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 ∼50Å. 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 300K. 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 temperaturedependent variation of the crystalline and disordered components that are substantially altered for the different filling-factors. *

Effect of aging temperature on the structure, pore morphology and bioactivity of new sol–gel synthesized bioglass

“…Taking into account the previous use of silica gel, Vycor and MCM-41 by our team as confinement matrix [36][37][38][39][40], we decided to elaborate a new mesoporous glass with molar composition close to 100% SiO 2 (molar composition of MCM-41). However, previous study showed that MCM-41 was not bioactive [41].…”

Pore Distribution Characteristics of Thawed Residual Soils in Artificial Frozen-Wall Using NMRI and MIP Measurements

“…Equation (2) indicates that there is a linear relation between the relaxation time T 2 and the pore diameter D. Thus, it is recognized that to calculate the pore diameters in a porous mediums based on its ρ 2 value. In several researches, ρ 2 values were calculated by comparing the T 2 spectrum curves with POSD results from mercury injection porosimetry technique as presented by Chen et al [33] and nitrogen gas adsorption technique as presented by Jahwar et al [34]. Besides, the ρ 2 values can be calculated by empirical formulas of permeability.…”

Quasi-Liquid Layer on Ice and Its Effect on the Confined Freezing of Porous Materials

“…Predicting the thickness of QLL always faces challenges especially when the phase transition from water to ice is beyond the bulk freezing. A pertinent example is the freezing of the water confined in thin pores, which can be influenced by many factors such as the geometry confinement that regulates the crystal anisotropy, the curvature effect that depresses the freezing temperature, and the actions of the pore walls on the pore fluid [1,3,11,[14][15][16][17]. Indeed, in chemical, environmental, and civil engineering, the confined freezing is more frequently quoted because almost all materials (except pure metals and crystals) are, more or less, in porous structures, and the phase transition of the water confined in the pores may occur in certain conditions during freezing [3,18].…”