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Effect of Pentanol Partitioning on Solubilization of Tetrachloroethylene and Gasoline by Sodium Dodecyl Sulfate Micelles
Journal of Colloid and Interface Science 241, 199–204 (2001) doi:10.1006/jcis.2001.7639, available online at http://www.idealibrary.com on
Effect of Pentanol Partitioning on Solubilization of Tetrachloroethylene and Gasoline by Sodium Dodecyl Sulfate Micelles1 Meifang Zhou∗ and R. Dean Rhue†,2 ∗ South Florida Water Management District, Chemistry Laboratory Section, 3301 Gun Club Road, P.O. Box 24680, West Palm Beach, Florida 33416-4680; and †Soil and Water Science Department, University of Florida, IFAS, P.O. Box 110290, Gainesville, Florida 32611 Received November 7, 2000; accepted April 13, 2000
The effect of interfacial pentanol concentrations on solubilization of tetrachloroethylene (PCE) and gasoline by sodium dodecyl sulfate (SDS) micelles was compared to that for dodecane solubilization, which had been measured in a previous study. The solubilization of PCE and gasoline reached their maximum values at a 1 : 3 SDS-to-pentanol molar ratio in the interface. As pentanol concentrations increased beyond that necessary for interfacial saturation, solubilization of PCE and gasoline decreased. This behavior was similar to that observed when dodecane was the oil phase. Electrical conductivity of aqueous SDS/pentanol solutions followed a trend similar to that for oil solubilization, reaching a maximum value at a 1 : 3 molar ratio of SDS to pentanol in the interface. The results of this and previous studies suggest that pentanol partitioning in SDS micelles can be described by a simple two-region model: Region I is the interface between the water–continuous phase and oil and Region II is the micelle inner core. When the mole fraction of pentanol in the interface is less than 0.75, pentanol partitions strongly into Region I, where it acts as a cosurfactant along with SDS and enhances oil solubilization. Above 0.75 mole fraction in the interface, pentanol partitions strongly into Region II, where it acts as a polar oil and competes with other oils for solubilization. ° C 2001
Academic Press
INTRODUCTION
In attempts to solubilize oil as a microemulsion, at least one alcohol is often included with the surfactant in the formulation (1–5). An alcohol acting as a cosurfactant in the microemulsion interface can increase oil solubilization and decrease the interfacial tension (IFT). Martel and co-workers (2) reported that the average optimum ratio of surfactant to cosurfactant on a weight basis was near 1 : 1, but the variability of the optimum ratio becomes quite large when different surfactant/cosurfactant concentrations are used in the microemulsion systems. This may be because only the cosurfactant that resides in the interface con1 This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series R-08054. 2 To whom correspondence should be addressed. E-mail: [email protected]. ufl.edu.
tributes to an increase in oil solubilization and a decrease in IFT, i.e., the molar ratio of surfactant to cosurfactant in the interface may be more important in a microemulsion than the total amount of cosurfactant in the system. For water-in-oil microemulsions with long chain alcohol cosurfactants (C6, or higher), the Schulman–Bowcott plot has been used to calculate the ratio of surfactant to alcohol in the interface (6–8). It has been reported that in this type of microemulsion, water solubilization reaches a maximum at a 1 : 3 molar ratio of interfacial potassium oleate to hexanol (6, 8). Likewise, microemulsion stability was found to be the greatest when the interfacial molar ratio of sodium dodecyl sulfate (SDS) to cetyl alcohol was 1 : 3 (9, 10). Shah (8) proposed that the hexagonal arrangement at a 1 : 3 molar ratio in the interface provides the closest packing of molecules, resulting in the highest solubilization and the greatest stability for the microemulsion system. The effect of interfacial ratios of surfactant to cosurfactant on oil solubilization for oil-in-water (o/w) microemulsions has not been studied extensively. Recently, equilibrium partitioning coupled with a “pseudophase” model was used to calculate the molar ratios of SDS to pentanol in the interface of o/w microemulsions and to examine the optimum surfactant to cosurfactant ratio for dodecane solubilization (11, 12). These authors found that the interface became saturated with pentanol at a molar ratio of SDS to pentanol of 1 : 3. Beyond this point, pentanol partitioned strongly into the dodecane. Solubilization of dodecane, a nonpolar oil, increased linearly up to the point of interfacial pentanol saturation, after which no further increase in solubilization occurred. Aoudia et al. (13) reported that butanol saturated the sodium 4-dodecylbenzenesulfonate (SDBS) micelle interface at a molar ratio of 1 : 3. This was deduced by Aoudia et al. by measuring the degree of fluorescence polarization emitted from the excimer species of the SDBS in the micellar interface. In this work, the effect of the interfacial ratio of SDS to pentanol on the solubilization of a polar oil, tetrachloroethylene (PCE), and a nonpolar/polar oil mixture, gasoline, has been evaluated and compared with that of dodecane. The results of this study along with those reported in a previous paper (11) support the concept of a simple two-region model for the effects of pentanol on oil solubilization by SDS micelles.
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C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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sample to flow through the capillary using the relationships
MATERIALS AND METHODS
Chemicals SDS was purchased from Fisher Scientific and used without further purification. Pentanol, isopropanol, NaCl, KCl, and CaCl2 were also obtained from Fisher Scientific and had purities of 99+%. Dodecane and PCE were purchased from Sigma at 99+% purity. Regular unleaded gasoline was obtained from a local service station. Oil Solubilization The maximum solubilization of dodecane, PCE, and gasoline as o/w microemulsions was determined by titration of aqueous mixtures containing various concentrations of SDS, salt, and alcohol. Table 1 lists the components that were used in six microemulsion systems from which data were obtained in this study. Five systems were used to measure oil solubilization, while the sixth consisted of aqueous SDS/pentanol mixtures used to measure conductivity and viscosity. Titrations with oil were conducted by first placing aqueous mixtures containing known amounts of SDS, alcohol, and either water or salt solution into a vial. The oil was titrated into these aqueous mixtures, drop by drop, forming clear o/w microemulsions. Titration was continued until the microemulsion became oil saturated, at which time the vials were weighed and the amount of oil that had been added to the point of saturation was calculated from the initial and final weights. The amounts of each component in the microemulsion are reported in units of weight percent (wt%). In those instances where a salt was included with the water component, the weight percent corresponds to the weight of the water plus salt in the system. Viscosity and Conductivity The viscosity of the surfactant/alcohol solutions was measured using an Ostwald capillary viscometer tube at 24◦ C. Viscosity was related to the time (t) required for a given volume of TABLE 1 Components Used in Six Microemulsion Systems
η b = vk = at + , ρ t
[1]
where η is the dynamic viscosity, ρ the density of the fluid, and vk the kinematic viscosity, and a and b are constants obtained by a least squares regression analysis. The last term is related to the kinetic energy correction and is negligible for flow times over about 2 min. The electrical conductivity of surfactant/alcohol solutions was determined with a digital conductivity meter (Model 1054, Amber Science Inc). Calculating Interfacial Concentrations of Pentanol The development of the pseudophase model for partitioning of alcohol among aqueous, oil, and interfacial phases in an anionic surfactant microemulsion system has been described in detail by Zhou and Rhue (11). Briefly, the pseudophase model assumes that a microemulsion can be represented as three phases: an aqueous phase, an oil phase, and the interface. The aqueous phase consists of water, salt (if present), and alcohol. The interface consists of surfactant and alcohol. The oil phase consists of oil and alcohol and includes oil solubilized inside the SDS micelles as well as any excess oil that may be in equilibrium with the water–continuous microemulsion as would occur, for example, in a Winsor Type I system. The model assumes that pentanol partitions freely among these three phases. The partitioning of pentanol between the aqueous and oil phases is complicated by the fact that alcohols, such as pentanol, self-associate in the oil phase; i.e., monomers combine to form dimers, trimers, etc. Taking the self-association into account, the coefficient for pentanol partitioning between aqueous and oil phases, K PC , was shown to be equivalent to the expressions (11) K PC =
CPo K K0 o C + K 0, w = CP 100ρ P
[2]
where CPo and CPw are the pentanol concentrations in the oil and aqueous phases, respectively, and ρ is the density of the pure alcohol. K is the equilibrium constant for self-association of pentanol in the oil phase,
Microemulsion component System
Oil phase
SDS (Wt.%)
Salt
Alcohol
Data obtained
I II III
Dodecane PCE PCE
3.0 2.5–3.5 1.0
Gasoline
0–10
Pentanol Pentanol Pentanol, IPA Pentanol
Oil solubilized Oil solubilized Oil solubilized
IV V
Gasoline
0–10
Pentanol
Oil solubilized
VI
None
None None 100 mM NaCl 50 mM KCl 10 mM CaCl2 None
Pentanol
Conductivity, viscosity
3.0
Oil solubilized
Ai + A1 = Ai+1 ,
[3]
where A1 represents a single molecule of alcohol and Ai and Ai+1 represent molecular aggregates containing i and i + 1 molecules of pentanol, respectively. K 0 in Eq. [2] represents the equilibrium between pentanol in the aqueous phase and pentanol monomers, A1 , in the oil phase. In the previous study, the partitioning of pentanol between the aqueous and oil phases was assumed to be the same whether surfactant was present or not. For the simplified three-component system consisting of water, oil, and pentanol, K PC was calculated
PENTANOL PARTITIONING AND OIL SOLUBILIZATION BY SDS MICELLES
201
from the measured concentrations of pentanol, CPo and CPw . From a plot of K PC against CPo , which was linear, K and K 0 were obtained from the resulting slope and intercept. For the aqueous SDS/pentanol/dodecane system, the following equation described the partitioning of pentanol between water and dodecane with an R 2 of 0.999 (11): K PC = 0.060 CPo + 0.49.
[4]
From this relationship, K and K 0 were found to be 99 ± 2 and 0.49 ± 0.01, respectively (11). Equation [4] was then used to partition pentanol between the oil and aqueous phases when SDS was present. Knowing the total amount of pentanol in the system and measuring the amounts of pentanol in the oil and aqueous phases, the pentanol associated with the interfacial region was calculated by difference based on a mass balance for the pentanol. The mole fraction–based partition coefficient for pentanol partitioning between the aqueous phase and the interface, K PX , is given by K PX =
X Pm , X Pw
[5]
where X Pm is the mole fraction of pentanol in the interface and X Pw is the mole fraction of pentanol in the aqueous phase. For the aqueous SDS/pentanol/dodecane system, K PX was found to be 225 ± 1 (11). This value was used in this study to partition pentanol between water and the interface when the oil phase was dodecane, PCE, or gasoline. Thus, it was assumed that the partitioning of pentanol between aqueous and interfacial phases was not affected by the type of oil used.
FIG. 1. Dodecane solubilized vs. pentanol.
after interfacial saturation as pentanol concentration increased further. PCE is more polar than dodecane, and the partitioning of pentanol into PCE following interfacial saturation had a more detrimental effect on solubilization than in the case of dodecane.
RESULTS AND DISCUSSION
The solubilization of dodecane and PCE as a function of the total pentanol concentration in the microemulsion is shown in Figs. 1 and 2. Arrows in these figures indicate the pentanol concentrations where the molar ratio of SDS to pentanol in the interface was calculated to be 1 : 3 based on the pseudophase model. Dodecane solubilization increased rapidly with pentanol concentration up to the point where the interface saturated with pentanol. Dodecane solubilization leveled off after this point and then decreased sharply as the pentanol concentration increased above about 6 wt%. Pentanol was shown previously to partition strongly into the dodecane after saturating the interface (11), thus competing with dodecane for space in the interior of the micelle. The plateau observed in Fig. 1 may have been the result of two separate processes: decreasing hydrophobicity of the oil phase with increasing pentanol and competition between dodecane and pentanol for space in the micelle inner core. In a similar manner, solubilization of PCE also increased with pentanol concentration, reaching a maximum at a molar ratio of SDS to pentanol in the interface of 1 : 3 (Fig. 2). However, unlike dodecane solubilization, PCE solubilization began to decrease immediately
FIG. 2. PCE Solubilized vs. pentanol.
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FIG. 3. Wt.% of 50 mMKCl solution.
Figures 3 and 4 represent the water-rich corners of two pseudo-ternary-phase diagrams for aqueous SDS/pentanol systems, for one of which the aqueous phase contained 50 mM KCl, for the other 10 mM CaCl2 . The water-rich corner of the
water/SDS/pentanol phase diagram is predominantly a singlephase region in which the SDS and pentanol form normal micelles dispersed in a water continuous background (referred to as the L1 domain). Water/SDS/pentanol compositions were
FIG. 4. Wt.% of 10 MmCaCl2 solution.
PENTANOL PARTITIONING AND OIL SOLUBILIZATION BY SDS MICELLES
selected from the L1 domain and titrated with gasoline to determine the maximum amount that these solutions could solubilize. Superimposed on the water/SDS/pentanol phase diagrams in Figs. 3 and 4 are isoconcentration contours representing the maximum amounts of gasoline, in wt%, that were solubilized. The presence of KCl and CaCl2 at these concentrations increased solubilization over that obtained with water alone without significantly affecting the size of the L1 domain (data for water not shown). The dotted line in each figure represents points where the interfacial ratio of SDS to pentanol was calculated to be 1 : 3 according to the pseudophase model. For a given amount of gasoline solubilized, the most effective combination of SDS and pentanol was that which resulted in a 1 : 3 molar ratio of SDS to pentanol in the interface. The solubilization data for gasoline as a function of pentanol concentration is similar to that for PCE in that it increases rapidly as the mole fraction of pentanol in the interface approaches saturation and then decreases sharply after interfacial saturation. The increased partitioning of pentanol into the gasoline after interfacial saturation was detrimental to the solubilization of gasoline. Above about 6 wt% SDS, the maximum solubilization of gasoline no longer corresponded to the 1 : 3 molar ratio (Figs. 3 and 4). The SDS micelle structure changes at concentrations above about 5 wt% from spherical to rod-shaped (12). There is no guarantee that partitioning will be the same for these new structures, and the 1 : 3 ratio for interfacial saturation may no longer be applicable at these higher surfactant concentrations. The addition of salt at pentanol concentrations below interfacial saturation increases oil solubilization, while at pentanol concentrations above interfacial saturation, salt decreases solubilization (11). Salt compresses the electric double layer at the membrane/water interface, resulting in closer packing of SDS and pentanol in the interface. Salt increases the viscosity of surfactant solutions and induces gel formation as pentanol concentrations approach interfacial saturation. The decrease in oil solubilization by salt coincides with the formation of these more rigid membrane structures. The effect of isopropanol (IPA) on PCE solubilization is shown in Fig. 5. Beginning at pentanol concentrations around 2 wt%, IPA resulted in a small decrease in oil solubilization. At the point where pentanol saturated the interface, solubilization decreased whether IPA was present or not, but it decreased at a slower rate in the presence of IPA so that, at pentanol concentrations greater than about 3 wt%, oil solubilization was greater with IPA present. Although maximum PCE solubilization was greater in the absence of IPA, the microemulsion phase formed a gel, as indicated in Fig. 5. Branched short chain alcohols such as IPA tend to destabilize the interface by decreasing the viscosity, thereby preventing gel formation. The effect of pentanol concentration on conductance and viscosity of aqueous SDS/pentanol solutions (no salt and no oil) is shown in Fig. 6. Electrical conductivity increased to a maximum that corresponded with pentanol saturation of the interface while viscosity continued to increase well beyond interfacial satura-
203
FIG. 5. PCE solubilized vs. pentanol.
tion. It has been shown that alcohols such as pentanol increase the degree of ionization of ionic micelles, increasing surface charge density and, thereby, increasing conductance (14, 15). The maximum conductivity for hexadecyltrimethyammonium
FIG. 6. Electrical conductivity (µmhos) vs. pentanol.
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the SDS micelles will remain spherical with increasing pentanol concentration as long as SDS concentrations remain below about 5 wt%. In this SDS concentration range, the micelles should simply increase in diameter as more pentanol is added to the system and finally break to form the L1 + L2 two-phase systems as the pentanol concentration exceeds some critical concentration. The viscosity data in Fig. 6 support this conclusion. The results of this and a previous study (11) suggest that pentanol partitioning in SDS micelles can be described by a simple two-region model (Fig. 7). In region I, which is the interface between the water–continuous phase and oil, pentanol acts as a cosurfactant along with SDS. When the mole fraction of pentanol in the interface is less than 0.75, i.e., a 1 : 3 SDS-to-pentanol molar ratio, pentanol partitions strongly into region I, where it enhances oil solubilization. At a mole fraction of 0.75, the interface has become saturated and no additional pentanol can enter the interfacial region. At interfacial mole fractions above 0.75, pentanol partitions strongly into region II, which is the micelle inner core. In region II, pentanol acts as a polar oil and competes with other oils for solubilization. REFERENCES
FIG. 7. Aqueous phase.
bromide/hexanol systems was also near the 1 : 3 surfactant-toalcohol molar ratio in the interface (16). The effect of pentanol on micelle surface charge might be expected to continue up to the point of interfacial saturation. Beyond this point, pentanol partitions into the micellar inner core, where it should have little or no effect on surface charge but would increase the size of the micelle and thereby decrease conductance. It has also been shown that alcohols decrease the critical micelle concentration (CMC) of surfactant solutions and increase the aggregate number of the micelles (15, 17, 18, 19). These latter effects should decrease the conductance but are apparently secondary to the effect on surface charge, as indicated by the increase in conductance up to interfacial saturation (Fig. 6). The increase in viscosity of aqueous SDS/pentanol solutions below interfacial saturation could be explained by the fact that the size of the micelles is increasing with pentanol concentration due to increasing aggregate number. The continued increase in viscosity above interfacial saturation with pentanol could be due to further increases in micelle size as pentanol begins to partition into the inner core. The L1 (normal micelle)/(L1 + L2) (normal micelle + reverse micelle) and L1 (normal micelle)/(L1 + D) (normal micelle + lamellar) phase behavior of water/SDS/pentanol systems (12, 20) suggests that
1. Desnoyers, J. E., Quirion, F., Hetu, D., and Perron, G., Can. J. Chem. Eng. 61, 672 (1983). 2. Martel, R., Gelinas, P. J., Desnoyers, J. E., and Masson, A., Ground Water 31, 789 (1993). 3. Martel, R., and Gelinas, P. J., Ground Water 34, 143 (1996). 4. Rhue, R. D., Annable, M. D., and Rao, P. S. C., J. Environ. Qual. 28, 1135 (1999). 5. Jawitz, J. W., Annable, M. D., Rao, P. S. C., and Rhue, R. D., Environ. Sci. Technol. 32, 523 (1998). 6. Bowcott, J. E., and Schulman, J. H., Z. Elektrochem. 59, 283 (1955). 7. Pithapurwala, Y. K., and Shah, D. O., Chem. Eng. Commun. 29, 101 (1984). 8. Shah, D., J. Colloid Interface Sci. 37, 744 (1971). 9. Choi, Y. T., El-Asser, M. S., Sudol, E. D., and Vanderhoff, J. W., J. Polym. Sci. Polym. Chem. Edn. 23, 2973 (1985). 10. Wang, C. C., Yu, N. S., Chen, C. Y., and Kuo, J. F., Polymer 37, 2509 (1996). 11. Zhou, M., and Rhue, R. D., J. Colloid Interface Sci. 228, 18 (1999). 12. Guerinand, G., and Bellocq, A. M., J. Phys. Chem. 92, 2550 (1988). 13. Aoudia, M., Rodgers, M. A. J., and Wade, W. H., J. Colloid Interface Sci. 144, 353 (1991). 14. Manable, M., Koda, M., and Shirahama, K., J. Colloid Interface Sci. 77, 189 (1980). 15. Attwood, D., Mosquera, V., Rodriguez, J., Garcia, M., and Suarez, M. J., Colloid Polym. Sci. 272, 584 (1995). 16. Vikholm, I., Douheret, G., Backlund, S., and Hoiland, H., J. Colloid Interface Sci. 116, 582 (1987). 17. Hayase, K., and Hayano, S., J. Colloid Interface Sci. 63, 446 (1978). 18. Muto, Y., Yoda, K., Yoshida, N., Esumi, K., Meguro, K., Binana-Limbele, W., and Zana, R., J. Colloid Interface Sci. 130, 165 (1989). 19. Forland, G. M., Samseth, J., Hoiland, H., and Mortensen, K., J. Colloid Interface Sci. 164, 163 (1993). 20. Clausse, M., Nicolas-Morgantini, L., Zradba, A., and Touraud, D., in “Microemulsion Systems” (H. L. Rosano and M. Clausse, Eds.), p. 15. Dekker, New York, 1987.
Effect of Pentanol Partitioning on Solubilization of Tetrachloroethylene and Gasoline by Sodium Dodecyl Sulfate Micelles1 Meifang Zhou∗ and R. Dean Rhue†,2 ∗ South Florida Water Management District, Chemistry Laboratory Section, 3301 Gun Club Road, P.O. Box 24680, West Palm Beach, Florida 33416-4680; and †Soil and Water Science Department, University of Florida, IFAS, P.O. Box 110290, Gainesville, Florida 32611 Received November 7, 2000; accepted April 13, 2000
The effect of interfacial pentanol concentrations on solubilization of tetrachloroethylene (PCE) and gasoline by sodium dodecyl sulfate (SDS) micelles was compared to that for dodecane solubilization, which had been measured in a previous study. The solubilization of PCE and gasoline reached their maximum values at a 1 : 3 SDS-to-pentanol molar ratio in the interface. As pentanol concentrations increased beyond that necessary for interfacial saturation, solubilization of PCE and gasoline decreased. This behavior was similar to that observed when dodecane was the oil phase. Electrical conductivity of aqueous SDS/pentanol solutions followed a trend similar to that for oil solubilization, reaching a maximum value at a 1 : 3 molar ratio of SDS to pentanol in the interface. The results of this and previous studies suggest that pentanol partitioning in SDS micelles can be described by a simple two-region model: Region I is the interface between the water–continuous phase and oil and Region II is the micelle inner core. When the mole fraction of pentanol in the interface is less than 0.75, pentanol partitions strongly into Region I, where it acts as a cosurfactant along with SDS and enhances oil solubilization. Above 0.75 mole fraction in the interface, pentanol partitions strongly into Region II, where it acts as a polar oil and competes with other oils for solubilization. ° C 2001
Academic Press
INTRODUCTION
In attempts to solubilize oil as a microemulsion, at least one alcohol is often included with the surfactant in the formulation (1–5). An alcohol acting as a cosurfactant in the microemulsion interface can increase oil solubilization and decrease the interfacial tension (IFT). Martel and co-workers (2) reported that the average optimum ratio of surfactant to cosurfactant on a weight basis was near 1 : 1, but the variability of the optimum ratio becomes quite large when different surfactant/cosurfactant concentrations are used in the microemulsion systems. This may be because only the cosurfactant that resides in the interface con1 This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series R-08054. 2 To whom correspondence should be addressed. E-mail: [email protected]. ufl.edu.
tributes to an increase in oil solubilization and a decrease in IFT, i.e., the molar ratio of surfactant to cosurfactant in the interface may be more important in a microemulsion than the total amount of cosurfactant in the system. For water-in-oil microemulsions with long chain alcohol cosurfactants (C6, or higher), the Schulman–Bowcott plot has been used to calculate the ratio of surfactant to alcohol in the interface (6–8). It has been reported that in this type of microemulsion, water solubilization reaches a maximum at a 1 : 3 molar ratio of interfacial potassium oleate to hexanol (6, 8). Likewise, microemulsion stability was found to be the greatest when the interfacial molar ratio of sodium dodecyl sulfate (SDS) to cetyl alcohol was 1 : 3 (9, 10). Shah (8) proposed that the hexagonal arrangement at a 1 : 3 molar ratio in the interface provides the closest packing of molecules, resulting in the highest solubilization and the greatest stability for the microemulsion system. The effect of interfacial ratios of surfactant to cosurfactant on oil solubilization for oil-in-water (o/w) microemulsions has not been studied extensively. Recently, equilibrium partitioning coupled with a “pseudophase” model was used to calculate the molar ratios of SDS to pentanol in the interface of o/w microemulsions and to examine the optimum surfactant to cosurfactant ratio for dodecane solubilization (11, 12). These authors found that the interface became saturated with pentanol at a molar ratio of SDS to pentanol of 1 : 3. Beyond this point, pentanol partitioned strongly into the dodecane. Solubilization of dodecane, a nonpolar oil, increased linearly up to the point of interfacial pentanol saturation, after which no further increase in solubilization occurred. Aoudia et al. (13) reported that butanol saturated the sodium 4-dodecylbenzenesulfonate (SDBS) micelle interface at a molar ratio of 1 : 3. This was deduced by Aoudia et al. by measuring the degree of fluorescence polarization emitted from the excimer species of the SDBS in the micellar interface. In this work, the effect of the interfacial ratio of SDS to pentanol on the solubilization of a polar oil, tetrachloroethylene (PCE), and a nonpolar/polar oil mixture, gasoline, has been evaluated and compared with that of dodecane. The results of this study along with those reported in a previous paper (11) support the concept of a simple two-region model for the effects of pentanol on oil solubilization by SDS micelles.
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C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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sample to flow through the capillary using the relationships
MATERIALS AND METHODS
Chemicals SDS was purchased from Fisher Scientific and used without further purification. Pentanol, isopropanol, NaCl, KCl, and CaCl2 were also obtained from Fisher Scientific and had purities of 99+%. Dodecane and PCE were purchased from Sigma at 99+% purity. Regular unleaded gasoline was obtained from a local service station. Oil Solubilization The maximum solubilization of dodecane, PCE, and gasoline as o/w microemulsions was determined by titration of aqueous mixtures containing various concentrations of SDS, salt, and alcohol. Table 1 lists the components that were used in six microemulsion systems from which data were obtained in this study. Five systems were used to measure oil solubilization, while the sixth consisted of aqueous SDS/pentanol mixtures used to measure conductivity and viscosity. Titrations with oil were conducted by first placing aqueous mixtures containing known amounts of SDS, alcohol, and either water or salt solution into a vial. The oil was titrated into these aqueous mixtures, drop by drop, forming clear o/w microemulsions. Titration was continued until the microemulsion became oil saturated, at which time the vials were weighed and the amount of oil that had been added to the point of saturation was calculated from the initial and final weights. The amounts of each component in the microemulsion are reported in units of weight percent (wt%). In those instances where a salt was included with the water component, the weight percent corresponds to the weight of the water plus salt in the system. Viscosity and Conductivity The viscosity of the surfactant/alcohol solutions was measured using an Ostwald capillary viscometer tube at 24◦ C. Viscosity was related to the time (t) required for a given volume of TABLE 1 Components Used in Six Microemulsion Systems
η b = vk = at + , ρ t
[1]
where η is the dynamic viscosity, ρ the density of the fluid, and vk the kinematic viscosity, and a and b are constants obtained by a least squares regression analysis. The last term is related to the kinetic energy correction and is negligible for flow times over about 2 min. The electrical conductivity of surfactant/alcohol solutions was determined with a digital conductivity meter (Model 1054, Amber Science Inc). Calculating Interfacial Concentrations of Pentanol The development of the pseudophase model for partitioning of alcohol among aqueous, oil, and interfacial phases in an anionic surfactant microemulsion system has been described in detail by Zhou and Rhue (11). Briefly, the pseudophase model assumes that a microemulsion can be represented as three phases: an aqueous phase, an oil phase, and the interface. The aqueous phase consists of water, salt (if present), and alcohol. The interface consists of surfactant and alcohol. The oil phase consists of oil and alcohol and includes oil solubilized inside the SDS micelles as well as any excess oil that may be in equilibrium with the water–continuous microemulsion as would occur, for example, in a Winsor Type I system. The model assumes that pentanol partitions freely among these three phases. The partitioning of pentanol between the aqueous and oil phases is complicated by the fact that alcohols, such as pentanol, self-associate in the oil phase; i.e., monomers combine to form dimers, trimers, etc. Taking the self-association into account, the coefficient for pentanol partitioning between aqueous and oil phases, K PC , was shown to be equivalent to the expressions (11) K PC =
CPo K K0 o C + K 0, w = CP 100ρ P
[2]
where CPo and CPw are the pentanol concentrations in the oil and aqueous phases, respectively, and ρ is the density of the pure alcohol. K is the equilibrium constant for self-association of pentanol in the oil phase,
Microemulsion component System
Oil phase
SDS (Wt.%)
Salt
Alcohol
Data obtained
I II III
Dodecane PCE PCE
3.0 2.5–3.5 1.0
Gasoline
0–10
Pentanol Pentanol Pentanol, IPA Pentanol
Oil solubilized Oil solubilized Oil solubilized
IV V
Gasoline
0–10
Pentanol
Oil solubilized
VI
None
None None 100 mM NaCl 50 mM KCl 10 mM CaCl2 None
Pentanol
Conductivity, viscosity
3.0
Oil solubilized
Ai + A1 = Ai+1 ,
[3]
where A1 represents a single molecule of alcohol and Ai and Ai+1 represent molecular aggregates containing i and i + 1 molecules of pentanol, respectively. K 0 in Eq. [2] represents the equilibrium between pentanol in the aqueous phase and pentanol monomers, A1 , in the oil phase. In the previous study, the partitioning of pentanol between the aqueous and oil phases was assumed to be the same whether surfactant was present or not. For the simplified three-component system consisting of water, oil, and pentanol, K PC was calculated
PENTANOL PARTITIONING AND OIL SOLUBILIZATION BY SDS MICELLES
201
from the measured concentrations of pentanol, CPo and CPw . From a plot of K PC against CPo , which was linear, K and K 0 were obtained from the resulting slope and intercept. For the aqueous SDS/pentanol/dodecane system, the following equation described the partitioning of pentanol between water and dodecane with an R 2 of 0.999 (11): K PC = 0.060 CPo + 0.49.
[4]
From this relationship, K and K 0 were found to be 99 ± 2 and 0.49 ± 0.01, respectively (11). Equation [4] was then used to partition pentanol between the oil and aqueous phases when SDS was present. Knowing the total amount of pentanol in the system and measuring the amounts of pentanol in the oil and aqueous phases, the pentanol associated with the interfacial region was calculated by difference based on a mass balance for the pentanol. The mole fraction–based partition coefficient for pentanol partitioning between the aqueous phase and the interface, K PX , is given by K PX =
X Pm , X Pw
[5]
where X Pm is the mole fraction of pentanol in the interface and X Pw is the mole fraction of pentanol in the aqueous phase. For the aqueous SDS/pentanol/dodecane system, K PX was found to be 225 ± 1 (11). This value was used in this study to partition pentanol between water and the interface when the oil phase was dodecane, PCE, or gasoline. Thus, it was assumed that the partitioning of pentanol between aqueous and interfacial phases was not affected by the type of oil used.
FIG. 1. Dodecane solubilized vs. pentanol.
after interfacial saturation as pentanol concentration increased further. PCE is more polar than dodecane, and the partitioning of pentanol into PCE following interfacial saturation had a more detrimental effect on solubilization than in the case of dodecane.
RESULTS AND DISCUSSION
The solubilization of dodecane and PCE as a function of the total pentanol concentration in the microemulsion is shown in Figs. 1 and 2. Arrows in these figures indicate the pentanol concentrations where the molar ratio of SDS to pentanol in the interface was calculated to be 1 : 3 based on the pseudophase model. Dodecane solubilization increased rapidly with pentanol concentration up to the point where the interface saturated with pentanol. Dodecane solubilization leveled off after this point and then decreased sharply as the pentanol concentration increased above about 6 wt%. Pentanol was shown previously to partition strongly into the dodecane after saturating the interface (11), thus competing with dodecane for space in the interior of the micelle. The plateau observed in Fig. 1 may have been the result of two separate processes: decreasing hydrophobicity of the oil phase with increasing pentanol and competition between dodecane and pentanol for space in the micelle inner core. In a similar manner, solubilization of PCE also increased with pentanol concentration, reaching a maximum at a molar ratio of SDS to pentanol in the interface of 1 : 3 (Fig. 2). However, unlike dodecane solubilization, PCE solubilization began to decrease immediately
FIG. 2. PCE Solubilized vs. pentanol.
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FIG. 3. Wt.% of 50 mMKCl solution.
Figures 3 and 4 represent the water-rich corners of two pseudo-ternary-phase diagrams for aqueous SDS/pentanol systems, for one of which the aqueous phase contained 50 mM KCl, for the other 10 mM CaCl2 . The water-rich corner of the
water/SDS/pentanol phase diagram is predominantly a singlephase region in which the SDS and pentanol form normal micelles dispersed in a water continuous background (referred to as the L1 domain). Water/SDS/pentanol compositions were
FIG. 4. Wt.% of 10 MmCaCl2 solution.
PENTANOL PARTITIONING AND OIL SOLUBILIZATION BY SDS MICELLES
selected from the L1 domain and titrated with gasoline to determine the maximum amount that these solutions could solubilize. Superimposed on the water/SDS/pentanol phase diagrams in Figs. 3 and 4 are isoconcentration contours representing the maximum amounts of gasoline, in wt%, that were solubilized. The presence of KCl and CaCl2 at these concentrations increased solubilization over that obtained with water alone without significantly affecting the size of the L1 domain (data for water not shown). The dotted line in each figure represents points where the interfacial ratio of SDS to pentanol was calculated to be 1 : 3 according to the pseudophase model. For a given amount of gasoline solubilized, the most effective combination of SDS and pentanol was that which resulted in a 1 : 3 molar ratio of SDS to pentanol in the interface. The solubilization data for gasoline as a function of pentanol concentration is similar to that for PCE in that it increases rapidly as the mole fraction of pentanol in the interface approaches saturation and then decreases sharply after interfacial saturation. The increased partitioning of pentanol into the gasoline after interfacial saturation was detrimental to the solubilization of gasoline. Above about 6 wt% SDS, the maximum solubilization of gasoline no longer corresponded to the 1 : 3 molar ratio (Figs. 3 and 4). The SDS micelle structure changes at concentrations above about 5 wt% from spherical to rod-shaped (12). There is no guarantee that partitioning will be the same for these new structures, and the 1 : 3 ratio for interfacial saturation may no longer be applicable at these higher surfactant concentrations. The addition of salt at pentanol concentrations below interfacial saturation increases oil solubilization, while at pentanol concentrations above interfacial saturation, salt decreases solubilization (11). Salt compresses the electric double layer at the membrane/water interface, resulting in closer packing of SDS and pentanol in the interface. Salt increases the viscosity of surfactant solutions and induces gel formation as pentanol concentrations approach interfacial saturation. The decrease in oil solubilization by salt coincides with the formation of these more rigid membrane structures. The effect of isopropanol (IPA) on PCE solubilization is shown in Fig. 5. Beginning at pentanol concentrations around 2 wt%, IPA resulted in a small decrease in oil solubilization. At the point where pentanol saturated the interface, solubilization decreased whether IPA was present or not, but it decreased at a slower rate in the presence of IPA so that, at pentanol concentrations greater than about 3 wt%, oil solubilization was greater with IPA present. Although maximum PCE solubilization was greater in the absence of IPA, the microemulsion phase formed a gel, as indicated in Fig. 5. Branched short chain alcohols such as IPA tend to destabilize the interface by decreasing the viscosity, thereby preventing gel formation. The effect of pentanol concentration on conductance and viscosity of aqueous SDS/pentanol solutions (no salt and no oil) is shown in Fig. 6. Electrical conductivity increased to a maximum that corresponded with pentanol saturation of the interface while viscosity continued to increase well beyond interfacial satura-
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FIG. 5. PCE solubilized vs. pentanol.
tion. It has been shown that alcohols such as pentanol increase the degree of ionization of ionic micelles, increasing surface charge density and, thereby, increasing conductance (14, 15). The maximum conductivity for hexadecyltrimethyammonium
FIG. 6. Electrical conductivity (µmhos) vs. pentanol.
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the SDS micelles will remain spherical with increasing pentanol concentration as long as SDS concentrations remain below about 5 wt%. In this SDS concentration range, the micelles should simply increase in diameter as more pentanol is added to the system and finally break to form the L1 + L2 two-phase systems as the pentanol concentration exceeds some critical concentration. The viscosity data in Fig. 6 support this conclusion. The results of this and a previous study (11) suggest that pentanol partitioning in SDS micelles can be described by a simple two-region model (Fig. 7). In region I, which is the interface between the water–continuous phase and oil, pentanol acts as a cosurfactant along with SDS. When the mole fraction of pentanol in the interface is less than 0.75, i.e., a 1 : 3 SDS-to-pentanol molar ratio, pentanol partitions strongly into region I, where it enhances oil solubilization. At a mole fraction of 0.75, the interface has become saturated and no additional pentanol can enter the interfacial region. At interfacial mole fractions above 0.75, pentanol partitions strongly into region II, which is the micelle inner core. In region II, pentanol acts as a polar oil and competes with other oils for solubilization. REFERENCES
FIG. 7. Aqueous phase.
bromide/hexanol systems was also near the 1 : 3 surfactant-toalcohol molar ratio in the interface (16). The effect of pentanol on micelle surface charge might be expected to continue up to the point of interfacial saturation. Beyond this point, pentanol partitions into the micellar inner core, where it should have little or no effect on surface charge but would increase the size of the micelle and thereby decrease conductance. It has also been shown that alcohols decrease the critical micelle concentration (CMC) of surfactant solutions and increase the aggregate number of the micelles (15, 17, 18, 19). These latter effects should decrease the conductance but are apparently secondary to the effect on surface charge, as indicated by the increase in conductance up to interfacial saturation (Fig. 6). The increase in viscosity of aqueous SDS/pentanol solutions below interfacial saturation could be explained by the fact that the size of the micelles is increasing with pentanol concentration due to increasing aggregate number. The continued increase in viscosity above interfacial saturation with pentanol could be due to further increases in micelle size as pentanol begins to partition into the inner core. The L1 (normal micelle)/(L1 + L2) (normal micelle + reverse micelle) and L1 (normal micelle)/(L1 + D) (normal micelle + lamellar) phase behavior of water/SDS/pentanol systems (12, 20) suggests that
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