Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy

We have studied the hydrophobic water/octadecyltrichlorosilane (OTS) interface by using the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS), and we obtained detailed structural information of the interface at the molecular level. Excess ions emerging at the interface were detected by changes of the surface vibrational spectrum induced by the surface field created by the excess ions. Both hydronium (H₃O⁺) and hydroxide (OH⁻) ions were found to adsorb at the interface, and so did other negative ions such as Cl⁻. By varying the ion concentrations in the bulk water, their adsorption isotherms were measured. It was seen that among the three, OH⁻ has the highest adsorption energy, and H₃O⁺ has the lowest; OH⁻ also has the highest saturation coverage, and Cl⁻ has the lowest. The result shows that even the neat water/OTS interface is not neutral, but charged with OH⁻ ions. The result also explains the surprising observation that the isoelectric point appeared at ∼3.0 when HCl was used to decrease the pH starting from neat water.