Langmuir 1992,8, 1885-1888
1885
The Role of Water in the Formation of Reversed Micelles: An Antimicellization Agent Zhi-Jian Yu, Nai-Fu Zhou,? and Ronald D. Neuman* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849 Received January 8,1992.I n Final Form: June 8,1992 Micellization of sodium bis(2-ethylhexyl) phosphate in n-heptane has been studied under controlled environmental conditions by dynamic and static light scattering. The results clearly show that a trace amount of water has a very dramatic effect on reversed micellization. In contrast with results in the literature, water can function as an antimicellizationagent. The generalityof and the evidencefor supporting the current view that water is a prerequisite for the formation of reversed micelles are discussed and criticized.
Introduction In spite of the increased attention which recently has been given to reversed micellar systems,l-1° the solution behavior of surfactants in apolar or nonpolar media is still incompletely understood. One of the unsolved fundamental questions is the effect of water on reversed micellization. Although it now appears to be taken for granted that water is required for the formation of micelles, very few studies have actually been made on the role of water in reversed micellization. Perhaps the limited number of reported investigations has been due, in part, to experimental difficulties associated with such measurements, for example, the determination of the critical micelle concentration (cmc) for surfactant/apolar solvent systems.11-13 Eicke and Christen considered the effect of water on surfactant aggregation in apolar solvents and, along the line of Zundel’s findings,14“showed” that the formation of reversed micelles depends on the presence of water in surfactant/apolar solvent systems.15 Apparently, after considering the problems of drying apolar solvents and performing subsequent measurements under controlled humidity conditions, Eicke and co-workers took another approach in studying the effect of water, i.e., an investigation of reversed micellar sizes of sodium bis(2-ethylhexy1)sulfosuccinate (AOT) and alkylated quaternary ammonium bis(2-ethylhexyl) sulfosuccinate in ben-
* To whom correspondence should be addressed.
Present address: Geological Survey of Alabama, Tuscaloosa, AL 3u82-9mo. (1) Smith, R. D.; Fulton, J. L.; Blitz, J. P.;Tingey,J. M. J. Phys. Chem. 1990,94,781. (2) Olesik, S. V.; Miller, C. J. Langmuir 1990, 6, 183. (3) Verbeeck, A.; Voortmane, G.; Jackers, C.; De Schryver,F. C. Langmuir 1989, 6,766. (4) Sahyun, M. R. V. J. Phys. Chem. 1988,92,6028. (6) Ueda, M.; Schelly, 2. A. Langmuir 1988,4,653. (6) Faure, A.; Tistchenko, A. M.; Chachaty, C. J. Phys. Chem. 1987, 91, 1827. (7) Verbeeck, A.; Gelade, E.; Schryver, F. C. De. Langmuir 1986,42, 448. (8) Luiei, P. L.; Straub, B. E. Reverse Micelles: Biological and TechmbgicalRelevance of Amphiphilic Structuresin Apolar Media;Plenum Press: New York, 1984. (9) Schelly, 2.A. In Aggregation Processes in Solution; Wyn-Jones, E., Gormally, J., E&; Elsevier Scientific: New York, 1983; p 140. (10) Eicke, H.F. Top. Curr. Chem. 1980,87, 85. (11) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980, 84, 1349. (12) Kertea, A. S.In Micellization, Solubilization and Microemulsiom; Mittal, K. L., Ed.; Plenum: New York, 1977; p 445. (13) Kertee, A. S.;Gutman, H. In Surface and Colloid Science; Matijevic, E., Ed.;Wiley: New York, 1976; Vol. 8, p 193. (14) Zundel, G. Hydration andzntermolecularlnteraction; Academic: New York, 1969. (15) Eicke, H. F.; Christen, H. Helv. Chim. Acta 1978, 61, 2258. f
zene.15J6 These workers, supposing a decreased hydration interaction between water and the alkylated ammonium ion compared with the sodium ion, concluded that water functions as a “gluing” agent or is prerequisite for micellization in apolar media. The “gluing” effect of water on reversed micellization has also been accepted by other i n v e s t i g a t ~ r s . ~In ~ ~aJ ~ study of AOT in apolar solvents, Schelly and co-workers emphasized that reversed micellar solutions are hygroscopic and water, which does not behave as a neutral impurity, is always p r e ~ e n t . They ~ ? ~ carefully investigated AOT/isooctane and AOT/cyclohexane systems by controlled partial pressure-vapor pressure osmometry and obtained a smaller micellar size5 than investigators18Jg who did not attempt to control the water content of the reversed micellar systems. This finding seems to support the proposition, as the authors stated in the literature review section of ref 5, that water promotes the aggregation of amphiphilic surfactants to reversed micelles and that part of the water present serves as a “gluing”agent between the polar head groups of the surfactant monomers in the core of the reversed micellar aggregates. Verbeeck and co-workers have also examined the effect of water on the aggregation of AOT and other surfactants in cyclohexane by fluorescence decay and UV absorption spectro~copy.~ These investigators concluded that water stimulates the aggregation of monomers at low surfactant concentration, but they then claimed that the addition of water has no influence on the critical micelle concentration (cmc). So far, previous investigations of the effect of water on reversed micellizationhave been mainly restricted to AOT/ apolar solvent systems. In order to test the generality of whether water promotes reversed micellization, it is necessary to investigate other classes of surfactants. Sodium bis(2-ethylhexyl) phosphate (NaDEHP), which is structurally similar to AOT, also readily forms reversed micelles in apolar solvents. Earlier research on NaDEHP, however, was carried out with wet solvents or the particular property measurements were performed in uncontrolled humidity environments,6pm26and to our knowledge studies (16) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979,83, 480. (17) Rosen, M.J. Surfactants and ZnterfacialPhenomena;John Wiley & Sons: New York, 1989, p 150. (18) Ueno, M.;Kiahimoto, H. Bull. Chem. SOC.Jpn. 1977,50,1631. (19) Kon-no, K.; Kitahara, A. J. Colloid Interface. Sci. 1971,35,636. (20) Faure, A.; Tistchenko, A. M.; Zemb, T.; Chachaty, C. J. Phys. Chem. 1985,89, 3373. (21) Faure, A.; Tistchenko, A. M.; Chachaty, C. In Reverse Micelles: Biological and Technological Relevance of Amphiphilic Structures in Apolar Media; Luisi, P. L., Straub, B. E., Eds; Plenum Press: New York, 1984; p 511. (22) Eicke, H.-F.; Christen, H. J. Colloid Interface Sci. 1974,48,281.
Q743-7463/92/24Q8-1885$Q3.oO/Q0 1992 American Chemical Society
1886 Langmuir, Vol. 8, No. 8, 1992
of NaDEHP or other surfactants in apolar solvents under truly dry conditions have not been reported in the literature. We have been interested in the aggregation behavior of metal-extractant complexes in an ongoing investigation of the interfacial chemistry in organophosphorus solvent extraction systems and have concluded that metal-extractant aggregation is analogous to that for classical surfactants in apolar s ~ l v e n t s . ~Therefore, ~-~~ in order to further understand the extraction of metals by bis(2-ethylhexyl) phosphoric acid (HDEHP), knowledge of the aggregation behavior of NaDEHP in apolar media is also important since NaDEHP forms in the liquid/liquid extraction system when the aqueous phase pH is adjusted with sodium hydroxide. Herein, we report our novel findings that water is not a prerequisite for reversed micellization in the NaDEHP/ n-heptane system. Instead, water can function as an antimicellization agent.
Experimental Materials and Methods HDEHP (Alfa, 96%) was purified followingthe procedures of ~~ was Partridge and JensenMas reported p r e v i ~ u s l y .NaDEHP synthesized by reflux of a HDEHPln-hexane solution over metallic sodium for 72 h. The n-hexane in the clear NaDEHP/ n-hexane solution was then removed in a rotary evaporator. The solid NaDEHP was further dried under high vacuum with the solvent being removed from the sample and trapped by activated carbon at the temperature of liquid nitrogen. The n-hexane (Phillips 66,99mol % ) was distilled twice. The n-heptane (Aldrich, HPLC grade)was twice distilled and dehydrated by metallic sodium. The NaDEHPln-heptane solutions were prepared by weighing proper amounts of NaDEHP and n-heptane. All operations for preparing “water-free” solutions and “optically clean” lightscattering samples were carried out within a glovebox in which an analyticalbalance and the optical cleaning apparatus described elsewhere32were installed. The chamber atmosphere of the glovebox was replaced by recirculating ultrahigh purity grade nitrogen which was dried by passage through columns of silica gel and sodium hydroxide. The chamber was further dried by P206which occupied about 50 % of the total area of the glovebox. The weight increase of one dish of PzO6 was traced by the analytical balance, and the removal of the moisture from the chamber was indicated when the weight of the PZOSbecame constant. The NaDEHP/ n-heptane solutions were transferred to Pyrex light-scattering cells, and any dust was removed by continuous filtration using the optical cleaning apparatus in the “dryn environment of the glovebox. The light-scattering cells were then sealed by tightly fitting Teflon caps which were further sealed with Parafilm. The samples so prepared were immediately subjected to lightscattering measurements. Dynamic and static light-scattering measurements were performed with a Brookhaven BI-2OOSM multiangle goniometer in conjunctionwith aLexelModel95-4 argon ion laser. The incident laser beam (A = 488 nm) was vertically polarized. A laser power of 200 mW was used in the light-scattering measurements reported herein. A Brookhaven 128-channelBI-2030ATdigital correlatorl (23) Eicke, H.-F.; Christen, H. J. Colloid Interface Sci. 1974,46,417. (24) Eicke, H. F.; Arnold, V. J. Colloid Interface Sci. 1974, 46, 101. (25) McDowell, W. J., Coleman, C. F. J.Inorg. Nucl. Chem. 1965,27, 1117.. (26) Myers, A. L.; Mcdowell, W. J.; Coleman, C. F. J. Znorg. Nucl. Chem. 1964,26, 2005. (27) Neuman, R. D.; Jones, M. A.; Zhou, N.-F. Colloids Surf. 1990,46, 45. (28) Neuman,R. D.;Zhou, N.-F.; Wu, J.; Jones, M.A.;Gaonkar,A. G.; Park, S. J.; Agrawal, M. L. Sep. Sci. Technol. 1990,25, 1655. (29) Neuman, R. D.; Park, S. J. J. Colloid Interface Sci., in press. (30) Partridge, J. A.; Jensen, R. C. J. Inorg. Nucl. Chem. 1969, 31, 2587. (31) Gaonkar, A. G.; Neuman, R. D. Sep.Purif.Methods 1984,13,141. (32) Yu, 2.-J.; Neuman, R. D. Langmuir 1992, 8, 2074.
Letters
a
[NaDEHP] (mM)
Figure 1. Scattered intensity as a function of surfactant concentration at 20 “C for solutions of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane prepared in the ambient room atmosphere. The scattering angle was 90°. computer was used to process the output signal of a Malvern RR51 photomultipler tube.
Results and Discussion It is to be emphasized that the novelty of the experimental findings presented herein would not be so dramatically evident if it were not for the fact that we were able to prepare optically clean samples for dynamic and static light-scattering measurements. It is very difficult to obtain reliable light-scattering measurements on dilute surfactant solutions, especially when only extremely small particles, e.g., surfactant aggregates,occur in apolar media. In fact, the scattered intensity of apolar surfactant solutions has been indistinguishable from that of the pure solvent in past light-scattering studies.’O Recently, however, we have developed a very simple and effective technique to prepare optically clean samples of surfactant solutions,32 thereby enabling high-quality lightscattering measurements on dilute surfactant systems where the elimination of dust is very critical. Figure 1shows the scattered intensity as a function of NaDEHP concentration at a temperature of 20 O C . The solutions were prepared without special control of humidity, i.e., the solvent was freshly distilled in the ambient room atmosphere and the solutions were prepared in the same atmosphere which was a t a relative humidity of about 60%. The water content of the n-heptane solvent was found to be 40 ppm by Karl-Fischer titration. The molar ratio of water to NaDEHP a t 2 mM NaDEHP was about 1.2. For the sake of convenience, we refer to the solutions prepared without humidity control as “wet” solutions throughout this paper. Two characteristic concentration regions can be clearly identified in Figure 1: in the low concentration region, the scattered intensity increases slowly with increasing concentration of NaDEHP; in the high concentration region, the scattered intensity increases tremendously. The abrupt change in scattered intensity which occurred at a NaDEHP concentration of 2 mM can be regarded as the onset of the formation of reversed micelles. Below this concentration the scattered intensity appears to increase almost linearly with increasing surfactant concentration, thereby indicating that the surfactant molecules in this concentration region exist as monomers or nuclei22in the surfactant solution. Above this concentration, however, the surfactant molecules or nuclei aggregate to form relatively large reversed micelles;for example, the apparent hydrodynamic radius measured by dynamic light scat-
Langmuir, Val. 8, No. 8,1992 1887
Letters 1
11
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[NaDEHP] (mM) Fiuure 2. Scattered intensity as a function of surfactant concentrationat 25 O C for solutionsof sodium bis(2-ethylhexyl) phosphate (NaDEHP)in n-heptane prepared under controlled humidity condition (0)and exposed to the ambient room atmosphere (+). The scattering angle was 90’. tering at 2.8mM is 33 nm (preliminary experiments suggest the shape of micellar aggregates is cylindrical), which markedly enhances the intensity of the scattered light. Figure 2 shows the relationship between scattered intensity and NaDEHP concentration at 25 “C under controlled humidity conditions; i.e., the samples (data points of “0”)were prepared following the procedures described in the Experimental Section. The term “dry” solution as used herein applies to those solutions prepared with the previously dried NaDEHP and n-heptane and under the controlled humidity environment. The water content of the “dry” solutions was found to be not measurable by Karl-Fischer titration, which indicates the water content to be
