Interaction of ions with phosphatidylcholine and phosphatidylethanolamine at the n-heptane/water interface : II. Hydrophobic ions

Electrocapillary phenomena at the n-heptane/water interface in the presence of phosphatidylcholine or phosphatidylethanolamine in alkane were studied by the drop-volume technique. The regulating role of the electric field was established in phospholipid interaction with inorganic ions: changes in interfacial pressure at the n-heptane/water interface with addition of electrolytes were observed only when an electric field was applied to the system of two immiscible liquids. It is suggested that ions can penetrate and bond by phospholipid functional groups at the phase boundary only after reorientation of the latter in the electric field. As in the case of ionic surfactants, change in interfacial pressure was directly proportional to the cube root of the concentration of electrolyte. It is assumed that as a result of preferential binding by phospholipid functional groups of ions of one sign, lipid molecules are charged and at the phase boundary behave themselves like ionic surfactants. Experiments with salts of multicharged ions, La³⁺, Fe(CN)₆]³⁻, and Fe(CN)₆]⁴⁻, have shown that mainly phospholipid functional groups bind cations.