Pseudo Phase Transition of Molecular Assembly in a Structure- Controlled Subnanospace Having Enhanced Potential.

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Molecular states of SO_2, NO, N_2, CH_4, and H_2O in carbon subnanospaces of slit shape were studied by heat of adsorption, in situ X-ray diffraction, high pressure adsorption, and ordinary molecular adsorption. Supercritical NO gas was changed into quasi-vapor through formation of (NO)_2 due to strong molecular field of the subnanospace with the aid of magnetic perturbation. Also supercritical CH_4 and N_2 was transformed to their quasi-vapor due to the subnanospace field and application of high pressure of adsorptives. The structural change of H_2O molecular assembly in the subnanospace was examined by high resolution X-ray diffraction anaysis, indicating presence of a distorted two-dimensional ice like structure.New phase transition of molecular assembly in the carbon nanospace was shown in the system of SO_2 and carbon slit pore of 0.8 nm width. The heat of SO_2 adsorption in the carbon subnanospace was divided into the constant region of enhanced potential field below phi=0.6 and an steep increase region above phi=0.6. The constant region was ascribed to enhanced SO_2-slit pore interactions (about 10 KJ/mol) including the induced dipole-permanent dipole interaction ; this indicated the parallel arrangement of dipoles of adsorbed SO_2 molecules. The steep increase was caused by the inter SO_2 molecular interaction which gives rise to conversion of the dipoles, forming the anti-parallel arrangement. The molecular potential calculation using the Lennard-Jones potential, the image potential approximation, and the Stockmayr potential supported this model. This orientation structure of the SO_2 dipoles in the carbon subnanospace depended on the micropore width ; in the greater carbon nanospace the above-mentioned steep transition was not observed. This research showed a new possibility of molecular chemistry controlled by the subnanospace field.