Structural analysis of water adsorbed in silica gel

The melting point, T_f of water in a pore decreases as the surface area to pore volume ratio of the pore decreases. Analysis of water absorbed in the pores of silica gels using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS) shows that the thickness of the bound, non-freezing water layer adjacent to the pore surface increases as its temperature increases, but that it is independent of the surface silanol concentration, [Si_sOH]. In contrast, the thickness decreases as the cylindrical pore radius r_H decreases. Thus, the increase in the bound water thickness from 0.45 nm for gels with r_H=1.2 nm to 1.2 nm for gels with r_H=7.5 nm is due to the increase from –53°C to –7°C of the temperature (e.g., the melting point T_f) at which the bound water thickness was measured, and not due to the increase in t_H or the decrease in [Si_sOH]. The T_f of bulk water measured in a DSC was –0.3°C. The boiling point T_v of bulk water measured in a DSC was 81.3°C. T_v increased to 94°C in 7.5 nm pores and to 109°C K in 1.2 nm pores.