Phase Transitions of Naphthalene and Its Derivatives Confined in Mesoporous Silicas

The phase transitions of naphthalene and its derivatives including 2-methylnaphthalene, 2-methyoxynaphthalene, and 2-chloronaphthalene confined in mesoporous silicas of spherical and cylindrical pore geometry were investigated by differential scanning calorimetry (DSC), and a depression of the phase transition temperature was observed. It was found that depression and freeze melt hysteresis are generally greater in spherical pores. Using powder X-ray diffraction, it was observed that the crystal structure of confined naphthalene is the same as the bulk. Using DSC and Raman spectroscopy, it was shown on both a thermodynamic and molecular level that there is a nonfreezing layer on the pore walls for each confined compound and its thickness depends on the pore diameter.