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.