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Ind. Eng. Chem. Res. 2000, 39, 2704-2708
Supersolubilization in Chlorinated Hydrocarbon Microemulsions: Solubilization Enhancement by Lipophilic and Hydrophilic Linkers Hirotaka Uchiyama,†,‡ Edgar Acosta,†,‡ San Tran,†,‡ David A. Sabatini,†,§ and Jeffrey H. Harwell*,†,‡ Institute for Applied Surfactant Research, School of Chemical Engineering and Material Science, and Department of Civil Engineering and Environmental Science, University of Oklahoma, Norman, Oklahoma 73019
In this paper, the effect of linker molecules on the solubilization capacity of an anionic surfactant system (sodium dihexyl sulfosuccinate) is studied. N-Alkyl alcohols are used as lipophilic linkers in middle-phase microemulsions of trichloroethylene, tetrachloroethylene, and hexane. The lipophilic linker effect increases the solubilization capacity of the anionic surfactant system. The solubilization parameter for both hydrocarbons and chlorinated hydrocarbon oil increases as a function of alcohol concentration. As the alkyl chain length of the alcohol linker molecule increases, the solubilization capacity increases. Moreover, the longer chain alcohol is more effective at linking oil molecules for hexane than for chlorinated hydrocarbon oils, indicating that the linker effect is more effective for higher equivilant alkane carbon number (EACN) oils. Sodium mono- and dimethylnaphthalenesulfonate is proposed as a hydrophilic linker to enhance the solubilization of lower EACN oils. The combination of lipophilic and hydrophilic linkers synergistically enhances the solubilization capacity of chlorinated hydrocarbon microemulsions. Introduction Chlorinated hydrocarbons such as trichloroethylene (TCE) and tetrachloroethylene (PCE) are widely used as industrial solvents and degreasers. As a result, groundwater and soil under major industrial sites are frequently polluted with TCE and PCE, posing a threat to local drinking water. Chlorinated hydrocarbon subsurface contamination has been addressed by using surfactants in a technique called surfactant-enhanced aquifer remediation (SEAR).1-3 Using this technique, the aqueous contaminant solubility in water is increased by the presence of surfactant micelles, enabling the removal of more contaminant with less water flushing through the contaminated area. Surfactants can also be used to promote contaminant displacement, where the capillary forces entrapping the contaminant are overcome by the ultralow interfacial tension achieved by optimal surfactant formulations. Numerous studies have evaluated the design of surfactant systems for SEAR application.4-8 Because every site and contaminant mixture has unique characteristics, the surfactant system must be tailored to achieve the maximum removal at the lowest cost (environmental and monetary). In general, a suitable surfactant for these purposes should have high coalescence rates, clear (neat) microemulsion phases, a high solubilization capacity, no precipitation, and a liquid crystal or gel formation at subsurface conditions and should be environmentally acceptable.8 There is considerable interest in increasing the solubilization capacity of surfactant systems for hydrocarbon contaminants. Salager, Graciaa, Lachaise, Cucuphat, * To whom all correspondence should be addressed. † Institute for Applied Surfactant Research. ‡ School of Chemical Engineering and Material Science. § Department of Civil Engineering and Environmental Science.
Figure 1. Schematic of molecular interactions including lipophilic linker molecules (4% AMA).
and Bourrel found that using long-chain alcohol and low-EON surfactants improves the solubilization of alkanes in microemulsions with ethoxylated nonylphenol surfactants.9-11 They proposed the “lipophilic linker” effect of the alcohol to improve the interaction between a surfactant and an alkane oil, therefore enhancing the solubilization capacity of surfactants. According to this approach, the linker provides an “extension” of the surfactant interaction deeper into the oil phase. The lipophilic linker is adsorbed “behind” the interface, in what is known as the palisade layer, also promoting some orientation of the oil molecules.12 Figure 1 shows a schematic of the lipophilic linker effect. Using hydrophobic polar molecules, such as a longchain alcohol, the surfactant interactions with the oil phase can be enhanced, thereby increasing the solubilization capacity. Continuing the search for new ways to formulate surfactant systems for SEAR purposes, this “linker” concept is applied here for chlorinated hydro-
10.1021/ie990819p CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000
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carbons and hexane using sodium dihexylsulfosuccinate (Aerosol-MA, AMA) as the main surfactant molecule. In the present work, middle-phase microemulsion formation is studied in the presence of normal alcohol as “lipophilic linker molecules” for chlorinated hydrocarbon oils. We report the effect of such linker molecules on chlorinated hydrocarbon microemulsion formation using anionic surfactants. We also propose a new approach to enhance the solubilization capacity of the microemulsion by introducing a “hydrophilic linker molecule”. The two effects are shown to be synergistic. Experimental Section Materials. TCE, PCE, hexane (99%+), n-alcohols C6-C18 (98+%), oleyl alcohol (85%), and sodium chloride were supplied by Aldrich and used without further purification. Sodium mono- and dimethylnaphthalenesulfonate (SMDNS) was obtained from Witco. AMA was supplied by CYTEC as an 80% solution (2-propanol,
