Behavior of mesogenic molecules deposited at the alumina-air interface: A deuteron NMR study

Thin molecular depositions of $4^{\prime }$-pentyl-4-cyonobiphenyl (5CB) mesogenic molecules are investigated via quadrupole-perturbed deuteron nuclear magnetic resonance (DNMR) spectroscopy. Uniform and controlled thickness molecular surface depositions are prepared on the inner cylindrical surfaces of Anopore membranes by the solvent-evaporation technique. As a result, 5CB molecules are found in two different configurations: a bulklike one with parallel axial arrangement, and a surface one with planar radial arrangement. If the 5CB surface coverage exceeds $c\simeq 0.35,$ only the bulk state is present. In the coverage range between 0.015 and 0.35, the bulklike state and the surface layer coexist, conforming to a typical dewetting scenario. Below $c\simeq 0.015,$ only the surface layer is present. The dilution of the surface deposition with decreasing coverage is manifested as an increase in the DNMR doublet frequency splitting. The surface orientational order parameter Q, the surface biaxiality $\eta ,$ and the diffusion coefficient $D_S$ are determined from the DNMR spectral patterns obtained at different sample orientations in the external magnetic field. These angular patterns prove that in highly diluted surface depositions the molecules lie flat on the surface. However, they are not frozen and their molecular axes rapidly reorient on the DNMR measurement time scale, typically ${10}^{-4}\hspace{0.5em}\mathrm{s},$ while remaining confined to the surface. Simultaneously, molecules diffuse over the surface with a surface diffusion constant on the order of ${10}^{-11}\hspace{0.5em}{\mathrm{m}}^2{\mathrm{s}}^{-1}.$ Such molecular diffusion is responsible for an effective biaxiality on the DNMR time scale. However, an inherent biaxiality cannot be completely ruled out and thus may play a minor role. The surface phase has a two-dimensional (2D) gas character with some (possible) indicators of 2D-liquid properties.