Molecular motions of small molecules in porous silica sol-gel glass

Abstract

Monolithic samples of sol-gel glass of pore diameters varing form 10 to 100 A have been produced. The highly porous samples have been impregnated by different fluids and molecular motions of confined molecules have been studied using Raman spectroscopy. Some studies have been extended to silica sol-gel glass of modified surfaces. The prefferential adsorption and translational diffusion have been investigated in mixtures of pyridine with polar and non-polar solvents. It has been confirmed that pyridine can be preferentially adsorbed on silica, and it has been suggested that the process results in a bilayer structure of the interface. Rotational relaxation of carbon disulfide, chloroform, acetonitrile, and sulfur hexafluoride inside pores of diameter a couple of times larger than diameter of molecules has been studied. High temperature and pressure study have been employed for SF$\sb6$ measurements. The effect of pore diameters and different surface coverage on rotational diffusion has been discussed. It has been shown that surface interactions, in particular, hydrogen bonding between the imbedded molecules and silanols groups, were responsible for slowing down the rotational relaxations within small pores. The experimental results have been compared with EDJ and FPL theoretical models. Vibrational dephasing, intermolecular energy exchange and intramolecular energy coupling have been investigated in chloroform, acetonitrile, nitromethane, acetone, and methyl iodide. The vibrational modulation times were obtained from the Kubo theoretical function and used to analyze molecular interactions near the silica surface. The Fermi resonance effect has been studied in acetonitrile and the coupling constant has been obtained from the standard quantum mechanics analysis. The non-coincidence effect has been measured in acetone within pores of different surface structure under different concentrations. The Schweizer-Chandler dephasing model and energy exchange model have been tested for methyl iodide. It has been found that the Harris energy exchange model cannot explain the mechanism responsible for the band shapes of any of the modes of methyl iodide adsorbed on silica.