Study of properties and behavior of surfactants and micelles at the solid/liquid interface using molecular dynamics simulations.

摘要

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic and hydrophobic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammoniumbromide) surfactant in an aqueous medium that is modeled with empirical potentials.; The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. In the presence of hydrophobic surface of graphite, the aggregate evolves in to hemicylindrical structure with tails of surfactants lying on the surface due to hydrophobic interaction. This finding is in agreement with experimental data.; The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements. The simulated indentation of micelle/graphite system causes breakage of micelle at an indentation force of about 1.25 nN, which is slightly above the force predicted to break the micelle structure on silica (1 nN). This difference can be explained by a stronger interaction (hydrophobic) between the absorbed structure and the graphite substrate.