VOLTA-POTENTIAL AND ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE STUDIES OF THE ION-EXCHANGE MEMBRANE-PROPERTIES OF THE (ALPHA-CYCLODEXTRIN POLYMER FILM)/(4-NITROPHENOL/4-NITROPHENOLATE) INCLUSION SYSTEM

The 4-nitrophenol/4-nitrophenolate guest system molecularly included in the alpha-cyclodextrin polymer (alpha-CDP) film, applied to the glassy carbon electrode, induces cation-exchange membrane properties of the film. These properties were revealed by measurements of the Volta-potential difference by means of the Kelvin dynamic capacitor method at the alpha-CDP film coated electrode, emersed under potentiostatic polarization from the electrochemical cell containing 1 mM 4-nitrophenol solutions of different KCl concentrations at different pH. The negative values of the surface potential difference of the coated electrode and solution calculated both for 4-nitrophenol and 4-nitrophenolate indicate that the positive end of the guest dipole, ie the nitro group point out from the polymer into the solution phase. Acid-base titration of the alpha-CDP film coated Au/quartz electrode in 5 mM 4-nitrophenol, 0.5 M LiCl, performed by using an electrochemical quartz crystal microbalance shows that 4-nitrophenolate is included more strongly than 4-nitrophenol. Also the apparent dissociation constant of 4-nitrophenol in this inclusion system, pK(a)(alpha-CDP/4-NP)pol = 6.55, is smaller than that of 4-nitrophenol/4-nitrophenolate in the absence, and larger than that in the presence, of alpha-cyclodextrin in the solution.