Equimolar mixtures of aqueous linear and branched SDBS surfactant simulated on single walled carbon nanotubes

In our previous simulation study [J. Phys. Chem. C, 2011, 115, 17286], branched sodium dodecyl benzene-sulfonate (SDBS) surfactants showed self-assembled structures on single-walled carbon nanotubes (SWNTs) that were strongly dependent on tube diameter. Those results suggested that branched SDBS, as opposed to their linear counterparts, could specifically stabilize SWNTs of narrow diameter. Experimental data, however, show that SDBS stabilizes aqueous SWNTs of many diameters. This discrepancy between simulated and experimental results could be explained by the fact that experimental SDBS samples are isomeric mixtures. To test this possibility we report here molecular dynamics (MD) simulation results for equimolar mixtures of aqueous linear and branched SDBS on (6,6) and (20,20) SWNTs at ambient conditions. Our results suggest that there is no strong effect due to nanotube diameter on the morphology of mixed SDBS surfactant aggregates, although the adsorbed aggregate structure strongly depends on surfactant coverage. In-plane radial distribution functions suggest that linear and branched molecules distribute evenly onto the surfaces of (6,6) SWNTs, while some evidence of segregation, in which branched SDBS predominantly pack near other branched molecules, was obtained on (20,20) SWNTs at high surface coverage. These results suggest that the lack of specificity in stabilizing aqueous dispersions of carbon nanotubes using SDBS surfactants is probably due to the presence of multiple isomeric molecules in commercial surfactant samples. Perhaps more importantly, these simulations suggest that using mixtures of surfactants could affect the structure of the adsorbed aggregates, and the stability of aqueous dispersion of carbon nanotubes

Molecular Self-Assembly of Surfactants on Solid Surfaces

Understanding adsorption and aggregation of surfactants on solid surfaces is of great importance to many applications. The aim of this thesis is to obtain molecular-level knowledge regarding the role of (1) surfactant-assisted aqueous dispersions of single-walled carbon nanotubes (SWNTs) and (2) surfactant adsorption on heterogeneous surfaces, using computer simulations. For the first objective, molecular dynamics simulations were employed to study the morphology of surfactants self-assembled on (6,6), (12,12), and (20,20) SWNTs. The results show that the surfactant molecular architecture significantly affects the packing of surfactants on SWNTs. The branched sodium dodecyl benzenesulfonate (SDBS) is more effective in stabilising dispersions of narrow SWNTs than its linear counterpart. There is no strong effect of the nanotube diameter seen on the morphology of mixed linear and branched SDBS. Comparing the self-assembled aggregates formed by caesium (Cs+) and sodium (Na+) dodecyl sulphate surfactants, Cs+ ions yield a more compact coverage on the (6,6) SWNT, compared to Na+. These outcomes could provide physical guidelines for designing surfactant formulations to improve the quality of the aqueous SWNT dispersions. For the second objective, dissipative particle dynamics simulations were used to investigate the adsorption of surfactants near patterned surfaces. The hydrophobic patterns on which the surfactants could adsorb are surrounded by surfaces that repel the surfactants. On the surfaces containing one hydrophobic stripe, as the stripe width decreases, monolayers become hemi-cylinders, hemi-spheres, and individual surfactants, a consequence of lateral confinement. When two hydrophobic stripes are present on the surfaces, there is evidence of cooperative effects (i.e., hemi-cylindrical shells or irregular structures formed). The morphological (width and depth) and chemical (fully and partially hydrophobic) properties of the trenches predominantly affect the self-assembled surfactant aggregates. These findings could assist in understanding of surfactant adsorption on heterogeneous surfaces and perhaps in facilitating new methods for the fabrication of nano-structured surfaces

Surfactant Aggregates Templated by Lateral Confinement

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

Self-assembled surfactants on patterned surfaces: confinement and cooperative effects on aggregate morphology

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

Time to Switch to Second-line Antiretroviral Therapy in Children With Human Immunodeficiency Virus in Europe and Thailand.

Background: Data on durability of first-line antiretroviral therapy (ART) in children with human immunodeficiency virus (HIV) are limited. We assessed time to switch to second-line therapy in 16 European countries and Thailand. Methods: Children aged <18 years initiating combination ART (≥2 nucleoside reverse transcriptase inhibitors [NRTIs] plus nonnucleoside reverse transcriptase inhibitor [NNRTI] or boosted protease inhibitor [PI]) were included. Switch to second-line was defined as (i) change across drug class (PI to NNRTI or vice versa) or within PI class plus change of ≥1 NRTI; (ii) change from single to dual PI; or (iii) addition of a new drug class. Cumulative incidence of switch was calculated with death and loss to follow-up as competing risks. Results: Of 3668 children included, median age at ART initiation was 6.1 (interquartile range (IQR), 1.7-10.5) years. Initial regimens were 32% PI based, 34% nevirapine (NVP) based, and 33% efavirenz based. Median duration of follow-up was 5.4 (IQR, 2.9-8.3) years. Cumulative incidence of switch at 5 years was 21% (95% confidence interval, 20%-23%), with significant regional variations. Median time to switch was 30 (IQR, 16-58) months; two-thirds of switches were related to treatment failure. In multivariable analysis, older age, severe immunosuppression and higher viral load (VL) at ART start, and NVP-based initial regimens were associated with increased risk of switch. Conclusions: One in 5 children switched to a second-line regimen by 5 years of ART, with two-thirds failure related. Advanced HIV, older age, and NVP-based regimens were associated with increased risk of switch

Surfactants adsorption on crossing stripes and steps

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 = 0.61LS, 1.22LS and 1.83LS, where LS 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