Langmuir 1998, 14, 4011-4016
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Indirect Evidence for Non-DLVO Forces in Emulsions J. Petkov,† J. Se´ne´chal,‡ F. Guimberteau,‡ and F. Leal-Calderon*,‡ Centre de Recherche Paul Pascal, Avenue Schweitzer, 33600 Pessac, France, and Laboratory of Thermodynamics and Physicochemical Hydrodynamics, Faculty of Chemistry, Sofia University, 1 James Bouchier Avenue, 1126 Sofia, Bulgaria Received November 24, 1997. In Final Form: February 20, 1998 We prepare quasi-monodisperse oil-in-water emulsions stabilized by tetradecyltrimethylammonium bromide and observe their behavior at various surfactant and salt concentrations. In the absence of salt, we observe a succession of dispersed and flocculated states as the surfactant concentration is increased. At constant surfactant concentration, increasing the amount of a monovalent salt (KF, KCl, KBr, ...) from 0 to 2 M leads to flocculation followed by a reentrant dispersed state. We examine the influence of the chemical nature of the counterion and temperature on the flocculation and redispersion phenomena. We also investigate the phase behavior of quasi-monodisperse emulsions in the presence of a divalent salt (MgSO4). Our observations reflect the predominance of different non-DLVO surface forces (structural, hydration, “thermal fluctuations” forces) in controlling the stability of emulsions at high salt and/or surfactant concentrations.
1. Introduction Emulsions are colloidal systems made of liquid droplets dispersed in a liquid continuous phase. The emulsification process provides the energy necessary to disperse one liquid into the other and to reach the final metastable state. The presence of surface active species that adsorb on the droplet surfaces may ensure long-term metastability as required for many practical and industrial applications. Once produced, emulsions may exhibit a large variety of both reversible and irreversible transformations. The coarsening of the emulsions is an irreversible transformation that can have two distinct origins: coalescence and Ostwald ripening. Coalescence consists of the rupture of the thin liquid film that separates two droplets in contact. Ostwald ripening consists of the diffusion of molecules through the continuous phase and is driven by the Laplace pressure difference between droplets having different radii. Reversible transformations result from the existence of attractive forces between the droplets or their surfaces. When the attractive interaction is approximately kT, the thermal energy, the emulsions exhibit two-phase coexistence.1-6 When the interaction is much larger than kT, then the emulsion transforms into a gel made of strongly interconnected droplets.6-8 Many different interactions produce reversible transitions, in that droplets weakly or strongly aggregated may be totally redispersed by changing some experimental parameters (composition of the continuous phase, temperature, etc.). A case of special interest concerns the aggregation of charged droplets at high electrolyte concentration. The † ‡
Sofia University. Centre de Recherche Paul Pascal.
(1) Aronson, M. P. Langmuir 1989, 5, 494. (2) Bibette, J.; Roux, D.; Nallet, F. Phys. Rev. Lett. 1990, 65, 2470. (3) Binks, B. P.; Fletcher, P. D. I.; Horsup, D. I. Colloids Surf. 1991, 61, 291. (4) Steiner, U.; Meller, A.; Stavans, J. Phys. Rev. Lett. 1995, 74, 4750. (5) Meller, A.; Stavans, J. Langmuir 1996, 12, 301. (6) Leal-Calderon, F.; Gerhardi, B.; Espert, A.; Brossard, F.; Alard, V.; Tranchant, J. F.; Stora, T.; Bibette, J. Langmuir 1996, 12, 872. (7) Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 981. (8) Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A. Langmuir 1993, 9, 3352.
stability of colloids in the presence of salt is in many respects reasonably well understood in terms of the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory,9,10 which takes into account two components in the total interaction between the particles: a dispersive (London van der Waals interaction) and an electrostatic one. Above a critical electrolyte concentration, irreversible aggregation (coagulation) is predicted. Although some empirical rules that govern the behavior of colloidal dispersions can be reasonably understood in terms of the classical theory, it is clearly insufficient for explaining many experimental facts that we propose to classify in two different categories: (i) One category is redispersion of colloids by dilution (“repeptization”) or inversely by salt addition as observed for charged and neutral latex particles or for hydrophilic silica sols.11-13 Specific ion effects (Hofmeister series) appear: differences between ions of the same valency are made manifest in the critical coagulation concentration.14 (ii) The other category is adhesion of liquid interfaces revealed by the presence of large contact angles between deformable emulsion droplets stabilized by anionic surfactants (with sulfate, sulfonate, or carboxylate polar heads).15 Specific ion effects (which may differ from Hofmeister series) also appear at constant electrolyte concentration, the magnitude of the adhesive energy strongly depends on the type of counterion;16 the adhesive energy decreases with temperature until complete redispersion.16 Observations (i) give indirect evidence of the so-called (9) Derjaguin, B. V.; Landau, L. D. Acta Physicochem. URSS 1941, 14, 633. (10) Verwey, E. J. W.; Overbeek, J. T. G. Theory of Stability of Lyophobic Colloids; Elsevier: New York, 1948. (11) Healy, T. W.; Homola, A.; James, R. O.; Hunter, R. J. Faraday Discuss. Chem. Soc. 1978, 65, 156. (12) Delgado-Calvo-Flores, J. M.; Peula-Garcia, J. M.; MartinezGarcia, R.; Callejas-Fernandez, J. J Colloid Interface Sci. 1997, 189, 58. (13) Frens, G.; Overbeek, J. T. G J. Colloid Interface Sci. 1972, 38, 376. (14) Overbeek, J. T. G. Colloid Science; Elsevier: Amsterdam, 1952; Vol. 1. (15) Aronson, M. P.; Princen, H. M. Nature 1980, 286, 370. (16) Aronson, M. P.; Princen, H. M. Colloids Surf. 1982, 4, 173.
S0743-7463(97)01280-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998
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hydration forces. Such forces arise whenever surfaces are highly hydrophilic or are covered by strongly hydrated molecules. Although the origin of structural forces still remains obscure, they have been, however, measured in detail between mica,17 silica surfaces,18 and liquid foam films.19 Experimentally, they appear to be exponentially decaying with a decay length of the order of 1 nm. The Hofmeister series (or the so-called lyotropic series) reveals the influence of adsorbed ions in regulating structural forces. Indeed, at high salt concentration, hydrated ions strongly bind to the surfaces and give rise to the structural repulsive forces. The repulsion is attributed to the energy needed to dehydrate the bound ions, which presumably retain some of their water of hydration on binding. Direct force measurements reveal that both the strength and range of structural forces increase with the polarizability or equivalently the hydration of the adsorbed ions.20 The efficiency of electrolytes in inducing colloidal aggregation reflects the same order: more hydrated ions are less efficient and produce flocculation at higher electrolyte concentration. For anions the order of effectiveness is F- < Cl- < Br- < I- while for cations it is Li+ < Na+ < K+.21 The origin of observations (ii) still remains an open question. When adhesive, fluid droplets separated by a thin liquid film are deformed exhibiting a nonzero contact angle between the liquid film and its associated Plateau border. This is a consequence of the reduction of the interfacial tension within the film due to strongly attractive interactions. Adhesion between surfactant monolayers was first recognized on soap films.22-25 It has been shown that the same adhesion exists between large direct emulsion droplets in the presence of the same surfactants and salts.15 The large contact angles (and hence the large adhesive energies) observed between oil droplets and their variation with temperature cannot be attributed to van der Waals attraction. Indeed, the same adhesion has been observed in oil/water/air interfaces where van der Waals forces are repulsive.26 The contact angle depends on the nature of the surfactant polar head and on the type of counterion: for sulfate surfactants the contact angle increases from Li to Na and K, while for carboxylate surfactants the order is exactly reversed.16 It is worth noting that the bulk precipitation of these two types of surfactants in the presence of salt identically depends on the type of counterion: for sulfate surfactants the solubility is reduced when changing the salt cation from Li to K and this order is reversed for carboxylate chains.27,28 The reversal in the effect of counterions observed in both the contact angles and the solubility suggests that the adhesion of the monolayers and the bulk precipitation of the surfactants may originate in the same molecular attractive interaction. Moreover, the same correlation between bulk precipitation and monolayer adhesion is observed as a function of temperature: by lowering the temperature ionic surfactants precipitate and the adhesive (17) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (18) Peschel, G.; Belouschek, P.; Mu¨ller, M. M.; Mu¨ller, M. R.; Konig, R. Colloid Polym. Sci. 1982, 260, 444. (19) Clunie, J. S.; Goodman, J. F.; Symons, P. C Nature (London) 1967, 216, 1203. (20) Pashley, R. M.; Quirk, J. P. Colloids Surf. 1984, 9, 1. (21) Israelachvili, J. N. Intermolecular and surface forces; Academic Press: London, 1992. (22) Princen, H. M. J. Phys. Chem. 1968, 72, 3342. (23) Huisman, F.; Mysels, K. J. J. Phys. Chem. 1969, 73, 489. (24) Kolarov, T.; Scheludko, A.; Exerowa, D. Trans. Faraday Soc. 1968, 64, 2864. (25) De Fejter, J. A.; Vrij, A. J. Colloid Interface Sci. 1978, 64, 269. (26) Poulin, P. Thesis, University of Bordeaux I, 1995. (27) Weil, I. J. Phys. Chem. 1966, 70, 133. (28) Goddard, E. D. Croat. Chem. Acta 1970, 42, 143.
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energy between monolayers increases.26 Recently, measurements of the aqueous film thickness between the adhesive monolayers in Newton black films and emulsions have been performed by using neutron scattering.29 They have revealed a two-dimensional structure with a constant thickness of about 32 Å as found in surfactant bulk aggregates again suggesting a unique origin for monolayer adhesion and surfactant bulk precipitation. It is well-known that contact angles rarely form with cationic alkyltrimethylammonium surfactants neither in foam films nor in emulsions.16,30 So far, the behavior of colloids stabilized by such surfactants at very high salt concentration remains essentially unexplored. The strong hydrophilicity of the trimethylammonium polar head (-N(CH3)3+) and the absence of bulk precipitation (at room temperature) in the presence of many different electrolytes make this type of surfactant a good candidate to probe the influence of hydration forces as described in (i). Moreover, quaternary ammonium ions are highly selective with respect to counterion binding,31 and differences in the stability of emulsions are expected by changing the type of counterion. In this paper, we report the flocculation behavior of quasi-monodisperse oil-in-water emulsions stabilized by tetradecyltrimethylammonium bromide (TTAB) as a function of surfactant concentration, salt concentration, counterion chemical nature, and temperature. The phase diagrams exhibit flocculated and reentrant dispersed domains that cannot be accounted for within the frame of the classical DLVO theory. We discuss the origin of the observed phenomena on the basis of surface force measurements and other experimental data reported in the literature. Experimental Section (a) Emulsion Preparation. We used hexadecane oil (Fluka) and tetradecyltrimethylammonium bromide (TTAB, Sigma) to prepare the emulsions. The different salts used, KF, KCl, NaCl, KBr, NaBr, NaI, KI, and MgSO4, were all purchased from Merck. Crude emulsions were prepared by gently shearing a highly viscous surfactant continuous phase together with the oil.32 The initial composition is 80% of hexadecane, 10% of TTAB, and 10% of water (in weight). We first obtained polydisperse emulsions with diameters ranging from 0.1 to 10 µm. A consecutive dilution and fractionation of the emulsions33 resulted finally in quasimonodisperse emulsions. The fractionation method consists in introducing excess surfactant in the continuous phase in order to produce flocculation. Indeed, the presence of a large amount of micelles induces an attractive interaction between the oil droplets, the so-called depletion force,1,2 which may be gradually increased by adding more surfactant. Since the depletion interaction is also proportional to the size ratio between droplets and micelles, the flocculation induced by excess surfactant allows a size separation in an initial polydisperse emulsion. The droplet size distribution was measured by means of a commercial granulometer (Coulter, LS230) and the average diameter was independently checked by dynamic light scattering experiments. Figure 1 shows a typical quasi-monodisperse emulsion with very narrow size distribution obtained after fractionation. The Ostwald ripening is negligible due to the very low solubility of hexadecane in water. Indeed, we did not observe any change in the average droplet size within a period of several months. After the emulsification and fractionation processes, the emulsions are centrifugated and their initial continuous phase is (29) Poulin, P.; Nallet, F.; Cabane, B.; Bibette, J. Phys. Rev. Lett. 1996, 77, 3248. (30) Princen, H. M.; Mason, S. G. J. Colloid Interface Sci. 1965, 29, 156. (31) Kellaway, L.; Warr, G. G. J. Colloid Interface Sci. 1997, 193, 312. (32) Aronson, M. P.; Petko, M. F. J. Colloid Interface Sci. 1993, 159, 134. (33) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474.
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Figure 1. Typical size distribution (granulometer Coulter, LS230) of a quasi-monodisperse hexadecane emulsion obtained by fractionation of a very polydisperse emulsion using the method described in ref 33. The average diameter is around 0.3 µm. Figure 3. Microscopic image of a flocculated hexadecane emulsion stabilized by TTAB. The surfactant concentration is equal to 30 cmc. This image reveals the existence of free droplets coexisting with dense aggregates suggesting a fluid-solid like equilibrium.
Figure 2. Phase diagram of an hexadecane emulsion stabilized by TTAB in the presence of KBr at room temperature (20 °C). The droplet diameter is 0.5 µm and the droplet volume fraction is set to 1% in volume. The surfactant concentration is expressed in terms of the number of cmc’s of TTAB (with appropriate correction due to the salinity). replaced by a solution of known composition. In all the experiments, the droplet volume fraction is set to 1%. (b) Results. We investigate the influence of both electrolyte and surfactant concentrations on the phase behavior of the emulsions stabilized by TTAB. Critical micellar concentrations (cmc) used and given on the abscisse of some figures are properly corrected with respect to the presence of electrolyte. For that purpose a set of measurements of cmc’s at different electrolyte concentrations were done by using the surface tension method (Wilhelmy plate). We detect the state of aggregation of the emulsions by means of a phase contrast optical microscope equipped with a Nomarsky contrast (Zeiss, Axiovert 100). The general behavior of emulsions stabilized by TTAB is given in Figure 2. We report the state of aggregation of quasimonodisperse hexadecane droplets of 0.5 µm in average diameter in the presence of KBr at room temperature (20 °C). When no electrolyte is added, the emulsions remain dispersed between 0.1 and 20 cmc. Flocculation occurs above 20 cmc leading to a macroscopic phase separation of the samples: a dense cream coexists with a dilute milky phase. Before the separation occurs due to gravity effects, the microscope observation (Figure 3) clearly reveals a dynamic exchange between free droplets and the ones trapped in the aggregates suggesting the existence of a fluid-solid like equilibrium.2 Surprisingly, this weak flocculation disappears if an additional amount of surfactant is added to the system above 40 cmc and appears again at concentrations above 100 cmc. If we now consider the influence of electrolyte addition, we observe a strong flocculation above 0.4 M of KBr, whatever the surfactant concentration is. The particles are all entrapped within large and tenuous clusters that fill the whole volume (Figure 4). A gel-like structure rapidly forms after shaking and further contracts upon the effect of gravity. If the cream obtained after a few hours of settling is diluted with water, one gets instantaneously an homogeneous emulsion showing that
Figure 4. Microscope image of a totally flocculated hexadecane emulsion (0.5 µm in diameter). The surfactant concentration is set to cmc and the KBr concentration to 0.75 M. The particles are all entrapped within large and tenuous clusters that fill the whole volume. the flocculation is reversible. Additional experiments with large droplets (more than 50 µm) show that there is no appreciable contact angle between the aggregated droplets thus confirming observations previously reported.16 Finally, increasing the amount of electrolyte above 1.3 M of KBr leads to total redispersion of the emulsion droplets (Figure 2). In Figure 5, we compare the ability of certain counterions to flocculate emulsions stabilized by TTAB. In these experiments, we use the same hexadecane emulsion of 0.5 µm in diameter with a fixed surfactant concentration equal to 3.5 × 10-3 M (1 cmc of TTAB in the absence of salt). The electrolyte concentrations necessary to induce flocculation are sufficiently large to neglect the influence of the bromide ions coming from TTAB. With F- counterion, a weak flocculation is detected between 0.7 and 0.8 M of KF. Unfortunately, it was impossible to explore the same emulsion under the action of NaF because of its low solubility in water. Next counterion Cl- was supplied with two different chloride salts: KCl and NaCl. A broadening of the region where the emulsion is flocculated is observed with no specific effect due to the co-ions. With Br- ions, we follow the same procedure, using KBr and NaBr salts. The tendency of the flocculated region to broaden continues with again no effect of the co-ion. Finally, I- ion was investigated using KI and NaI. Something remarkable in this case is the impossibility to get redispersed emulsions neither by water dilution nor further salt addition once the critical flocculation concentration is reached.
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Figure 5. Flocculated regions of hexadecane emulsion (stabilized by TTAB, 0.5 µm in diameter, droplet volume fraction ) 1%) in the presence of different halogenide ions at room temperature. The lines represent the domains where the emulsion is flocculated. The surfactant concentration is equal to 3.5 × 10-3 M. In the case of iodide, we observe both coalescence and flocculation with no redispersion by water dilution or by salt addition.
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Figure 7. Redispersion temperatures of quasi-monodisperse wax oil emulsion (stabilized by TTAB at 3.5 × 10-3 M, 0.5 µm in diameter, droplet volume fraction ) 1%) as a function of KBr concentration. The measurements are carried out as described in Figure 6.
Figure 8. Magnification of Figure 7 corresponding to the lowest electrolyte concentration. The horizontal dashed lines delimit the domain of melting of the wax oil. Figure 6. Redispersion temperatures of hexadecane emulsion (stabilized by TTAB at 3.5 × 10-3 M, 0.5 µm in diameter, droplet volume fraction ) 1%) as a function of KBr concentration. Starting at room temperature, the emulsion is heated at very low rate (1 °C/mn) and we determine with a microscope the temperature of redispersion. The empty squares stand for the temperature T1 at which redispersion starts. Solid triangles correspond to the temperature T2 at which total redispersion is achieved. A coexisting state is observed between T1 and T2. Moreover surfactant precipitation takes place above 0.02 M KI and partial coalescence of the droplets occurs. We now examine the behavior of flocculated emulsions under the influence of temperature. We perform some experiments in the presence of KBr in the region where the emulsion is flocculated at room temperature, namely, between 0.4 and 1.3 M. We slowly heat the samples (1 °C/min, with a Mettler microscope oven), and at the same time we observe with an optical microscope the changes that take place during this process. Our observations are summarized in Figure 6. The squares denote the temperature T1 at which the flocs start to redisperse, and the solid triangles the temperature T2 at which no more flocs are present. Between T1 and T2 we observe coexistence between flocs and free droplets. The coexistence is not a consequence of a slow redispersion process but can be considered as a fluid-solid thermodynamic equilibrium. Indeed, if a macroscopic sample is stored for a very long period of time (24 h) between T1 and T2, then we observe the coexistence between two phases with different oil volume fractions. As can be seen, the redispersion temperatures increase with the electrolyte concentration. However, once 1.0 M of KBr is reached, the temperatures start to decrease and at 1.3 M KBr they steeply fall down to room temperature. To examine how the physical state of the dispersed droplets (fluid or solid) may affect this temperature-driven transition, we prepare a quasi-
monodisperse emulsion of wax oil of the same average droplet diameter (0.5 µm). The wax oil used (from Merck) is a mixture of different paraffinic chains and has a melting temperature domain between 39 and 43 °C. In Figure 7, we plot the behavior of the wax oil emulsion as a function of salt concentration (KBr) and temperature. We first note a steplike transition around 0.3 M of KBr: the redispersion temperature suddenly jumps and reaches a value corresponding to the melting temperature domain of the wax oil (see Figure 8). Between 0.3 and 0.4 M, T1 and T2 remain close to each other and included in the melting domain of the wax oil. In other words, in this concentration range the emulsion is totally dispersed when the droplets are liquid and aggregated when they are solid. Above 0.4 M KBr, T1 and T2 increase and the coexistence domain located between these two temperatures becomes broader. Finally, up to 1 M, T1 and T2 start to decrease as for hexadecane. To explore the possible role of the surfactant chain length, we also perform experiments with hexadecyltrimethylammonium bromide (HTAB). We find for this surfactant the same phase diagram (surfactant/salt) as for TTAB with approximately the same temperatures of redispersion. This result indicates that the behavior of the surfactant-covered droplets is primarily governed by the trimethylammonium surfactant head and does not depend much on the hydrocarbon chain length. Finally, we investigate the role of counterion valency. The emulsion used this time is made of hexadecane droplets with 0.3 µm in average diameter and the electrolyte used is MgSO4 (a 2:2 symmetrical electrolyte). On the ordinate of Figure 9, the electrolyte concentration is expressed in terms of ionic strength I ) 4CMgSO4, where CMgSO4 is the molar salt concentration. We observe some differences compared with the case where the electrolyte used was 1:1 as KBr. At surfactant concentrations well below cmc, even in the presence of a small amount of
Non-DLVO Forces in Emulsions
Figure 9. Phase diagram of a quasi-monodisperse hexadecane emulsion stabilized by TTAB in the presence of MgSO4 at room temperature (the average droplet diameter is 0.3 µm and the droplet volume fraction is 1%). electrolyte, we detect not only flocculation but also partial coalescence revealed by the presence of a nonnegligible fraction of droplets larger than 0.3 µm. At cmc/2, an addition of MgSO4 (above 0.4 M of ionic strength) leads to coalescence. Surprisingly, an increase of the electrolyte at the same surfactant concentration made the emulsion only flocculated without any coalescence (as deduced after dilution with pure water). The emulsion remains dispersed and stable against coalescence in a domain of relatively low salt and surfactant concentration. A flocculated region (without coalescence) appears for I g 0.5 M, but in this case we do not observe any redispersion by further salt addition even at very high ionic strength. The samples within this flocculated region are weakly aggregated except for 1 M e I e 2 M where they are totally aggregated.
3. Discussion The phase diagrams previously described are quite complex and cannot be totally understood within the frame of the classical DLVO theory. We aim now to discuss some of the most important features of these phase diagrams and to propose some qualitative interpretations based on surface force measurements or other data reported in the literature. The results shown in Figure 2 are a good example for the action of non-DLVO forces. If we consider the row of points where no electrolyte is added, we suspect the occurrence of oscillatory structural forces due to the presence of surfactant micelles. Indeed, such forces may appear when thin liquid films contain tiny colloidal particles such as macromolecules or micelles. Structural forces are a consequence of variations in the density of packing of small particles around a surface on the approach of a second one.34 Theoretical models35 predict that at high particle concentration, the structural forces have an oscillatory profile: the force varies between attraction and repulsion with a periodicity close to the mean diameter of the small particles. At low particle concentration, the structural forces transform into the so-called depletion attraction. In this limit, the force between the surfaces is monotically attractive. The attraction is due to the fact that when two surfaces approach, the small particles are expelled leading to an uncompensated osmotic pressure within the depleted region.1-3 As the particle volume fraction increases, the depletion attraction is still present at short separations and the force begins to oscillate at larger separations, the amplitude of the oscillations increasing with the particle volume fraction. Experi(34) Kjellander, R.; Sarman, S. Chem. Phys. Lett. 1988, 149, 102. (35) Henderson, D. J. Colloid Interface Sci. 1988, 121, 486. Kralchevsky, P. A.; Denkov, N. D. Chem. Phys. Lett. 1995, 240, 385.
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mentally, oscillatory structural forces between mica surfaces in the presence of HTAB micelles have been measured by Richetti et al. using the surface force apparatus technique.36 As predicted, at high enough micellar concentration, they observe the addition of an oscillatory potential to a depletion minimum. The authors suggest the possibility of observing a reentrant transition phase sequence in highly concentrated micellar solutions. Indeed, due to the oscillatory shape of the force profile, the repulsive barrier preceding the depletion minimum may be enough to prevent any flocculation at low micellar concentration, while at higher concentration the preceding minimum due to structural effects might be deep enough to favor a new flocculation. The succession of flocculated and dispersed states that we observe as the TTAB concentration is increased seems to confirm that prediction based on surface force measurements. We now examine the influence of electrolyte at constant surfactant concentration. The flocculation occurring above 0.4 M KBr in Figure 2 is probably driven by van der Waals attraction. However, the redispersion observed at higher electrolyte concentration is another manifestation of nonDLVO forces. One plausible explanation for such redispersion should be the occurrence of hydration forces. If we consider the behavior of the hexadecane emulsion under addition of different electrolytes (Figure 5), we see that there is a correlation between the ability of the counterions to be hydrated and the extension of the flocculated domain. Indeed, the critical flocculation concentration decreases from the most hydrated ion F- to the least hydrated one Br- (we do not consider here the case of iodide counterion for which we observe surfactant precipitation and coalescence). At the same time, the critical redispersion concentration increases resulting in a broadening of the flocculated domain from F- to Br-. Similar results have already been reported by Healy et al.11 who investigated the stability of amphoteric latexes against coagulation in the presence of different monovalent ions at comparatively high ionic strengths. The latexes are extremely stable in the presence of Li+ ions, which are known to be strongly hydrated, and unstable if the Li+ ions are replaced by weakly hydrated Cs+ ions. Our results are also in a good agreement with observations reported by Pashley et al.37 who measured the strength of hydration forces between mica surfaces induced by different cations. The measured strengths for surfaces with adsorbed monovalent and divalent cations follow closely their hydration (Mg2+ > Ca2+ and Li+ ∼ Na+ > K+ > Cs+) and increase with their concentration in water, being zero at low concentrations and becoming important at higher ones. Empirically, the hydration potential between planar surfaces can be approximated by an exponential dependence:21
uhyd ) P0 exp(-h/λ) where h is the distance between the surfaces and P0 and λ are two experimental parameters that characterize the amplitude and range of the repulsion. Delgado et al.12 have calculated the interaction between anionic latexes at high electrolyte concentrations (KBr) using the DLVO theory extended with the expression for the hydration repulsion. Taking P0 and λ values from literature, they show that at very high ionic strength, a repulsive barrier due to hydration forces arises leading to redispersion. (36) Richetti, P.; Kekicheff, P. Phys. Rev. lett. 1992, 68, 1951. (37) Pashley, R. M.; Israelaschvili, J. N J. Colloid Interface Sci. 1984, 97, 446.
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Coming back to our results, the broadening of the flocculated region from F- to Br- (Figure 5) may result from a complex interplay between counterion binding and hydration forces. It is known that the degree of ion binding to quaternary ammonium surfactants is increasing from F- to Br-,31 which could result in lower electrostatic surface potentials and hence lower critical flocculation concentration along this series. Moreover, since both the strength and range of hydration forces increase with the degree of hydration of counterions, one may expect higher critical redispersion temperatures from F- to Br- as found experimentally. The case of the I- ion has to be considered separately since its binding to the surfactant polar head is so strong that it produces surfactant precipitation even at very low electrolyte concentration. The resulting interaction between the droplet surfaces is modified with respect to the previous counterions as revealed by the partial coalescence and also by the irreversibility of the flocculation process. Within the hydration model, the degree of hydration of ions should diminish and thus the total attraction between the oil droplets should increase with temperature. On the contrary, Figure 6 shows that by raising the temperature the system turns from a totally aggregated emulsion into a totally dispersed one. Clearly, this result is not consistent with a purely hydration origin for the repulsive forces. We believe our results originate in the existence of thermal fluctuation interactions.38 The droplet surfaces in emulsions are thermally mobile39 or fluidlike and may be submitted to molecular-scale fluctuations (protusions) and large-scale undulations.21 When two such surfaces approach to each other, their undulations and/or protusions become increasingly confined and a repulsive force arises due to the unfavorable entropy of confinement. Such a repulsion generally increases with temperature21 and is restricted to fluid interfaces. We can see that the redispersion transition observed with hexadecane emulsion (Figure 6) does not occur in the case of wax oil emulsion as long as the droplets are solid (Figures 7 and 8). Indeed, between 0.3 and 0.4 M KBr, the temperature of transition is not continuously changing but steeply jumps and reaches a plateau value that roughly corresponds to the melting temperature of the wax oil. Above 0.4 M KBr, the temperature of transition becomes larger than the melting temperature and the behavior of the wax liquid droplets becomes identical to that observed with hexadecane droplets. The redispersion occurring at high temperature with liquid droplets and the absence of (38) Israelachvili, J. N.; Wennestro¨m, H. Langmuir 1990, 6, 873. Israelachvili, J. N.; Wennestro¨m, H. J. Phys. Chem. 1992, 96, 520. (39) Gang, H.; Krall, A. H.; Weitz, D. A. Phys. Rev. E 1995, 52, 6289.
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redispersion with solid droplets strongly suggest the existence of thermal fluctuation interactions in emulsions. Our results are also consistent with measurements of the short-range repulsion performed between other interfaces such as phospholipid bilayers:40 on increasing the temperature from the frozen to the liquid state of the chains, the repulsion between bilayers increases. In the case of MgSO4 (Figure 9), no redispersion phenomena were detected although the sulfate ion is strongly hydrated. The possible reason might be the interplay between classical double-layer forces (i.e., electrostatic and van der Waals), short-range hydration interaction, and ionic-correlation attractive interaction.41,42 This attraction originates in the energy of formation of the counterion atmosphere around each ion. Theoretical calculations show that ion correlation effects do not contribute much in the presence of 1:1 electrolyte to the overall interaction potential energy. If, however, the electrolyte is multivalent like MgSO4, they may become important and produce flocculation.41 4. Conclusion In this paper, we have examined complex systems and given another indirect evidence for non-DLVO forces that exist in emulsions. Different phase diagrams have been described in which flocculated emulsions are surprisingly redispersed when salinity, temperature, or surfactant concentration increases. In particular, we have shown how the microscopic properties that govern the hydration of counterions and their binding to the surfactant polar heads may be used to induce colloidal phase transitions. Our qualitative discussion of the results was tied to published measurements of surface forces. More experimental and theoretical work is still necessary to confirm and quantify the forces that are at the origin of the observed phenomena. The striking stability obtained with some surfactant/salt associations may be useful for practical applications where high salinity is required, and we hope our results will provide some guidance in the field of emulsion formulation. Acknowledgment. The authors gratefully acknowledge Dr. P. Poulin and Dr. O. Mondain-Monval for fruitful discussions. LA9712808 (40) Marra, J.; Israelachvili, J. N. Biochemistry 1985, 24, 4608. (41) Attard, P.; Mitchell, D. J.; Ninham, B. W. J. Chem. Phys. 1988, 89, 4358. (42) Kralchevsky, P. A.; Paunov, V. N. Colloids Surf. 1992, 64, 245.
