Discipline: Chemistry and Chemical Sciences
Subcategory: Chemistry (not Biochemistry)
Room: Exhibit Hall
Lauren Williams - Carthage College
Co-Author(s): Kevin Morris, Carthage College, Kenosha, WI.; Yayin Fang, Howard University College of Medicine, Washington, DC; Eugene Billiot and Fereshteh Billiot, Texas A&M University-Corpus Christi, Corpus Christi, TX.
Surfactants are used in commercial cosmetics and detergents. A surfactant molecule has a hydrophilic headgroup bound to a hydrophobic hydrocarbon tail. They aggregate in aqueous solutions into spherical structures called micelles. The surfactants studied here contain the amino acid phenylalanine in the hydrophilic headgroup. Phenylalanine-containing surfactants are biodegradable and have antimicrobial properties. The surfactants studied were negatively charged, therefore, positive ions bind to the negative micelle surface. Our goal was to characterize this micelle-counterion intermolecular interaction. Our hypothesis was that the interaction should depend on solution pH because pH affects the charges of both the counterions and surfactants. The surfactants studied contained either an Alanine-Phenylalanate or a Glycine-Phenylalanate headgroup attached to a hydrocarbon chain. The positive counterions were the L-Lysine and 1,4-diaminobutane. Aqueous solutions containing a surfactant and a counterion were prepared from pH 7.5 to 11.5 and nuclear magnetic resonance (NMR) spectroscopy was used to study these solutions. Changes in the chemical shifts of the NMR peaks were used to measure the critical micelle concentration (CMC) of the surfactants. This value is the concentration at which surfactants first aggregate into micelles. NMR spectroscopy was also used to measure the diffusion rates of micelles and counterions. Changes in the counterion diffusion rate were used to calculate the mole fraction of counterion molecules bound to the micelles. Finally, two-dimensional NMR was used to investigate the structures of the micelle-counterion complexes.The results showed that both L-Lysine and 1,4-diaminobutane bound to the micelle below pH 10, but dissociated from the micelle surface above this pH because of a reduction of the counterion charge. It was also observed that when the counterions interacted with the micelles, they bound parallel to the micelle surface. Finally, the CMC measurements showed that the CMC was lower below pH 10.0 when the counterions were bound and higher above pH 10 when the counterions were not associated with the micelles.Our conclusion is that pH affects the structures of micelle-counterion complexes and the surfactants’ CMC. Therefore, pH conditions must be carefully considered when surfactants are used in commercial products. Our future work will investigate surfactants with different amino acids in their headgroups and other positive counterions like L-Arginine, 1,6-diaminohexane and choline.1.Rothbauer, G.A.; Rutter, E.A.; Reuter-Seng, C.; Vera, S.; Billiot, E.J.; Fang, Y.; Billiot, F.H.; Morris, K.F. J. Surfact. Deterg. 2018, 21, 139–153.2.Pinazo, A.; Manresa, M.A.; Marques, A.M.; Bustelo, M.; Espuny, M.J.; Perez, L. J. Colloid Interface Sci. 2016, 228, 17-39.
Funder Acknowledgement(s): We thank the NSF-REU program (Grant# 1709394) for supporting this research. We also acknowledge the generosity of the Ralph E. Klingenmeyer family.
Faculty Advisor: Dr. Kevin Morris, email@example.com
Role: Preparation of solutions for analysis, preforming and analyzing NMR experiments, interpreting results and designing new studies.