Characterization of Micelle Formation by the Single Amino Acid-Based Surfactants Undecanoic L-Isoleucine and Undecanoic L-Norleucine in the Presence of Diamine Counterions with Varying Chain Lengths

Abstract

The binding of linear diamine counterions with different methylene chain lengths to the amino-acid-based surfactants undecanoic L-isoleucine (und-IL) and undecanoic L-norleucine (und-NL) was investigated with NMR spectroscopy. The counterions studied were 1,2-ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, and 1,6-diaminohexane. These counterions were all linear diamines with varying spacer chain lengths between the two amine functional groups. The sodium counterion was studied as well. Results showed that when the length of the counterion methylene chain was increased, the surfactants’ critical micelle concentrations (CMC) decreased. This decrease was attributed to diamines with longer methylene chains binding to multiple surfactant monomers below the CMC and thus acting as templating agents for the formation of micelles. The entropic hydrophobic effect and differences in diamine counterion charge also contributed to the size of the micelles and the surfactants’ CMCs in the solution. NMR diffusion measurements showed that the micelles formed by both surfactants were largest when 1,4-diaminobutane counterions were present in the solution. This amine also had the largest mole fraction of micelle-bound counterions. Finally, the und-NL micelles were larger than the und-IL micelles when 1,4-diaminobutane counterions were bound to the micelle surface. A model was proposed in which this surfactant formed non-spherical aggregates with both the surfactant molecules’ hydrocarbon chains and n-butyl amino acid side chains pointing toward the micelle core. The und-IL micelles, in contrast, were smaller and likely spherically shaped.

An Investigation of the Effect of pH on Micelle Formation by a Glutamic Acid-Based Biosurfactant

“…Another notable difference between the binding of 1,4-diaminobutane and 1,6diaminohexane counterions to the tridecanoic-Glu micelles is that the maximum f b,counterion value for 1,4-diaminobutane was 0.9, while the maximum f b,counterion for 1,6-diaminohexane was 0.6. A similar result was reported by Maynard-Benson, et al in a study of linear diamine counterions binding to undecanoic L-norleucine micelles [17]. Previous studies have shown that these linear diamine counterions bind parallel to the surface of amino acid-based micelles, allowing the two amine function groups to interact with multiple surfactant unimers.…”

“…These MD simulations are underway. Finally, since R h includes both the micelle and counterions, comparing R h values at pHs where the mole fraction of bound counterions is high to pHs where these values are low provides insight into the structure of the micelle-counterion complex [15][16][17]26]. This method will be employed below to interpret the results shown in Figure 5.…”

“…pHs where these values are low provides insight into the structure of the micelleterion complex [15][16][17]26]. This method will be employed below to interpret the r shown in Figure 5.…”

“…Previous studies have shown that these linear diamine counterions bind parallel to the surface of amino acid-based micelles, allowing the two amine function groups to interact with multiple surfactant unimers. Maynard-Benson, et al suggested that f b,counterion values were larger for 1,4-diaminobutane and smaller for 1,6-diaminohexane because the spacing between the amine functional groups in the former counterion was optimal for the counterion to bridge between two surfactant monomers [17]. A similar effect likely explains why the maximum f b,counterion values were also larger for 1,4-diaminobutane counterions than for 1,6-diaminohexane when these two diamines bound to the tridecanoic-Glu micelles.…”

“…The goal of this effort is to identify the fundamental factors responsible for the self-assembly, structure, and function of micelles and other supramolecular structures formed by these compounds. To date, work has focused on surfactants containing nonpolar amino acids like L-Leucine, L-Phenylalanine, and L-Isoleucine [15][16][17]. In this study, however, a linear amino acid-based surfactant containing the acidic amino acid Glutamic acid was investigated.…”