Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, variously, 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) and area per molecule (A min) are shown to deviate from the expected patterns of behavior as the number of carbon atoms in the alkyl chain (n) increases beyond a certain maximum. This aberrant behavior is observed at the hydrocarbon/water as well as the aqueous/air interface. The unexpected values of the physicochemical parameters at long alkyl chain length have been interpreted on the basis of a concentration region in which submicellar or multilayer 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 monoquaternary analogues demonstrates that there is a continuously changing shape with change in n for the geminis, whereas the profiles for nongeminis are invariant. Long-chain geminis exhibit a gradually sloping sigmoidal profile, indicating a variety of environments experienced by pyrene-3-carboxaldehyde between the totally aqueous environment at low surfactant concentration and the hydrophobic (entirely micellar) environment at high surfactant concentration. The large variation in the polarity of the probe environment between these two extremes may be attributed to the formation of submicellar structures.

The aggregation behavior of four series of bis(quaternary ammonium halide) surfactants (gemini surfactants) having diethyl ether, dihydroxybutyl, monohydroxypropyl, and dimethylene phenylene spacer groups has been studied using steady-state and time-resolved fluorescence spectroscopy. Aggregation numbers were determined using the time-resolved single photon counting method with pyrene as the probe. At certain surfactant concentrations, aggregation numbers of 2, expressed as gemini molecules per micelle, were obtained in all four series when the number of carbon atoms in the alkyl chain length (n) increased beyond a maximum. These long-chain geminis also have a critical micelle concentration (cmc) greater than expected on the basis of plots of log cmc vs n for the shorter chain homologues. This deviation has been attributed to the formation of premicellar aggregates in the surfactant concentration region between the observed and the expected cmc values. The aggregation numbers obtained here indeed point to the existence of dimers in this region. Steady-state fluorescence measurements of I1/I3 values, which are indicative of the polarity of the probe environment, are followed as a function of surfactant concentration. These ratios are used to compare cmc values obtained by surface tension methods as well as to confirm the more hydrophilic nature of systems where dimers are thought to be present.

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