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Effects of Interfacial Specific Cation and Water Molarities on AOT Micelle-to-Vesicle Transitions by Chemical Trapping: The Specific Ion-Pair/Hydration Model

Abstract

Salt induced micelle-to-vesicle transitions of ionic surfactants depend on surfactant chain length, headgroup structure, counterion type and concentration, but the interfacial molarities of counterions and water that balance the hydrophobic effect are difficult to determine. In anionic micelles of twin-tailed sodium bis (2-ethylhexyl)sulfosuccinate (AOT), the chemical trapping (CT) method provides estimates of the interfacial molarities of anionic headgroups (RSO3–m) and neutral (H2Om) nucleophiles during salt induced transitions of AOT micelles to vesicles. Product yields were measured by HPLC from the competitive dediazoniation reaction using a specially designed hydrophobic probe, 4-hexadecyl-2,6-dimethylbenzenediazonium cation, 16-ArN2+. The reactions were run at constant, 15 mM AOT mixed with 0 to 50 mM added salts, containing cations of different sizes and valences including tetraalkylammonium cations (MR4+, R = 1-4) and metal cations (M1-3+). Parallel reactions in aqueous salt solutions with a short chain analog, 1-ArN2+, were used as references to calculate interfacial molarities. Aggregates were structurally characterized by TEM and DLS. Typically, interfacial RSO3– molarities increase with added salt from 1 to 2 M and water molarities decrease from about 40 to 20 M as the micelles transition to vesicles. These changes are consistent with the Ion-Pair/Hydration model, in which the added cations form neutral but polar ion-pairs with RSO3– that had a lower demand for hydration and water was released into the surrounding aqueous phase. The extent of ion-pairing increases with cation size, charge and hydrophobicity and decreases in interfacial water molarity, which permits tighter interfacial packing and vesicle formation at lower added salt concentrations.

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Publication details

The article was received on 23 Sep 2018, accepted on 12 Feb 2019 and first published on 13 Feb 2019


Article type: Paper
DOI: 10.1039/C8CP05987J
Citation: Phys. Chem. Chem. Phys., 2019, Accepted Manuscript

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    Effects of Interfacial Specific Cation and Water Molarities on AOT Micelle-to-Vesicle Transitions by Chemical Trapping: The Specific Ion-Pair/Hydration Model

    C. Liu, Y. Wang, Y. Gao, Y. Zhang, L. Zhao, B. Xu and L. S. Romsted, Phys. Chem. Chem. Phys., 2019, Accepted Manuscript , DOI: 10.1039/C8CP05987J

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