Interfacial Water Structure and Effects of Mg2+and Ca2+Binding to the COOH Headgroup of a Palmitic Acid Monolayer Studied by Sum Frequency Spectroscopy

Abstract

The interfacial hydrogen-bonding network that uniquely exists in between a palmitic acid (PA) monolayer and the underneath surface water molecules was studied using vibrational sum frequency generation (VSFG) spectroscopy. Perturbations due to cation binding of Mg(2+) and Ca(2+) were identified. The polar ordering of the interfacial water molecules under the influence of the surface field of dissociated PA headgroups was observed. Only a fraction of PA molecules are deprotonated at the air/water interface with a neat water subphase, yet the submonolayer concentration of negatively charged PA headgroups induces considerable polar ordering on the interfacial water molecules relative to the neat water surface without the PA film. Upon addition of calcium and magnesium chloride salts to the subphase of the PA monolayer, the extent of polar ordering of the interfacial water molecules was reduced. Ca(2+) was observed to have the greater impact on the interfacial hydrogen-bonding network relative to Mg(2+), consistent with the greater binding affinity of Ca(2+) toward the carboxylate group relative to Mg(2+) and thereby modifying the interfacial charge. At high-salt concentrations, the already disrupted hydrogen-bonding network reorganizes and reverts to its original hydrogen-bonding structure as that which appeared at the neat salt solution surface without a PA monolayer.