The influence of cationic tetrapropoxycalix[4]arene choline on protolytic equilibria of acid-base indicators in aqueous solutions

Water-soluble ionic calixarenes belong to very promising macrocyclic molecules owing to their applications in biochemistry. However, the state of such substances in aqueous media is still unclear. The present paper is devoted to gaining insight into the properties of aqueous solutions of a cationic calixarene, 5,11,17,23-tetra(N, N-dimethyl-N-hydroxyethylammonium)-methylene-25,2 6,27,28-tetrapropoxycalix[4]arene tetrachloride, within a wide concentration range. For this purpose. we examined the influence of the above system on the equilibrium of sixteen acid-base indicator dyes, with various charge types of acid-base couples, by means of vis-spectroscopy. The deviations of the 'apparent' ionization constants, K-a(a), from the values in pure water, K-a(w), and the shifts of the absorption bands of the dyes were compared with those in micellar solutions of common cationic surfactants. This confrontation, as well as dynamic light scattering measurements, confirms the existence of small positively charged aggregates. Several experiments were made with anionic and nonionic water-soluble calixarenes. The performed study allows to make three conclusions about ionic calixarenes (i) in aqueous media, they form micelle-like aggregates, (ii) such aggregates are able to strongly modify protolytic properties of other dissolved substances and (iii) the calixarene+dye associations are governed rather by electrostatic and hydrophobic interactions, than by expected 'Hosts-Guest' ones. (C) 2008 Elsevier B.V. All rights reserved.