(30e) Phase Transitions of Liquids Confined in Nano-Pores; The Novel Ice Structures

Melting of  fluids confined in nano-porous materials has been extensively studied using both experiments and simulations . This effort is relevant to the fundamental understanding of  the behavior of nano-dimensional fluids and solids, and the influence of surface forces on such materials. Nano-porous materials, such as  activated carbons, silicas, etc., play a prominent role in chemical processing, particularly in separations and as catalysts and catalyst supports. Fluids confined in such porous materials possess many novel properties that can form the basis of future nanotechnologies. Recent studies for pores of simple geometry have shown a rich phase behavior associated with melting in confined systems. The melting temperature may be lowered or raised relative to the bulk melting point, depending on the nature of the adsorbate and the porous material. Much of the apparently complex phase behavior is a result of competition between the fluid-wall and fluid-fluid intermolecular interactions. Of particular interest have been the effects of confinement on phase transitions, on the structure of water and ice in nano-pores , and on dynamical properties of water in one-dimensional pores considered as a model of water channels.  

We  report experimental results on the structure and melting behavior of ice confined in multi-walled carbon nanotubes (MWCN) and ordered mesoporous carbon CMK-3, which is the carbon replica of SBA-15 silica template. The silica template has cylindrical mesopores with micropores connecting the walls of neighboring mesopores. The structure of the CMK-3 consists of carbon rods connected by smaller side-branches, with quasi-cylindrical mesopores of average pore size 4.5 nm and micropores of 0.6 nm. Neutron diffraction, dielectrics relaxation 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 MWCN of inner diameters of 2.4 nm and 4 nm studied by the same methods.  For water in CMK-3 we observe the existence of cubic ice, tetragonal : ice VIII and ice IX nanocrystal forms, while we find cubic ice in the case of MWCN and CMK-3 mesopores. These crystal forms, which occur in bulk water only at temperatures below 180 K in the case of cubic ice, and at pressures of hundreds or thousands of MPa in the case of ice VIII and IX, are stabilized by the confinement.