Micellization of an anionic gemini surfactant having N,N-dialkylamide, carboxyl, and carboxylate groups in aqueous NaCl solutions

The micellization of an anionic gemini surfactant, (CH2)(2)[N(COC11H23)CH(CO2H)CH2(CO2H)](2).2NaOH, abbreviated as GA, having N,N-dialkylamide, carboxyl, and carboxylate groups, in aqueous solutions of NaCl at pH 5.0 was investigated by means of surface tension and of fluorescence, using pyrene as a probe. The I-1/I-3 vs. log surfactant molar concentration (C) plots showed two values of critical micelle concentration (cmc). Of the two values, the higher cmc value was close to that measured by the surface tension method, meaning that the lower cmc value may be caused by the formation of small, non-surface-active soluble premicellar aggregates. Evidence for the existence of these premicellar aggregates includes the fact that the lower of the two cmc values for GA was not observed in the I-1/I-3 vs. log C plots for (CH2)(2)[N(COC11H23)- CH2CH2CO2Na](2), abbreviated as 212, in 0.001-0.3 M NaCl at pH 11.0, since the only cmc value obtained was close to that from surface tension data. Plots of log molar concentration of Na+ in the solutions vs. log cmc, or Corrin-Harkins plots, revealed that the lower cmc values decrease with an increase in Na+ concentration, and when [Na+] = 1 M, the values reach the decreasing higher cmc value. This means two mechanisms of GA aggregation occured in at the same aqueous solutions of NaCl. In the first one, monomeric form --> micelle I --> micelle II, and in the second, monomeric form --> premicelle --> micelle II. The degree of micelle ionization, or beta, for the higher cmc values is 0.97 at 10(-2) M less than or equal to [Na+] less than or equal to 0.1 M. This large beta value indicates that the GA molecules, as a result of the structural characteristic attributable to the simultaneous presence of N,N-dialkylamide and carboxylate groups in the molecule, may be almost fully dissociated even at the cmc. The beta value for the lower cmc is 0.75. This indicates that the existence of a non-monomeric state at concentrations below the cmc determined from surface tension data, which is additional evidence for the premicellar aggregation.