Aberrant aggregation behavior in cationic gemini surfactants investigated by surface tension, interfacial tension, and fluorescence methods

Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, va riously, diethyl ether, monohydroxypropyl, and dihydroxybutyl spacer groups have been investigated by surface tension, interfacial tension, and steady -state fluorescence techniques. The critical micelle concentration (cmc) an d area per molecule (A(min)) are shown to deviate from the expected pattern s of behavior as the number of carbon atoms in the alkyl chain (n) increase s beyond a certain maximum. This aberrant behavior is observed at the hydro carbon/water as well as the aqueous/air interface. The unexpected values of the physicochemical parameters at long alkyl chain length have been interp reted on the basis of a concentration region in which submicellar or multil ayer structures are forming. Fluorescence measurements provide confirmation of cmc values by an alternative technique. Comparison of the fluorescence emission maxima profiles of the gemini surfactants with those of their mono quaternary analogues demonstrates that there is a continuously changing sha pe with change in n for the geminis, whereas the profiles for nongeminis ar e invariant. Long-chain geminis exhibit a gradually sloping sigmoidal profi le, indicating a variety of environments experienced by pyrene-3-carboxalde hyde between the totally aqueous environment at low surfactant concentratio n and the hydrophobic (entirely micellar) environment at high surfactant co ncentration. The large variation in the polarity of the probe environment b etween these two extremes may be attributed to the formation of submicellar structures.