Colloidal Nature of Cationic Calix[6]arene Aqueous Solutions

The study of the aqueous solutions of the cationic hexameric calixarene 5,11,17,23,29,35-hexakis (N,N-dimethyl-N-hydroxyethylammoniummethylene)-37,38,39,40,41,42-hexametoxycalix­[6]­arene hexachloride demonstrates their colloidal nature. Within the concentration range from 1 × 10^–5 to 0.01 M, this compound forms positively charged aggregates of a much larger size as compared with those of tetrameric cationic calix[4]­arene previously studied by us. The aggregation was confirmed both by DLS and TEM experiments; the last method revealed that the 10² to 10³ nm sized particles consist of smaller primary aggregates. These aggregates manifest themselves in displaying alteration of absorption spectra and marked shifts of apparent ionization constants of common acid–base indicators. These shifts resemble those registered in micellar solutions of cationic surfactants and cationic polyelectrolyte brushes. The pCl measurements also confirm the formation of calixarene aggregates. The dye associations with the calixarene are governed rather by interactions with the aggregates of the macrocycles than by expected inclusion of the guest into the hosts cavity. Hence, on using ionic calixarenes in aqueous media, not only inclusion phenomena but also micellar effects should be taken into account.