Discipline: Chemistry and Chemical Sciences
Subcategory: Chemistry (not Biochemistry)
Room: Exhibit Hall
Jacob Mayer - 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 headgroup bound to a hydrocarbon tail. In aqueous solution, surfactants aggregate into spherical structures called micelles by placing their headgroups at the micelle surface and their non-polar tails in the micelle core. This project used nuclear magnetic resonance (NMR) spectroscopy to study micelle formation by an amino acid-based surfactant which is naturally derived and a green alternative to the surfactants used in cosmetics, pharmaceuticals, and detergents.The surfactant studied contained a glutamic acid headgroup attached to a hydrocarbon chain. Our goal was to characterize how the properties of micelles formed by this surfactant changed with solution pH. We also studied the association of positively charged ions with the negative micelle surface. Our hypothesis was that since the surfactant contained multiple carbocyclic acid functional groups, the sizes and physical properties of micelles formed by the surfactant should depend on solution pH.Solutions containing the surfactant and positive ions were prepared in the pH range 5 to 11.5. NMR spectroscopy was used to measure micelle and counterion diffusion coefficients. These values were used to calculate the radii of the micelles and the fraction of counterion molecules bound to the micelle surface at each pH. Titration experiments were performed to determine the pKa of the surfactant’s acidic protons.Titration measurements showed that the pKa of the side-chain carbocyclic acid in the glutamic acid surfactant’s headgroup was 6.4. Diffusion experiments showed that when counterions with a +1 charge were in solution, micelles only formed below pH 6. In contrast, in the presence of +2 counterions micelles formed between pH 5 and 10.5. Finally, in the latter solutions, the micelles were larger below pH 6, smaller from pH 6 to 10, and smaller still above pH 10. Our conclusions were that below pH 6, the surfactant headgroup had a -1 charge. At these lower pH’s the micelles were larger and were able to bind +1 and +2 counterions. Above pH 6, though, the surfactant headgroup had a -2 charge. Unfavorable electrostatic repulsion between the negative headgroups prevented micelle formation when +1 counterions were in solution. However, +2 counterions were better able to balance the surfactant’s negative charge, therefore micelles formed above pH 6 in solutions containing +2 counterions. Future work will investigate solutions containing surfactants with an aspartic acid headgroup and study the effect that other positive ions like choline and cyclohexanediamines have on micelle formation. 1.Chandra, N.; Tyagi, V.K. J. Dispers. Sci. Technol. 2013, 34, 800-808.2.Lewis, C.; Hughes, B.H.; Vasquez, M.; Wall, A.M.; Northrup, V.L.; Witzleb, T.J.; Billiot, E.J.; Fang, Y.; Billiot, F.H; Morris, K.F. J. Surfact. Deterg. 2016, 19, 1175–1188.Funding Acknowledgement: 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/Mentor: Kevin Morris, Carthage College, firstname.lastname@example.org
Funder Acknowledgement(s): Funding Acknowledgement: 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, performing and analyzing NMR experiments, interpreting results, and designing new studies.