Aggregates of cationic calix[4]arenes in aqueous solution as media for governing protolytic equilibrium, fluorescence, and kinetics

Abstract

Many water-soluble ionic calixarenes readily form micelle-like aggregates in aqueous solution. Presently, it is firmly proved that such aggregates tend to shift the state of the acid-base equilibrium of indicator dyes just in the same manner as cationic surfactant micelles do. In this paper, we further the corresponding studies and report the peculiarities of several kinds of solvatochromic effects, fluorescence, acid-base equilibrium and kinetic processes in aqueous solutions of four cationic amphiphilic calix[4]arenes bearing hydrophilic choline groups at the upper rim: 5,11,17,23-tetra(N,N-dimethyl-N-hydroxyethylammonium)methylene-25,26,27,28-tetraalkyl-oxycalix[4]arene tetrachlorides. n-Propyl, n-hexyl, n-octyl, and n-dodecyl were used as alkyls, and the calixarenes were designated as of Cn-C4A-Ch, where n = 3, 6, 8, and 12, respectively. The principal colloidal properties, i.e., particle size and electrophoretic mobility of aggregates were characterized via dynamic light scattering. In parallel, the results were compared with those obtained in the presence of micelles of the common cationic surfactant cetyltrimethylammonium bromide and of large-sized aggregates of the calix[6]arene with six methoxy groups at the lower rim. As tools, molecular probes of triphenylmethine and fluorescein series, as well as the solvatochromic Reichardt’s dye were used. The obtained block of data sheds light upon the properties of calixarene aggregates as tools for governing chemical processes.