(705e) Ion-Specific Effects in Freezing/Melting of Salt Solutions in the Cylindrical Nanochannels of SBA-15 Silica

Crystallization of salt within the pores of stone is a major reason for destruction of monuments and building materials. While many studies addressed salt crystallization induced by water evaporation from the pores, less attention has been paid to salt crystallisation caused by water freezing. In narrow pores, the observed melting point depression relative to bulk water represents a combination of the familiar colligative effect and the shift in the phase transition due to confinement in the pores.

We used differential scanning calorimetry (DSC) to determine the melting point depression of eutectic water + alkali halide systems in the cylindrical nanochannels of SBA-15 silica. The experiments yield ΔT_p, the depression of the eutectic temperature in the pores relative to the bulk eutectic temperature. This thermodynamic quantity can be related to a nanoscale length, namely the thickness of a layer of non-freezing water at the wall of the cylindrical channels. Specific ion adsorption at the silica/water and ice/water interfaces of this thin layer will affect the thickness of the non-freezing layer and in turn the eutectic temperature in the pores. The observed trends in  within the cationic and anionic series of the alkali havides correlates with expectation on the basis of ionic size and the Hofmeister series.