(181g) Freezing/melting of Liquids In Cylindrical Nanopores

We report a study of the melting and freezing processes of CCl4 and H2O in carbon nanotubes of different diameters. The phase transitions in these substances have been observed by dielectric spectroscopy (DS) and differential scanning calorimetry (DSC), which permitted determination of the phase transition temperatures. The structure of the new phases has been studied by neutron diffraction (ND).

A shift between the melting points determined for the bulk liquid and the confined liquid was observed. The magnitude of the shift depends on the diameter of the carbon nanotubes and the properties of the liquid confined in the pores. For CCl4 in nanotubes of diameter <5nm a single phase transition is observed at a temperature lower than that of the bulk liquid. For CCl4 in tubes of diameter greater than 5nm, the melting point of the contact layer was at a temperature higher than that of the bulk liquid (typical of contact layers), while the melting point of the inner layers was lower than that of the bulk liquid; these results are consistent with our theoretical predictions [1]. For H2O in nanotubes of diameters 2.8 and 4 nm, a single phase transition was observed at a temperature lower than that of the bulk liquid. This result suggests that the arrangement of water molecules in the nanotubes may not form a system of layers, as then separate phase transitions typical of the contact and inner layers would be observed. The presence of crystalline cubic structure of ice in the nanotubes, different than that in bulk water, observed by the ND method, has confirmed this conclusion. A decrease in the melting point of water confined by graphene surfaces can be explained by a low value of the a parameter, being a measure of the liquid-pore wall and liquid-liquid interactions for water, for which it takes values below 1 ( α = 0.51) [2]. For water adsorbed in silica glasses the α value is larger; the behavior of melting of water observed by dielectric methods will also be discussed for this case.

[1] M.Jażdżewska, F.Hung, K.E.Gubbins, M.Śliwińska-Bartkowiak – „An experimental study of melting of

CCl4 in carbon nanotubes”, Phys. Chem. Chem. Phys., 2005,7,3884-3887

[2] R.Radhakrishnan, K.E.Gubbins, M.Sliwinska-Bartkowiak, “Global Phase Diagrams for Freezing in Porous

Media”, J.Chem.Phys., 116, 1147 (2002)