Micelle ionization degree of anionic gemini surfactant having N,N-dialkylamide and carboxylate groups

The critical micelle concentration (cmc) behavior in aqueous NaCl solutions of the anionic gemini surfactant N,N′-ethylenebis(sodium N-dodecanoyl-β-alanate), (CH2)2[N-(COC11H23)CH2CH2(CO2Na)]2 (212), having N,N-dialkylamide and carboxylate groups in a molecule was investigated to measure the value of micelle ionization degree (α). For comparison, the behavior of sodium N-dodecanoyl-N-methyl-β-alanate (SDMA), a monomer corresponding to 212, also was investigated. The α value obtained from the Corrins-Harkins plot for 212 is 0.97 when [Na+]≤0.025 M and the value for SDMA is 0.51 when [Na[+]≤0.1 M. That the α for 212 is larger than that for SDMA reflects the higher pKa1 value for 212 than the pKa value of SDMA. For 212, when [Na+]≤0.025 M the amount of Na+ released during micellization is so large that the Na+ concentration above the cmc is almost equal to that below the cmc, and therefore 212 nearly behaves as a nonionic surfactant.