The nature of aqueous solutions of a cationic calix[4]arene: A comparative study of dye-calixarene and dye-surfactant interactions

Among different types of calixarenes, the water-soluble ones are of especial interest because of their possible applications in biochemical research. In order to elucidate the nature of aqueous solutions of a cationic amphiphilic calixarene, substituted tetrapropoxy-calix[4]arene bearing hydrophilic choline groups at the upper rim, we studied vis spectroscopically the influence of the above system on the acid-base behavior of three indicator dyes, namely, 2,4-dinitrophenol, bromophenol blue, and N,N'-dioctadecylrhodamine, at constant ionic strength of 0.05 M, maintained with NaCl addition. Simultaneously, 'apparent' ionization constants, K-a(a), of the same dyes were determined in the presence of common cationic surfactant micelles. Within the concentration range from 1.0x10(-5) to 0.01 M, the aforementioned water-soluble calixarene displays effects similar to those of micelles of cetyltrimethylammonium bromide (or chloride). The shifts of the absorption and emission bands in the visible region, as well as the alterations of the K-a(a) values against the 'aqueous' ones appeared to be very similar in aqueous solutions of both calix[4]arene and cationic surfactant. A conclusion can be made about aggregation (or association), i.e., micelle formation of the cationic calix[4]arene under study.