Structure and Dynamics of Surfactant and Hydrocarbon Aggregates on Graphite: A Molecular Dynamics Simulation Study

Abstract

We have examined the structure and dynamics of sodium dodecyl sulfate (SDS) and dodecane (C 12 ) molecular aggregates at varying surface coverages on the basal plane of graphite via classical molecular dynamics simulations. Our results suggest that graphite-hydrocarbon chain interactions favor specific molecular orientations at the single-molecule level via alignment of the tail along the crystallographic directions. This orientational bias is reduced greatly upon increasing the surface coverage for both molecules due to intermolecular interactions, leading to very weak bias at intermediate surface coverages. Interestingly, for complete monolayers, we find a re-emergent orientational bias. Furthermore, by comparing the SDS behavior with C 12 , we demonstrate that the charged head group plays a key role in the aggregate structures: SDS molecules display a tendency to form linear file-like aggregates while C 12 forms tightly bound planar ones. The observed orientational bias for SDS molecules is in agreement with experimental observations of hemimicelle orientation and provides support for the belief that an initial oriented layer governs the orientation of hemimicellar aggregates.

Controlling Carbon-Nanotube—Phospholipid Solubility by Curvature-Dependent Self-Assembly

“…Similar findings have been reported for SDS monolayers. 27 In total, our findings on adsorption morphology changes with curvature above are in full aggreement with the MD simulations of Tummala et al on SDS behavior. 37 In their work, SDS formed micellar structures on planar substrate but positively curved substrate induced lamellar and negatively curved substrate cylindrical lamellar aggregates.…”

“…This results from the chain symmetry, and has been previously reported for, e.g., SDS in bulk and at interfaces. 27,68 Furthermore, the lipid head group interactions influence the lipid packing on the substrate. Simulations of HD isolate this effect as they represent just the tail interactions with the substrate.…”

“…Similar substrate induced nucleation and freezing upon adsorption in comparison to bulk aqueous aggregates has been reported for anionic SDS. 27 Additionally, we examined the aqueous dispersion efficiency of the lipid coatings directly by free energy calculations, and indirectly by assessing the relation between head-group hydration and the morphology geometry. While charged lipids repel each other by electrostatic double-layer forces, phosphadidylcholine lipids are zwitterionic and have very small electrostatic repulsion.…”

Surfactant and Hydrocarbon Aggregates on Defective Graphite Surface: Structure and Dynamics

“…Often, an important aspect of surface aggregation is overlooked: a surface is rarely, if ever, smooth and defect-free, and the presence of irregularities is expected to influence the aggregation process. Here, we continue our studies of sodium dodecyl sulfate (SDS) and hydrocarbon aggregation on an ideal graphite surface 1 by focusing on the effects of surface defects (vacancies and steps) on the aggregation kinetics of SDS and dodecane (C 12 ) monolayers at varying coverages on graphite.…”

Orientational Order of Molecular Assemblies on Rough Surfaces

“…So far, however, a satisfactory computational method has not been demonstrated for a system large enough to exhibit the micelle formation and orientation. [8][9][10] To treat the same system at the micellar (colloidal) length scale with a simpler approach, we recently chose to view a micelle as a macroscopic "rod" and have shown that anisotropic van der Waals torque between rod-like surfactant micelles and a graphite substrate is large enough to orient the micelles perpendicular to the symmetry axes of the graphite lattice. 11,12 In more recent experiments, we also studied the aggregates of sodium dodecyl sulfate (SDS) on Au(111), a system that is well-known 7,13 to recognize the lattice symmetry axes.…”