Pervaporation properties of fluoroalkylsilane (FAS) grafted ceramic membranes

Changing the hydrophilic character of ceramic membranes into hydrophobic ones is nowadays of particular interest. The originally hydrophilic character of ceramic membranes can be changed by grafting hydrophobic molecules on the surface. Fluoroalkylsilanes (FAS) are a group of compounds which can be efficiently used to create the hydrophobic character of different surfaces. The grafting process can be performed by a reaction between -OH surface groups of the ceramics and ethoxy groups (-O-Et) presented in organosilane compounds. The aim of this work was to prepare FAS-modified pervaporation ceramic membranes by grafting alumina and/or titania ceramic nanofiltration membranes with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (C8) molecules and to determine the properties of such modified hydrophobic ceramic membranes in the pervaporation process used for the desalination of different salt solutions and for the removal of organic solvents of different hydrophobicity [i.e., ethanol (EtOH) and methyl t-butyl ether (MTBE)] from water. It was found that transport and selective properties of grafted membranes depended on the type of ceramics and the grafting conditions. In general, FAS grafted titania membranes were more permeable and more selective than alumina ones. In the case of desalination it was found that salt rejection was practically 100%; there was no salt found in the permeate. Pervaporation flux in contact with an aqueous salt solution was dependent on the kind of salt, its concentration and feed temperature. FAS grafted membranes prepared from titania showed much higher permeation fluxes compared with alumina ones. Both membranes showed high selectivity towards the organic component of water–organic mixtures. The selectivity factor of FAS-TiO2 (grafted in 0.1 M C8), FAS-TiO2 (grafted in 0.01 M C8) and FAS-Al2O3 (grafted in 0.01 M C8) membranes in contact with water–MTBE (2 wt.% MTBE) solution was 95, 45 and 23, respectively. The C8 molecules grafted on the titania ceramics underwent conformation change from a tangled form into the straightened one when the C8 environment changed from pure water to a water–organic or water–electrolyte solution. The concentration range in which this conformation change occurred was dependent on the hydrophobicity of organic solvent molecules presented in the solution. In the case of more hydrophobic MTBE molecules the tangled–straightened form change of FAS molecules occurred in a concentration range 0–0.1 mol. % MTBE.