Fourier transform carbon-13 relaxation and self-diffusion studies of microemulsions

Abstract

Multi-field ¹³C n.m.r. relaxation (spin–lattice relaxation and nuclear Overhauser enhancement) and multi-component Fourier-transform n.m.r. self-diffusion measurements have been applied to provide information on different aspects of the dynamic structure of microemulsions. Studies were performed over extensive concentration regions for four-component systems composed of sodium p-octylbenzene sulphonate, butan-l-ol, water and n-decane or toluene while limited investigations were made for other systems. The ¹³C relaxation data were analysed on the basis of a two-step model of relaxation to provide information on short- and long-range molecular dynamics and on the degree of order of the alkyl chains with respect to hydrophobic–hydrophilic interfaces. The local motion of the alkyl chains is generally about as rapid (10^–11 s) as that of liquid hydrocarbons. The effect of the organization in the system and the creation of hydrophobic–hydrophilic interfaces is a slight anisotropy in the molecular motion. This anisotropy is similar in magnitude to that of liquid-crystalline phases, but for the microemulsions the lifetime of the anisotropy is very short, one or two nanoseconds. According to the self-diffusion results, a closed water droplet structural model may apply for alcohols with longer alkyl chains. For the above-mentioned systems with butanol as cosurfactant the self-diffusion coefficients of both water and hydrocarbon are quite high and inconsistent with structurally segregated domains, unless these are very short-lived.