Physisorption Gives Narrower Orientational Distribution than Chemisorption on a Glass Surface: A Polarization-Sensitive Linear and Nonlinear Optical Study

Abstract

Linear and nonlinear polarization-sensitive spectroscopic techniques were employed to determine the orientational distribution of a probe molecule physisorbed and chemisorbed on a fused-silica glass surface. The chemisorption was through a well-known specific reaction between the maleimide group of the probe molecule and the thiol group of the surface. In the physisorption, the molecule was bound to the surface by the weaker van der Waals forces, dipole−dipole interaction, or both. Despite the absence of the strong interaction between the molecule and the surface in the physisorption, it was found that the physisorption gives narrower orientational distribution than the chemisorption. The strong covalent bond in the chemisorption is not effective to increase the orientational order. It is highly likely that the present conclusion is applicable to a variety of probe molecules having a functional group for a specific reaction of chemisorption.