Fluorocarbon surfactant/water mesomorphic phases. Part 2.—A 19F and 1H pulsed n.m.r. study of alkyl chains in the lamellar phase of the ammonium perfluoro octanoate + water system

Abstract

Fluorine spin lattice and spin-spin relaxation time measurements have been made for the lamellar phase of the ammonium perfluoro octanoate + water system. The values are qualitatively similar to relaxation rates of alkyl chain protons in similar systems. The T₁ values indicate that rotation of CF₂ groups about the long axis of the chain is slower at 298 K than is observed in hydrocarbon systems but that the activation energy for this process is of the same order or smaller than is found for hydrocarbons. The wagging and bending motions of the fluorocarbon chains are less than those of hydrocarbon chains. The proton T₁ values of solubilized hydrocarbon chain additives have been measured and indicate that CH₂ rotational motion becomes more restricted with increasing chain length. The effect of chain length is much larger than the effect of charge attraction or hydrogen bonding. No large distribution of alkyl group rotational correlation times is observed as the distance between the CH₂ group and the head group is increased.