The difference between the aggregates of short-tailed and long-tailed cationic calix[4]arene in water as detected using fluorescein dyes

Water-soluble cationic calixarenes readily form aggregates in water. The properties of these aggregates, however, markedly vary along with alterations in the molecular structure. In a set of our recent studies, such aggregates were examined by various methods, and first of all using acid-base indicator dyes. The latter acted in fact as monoprotic acids exhibiting one color transition, and their pKa shifts against the values in water were relatively close for different calixarenes. In the present paper, we report the behavior of a much more complicated dye, fluorescein, and two relative compounds, in aqueous solutions of two cationic calix[4]arenes. The last named possess four choline groups on the upper rim and the sole difference is the length of the alkoxy groups at the lower rim: O(CH2)2CH3 and O(CH2)7CH3. In water, both calixarenes form aggregates of similar size and with similar zeta-potential. However, their influence on the acid-base and tautomeric equilibria of fluorescein clearly demonstrate that the aggregates of the long-tailed calixarene resemble the common micelles of colloidal surfactants to a higher degree than those of tetrapropyloxy compound.