(432f) DNA Surfactants as Probes of Monomer Exchange Dynamics in Self-Assembled Systems

The dynamics of self-assembly processes have been studied by stopped-flow and phosphorescence methods, to name a few. Knowledge of these time scales is critical when predicting size distributions and solubilization capacity in surfactant microstructures. These data vary significantly with hydrophile-lipophile balance, chain length, and the presence of specific interactions, to name a few.

We present a novel electrophoretic method to characterize the kinetics of monomer exchange using surfactant tails conjugated to DNA oligomers of a precise molecular weight. Here, DNA surfactants are introduced into a capillary and electrophoretically eluted in the presence of nonionic surfactant micelles. Nonionic micelles transiently interact with the DNA surfactant, retarding its electrophoretic mobility. During the run, the highly monodisperse DNA surfactant exchanges among many distinct nonionic micelles, each with widely varying size. The elution time for the DNA surfactant can be mapped to the average size of the micelles and their partition coefficient. The broadness of the peak can be traced to several different mechanisms, including the polydispersity of the micelle population and the number of exchanges that occur. These two effects compete: a large polydispersity is overcome by excessive exchanges, and vice versa. As such, an independent measurement of micelle polydispersity by light scattering, combined with peak broadness from capillary electrophoresis, gives the monomer exchange rate.

We demonstrate that the presence of DNA does not impact the free energy of transfer for n-alkyl surfactant tails with lengths in the range of 12-18 carbons. A detailed model, accounting for effects of diffusion, injection, and detection on peak broadening will be presented along with exchange kinetics for various spherical and rod-like micelles. These are obtained from mixtures of the poly (oxyethylene) surfactants Triton X-100 and C16E6. We also present adsorption-desorption kinetics for DNA surfactants interacting with oil-in-water nanoemulsions and carbon nanotubes obtained using the CE method.