The adsorption isotherms of water on MCM-41. Each curve is shifted by 0.5 for clarity. The arrows indicate the pressures for monolayer and capillary filling used for the C p measurements. The adsorbed amounts between the arrows were used to estimate the amount of internal water in the pores. The monolayer values were assigned to the interfacial water and used in the separation of the contributions of the interfacial (C m) and internal (C i) water from the measured heat capacity C p.
(a) Temperature change of the heat capacity (open symbols) C p and C i (closed symbols) of pore and internal water confined in MCM-41, respectively, and C p of the monolayer (ML) water adsorbed onto the pore surface of sample C16. The broken lines denote those of bulk water (ice Ih and liquid water). The values of C i were obtained by assuming that the pore water in the capillary filled state composes of both interfacial and internal water and that the contribution of the interfacial water is equal to that of the monolayer water. (b) DSC curves of water filled in the mesopores of MCM-41 (C12–C16) representing the L–L transitions before and after crystallization and melting of pore water. For sample C10, only L–L transition is detectable. The data of water contacted with crushed quartz without pores are also shown to confirm the stability of the base line of the DSC measurements. The scanning rate was 2 K min−1 in the measurements.
(a) A comparison of the C p curve (lines with symbols) and the DSC curve (solid lines) in the warming direction of water confined in MCM-41. Both values were arbitrarily magnified to make the comparison clear. The DSC curves were measured at a heating rate of 2 K min−1. The onset temperature does not vary when the scanning rate was below 7 K min−1. The melting point, mp (DSC), was designated at a crossing point between the base line and the tangential line on the left-hand side of the peak, i.e., onset temperature of melting. The value of melting point, mp(C p), was determined as the peak position of the C p curves. (b) Plots of the melting point of water determined from the C p and DSC measurements as a function of the inverse of the effective pore radius. The melting point of samples C22 and SBA-15 are added from a previous work. 10 The terms r and t denote the pore radius and the thickness of interfacial water (0.38 nm), respectively.
Entropy values estimated from the C i values of the internal water confined in MCM-41, where the starting value was fixed arbitrarily on the ordinate since the residual entropy of the confined water in the mesopores of MCM-41 is available. The line in black is for Ih.
Temperature dependence of the INS spectra of water confined in MCM-41: (a) sample C10, (b) C18, and (c) in the monolayer water adsorbed on the pore surface of C18. The solid lines were estimated by fitting 2 or 3 Gaussian functions using the fitting program ORIGIN (Microcal, USA). The dashed lines are drawn to clarify the change in peak position with temperature.
INS spectra of ice: (a) amorphous, Ih, and Ic, 45 and (b) water capillary-filled in samples C10 and C18, and the monolayer water adsorbed on the pore surface of C18. The spectra are displaced upward arbitrarily for clarity.
Scheme of (a) enthalpy and (b) C p changes of confined water with temperature in the region of the phase changes. Line number 1 is for water in mesopores presenting freezing temperature higher than HNT (e.g., C18); the enthalpy change for homogeneous nucleation crystallization is shown by a broken line. Line number 2 is for water confined obeying the G–T relation (e.g., C12). Line 3 is for water which does not crystallize until very low temperature at around 190 K (C10).
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