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ABSTRACT:
Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilize styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCI). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1:3; however, the SHDPDS/DPCI mixture at a ratio of 3:1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCI adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHDPDS/DPCI systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle.
Paper no. S1509 in JSD 9, 29-37 (Qrt. 1, 2006).
KEY WORDS: Adsolubilization, anionic surfactant, cationic surfactant, ethylcyclohexane, mixed surfactant, silica, solubilization, styrene.
Abbreviations: CMC, critical micelle concentration; DPC1, dodecylpyridinium chloride; GC, gas chromatography; MSR, molar solubilization ratio; PODD, pentamethyl-octadecyl-1,3-propane diammonium dichloride; SDS, sodium dodecyl sulfate; SHDPDS, sodium hexadecyl-diphenyloxide disulfonate.
Micelles are formed when the surfactant concentration exceeds the critical micelle concentration (CMC). Above the CMC, surfactant monomers associate with one another to form micelles, which have a hydrophobic interior. When an oil phase is in contact with an aqueous micellar solution, oil molecules partition into the hydrophobic core of these micelles, a process known as solubilization (1-3). Solubilization is sometimes expressed as the amount of oil solubilized per mass, volume, or moles of surfactant present in micelles at saturation (1) (Fig. 1). Surfactant systems with higher solubilization capacities are desirable, as they reduce surfactant requirements and formulation costs in applications such as surfactant-based separation...