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Comparison of the Titration and Contact Methods for the Water Solubilization Capacity of AOT Reverse Micelles in the Presence of a Cosurfactant
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
189, 208–215 (1997)
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Comparison of the Titration and Contact Methods for the Water Solubilization Capacity of AOT Reverse Micelles in the Presence of a Cosurfactant HAMID R. RABIE, DAMRESS HELOU, MARTIN E. WEBER,
AND
JUAN H. VERA 1
Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2A7, Canada Received May 17, 1996; accepted January 21, 1997
The method used to form a reverse micellar phase determines the amount of water solubilized in the organic phase. The effect of cosurfactant and salt on the water solubilization capacity per mole of surfactant was determined for two methods of solubilization: contact with an excess aqueous phase and the titration method. The two methods are intrinsically different. While in the titration method, there is a transition from a Winsor IV system to either Winsor I or II system, in the contact method, the experiments are always in a Winsor II system. When the titration method was used at low salt concentrations, or even without salt, the water solubilized increased with the concentration of cosurfactant, at low cosurfactant concentrations. When the contact method was used, the water solubilized per mole of surfactant decreased with an increase in the cosurfactant concentration. When titration was used, at a fixed cosurfactant-to-surfactant molar ratio, several solubilization capacities were obtained at low salt concentrations, whereas a single solubilization capacity was found when the titrant had a higher salt content. With the contact method, a single solubilization capacity was observed for all salt concentrations at which reverse micelles were formed. When the same salt solution was used as the titrant in the titration method and the initial excess aqueous phase in the contact method, a larger amount of water was solubilized by the latter method. q 1997 Academic Press Key Words: reverse micelles; microemulsions; water uptake; surfactants.
1. INTRODUCTION
Reverse micelles are aggregates of surfactant molecules in an apolar solvent that contain microscopic polar cores of solubilized water (water pools). They can solubilize large amounts of water, forming isotropic solutions, sometimes called water-in-oil microemulsions (1). Sodium bis-2(ethylhexyl) sulfosuccinate (AOT) has been used widely as a surfactant because of its ability to form reverse micelles containing large amounts of water without the addition of a cosurfactant (2–5). On the other hand, cationic surfactants 1
To whom all correspondence should be addressed.
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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usually require a cosurfactant to form reverse micelles (6– 18). Cosurfactants are some additives that are generally understood to be amphiphilic molecules assisting the emulsifying property of surfactants (19). Based on this definition, different compounds such as alcohols with relatively long chains, benzene, carbon tetrachloride, and nitrobenzene are considered cosurfactants (19). Reverse micelles can be formed in an organic phase by the contact method or by the titration method. The experimental procedure for the contact method is depicted in Fig. 1a and that for the titration method in Fig. 1b. Contact method. A bulk excess aqueous phase is contacted with an organic solvent containing the surfactant and cosurfactant. The excess aqueous phase must contain an electrolyte to prevent the transfer of the surfactant from the organic phase to the aqueous phase (20). After mixing and settling, the organic phase is in equilibrium with an excess aqueous phase, thus forming a Winsor II system (1). The amount of water solubilized is obtained by measuring the water content of the organic phase, usually by Karl Fischer titration. The composition of the water pool is determined by the exchange equilibrium of ions and solutes between the phases. Titration method. An aqueous solution is titrated into an organic solvent containing a surfactant and a cosurfactant. The maximum amount of aqueous solution that can be solubilized is determined from the appearance of permanent turbidity. In this method there is no excess aqueous phase in equilibrium with the organic phase containing the reverse micelles. Since no excess aqueous phase is present, the composition of the water pool is identical to the composition of the titrant. The use of an electrolyte is not required in this method, but, if present in the titrant, its concentration affects the maximum water solubilization. The titration method has been used widely to form reverse micelles as injection fluids for a number of tertiary oil recovery processes (21) and to study different types of reactions inside the reverse micelles (5). The contact method, on the other hand, is identical to the basic step used in extracting
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FIG. 1. Diagram of experimental procedures with (a) contact and (b) titration methods.
solutes from water into a reverse micellar organic phase. In the contact method, it is possible to use an additional region of the phase diagram and not to be limited to the interior of a Winsor IV system, as is the case in the titration method. The material balances for all species, together with the consideration of electroneutrality in the two phases, provide valuable information on the distribution of solutes between the excess aqueous phase and the reverse micellar phase present in the contact method. Previous studies (18, 21–23) have shown that in the contact method, the solubilization capacity for water, or water uptake, is a strong function of the identity of the surfactant counterion. The water uptake is also a function of the ionic strength of the excess aqueous phase and of the nature of the solvent. As well, it depends on the surfactant and cosurfactant type and concentrations and on the temperature. Although many studies have described the effect of different variables on the water uptake obtained by either of these two methods (18–26), no single work compares the water uptakes obtained by the titration and contact methods under similar conditions. The key question is: Do we have similar effects of a particular variable on the water uptake using the titration and contact methods? In some published work, the conclusions drawn from the results obtained with one of these methods were used to explain the results obtained with the other method. For ex-
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ample, when extracting an amino acid using the contact method, Leodidis and Hatton (27) found that the water uptake in the organic phase increased linearly with the initial amino acid concentration. They concluded that this was due to the cosurfactant effect of the amino acid because the water uptake had been shown to increase in the presence of a cosurfactant (19); however, this increase in water uptake with cosurfactant concentration was observed by Eicke (19) using the titration method rather than the contact method used by Leodidis and Hatton (27). We will show that the effect of a cosurfactant is different when the contact method is used. A second example is the recent work of Ashrafizadeh and Demopoulos (28), who studied the effect of tridecanol on the water solubilization capacity of reverse micelles formed by Kelex in kerosene using the contact method. They observed that the water uptake decreased with tridecanol concentration. They stated that tridecanol is a cosurfactant, and thus, the water solubilization capacity should increase initially with tridecanol concentration and decrease with further addition of tridecanol, as observed by Leung and Shah (29). Furthermore, they speculated on this discrepancy. It is important to note that Leung and Shah used the titration method in their experiments rather than the contact method. We believe that the experimental results of Ashrafizadeh and Demopoulos (28) are correct and that there is no reason to expect a maximum in the water solubilization capacity as a function of tridecanol concentration. The objective of this work was to compare the effect of cosurfactant concentration on the water solubilization capacity using the titration and the contact methods under similar conditions. We have chosen the AOT–heptanol–decane– water–salt system as the model system for this study. Some experimental results are also presented for the system of AOT–benzene–isooctane–water–NaCl, the system studied by Eicke (19). In preliminary experiments using the titration method for the AOT–heptanol–decane–water–salt system, we found that different amounts of water could be solubilized in the organic phase, for a fixed initial composition of the organic phase. On the other hand, the contact method produces a single value for the water solubilized in the organic phase, for fixed initial compositions of the phases. The significance of these results, which are reported here for the first time, is discussed in Section 3. Therefore, a second objective of this work was to explore these multiple water solubilization capacities that exist when the titration method is employed. 2. EXPERIMENTAL METHODS
AOT of 99% purity was obtained from Sigma (St. Louis, MO) and used without further purification. Karl Fischer solvent was purchased from BDH Inc. (Toronto, ON), reagentgrade isooctane from Fisher Scientific (Montreal, QC). All
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other chemicals were obtained from Anachemia (Montreal, QC). The water used for experiments was distilled and deionized, with an electrical conductivity lower than 0.8 mS/cm. In the contact method experiments, an excess aqueous NaCl solution was contacted with an organic phase containing AOT in a mixture of heptanol and decane or benzene and isooctane. The initial volumes of the organic phase and the excess aqueous phase were equal. The two phases were shaken vigorously at constant temperature for 2 h and then left to stand for 2 weeks at the same temperature. At equilibrium, there were two clear homogenous phases, an aqueous and an organic phase, in contact. The water content of the organic phase was measured by Karl Fischer titration using a Model 701 titrator (Metrohm Ltd., Herisau, Switzerland). The reproducibility of the solubilization capacity, expressed as the molar ratio of water to surfactant in the organic phase, was about {1. In the titration method experiments, water or an aqueous NaCl solution was titrated directly into a prepared organic phase containing AOT in a mixture of heptanol and decane or benzene and isooctane, under vigorous stirring and constant temperature. The onset of permanent turbidity was taken to indicate the solubilization capacity. Therefore, the maximum water solubilization capacity obtained with the titration method corresponds to a single homogenous organic phase containing solubilized water and no excess aqueous phase is present. The reproducibility of the solubilization capacity, expressed as the molar ratio of water to surfactant in the organic phase, was about {3.5. 3. RESULTS AND DISCUSSION
The experimental data for water solubilization capacity are presented as moles of water per mole of surfactant solubi-
FIG. 2. Effect of heptanol on water uptake for various initial salt concentrations using the contact method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: m, 70 mM NaCl; j, 97 mM NaCl.
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FIG. 3. Effect of heptanol on water uptake for various initial surfactant concentrations using the contact method at 257C. Initial organic: j, 410 mM AOT; or l, 100 mM AOT in decane. Initial aqueous: 97 mM NaCl.
lized in the organic phase, w0 . In computing w0 for the contact method, it was assumed that all of the surfactant was in the organic phase. This is a good assumption because the excess aqueous phase was a salt solution (20, 30). At low salt concentrations in the excess aqueous phase, the surfactant migrates to the aqueous phase and no reverse micelles are formed. 3.1. Results for the Contact Method All the results presented in this section were obtained in a Winsor II system. Figure 2 shows the equilibrium water uptake per mole of surfactant, w0 , as a function of the initial molar ratio of heptanol to AOT in decane for two different initial NaCl concentrations in the aqueous phase. The initial concentration of surfactant was 410 mM. The water uptake decreased with the addition of heptanol for both initial salt concentrations; however, the difference in the water uptake for different salt concentrations decreased with further addition of heptanol. At low salt concentrations, below approximately 60 mM, almost all of the surfactant migrated to the aqueous phase and no reverse micelles were formed (20, 30). A minimum amount of salt is necessary to salt out the surfactant from the excess aqueous phase into the organic phase, to form reverse micelles. Figure 3 displays similar results for two initial surfactant concentrations. For both surfactant concentrations, the water uptake per mole of surfactant decreased, from essentially the same value, with an increase of the initial molar ratio of heptanol to AOT. As the molar ratio of heptanol to AOT increased, lower water uptakes were obtained with the higher surfactant concentration. Similar results for the effect of different alcohols on the water uptake have been reported for other systems (17).
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FIG. 4. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: distilled water. l, Present data; s, Ref. (25).
Following Wang et al., (17), we consider the following phenomena to explain the decrease of water uptake with alcohol concentration: AOT is an anionic surfactant; therefore, there is an equilibrium between the ionic (S 0 ) and nonionic (SNa) forms of the surfactant: S 0 / Na / S SNa.
[1]
The ionic form of AOT resides at the reverse micellar interface and in the water pool, while the undissociated from is present in the bulk organic phase as well as at the reverse micellar interface. The presence of the alcohol in the bulk organic phase makes it a better solvent for the undissociated form of the surfactant; thus, less surfactant is available to form reverse micelles. In this case, the alcohol acts as a cosolvent. 3.2. Results for the Titration Method Figure 4 presents the molar ratio of water to surfactant in the organic phase as a function of the initial molar ratio of heptanol to AOT in decane using the titration method at 257C with distilled water as the titrant. The results of Hou and Shah (25) are shown for comparison. The maximum amount of water solubilized in the organic phase increased initially with the alcohol concentration and decreased with further addition of alcohol. The region below the solid line in Fig. 4 is a Winsor IV system, whereas the region above the line is a Winsor I system (1). We emphasize that while a minimum concentration of salt is needed in the aqueous phase to form reverse micelles using the contact method, reverse micelles can be formed without salt using the titration method. These results are explained below.
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When the contact method is used with salt concentrations below a certain minimum, the surfactant is solubilized in the excess aqueous phase and no reverse micelles form. When the titration method is used, however, there is no excess aqueous phase and the surfactant remains in the organic phase. Therefore, reverse micelles can be formed with distilled water using the titration method but not the contact method. The surfactant in the organic phase can be solubilized in three different regions: (i) the bulk organic phase, (ii) the reverse micellar interface, and (iii) the water pool in the reverse micelles. Without salt, or at low salt concentrations, a fraction of surfactant is solubilized in the water pool as well as the other two regions. The effect of addition of alcohol in the case of titration can be interpreted as the result of two opposing trends. A small concentration of alcohol in the organic phase will favor the participation of the alcohol at the reverse micellar interface. In this case, the alcohol molecules shield the repulsive forces between the surfactant head groups, thus favoring the migration of surfactant molecules from the water pool to the reverse micellar interface. More surfactant is available for formation of reverse micelles; thus, the water uptake increases. At larger concentrations of alcohol, however, the cosolvent effect of alcohol, as explained in Section 3.1, predominates and the water uptake decreases with an increase in the alcohol concentration. These two opposing trends are responsible for the maximum shown in Fig. 4. Similar maxima in water solubilization capacity were obtained by Hou and Shah (25) using the titration method for the system AOT–alcohol–solvent–water–NaCl. 3.3. Multiple Solubilization Capacity in the Titration Method If extra water is added (by titration) beyond the point at which turbidity appears, shown by solid line in Fig. 4, the extra water will separate and form an excess aqueous phase. If, however, one continues titrating water into the mixture, all the extra water will eventually become solubilized in the organic phase, again forming a single clear isotropic phase. Therefore, at a fixed molar ratio of heptanol/AOT, the system may exhibit more than one transformation from a Winsor IV system to a Winsor I system, solubilizing different amounts of water. We denote this phenomenon as multiple solubilization capacity. These new results, obtained at 257C, are presented in Fig. 5, along with the data from Fig. 4. In Fig. 5, several regions are labeled as Winsor IV and Winsor I. For certain molar ratios of heptanol to AOT, a Winsor I system was transformed into a transparent solution, a new Winsor IV system, when more water was titrated. For example, at a molar ratio of heptanol to AOT of 0.2, the solution remained clear (in the region labeled Winsor IVA) until w0 É 70, where turbidity was observed, as the
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FIG. 5. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: distilled water.
mixture turned into a Winsor I system. The solution remained cloudy as more water was added (Winsor I-B) until, at w0 É 120, the solution became transparent and homogenous again (Winsor IV-B). Adding still more water turned the solution cloudy again, thus entering into the region Winsor I-C. Figure 5 shows that at a fixed molar ratio of heptanol to AOT, multiple solubilization capacities may be observed. Values of w0 as high as 170 mol of water per mole of surfactant can be obtained in a Winsor IV system. At such high water uptakes, the solution became very viscous and no attempt was made to solubilize more water. Similar results were obtained at 357C. The existence of transparent but slowly separating solutions was reported by Delord and Larche´ (31) for the system of AOT–decane–water; however, no results are available for a similar system in the presence of alcohol. They stated that, ‘‘in the early stages (of settling), it is sufficient to shake the tube to reconstitute what appears to be a transparent, uniform solution.’’ They further explained that, ‘‘upon allowing the samples to stand for a period of 1 to 3 months, the phase separation which occurs is no longer reversible, that is, shaking the tube will not reconstitute a single transparent phase.’’ For comparison, samples of the Winsor IV-A region of Fig. 5 having both low and high ratios of heptanol to AOT were left to settle for 75 days. After about 2 weeks, the samples began to separate into two clear phases; however, agitation reconstituted the clear homogenous solution. As time passed, the size of the aqueous phase separating from the initially homogenous sample grew; nevertheless, even after 75 days, agitation of the sample reconstituted the single clear phase. Figure 6 shows the water uptake for the titration of an
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FIG. 6. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: 12.5 mM NaCl.
aqueous solution containing 12.5 mM NaCl into an organic phase consisting of heptanol and AOT in decane. A comparison of Figs. 5 and 6 shows that the addition of salt displaced the Winsor IV regions upward and to the left in the diagram. The use of a more concentrated salt solution as titrant eventually caused the disappearance of the newly found regions and only one Winsor IV and one Winsor II region remained, as shown in Fig. 7 for 50 mM NaCl as the titrant. It is important to note that the region above the curve in Fig. 7 is Winsor II and not Winsor I. Since the salt concentration is higher than the minimum required to form reverse micelles, the separation of an aqueous phase will generate a Winsor II system above the solid line in Fig. 7. Only at salt
FIG. 7. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: 50 mM NaCl.
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FIG. 8. Effect of benzene on water uptake using the titration method at 227C. Initial organic: 40 mM AOT in isooctane. Initial aqueous: distilled water [ s, Ref. (19); n, this work]; 100 m M NaCl ( l ).
concentrations lower than the minimum, will the surfactant migrate to the aqueous phase and form a Winsor I system. At this salt concentration (50 mM NaCl), the water uptake decreased continuously with the molar ratio of heptanol to AOT. We postulate that there is sufficient salt to salt out the surfactant molecules from the water pool. Under this condition, the surfactant molecules are either at the reverse micellar interface or in the bulk organic phase. Therefore, the effect of alcohol is only that of a cosolvent, and the water uptake decreases continuously with alcohol concentration. The existence of several Winsor IV and Winsor I regions in the AOT–solvent–alcohol–salt–water system has interesting consequences. Although these newly observed regions might be metastable, they may be useful for cutting oils and in oil recovery, where the solubilization capacity of water plays an important role; however, these considerations are beyond the scope of this work. The main objective is to show that results obtained from the titration method are very different from those obtained from the contact method and the multiple solubilization capacity is just one of them. To confirm these findings, we studied the effect of benzene, which Eicke (19) considered as a cosurfactant. Figure 8 shows water uptake data for the system AOT–benzene– isooctane–water–NaCl as a function of the molar ratio of benzene to AOT for titration with either distilled water or 100 mM NaCl. This system, without salt, was investigated by Eicke (19), whose data for titration with distilled water are denoted by open circles. Two points measured in this work are shown as open triangles. The filled circles show the results we obtained by titration with 100 mM NaCl. Similar to what we found previously, the water uptake with distilled water as titrant increased with small amounts of benzene and then decreased with further addition of benzene. Thus, as Eicke (19) observed, the effect of benzene is similar
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to that of alcohol in the titration method using distilled water. When NaCl was present in the titrant at a concentration of about 100 mM, the water solubilization decreased continuously with the addition of benzene. The reasons for this behavior are similar to those for alcohol with one major difference. Since benzene is more hydrophobic than alcohol, it cannot approach the surfactant head groups as closely as alcohol can; however, benzene can participate in the interfacial region by being located mainly between the chains of the surfactant molecules. Due to the steric effect, the surfactant molecules are pushed apart, thus favoring the participation of more surfactant at the interface. The surfactant migrates from the water pool to the reverse micellar interface; more surfactant is available for formation of reverse micelles; thus more water is solubilized. Similar to alcohol, however, the cosolvent effect of benzene will predominate at higher benzene concentrations, and the water uptake decreases with an increase in benzene concentration. Therefore, a maximum in water solubilization capacity is observed with distilled water as titrant. At sufficiently high salt concentration, however, the surfactant molecules are salted out from the water pool, and the effect of benzene, similar to that of alcohol, is only that of a cosolvent. Thus, the water uptake decreases continuously with benzene concentration. 3.4. Comparison of the Contact and Titration Methods for Aqueous Salt Solutions Figure 9 shows the water uptake results obtained by titration and by contact using a 100 mM NaCl solution, as the titrant in the titration method and as the initial excess aqueous phase in the contact method, for the AOT–benzene– isooctane system. At this high salt concentration, for both
FIG. 9. Comparison of the contact ( s ) and titration ( l ) methods for the effect of benzene on water uptake at 227C. Initial organic: 40 mM AOT in isooctane. Initial aqueous: 100 mM NaCl.
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4. CONCLUSIONS
FIG. 10. Comparison of the contact ( s ) and titration ( l ) methods for the effect of salt on water uptake at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: NaCl.
methods, the water uptake decreased continuously. For all ratios of benzene to AOT, the contact method gave higher w0 values. Figure 10 presents the water uptakes obtained by titration and by contact for AOT in decane using various NaCl solutions as titrant and as the initial excess aqueous phase. As in Fig. 9, higher water uptakes are obtained by the contact method. This is explained below. In the contact method, the partition of salts from an excess aqueous phase to the water pool of reverse micelles is governed by the partition coefficient, Li , for each salt, Li Å CU i /Ci ,
[2]
where CV i is the concentration of salt in the water pool, and Ci is the concentration of salt in the excess aqueous phase. In fact, for reverse micelles of AOT in n-heptane, formed by the contact method, it has been shown (32) that the sodium chloride partitions weakly in favor of the bulk aqueous phase. If the value of Li is less than unity, the ionic strength inside the water pool is less than that in the initial aqueous solution used for the contact experiment. In the titration method, however, the composition and, thus, the ionic strength of the water pool are identical to those of the titrant. Therefore, the ionic strength of the water pool formed in the titration method is higher than the ionic strength of the water pool formed in the contact method for the same initial salt concentration. The increase in the ionic strength of the water pool produces a higher charge density inside the reverse micelles. The repulsive forces between surfactant charged heads are reduced; they come closer and the size of the reverse micelles decreases, thus reducing the water uptake (18, 29).
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We have demonstrated the differences in water solubilization capacity that result when the titration and contact methods are used to form reverse micelles. We have also proposed phenomenological mechanisms for the findings, which, although not fully tested, explain the experimental results. Even when some of the differences found in water solubilization capacity are the product of presenting two-dimensional sections of three- and four-dimensional spaces, the fact remains that the results obtained with one method cannot be directly extended to the other method. The titration and contact methods gave different water solubilization capacities per mole of surfactant. While a minimum concentration of salt is needed to form reverse micelles by contact, reverse micelles can be formed by titration with distilled water. A single water solubilization capacity was obtained by the contact method, whereas the results obtained by titration are quite different at various salt concentrations. Multiple water solubilization capacities were obtained by titration with distilled water or with salt solutions of low concentration. The Winsor IV phase obtained after titrating distilled water into the organic phase separated into two phases after a period of about 2 weeks. These two phases recombined when agitated. The two equilibrated phases obtained in the contact method remained the same even after many weeks. With titration, the multiple solubilization capacities and the maxima in water uptake as a function of cosurfactant concentration disappear when sufficient salt is present in the titrant. The initial increase in the water uptake with an increase in cosurfactant concentration, in the absence of an electrolyte, is a characteristic of the titration method, but is not obtainable using the contact method. When reverse micelles were formed by the titration and contact methods using identical aqueous solutions, higher water uptakes were obtained via contact. The answer to the question ‘‘Do we have similar effects of a particular variable on the water uptake using the titration and contact methods?’’ is definitely no. Specifically, for the system studied by Leodidis and Hatton (27), we conclude that the increase in water uptake with amino acid concentration observed in their contact method experiments cannot be due to a cosurfactant effect of the amino acid. In the contact method, water uptake decreases with the cosurfactant concentration, at any salt concentration. Water uptake increases with the addition of cosurfactant only with the titration method and only at low salt concentrations or with no salt added. ACKNOWLEDGMENT The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support.
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CS974785
Comparison of the Titration and Contact Methods for the Water Solubilization Capacity of AOT Reverse Micelles in the Presence of a Cosurfactant HAMID R. RABIE, DAMRESS HELOU, MARTIN E. WEBER,
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JUAN H. VERA 1
Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2A7, Canada Received May 17, 1996; accepted January 21, 1997
The method used to form a reverse micellar phase determines the amount of water solubilized in the organic phase. The effect of cosurfactant and salt on the water solubilization capacity per mole of surfactant was determined for two methods of solubilization: contact with an excess aqueous phase and the titration method. The two methods are intrinsically different. While in the titration method, there is a transition from a Winsor IV system to either Winsor I or II system, in the contact method, the experiments are always in a Winsor II system. When the titration method was used at low salt concentrations, or even without salt, the water solubilized increased with the concentration of cosurfactant, at low cosurfactant concentrations. When the contact method was used, the water solubilized per mole of surfactant decreased with an increase in the cosurfactant concentration. When titration was used, at a fixed cosurfactant-to-surfactant molar ratio, several solubilization capacities were obtained at low salt concentrations, whereas a single solubilization capacity was found when the titrant had a higher salt content. With the contact method, a single solubilization capacity was observed for all salt concentrations at which reverse micelles were formed. When the same salt solution was used as the titrant in the titration method and the initial excess aqueous phase in the contact method, a larger amount of water was solubilized by the latter method. q 1997 Academic Press Key Words: reverse micelles; microemulsions; water uptake; surfactants.
1. INTRODUCTION
Reverse micelles are aggregates of surfactant molecules in an apolar solvent that contain microscopic polar cores of solubilized water (water pools). They can solubilize large amounts of water, forming isotropic solutions, sometimes called water-in-oil microemulsions (1). Sodium bis-2(ethylhexyl) sulfosuccinate (AOT) has been used widely as a surfactant because of its ability to form reverse micelles containing large amounts of water without the addition of a cosurfactant (2–5). On the other hand, cationic surfactants 1
To whom all correspondence should be addressed.
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usually require a cosurfactant to form reverse micelles (6– 18). Cosurfactants are some additives that are generally understood to be amphiphilic molecules assisting the emulsifying property of surfactants (19). Based on this definition, different compounds such as alcohols with relatively long chains, benzene, carbon tetrachloride, and nitrobenzene are considered cosurfactants (19). Reverse micelles can be formed in an organic phase by the contact method or by the titration method. The experimental procedure for the contact method is depicted in Fig. 1a and that for the titration method in Fig. 1b. Contact method. A bulk excess aqueous phase is contacted with an organic solvent containing the surfactant and cosurfactant. The excess aqueous phase must contain an electrolyte to prevent the transfer of the surfactant from the organic phase to the aqueous phase (20). After mixing and settling, the organic phase is in equilibrium with an excess aqueous phase, thus forming a Winsor II system (1). The amount of water solubilized is obtained by measuring the water content of the organic phase, usually by Karl Fischer titration. The composition of the water pool is determined by the exchange equilibrium of ions and solutes between the phases. Titration method. An aqueous solution is titrated into an organic solvent containing a surfactant and a cosurfactant. The maximum amount of aqueous solution that can be solubilized is determined from the appearance of permanent turbidity. In this method there is no excess aqueous phase in equilibrium with the organic phase containing the reverse micelles. Since no excess aqueous phase is present, the composition of the water pool is identical to the composition of the titrant. The use of an electrolyte is not required in this method, but, if present in the titrant, its concentration affects the maximum water solubilization. The titration method has been used widely to form reverse micelles as injection fluids for a number of tertiary oil recovery processes (21) and to study different types of reactions inside the reverse micelles (5). The contact method, on the other hand, is identical to the basic step used in extracting
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FIG. 1. Diagram of experimental procedures with (a) contact and (b) titration methods.
solutes from water into a reverse micellar organic phase. In the contact method, it is possible to use an additional region of the phase diagram and not to be limited to the interior of a Winsor IV system, as is the case in the titration method. The material balances for all species, together with the consideration of electroneutrality in the two phases, provide valuable information on the distribution of solutes between the excess aqueous phase and the reverse micellar phase present in the contact method. Previous studies (18, 21–23) have shown that in the contact method, the solubilization capacity for water, or water uptake, is a strong function of the identity of the surfactant counterion. The water uptake is also a function of the ionic strength of the excess aqueous phase and of the nature of the solvent. As well, it depends on the surfactant and cosurfactant type and concentrations and on the temperature. Although many studies have described the effect of different variables on the water uptake obtained by either of these two methods (18–26), no single work compares the water uptakes obtained by the titration and contact methods under similar conditions. The key question is: Do we have similar effects of a particular variable on the water uptake using the titration and contact methods? In some published work, the conclusions drawn from the results obtained with one of these methods were used to explain the results obtained with the other method. For ex-
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ample, when extracting an amino acid using the contact method, Leodidis and Hatton (27) found that the water uptake in the organic phase increased linearly with the initial amino acid concentration. They concluded that this was due to the cosurfactant effect of the amino acid because the water uptake had been shown to increase in the presence of a cosurfactant (19); however, this increase in water uptake with cosurfactant concentration was observed by Eicke (19) using the titration method rather than the contact method used by Leodidis and Hatton (27). We will show that the effect of a cosurfactant is different when the contact method is used. A second example is the recent work of Ashrafizadeh and Demopoulos (28), who studied the effect of tridecanol on the water solubilization capacity of reverse micelles formed by Kelex in kerosene using the contact method. They observed that the water uptake decreased with tridecanol concentration. They stated that tridecanol is a cosurfactant, and thus, the water solubilization capacity should increase initially with tridecanol concentration and decrease with further addition of tridecanol, as observed by Leung and Shah (29). Furthermore, they speculated on this discrepancy. It is important to note that Leung and Shah used the titration method in their experiments rather than the contact method. We believe that the experimental results of Ashrafizadeh and Demopoulos (28) are correct and that there is no reason to expect a maximum in the water solubilization capacity as a function of tridecanol concentration. The objective of this work was to compare the effect of cosurfactant concentration on the water solubilization capacity using the titration and the contact methods under similar conditions. We have chosen the AOT–heptanol–decane– water–salt system as the model system for this study. Some experimental results are also presented for the system of AOT–benzene–isooctane–water–NaCl, the system studied by Eicke (19). In preliminary experiments using the titration method for the AOT–heptanol–decane–water–salt system, we found that different amounts of water could be solubilized in the organic phase, for a fixed initial composition of the organic phase. On the other hand, the contact method produces a single value for the water solubilized in the organic phase, for fixed initial compositions of the phases. The significance of these results, which are reported here for the first time, is discussed in Section 3. Therefore, a second objective of this work was to explore these multiple water solubilization capacities that exist when the titration method is employed. 2. EXPERIMENTAL METHODS
AOT of 99% purity was obtained from Sigma (St. Louis, MO) and used without further purification. Karl Fischer solvent was purchased from BDH Inc. (Toronto, ON), reagentgrade isooctane from Fisher Scientific (Montreal, QC). All
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other chemicals were obtained from Anachemia (Montreal, QC). The water used for experiments was distilled and deionized, with an electrical conductivity lower than 0.8 mS/cm. In the contact method experiments, an excess aqueous NaCl solution was contacted with an organic phase containing AOT in a mixture of heptanol and decane or benzene and isooctane. The initial volumes of the organic phase and the excess aqueous phase were equal. The two phases were shaken vigorously at constant temperature for 2 h and then left to stand for 2 weeks at the same temperature. At equilibrium, there were two clear homogenous phases, an aqueous and an organic phase, in contact. The water content of the organic phase was measured by Karl Fischer titration using a Model 701 titrator (Metrohm Ltd., Herisau, Switzerland). The reproducibility of the solubilization capacity, expressed as the molar ratio of water to surfactant in the organic phase, was about {1. In the titration method experiments, water or an aqueous NaCl solution was titrated directly into a prepared organic phase containing AOT in a mixture of heptanol and decane or benzene and isooctane, under vigorous stirring and constant temperature. The onset of permanent turbidity was taken to indicate the solubilization capacity. Therefore, the maximum water solubilization capacity obtained with the titration method corresponds to a single homogenous organic phase containing solubilized water and no excess aqueous phase is present. The reproducibility of the solubilization capacity, expressed as the molar ratio of water to surfactant in the organic phase, was about {3.5. 3. RESULTS AND DISCUSSION
The experimental data for water solubilization capacity are presented as moles of water per mole of surfactant solubi-
FIG. 2. Effect of heptanol on water uptake for various initial salt concentrations using the contact method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: m, 70 mM NaCl; j, 97 mM NaCl.
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FIG. 3. Effect of heptanol on water uptake for various initial surfactant concentrations using the contact method at 257C. Initial organic: j, 410 mM AOT; or l, 100 mM AOT in decane. Initial aqueous: 97 mM NaCl.
lized in the organic phase, w0 . In computing w0 for the contact method, it was assumed that all of the surfactant was in the organic phase. This is a good assumption because the excess aqueous phase was a salt solution (20, 30). At low salt concentrations in the excess aqueous phase, the surfactant migrates to the aqueous phase and no reverse micelles are formed. 3.1. Results for the Contact Method All the results presented in this section were obtained in a Winsor II system. Figure 2 shows the equilibrium water uptake per mole of surfactant, w0 , as a function of the initial molar ratio of heptanol to AOT in decane for two different initial NaCl concentrations in the aqueous phase. The initial concentration of surfactant was 410 mM. The water uptake decreased with the addition of heptanol for both initial salt concentrations; however, the difference in the water uptake for different salt concentrations decreased with further addition of heptanol. At low salt concentrations, below approximately 60 mM, almost all of the surfactant migrated to the aqueous phase and no reverse micelles were formed (20, 30). A minimum amount of salt is necessary to salt out the surfactant from the excess aqueous phase into the organic phase, to form reverse micelles. Figure 3 displays similar results for two initial surfactant concentrations. For both surfactant concentrations, the water uptake per mole of surfactant decreased, from essentially the same value, with an increase of the initial molar ratio of heptanol to AOT. As the molar ratio of heptanol to AOT increased, lower water uptakes were obtained with the higher surfactant concentration. Similar results for the effect of different alcohols on the water uptake have been reported for other systems (17).
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FIG. 4. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: distilled water. l, Present data; s, Ref. (25).
Following Wang et al., (17), we consider the following phenomena to explain the decrease of water uptake with alcohol concentration: AOT is an anionic surfactant; therefore, there is an equilibrium between the ionic (S 0 ) and nonionic (SNa) forms of the surfactant: S 0 / Na / S SNa.
[1]
The ionic form of AOT resides at the reverse micellar interface and in the water pool, while the undissociated from is present in the bulk organic phase as well as at the reverse micellar interface. The presence of the alcohol in the bulk organic phase makes it a better solvent for the undissociated form of the surfactant; thus, less surfactant is available to form reverse micelles. In this case, the alcohol acts as a cosolvent. 3.2. Results for the Titration Method Figure 4 presents the molar ratio of water to surfactant in the organic phase as a function of the initial molar ratio of heptanol to AOT in decane using the titration method at 257C with distilled water as the titrant. The results of Hou and Shah (25) are shown for comparison. The maximum amount of water solubilized in the organic phase increased initially with the alcohol concentration and decreased with further addition of alcohol. The region below the solid line in Fig. 4 is a Winsor IV system, whereas the region above the line is a Winsor I system (1). We emphasize that while a minimum concentration of salt is needed in the aqueous phase to form reverse micelles using the contact method, reverse micelles can be formed without salt using the titration method. These results are explained below.
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When the contact method is used with salt concentrations below a certain minimum, the surfactant is solubilized in the excess aqueous phase and no reverse micelles form. When the titration method is used, however, there is no excess aqueous phase and the surfactant remains in the organic phase. Therefore, reverse micelles can be formed with distilled water using the titration method but not the contact method. The surfactant in the organic phase can be solubilized in three different regions: (i) the bulk organic phase, (ii) the reverse micellar interface, and (iii) the water pool in the reverse micelles. Without salt, or at low salt concentrations, a fraction of surfactant is solubilized in the water pool as well as the other two regions. The effect of addition of alcohol in the case of titration can be interpreted as the result of two opposing trends. A small concentration of alcohol in the organic phase will favor the participation of the alcohol at the reverse micellar interface. In this case, the alcohol molecules shield the repulsive forces between the surfactant head groups, thus favoring the migration of surfactant molecules from the water pool to the reverse micellar interface. More surfactant is available for formation of reverse micelles; thus, the water uptake increases. At larger concentrations of alcohol, however, the cosolvent effect of alcohol, as explained in Section 3.1, predominates and the water uptake decreases with an increase in the alcohol concentration. These two opposing trends are responsible for the maximum shown in Fig. 4. Similar maxima in water solubilization capacity were obtained by Hou and Shah (25) using the titration method for the system AOT–alcohol–solvent–water–NaCl. 3.3. Multiple Solubilization Capacity in the Titration Method If extra water is added (by titration) beyond the point at which turbidity appears, shown by solid line in Fig. 4, the extra water will separate and form an excess aqueous phase. If, however, one continues titrating water into the mixture, all the extra water will eventually become solubilized in the organic phase, again forming a single clear isotropic phase. Therefore, at a fixed molar ratio of heptanol/AOT, the system may exhibit more than one transformation from a Winsor IV system to a Winsor I system, solubilizing different amounts of water. We denote this phenomenon as multiple solubilization capacity. These new results, obtained at 257C, are presented in Fig. 5, along with the data from Fig. 4. In Fig. 5, several regions are labeled as Winsor IV and Winsor I. For certain molar ratios of heptanol to AOT, a Winsor I system was transformed into a transparent solution, a new Winsor IV system, when more water was titrated. For example, at a molar ratio of heptanol to AOT of 0.2, the solution remained clear (in the region labeled Winsor IVA) until w0 É 70, where turbidity was observed, as the
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FIG. 5. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: distilled water.
mixture turned into a Winsor I system. The solution remained cloudy as more water was added (Winsor I-B) until, at w0 É 120, the solution became transparent and homogenous again (Winsor IV-B). Adding still more water turned the solution cloudy again, thus entering into the region Winsor I-C. Figure 5 shows that at a fixed molar ratio of heptanol to AOT, multiple solubilization capacities may be observed. Values of w0 as high as 170 mol of water per mole of surfactant can be obtained in a Winsor IV system. At such high water uptakes, the solution became very viscous and no attempt was made to solubilize more water. Similar results were obtained at 357C. The existence of transparent but slowly separating solutions was reported by Delord and Larche´ (31) for the system of AOT–decane–water; however, no results are available for a similar system in the presence of alcohol. They stated that, ‘‘in the early stages (of settling), it is sufficient to shake the tube to reconstitute what appears to be a transparent, uniform solution.’’ They further explained that, ‘‘upon allowing the samples to stand for a period of 1 to 3 months, the phase separation which occurs is no longer reversible, that is, shaking the tube will not reconstitute a single transparent phase.’’ For comparison, samples of the Winsor IV-A region of Fig. 5 having both low and high ratios of heptanol to AOT were left to settle for 75 days. After about 2 weeks, the samples began to separate into two clear phases; however, agitation reconstituted the clear homogenous solution. As time passed, the size of the aqueous phase separating from the initially homogenous sample grew; nevertheless, even after 75 days, agitation of the sample reconstituted the single clear phase. Figure 6 shows the water uptake for the titration of an
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FIG. 6. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: 12.5 mM NaCl.
aqueous solution containing 12.5 mM NaCl into an organic phase consisting of heptanol and AOT in decane. A comparison of Figs. 5 and 6 shows that the addition of salt displaced the Winsor IV regions upward and to the left in the diagram. The use of a more concentrated salt solution as titrant eventually caused the disappearance of the newly found regions and only one Winsor IV and one Winsor II region remained, as shown in Fig. 7 for 50 mM NaCl as the titrant. It is important to note that the region above the curve in Fig. 7 is Winsor II and not Winsor I. Since the salt concentration is higher than the minimum required to form reverse micelles, the separation of an aqueous phase will generate a Winsor II system above the solid line in Fig. 7. Only at salt
FIG. 7. Effect of heptanol on water uptake using the titration method at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: 50 mM NaCl.
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FIG. 8. Effect of benzene on water uptake using the titration method at 227C. Initial organic: 40 mM AOT in isooctane. Initial aqueous: distilled water [ s, Ref. (19); n, this work]; 100 m M NaCl ( l ).
concentrations lower than the minimum, will the surfactant migrate to the aqueous phase and form a Winsor I system. At this salt concentration (50 mM NaCl), the water uptake decreased continuously with the molar ratio of heptanol to AOT. We postulate that there is sufficient salt to salt out the surfactant molecules from the water pool. Under this condition, the surfactant molecules are either at the reverse micellar interface or in the bulk organic phase. Therefore, the effect of alcohol is only that of a cosolvent, and the water uptake decreases continuously with alcohol concentration. The existence of several Winsor IV and Winsor I regions in the AOT–solvent–alcohol–salt–water system has interesting consequences. Although these newly observed regions might be metastable, they may be useful for cutting oils and in oil recovery, where the solubilization capacity of water plays an important role; however, these considerations are beyond the scope of this work. The main objective is to show that results obtained from the titration method are very different from those obtained from the contact method and the multiple solubilization capacity is just one of them. To confirm these findings, we studied the effect of benzene, which Eicke (19) considered as a cosurfactant. Figure 8 shows water uptake data for the system AOT–benzene– isooctane–water–NaCl as a function of the molar ratio of benzene to AOT for titration with either distilled water or 100 mM NaCl. This system, without salt, was investigated by Eicke (19), whose data for titration with distilled water are denoted by open circles. Two points measured in this work are shown as open triangles. The filled circles show the results we obtained by titration with 100 mM NaCl. Similar to what we found previously, the water uptake with distilled water as titrant increased with small amounts of benzene and then decreased with further addition of benzene. Thus, as Eicke (19) observed, the effect of benzene is similar
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to that of alcohol in the titration method using distilled water. When NaCl was present in the titrant at a concentration of about 100 mM, the water solubilization decreased continuously with the addition of benzene. The reasons for this behavior are similar to those for alcohol with one major difference. Since benzene is more hydrophobic than alcohol, it cannot approach the surfactant head groups as closely as alcohol can; however, benzene can participate in the interfacial region by being located mainly between the chains of the surfactant molecules. Due to the steric effect, the surfactant molecules are pushed apart, thus favoring the participation of more surfactant at the interface. The surfactant migrates from the water pool to the reverse micellar interface; more surfactant is available for formation of reverse micelles; thus more water is solubilized. Similar to alcohol, however, the cosolvent effect of benzene will predominate at higher benzene concentrations, and the water uptake decreases with an increase in benzene concentration. Therefore, a maximum in water solubilization capacity is observed with distilled water as titrant. At sufficiently high salt concentration, however, the surfactant molecules are salted out from the water pool, and the effect of benzene, similar to that of alcohol, is only that of a cosolvent. Thus, the water uptake decreases continuously with benzene concentration. 3.4. Comparison of the Contact and Titration Methods for Aqueous Salt Solutions Figure 9 shows the water uptake results obtained by titration and by contact using a 100 mM NaCl solution, as the titrant in the titration method and as the initial excess aqueous phase in the contact method, for the AOT–benzene– isooctane system. At this high salt concentration, for both
FIG. 9. Comparison of the contact ( s ) and titration ( l ) methods for the effect of benzene on water uptake at 227C. Initial organic: 40 mM AOT in isooctane. Initial aqueous: 100 mM NaCl.
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4. CONCLUSIONS
FIG. 10. Comparison of the contact ( s ) and titration ( l ) methods for the effect of salt on water uptake at 257C. Initial organic: 410 mM AOT in decane. Initial aqueous: NaCl.
methods, the water uptake decreased continuously. For all ratios of benzene to AOT, the contact method gave higher w0 values. Figure 10 presents the water uptakes obtained by titration and by contact for AOT in decane using various NaCl solutions as titrant and as the initial excess aqueous phase. As in Fig. 9, higher water uptakes are obtained by the contact method. This is explained below. In the contact method, the partition of salts from an excess aqueous phase to the water pool of reverse micelles is governed by the partition coefficient, Li , for each salt, Li Å CU i /Ci ,
[2]
where CV i is the concentration of salt in the water pool, and Ci is the concentration of salt in the excess aqueous phase. In fact, for reverse micelles of AOT in n-heptane, formed by the contact method, it has been shown (32) that the sodium chloride partitions weakly in favor of the bulk aqueous phase. If the value of Li is less than unity, the ionic strength inside the water pool is less than that in the initial aqueous solution used for the contact experiment. In the titration method, however, the composition and, thus, the ionic strength of the water pool are identical to those of the titrant. Therefore, the ionic strength of the water pool formed in the titration method is higher than the ionic strength of the water pool formed in the contact method for the same initial salt concentration. The increase in the ionic strength of the water pool produces a higher charge density inside the reverse micelles. The repulsive forces between surfactant charged heads are reduced; they come closer and the size of the reverse micelles decreases, thus reducing the water uptake (18, 29).
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We have demonstrated the differences in water solubilization capacity that result when the titration and contact methods are used to form reverse micelles. We have also proposed phenomenological mechanisms for the findings, which, although not fully tested, explain the experimental results. Even when some of the differences found in water solubilization capacity are the product of presenting two-dimensional sections of three- and four-dimensional spaces, the fact remains that the results obtained with one method cannot be directly extended to the other method. The titration and contact methods gave different water solubilization capacities per mole of surfactant. While a minimum concentration of salt is needed to form reverse micelles by contact, reverse micelles can be formed by titration with distilled water. A single water solubilization capacity was obtained by the contact method, whereas the results obtained by titration are quite different at various salt concentrations. Multiple water solubilization capacities were obtained by titration with distilled water or with salt solutions of low concentration. The Winsor IV phase obtained after titrating distilled water into the organic phase separated into two phases after a period of about 2 weeks. These two phases recombined when agitated. The two equilibrated phases obtained in the contact method remained the same even after many weeks. With titration, the multiple solubilization capacities and the maxima in water uptake as a function of cosurfactant concentration disappear when sufficient salt is present in the titrant. The initial increase in the water uptake with an increase in cosurfactant concentration, in the absence of an electrolyte, is a characteristic of the titration method, but is not obtainable using the contact method. When reverse micelles were formed by the titration and contact methods using identical aqueous solutions, higher water uptakes were obtained via contact. The answer to the question ‘‘Do we have similar effects of a particular variable on the water uptake using the titration and contact methods?’’ is definitely no. Specifically, for the system studied by Leodidis and Hatton (27), we conclude that the increase in water uptake with amino acid concentration observed in their contact method experiments cannot be due to a cosurfactant effect of the amino acid. In the contact method, water uptake decreases with the cosurfactant concentration, at any salt concentration. Water uptake increases with the addition of cosurfactant only with the titration method and only at low salt concentrations or with no salt added. ACKNOWLEDGMENT The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support.
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