A study of freezing–melting hysteresis of water in different porous materials. Part I: Porous silica glasses

The freezing and melting temperature hysteresis of water in Vycor porous glass and controlled pore glass (CPG) with the nominal pore diameters 7.0 and 7.5 nm, respectively, has been explored by NMR cryoporometry and NMR relaxometry techniques. The freezing branch of the hysteresis in Vycor was found to be much steeper than the melting one, presumably due to the pore blocking on freezing. This is not observed in CPG, which exhibits parallel freezing and melting branches. Sub-loops recorded starting from partially frozen states are self-similar with respect to the amount of unfrozen water, in both CPG and Vycor. On the other hand, sub-loops starting from partially molten states are self-similar only in CPG, while in Vycor, their shape does depend on the amount of unfrozen water. T2 relaxation measurements undertaken along the freezing branch reveal in both glasses a slow (on an NMR timescale) exchange of water molecules between frozen and unfrozen pore domains, indicating a good segregation of those domains on freezing. On melting, however, the regime of slow exchange persists only for CPG, while in Vycor, a quasi-exponential T2 relaxation is observed, characteristic of fast exchange. This suggests that frozen and molten regions in Vycor are well dispersed on melting, while in CPG they present extended domains. We explain such a difference assuming a worse interconnection of a Vycor’s pore space as compared to CPG, taking into account its essentially lower porosity (0.28 versus 0.51 in CPG).

Water confined in controlled porous glass (CPG) and Vycor exhibits freezing–melting hysteresis of different shape (H1 versus H2). Analysis of the hysteresis and NMR relaxation data suggests that water in CPG freezes and melts on same length scales, while in Vycor, the length scale for melting is smaller than for freezing. This difference is explained by a worse pore connectivity in Vycor as compared to CPG.Figure optionsDownload as PowerPoint slideResearch highlights
► Freezing and melting in controlled pore glass (CPG) occur on the same length scale.
► In Vycor pore glass, the length scale of melting is smaller than that of freezing.
► The difference between CPG and Vycor is explained by that in pore connectivity.