Structure and thermal stability of dichain sugar surfactants at the solid/water interface studied by sum-frequency vibrational spectroscopy

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 degrees 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 d(30)-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 solution concentration and temperature was elucidated, focusing on the di-(C6-Glu) species. Conformational disorder in the tail group increases with decreasing solution concentration below the critical micelle concentration (cmc). However, even with di-(C6-Glu) at 1/1000 cmc, the terminal methyl groups retain some orientational order. With a saturated di-(C6-Glu) monolayer, spectra at 95 degrees C are indistinguishable from those recorded at room temperature. Even when adsorbed from a 1/1000 cmc solution (at 20 degrees C), a partial monolayer of di-(C6-Glu) is robust to 65 degrees C. Di-(C6-Glu) is typical of the di-(Cn-Glu) series in its concentration and temperature behavior. The exceptional thermal stability of di-(Cn-Glu) monolayers on hydrophobic substrates suggests that they may be suitable candidates for forming temperature-insensitive microemulsions with applications in enhanced oil recovery.