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Langmuir 1995,11, 1162-1169
A Chemical Theory for Ion Distribution Equilibria in Reverse Micellar Systems. New Experimental Data for Aerosol-OT-Isooctane-Water-Salt Systems Hamid R. Rabie and Juan H. Vera* Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 Received October 31, 1994. In Final Form: January 12, 1995@ Aerosol-OT and some other surfactant molecules form reverse micelles in an organic phase in contact with an aqueous salt solution. Experimental data of the distribution of cations in Aerosol-OT-isooctanewater-salt systems have been measured at 23 "C for different chloride salts. The effect of different variables on the equilibriumion distribution between the organic phase, containingreverse micelles, and an aqueous phase, containingdifferentsalts, has been formulated in terms of dimensionless groups. These variables are (i)the initial surfactant concentration,(ii)the nature and initial salt concentrationof each salt, and (iii) the initial volume ratio of the two phases. The model distinguishes between different ions via the equilibriumconstants of their ion exchange reactions with the surfactant counterion. It accurately predicts the distribution of single charge and multicharge ions between the two phases of the system. A model formulated in dimensionless form, with few independent dimensionless groups rather than many independent variables, has definite advantages for the treatment of complex systems. In this work, the predictions of the model are compared with experimental results and with the predictions of other phenomenological models published in the literature for reverse micellar systems.
Introduction Reverse micelles have attracted much interest over the past few decades because of their extensive technological and biological importance for oil recovery, lubrication, detergency, catalysis, metal and biomolecule extraction, or use for probes in immunological processes and drug de1ivery.l Electrolytes are important components in the two-phase systems under consideration. It has been long established that a high salt concentration is a prerequisite for the formation of a Winsor I1 ~ y s t e m which , ~ , ~ is a waterin-oil (W/O)microemulsion in equilibrium with an excess aqueous phase. An understanding of the behavior of the system with different electrolytes, coupled with a knowledge of the ion distribution in the two-phase system, is desirable if the solubilization characteristics of solutes (metals, biomolecules,etc.) are to be interpreted correctly. The effect of electrolytes on reverse micelles has been studied by titration experiments which give a different perspective than that of the present study. The effects of temperature, salt type, and concentration on the maximum water uptake by the surfactant Aerosol-OT (AOT)reverse micellar phases before any excess aqueous phase is formed, was investigated by Kon-No and Kitahara.4-8 A similar study was undertaken by Chou and Shah.g In more recent years, Kunieda and Shinodalo-l2 and Gosh and Miller13 focused on the influence of salinity on the phase behavior of AOT/water/oil systems. Tosch et al.,14 Fletcher,15and Aveyard et a1.16have measured the distribution of sodium
* To whom correspondence should be addressed. Abstract published inAduameACSAbstracts, March 15,1995. (1)Luzar, A,; Bratko, D. J. Chem. Phys. 1990,92, 642. (2) Akoum, F.; Parodi, 0. J. Phys. (Les Uis, Fr.) 1986, 46, 1675. (3) Lampert, A.; Martinelli, R. U. Chem. Phys. 1984, 88, 399. (4) Kitahara, A.; Watanabe, K.; Kon-No, K.; Ishikawa, T. J.Colloid Interface Sci. 1969, 29, 48. ( 5 ) Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1971,35,403. (6) Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1971,35,636. (7) Kon-No, K.; Kitahara, A. J . Colloid Interface Sci. 1971,37,469. (8)Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1972,41, 47. (9) Chou, S. I.; Shah, D. 0. J . Colloid Interface Sci. 1981, 80, 311. (10)Kunieda, H.; Shinoda, K. J.Colloid Interface Sci. 1979,70,577. (11)Kunieda, H.; Shinoda, K. J.ColloidInterface Sci. 1980,75,601. (12) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1987, 118, @
586.
(13) Ghosh, 0.; Miller, C. A. J. Phys. Chem. 1987,91, 4528.
0743-7463/95/2411-1162$09.00/0
salts between an aqueous electrolyte solution and a W/O microemulsion in equilibrium. They also measured the water uptake by the reverse micelles and reported the AOT distribution between the two phases, the size of the reverse micelles, and the values of the interfacial tension. Studies more relevant to this work have been presented by Leodidis and Hatton," Vijayalakshmi et al.,18J9and Plucinski et a1.20 A phenomenological model for the selective solubilization of cations in the reverse micelles ofAOT surfactant was presented by Leodidis and Hatton.l' Their primary goal was to model the large differences in water uptake and the ion distribution. They used the modified Poisson-Boltzmann theory with the assumption that the chemical potential for the ions can be expressed as a linear combination of a series of different interaction terms, while neglecting the ion-ion terms. Their model distinguished the different cations through their charge, hydrated size, and electrostatic free energy of hydration. Vijayalakshmi et a1.18 and Vijayalakshmi and Gularilg proposed a model which assumed that the adsorption of counterions onto the surfactant surface of the reverse micelle is described by the Stern double layer model. Their model is simpler than the Leodidis-Hatton model, but it cannot distinguish between different ions with the same charge. Plucinski et a1.20studied the specific ion effect of the ion exchange in the AOT reverse micellar system in equilibrium as well as in kinetic experiments. They used the Graham model of metal ion adsorptionz1to express the adsorption ofmetal ions in the inner Helmholtz plane. We present here a theoretical model in which the specific character of each exchangeable counterion (hydrated size, free energy of hydration and electronic properties) is (14) Tosch, W. C.; Jones, S. C.; Adamson, A. W. J.Colloid Interface Sci. 1989, 31, 297. (15)Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2651.
(16)Aveyard, R.; Binks, B. P.; Clark, S.; Mead, J. J. Chem. Soc., Faraahy Trans. 1 1986,82, 125. (17) Leodidis, E. B.; Hatton, T. A. Langmuir 1989, 5 , 741. (18) Vijayalakshmi, C. S.; Annapragada, A. V.; Gulari, E. Sep. Sci. Technol. 1990,25, 711. (19) Vijayalakshmi, C. S.; Gulari, E. Sep. Sci. Technol. 1991,26,291. (20) Plucinski, P.; Nitsch, W. Langmuir 1994, 10,371. (21) Hunter, R. J. Foundations ofcolloid Science; Clarendon Press: Oxford, 1989.
0 1995 American Chemical Society
Ion Distribution Equilibria
Langmuir, Vol. 11,No. 4,1995 1163
I
16 14
g 12 Centrifuge
10
NiBr
h
N$
W
2c. 8
[water.
Mixing
Phase Separation
a 2 6 c
$ 4
Aqueous phase
2
n
-0
0.2 0.4 0.6 0.8 1 1.2 Sodium concentration (M)
1.4
Figure 2. Water uptake as a function of initial sodium concentration for salts with different anions (initial organic, 0.1 M AOT; initial aqueous, salt).
IWater
I
Two-phase system
Figure 1. Diagram of the experimental procedure.
introduced through a single equilibrium constant. The model is tested against literature data and new data measured in this work. Chemicals and Experimental Method Aerosol-OT (bis(2-ethylhexyl) sodium sulfosuccinate), AOT, of 99% purity was obtained from Sigma (Saint Louis, MO) and used without further purification. Interfacial tension results with Sigma AOT were undistinguishable from those measured for a highly purified sample of AOT.16 Reagent grade isooctane was purchased from Fisher Scientific (Montreal, PQ),Karl Fischer solvent from BDH Inc. (Toronto, ON) and Hyamine 1622 surfactant electrode titrant from Orion Research Inc. (Cambridge, MA) were used. All other chemicals were obtained from A&C American Chemicals Ltd. (Montreal, PQ). The water used for preparation of electrolyte solutions was distilled and deionized and had a n electrical conductivity lower than 0.8 ,uS/cm. An aqueous electrolyte solution containing the salts was contacted with a solution ofAerosol-OT in isooctane. The volume ratio of two phases was set a t a fixed value. The phases were vigorously shaken at 200 rpm for 2 h a t 23 "C and then left to stand for 2 weeks at the same temperature. The phases were then carefully separated for analysis. Water uptake and ion distribution measurements over time showed that this 2-week equilibration period was adequate for complete phase separation and for the achievement of thermodynamic equilibrium. Of the two phases formed, the lower (denser) phase was a n electrolyte solution containing a negligible amount of AOT, the cations Mzl+ and Na+, and one anion. The upper phase was a reverse micellar microemulsion under the conditions employed. The water content in the organic phase was measured by Karl Fischer titration using a Model 701 titrator (Metrohm Ltd., Herisau, Switzerland). The concentrations of the cations in the final aqueous phase were determined by atomic absorption on a Model Smith-Hieftje I1 (Therm0 Jarrell Ash, Franklin, MA) spectrophotometer. The surfactant concentration in the aqueous phase was measured by Hyamine 1622 titration with a 93-42 Orion surfactant electrode (Orion Research Inc., Cambridge, MA).
Experimental Results As shown in Figure 1the system consists of an organic phase, containing a surfactant, in equilibrium with an aqueous phase containing salts. In the initial aqueous phase, there are n different salts with different cations and anions. Superscript zero, for all symbols, refers to the initial condition.
We have chosen the sodium bis(2-ethylhexyl sulfosuccinate (A0T)-water-in-oil system in contact with an excess aqueous phase containing ( n- 1)chloride salts, MCl,,, in addition to NaCl, to test our theoretical model. In this system, the following charged species are found in each micelle: (i) surfactant head groups, each with a charge of -1for AOT; (ii)counterions bound to the surfactant head groups, Na+ and (n - 1)Mp+ ions; and (iii)the free ions in the water pool solution not bound to the surfactant head groups (free cations and anions). Our experimental results, depicted in Figure 2, indicate that the anion of a salt does not have any effect on the water uptake of the AOT system, as long as all the salts have the same cation. In this figure, the effect of F-,C1-, Br-, I-, NO3-, and are shown. Results using SCN-, Cos2-, HP042-, and Po43- not reported here were similar to those shown in Figure 2. Similar results have been found for DODMAC, a cationic surfactant, for the effect of cations in the aqueous solution.22 These results indicate that only the ions which are exchangeable with the surfactant counterion alter the water uptake. Thus, for AOT studies, the use of chloride salts is a useful simplification. For AOT systems, the nature of the cation of the salts is determinant. In this work we studied the cations sodium, potassium, rubidium, cesium, calcium, strontium, barium, zinc, and copper. In all cases we used the chloride salts. The experimental results are shown in Figures 3 to 12 and discussed together with those of the theoretical treatment. Theory The extraction of ions to the water pools of the reverse micelles is interpreted here using a thermodynamic approach similar to the one used for ion exchange with resins. The ions in the excess aqueous phase are extracted into the water pools by solvent extraction and then from the water pools onto the surfactant surface of the reverse micelles by ion exchange. For AOT, since the surfactant, S, is anionic, Mp+is the exchangeable cation with the original surfactant counterion, Na+. The partitioning of cations is conveniently summarized as a partition coefficient, Ai, for each cation including sodium
Ai = ci,f/Ci where Ci is the molar concentration of cation i in the (22) Rabie, H. R.; Weber, M. E.; Vera, J. H. J. Colloid Interface Sei., in press.
1164 Langmuir, Vol. 11, No. 4, 1995
Rabie and Vera
aqueous phase, and ci,f is the molar concentration of the free cation i in the water pools. The free cations in the water pools are assumed to undergo a reaction of ion exchange with the cations bound to the surfactant head groups. Following Shallcross et al.23 and Allen and Addison,24the ion exchange for an anionic surfactant is represented by a reversible reaction of the form
unit volume oforganic phase. The mass balance of sodium is then formulated as:
(7)
(2)
where w is the volume ratio of water in the organic phase at equilibrium and it is only a function of final ion concentrations at equilibrium. For each of the other cations present in the initial aqueous phase we write:
where the free ion M i , f in the water pools replaces the surfactant bound counterion Na+ in the reverse micelles. In eq 2, zi is the charge number of ion M i and subscript "f' refers to the free ions inside the water pools. The equilibrium constant KN:, in terms of concentrations, for this reaction is
Note that in eq 7, the surfactant solubilized in the aqueous phase is assumed to be completely dissociated. Assuming that in the reverse micelles all the surfactant sites are filled with either sodium or other cations, we write
ziSNa
+ Mi,?+ == SzMi + ziNaf+
(n-1)
(3)
(9)
-
i=l
where c i , b and c N a , b are the molar concentrations ofcation i and sodium, respectively, bound to the surfactant head group in the reverse micellar phase. Equations 2 and 3 are written for an anionic surfactant; however, they can be easily applied for cationic surfactants as well. In a ternary surfactant counterion system, the three equilibrium constants must satisfy a so-called triangular relat i ~ nof~ the ~ , form
(Kj)zr(K$i(
