A theory of ion binding and phase equilibria in charged lipid membranes. I. Proton binding

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Abstract
A statistical mechanical theory is developed for the binding of protons to charged, acidic phospholipid membranes in contact with an aqueous solution, and the effects of such ion binding on phase transitions within a membrane are considered. Poisson–Boltzmann approximations are used to derive a law of mass action that contains an additional ionic strength dependent factor not found in the laws of mass action used by earlier workers. An equation is obtained for the equilibrium area of a membrane, and expressions are developed for a number of important thermodynamic properties, including the lipid chemical potential. These results are used to obtain equations for the membrane phase transition temperature. A correspondence between the lipid chemical potential and the chemical potential of protonated lipid is employed to explain the form of these equations. Calculated curves of transition temperature versus pH are presented and discussed for a number of different monovalent salt concentrations, and the results of the theory are compared with results of previous theories.