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

Abstract
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.