Freezing and melting of methanol in a single cylindrical pore: Vitrification of methanol

To study the freezing/melting behavior of a confined CH3OH, we performed x-ray diffraction measurements of CH3OH confined inside the cylindrical pores of seven kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii (r = 1.2, 2.1, 2.5, 3.9, 4.5, 5.3, and 7.0 nm) as a function of temperature. The freezing/melting behavior depends markedly upon the pore size. Within the pores of r≤3.9 nm, the confined CH3OH vitrifies on freezing. On the other hand, cooling of the CH3OH confined to the pores of r≥4.5 nm results in crystallization of the liquid. Within the pores of r = 5.3 nm, the crystallization proceeds in two steps: prefreezing first occurs and then it transforms into a crystalline solid with the same structure as that of the bulk α phase. The prefreezing temperature seems to lower steeply with decreasing pore-size and to approach the freezing temperature for the pores of r = 4.5 nm. Cooling of the CH3OH confined to the pores of r = 7.0 nm results in formation of a crystalline solid with the same structure as that of the bulk β phase and it does not transform into the low temperature α phase on further cooling down to 30 K. A large hysteresis effect between freezing and melting is observed. A mechanism of the vitrification is discussed.