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From nanoscale modification to separation - The role of substrate and modifiers in the transport properties of ceramic membranes in membrane distillation

Kujawa, Joanna
Journal of membrane science 2019 v.580 pp. 296-306
Gibbs free energy, aluminum oxide, artificial membranes, ceramics, chlorine, contact angle, distillation, fluorine, hydrophobicity, hysteresis, mechanical properties, moieties, silane, surface tension, titanium dioxide
The set of various ceramic substrates Al2O3-100nm, TiO2-75nm, ZrO2-3nm, and ZrO2-200nm were used in the hydrophobization process with different silane-based modifiers fluorinated and fluorine-free, possessing different reactive groups (ethoxy, methoxy, and chlorine) and different length of the alkyl-chain (C6 and C16). It was investigated how these variables can influence the features of the material and then transport and separation properties. A comprehensive material study including evaluation of morphological, wetting and structural properties was performed and referred to the membrane performance in membrane distillation. Transport features, particularly various driving forces (range 105–685 mbar) and their impact on the overall membrane performance were studied. The highest grafting effectiveness was observed for superhydrophobic Al2O3-100nm-C16Cl3 (contact angle 154°, hysteresis 10°) possessing critical surface tension of 12.4 mN m−1 and polar part of surface free energy 1.1 mN m−1. Improvement in mechanical properties determined by nanoindentation of all tested ceramics after hydrophobization was observed. Al2O3-100nm-C6OEt3 has the best transport properties. Such results were ensured by an appropriate substrate selection with the highest level of hydroxyl groups ready for modification on pristine material and lower reactivity of ethoxy groups in comparison to chlorine one. All membrane possessed a very high salt rejection coefficient ca.99%.