The solid–liquidphase transition temperatures and heats of fusion ΔH f of nonpolar organic solids confined in the pores of controlled pore glasses were measured by differential scanning calorimetry. The pore diameters d were in the range of 40–730 A and the organics studied were cis‐decalin, trans‐decalin, cyclohexane, benzene, chlorobenzene, naphthalene, and heptane. In accordance with previous reports on studies of primarily inorganic materials, the melting point of the pore solidT(d) decreased with decreasing pore diameter. In addition, a large reduction in the bulk enthalpy of fusion ΔH f of the pore solid was measured, which apparently has not been studied in detail by other workers. A linear correlation was found between the melting point depression (ΔT m ) and the reciprocal diameter, as predicted by theories of solidification in a capillary. The calculated values of the solid–liquid interfacial energy σsl were in reasonable agreement with values reported in the literature based on other methods of measurement.

/pdf/the-melting-behavior-of-organic-materials-confined-in-porous-315yrxd5fm.pdf

The melting behavior of organic materials confined in porous solids

A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.

https://iopscience.iop.org/article/10.1088/0953-8984/13/11/201/pdf

Confinement effects on freezing and melting

Experimental studies on the applicability of the Kelvin equation to highly curved concave menisci

In order to clarify the origin of the hysteresis between freezing and melting of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of seven kinds of siliceous MCM-41 (a member of ordered mesoporous materials denoted by Mobil Oil researchers) with different pore radii (1.2–2.9 nm) and the interconnected pores of Vycor glass as a function of temperature. The hysteresis effect depends markedly on the size of the cylindrical pores: the hysteresis is negligibly small in smaller pores and becomes remarkable in larger pores. This strongly suggests that the hysteresis is arisen from size-dependent supercooling of water confined to the mesopores. For the water confined to the mesopores with pore radius of 1.2 nm, a continuous transition between a liquid and a solid precedes the first-order freezing transition of the pore water which would occur by the same mechanism as in bulk water.

Freezing and melting of water in a single cylindrical pore: The pore-size dependence of freezing and melting behavior

The effect of finite size on the solidification of o-terphenyl and benzyl alcohol confined in model controlled pore glass (CPG) materials is described. These two organic liquids form either amorphous glasses or crystalline solids in the bulk upon cooling, depending on the rate of cooling and other factors. The solidification behavior of the liquid in the pores was studied as a function of pore diameter (4−73 nm), chemical surface treatment of the CPG and the degree of pore filling, by differential scanning calorimetry (DSC). We observe that the glass transition, Tg, shifts to a lower temperature as pore size decreases. This shift is independent of the degree of pore filling for both o-terphenyl and benzyl alcohol, suggesting that a reduction in bulk density or a negative pressure effect is not the cause of the observed shift. The crystallization behavior of o-terphenyl and benzyl alcohol is also altered by confinement and strongly depends on the pore size and degree of pore filling.

/pdf/vitrification-and-crystallization-of-organic-liquids-4xt25tzrry.pdf

Vitrification and Crystallization of Organic Liquids Confined to Nanoscale Pores

Differential scanning calorimetry (d.s.c.) and nuclear magnetic resonance (n.m.r.) techniques were used to study the melting of ice in porous solids.At low water contents (less than about three monolayers) no freezing or melting was observed. As the water content was increased, a single melting peak was observed as the pores filled. The amplitude of this peak reached a constant value when the pores were completely filled, and then at higher water contents a second peak was observed increasing in amplitude as more water was added. The lower melting point was characteristic of the pore water and this melting point decreased with decresing pore radius. The higher melting point (0°C) was that of the non-pore, bulk, water. The pore volume determined calorimetrically agreed with the quoted pore volume. A coefficient of 0.9 for the linear correlation between melting point depression (ΔT) and the reciprocal radius (1/r) confirmed the applicability of the Kelvin equation. It is concluded that the observed properties of water in pores are essentially bulk properties at distances of more than about 10 A from the surface, and influenced more by the Kelvin effect, i.e., capillarity, than by the surface of the silica. D.s.c. can be used as a rapid method to characterise these silicas. Two simple measurements on silica samples containing water allow an estimation of the surface area, pore volume and an average pore size to be made.

George K. Rennie

Papers

Melting of ice in porous solids

Melting of ice in porous solids