The CH stretch vibrational spectra of the trisiloxane superspreading surfactant Silwet L-77 ((CH3)3SiOSi(CH3)(C3H6)(OCH2CH2)78OCH3)OSi(CH3)3) at the air/water interface are measured with the surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). The spectra are dominated with the features from the SiCH3 groups around 2905 cm1 (symmetric stretch or SS mode) and 2957 cm1 (mostly the asymmetric stretch or AS mode), and with the weak but apparent contribution from the OCH2 groups around 2880 cm1 (symmetric stretch or SS mode). Comparison of the polarization dependent SFG spectra below and above the critical aggregate or micelle concentration (CAC) indicates that the molecular orientation of the CH related molecular groups remained unchanged at different surface densities of the Silwet L-77 surfactant. The SFG-VS adsorption isotherm suggested that there was no sign of Silwet L-77 bilayer structure formation at the air/water interface. The Gibbs adsorption free energy of the Silwet surfactant to the air/water interface is 42.20.8kcal/mol, indicating the unusually strong adsorption ability of the Silwet L-77 superspreading surfactant.

Topics
Free energy, Vibrational spectra, Adsorption isotherm, Sum frequency generation spectroscopy, Air-water interface, Supramolecular assembly, Surfactants, Chemical compounds
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