In situ sum-frequency spectroscopic studies of adsorbed molecules at liquid and solid surfaces

Vibrational sum-frequency (SF) spectroscopy is a nonlinear optical technique which is highly surface sensitive and therefore very well suited for investigating molecular monolayers adsorbed on liquid and solid surfaces. We studied long chain fatty alcohols and amines physisorbed at the interface liquid/air and of octadecyltrichlorsilan (OTS) chemisorbed on glass in situ, i.e., in normal laboratory environment. Information on the molecular conformation and orientation is obtained from the SF spectra. The amplitudes of the degenerate methyl stretching mode for different polarization combinations of the incident and radiated fields is used to determine the molecular orientation with increased accuracy. Varying the pH-value of the aqueous subphase of a Langmuir film formed by octadecylamin, characteristic changes in the CH- and NH-spectral region are observed. The occurrence of CH₂-resonances indicates conformational deformations dependent on bulk pH. In this way SF spectroscopy is demonstrated to be an independent method for determining the surface pK of surfactants. Chemisorbed films of OTS have been prepared by polymerization of OTS on a glass substrate in anhydrous solution. Vanishing CH₂-resonances indicate straight chains of the adsorbed molecules, whereas from the line strength of residual Si-OH vibrations information on the degree of polymerization may be obtained.