solvent P. Poulin* and J. Bibette Centre de Recherche Paul Pascal/CNRS, Av A. Schweitzer, 33600 Pessac, France Received February 5, 1998. In Final Form: August 19, 1998 We measure contact angles between adhesive water droplets which are stabilized in oil mixtures by amphiphilic molecules. The energy of adhesion, deduced from the contact angles, depends strongly on the solvent quality; this is controlled by changing the ratio of two different oils. In particular, the energy of adhesion sharply rises when the amphiphiles become insoluble within the oil mixture.

We investigate the attractive interaction between surfactant monolayers that are adsorbed at the interface of water droplets suspended in organic solvent. The attraction results from molecular interactions between the surfactant tails that are in the organic medium. This kind of interactions is involved in the behavior of a wide variety of organic systems such as inverted micelles1 or coated silica particles in organic solvent.2,3 Regimes of attraction were deduced in such systems from lightscattering experiments and phase diagram analysis.1-3 However, the range of interaction that can be probed from such approaches is on the order of kT per colloidal particles, where kT is the thermal energy. In this work, by measuring contact angles between water droplets, we can explore surface attractions in organic solvent that are much stronger, up to kT per adsorbed surfactant molecules. Such limits can be reached by changing the solvent quality. This is achieved by mixing two different oils, a first one in which the surfactant is soluble and a second one in which the surfactant is insoluble. The adhesion between the surfactant monolayers strongly varies as a function of the composition of the oil mixture. The energy of adhesion is essentially zero in good solvent whereas it sharply rises when the amphiphilic molecules become insoluble within the oil mixture. Although involving short molecules, such behavior is somehow reminiscent to the behavior of particles or interfaces that are covered by long polymer chains and that become attractive when the solvent quality is poor.4,5 Finally, we observe that the energy of adhesion keeps increasing well above the solubility threshold of the amphiphile. Water-in-oil droplets of about 50 µm are formed under slow mixing in the presence of amphiphilic molecules. We find that such water droplets can be strongly adhesive without coalescing as already observed.6,7 Figure 1 shows a microscopic picture of such adhesive droplets. This system consists of droplets of salted water (MgSO4 2%) dispersed in a mixture of 70% by weight silicone oil (Poly Dimethyl Siloxane, viscosity of about 3.5 Pa/s) in ether in the presence of 0.2% of egg lecithin (Egg PC), a phospholipid that stabilizes the droplets. Egg PC is insoluble in silicone oil, whereas it is soluble in ether. A large contact angle is observed in Figure 1; this reflects a strong (1) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983, 87, 1023. (2) Jansen, J. W.; DeKruif, C. G.; Vrij, A. J. Colloid Interface Sci. 1986, 114, 471. (3) Jansen, J. W.; DeKruif, C. G.; Vrij, A. J. Colloid Interface Sci. 1986, 114, 492. (4) Klein, J.; Pincus, P. Macromolecules 1982, 15, 1129. (5) Klein, J. Adv. Colloid Interface Sci. 1982, 16, 101. (6) Aronson, M.; Princen, H. M. Nature 1980, 286, 370. (7) Leermakers, F. A. M.; Sdranis, Y. S.; Lyklema, J.; Groot, R. D. Colloids Surf. 1994, 85, 135.

Figure 1. Microscope picture of two adhesive droplets stabilized by Egg PC in a mixture of silicone oil (70% by weight) and ether. The contact angle formed by the droplets is about 40° (white bar 50 µm).

attraction between the facing monolayers that adhere to each other to form a bilayered structure such as in biological membranes8 or in the adhesive films between oil in water droplets.9 To determine the energy of adhesion between the droplets, we measure both the contact angle θ and the oil-water interfacial tension γ. As seen in Figure 1, the contact angle is large enough to be directly measured. However, when the contact angle is lower than 30°, we better determine the contact angle by taking advantage of the transparency of the thin bilayer that forms between adhesive droplets. Indeed, when observed by using direct transmission optical microscopy, the thin bilayer is bright because it is too thin to reflect the light. By contrast, the surrounding isolated interfaces appears darker because they reflect the light. Figure 2 shows a microscopic picture of a collection of adhesive droplets (of about 50 µm in diameter) when the solvent is composed of 50% of silicone oil in ether and still in the presence of 0.2% of Egg PC. Many white (transparent) ellipsoids are visible and consist in the projection of the circular flat bilayers (separating two adhesive droplets) onto the microscope focus plane. By directly measuring, for a pair of adhesive droplets, the two radii of the droplets R1, R2 and the larger axis of the ellipsoidal transparent zone r, we accurately deduce the value of the contact angle from the following expression:10 2θ ) sin-1(r/R1) + sin-1(r/R2) (see Figure 3). We deduce the energy of adhesion ∆F from the Young-Dupre` equation ∆F ) 2 γ(1 - cos(θ)). We perform contact angle (8) Gennis, R. B. Biomembranes; Springer-Verlag: New York, 1989. (9) Poulin, P.; Nallet, F.; Cabane, B.; Bibette, J. Phys. Rev. Lett. 1996, 77, 3248. (10) Princen, H. M. Colloids Surf. 1984, 9, 47.

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Figure 2. Collection of adhesive droplets stabilized by Egg PC in a mixture of silicone oil (50% by weight) and ether. A white ellipsoid can be noticed between superimposed droplets. This ellipsoid is the projection of the adhesive film between the droplets. The contact angle is about 29° (white bar 50 µm).

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Figure 5. Microscope picture of droplets in the presence of a large amount of silicone oil. The droplets are strongly adhesive. Precipitates of surfactant can be clearly seen in the bulk as at the surface of the droplets. In such conditions, it is rather difficult to measure the contact angle (white bar 50 µm).

Figure 3. Schematic representation of two adhesive droplets viewed under perspective as in the experimental picture of Figure 2.

Figure 6. Energy of adhesion between Span 80 monolayers in a silicone oil/dodecane mixture. The arrow indicates the insolubility threshold of the amphiphile.

Figure 4. Energy of adhesion between Egg PC monolayers in a silicone oil/ether mixture. The arrow indicates the insolubility threshold of the amphiphile.

and surface tension measurements for various ratios of ether over silicone oil. We observe that adhesion is essentially absent in ether whereas it is very strong in silicone oil. Figure 4 shows the evolution of the adhesion energy as a function of the oil composition. From 0 to 25% in silicone oil we find a slight increase of the adhesion energy whereas from 25 to 100% there is a much faster rise. We also independently identify the composition of the oil phase at which the amphiphilic molecules become insoluble and find a value of 25%. We show in Figure 5 a microscope picture of adhesive droplets when the solvent contains 95% of silicon oil and few percent of Egg PC. In

these conditions, the surfactant precipitates at the surfaces of the droplets and the droplets exhibit very large contact angles. We have observed similar behavior with sorbitan monooleate (Span 80), another surfactant that allows water droplets to be stabilized in organic solvent. We have used, with this surfactant, a mixture of dodecane (in which Span 80 is soluble) and silicone oil (in which Span 80 is insoluble) as solvent. As shown in Figure 6, we clearly observe the same trend as previously described: for an oil mixture composed of 35% silicone oil in dodecane we both observe the insolubility of Span 80 and a sharp rise of the energy of adhesion. From Figure 4 we extrapolate the energy of adhesion between two egg PC monolayers in pure silicone oil. It should be at least 3 mN/m. Therefore, we can argue about the magnitude of the adhesion energy between two monolayers into a very poor solvent (silicone oil) and found a value of about a few tenths of kT per amphiphilic molecule (by assuming an area per polar head of about 0.5 nm2). Water can be considered as an even worse solvent for the surfactant tails. We can therefore expect that the formation of a bilayer from two monolayers in water should involve energies much larger than kT per molecules. This is in agreement with the fact that Egg PC or Span 80 spontaneously assemble in water under

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the form of lamellar aggregates including unilamellar vesicles and multilamellar vesicles. We believe that the molecular mechanism of this adhesion is mainly entropic in origin. Two types of entropic effects may be evoked: One is a depletion mechanism already reported for colloidal fluids such as micelles or nonadsorbing polymers.11,12 Indeed, the adhesion of monolayers may identically involve the overlap of solvent depleted zones as the adsorbed amphiphile chains do not mix with the solvent. The other consists of structural modifications of the solvent molecules organization at the interfaces as compared to their organization in the bulk. This mechanism was already reported in

order to describe the insolubility of hydrophobic species in water.13,14 Finally, there is adhesion between oil droplets resulting from the adhesion between monolayers which form a bilayer; these phenomena can provide some insights into the mechanisms of cohesion of bilayer vesicles in water. Indeed, the existence of such bilayers and the adhesion in emulsion may have a similar origin. In this respect, it can be speculated that the cohesion of bilayers in water may have an important entropic contribution as well.

(11) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255. (12) LealCalderon, F.; Bibette, J. To be submitted for publication.

(13) Kauzmann, W. Adv. Protein Chem. 1959, 14, 1. (14) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1980.

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