Adsorption and distribution of ions to a Bilayer lipid membrane

Unlike neutral molecules, the distribution and membrane permeation of ionic species are significantly affected by coexisting ions in order to maintain electrical neutrality in the membrane and in an aqueous phase. Therefore, it is difficult to evaluate the membrane permeability of basic or acidic drugs that are ionized depending on the pH. By assuming that the distribution and adsorption of these ionic species to the bilayer lipid membrane (BLM) are similar to those occurring in the water-organic solvent interface, we analyzed the distribution and adsorption of ions to a liposome membrane and the influence of coexisting ion on those. The ionic distribution to BLM depended on the hydrophobicity of the coexisting ion and its concentration, but the ionic adsorption to BLM was independent. We propose a model for the distribution and adsorption of ionic species to BLM. According to this model, the distribution constant (K _D ) of ionic species and coexisting anions in the water-BLM interface, the ion-pair formation constant (K _ip ) of ionic species in BLM and the adsorption constant (K _ad ) of ionic species to the BLM surface were experimentally evaluated.