The structures of micelles of alkytrimethylammonium perfluorocarboxylates and of their adsorbed layers at the air/water interface

Abstract

The structures of adsorbed layers of dodecyl (C₁₂)- and hexadecyl(C₁₆)-trimethylammonium perfluoroacetate, perfluoropropionate and perfluorobutyrate have been studied at the air/water interface by neutron reflection. In general, as the size of the fluorocarbon counterion increases the surfactants become more surface active, as manifested by the values of the critical micelle concentration (c.m.c.). However, the area occupied per surfactant ion pair increases with size of the perfluoro counterion. For the dodecyl compound the increase in the limiting area at the c.m.c. is from 50 through 63 to 64 Ų. The perfluorocarboxylate ions are incorporated more readily into the C₁₆ rather than the C₁₂ layer, but only in the case of C₁₆trimethylammonium perfluorobutyrate could the position of the centre of the counterion distribution be determined from the reflection experiment. The micellar aggregation of the six compounds has also been studied by neutron small angle scattering. The main trends are that the change in counterion from perfluoroactetate to perfluorobutyrate leads to a marked increase in the ellipticity of the aggregates, the C₁₆ aggregates are substantially larger and more elliptical than the C₁₂ aggregates for the CF₃COO and C₂F₅COO ions, but they are comparable (with a large scatter) for the C₃F₇COO ion, and an increase in temperature from 25 to 40 °C causes the aggregation number to decrease. These results are interpreted in terms of varying penetration of counterions into the hydrophobic region of the micelle using the results from the adsorbed layer to help to interpret the variations in the packing parameter. It is concluded that, although these counterions do act somewhat like cosurfactants, they do not penetrate the hydrophobic region as much as ions resembling tosylate and this is the reason that they do not induce viscoelastic behaviour in the micellar solutions.