Properties of the transport of alkali metal salts through polymeric membranes containing benzo-18-crown-6 crown ether functional groups

Lightly crosslinked polymeric membranes were created by photopolymerization of di(ethylene glycol) ethyl ether acrylate, 4'-vinylbenzo-18-crown-6 and a small amount of hexamethylene diacrylate monomer. These membranes were ut ilized to gain an improved understanding of the behavior of fixed site carr ier membranes (FCMs) under ion transport conditions. Crown ether groups wer e ideal reactive sites because of the extensive studies which have been com pleted on their complexation, and the one chosen for these studies provides selective complexation for potassium over other alkali metal ions. Several classes of parameters were investigated, with primary emphasis on the comp osition of the solution in contact with the membrane, and separately, the m embrane composition. The effects of some of the material properties of the membrane polymers were also investigated, with the most notable being the m acroscopic swelling. The results of this study illustrate that the flux of the salts through these solid membranes is larger than that achieved with c omparable liquid membrane (LM) and polymer inclusion membrane (PIM) systems in experiments with comparable solute concentrations and carrier species. The increase in transport rate is presumably due to the extremely large con centration of reactive sites that are able to be incorporated into the memb ranes because of their production method. The normalized flux values (depen ding on the level of functionalization) ranged up to 58 000-72 000 mol(m(2) s)(-1)(mol/l)(-2) when multiplied by 10(7). These normalized flux values a re up to 3 orders of magnitude larger than those obtained under similar exp erimental conditions with LM and PIM systems. The large reactive site conce ntration and highly hydrated nature of these membranes does produce reduced values of selectivity than those with LM and PIM systems. The hydrated nat ure of the membrane interior provides radically different behavior from tra ditional LM and PLM systems with variation of the anion accompanying the ca tion during transport. In fact the trend of flux as a function of anion hyd rophilicity is contrary to that obtained in LM systems. (C) 1999 Elsevier S cience B.V. All rights reserved.