Microemulsion structure in four-component systems for different surfactants

Abstract

The microstructure of four-component microemulsions composed of ionic surfactant, cosurfactant hydrocarbon and water was studied as a function of surfactant polar head group and alkyl chain length using the multi-component self-diffusion approach, the self-diffusion coefficients being measured by the Fourier transform pulsed-gradient spin-echo 1H-NMR technique. The study concerned both cationic and anionic surfactants and was performed for two cosurfactants, i.e. pentanol and octanol, and a number of hydrocarbons (aliphatic and aromatic). The results obtained demonstrate that in every case the structure is very strongly influenced by the chain length of the alcohol, a longer cosurfactant favouring a droplet-type structure and a shorter one a bicontinuous structure; this occurs irrespectively of the chemical structure of the surfactant and the nature of the oil used. The main effect of the hydrocarbon and surfactant chain lenghts is to influence the distribution of molecules between different domains. The hydrocarbon chain length determines the distribution of oil and alcohol between the medium and the surfactant layers through a competition process that is not affected by the surfactant chemical structure. Furthermore, in a water-in-oil droplet situation, in addition to the larger degree of penetration for shorter alkanes between the surfactant molecules in comparison with the corresponding bicontinuous situation, we have an increased interfacial location of the alcohol with an increase of the surfactant tail length.