Heteronuclear Overhauser effect measurements in surfactant systems. 3. Average location of water with respect to surfactant polar head in micellized sodium octanoate and sodium dodecanoate

On the measurement of intermolecular heteronuclear cross relaxation rates in ionic liquids

The HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiment is commonly used to study interactions and structuring in ionic liquids (ILs) via the measurement of the cross relaxation rate σ between two spins. In the intermolecular case, σ is proportional to r-n, where r is the internuclear distance and n can vary between 1 and 6 depending on the frequency of the nuclei and their dynamics, thus σ can potentially provide detailed information on the liquid phase structure. However, in HOESY studies of ILs only relative values for σ are typically reported, making comparisons between different samples difficult. Herein we discuss the quantitative measurement of intermolecular cross relaxation rates based on the normalisation of HOESY signal intensities to the nuclear Boltzmann polarisation, demonstrated for 7Li-1H spin pairs in a lithium-containing pyrrolidinum-based ionic liquid electrolyte. We also use a simple model based on diffusing hard spheres for interpreting these quantities in terms of a distance of closest approach.

Phase equilibriums, self-assembly and interactions in two-, three- and four medium-chain length component systems

The Scandinavian surface (surfactant) and colloid science owes much of its success to Per Ekwall and Björn Lindman. In this review the main topics shared by their research groups at Åbo Akademi University in Finland and at Lund University in Sweden are described. The nature of surface active substances (cosolvents, co-surfactants and surfactants) and microemulsions are evaluated. It is shown that the properties of medium-chain length surfactants differ dramatically from long-chain surfactants. The phase equilibriums of binary systems are related to the phase equilibriums of ternary and quaternary systems referred to as microemulsions or more recently also as nanoemulsions. A distinction is made between hydrotrope liquids, detergentless microemulsions, surfactant mixture systems and microemulsions. Three component systems are assembled to "true" quaternary microemulsions. An exceptionally comprehensive network of thermodynamic parameters describing molecular site exchange and micelle formation are derived and related mutually. Gibbs free energy, enthalpy, entropy, volume, heat capacity, expansivity and compressibility can be used to illustrate the degree of aggregation cooperativity and to evaluate whether micelle formation is of a first-, second- or intermediate order phase transition. Theoretical simulations and experimental results show that the associate structures of medium-chain length surfactants are quite open and may be deformed due to small aggregation numbers. The self-assembly occurs over a number of distinct steps at a series of experimentally detectable critical concentrations. Despite the low aggregation tendency their phase behavior equals those of long-chain homologs in surfactant mixture and microemulsion systems. A number of models describing the self-assembly are reviewed. Nuclear magnetic resonance (shift, relaxation rate and diffusion), Laser Raman and infrared spectroscopies were chosen as key instruments for molecular interaction characterization since they were used in the collaboration between the research groups in Åbo and in Lund. A new method is introduced in order to evaluate the traditional procedure for extracting limiting parameters which also enables an illustration of the degree of cooperativity. The focus is laid mainly on aqueous, alcoholic, saline and, to a limited extent oil phases of one-, two-, three- and four component systems of water-sodium carboxylates-alcohol-oil. The extensive thermodynamic characterization of these liquid phases and liquid crystalline phases is left out due to space restrictions.

Solute-solvent contact by intermolecular cross relaxation. I. The nature of the water-hydrophobic interface

Intermolecular cross-relaxation rates between solute and solvent were measured by {1H} 19F nuclear magnetic resonance experiments in aqueous molecular solutions of ammonium perfluoro-octanoate and sodium trifluoroacetate. The experiments performed at three different magnetic fields provide frequency-dependent cross-relaxation rates which demonstrate clearly the lack of extreme narrowing for nuclear spin relaxation by diffusionally modulated intermolecular interactions. Supplemented by suitable intramolecular cross-relaxation, longitudinal relaxation, and self-diffusion data, the obtained cross-relaxation rates are evaluated within the framework of recent relaxation models and provide information about the hydrophobic hydration. In particular, water dynamics around the trifluoromethyl group in ammonium perfluoro-octanoate are more retarded than that in the smaller trifluoroacetate.

Water-surfactant contact studied by 19F-1H heteronuclear overhauser effect spectroscopy

Intermolecular 19F-1H cross-relaxation is measured using heteronuclear Overhauser effect NMR spectroscopy (HOESY) in the micellar solution of cesium pentadecafluorooctanoate. The results are analyzed in terms of a weak 1H-19F cross-relaxation between the water protons and the fluorines in the fluoroalkyl chain and a strong 19F-19F cross-relaxation within the fluoroalkyl chain. The water-surfactant cross-relaxation indicates a water approach to the first CF2 segment in the order of 2.0 A and a short (<<ns) water residence time. Evidence of fluorine hydration further inside the micelle is presented. Copyright 1998 Academic Press.