On the determination of polarity parameters of silica by means of solvatochromic probe dyes

Abstract

Empirical “solvent” parameters of silica surfaces are determined by means of solvatochromic dyes, e.g., Cu(tmen) (acac)+ ClO 4 − /B(C6H5) 4 − , Fe(phen)2(CN)2, and Michlers Ketone dissolved in 1,2-dichloroethane or cyclohexane. The values obtained by UV/VIS-measurements are compared with both the polarity scales of Kamlet and Taft and with Gutmann's donor and acceptor numbers. Kamlet and Taft's α-parameter (the hydrogen-bond donation capacity of a solvent) is determined independently by means of salicylideneaniline. The π* of silica is determined using 5-N,N-dimethylamino-5′-nitro-2,2′-bithiophene and the complex of tetracyanoethylene with Michlers Ketone, respectively. Further on the reliabilities of the obtained Kamlet-Taft parameters α, β, and π* are checked by comparism with experimental and calculated values of theET(30)-parameter of silica by means of multi-parameter regression analysis. The surface properties of silica are influenced mainly by HBD-properties α (75%) and dipolarity/polarizability π* (20%), but hardly by the HBA-properties β (<5%). UV/VIS-measurements were carried out in a special equipment with glass fiber optics, A drawing of the apparatus is given.