SOLID-SOLID PHASE-TRANSITIONS OF CYCLOHEXANE IN POROUS SOL-GEL GLASS

Raman spectra of solid cyclohexane inside porous sol-gel glass were measured as a function of temperature and pore diameter. Samples with average pore diameters of 3.5 and 22 nm were used. The transition between the cubic and monoclinic phases of cyclohexane was accompanied by changes in the v(21) band shape. From those changes, the depression of the phase transition temperature was estimated and was found to depend on the size of the pores. At low temperatures, cyclohexane in the porous glass formed a mixture of monoclinic crystallites and an amorphous phase. It is suggested that the amorphous structure is formed by molecules within a surface layer near the pore walls. It is shown that the extent of the amorphous structure can be altered by replacing surface hydroxyl groups with trimethylsilyl groups, i.e., by modifying surface interactions.