Phase Separation of Phosphatidylcholine–Water Systems below the Main Transition Temperature Induced by Monovalent Ions

Effects of 1:1 electrolytes (LiCl, NaCl, NaBr, KCl, RbCl, and NH4Cl) on the stacked multilamellar structure of phosphatidylcholine–water systems below the acyl-chain melting temperature were examined by means of small-angle X-ray scattering and differential scanning calorimetry. For the ternary DPPC–salt–water system at 20 °C, the lamellar repeating distance, D, jumped from 64 to 72 Å at certain thresholds of electrolyte concentration; 4.0, 2.1, and 1.4 mol kg−1 when LiCl, NaCl, or KCl were added, respectively. The temperature-dependent D jump from 72 to 64 Å at 20 °C was also confirmed for DPPC in 2.1 mol kg−1 NaCl accompanying the small latent heat of ca. 0.033 J g−1. For the DMPC–NaCl–water system at 5 °C, the salt-induced D jump, at a threshold NaCl concentration of 1.2 mol kg−1, was also observed. A triangular phase diagram of DPPC–NaCl–water at 20 °C was constructed based on X-ray scattering data. It was confirmed that the DPPC–NaCl–water system had a three-phase coexisting region, i.e., two lamellar phases with different repeating distances of 64 or 72 Å, respectively, and the NaCl bulk solution. Some qualitative aspects are discussed concerning the interpretation of the phase separation into the two different lamellar phases based on the lipid–monovalent ion interactions.