Interaction of Na+, K+, Li+ and Ca2+ ions in Tris-HCl buffer substrate with monolayers of phosphatidic acid and homologous phosphatidic acid alkyl esters

The interaction of ions (Na⁺, K⁺, Li⁺, Ca²⁺) with monolayers of phosphatidic acid alkyl esters (alkyl = methyl, ethyl,n-propyl,n-pentyl) were investigated at the air/water interface on Tris-HCl buffer, as well as on the electrolytes containing subphases.Qualitatively it can be stated that there are no considerable interactions between Na⁺ ions in the substrate and the head groups of phosphatidic acid esters in the monolayers. On the whole, the modification of the shape in the /a and v/a isotherms (^s = film pressure, v^s = film potential) of the homologous phosphatidic acid esters as a function of the length of the ester group on the subphase containing NaCl, KCl, and LiCl corresponds to that on Tris-HCl buffer without admixture of electrolytes.On the other hand the strength of interaction between Ca²⁺ ions and the homologous phosphatidic acid esters depends on the length of the ester group. The film-condensing effect of Ca²⁺ ions becomes smaller with increasing length of the ester group.