SILVER ION COORDINATION IN MEMBRANES FOR FACILITATED OLEFIN TRANSPORT

Silver K-edge XAFS (X-ray absorption fine structure) was used to determine the coordination of silver ions in synthetic polymeric membranes of interest for facilitated olefin transport. The Ag+ ions in membranes treated with aqueous solutions of AgNO3, AgClO4, AgBF4, and AgF are three-coordinate with an average Ag-O bond length (2.22 +/- 0.04 angstrom) that is independent of the nature of the counteranions (i.e., nitrate, perchlorate, tetrafluoroborate, and fluoride) and significantly shorter than that (2.34 +/- 0.04 angstrom) for four-coordinate Ag+ in aqueous solution. Similarly, the silver ion coordination is independent of the nature (i.e., composition, thickness, manufacturer, etc.) of the membrane. The aqueous environment in these geltype membranes is sufficiently different from bulk water, such that it causes a change of coordination of the Ag+ ions. In contrast, the coordination of the linear [NC-Ag-CN]- anion is the same in both bulk solution and membrane.