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

The carbons involved in many industrial applications take many diverse forms ranging from well-developed crystalline structures, such as graphite and diamond, to the so-called amorphous varieties such as carbon blacks, cokes, chars, activated carbons.

Textural Characterization of Porous Carbons by Physical Adsorption of Gases

The heat capacity of nitrogen adsorbed on titanium dioxide has been measured for amounts adsorbed corresponding to 2.2, 3.1, 4.0, and 4.8 molecular layers in the temperature range 14 ° to 75 °K. Th...

Phase transitions in multimolecular layers of adsorbed nitrogen

The adsorption characteristics of alkanes on high surface area cellulose were investigated by inverse gas chromatography. The enthalpy and entropy of adsorption of alkanes on the solvent exchanged cellulose were greater than the values on non-porous cellulose. The difference in the heats of adsorption of alkanes between the non-porous and the solvent exchanged cellulose was greater for larger molecules indicating stronger interaction for the higher alkanes in the pore structure. For n-hexane the differences in enthalpy and entropy of adsorption were 17 kJ/mole and 52 J/mole K, respectively. The London component of the surface free energy of adsorption was 116 mN/m for the solvent exchanged cellulose.

Analysis of the Adsorption of Alkanes on High Surface Area Cellulose by Inverse Gas Chromatography

Es wird festgestellt, dass organische Substanzen, wenn sie auf einer festen Oberflache in einer Schichtdicke bis zu 100 A ausgebreitet sind, einen tieferen Smp. besitzen als die kompakte Substanz. Die beobachteten Smp.-Erniedrigungen variieren bis zu 30°, je nach Substanz und Unterlage; sie verschwinden bei Dicken, welche sich von 15 zu 100 A erstrecken konnen. Die beobachteten Smp.-Erniedrigungen wachsen linear mit der Zahl der π-Elektronen der ausgebreiteten Verbindung. Im Falle einer Legierung des Hume-Rothery-Typus (Silber-Zinn) als Unterlage ist die Smp.-Erniedrigung um so grosser, unausgefullter die Brillouin-Zone des Elektronengases des Metalls ist. So zeigen die γ-Phasen, die fast vollstandig besetzte Energieniveaus haben, ein Minimum der Smp.-Depression. Diese Befunde werden in der Weise gedeutet, dass die den Gitterzusammenhalt be-wirkenden Elektronen der ausgebreiteten Substanz in das Elektronen-gas der Metallunterlage eintauchen, wodurch ihre Dichte in der dun-nen Schicht verringert, und damit der Smp. der letzteren erniedrigt wird. Befunde und Deutung stehen in guter Ubereinstimmung mit den auf dem Gebiete der heterogenen Katalyse existierenden Erfahrungen.

Über eine Schmelzpunktserniedrigung organischer Substanzen in dünnen Schichten. III. Mitteilung

Batchelor and Foster1 found, by examination of the vapour pressure – temperature curves, that dioxan adsorbed on a ferric oxide gel of pore radius ∼ 100 A. melts sharply about 6° C. below the normal melting point of the bulk liquid. During a recent investigation of the sorption of amines by silica gel, a similar effect has been observed with ethylenediamine.

/pdf/melting-point-of-adsorbed-liquids-phoogoqm1n.pdf

Marjorie J. Brown

Papers

Melting Point of Adsorbed Liquids

The Sorption of Amines by Silica Gel

200. The sorption of vapours by ferric oxide gel. Part III. Aliphatic amines