Specific Heat and Transformations of Water in 1.4 and 1.8 nm Pore-MCMs

Thermodynamic changes on transformations of water in MCMs of 1.4 and 1.8 nm diameter cylindrical pores were investigated by measuring the specific heat, C-p,C-app, during cooling and during heating at 12 K/h rate in the 160-280 K range. The nature of the slow transformation was further investigated by measuring the dynamic specific heat by using temperature-modulated calorirnetry. It led us to establish whether or not contributions to C-p,C-app are thermally reversible. We found that a thermally reversible, monotonic change in C-p,C-app of water occurs in 1.4 nm pore-MCM and a sigmoid-shape change occurs in 1.8 nm pore-MCM. C-p,C-app measured during cooling and during heating is found to be the same at T < 227 K for 1.4 nm pore-MCM, and at T < 232 K for 1.8 nm pore-MCM, as is the real component of the complex specific heat, C-p'. This shows that the sigmoid-shape change in C-p,C-app is not due to glass liquid transition of water in the pores. This is further confirmed by showing that the C-p,C-app features observed for water differ characteristically from the C-p,C-app features of glycerol measured at the same 12 K/h rate. The continuous change in C-p,C-app of water in the MCMs with change in T is reversible, and this indicates that water gradually converts to distorted ice-like structures on cooling, a process that is reversed on heating, until their fractional amounts reach a reversible equilibrium. The large sigmoid shape increase in C-p,C-app on heating is attributed to the latent heat, similar to that seen on premelting of fine grain crystals. The findings put into question a conclusion from neutron scattering and NMR studies that water in 1.8 nm pores undergoes a structural and kinetic transition at similar to 225 K, while remaining a liquid. Thermodynamic indication for a reversible low-density to high-density transition in water is not found.