Abstract

Nuclear Magnetic Resonance (NMR) diffusion experiments were conducted to measure the radius of undecyl-leucine-valine (und-LV) micelles and the fraction of micelle-bound counter-ions as a function of pH. L-arginine was found to bind to the und-LV micelles at low pH, but dissociates from the micelle surface above pH 10. Furthermore, the micelle radius in solutions containing L-arginine counterions was found to change very little with solution pH. Similar results were obtained with the counterions L-canavanine and L-homoarginine. These results suggest that these counterions embed themselves into the micelle headgroup region. Similarly, 1,4- and 1,2-cyclohexanediamines were found to associate with the micelles at low pH, but dissociate from the micelle surface when pH was increased. Unlike in the L-arginine study though, the micelle radii in the presence of the diamine cations were larger when the counterions were bound and smaller when they were not. This result suggested that the diamine counterions bound perpendicular to the micelle surface. Finally, placement of the amine functional groups was found to affect their binding to the micelles with the 1,4-diamines binding more strongly than the corresponding 1,2-diamines at low pH.