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, mono hydroxypropyl, and dimethylene phenylene spacer groups has been studied usi ng steady-state and time-resolved fluorescence spectroscopy. Aggregation nu mbers were determined using the time-resolved single photon counting method with pyrene as the probe. At certain surfactant concentrations, aggregatio n 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 critic al 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 a ttributed to the formation of premicellar aggregates in the surfactant conc entration region between the observed and the expected cmc values. The aggr egation numbers obtained here indeed point to the existence of dimers in th is 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 h ydrophilic nature of systems where dimers are thought to be present.