© Copyright 2000 American Chemical Society
FEBRUARY 8, 2000 VOLUME 16, NUMBER 3
Letters Droplet-Droplet Interactions in Both Direct and Inverse Emulsions Stabilized by a Balanced Amphiphilic Polyelectrolyte Patrick Perrin Ecole Supe´ rieure de Physique et Chimie de Paris (ESPCI), CNRS, Universite´ Pierre et Marie Curie (UPMC), UMR 7615, Physico-Chimie des Polyme` res, 10, rue Vauquelin, 75005 Paris, France Received July 9, 1999. In Final Form: November 10, 1999 An original amphiphilic polyelectrolyte was used as an emulsifier of the n-hexadecene-water system. The balanced hydrophile-lipophile properties of the polymeric surfactant allow the preparation of both oil and water continuous emulsions with dispersed phase volume fractions of up to 0.9. The oil and water droplet interactions were investigated by dynamic rheology and optical microscopy. In contrast to oil droplets, water droplets are strongly adhesive so that fractal gels can form in inverse emulsions. Hence, the Princen theoretical model on emulsion elasticity gives adequate fitting of the rheological behavior of direct emulsions but fails in describing the behavior of inverse emulsions.
Emulsions consist of droplets of one liquid dispersed in a second immiscible liquid. Not being at thermodynamic equilibrium, the liquid-liquid dispersion tends to separate over time. Emulsifiers, usually (small molecule) surfactants, are hence added to the system to slow emulsion breaking mechanisms such as coalescence.1 In sharp contrast to surfactants, studies dealing with the behavior of emulsions stabilized by polymeric surfactants are scarce.2-9 As a consequence, we pursue our recent investigations regarding both the stabilization of mac(1) Becher, P. Encyclopedia of Emulsion Technology; Dekker: New York, 1983; Vol. 1. (2) Pons, R.; Solans, C.; Tadros, Th. F. Langmuir 1995, 11, 1966. (3) Pons, R.; Taylor, P.; Tadros, Th. F. Colloid Polym. Sci. 1997, 275, 769. (4) Piirma, I. Polymeric Surfactants; Surfactant Sci. Ser.; Dekker: New York, 1992; Vol. 72. (5) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1. (6) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Nature 1998, 392, 367. (7) March, G. C.; Napper, D. H. J. Colloid Interface Sci. 1977, 61, 383. (8) Marti, S.; Nervo, J.; Riess, G. Prog. Colloid Polym. Sci. 1975, 58, 114. (9) Lochhead, R. Y.; Rulinson, C. J. Colloids Surf., A 1994, 88, 27.
roemulsions by means of amphiphilic polyelectrolytes10-14 and the behavior of these copolymers at interfaces.15,16 For instance, it was shown for the first time that a small amount of associating polyelectrolytes leads to the rapid formation of ordered monodisperse emulsions10 and that the well-balanced hydrophile-lipophile (HL) properties of highly hydrophobically modified polyelectrolytes can adequately be used to monitor emulsion type (oil in water, O/W, or water in oil, W/O) by simply changing external parameters such as pH and ionic strength.13,14 The focus of this paper is the formation of both inverse (W/O) and direct (O/W) emulsions covering a broad range of dispersed phase volume fractions (up to φ ) 0.9). The emulsifier (10) Perrin, P. Langmuir 1998, 14, 5977. (11) Perrin, P.; Lafuma, F. J. Colloid Interface Sci. 1998, 197, 317. (12) Perrin, P.; Lafuma, F.; Audebert, R. Prog. Colloid Polym. Sci. 1997, 105, 2228. (13) Perrin, P.; Monfreux, N.; Dufour, A.; Lafuma, F. Colloid Polym. Sci. 1998, 277, 89. (14) Perrin, P.; Monfreux, N.; Lafuma, F. Colloid Polym. Sci. 1999, 276, 945. (15) Millet, F.; Nedyalkov, M.; Renard, B.; Perrin, P.; Lafuma, F.; Benattar, J. J. Langmuir 1999, 15, 2112. (16) Millet F.; Benattar, J. J.; Perrin, P. Phys. Rev. E 1999, 60, 2045.
10.1021/la990900x CCC: $19.00 © 2000 American Chemical Society Published on Web 01/08/2000
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used in all emulsions is an original amphiphilic polyelectrolyte having balanced HL properties. The oil and water droplet interactions were studied by optical microscopy and dynamic rheology. The linear viscoelastic properties of the dispersions measured as a function of φ clearly indicate that the droplet-droplet interactions encountered in direct and inverse emulsions are of different types. More specifically, the Princen theoretical model17,18 on emulsion elasticity provides a good description of the dynamic rheological behavior of direct emulsions thereby evidencing only repulsive interactions between oil droplets. In contrast, as demonstrated by optical microscopy, the water droplets are adhesive so that the Princen approach fails in describing the elastic modulus-internal phase volume fraction dependence in the case of inverse emulsions. The polymeric surfactant is a hydrophobically modified poly(sodium acrylate) with the following chemical structure:
with t, the degree of grafting, in mol %. Copolymers with a wide range of t were synthesized.11,14,19-21 However, the experiments described in this paper are restricted to the polymeric surfactant with t ) 60% and a molecular weight of 50 000. The modification degree of the polymer was determined by carboxylate group titration, elemental analysis, and 13C NMR spectroscopy.13,14,21 By analogy with the corresponding low hydrophobically modified polymers (typically t e 20%), the chemically grafted alkyl chains of the copolymer with t ) 60% are likely to be “randomly distributed” along the poly(sodium acrylate) backbone.21 As discussed previously,13,14 the amphiphilic properties of this copolymer are balanced so that direct and inverse emulsions (φ ) 0.5) can be obtained at low and high ionic strengths, respectively. This unique feature was further used to prepare both types of emulsion with higher internal phase volume fraction (φ > 0.5) as explained hereafter. The method of preparation of direct (or inverse) concentrated emulsions is described as follows. Concentrated polymer solutions, 4% (weight of polymer/volume of solvent), were prepared by swelling the polymer (0.16 g) in 4 mL of double-distilled deionized water with a Milli-Q system from Millipore (inverse: in 4 mL of n-hexadecene used without further purification, Prolabo). Then, 4 mL of n-hexadecene (inverse: 4 mL of a 3.4 M concentrated NaCl solution) was added to the aqueous phase (inverse: oil phase) prior to a one-step emulsification process. The two phases were mixed using a rotor-stator type of homogeneizer (Heidolph DIAX 600) for 5 min at 24 000 rpm to form direct (or inverse) emulsions with φ ) 0.5. Higher oil (inverse: brine) volume fractions were obtained by progressively adding n-hexadecene (inverse: NaCl solution) to the premixed emulsion (φ ) 0.5) in a way similar to the preparation of a mayonnaise. Added oil (or brine) was fragmented into droplets by stirring the dispersions for 1 min at 8000 rpm, 2 min at 9500 rpm, and 2 min at 13 500 rpm (inverse: 5 min at 8000 rpm). The (17) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427. (18) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160. (19) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1989, 20, 577. (20) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers: Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; p 218. (21) Monfreux, N. Ph.D. Thesis, Universite´ Pierre et Marie Curie (Paris 6), 1998.
Figure 1. Frequency dependence of the elastic modulus (G′) for both direct (A) and inverse (B) emulsions. The internal phase volume fractions are indicated above.
emulsification procedure used here allows the preparation of emulsions with inner phase volume fractions up to 0.9. A large number of direct and inverse emulsion samples with various internal phase contents were prepared. Viscoelastic experiments were performed at 298 K, immediately after the preparation of the samples on a straincontrolled rheometer (Rheometrics RFS II) equipped with a cone-plate geometry (2°, diameter ) 2.5 or 5 cm). As shown in Figure 1, the elastic modulus, G′, was measured for each sample as a function of the frequency, 0.1 < ω (rad/s) < 100, in the linear viscoelastic domain. The lower the inner phase content, the larger the dependence of G′ on frequency, as expected. However, this effect is more pronounced for direct (Figure 1A) than for inverse emulsions (Figure 1B). Since G′ increases regularly with φ, it is worth analyzing our results with the theoretical model of Princen.17,18 According to the model, the elastic modulus is given by
G′ ) aγ/R32φ1/3(φ - φC)
(1)
R32 is the volume-surface mean radius, γ the interfacial tension, and φ the dispersed phase volume fraction. The parameters a and φC, 1.77 and 0.712, respectively, were determined by Princen from experiments on concentrated direct emulsions. φC is related to the close-packing volume fraction of droplets. The increase of G′ with φ originates from an increasing compression of the droplets as the internal phase volume fraction becomes larger than φC. Since the dependence of γ/R32 with φ is not known for this system, the complete analysis of our experimental data using the Princen model (eq 1) was not possible.
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Figure 2. G′/φ1/3 versus φ plots in the case of direct (A) and inverse (B) emulsions. Emulsions were all prepared from φ ) 0.5 premixed emulsions (filled circles) except the one at φ ) 0.3 prepared from a φ ) 0.2 premixed emulsion (filled square).
However, much information can be obtained from the G′/ φ1/3 versus φ plots presented in Figure 2. In the case of direct emulsions (Figure 2A), low and almost constant values of G′/φ1/3 were recorded up to φ ≈ 0.7. Above this threshold value, G′/φ1/3 increases markedly and linearly up to 0.9. These observations are consistent with the model proposed by Princen and indicate that the ratio γ/R32 does not depend on the oil volume fraction. In contrast, high G′/φ1/3 values between 0.5 and 0.7 were measured for the inverse emulsions (Figure 2B). This cannot be explained within the framework of the Princen concept. Consequently, the nature of the droplet-droplet interactions in both kinds of emulsion, direct and inverse, are different even if the same emulsifier was used to stabilize both oil-water and oil-brine interfaces. At this point, it is appropriate to reconsider the ratio γ/R32 in order to continue our discussion. As a result of its well-balanced HL properties, the polymer is not completely soluble in either oil, freshwater, or brine. As a consequence, interfacial and surface tension measurements would certainly be difficult to carry out. To give an estimate of the droplet diameter, a series of pictures of emulsion samples were taken using an optical microscope (Olympus BH-2) equipped with a camera. Characteristic photographs of both direct and inverse concentrated emulsions are given in Figure 3. In fact, it was not possible with this method to observe any significant change in the mean droplet size with increasing φ from 0.5 to 0.9 regardless of emulsion type. However, the diameter of brine droplets was found to be larger than that of oil droplets. With these remarks, we consider that the values of the diameters, D, are 10 ( 5 and 15 ( 5 µm for direct and inverse emulsions
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Figure 3. Concentrated direct (φ ) 0.84) (A) and inverse (φ ) 0.9) (B) emulsions. Optical microscope scale (white bar) is 10 µm.
(0.5 e φ e 0.9), respectively. With Doil ) 10 ( 5 µm, the interfacial tension was calculated according to eq 1 from the linear regression presented in Figure 2A. The reasonable values of γ ) 5.0 ( 2.5 mN/m and φC ) 0.732 indicate that the Princen model nicely describes the behavior of direct emulsions. Consequently, the interactions between oil droplets reduce to droplet compression which can only occur above φ ≈ 0.7. As demonstrated above, inverse emulsions exhibit a completely different behavior that deserves further comments. To better understand the observations of nonvanishing G′ values below 0.7, the properties of emulsions with a brine volume fraction of 0.3 were investigated. The dispersions were prepared from a φ ) 0.2 (instead of 0.5) premixed emulsion using exactly the same emulsification procedure. Optical microscopy photographs (Figure 4) reveal unambiguously the formation of fractal structure resulting from droplet aggregation. The structure is very similar to that observed by Princen and co-workers (Figure 10B in ref 22) for direct emulsions (paraffin oil droplets) stabilized by sodium dodecyl sulfate in the presence of an electrolyte (NaCl, 0.6 M). The contact angles between droplets were found to be very dependent on the type and concentration of added electrolytes, nature of the surfactant headgroup, and temperature.22-24 Water droplets in benzene containing 2% of catafor-02-laurate are also strongly aggregated and exhibit a similar structure.25 Contact angle measurements in thin liquid films separating neighboring droplets22-24 as well as the mechanisms (22) Princen, H. M.; Aronson, M. P.; Moser, J. C. J. Colloid Interface Sci. 1980, 75, 246. (23) Aronson, M. P.; Princen, H. M. Nature 1980, 286, 370. (24) Aronson, M. P.; Princen, H. M. Colloids Surf. 1982, 4, 173. (25) Davies, J. T. Recent Prog. Surf. Sci. 1964, 2, 129.
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Figure 4. Optical photographs of an inverse sticky emulsion with brine volume content of 0.3. Droplet aggregation (A) leads to the formation of fractal gels (B). Pictures were taken at two different magnifications: (A) the scale bar is 10 µm; (B) the scale bar is 50 µm.
of aggregation in sticky emulsions discussed recently26 are far beyond the scope of this paper. Since the elastic modulus of the emulsion is low, we come to the conclusion that for a brine content of 0.3 the fractal gel is weak. Consequently, since the continuous phase exhibits no elasticity, the higher G′ values observed between 0.5 and 0.7 can reasonably be attributed to the formation of stronger gels arising from the decrease of the interparticle distance with increasing φ. At φ ) 0.5, the observation of the gel structure is prevented by the too high droplet concentration. However, the macroscopic aggregates can be visualized when the system is diluted. As a final remark, the linear variation of G′/φ1/3 over the entire range of investigated brine volume fraction (0.5 e φ e 0.9) is obviously an interesting feature. No satisfactory explanation can be given at the present time. Curiously, the (26) Poulin, P.; Bibette, J.; Weitz, D. A. Eur. Phys. J. B 1999, 7, 277.
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expected extra contribution to G′ due to the packing and deformation of droplets above φC is not apparent. To summarize, the work described in this paper gives striking evidence of the great emulsifying potential of macromolecules. Copolymers have the advantage that their amphiphilic properties can continuously be adjusted during synthesis. As a result of this, a balanced polyelectrolyte was synthesized and used as emulsifier of the water-n-hexadecene system. Furthermore, the finetuning of the HL properties of the system achieved by changing ionic strength allows the formation of both oil and water continuous emulsions over a wide range of dispersed phase volume fraction (up to 0.9). In other words, the macrosurfactant used here is able to stabilize the interfaces of both concentrated direct and inverse emulsions. Furthermore, advantage was taken from preparing this original system to compare oil and water droplet interactions. As shown from optical microscopy and dynamic rheology, brine droplets are adhesive and thus start to interact strongly at concentrations far below the close-packing volume fraction. In contrast, oil droplets are not sticky. The interactions in direct emulsions are well-described by the Princen model since they arise solely from the compression of the droplets above the closepacking concentration. To generalize the discussion, this work provides an original model system for studying polymer adsorption at interfaces. The structure of the liquid interface which separates droplets and protects them against coalescence is a fundamental question. Much work still remains to be done to solve the problem. The macroscopic observations presented in this paper are important steps in this direction. First, ionic strength changes emulsion type and hence the spontaneous curvature of the polymeric surfactant,27 a concept which is not very clear in the case of grafted polymer. Second, the structure of the interface prevents oil droplets but not brine droplets from flocculation. Third, regardless of emulsion type, both water and oil thin layers separating two neighboring droplets are resistant enough to support pressure encountered in concentrated emulsions at least up to a dispersed phase content of 0.9. Fourth, since the emulsifier is not soluble either in oil, water, or brine, the polymer “conformation” at interfaces must certainly be considered at different scales ranging from single chains to globules. It is remarkable that the nonsolubility of the emulsifier in the continuous phases does not prevent emulsion stabilization. Acknowledgment. The author thanks Franc¸ ois Lequeux for helpful discussions and critical reading of the manuscript. LA990900X (27) Kabalnov, A.; Wennerstrom, H. Langmuir 1996, 12, 276.
