Fluorescence study of premicellar aggregation in cationic gemini surfactants

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 I-1/I-3 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 emc values obtained by surface tension methods as well as to confirm the more hydrophilic nature of systems where dimers are thought to be present.