Discipline: Chemistry and Chemical Sciences
Subcategory: Chemistry (not Biochemistry)
Room: Exhibit Hall A
Jessica Fletcher - 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.
Surfactant molecules contain a charged or polar headgroup bound to a hydrocarbon tail. Surfactants aggregate into roughly spherical structures called micelles by placing their polar headgroups at the micelle surface and extending their non-polar tails into the micelle core. Applications of surfactants include cosmetics, pharmaceuticals, and detergents. This project used nuclear magnetic resonance (NMR) spectroscopy to study micelle formation by an amino acid-based surfactant. This class of molecules are biodegradable, naturally derived, and have low toxicities, therefore, they are a greener alternative to the surfactants used in many consumer goods. The surfactant studied here contained a Leucine-Valinate headgroup attached to a hydrocarbon chain. Our goal was to characterize how the properties of micelles formed by this surfactant changed with solution pH and the positively charged counterion bound to the negative micelle surface. Both cyclic and linear diamine counterions were investigated. Our hypothesis was that solution pH affects the conditions under which the counterions are charged and thus their binding to the micelles. Solutions containing our surfactant and positive counterions were prepared in the pH range 7.5 to 11.5. NMR spectroscopy was then used to measure micelle and counterion diffusion coefficients. These values were in turn used to calculate the radii of the surfactant micelles and the fraction of counterion molecules bound to the micelle surface at each pH. Finally, two-dimensional NMR was used to investigate intermolecular interactions in the counterion-surfactant complexes. NMR experiments showed that the diamine counterions bound to the micelle below pH 9.0, but at higher pH each counterion dissociated from the micelle surface. In addition, linear diamines with longer alkyl chains bound to the micelles more strongly than amines with shorter alkyl chains. Finally, two-dimensional NMR spectra showed that the linear diamines bound parallel to the micelle surface with the amine functional groups interacting with multiple surfactant molecules. Solutions containing cyclic diamines were only water-soluble at high pH and cyclic diamines, unlike linear compounds, bound perpendicular to the micelle surface. Our conclusions are that solution pH has an important impact on the structures of micelle-counterion complexes. Therefore, pH conditions must be carefully considered when these surfactants are used in commercial formulations. Our future work will investigate amino acid-based surfactants with different headgroups and study the conformation of the surfactant molecules within the micelles. References: Lewis, C. et. al. Effect of pH on the Binding of Sodium, Lysine, and Arginine Counterions to L-Undecyl Leucinate Micelles. J Surfact Deterg, 19: 1175-1188. Chandra, N. et. al. Synthesis, Properties, and Applications of Amino Acid Based Surfactants: A Review. J Disper Sci Technol 34: 800-808.
Funder Acknowledgement(s): We thank the NSF-RUI program (Grant# 1709394) for supporting this research. We also acknowledge the generosity of the Ralph E. Klingenmeyer family.
Faculty Advisor: Kevin Morris, email@example.com
Role: During the summer of 2019, I completed all of the experiments outlined in the abstract. My daily activities included preparing surfactant-counterion solutions at specific pH, collecting NMR spectra of the solutions, and analyzing, processing, and interpreting NMR data and results. I am continuing this work during the 2019-2020 academic year. Our goal is to complete work with the surfactant we studied last summer and then move on to investigate new surfactant systems with NMR.