The pH dependence of calcium adsorption onto anionic phospholipid monolayers

The adsorption of ⁴⁵Ca to monolayers of phosphatidylinositol and dicetylphosphoric acid has been measured as a function of subphase pH with simultaneous recordings of surface pressure and interfacial potential. Below pH 3 little calcium was adsorbed and the films are assumed to be unionized. With acid subphases between pH 3 and 6.5 adsorption of calcium occurred initially, but it was then gradually lost due to an ageing process in the films. This time dependent change in the properties of the film was independent of the presence of Ca²⁺, but was dependent on the H⁺ concentration in the subphase; it was however not due to an acid hydrolysis of the monolayer. Ca²⁺ was permanently adsorbed at pH values above 6.5 with an increasing affinity up to pH 11.