Studies of water and ice in hydrophilic and hydrophobic mesoporous silicas: pore characterisation and phase transformations

Abstract

A study has been made as a function of temperature of the phase transformation of water and ice in two samples of mesoporous silica gel with pore diameters of ∼50 Å. One sample was modified by coating with a layer of trimethylchlorosilane, giving a predominantly hydrophobic internal surface, whereas the unmodified sample has a hydrophilic interface. The pore structure was characterised by nitrogen gas adsorption and NMR cryoporometry and the melting/freezing behaviour of water and ice in the pores was studied by DSC and neutron diffraction for cooling and heating cycles, covering a range of 200 to 300 K. Measurements were made for several filling-factors in the range 0.2 to 0.9. The results show a systematic difference in the form of ice created in each of the samples. The non-modified sample gives similar results to previous studies with hydrophilic silicas, exhibiting a defective form of cubic ice superimposed on a more disordered pattern that changes with temperature and has been characterised as ‘plastic’ ice [Liu et al., 2006, Webber et al., 2007]. The modified sample has similar general features but displays important variability in the ice transformation features, particularly for the case of the low filling-factor (f = 0.2). The results exhibit a complex temperature-dependent variation of the crystalline and disordered components that are substantially altered for the different filling-factors.