Aggregation Behavior in Water of Monomeric and Gemini Cationic Surfactants Derived from Arginine

A new class of gemini surfactants, bis(Args) (C n (LA)2, where n = 3, 6, and 9), has been synthesized. Their solution and tension behaviors were studied and compared to those of the corresponding monomeric surfactant, LAM (N α-lauroylarginine methyl ester) and of a common cationic surfactant, CPC (cetylpyridinium chloride). Bis(Args) are made up of two symmetrical long chain N α-acyl-l-arginine residues of 12 carbon atoms linked by amide covalent bonds to an α,ω-alkylidenediamine spacer chain of varying length (n). By being produced from amino acid sources (arginine), these surfactants are biocompatible and less toxic to the environment. The solution behavior is also important for potential applications in foaming, agrichemical spreading aids, and cleaning processes, and in understanding the interfacial behavior. Strong evidence of two cmc's with different characters of aggregates was obtained from different techniques for the gemini surfactants but not for the monomeric surfactants. Surface tensiometry indicates that the geminis form aggregates of substantial size at 0.001−0.01 mM (at 25 °C) or at concentrations about 3 orders of magnitude lower than that of LAM. Fluorescence results and lower chloride counterion binding than that for LAM suggest that the aggregates are nonglobular. These methods reveal also a second cmc for larger globular aggregates at 0.09−0.5 mM. Conductivity measurements and calculations are consistent with the above inferences and were used to estimate the aggregation number N and the counterion binding parameter β. The nonglobular aggregates have lower β and smaller N values than the globular aggregates (micelles), and unlike conventional micelles, they tend to increase the molar conductivity compared to that of the pre-cmc solution.