THERMODYNAMIC STUDIES OF BIS(QUATERNARYAMMONIUM) AND (N-ALKYL-) IONIC SURFACTANTS
Thermodynamic properties and aggregation behaviour of n-alkyl m-3-m (where m = 8, 12, 16 carbon atom chains) and cyclododecyl c12-s-c12 (where s = 3,4,6) gemini surfactants containing dimethylammonium bromide head groups have been studied. The synthesized surfactants have been characterized according to their cmc, a, and surface coverage at the air-water interface. The cmc is found to decrease with increasing chain tail length in the m-3-m salt series. The cmc and counterion association to the micelle reaches a maximum value in the c12-s-c12 surfactant series for a butyl head group spacer, whereas the area per head group was observed to increase as the linker spacer length increased. Experimental apparent molar vol urnes of the binary aqueous surfactant systems were measured in the post-micelle region and an additivity scheme was used to estimate the infinite dilution apparent molar volumes. These data provided estimates of the volumes of micellization for these salts. The micellization volumes of the c12-s-c12 surfactant series are typically small, indicating no micelles or very small micelles are produced by these surfactants in aqueous medium. Mean aggregation numbers, Ns, obtained from time resolved fluorescence quenching (TRFQ) show that N, increases with increasing chain length for the m-3-m gemini surfactants. In contrast, low mean aggregation numbers were obtained for the c12-s-c12 salt series. Distribution coefficients, Kd, of mixed micelle systems consisting of sodium alkylcarboxylate salts, CnNa (where n = 10, 12), alkyltrimethyl- ammonium bromide salts. RNMe3Br (where R = 10, 12), and ethoxylated alcohols, C EO (where EO = ethylene oxide and n = 0, 1,3) were studied using 4 n an runr T} relaxation method. Results were compared against a thermodynamic method for determining Kd and found to be in agreement with this data. The presence of EO groups on the alcohol favors its solubility in anionic micelles, indicating that the interaction between the anionic micelle and the alcohol involves the EO groups of the alcohol. For the cationic systems, an increase in the number of EO groups has no effect on K, implying that the EO groups do not contribute to the interactions occurring in mixed micelles of cationic surfactants and alcohol's containing EO groups.