Molecular mechanism of charge inversion revealed by polar orientation of interfacial water molecules: A heterodyne-detected vibrational sum frequency generation study.

"Charge inversion" is a phenomenon in which multivalent counterions overcompensate for interfacial charges and invert the sign of the net charge near a surface. This phenomenon is believed to be relevant to biologically important processes such as DNA condensation, and hence it has attracted much attention. We investigated the polar orientation of interfacial water molecules at two different negatively charged interfaces in the absence and presence of La using heterodyne-detected vibrational sum frequency generation spectroscopy, which can directly determine the up/down orientation of interfacial molecules. It was found that the orientations of water molecules at a bio-relevant phospholipid interface change from the hydrogen-up to the hydrogen-down with the addition of 10 µM La. This change of water orientation indicates that the net charge at the phospholipid interface is inverted by adsorption of La to the phosphate headgroup. By contrast, at an alkylsulfate interface, the majority of the interfacial water molecules remain hydrogen-up orientated even in the presence of 25 mM La, indicating that the sulfate headgroup is still solvated by up-oriented water. The observed headgroup specificity suggests that charge inversion at the phospholipid interface originates primarily from the chemical interaction between the phosphate and La ion.