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Anatomy of a Coacervate Fredric M. Menger* and Bridget M. Sykes Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received February 20, 1998. In Final Form: May 28, 1998 Coacervates, prepared by adding inorganic and organic salts to aqueous solutions of Aerosol OT (AOT), were analyzed for their AOT, salt, and water content. In addition, the volumes of the coacervates generated under standard conditions were determined. It was found that for inorganic salts, there is a “critical salt concentration” below and above which the volume of coacervate is very small. For example, NaCl has a critical concentration of 0.3 M, a concentration that will convert 50 mL of 0.02 M AOT into more than 7 mL coacervate (the remainder being equilibrium liquid that floats above the immiscible coacervate phase). At 0.2 or 0.6 M NaCl, the coacervate volume is reduced to 2 mL or less. The coacervate formed at the critical salt concentration has an [AOT] ) 0.2 M and a [NaCl] ) 0.3 M. Thus, a coacervate of 0.2 M AOT and 0.3 M NaCl is immiscible with 0.3 M NaCl despite the highly aqueous nature of both. This is attributed to the enthalpic requirements for breaking up three-dimensional AOT structures composed of bilayers. Critical concentrations of the alkali metals Li+, Na+, K+, Rb+, and Cs+ are 1, 0.3, 0.1, 0.1, and 0.1 M, respectively. Organic coacervators, such as n-octylammonium chloride and used in place of NaCl, have a profound effect upon the coacervation process. Thus, n-octylammonium ion has no critical salt maximum. Instead, the coacervate volume remains small until about 0.25 M added salt, whereupon the insoluble layer increases dramatically in volume to become the dominate phase. Coacervation is rationalized in terms of positiveto-negative changes in the spontaneous mean curvature of bilayers.
Introduction Coacervates are among the more exotic systems in colloid chemistry. The word derives from the Latin “co” (together) and “acerv” (a heap). “Acerv” refers to the colloidal molecules that phase-separate from an aqueous medium during formation of a second aqueous layer. Since the second colloid-rich layer (the “coacervate”) is immiscible with the colloid-poor supernatant (the “equilibrium liquid”), there exists a remarkable situation in which two largely aqueous layers do not freely mix. A coacervate is, by definition, incompatible with its own solvent. A Dutch chemist, Bungenberg de Jong, must be considered the father of coacervate chemistry.1,2 Indeed, he coined the term. He also recognized the relevance of coacervates to biology, writing in 1936 a monograph entitled “Les Coacervates et Leur Importance en Biologie”. But it was Oparin, a Russian biochemist, who popularized coacervates among those outside the colloid arena; he proposed that life first formed in coacervate droplets.3 Coacervate research in the 20th century, which has included several decades of relative inactivity, is summarized in several lead references4-11 and in the papers cited therein. For the most part, information on coacervates is distributed among the colloid, polymer, physical chemical, and pharmaceutical literature. (1) de Jong, H. G. Bungenberg; Kruyt, H. R. Kolloid Z. 1930, 50, 39. (2) de Jong, H. G. Bungenberg In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1949; Vol. 1. (3) Oparin, A. I.; Gladilin, K. L.; Kirpotin, D. B.; Chertibrim, G. V.; Orlovsky, A. F. Dokl. Acad. Nauk. SSSR 1977, 232, 485. (4) Sperber, G. O. Acta Physiol. Scand. 1977, 99, 129. (5) van Oss, C. J. J. Dispersion Sci. Technol. 1988, 9, 561. (6) Li, Y.; Dubin, P. L.; Havel, H. A.; Edwards, S. L.; Dautzenberg, H. Langmuir 1995, 11, 2486. (7) McMullen, J. N.; Newton, D. W.; Becker, C. H. J. Pharm. Sci. 1982, 71, 628. (8) Voorn, M. J. Recl. Trav. Chim. Pays-Bas 1956, 75, 1021. (9) Stassen, S.; Nihant, N.; Martin, V.; Grandfils, C.; Je´roˆme, R. Teyssie´, Ph. Polymer 1994, 35, 777. (10) Burgess, D. J.; Kwok, K. K.; Megremis, P. T. J. Pharm. Pharmacol. 1991, 43, 232. (11) Smith, A. E.; Bellware, F. T.; Silver, J. J. Nature 1967, 214, 1038.
Four specific examples of coacervates serve to define further this strange form of matter: (1) Aqueous solutions of long-chain quaternary ammonium salts, above their critical micelle concentration, can separate into a coacervate layer upon addition of inorganic salts.12 (2) Benzyltriphenylphosphonium chloride (1.0 mM) mixed in water with sodium dodecyl sulfate (0.1-0.5 mM) forms insoluble droplets that, upon microscopic examination, are found to contain “vacuoles” of equilibrium liquid.13 The droplets coalesce into a homogeneous layer on standing. (3) Acacia (a polysaccharide) mixed with gelatin at pH ) 3-5, where the former bears a negative charge and the latter a positive charge, produce a coacervate.14 (4) Interaction in water between trimethyldodecylammonium bromide and amaranth (an azo dye with three sulfonate groups) leads to a gelatanous red coacervate.15 Although the distinction is fuzzy, coacervates have been subdivided into “simple” and “complex”. In simple systems, addition of salt promotes coacervation as in the first example above. In complex systems, two oppositely charged species (often polyions) are involved in an associative phase-separation that is inhibited by salt.16 The final three of the above examples fall into this category. With both simple and complex coacervates, the key question is the same: How does intermolecular association or “internal adherence”, which purportedly exists among the colloidal components of a coacervate,4 impede or prevent the dilution of the coacervate? We selected for our investigations a simple coacervate: Aerosol OT (or “AOT”) plus NaCl or organic salts in water. By focusing on a simple coacervate, we avoided the uncertainties associated with the ill-defined structures of polyions such as acacia and gelatin. On the other hand, (12) Cohen, I.; Vassiliades, T. J. Phys. Chem. 1961, 65, 1774. (13) Mubzhager, G. I.; Davis, S. S. J. Colloid Interface Sci. 1978, 66, 110. (14) Burgess, D. J.; Carless, J. E. J. Colloid Interface Sci. 1984, 98, 1. (15) Barry, B. W.; Russell, G. F. J. J. Pharm. Sci. 1972, 61, 502. (16) Overbeek, J. T. G.; Voorn M. J. J. Cell. Comp. Physiol. 1957, 49 (suppl. 1), 7.
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extensive prior work17-20 has shown that the AOT/NaCl/ H2O system is anything but simple. For example, a ternary phase diagram possesses multiple zones including three isotropic liquid phases, a lamellar liquid crystalline phase, a viscous isotropic cubic phase, and a reversed hexagonal liquid crystalline phase.18 As many as 15 phases have been detected at the AOT corner of the phase diagram alone! In more recent times, the word “coacervate” has been lost in this sea of phases. “Coacervate” has seemingly become replaced by the so-called “anomalous” or “L3” phase.19,20 In an L3 phase, AOT does not form discrete micellar aggregates but, instead, assembles into interconnected bilayers to create a spongy, spacefilling structure. More will be said of the L3 phase later when we draw on the work of others to supplement our own. Our approach was somewhat different from those of the past. Thus, both the equilibrium liquid and the coacervate phases were analyzed for AOT (spectrophotometically), for NaCl (with an ion-selective electrode), and for water (via Karl Fisher titrations). These analyses, plus the measured volume of the coacervate, enabled us to characterize each system with seven numbers as various parameters were systematically varied. For reasons of history and euphony, the word “coacervate” was retained throughout the text. Experimental Section Purification of AOT. Aerosol OT, supplied by Fisher, was purified by dissolving 60 g of the compound in 150 mL of hot methanol. The cloudy mixture was filtered, and the filtrate was stirred with 1 g of decolorizing charcoal. After another filtration, the methanol was removed under reduced pressure to produce a white solid that was dried for 24 h in a vacuum desiccator over P2O5. The critical micelle concentration (cmc) of the purified AOT, determined by tensiometry, agreed with the value of 2.5 mM (aqueous solutions, ambient temperature) reported in the literature.21 Purification of Amine Salts. Alkylammonium chlorides (purchased or prepared from the amine plus aqueous HCl) were crystallized from Et2O/MeOH and dried under reduced pressure. Determination of AOT. The concentration of AOT in the phases was determined from the absorbance of the solution at 230 nm, relative to pure solvent, where AOT has an extinction coefficient of 86.1 mol-1 cm-1 (H2O) and 100.1 mol-1 cm-1 (MeOH). Measurements were carried out using a Varian DMS 200 spectrophotometer with 1 cm quartz cuvettes. In a few samples (e.g., when the AOT concentration was very low in the equilibrium liquid), the presence of added salts interfered with the spectral detection of AOT. Determination of Cl- and Br-. Concentrations of Cl- and Br- were determined with the aid of ion-selective electrodes and a double-junction reference electrode fixed to a pH meter. The (17) Acharya, R.; Ecanow, B.; Balagot, R. J. Colloid Interface Sci., 1972, 40, 125. (18) Fontell, K. In Colloid Dispersions and Micellar Behavior; American Chemical Society Symposium, Series 9; American Chemical Society: Washington, DC, 1975; p 270. (19) Skowi, M.; Marignan, J.; May, R. Colloid Polym. Sci. 1991, 269, 929. (20) Balinov, B.; Olsson, U.; So¨derman, O. J. Phys. Chem. 1991, 95, 5931 and references therein. For a recent review, see: Laughlin, R. G. In Micelles, Microemulsions, and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; pp 73-99. (21) Williams, E. F.; Woodberry, N. T.; Dixon, J. K. J. Colloid Sci. 1957, 12, 452.
Menger and Sykes electrodes were always calibrated using a series of sodium halide standards prior to taking measurements. Coacervate and equilibrium liquid samples were diluted with deionized water to bring the concentrations within range of the electrode calibrations. When the AOT concentration was high relative to that of the halide ion, the readings showed some instability leading to an uncertainty of about (5% in the data. Determination of H2O. The concentration of water in coacervate samples was measured by injecting a 5 µL sample into a Fisher coulomatic titrimeter, model 447. Determination of Alkylammonium Chlorides. The concentrations of n-octylammonium chloride were determined by the precipitation-titration method of Metcalfe et al.22 using tetraphenyl borate. The endpoint of the titration was detected conductometrically. Certain samples containing alkylammonium chloride were water insoluble in which case an NMR method was used to obtain [AOT]/[RNH+3] ratios. Thus, the AOT peak area at 4 ppm of the -CH2-OCO protons vs the peak area at 8 ppm of the alkylammonium protons were compared with calibration plots based on known [AOT]/[RNH3+] ratios. Preparation of Coacervates. Coacervates were prepared by addition of weighed amounts of solid NaCl or alkylammonium chloride to a given volume (generally 50 mL) of aqueous AOT in a 50 mL conical, graduated centrifuge tube. Samples were capped and shaken following the addition of salt and then centrifuged (IEC benchtop clinical centrifuge) to hasten phase separation. Phase separation was considered to be complete when the volumes of the coacervate and equilibrium liquid remained constant over several days. Care had to be exercised in selecting centrifugation times. Thus, extended centrifugation (12-24 h) resulted in the formation of inhomogeneous coacervate phases. For this reason the centrifugation time was limited to 1 h followed by 23 h of thermostating at 20.0 °C in a water bath. When this protocol was followed, the water concentration of a coacervate was independent of the location within the tube from which the test sample was removed, an indication of coacervate homogeneity.
Results and Discussion Inorganic Coacervators. The definitive work on phase equilibria in the AOT/water/salt system, carried out by Fontell,18 revealed the following as quoted from his summary: “Addition of sodium chloride to the binary system of di-2-ethylhexylsulfosuccinate (Aerosol OT) and water causes the occurrence of a new phase in addition to those already existing in the system. The new phase has an isotropic liquid structure. Its region of existence is a narrow band running parallel with the water-Aerosol OT axis of the triangular phase diagram. Its content of sodium chloride amounts to about 1.6%, while the content of Aerosol OT ranges from about 3 to 58% and the content of water from about 40 to 95%. The appearance of the phase is a somewhat oily liquid which exhibits thixotropic behavior. At high contents of water the phase has a bluish tint. The structure of the colloidal aggregates of the phase is still not established.” Fontell determined that the molecular composition at the water-rich end of the extremely narrow coacervate zone contains 1 mol of AOT, 2.8 mol of NaCl, and 600 mol of water.18 This corresponds to 0.092 M AOT and 0.26 M NaCl. The phase is in equilibrium with aqueous sodium chloride solutions of concentrations between 0.17 and 1.4 M. It was this dilute coacervate region that interested us. How could an aqueous solution of 98% water be immiscible with another aqueous solution also of > 98% water? This question prompted us to examine AOT/water/ NaCl coacervates more closely, and we report the results herein. As will be seen, our approach was much like that of a physical organic chemist with more emphasis on two(22) Metcalfe, L. D.; Martin, R. J.; Schmitz, A. A. J. Am. Oil Chem. Soc. 1996, 43, 355.
Anatomy of a Coacervate
Figure 1. Coacervate volume as a function of total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
variable graphs and tables than on phase diagrams. The former have certain advantages; for example, they can include important volume data usually absent in phase diagrams. A series of solutions was prepared having a volume of 50 mL and a constant AOT concentration of 0.020 M. To these was added solid NaCl such that the total salt concentration varied from 0.1 to 1.4 M. An NaCl-induced phase separation occurred with the AOT-rich coacervate phase at the bottom and the equilibrium liquid at the top. After the volumes of the phases became constant with time, the concentrations of AOT, Cl-, and water were measured at 20 °C along with the volume of the coacervate. It was found that the coacervate volume reached a sharp maximum of 7.4 mL at [NaCl] ) 0.3 M (Figure 1). It was as if there were a “critical NaCl concentration” above and below which coacervates tend to form only with low volumes or not at all. Such information, incidentally, is usually obscure in articles dealing with phase diagrams. In any event, one is struck by the remarkably low concentrations of AOT and NaCl (0.02 and 0.3 M, respectively) that can cause water to separate into two layers that do not mix. As detailed in the Experimental Section, we measured the AOT, Cl-, and water concentrations in both the coacervate and equilibrium liquid phases. Figures 2, 3, and 4 show how the [AOT]coac, [Cl-]coac, and [H2O]coac, respectively, vary as the amount of added NaCl was increased. Once again, the [AOT] total was initially fixed at 0.020 M in 50 mL total volume at 20 °C. In each of the three plots the critical [NaCl] of 0.3 M is marked with an arrow. Figure 2 shows a smooth increase in the [AOT]coac, varying from 0.02 to 1.3 M, as the total [NaCl] was increased from 0.2 to 1.2 M. Interestingly, at the [NaCl]crit of 0.3 M, where the volume of coacervate is at its maximum, the [AOT]coac is only about 0.15 M. At greater additions of NaCl (0.4-1.2 M) there is less coacervate, but the coacervate that does form has a considerably enhanced AOT content. Figure 3 is a plot of [Cl-]coac vs [NaCl]total. In contrast to the previous AOT plot, the NaCl concentration among the various coacervates varies little (only from 0.23 to 0.35 M). Thus one can make a relatively large volume of coacervate (i.e., at the [NaCl]crit) with a composition of 0.2 M AOT and 0.30 M NaCl. Or one can make a small volume
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Figure 2. Concentration of AOT in the coacervate as a function of total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
Figure 3. Chloride concentration in the coacervate as a function of total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
of coacervate with a composition of, for example, 1.3 M AOT and 0.35 M NaCl. Note that all coacervates have a salt concentration approximating that found at the critical point. Figure 4 gives the water concentration in g L-1 in the coacervate phase as the total NaCl added to the initial 50 mL of 0.02 M AOT is systematically increased. As one would expect from the Figure 2, the water content continuously decreases from about 950 g L-1 at 0.2 M AOT (0.2 M added NaCl) to 500 g L-1 at 1.2 M AOT (1.2 M added NaCl). An interesting behavior can be deduced from Figures 1 and 2: The amounts of AOT expelled from the water, with different amounts of added NaCl, are relatively constant. For example, 0.3 M [NaCl]total creates a coacervate with a volume of 7.4 mL and with an [AOT]coac of 0.18 M. In contrast, 1.2 M [NaCl]total creates a coacervate with a volume of only 1.0 mL but with an [AOT]coac of 1.3 M. Since (7.4 × 0.18) ) (1.0 × 1.3), there is a tendency for the salt, regardless of its concentration, to cause the phase-separation of a given quantity of AOT. Whether this observation can be expanded into a generalization is
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Figure 4. Water concentration in the coacervate as a function of total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
not known. One can, however, answer affirmatively to the question of whether the constant amount of AOT is qualitatively understandable: The greater the amount of added NaCl, the stronger the hydrophobic association of the AOT into the three-dimensional structure that obviously pervades the coacervates. But, in addition, the greater the amount of added NaCl, the more difficult it is to create a new phase with the low 0.3 M NaCl “desired” by the coacervate. Thus, a compromise is reached. At a high [NaCl]total, the volume of the coacervate is markedly diminished, but this is accompanied by a high concentration and degree of aggregation. The explanation is vague and, no doubt, incomplete, yet it has an appealing ring of truth. Note that in all coacervates formed above [NaCl]crit, the [AOT]coac exceeds the solubility of AOT in pure water (ca. 0.04 M). We now move on to the less dense equilibrium phase. The volumes of this phase are not listed or graphed here, but they can be easily calculated by subtracting the coacervate volumes in Figure 1 from 50 mL. The concentration of water in all the equilibrium liquids was essentially 1000 g/L (i.e., within experimental error of pure water). Figures 5 and 6 depict the dependence of [AOT]equil and [Cl-]equil on the [NaCl]total, respectively. In summary, the plots show that as the [NaCl]equil increases linearly in the equilibrium phase, the AOT diminishes precipitously at or near the [NaCl]crit. Little AOT remains in the equilibrium liquid above 0.4 M added NaCl. Visually, both the coacervate and equilibrium phases are clear and homogeneous. At [NaCl]total ) 0.30 M, the coacervate has a particularly noticeable blue sheen. When [NaCl]total ) 1.4 M, a waxy solid phase rather than a coacervate separates from the aqeous solution. Fontell18 called this waxy solid a “viscous isotropic phase” and proposed that its structure consisted of short, rod-shaped aggregates that form two independent but interpenetrating networks. We did not investigate the material further. The time required for complete phase separation is dependent upon the total concentration of salt; the more [NaCl]total exceeded [NaCl]crit, the faster the phase separation occurred. This, of course, is related to the greater densities for the more concentrated coacervates (e.g., 1.03 and 1.10 g/mL for coacervates made from an [NaCl]total of 0.3 and 1.0 M, respectively). Coacervate formation was observed “in situ” by light microscopy via the addition of a small amount of solid NaCl to 0.02 M AOT placed on a
Menger and Sykes
Figure 5. Concentration of AOT in the equilibrium liquid as a function of the total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
Figure 6. Chloride concentration in the equilibrium as a function of the total concentration of NaCl added to 50 mL of 0.020 M AOT at 20 °C.
slide. Many small droplets were seen teeming away from the surface of the NaCl crystals. These droplets proceeded to coalesce into increasingly larger drops that ultimately settled into large coacervate “globs” on the bottom of the slide. The experiments were also carried out in a different mode: Coacervates were prepared by adding varying amounts of AOT to an initial volume of 50 mL of 0.30 M NaCl. Again, the concentrations of AOT, Cl-, and water were measured. It was found that the coacervate volume was directly proportional to the [AOT]total (Figure 7). Levels of AOT, Cl-, and water were relatively constant as the [AOT]total was varied from 0.005 to 0.025 M. Thus, [AOT]coac, [Cl-]coac, and [H2O]coac remained at roughly 0.15 M, 0.26 M, and 925 g/L, respectively. In summary, only the coacervate volume was responsive to the initial AOT concentration prior to addition of salt; all components within those varying volumes of coacervates assumed concentrations observed at the [NaCl]crit (Figure 2). Stated in another way, the amount of AOT in the coacervate depends on the total concentration of added salt rather than upon the ratio of salt to AOT.
Anatomy of a Coacervate
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Table 1. Critical Salt Concentrations and Concentrations of AOT, Cl-, and Water in the Coacervate Phase Induced by Various Alkali Metal Salts; Estimated AOT Concentrations in the Equilibrium Phase are also Listed salt
[MCl]crit, M
[AOT]coac,a M
[Cl-]coac, M
[H2O]coac,a g/L
[AOT]equil, M
LiCl NaCl KCl RbCl CsCl
1 0.3 0.1 0.1 0.1
0.21-0.97 0.088-1.3 0.095-1.2 0.097-1.3 0.17-1.0
0.92 ( 0.02 0.31 ( 0.01 0.15 ( 0.03 0.084 ( 0.01 0.12 ( 0.008
890-595 933-505 928-464 924-498 929-558
(2.8 ( 0.8) × 103 (6 ( 1) × 10-4 (8.1 ( 0.7) × 10-4 b b
a Values represent a range of concentrations found over addition of 0.2-1.4 M salt. b Not quantified owing to interference in spectral determinations by the metals.
Figure 7. Coacervate volume as a function of the total concentration of AOT added to 50 mL of 0.030 M NaCl at 20 °C.
Values of the surface tension of the coacervates are typical of aqueous solutions of AOT above its cmc: 27 ( 0.5 dyn/cm regardless of the AOT content. The temperature stability of the coacervates was examined briefly. Thus, a coacervate was prepared from [AOT]total ) 0.02 M and [NaCl]total ) 0.4 M at 20 °C. The coacervate was then separated from the upper equilibrium liquid (with the aid of a syringe) and placed in a thermostated test tube. Changes in this isolated coacervate with temperature were noted: 2 °C, a thick white precipitate suspended in clear liquid; 3-14 °C, two clear liquid phases; 24-30 °C, one homogeneous, clear liquid phase; 40 °C, two clear phases, the lower phase being a loose gel and the upper phase a free-flowing liquid; 61 °C, two clear phases, the bottom one being highly viscous. Although we do not understand the structural basis of these observations, they serve to emphasize, as do the phase diagrams, the extreme complexity of the situation. Since the total NaCl concentration added to the solutions dictates the AOT content of the resulting coacervate, it was worthwhile to inquire as to how coacervation depends on the type of salt. For this reason we investigated the ability of LiCl, KCl, RbCl, and CsCl to induce coacervate formation. Experiments were carried out as before: varying amounts of salt were added to 50 mL of 0.020 M AOT. Once the volumes of the resulting two phases were constant, the concentrations of AOT, Cl-, and water were determined. The 0.020 M Na+ present in all the solutions was considered insignificant relative to the great excess of other cations. It was found that each alkali metal has a critical concentration at which the coacervate volume reaches a sharp maximum (Figure 8). The plots of volume vs [MCl]total show that the critical concentrations required
Figure 8. Coacervate volume as a function of total concentration of LiCl, NaCl, KCl, RbCl, or CsCl added to 50 mL of 0.020 M AOT at 20 °C.
to induce significant coacervation in 0.020 M AOT decreases in the series Li+ > Na+ > K+ ≈ Rb+ ≈ Cs+. Table 1 enumerates the various critical concentrations (varying from 1 M for Li+ to 0.1 M for the heaviest three metals). Table 1 also shows that the concentration of [AOT]coac covers similar ranges for the five metals. And the cation concentration within the coacervates always approximates the critical concentration no matter what the particular volume or AOT content of the coacervate. Dicationic metals (i.e., Mg2+ and Ca2+) caused thick white precipitates to fall from solution. Differing volumes of 5.5 M HCl were added to 50 mL aliquots of 0.020 M AOT in volume-calibrated centrifuge tubes. Coacervation occurred from initially cloudy solutions. Two points are worthy of mention: (a) The coacervate volume decreased uniformly from 13 mL to about 1.8 mL as the [HCl]total increased from 0.02 to 0.6 M. If HCl has a critical concentration, it must be less than 0.02 M (which indicates that H+ is a far better coacervant than any of the metals). (b) The coacervates are unstable, slowly disappearing with time. Undoubtedly, an acid-catalyzed hydrolysis of AOT ester moieties brings about the destruction of the coacervates. NaCl and NaBr behave identically with regard to their ability to induce coacervates of given concentrations. This affirms the above conclusion that coacervation is in large measure an effect of cations. Listed below are the main features of the L3 or “anomalous” phase that we equate with the coacervate. The information has been drawn heavily from papers by Olsson et al.20 and Anderson et al.23 in particular. (23) Anderson, D.; Wennerstro¨n, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243.
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(1) The L3 phase is an isotropic liquid in which the surfactant bilayer constitutes the main structural unit. Field gradient spin-echo data20 are inconsistent with a model based on discrete micellar aggregates. (2) Bilayer assemblies extend “infinitely” and are highly interconnected. L3 might be considered a disordered lamellar phase where the normally flat bilayer sheets have a high density of defects including saddlelike deformations. There is no long-range order. (3) L3 is bicontinuous in that it separates the solvent into two domains. (4) Interactions among the various bilayer sections are weak, thereby inhibiting short interbilayer distances found in the bilayer stacks of the lamellar phase. (5) The main impetus for L3 formation can be understood in terms of a change in the spontaneous mean curvature, Ho. Thus, in the absence of salt, repulsive electrostatic forces among the headgroups favors a positive Ho (i.e., a curvature “toward the apolar region” as shown in the bilayer leaflet below). When salt is added, the electrostatic contribution to the curvature free energy diminishes, and Ho is driven to negative values (i.e., a curvature “toward the solvent” as also shown below). The positive-to-negative change in Ho has less to do with entropic considerations than with the surfactant seizing the opportunity to obtain optimal curvature. In the AOT case in which the [NaCl] ) ∼0.3 M, spontaneous curvature is expected to be toward water over the whole L3 stability range.
Menger and Sykes Table 2. Critical Salt Concentration and Concentrations of AOT, Cl-, and Water in Coacervate Phase Induced by Three Ammonium Salts salt
[MCl]crit, M
NH4Cl C3H7NH3Cl (C2H5)3NHCl
0.3 0.075 0.45
[AOT]coac,a M
[Cl-]coac, M
0.125-1.12 0.25 ( 0.03 0.040-1.24 0.055 ( 0.009 0.043-1.17 0.7 ( 0.2
[H2O]coac,a g/L 914-492 966-134 900-299
a Values represent a range of concentrations found over addition of 0.06-0.45 M salt.
Figure 9. Coacervate volume as a function of total concentration of n-octylammonium chloride added to 25 mL of 0.020 M AOT at 23 °C.
(6) It will be noticed that we have refrained from giving a detailed pictorial representation of the L3 structure. Suffice it to say that it is safer to rely upon accurate verbal descriptors than upon speculative drawings. According to the above analysis, the stronger the binding of a metal cation to the anionic surfactant, the more effective the shielding of the electrostatic repulsion among the anionic headgroups, and the more readily a leaflet will “invert” to a negative Ho where the headgroups are more tightly packed. Our observed dependence of [MCl]crit upon the nature of the metal is consistent with this model. It has been shown that counterion binding strength decreases in the order Cs+ ≈ K+ > Na+ > Li+ as reflected in a decrease in cmc of dodecyl sulfate (DS) on going from lithium DS to sodium DS to cesium DS.24,25 Thus, the effective binding of the heavier alkali metal cations to the surface of the leaflets permits closer packing of the AOT sulfonate headgroups and, thereby, conversion to the negative Ho necessary for coacervate formation. Organic Coacervators Cation binding is, clearly, key to the curvature changes essential for coacervation. This led us to wonder what would happen if the cation possessed hydrophobic character and, therefore, an added incentive to bind to the AOT bilayers. For this reason we investigated a series of amine salts: CH3(CH2)nNH3Cl, where n ) 2 and 7. The remainder of the paper will focus on these aminecontaining systems. (24) Oh, S. G.; Shah, D. O. J. Phys. Chem. 1993, 97, 284. (25) Lu, J. R.; Marrocco; A.; Su; T. J.; Thomas; R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303.
For comparison purposes we began by determining the AOT, Cl-, and water content of the coacervate and equilibrium phases induced by NH4Cl. Suffice it to say, without presenting all the specifics, that NH4Cl behaves similarly to NaCl. For example, it has a critical concentration of 0.3 M where the coacervate volume has a sharp maximum of 7.5 mL. The AOT content of the coacervates increases from 0.1 to 1.12 M as the [NH4Cl]total is increased from 0.3 to 1.4 M. n-Propylammonium chloride (C3H7NH3Cl) was found to be a far more effective coacervator under our standardized conditions (50 mL of 0.02 M AOT). As seen in Table 2, C3H7NH3Cl has a 4-fold lower critical concentration than NH4Cl or NaCl. Although at high added salt concentrations the AOT content of the coacervates are the same (1.2 ( 0.1 M), the Cl- concentrations differ, mirroring the critical salt concentrations. (Note once again that we did not differentiate between sodium and ammonium cations; it is reasonable to assume, however, that ammonium ion is the dominant counterion in the coacervate owing to its great excess and to its affinity for the AOT bilayer.) In contrast to n-propylammonium chloride, triethylammonium chloride (Et3NHCl) does not manifest a lowering of the critical concentration relative to NH4Cl. Perhaps hydrophobic binding to the AOT bilayers is sterically impeded here. n-Octylammonium chloride experiments were carried out with only 25 mL of 0.02 M AOT to which was added a varying amount of C8H17NH3Cl. Once the volumes of the phases became constant, the concentration of the components (AOT, water, and C8H17NH3+) was measured in addition to the coacervate volumes. Figure 9 shows a unique and unexpected behavior. At 0.01-0.02 M ammonium salt, a small amount of waxy
Anatomy of a Coacervate
Langmuir, Vol. 14, No. 15, 1998 4137
Table 3. Concentrations of AOT, n-Octylammonium Salt, and Water in the Coacervate Phase and AOT and n-Octylammonium Salt in the Equilibrium Liquid, as a Function of the Total AOT Added to 0.05 M n-Octylammonium Salt [AOT]total, M
[AOT]coac, M
[C8NH3+]coac, M
[H2O]coac, g/L
[AOT]equil, M
[C8NH3+]equil, M
0.02 0.04 0.05 0.10
1.58 1.47 1.47 1.38
0.74 0.81 0.97 0.74
109 105 119 220
5 × 10-4 1.1 × 10-3 5.4 × 10-3 0.017
0.031 0.011 0.0 0.0
solid separated from the liquid. At values of [C8H17NH3Cl]total ) 0.06-0.23 M, only a small coacervate volume was produced (
