Novel ice structures in carbon nanopores: pressure enhancement effect of confinement

Abstract

We report experimental results on the structure and melting behavior of ice confined in multi-walled carbon nanotubes and ordered mesoporous carbon CMK-3, which is the carbon replica of a SBA-15 silica template. The silica template has cylindrical mesopores with micropores connecting the walls of neighboring mesopores. The structure of the carbon replica material CMK-3 consists of carbon rods connected by smaller side-branches, with quasi-cylindrical mesopores of average pore size 4.9 nm and micropores of 0.6 nm. Neutron diffraction and differential scanning calorimetry have been used to determine the structure of the confined ice and the solid-liquid transition temperature. The results are compared with the behavior of water in multi-walled carbon nanotubes of inner diameters of 2.4 nm and 4 nm studied by the same methods. For D(2)O in CMK-3 we find evidence of the existence of nanocrystals of cubic ice and ice IX; the diffraction results also suggest the presence of ice VIII, although this is less conclusive. We find evidence of cubic ice in the case of the carbon nanotubes. For bulk water these crystal forms only occur at temperatures below 170 K in the case of cubic ice, and at pressures of hundreds or thousands of MPa in the case of ice VIII and IX. These phases appear to be stabilized by the confinement.

Structural analysis of water and carbon tetrachloride adsorbed in activated carbon fibres

“…The structure of confined ice has been studied in carbon nanotubes of 0.7-1.5 nm diameter using molecular simulation 8,9 and experiment, 5,10 and provides convincing evidence for the formation of various kinds of ice nanocrystals, including square-gonal ice as ice VIII and ice IX in nanotubes of small diameters. 11 Similar quasi-high pressure effects due to confinement are observed in chemical reactions in pores. Reactions that only occur at high pressures in the bulk phase occur readily at normal bulk phase pressures in the confined phase as has been observed in experiments 12 and molecular simulations.…”

Pressure Profile for an Associating Lennard-Jones Fluid Confined in a Spherical Cavity

“…A fluid confined in a nanopore would become inhomogeneous, and as a result, some physical properties can be influenced by the dimension of the cavity. − ,,,, Therefore, it is of interest to investigate the effect of radius of the cavity on the pressure profiles. Figure a,b shows the pressure profiles of the associating LJ fluid under the conditions of f = 4, ρ b σ 3 = 0.5, and βε ass = 5 for R increasing from 2.5σ to 20.0σ with an interval of 2.5σ.…”

“…On the basis of the existing experiments and simulations on various properties of confined fluids, it was shown that some phenomena that can only be observed at a very high pressure in the bulk fluid can take place at a pressure that is orders of magnitude lower. Such an enhancement of pressure due to the nanoconfinement has attracted increasing interest. ,,− Thus, one can expect that fluids within nanopores or porous materials have the scope for rapid development in the field of novel functional materials.…”

“…The physical behavior of a fluid confined within nanopores (nanoslit, nanotube, and nanocavity) are significantly different from that of the bulk fluid. − Experimental evidence for this kind of difference can be found from the high pressures and reaction yields within a nanopore, − in which the enhancement of pressure due to the confinement can change by orders of magnitude. Moreover, some striking changes in dynamic behavior, phase transition, and elastic properties of the confined water − are also observed.…”

“…For an inhomogeneous fluid, the pressure tensor depends explicitly on the position, and hence is called the local pressure tensor. The local pressure tensor of a confined fluid has, in addition to the above-mentioned two parts, an extra part contributed by either an external potential or fluid–pore interaction. − On the basis of these considerations, there have been many efforts over the last several decades to study the pressure tensor for confined fluids. ,− ,− …”