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 so lid hydrophobic substrate. Sum-frequency (SF) spectra of di-(C6-Glu) demons trate that the effectiveness and efficiency of adsorption is only marginall y affected by temperature up to 95 degrees C. The line shapes of SF resonan ces in the C-H stretching region were modeled with use of a refined Levenbe rg-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 incre ase with di-(Cn-Glu) tail length due to more gauche defects while the methy l 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 t emperature was elucidated, focusing on the di-(C6-Glu) species. Conformatio nal disorder in the tail group increases with decreasing solution concentra tion below the critical micelle concentration (cmc). However, even with di- (C6-Glu) at 1/1000 cmc, the terminal methyl groups retain some orientationa l order. With a saturated di-(C6-Glu) monolayer, spectra at 95 degrees C ar e indistinguishable from those recorded at room temperature. Even when adso rbed from a 1/1000 cmc solution (at 20 degrees C), a partial monolayer of d i-(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 sugge sts that they may be suitable candidates for forming temperature-insensitiv e microemulsions with applications in enhanced oil recovery.