Articles you may be interested inOn the state of water in 2.4 nm cylindrical pores of MCM from dynamic and normal specific heat studies J. Chem. Phys. 139, 064507 (2013); 10.1063/1.4817333 Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt Universal scaling, dynamic fragility, segmental relaxation, and vitrification in polymer melts Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation J. Chem. Phys. 114, 950 (2001); 10.1063/1.1329343Freezing and melting of water in a single cylindrical pore: The pore-size dependence of freezing and melting behaviorTo study the freezing/melting behavior of a confined CH 3 OH, we performed x-ray diffraction measurements of CH 3 OH confined inside the cylindrical pores of seven kinds of regular mesoporous adsorbents ͑MCM-41 and SBA-15͒ with different pore radii (rϭ1.2, 2.1, 2.5, 3.9, 4.5, 5.3, and 7.0 nm͒ as a function of temperature. The freezing/melting behavior depends markedly upon the pore size. Within the pores of rр3.9 nm, the confined CH 3 OH vitrifies on freezing. On the other hand, cooling of the CH 3 OH confined to the pores of rу4.5 nm results in crystallization of the liquid. Within the pores of rϭ5.3 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 ␣ phase. The prefreezing temperature seems to lower steeply with decreasing pore-size and to approach the freezing temperature for the pores of rϭ4.5 nm. Cooling of the CH 3 OH confined to the pores of rϭ7.0 nm results in formation of a crystalline solid with the same structure as that of the bulk ␤ phase and it does not transform into the low temperature ␣ phase on further cooling down to 30 K, leading to the appearance of a glassy crystal with the ␤ phase structure. A large hysteresis effect between freezing and melting is observed. A mechanism of the vitrification is discussed.

To study the freezing/melting behavior of a confined N2, CO, and Kr, we performed X-ray diffraction measurements of N2, CO, and Kr confined inside the cylindrical pores of five kinds of siliceous MCM-41 with different pore radii (R = 1.45−2.9 nm) as a function of temperature. Freezing and melting of these three molecules were almost reversible for all the MCM-41 samples used here. Freezing of the CO confined to the mesopores resulted in the appearance of the orientationally ordered α phase, whereas the N2 solidified into the orientationally disordered phase analogous to the bulk β phase. The solid Kr confined to the mesopores contains a considerable amount of random stacking faults. The freezing-point depressions of N2 and CO relative to the bulk melting point were nearly the same and were slightly larger than that of Kr. Upon cooling, both molecular liquids of N2 and CO confined to the cylindrical mesopores of R = 2.1 nm vitrify, whereas liquid Kr crystallizes within the mesopores of R = 1.8 nm and vitrifies within the mesopores of R = 1.45 nm. The mechanism of vitrification of a confined liquid upon cooling is considered.

To study the freezing/melting behavior of a confined CH3Cl, we performed X-ray diffraction measurements of CH3Cl confined inside the cylindrical pores of six kinds of siliceous MCM-41 with different pore radii (R = 1.2−2.9 nm) as a function of temperature. The pore-size dependence of the phase-transition temperature suddenly changes between R = 2.5 and 2.1 nm and between R = 1.8 and 1.45 nm. The CH3Cl confined to the mesopores of R ≥ 1.8 nm shows a large hysteresis effect between freezing and melting and crystallizes on freezing, while the CH3Cl confined to the mesopores of R ≤ 1.45 nm vitrify on cooling. Although the CH3Cl confined to the mesopores of R ≥ 2.5 nm freezes into a solid with the same ferroelectric structure as the bulk, the CH3Cl confined to the mesopores of R = 1.8 and 2.1 nm crystallizes into a structure that is different from the bulk. The results are discussed on the basis of a homogeneous nucleation, as well as the effects of finite size on melting point depression and the structure of a confined solid.

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