Origin of the enthalpy features of water in 1.8 nm pores of MCM-41 and the large C(p) increase at 210 K.
It is shown that exothermic and endothermic features of dH(m)/dt observed on heating rapidly precooled and slowly precooled states of water in 1.8 nm pores of MCM-41 and the unusually large increase in the specific heat in the 210-230 K range [M. Oguni, Y. Kanke, S. Namba, and AIP Conf, Proc. 982, 34 (2008)] are inconsistent with kinetic unfreezing of a disordered solid, or glass softening. The exotherm is attributable to the melt's gradual conversion to distorted icelike structures and the endotherm to the reverse process until their fractional amounts reach a reversible equilibrium on heating. The large increase in C(p,m) with T is attributed to the latent heat, similar to that seen on premelting of fine grain crystals. The available calorimetric data on freezing and melting and the pore-size dependence of the features support this interpretation. The findings also put into question a conclusion from neutron scattering studies that in 1.8 nm pores water undergoes a structural and kinetic transition at approximately 225 K while remaining a liquid.
Johari
G P
GP
Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada. joharig@mcmaster.ca