SUM-FREQUENCY SPECTROSCOPY OF MONOLAYERS OF ALKOXY-TERMINATED ALKANETHIOLS IN CONTACT WITH LIQUIDS

The microscopic interactions between organic monolayers and various liquids have been studied by infrared-visible sum-frequency spectroscopy. The spectra of a series of alkoxy-terminated hexadecanethiols (CH3(CH2)nO-(CH2)16SH, n = 0-3) on gold and silver were recorded under air, hexane, acetonitrile, and water in the infrared C-H stretching region. The spectra of monolayers of octadecanethiol (ODT), which contains pure linear hydrocarbon chains, were also recorded for purposes of comparison. Intermolecular forces between the liquids and the polar ether group change the positions, line widths, and intensities of the terminal methyl group resonances in monolayers of the methyl ether (CH3O-(CH2)16SH). These changes were most pronounced under water and are consistent with the formation of hydrogen bonds between water and the oxygen atom of the ether. The spectra of the longer ether analogues reveal the dependence of these hydrogen bonding interactions on the depth of the ether oxygen atom beneath the monolayer surface. As the oxygen atom was progressively ''buried'', its influence became less pronounced and it no longer had an observable effect on the spectra of the butyl ether (CH3(CH2)3O-(CH2)16SH). The microscopic interactions inferred from the sum-frequency spectra are broadly consistent with macroscopic contact angle measurements.