(534g) Self-Assembly and Charging of Biomimetic Surfactants in Nonpolar Solvents

Lipophilic modification of peptides to form peptide amphiphiles allows for the controlled, responsive presentation of these bioactive molecules for applications in drug delivery, biomaterials, biosensing, and bioseparations. While nearly all studies of peptide-amphiphile assembly have been performed in aqueous solution, use of nonpolar solvents may provide an avenue to the assembly of more complex structures as nondirectional hydrophobic forces are replaced by orientation-specific dipolar interactions. We have recently synthesized a library of amino-acid-linked surfactants to better understand the relative importance of headgroup chemistry, tail architecture, and chirality on self-assembly in nonpolar solvents. The surfactants assemble into small, stable inverse micelles in the absence of water. When equilibrated with ambient water vapor, much larger structures are formed that ultimately settle out of solution. Using a combination of electrochemical impedance spectroscopy (EIS), neutron scattering, and zeta potential measurements, we find that the solutions exhibit a decrease in their apparent ionic strength with increased surfactant concentration. This is the opposite of what is expected in aqueous solutions of charged surfactant, or for the disproportionation model of micelle-assisted charge generation in nonpolar solvents. We determine that the process is due to a decrease in inverse micelle size with increasing concentration, as verified by light scattering. We further demonstrate that the fraction of micelles charged increases by an order of magnitude when larger, organic counter ions are used, suggesting that the other charging mechanisms are at play.