Characterization of single amino acid based surfactants undecanoic L-Isoleucine and undecanoic L-Norleucine in the presence of diamine counterions with varying chain lengths

To understand chiral recognition and discrimination, single amino-acid-based surfactants undecanoic L-isoleucine and L-norleucine were synthesized and examined at various pHs with different counterions. Analysis was conducted utilizing different analytical instruments and techniques such as NMR and Capillary Electrophoresis (CE). Knowledge gained from this research will later be used to develop a database that will recognize behaviors of micelle systems and determine which parameters and materials will be most effective for enantiomeric separation.1 The focus of this particular study was to determine how varying the chain length of diamine counterions affects the properties of the surfactant. The following six counterions were investigated: 1,2-ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and sodium. Sodium was employed as the standard counterion for comparison. Early work was performed using arginine and lysine as counterions before shifting focus to the diamines; this data is also included as supplemental information. Data from this project has shown that the Critical Micelle Concentration (CMC) of surfactants is dependent upon the chain length of the diamine counterions. As the counterion chain length was increased, the CMC decreased. At pH 9 the CMC decreased from 12.19 mM undecanoic L-isoleucine with 1,2 ethylenediamine to a CMC of 2.00 mM for undecanoic Lisoleucine with 1,6 diaminohexane as the counterion. CE data showed that as the chain length of the counterion was increased, the enantiomers were resolved at a lower surfactant-counterion concentration. There was also improved chiral selectivity of BNA enantiomers in the presence of diamine counterions compared to the standard sodium. Based on previous literature, the micellar size also increased with a decrease in CMC as expected. The average hydrodynamic radius of undecanoic L-norleucine with 1,2-ethylenediamine at pH 10 was 9.17 Å compared to 24.52 Å for L-norleucine with 1,6 diaminohexane at pH 10.