Relation between surfactant structure and properties of spherical micelles. 1-Alkyl-4-alkylpyridinium halide surfactants

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This paper describes a detailed study of the properties of spherical micelles formed from 18 1-alkyl-4-alkylpyridinium iodides. Structural variations in the surfactants include (i) branching of the 4-alkyl chain while keeping the number of carbons in the chain invariant and (ii) variation of the lengths of the 1- and 4-alkyl groups. It is found that the free energy of transfer of a methylene group from water to a spherical micelle depends on the position of the CH2 group in the aggregate (Stern region vs core) and on whether the methylene group is located in the main or in the side chain. Interestingly, the thermodynamic stability of a micelle appears to be little affected by changes in the packing of the alkyl chains in the core of a micelle. The micropolarity in the Stern region of the spherical micelles is ethanol-like and is not affected by (i) the shape of the surfactant monomer, (ii) the length of the 4-alkyl chain, and (iii) the hydrophobicity of the 1-alkyl chain. The catalytic efficiency of a spherical micelle in the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate does not depend on the alkyl chain length and is barely dependent on the degree of branching of the 4-alkyl chain. The rate constant in the micellar phase points also to an ethanol-like micropolarity in the Stern region. In contrast, the catalytic efficiency is strongly influenced by the hydrophobicity of the 1-alkyl chain and the substitution pattern of the pyridinium ring. Different degrees of shielding of the probe molecule from water by part of the surfactant may account for this effect. The dependence of the catalytic efficiency on the morphology of the aggregate stems probably from different locations of the probe in different aggregates. The small changes in surface potential reflect minor variations in the surface charge density and critical micelle concentration upon alkyl-chain branching. Finally, a fast and efficient route for the synthesis of branched surfactants, starting from lactones, is presented.