Probing surface acidity, basicity, and dipolarity/polarizability of 12-tungstophosphoric acid by means of solvatochromic dyes

Abstract

Empirical surface polarity parameters α, β, and π* in terms of the Kamlet–Taft scale as well as Gutmann's acceptor number (AN) of 12-tungstophosphoric acid (H₃PW), pre-treated at various temperatures, have been estimated using solvatochromic dyes as probes. α (hydrogen-bond donating acidity) or AN and π* (dipolarity/polarizability), respectively, have been investigated by means of Fe(phen)₂(CN)₂ [cis-dicyanobis(1,10-phenanthroline)iron(II)] (1) and Michler's ketone [4,4′-bis(dimethylamino)benzophenone] (2) as solvatochromic surface polarity indicators. Apparent β (hydrogen-bond accepting ability) parameters have been evaluated by means of an aminobenzodifuranone dye. Also, coumarin 153 has been checked to analyse the surface polarity of H₃PW. The outstanding surface properties of H₃PW when pre-treated at 250 °C are supported by UV/vis spectroscopic results of triphenylmethylium/H₃PW adducts. The chemical interpretation of the polarity parameters and nature of interactions which they reflect are discussed.