Hydrophobic collapse of a stearic acid film by adsorbed l-phenylalanine at the air-water interface.

The surface morphology of atmospheric aerosol particles can influence the particle's overall effect on climate through enhancing or impeding its ability to uptake and evaporate water. In the work presented here, complementary surface-sensitive information from π-A isotherms, Brewster angle microscopy (BAM), and infrared reflection-absorption spectroscopy (IRRAS) are used to monitor the induced hydrophobic collapse of a surfactant film by an adsorbed amino acid at the air-water interface. The stearic acid film studied here is well-known to form a very stable floating monolayer at the air-water interface, and is shown in this work to withstand isotherm compression-expansion cycles without any premature collapse. With the presence of the water-soluble amino acid l-phenylalanine, however, significant disruption is observed of the stearic acid film, evidenced by the disappearance of its liquid-condensed phase from the isotherm cycles, as well as premature collapse structures observed in the BAM images and a change in intensity in stearic acid's C-H stretching region in the IRRAS spectra. Throughout this process, the surface layer is transformed from a homogeneous hydrophobic surface to an inhomogeneous surface with three-dimensional hydrophobic aggregates as well as hydrophilic "holes" with minimum surfactant coverage.