NMR Studies on the Aggregation of Mononuclear and Dinuclear Cobalt(III) Amphiphilic Complexes Having Alkyl Chains

The aggregation of novel type cobalt(III) surfactants, such as [Co(en)2{NH2(CH2)2S(1-CnH2n+1)}]3+ (n = 6, 8, and 12) and [(en)2Co{NH2(CH2)2 S(CH2)mS(CH2)2NH2}Co(en)2]6+ (m = 8 and 12), was studied in water using multinuclear NMR spectroscopies, 1H NMR self-diffusions, and 1H NMR chemical shifts. The 59Co linewidths of the surfactants were useful to monitor the premicellar and micellar formation. It was revealed that the tripositive mononuclear surfactants have comparable cmc (critical micelle concentration) values to those for the usual monovalent surfactants having the same alkyl chains. The 35Cl longitudinal relaxation rates for the counterions were useful to monitor the two kinds of ionic interactions, i.e., those with the surfactant monomers and with the micelles. We determined the cmcs for the premicellar formation in the hexyl mononuclear and octyl dinuclear complex systems. The determination of this kind of cmc has become possible by using such highly charged complexes as the cystam complex and by measurements of the 59Co linewidths and of the 35Cl longitudinal relaxation rates. The self-diffusion coefficient for the surfactants revealed that the aggregation behavior is similar in comparison between the hexyl mononuclear and octyl dinuclear complexes or between the octyl mononuclear and dodecyl dinuclear complexes.