Determination of water intrusion heat in hydrophobic microporous materials by high pressure calorimetry

The understanding of interactions between a solid surface and a non-wetting liquid still remains of fundamental interest in numerous research fields, from chemistry to biology. This work focuses on the mechanisms of water intrusion in hydrophobic microporous materials through the thermal analysis of the phenomenon. A specific calorimetric technique coupled to high pressure equipment has been developed to investigate equilibrium thermal effects in such thermodynamic systems from 0 to 400 MPa under isothermal conditions. First validation tests of this method were carried out by compressing degassed water in a constant volume V with successive small pressure increments dp. At equilibrium, the integration of the heat flow as a function of time leads to the differential heat of compression of liquid water δQ/dp from which the thermal isobaric dilatation coefficient of bulk liquid water is calculated. Results show a good balance between experimental and predicted isobaric dilatation coefficients given in literature. Then, the use of this calorimetric device to measure thermal effects of liquid water intrusion in a pure siliceous microporous zeolite, silicalite-1, displays an endothermic effect around 100 MPa, as predicted by other authors using GCMC simulations. The calculation of the intrusion heat, by subtracting the thermal effect of water compression around the porous material, is also validated by comparing microporous versus non-porous silica material. This work opens thus on a powerful method to qualify and quantify mechanical and thermal effects associated with water intrusion in microporous materials under high pressure.