Structure and Thermal Stability of Dichain Sugar Surfactants at the Solid/Water Interface Studied by Sum-Frequency Vibrational Spectroscopy

Abstract

IR−visible sum-frequency spectroscopy (SFS) has been used to study adsorbed monolayers of a homologous series of dichain sugar surfactants, denoted di-(Cn-Glu), in situ at the interface between the bulk aqueous phase and a solid hydrophobic substrate. Sum-frequency (SF) spectra of di-(C6-Glu) demonstrate that the effectiveness and efficiency of adsorption is only marginally affected by temperature up to 95 °C. The line shapes of SF resonances in the C−H stretching region were modeled with use of a refined Levenberg−Marquardt fitting program to aid interpretation. Spectra of a partially deuterated d30-di-(C6-Glu) monolayer show that methylene resonances arise solely from the tail groups. The methylene mode amplitudes generally increase with di-(Cn-Glu) tail length due to more gauche defects while the methyl mode amplitudes remain nearly constant. This trend is discussed in terms of statistical considerations and types of gauche defect in dialkyl chains. The dependence of monolayer structure on bulk solutio...