Surface pH and Ion Affinity at the Alcohol-Monolayer/Water Interface Studied by Sum-Frequency Spectroscopy

Phase-sensitive sum-frequency vibrational spectroscopy was used to probe interfaces of a long-chain alcohol monolayer with various electrolytic solutions, chosen as a prototype for nonionic organic/water interfaces. Spectra in the OH stretching range were observed to be under the influence of ions emerging at the interfaces. Analysis of the spectra with the help of Levin’s theory allowed us to quantitatively find the surface densities of various ions and, hence, the ion affinities and surface pH/pOH at the alcohol/water interface. For the ions studied, the interface affinity has the ranking order of OH^– > I^– > Cl^– ∼ H₃O⁺ > Na⁺, the same as that at the air/water interface except that OH^– and H₃O⁺ have their places interchanged. Significantly stronger affinity of OH^– than H₃O⁺ with the result of surface pH/pOH extrapolating to bulk pH 7 suggests that the alcohol/neutral water interface is weakly basic.