Spread monolayer of a hydrophobic polyacid. II. Surface potential and calcium binding by ion exchange

Abstract

The surface potential ΔV of monolayers of a hydrophobic polyacid HPA, the alternate copolymer of maleic acid and of hexadecylvinylether, spread at the surface of aqueous solutions, has been studied as a function of pH, ionic strength, and Ca 2+ concentration. In the absence of Ca 2+ ions, the ΔV—pH plots resemble acid—base titrations curves, and ΔV is practically independent of the ionic strength. In the presence of Ca 2+ ions, ΔV increases slightly when the amount of bound Ca 2+ ions is increased by varying either the bulk ratio Ca 2+/Na + or the bulk pH. It is assumed that the majority of the counterions Na + or Ca 2+ is paired to the carboxylates of the dissociated HPA and that the contribution to ΔV of the diffuse part of the double layer is negligible. Then, the variation of the degree of dissociation and the p K of the maleic acid residues of HPA are deduced in the absence of Ca 2+ ions. It is found that only one carboxyl group of the maleic acid residue is dissociated when the bulk pH < 10. A relation is found between ΔV and the p K of dissociation. It shows that the variations of ΔV with pH, at constant surface density of HPA, parallel those of the carboxylate electrostatic free energy originating in the increase of the surface charge density of the monolayer with the bulk pH. An analogous increase of the surface charge density of the monolayers follows the exchange Ca 2+/Na + owing to the replacement of one Na + by one Ca 2+. This “charging” of the surface seems to be responsible of the increase in ΔV. Furthermore, in this case, and when the amount of bound Ca 2+ ions is high, the presence of positively charged sites next to the second undissociated carboxyl group of the maleic acid residue, may lower the p K of the latter and increase the degree of dissociation of the monolayer at a constant pH. Finally, it is found that the surface potential and the degree of dissociation of HPA monolayers reproduce the surface potential of the outer face of the nerve membranes and the variation of the sodium conductance with pH and pCa.