Ion Diffusion Selectivity in Lecithin-Water Lamellar Phases

The diffusion coefficients of Na⁺, Rb⁺, and cl^- were determined in lecithin-water lamellar phases at 18°C as a function of phase hydration. Diffusion was measured within the phase with no transfer between phase and bulk aqueous medium. The relative diffusion coefficients of anion and cation depended strongly on phase hydration. At low water content, the diffusion coefficient of Cl^- was greater than that of Na⁺ or Rb⁺ whereas at high water content both cations diffused faster than the anion. The change in relative diffusion coefficient occurred at 0.24 g water/g phase (24% water). The possibility that a change in conformation of the lecithin polar head occurs at a phase water content of 24% is considered. The diffusion coefficients of all three ions decreased at the water content where the relative diffusion rates inverted. Freeze fracture and polarizing microscopy studies were carried out to obtain information on phase structure. The latter study indicated that a change in long-range organization of the phase occured at 24% water. This change accounts for the decrease in the ion diffusion coefficients at this water content. The change in conformation of the choline phosphate group proposed as an explanation for the change in ion selectivity could lead to changes in long-range organization of the phase as a second order-effect.