Control of the capillary condensation/evaporation of water in porous media by thermosensitive coating
Final Report Abstract
The development of new materials for thermal storage, based on the condensation/evaporation transition of water in thermo-responsive pores, was the main aim of this project. The synthesis of some thermo-responsive porous materials has just recently been reported, but the adsorptive properties of such materials have not been studied yet, neither theoretically nor experimentally. In our project, we have performed the first simulation studies of the phase behaviour of water in thermo-responsive pores. The theoretically predicted condensation/evaporation transition of confined water in equilibrium with an external bulk reservoir upon varying strength U0 of the water-pore interaction has been observed in simulations. This transition occurs, when U0 achieves some critical value U0 c, which practically does not depend on temperature, but depends on the pore size. In spherical pores, U0 c becomes weaker upon increasing pore size: it changes from -1.75 kcal/mol to -0.94 kcal/mol, when the pore radius Rp increases from 9 Å to 25 Å. Extrapolation to macrosopic pores, gives U0 c = -0.61 kcal/mol. The dependence of U0 c on pore size was attributed to the size dependence of the contact angle, which is due to a line tension effect. The obtained results show that even in strongly hydrophilic porous material, water vapour should not condense in nano-cavities. Besides, they evidence that nano-structuring of even strongly hydrophilic surfaces can be used to make them more hydrophobic. Our simulations have shown that the heat storage density of the thermo-responsive porous materials can exceed ~1000 MJ/m3 at temperatures t < 100°C. Thermo-sensitivity of the porous material leads to a weaker dependence of the transition temperature on the pore size and on the humidity of the external bulk vapour. Currently, the grafting of the inner pore surface with thermo-sensitive macromolecules is the most developed method of making thermoresponsive porous materials. We have found, that the hydrophilic/hydrophobic transition of such macromolecules occurs at thermal disruption of the H-bonded network of hydration water along the polymer chain. The cooperativity of this phenomenon depends on the chemical structure of the polymer backbone. This finding gives the possibility to determine macromolecules, applicable as pore wall coating for thermo-responsive materials. Interestingly, polypeptides also undergo hydrophilic/hydrophobic transition at about 37°C and their use in heat storage applications is possible.
Publications
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Water condensation in pores for heat storage: theoretical limits and prerspectives. In Proceedings of the 23th European Symposium on Applied Thermodynamics, Cannes, France, 2008
I. Brovchenko, A. Oleinikova and A. Geiger
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“Condensation/Evaporation Transition of Water in Spherical Pores in Equilibrium with Saturated Bulk Water”, J. Phys. Chem. B , 114, 16494–16502 (2010)
I. Brovchenko and A. Oleinikova
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What Determines the Thermal Stability of the Hydrogen-Bonded Water Network Enveloping Peptides? J. Phys. Chem. Lett. 2, 765-769 (2011)
A. Oleinikova and I. Brovchenko
