Origin of the sphere-to-rod transition in cationic micelles with aromatic counterions: specific ion hydration in the interfacial region matters |
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Authors: | Geng Yan Romsted Laurence S Froehner Sandro Zanette Dino Magid Linda J Cuccovia Iolanda M Chaimovich Hernan |
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Affiliation: | Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. |
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Abstract: | Sphere-to-rod transitions of cetyltrimethylammonium (CTA+) micelles with dichlorobenzoate counterions are remarkably substituent dependent. Simultaneous estimates of the interfacial molarities of H2O, MeOH, and Cl- and 2,6- and 3,5-dichlorobenzoate (2,6OBz and 3,5OBz) counterions were obtained by the chemical trapping method in mixed micelles of CTACl/CTA3,5OBz and CTACl/CTA2,6OBz without added salt. Increasing the CTA3,5OBz mole fraction produces a marked concurrent increase in interfacial 3,5OBz- and a decrease in interfacial H2O concentrations through the sphere-to-rod transition. No abrupt concentration changes are observed with increasing CTA2,6OBz mole fraction. Counterion-specific changes in the interfacial water concentration may be a major contributor to the delicate balance of forces governing micellar morphology. |
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