Abstract

NMR spectroscopy was used to investigate the physical properties of two amino acid based biosurfactants: undecyl t-butyl-Leucinate and undecyl Alanate. Bio-based surfactants are naturally sourced, biodegradable, biocompatible, and do not use petroleum in their synthesis. The binding of several organic cations (1,4-diaminobutane, L-Arginine, L-Lysine, 1,6-diaminohexane, 2-(2-Aminoethoxy) ethyl amine, and Agmatine) to micelles formed by these two surfactants was investigated. NMR diffusion experiments were used to study changes in micelle radii and the mole fraction of micelle-bound counterions with solution pH. Two-dimensional NMR experiments were used to study the structures of the counterion-micelle complexes. These experiments showed that the counterions bound parallel to the micelle surface and interacted with multiple surfactant monomers. In experiments with undecyl t-butyl-Leucinate, counterions were most strongly bound at pH below 9 where their charges were +1 or +2. Binding strength was correlated strongly with counterion charge, in that as charge decreased, so did the fraction bound. Counterions interestingly remained bound to undecyl t-butyl-Leucinate micelles at pH above 10 despite being neutrally charged due to hydrophobic interactions between the surfactant’s t-butyl headgroups and the counterion’s alkyl chain. In experiments with undecyl Alanate, counterion binding was also strongly correlated with counterion charge. As counterions deprotonated to become neutrally charged, the mole fraction of counterions bound to the micelle however, dropped to nearly zero with undecyl Alanate. It was hypothesized that interactions with methyl groups at the surface of the micelle were not strong enough for the neutral counterions to associate with the micelles.