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Effects of Volume Ratio and of Surfactant, Salt, and Alcohol Concentrations on the Ion Distribution of Dioctyldimethylammonium Chloride Reverse Micelles in Isooctane Hamid R. Rabie and Juan H. Vera* Chemical Engineering Department, McGill University, Montreal, Quebec H3A 2A7, Canada Received January 29, 1996. In Final Form: April 30, 1996X The effects of volume ratio, of surfactant and alcohol concentrations, and of different ions on the ion distribution of a dioctyldimethylammonium chloride-decanol-isooctane reverse micellar phase in equilibrium with an aqueous phase were studied. The initial organic phase containing the surfactant and decanol, as cosurfactant, in isooctane was contacted with an aqueous phase containing electrolytes. Experimental data of the distribution of anions in this system have been measured at 23 °C. A mathematical model has been developed which accounts for the exchange of the surfactant counterion with the anions in the aqueous phase. This model uses one single parameter for each anion, which depends only on the nature of the anion. It accurately represents the distribution of anions between the two phases of the system. Dimensionless groups have been employed to combine the effect of several independent variables into a few groups.
1. Introduction When an organic phase containing ionic surfactants is exposed to an aqueous phase containing salts, thermodynamically stable reverse micelles may form.1 The utilization of reverse micelles, which are the result of the aggregation of surfactant molecules in organic solvents,2 permits the solubilization of different biomolecules and salts. Reverse micelles generally consist of four components: surfactant (e.g. alkylammonium salts; alkali metal salts of long chain carboxylic, sulfonic, and phosphoric acids), cosurfactant (e.g. an alcohol), oil (hydrocarbon diluent), and water.3-5 Three-component systems of surfactant, oil, and water are also known. Sodium bis-(2-ethylhexyl)sulfosuccinate (also called Aerosol-OT) has been used widely as a surfactant because of its ability to form microemulsions containing large amounts of water without addition of cosurfactant.6-9 On the other hand, cationic surfactants usually require a cosurfactant in order to form reverse micelles10-23 although * To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems: Formation, Solvency, and Physical Properties; Surfactant Science Series, Vol. 30; Decker: New York, 1988. (2) Eicke, H. F. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1982. (3) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (4) Luisi, P. L.; Straub, B. E. Reverse MicellessTechnological and Biological Relevance; Plenum: New York, 1984. (5) Friberg, S. E.; Bothorel, P. Microemulsions: Structure and Dynamics; CRC Press: Boca Raton, FL, 1987. (6) Haering, G.; Luisi, P. L.; Hauser, H. J. Phys. Chem. 1988, 92, 3574. (7) McFann, G. J.; Johnston, K. P. J. Phys. Chem. 1991, 95, 4889. (8) Johnnsson, R.; Almgren, M.; Alsins, J. J. Phys. Chem. 1991, 95, 3819. (9) Jolivalt, C.; Minier, M.; Renon, H. In Downstream Processing and Bioseparation; Hamel, J. F. P.; Ed.; ACS Symposium Series, Vol. 419; American Chemical Society: Washington, DC, 1990. (10) Lang, J.; Lalem, N.; Zana, R. J. Phys. Chem. 1991, 95, 9533. (11) Jada, A.; Lang, J.; Zana, R.; Marhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (12) Verbeeck, A.; Voortmans, G.; Jackers, C.; De. Schryver, F. C. Langmuir 1989, 5, 766. (13) Sjo¨blom, J.; Skurtveit, R.; Saeten, J. O.; Gestblom, B. J. Colloid Interface Sci. 1991, 141, 329. (14) Eastoe, J. Langmuir 1992, 8, 1503. (15) Lang, J.; Mascolo, G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3069.
S0743-7463(96)00090-X CCC: $12.00
some ternary water-cationic surfactant-oil systems24-27 are known. The three-tailed surfactant trioctylmethylammonium chloride or TOMAC is the most widely studied cationic surfactant. Recent studies21,22 on two cationic surfactants, dioctylmethylammonium chloride and bromide (or DODMAC and DODMAB), have shown that these surfactants can form reverse micelles. DODMAC was shown to solubilize considerably more water than TOMAC. Laughlin et al.23,28 previously used the DODMAC abbreviation for dioctadecyldimethylammonium chloride. The effect of electrolytes on reverse micelles has been studied by titration experiments which give a different perspective than that of the contacting method.22 Tosch et al.,29 Fletcher,30 and Aveyard et al.31 have measured the distribution of sodium salts and AOT between an aqueous electrolyte solution and a water in oil microemulsion in equilibrium. Studies more relevant to this (16) Ninham, B. W.; Chen, S. J.; Evans, D. F. J. Phys. Chem. 1984, 88, 5855. (17) Jolivalt, C.; Minier, M.; Renon, H. J. Colloid Interface Sci. 1990, 135, 85. (18) Krei, G. A.; Hustedt, H. Chem. Eng. Sci. 1992, 47, 99. (19) Brandani, V.; Giacomo, G. D. Process Biotechnol. 1993, 28, 411. (20) Hano, T.; Ohtake, T.; Matsumoto, M.; Kitayama, D.; Hori, F.; Nakashio, F. J. Chem. Eng. Jpn. 1991, 24, 20. (21) Wang, W.; Weber, M. E.; Vera, J. H. J. Colloid Interface Sci. 1994, 168, 422. (22) Rabie, H. R.; Weber, M. E.; Vera, J. H. J. Colloid Interface Sci. 1995, 174, 1. (23) Laughlin, R. G.; Munyon, R. L.; Burns, J. L.; Coffindaffer, T. W.; Talmon, Y. J. Phys. Chem. 1992, 96, 374. (24) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (25) Zemb, T. N.; Hyde, S. T.; Derian, P.-J.; Barnes, I. S.; Ninham, B. W.; J. Phys. Chem. 1987, 91, 3814. (26) Eastoe, J.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1994, 90, 487. (27) Eastoe, J.; Dong, J.; Hetherington, K. J.; Steytler, D.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1996, 92, 65. (28) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (29) Tosch, W. C.; Jones, S. C.; Adamson, A. W. J. Colloid Interface Sci. 1969, 31, 297. (30) Fletcher, P. D. I. J. Chem. Soc.; Faraday Trans. 1 1986, 82, 2651. (31) Aveyard, R.; Binks, B. P.; Clark, S.; Mead, J. J. Chem. Soc.; Faraday Trans. 1 1986, 82, 125. (32) Leodidis, E. B.; Hatton, T. A. Langmuir 1989, 5, 741. (33) Vijayalakshmi, C. S.; Annapragada, A. V.; Gulari, E. Sep. Sci. Technol. 1990, 25, 711. (34) Plucinski, P.; Nitsch, W. Langmuir 1994, 10, 371. (35) Rabie, H. R.; Vera, J. H. Langmuir 1995, 11, 1162.
© 1996 American Chemical Society
Dioctyldimethylammonium Chloride Reverse Micelles
work have been presented by Leodidis and Hatton,32 Vijayalakshmi et al.,33 Plucinski et al.,34 and Rabie and Vera.35 We have recently proposed a chemical theory35 for the distribution of ions between an excess aqueous phase and a reverse micellar phase. In this model, the specific character of each exchangeable counterion (hydrated size, free energy of hydration, and electronic properties) has been introduced through a single equilibrium constant of its ion exchange reaction with the original surfactant counterion. The predictions of the model have been found to be in excellent agreement with the experimental results for the AOT system. This model has proved to give better predictions than other existing models published in the literature.32-34 Dimensionless groups were defined to regroup several independent variables. In this work, we study the ion distribution in the DODMAC reverse micellar system considering the effects of volume ratio and of surfactant, salt, and alcohol concentrations. We also derive the necessary equations from the chemical theory for reverse micelles35 for the predictions of ion distribution in such a system and compare the predictions with the experimental results. A method is described to determine the concentrations of all species in both phases.
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Figure 1. Water uptake as a function of initial hydroxide concentration for salts with different cations: initial organic phase, 100 mM DODMAC, 250 mM decanol; initial aqueous phase, hydroxide; initial volume ratio, unity.
2. Materials and Methods The commercial surfactant LF-80 was obtained from Lonza Inc. (Fair Lawn, NJ). This surfactant contains 80 wt % dioctyldimethylammonium chloride (DODMAC) in an ethanolwater solution. It was concentrated by vacuum evaporation at 40 mmHg for 10 h. The purity of the concentrated materials was at least 99 wt %, as determined by sodium lauryl sulfate titration with a Model 93-42 Orion surfactant electrode (Orion Research Inc., Cambridge, MA). Reagent grade isooctane and decanol were purchased from Fisher Scientific (Montreal, QC), Karl Fischer solvent was purchased from BDH Inc. (Toronto, ON), and Hyamine 1622 surfactant electrode titrant was purchased from Orion Research Inc. (Cambridge, MA). All other chemicals were received from A & C American Chemicals Ltd. (Montreal, QC). Deionized water, with an electrical conductivity lower than 0.8 µS/cm, was used for all experiments. The initial organic phase was prepared by adding sufficient purified surfactant to decanol to obtain the desired molar ratio of decanol to DODMAC (2.5/1 in most experiments). Isooctane was then added to make up the required volume. An aqueous electrolyte solution containing salts was then contacted with the organic solution. The volume ratio of the two phases was set at a fixed value. The phases were vigorously shaken for 2 h at 23 °C and then left to stand for 1 week at the same temperature. The phases were then separated for analysis. The settling time used here ensured equilibrium. Some samples were analyzed after 3 or 4 weeks, and the results of the analysis were unchanged. The water content in the organic phase was measured by a Karl Fischer titrator Model 701 (Metrohm Ltd., Herisau, Switzerland). The pH of the aqueous phase was measured by a Model 691 pH meter (Metrohm, Ltd.). The concentration of surfactant in the aqueous phase was measured by a surfactant electrode, as mentioned earlier. The anion concentration in the aqueous phase was measured by a DX 500 ion chromatograph (Dionex Canada Ltd., Brossard, QC). The concentrations of the cations in the final aqueous phase were determined by atomic absorption on a Model Smith-Hieftja II (Thermo Jarrell Ash, Franklin, MA) spectrophotometer.
3. Experimental Results In this study, the contacting method, or phase-transfer experiment, is employed for the formation of reverse micelles. This method has some advantages which are discussed elsewhere.36 The main advantage is that the material balances and other constraints for all species between the two phases of the system can provide valuable (36) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6400.
Figure 2. Final chloride concentration in the aqueous phase as a function of initial hydroxide concentration for salts with different cations: initial organic phase, 100 mM DODMAC, 250 mM decanol; initial aqueous phase, hydroxide; initial volume ratio, unity. The solid line represents the prediction of the model.
information on the distribution of solutes in reverse micellar systems. In addition, for some practical purposes such as extraction, only the contacting method can give useful information. In the case of anions, chloride and hydroxide are chosen as model ions for the study of ion distribution. One of the anions has to be the same as the original surfactant counterion, which is chloride for DODMAC. Even if this anion is not initially added to the aqueous phase, it will be present in the final aqueous phase due to the ion exchange. Hydroxide is chosen, since it is a common anion and it is also an essential component for extraction of amino acids, proteins, enzymes, and some metals with cationic surfactants. Thus, the five ionic species present in the final aqueous phase of this study were measured. They were Cl-, OH-, H+, surfactant, and a cation (either Na+, K+, or Ca2+). The difference between the measured equivalents of anions and cations was less than 10% of the total ions in the aqueous solution. The total concentration of each species in the organic phase was calculated from mass balance. Figure 1 shows the effect of cations on the water uptake for the hydroxide salts of sodium, potassium, and calcium. No chloride was initially added to the aqueous phase. When data were plotted as a function of initial hydroxide concentration, the results for different salts collapsed onto one single curve. Thus, the identity of the anion, which is the exchangeable counterion in cationic surfactants, determined the water uptake. Similar results were obtained for the variation of chloride concentration in the final aqueous phase depicted in Figure 2. The appearance of a maximum in the water uptake with salt concentration is a very interesting finding which was first reported by
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Figure 3. Surfactant distribution as a function of initial hydroxide concentration for salts with different cations: initial organic phase, 100 mM DODMAC, 250 mM decanol; initial aqueous phase, hydroxide; initial volume ratio, unity.
Rabie et al.22 The discussion and explanation of the appearance of this maximum have been presented elsewhere.22 As shown in Figure 2, chloride was present in the final aqueous phase, although no chloride was initially added, and its concentration increased with initial hydroxide concentration. This is due to the fact that the hydroxide ions can be exchanged with the counterion of the surfactant, chloride. A simple model which takes into account this observation is presented later. Figure 3 presents the distribution of surfactant between two phases, observed when NaOH, KOH, or Ca(OH)2 was added to the aqueous phase. At salt concentrations below 20 mM, almost all of the surfactant was in the aqueous phase, making it cloudy. Over a narrow range of salt concentration, at around 20 mM, the surfactant was salted out of the aqueous phase, into the organic phase, where it formed reverse micelles. Similar results were obtained for other conditions and for other salts. The results reported in Figures 1, 2, and the ones that follow were obtained in Winsor type II systems where two clear phases were formed. As discussed earlier, under these conditions, about 95% of the initial amount of surfactant was present in the organic phase. Another interesting finding is the distribution of cations in this system. The final concentrations of cations (sodium, potassium, or calcium) in the excess aqueous phase were almost the same as their initial values. A slight decrease was observed, but it was not significant. This justifies the useful assumption that the concentrations of ions in the water pools is the same as that in the coexisting aqueous phase. This is discussed with more detail in the next section. 4. Modelling A number of different species are found in each reverse micelle. These are (i) surfactant head groups, each with a charge of +1; (ii) counterions Cl- and OH-, bound to the surfactant head groups; and (iii) free ions such as Na+, Cl-, and OH-, which are in solution in the water pool. The extraction of ions to the reverse micellar phase is interpreted here using a thermodynamic approach similar to the one used in our previous work,35 which is based on the ion exchange with resins. The ions are extracted from the aqueous phase to the water pools of the reverse micelles in the organic phase. We assume that the concentration of any species in the water pool is the same as that in the excess aqueous phase. We have reasons to believe that this assumption is valid in our work. In a cationic surfactant system, the cations cannot interact with the reverse micellar interface due to the strong repulsive forces. Thus, they are mainly in the water pools of the reverse micelles. If the concentration
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of a cation in the water pool is different from that in the aqueous phase at equilibrium, a difference will be detected between its initial and final concentrations in the aqueous phase at moderate water uptakes. As mentioned before, no significant difference was observed for a wide range of water uptakes. In addition, Wang et al.37 found that, for the same system, when the water uptake is greater than 5 wt %, this assumption is valid. Furthermore, the ions in the water pools are exchanged with the original surfactant counterion at the reverse micellar interface to produce new forms of the surfactant. A treatment for the case of anionic surfactants is given elsewhere.35 However, for completeness and clarity, the basic equations are repeated in this work for DODMAC, a cationic surfactant. In addition, the definition of concentrations at the reverse micellar interface is different in this work from that used in our previous study. This difference is explained below. Following our previous work,22,35 the overall ion exchange reaction between the hydroxide in the aqueous phase and the counterion of the surfactant, chloride, at the reverse micellar interface is represented by a reversible reaction of the form:
SCl + OH- a SOH + Cl-
(1)
SCl and SOH represent unionized surfactant in the chloride and hydroxide forms, respectively. In terms of concentrations, the equilibrium constant of this reaction, Ks, is given by
Ks )
[SOH][Cl-] [SCl][OH-]
(2)
The value of the equilibrium constant for this reaction was found previously to be 0.096.22 Here we use the same value for our calculations. This is the only parameter needed for calculations of ion distributions for totally different conditions of volume ratio and surfactant, chloride, hydroxide, and alcohol concentrations. Assuming that all the counterion sites of the surfactant at the reverse micellar interface are filled with either chloride or hydroxide, the mass balance of the surfactant is then formulated as
[SCl] + [SOH] ) [S]0
(3)
where S stands for the surfactant in any of its forms and the superscript “0” refers to the initial value. Thus, the term on the right hand side of eq 3 is the initial surfactant concentration. Equation 3 assumes that all the surfactant in the organic phase is found in the reverse micellar interface, which is a reasonable assumption in light of the results of Aveyard et al.27 This equation also assumes that only a small fraction of the surfactant remains in the aqueous phase at equilibrium, which was shown to be true in this study. The concentrations of the surfactant, bound chloride, SCl, and bound hydroxide, SOH, are defined as moles of surfactant or bound ion per unit volume of water-free organic phase. This definition is different from the one we used in our previous work,31 which was based on the total organic phase volume. This definition gives better results at high water uptakes than the previous one. Using this definition, the mass balance of chloride takes the form
[Cl-] + r[SCl] ) [Cl-]0 + r[S]0
(4)
where r is the initial volume ratio of organic to aqueous (37) Wang, W.; Weber, M. E.; Vera, J. H. Biotechnol. Bioeng. 1995, 46, 343.
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phase defined as
r)V h 0/V0
(5)
In eq 5, V h 0 and V0 are the initial volumes of the organic and aqueous phases, respectively. Since the organic solvent, isooctane, is insoluble in water and since the fraction of surfactant left in the aqueous phase at equilibrium is negligible, the volume water-free organic phase remains unchanged. In addition, the sum of the final aqueous phase volume and the volume of water in the organic phase will be equal to the initial volume of the aqueous phase. These constraints have been considered in eq 4, in addition to the assumption of having the same concentration for any species in the water pool as that in the excess aqueous phase at equilibrium. Similarly, the mass balance of hydroxide reduces to
[OH-] + r[SOH] ) [OH-]0
(6)
Combining eqs 2-4 and 6 yields the following solution:
x
2
+ Ks(δ - 1)) +
4Ks(R0OH
+ δ) (7)
where ROH is the dimensionless group describing the equilibrium state and is defined as
ROH ) [Cl-]/[OH-]
(8)
0 has a similar definition but for the initial values. and ROH The other dimensionless group which describes the initial state is δ and is defined as
δ)
V h 0[S]0 V0[OH]0
(9)
The equilibrium concentration of chloride in the aqueous phase is then calculated using the following equation:
[Cl-] )
[Cl-]0 + [OH-]0 1 + 1/ROH
(10)
and the equilibrium concentration of hydroxide in the aqueous phase is calculated from
[OH-] )
Table 1. Effect of Decanol Concentration on the Equilibrium Concentration of Chloride in the Excess Aqueous Phase [Cl-]0
2ROH ) R0OH + Ks(δ - 1) + (R0OH
Figure 4. Distribution of ions as a function of initial hydroxide concentration: initial organic phase, 100 mM DODMAC, 250 mM decanol; initial aqueous phase, NaOH; initial volume ratio, unity. Solid lines represent the predictions of the model.
[Cl-]0 + [OH-]0 1 + ROH
(11)
The concentrations of bound chloride and hydroxide can then be calculated from eqs 3, 4, and 5, respectively. 5. Results and Discussion The predictions of the model proposed in this work are compared with experimental data for the chloride and hydroxide distribution in the DODMAC reverse micellar system under various concentrations of chloride and hydroxide salts and various concentrations of surfactant and alcohol and at different volume ratios of the two phases. 5.1. Effect of Alcohol Concentration. Table 1 presents the experimental results of the equilibrium concentrations of chloride in the excess aqueous phase in millimolar for various concentrations of decanol and NaCl. The initial concentration of NaOH was 200 mM with 100 mM DODMAC and an initial volume ratio of unity. The results of this table clearly show that there is no significant effect of alcohol on the ion distribution. Similar results were obtained for different NaOH concentrations. Fluc-
[NaCl] (mM)
[decanol] ) 200 mM
[decanol] ) 250 mM
[decanol] ) 300 mM
[decanol] ) 350 mM
50 100 200
67 111 209
67 115 212
72 116 212
69 115 210
tuations of the final concentrations are within the experimental error. As can be seen from the derivation of the model, no effect of alcohol concentration is expected on the distribution of ions. Although an alcohol has a definite effect on the water uptake and size of the reverse micelles, it does not have any significant effect on the ion exchange reaction, happening inside the reverse micelles, between different ions and the surfactant head groups. Thus, the value of the equilibrium constant obtained for hydroxide-chloride can be used for any alcohol concentration. 5.2. Effects of Salt Type and Concentration. Figure 4 shows the effect of initial NaOH concentration on the distribution of chloride and hydroxide between the two phases. As mentioned before, the equilibrium concentrations of chloride and hydroxide in the aqueous phase were measured while the corresponding organic concentrations were obtained from mass balance. These later concentrations refer to the chloride or hydroxide bound to the surfactant at the reverse micellar interface. No chloride was initially added to the aqueous phase. In Figure 4, the chloride concentration in the aqueous phase and the bound hydroxide concentration in the organic phase increased with NaOH and reached a plateau at higher NaOH concentrations, which corresponds to a saturation of surfactant. Notably, under the conditions studied, these two concentrations fell on the same curve. However, the concentration of bound chloride in the organic phase almost reached zero. The hydroxide concentration in the aqueous phase increased almost linearly with NaOH concentration, and it was lower than the initial hydroxide concentration. All of these findings agree with the explanation of the ion exchange between the chloride of the surfactant and hydroxide. The solid lines which show the predictions of the present model represent well the experimental data. Figure 5 depicts the equilibrium concentration of chloride in the aqueous phase as a function of the initial ratio of chloride to hydroxide concentration for various initial NaOH concentrations. The experiments were carried out by varying the NaCl concentration for each NaOH concentration. The initial surfactant concentration was 100 mM with 250 mM decanol and a volume ratio of
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Figure 5. Final chloride concentration in the aqueous phase as a function of initial molar ratio of chloride to hydroxide for different initial NaOH concentrations: initial organic phase, 100 mM DODMAC, 250 mM decanol; initial aqueous phase, NaCl, NaOH; initial volume ratio, unity. Solid lines represent the predictions of the model.
Figure 6. Final chloride concentration in the aqueous phase as a function of the (r[S]0) term for different initial NaCl concentrations: initial organic phase, DODMAC, 250 mM decanol; initial aqueous phase, NaCl, 100 mM NaOH. The open symbols refer to the variation in surfactant concentration while the closed symbols refer to the variation in volume ratio. Solid lines represent the predictions of the model.
unity. The equilibrium chloride concentration increased linearly with the initial ratio of hydroxide to chloride for any NaOH content. The slope of the line is larger for higher NaOH concentrations. The solid lines are the predictions of the model. An excellent agreement was found between the predictions of the model and the experimental results. 5.3. Effects of Initial Volume Ratio and Surfactant Concentration. Figure 6 presents the results of the equilibrium chloride concentration in the aqueous phase as a function of the (r[S]0) term for two different initial NaCl concentrations. The initial decanol concentration was fixed at 250 mM with a NaOH concentration of 100 mM. The variation in the (r[S]0) term was originated from a variation in initial surfactant concentration or initial volume ratio. When the surfactant concentration was changed, the initial volume ratio was fixed at unity. However, when the volume ratio was changed, the initial surfactant concentration was fixed at 100 mM. The data of the two different sets of experiments followed the same variation. Increasing the (r[S]0) term increased the chloride concentration for different initial NaCl concentrations. The slopes of both curves approached zero at high (r[S]0), corresponding to the extraction of almost all the hydroxide from the aqueous phase. 5.4. Dimensionless Form Representation. All the results obtained under different conditions can be condensed in one figure using the dimensionless groups. In Figure 7, the dimensionless form of the model is used to
Rabie and Vera
Figure 7. Equivalent ratio of chloride to hydroxide in the aqueous phase vs the δ parameter for different initial equivalent ratios of chloride to hydroxide in the aqueous phase. Variation of the δ parameter originates from variation of salt and surfactant concentrations and of the phase volume ratio.
show the predictions of the model. The equilibrium equivalent ratio of chloride to hydroxide in the excess aqueous phase (ROH) is plotted vs δ (the initial equivalent ratio of surfactant in the organic phase to hydroxide in 0 (the initial equivalent the aqueous phase) for various ROH ratio of chloride to hydroxide in the aqueous phase). The value of δ was changed in the following ways: (i) by changing the initial concentration of NaOH while keeping the initial surfactant and NaCl concentrations constant with the volume ratio set at unity; (ii) by changing the initial surfactant concentration in the organic phase, while keeping the initial NaOH and NaCl concentrations in the aqueous phase constant with the volume ratio set at unity; (iii) by changing the volume ratio, while keeping constant the initial concentration of surfactant in the organic phase and the initial NaOH and NaCl concentrations in the aqueous phase. As shown in Figure 7, the data of three different 0 . For experiments collapse onto a single curve for any ROH 0 any value of ROH, the curve approaches the “uniform distribution” or “no selectivity” curve at low values of δ. The agreement between the model and experiment is excellent (within 1-3%). 6. Conclusions For DODMAC surfactant, the distribution of ions at equilibrium was found to be controlled by the ion exchange reaction between the anions initially present in the aqueous phase and the counterion of the surfactant, chloride. The extraction of ions into the water pool had no significant effect on the ion distribution, since the solubilization capacity of the water pool was found to be very similar to that of the excess aqueous phase. This conclusion regarding the solubilization capacity was drawn from the distribution of cations at equilibrium. The alcohol concentration had no significant effect on the ion distribution, since it did not interfere with the ion exchange reaction. Other variables like surfactant concentration, salt type and concentration, and volume ratio, altered the ion distribution. In these cases the effects were in accord with the predictions of the model based on the ion exchange. In this model, dimensionless groups were employed to reduce the number of independent variables. Excellent agreement was found between the predictions of the model and the experimental results. Acknowledgment. The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support. LA960090E
