Quaternary 'N-alkylaldonamide-brine-decane-alcohol' systems.: Part II:: microstructure of the one-phase microemulsion by NMR spectroscopy

The 'N-octylribonamide-brine-decane-butanol' system has been investigated by proton nuclear magnetic resonance. Although the microstucture of microemulsions involved in the different multiphase equilibra of the system has been well defined previously, that of the large monophasic domain was still unknown. Mapping of classical nuclear magnetic resonance (NMR) parameters such as the chemical shift, the relaxation time of the protons of the different components and the proton exchange rate between water and butanol in a broad composition range has allowed to identify three zones in the monophasic region. The first one, zone I, is characterized by small surfactant aggregates solvated by water and butanol. In this structure, the chemical shift of the surfactant varies with the composition of its solvation shell and its degree of self-association; the relaxation time has been correlated to the size of the aggregates. In zone II, when the surfactant concentration is larger than 25%, its chemical shift becomes constant while its relaxation time decreases. At the same time, the chemical shift of water protons shows the formation of a water core. These results denote the formation of an inverse microemulsion where the surfactant is immobilized in the interface. In zone III, located in the 'fish tail', the chemical shift does not change but the relaxation time increases. The structure has been determined as a bicontinuous microemulsion characterized by a fluid interface where the molecules involved in the interface have the same environment but have a higher mobility. The proton exchange rate between water and butanol measured in the different structures has led to a scheme of cyclic transfer between the two components that took place at and through the surfactant. (C) 1999 Elsevier Science B.V. All rights reserved.