Adsorption of polymers on inorganic solid acids investigated by means of coadsorbed solvatochromic probes

The adsorption of five different synthetic polymers, namely, polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly(4-vinylpyridine) (PVPy), and poly(N-vinylpyrrolidone) (PVP) in a 1,2-dichloroethane or dichloromethane slurry has been studied by measuring the UV/vis spectra of coadsorbed solvatochromic probes. Fe(phen)(2)(CN)(2) [cis-dicyano-bis-(1,10)-phenanthroline-iron(II), (1)] and Michler's ketone [4,4'-bis(NN-dimethylamino)-benzophenone, (2)] were used as solvatochromic surface polarity indicators. The UV/vis transmission spectra of the two surface polarity indicators 1 and 2 adsorbed on silica and alumina, respectively, were measured as a function of the adsorption time of the coadsorbed polymer in slurries in dichloromethane or 1,2-dichloroethane. Kamlet-Taft's alpha (hydrogen-bond acidity) parameter of the polymer/solid acid interfaces has been calculated using the linear function of (nu) over tilde (max) (1) with alpha of solid acids after Spange, S.; Vilsmeier, E.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6417. The alpha value of alumina significantly decreases on increasing the HBA (hydrogen-bond accepting) capacity of the adsorbed polymer PS < PMMA < PEO < PVPy approximate to PVP. The basic polymer adsorbs preferentially on the Lewis acidic sites of alumina. Adsorption of polymers on silica causes similar effects on (nu) over tilde (max) (1) as observed for alumina, but the effects cannot be classified in terms of an acid-base interaction model for all polymers, because PVPy and PS show anomalous behavior on adsorption. Determination of pi* (dipolarity/polarizability) parameters of polymer/solid acid interfaces is not possible because of the large pi* value of the solvents used.