No abstract

The adsorption and self-assembly of surfactants are ubiquitous processes in several technological applications, including the manufacture of nano-structured materials using bottom-up strategies. Although much is known about the adsorption of surfactants on homogeneous flat surfaces from experiments, theory, and simulations, limited information is available, in quantifiable terms, regarding the adsorption of surfactants on surfaces with chemical and/or morphological heterogeneity. In an effort to fill this knowledge gap, we report here results obtained using equilibrium dissipative particle dynamics (DPD) simulations for the adsorption of model surfactants onto patterned flat surfaces (i.e., flat surfaces with chemical heterogeneity). The patterns consist of one or two stripes of variable width on which the surfactants could adsorb. The adsorbing stripes are surrounded by a surface that effectively repels the surfactants. This repelling surface, perhaps not realistic, allows us to quantify the effect of lateral confinement on the morphology of surfactant aggregates. When the stripe width is large (effectively providing a homogeneous flat surface), the surfactants yield a flat monolayer. Our simulations suggest that the flat monolayers become hemi-cylinders, hemi-spheres, and individual surfactants as the stripe width decreases, a consequence of lateral confinement. In some cases our simulations show evidence of cooperative effects when two adsorbing stripes are present on the surface. If the distance between the stripes and the widths of the stripes are both less than about one surfactant length, hemi-cylindrical shells and irregular structures are observed because of cooperativity; otherwise the results match those found for a single isolated stripe. Our predictions could be useful for the design of new nano-structured materials and coatings, for applications ranging from nano-fluidic devices to nano-reactors.

Using coarse-grained dissipative particle dynamics (DPD) simulations, we systematically study the effect of surface heterogeneity on surfactant adsorption. Here we investigate the adsorption and aggregation of surfactants on hydrophobic stripes crossing each other perpendicularly (i.e., crossing stripes) and on hydrophobic steps. The results are compared with those obtained for isolated stripes. We find that on crossing stripes of moderate stripe widths (e.g., L = 0.61L, 1.22L and 1.83L, where L is the length of one surfactant molecule) the crossing region hinders the formation of defect-free adsorbed surfactant structures. By increasing the stripe width and/or by increasing the length of one of the two perpendicularly crossing stripes (i.e., lowering the surface density of defects/intersections), the crossing region is found to have a weaker effect on the features of the adsorbed structures. Regarding surfactant adsorption on steps, our simulation results show that the self-assembled aggregates can be stretched along the step corner, and the resultant elastic deformation can hinder adsorption. This qualitative observation can facilitate a description of surfactant adsorption that takes into consideration also the deformation of the self-assembled film. As suggested by such a general model, increasing the convex angle of the step, increasing the size of the surfactant head groups, and changing other physical parameters can reduce the elastic energy penalty, and yield larger amounts of surfactants adsorbed. The results presented could assist in understanding and sometimes predicting surfactant adsorption on heterogeneous surfaces, suggest methods to formulate surfactant mixtures to control surface coverage on heterogeneous surfaces, and perhaps facilitate new methods for the fabrication of nano-structured surfaces.

Tremendous progress has been made to stabilize carbon nanotube dispersions using surfactants, although many questions await answer to design surfactant formulations that selectively stabilize nanotubes monodispersed in diameter and chirality. Stimulated by recent experimental observations [J. Am. Chem. Soc., 2010, 132, 16165-16175], we attempt here to quantify how changing the counter-ion (Cs + instead of Na + ) affects the morphology of dodecyl sulfate surfactants adsorbed on carbon nanotubes. Using atomistic molecular dynamics we simulated aqueous cesium dodecyl sulfate (CsDS) adsorbed on (6,6), (12,12), and (20,20) single-walled carbon nanotubes (SWCNTs) at ambient conditions. When compared to results for sodium dodecyl sulfate (SDS), our results suggest that surface aggregates with Cs + ions, compared to Na + , yield a more compact coverage of the nanotubes at the surfactant surface coverage of 0.25 nm 2 per headgroup, with the surfactant heads extended towards the bulk aqueous solution, and prevent water from accessing the nanotube surface. These morphological results suggest that CsDS should be more effective than SDS at stabilizing aqueous carbon nanotubes dispersions. More importantly, these results were obtained only for the (6,6) nanotubes simulated. For the wider nanotubes our simulations show limited, if any, differences in the morphology of the surfactant aggregates when the Na + ions are substituted with Cs + ones. To validate our results we measured experimental UV-Vis-NIR absorbance spectra for aqueous carbon nanotubes with diameters similar to that of (6,6) and of (12,12) nanotubes stabilized by SDS at increasing salt concentration (CsCl vs. NaCl).The results are indicative of changes in the surfactant self-assembled structure on the narrow nanotubes in the presence of Cs + ions, while data for the wider tubes only suggest salt-screening effects for both Na + and Cs + ions. The different salt-specific behavior observed for the surfactants adsorbed on narrow vs. wide carbon nanotubes could be exploited for the selective stabilization of mono-dispersed carbon nanotube samples, although a surfactant more effective than SDS should be used.

Self-assembly is widely seen as the method of choice for the bottom-up manufacture of supra-colloidal aggregates. Surfactants have been used extensively to appreciate qualitatively and quantify driving forces and methodologies for controlling self-assembling processes and the resultant self-assembled aggregates. However, not much is known regarding self-assembled surfactant aggregates formed on heterogeneous surfaces. If heterogeneous surface features affect the morphology of surfactant aggregates, it is possible that new templating methodologies could be designed by engineering surfaces. Here we report equilibrium dissipative particle dynamics simulation results for surfactants adsorbed on model heterogeneous surfaces. Our simulation results reveal that, depending on the morphological and chemical properties of the solid substrate, a number of not-before-reported structures can be obtained for the self-assembled aggregates. The results presented could be useful for the manufacture of new coatings and materials, e.g., via the admicellar polymerization procedure, as well as for interpreting experimental data for surfactant adsorption on heterogeneous surfaces.