Better understanding and control of adsorption and self-assembly processes of molecules at interfaces has both fundamental and practical importance. Mica is one of the most commonly used substrates for investigating the dynamics and outcomes of molecular adsorption, because large, atomically flat surfaces can be easily prepared. Studies of assembly by large bio-molecules and organic semiconductors on mica surfaces have revealed interesting adsorption behaviors and morphologies that depend on the type of micas and solvent environments. Epitaxial relation, or lack thereof, between the mica surface and the adsorbed molecules have been speculated to be either due to the presence of electric dipole fields or geometric effects (e.g., corrugations). Also, whether smaller water molecules also follow epitaxial growth relationships is unclear. Here, by combining two surface sensitive techniques: vibrational sum frequency generation (vSFG) spectroscopy and high resolution atomic force microscropy (AFM), we are able to first determine the absolute orientation of the outermost, buried hydroxyl groups of muscovite mica and show that it lies along one of the three nearest neighbor lattice direction, thereby breaking the trigonal symmetry of the surface. Using azimuthally-resolved vSFG spectroscopy, we investigate the water-structure above two distinct types of micas (muscovite and fluorophlogopite). Our results indicate that the vSFG signal response from the muscovite-water interface is rotationally anisotropic and is correlated to the orientation of the sub-surface hydroxyl groups. In the case of hydroxyl absent fluorophlogopite, the vSFG signal is rotationally isotropic. The anisotropic vSFG signal is found to be due to bulk muscovite, although its exact origin is still unclear. Moreover, we find that the non-hydrogen bonded water exists at the muscovite-water interface, but absent at the fluorophlogopite-water interface. Additionally, we also observe a direct correlation between the buried hydroxyls and the preferred orientation of the proteins on muscovite, and the lack thereof on fluoro-phlogopite. Our results indicate that the presence of hydroxyl groups in mica is critical to how molecules, irrespective of size, arrange next to its surface. In order to understand the origin of correlation between hydroxyl groups and molecular anisotropy, we have started to investigate the geometrical effect vs. electric-dipole effect using density functional theory simulations.

Understanding molecular assembly on mica surfaces: Influence of buried hydroxyls

2019 Fall National Meeting

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