The temperature dependence of water and counter ion order in soap-water mesophases. A deuterium and sodium NMR study

Abstract

Deuterium magnetic resonance (DMR) spectra of the water and hydrocarbon chains in potassium and sodium palmitates and sodium magnetic resonance spectra of sodium in sodium palmitate demonstrate correlations between water, hydrocarbon chain and counter ion order. In the lamellar phase of potassium palmitate the order parameters inferred from DMR splittings of D 2O and the first few methylene chain segments initially increase and then decrease with increasing temperature. This is explained in terms of a model where the lipid-water structure at low temperatures imposes a direction for which all the order parameters are smaller than for the higher temperature structure for purely geometric reasons. As temperature increases the structuring effect of water decreases and there is an “apparent” increase in order until at even higher temperatures there is an intrinsic decrease in order parameter. In addition, for potassium palmitate the DMR splittings of D 2O and the first few methylene segments indicate a “phase transition” within the liquid crystalline phase.