Vibrational spectra and adsorption of trisiloxane superspreading surfactant at air/water interface studied with sum frequency generation vibrational spectroscopy

The C-H stretch vibrational spectra of the trisiloxane superspreading surfactant Silwet L-77 ((CH3)(3)Si-O-Si(CH3)(C3H6)(OCH2CH2)(7-8)OCH3)-O-Si(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 -Si-CH3 groups around 290.5 cm(-1) (symmetric stretch or SS mode) and 2957 cm(-1) (mostly the asymmetric stretch or AS mode), and with the weak but apparent contribution from the -O-CH2- groups around 2880 cm(-1) (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 C-H related molecular groups remained unchanged at different surface densities of the Silwet L-77 surfactant. The SFG-VS adsorption isotherm suggested that there was 110 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 -42.2 +/- 0.8kcal/mol, indicating the unusually strong adsorption ability of the Silwet L-77 superspreading surfactant.