Surface and aqueous properties of anionic gemini surfactants having dialkyl amide, carboxyl, and carboxylate groups

A systematic study of the equilibrium surface properties (in water and in the presence of 10−2 M NaCl) of a novel series of anionic gemini surfactants, (CH2)2[N(COCnH2n+1)CH(COOH)CH2COOH]·2NaOH (GA), where (n+1)=8, 10, 12, 14, and 16, was investigated. The responses of humans to closed patch tests with (CH2)2[N(COC11H23)CH(COOH)−CH2COOH]2·2NaOH (GA‐12) were also investigated. Premicellar self‐aggregation (both in water and 10−2 M NaCl) occurred when the N‐acyl group contained more than 14 carbon atoms, since the critical micelle concentration (CMC) values decreased and the pC20 values increased as (n+1) increased for (n+1) ≤14; the CMC values increased and the pC20 values decreased as (n+1) increased for (n+1)>14, both in water and in 10−2 M NaCl. The absence of a break in the specific conductance‐surfactant molar concentration plots for the GA homologs indicates protonation of the carboxylate group and strong Na+ release during micellization. This is a structural characteristic of the anionic geminis having N‐dialkylamide and carboxylate groups in a molecule. The skin irritation potential of GA‐12 is lower than that of the corresponding “monomer”, C11H23CON(CH3)CH(COOH)CH2(COOH)·NaOH, and the analog, C11H23CON(CH3)CH2COONa·H2O.