Membrane phenomena in nonisothermal systems .2. Effect of hydrophobicity of ion-exchange groups of anion-exchange membranes on thermoosmosis

Solvent transport across poly(styrene)-based anion-exchange membranes with various ion-exchange groups was measured under a temperature gradient to discuss the effect of hydrophobicity of the membranes on thermoosmosis. Two varieties of poly(styrene)-based copolymer membranes with 10 and 14% divinylbenzene as cross-linking agent were prepared as base membranes. Benzyltrimethylammonium, benzyltriethylammonium, benzyltripropylammonium, benzyltributylammonium, and benzyltripentylammonium were introduced in the base membranes as anion-exchange groups. The thermoosmotic volume flux toward the hot solution side decreased with increasing the length of alkyl groups (increasing hydrophobicity of the groups), and at last, for the membranes with benzyltripentylammonium, the direction of thermoosmosis reversed: from the hot to the cold solution side as is observed for usual hydrophobic membranes. The transported energy and the mean transported entropy of water in the membranes estimated from the data of thermoosmosis and that of osmosis measurements increased with increasing the hydrophobicity of the ion-exchange groups introduced in the membranes.