APPARENT PK(A) OF THE FATTY-ACIDS WITHIN ORDERED MIXTURES OF MODEL HUMAN STRATUM-CORNEUM LIPIDS

Purpose. The apparent pK(a) of the fatty acids within hydrated ( 30% w/w) model human stratum corneum (SC) lipid mixtures should be measured. Methods. The degree of ionisation of the fatty acids was calculated as a function of pH using Fourier transform infra-red spectroscopy. The relative intensity of the stretching bands of the unionized and ionized carboxylic groups was determined and fitted to the relevant expression for ionic equilibrium of a monoprotic acid. The pK(a) was then calculated for increasing proportion of unsaturated fatty acid in the lipid mixture. Results. Values for pK(a) in the range 6.2-7.3 were found, increasing with greater proportion of oleic acid. These are some 1.5-3 pH units higher than the pK(a)s of fatty acids in molecular solution. Conclusions. As there exists a pH-gradient across the SC, the degree of ionisation will also vary. In the innermost SC layers, a pH of 7 will produce 90% ionization of the fatty acids and head-group repulsion will be great. At the SC surface, the pH of 5 will cause almost minimal head-group repulsion, tending to increase crystallinity and promote a bilayer structure.