FTIR-ATR spectroscopic analysis of bis-crown-ether based PVC-membrane surfaces

Abstract

Model membranes have been prepared from PVC (65 mg), dioctyl sebacate (120 mg) and the crown ether BME-44 (4 mg) in 2 ml THF solution and analyzed by the molecular-specific surface technique Attenuated Total Reflectance (ATR) in the infrared region in order to better understand the complexation reactions at the interface between the membrane and aqueous solutions of NaCl, KCl, KSCN and NH4SCN. The information depth provided by this method was between 0.1 and 3 μm and could be changed by variation of the angle of incidence of the IR-radiation. The study of the reactions was carried out ex-situ (at the dried membrane surfaces) after contact times of 1 h with 0,1-2 M aqueous solutions and subsequent rinsing with H2O for 0-100 s. It was found that KSCN- and NH4SCN-contact results in significant complex formation with the membrane, whereas KCl and of course NaCl do not react in such a way (under diffusion of the complex into the bulk of the membrane). Through rinsing with H2O and variation of the angle of incidence experimental evidence was found that KCl forms a complex only in the outmost surface region of the membrane. This complex cannot penetrate into the membrane because of the lipophobic nature of Cl-. These findings support the "space-charge effect" model for the explanation of the potential generation since the electrochemical behaviour of the electrode membrane described is nearly equal for KCl and KSCN in low concentrations (-58.5 mV/decade) while it loses its sensitivity for KSCN (not for KCl) at higher concentrations. © 1985 Springer-Verlag.