Competitive surfactant adsorption of anionic surfactant AOT and nonionic surfactant Tween 20 on gold was investigated by using a quartz crystal microbalance with dissipation (QCM-D) at 25 °C. The adsorption isotherm of pure AOT did not reach a plateau at the CMC, but rather adsorption continued to increase gradually at concentrations higher than the CMC before reaching a plateau. This behavior is evidence of competitive adsorption between AOT and impurities. The adsorbed layer of AOT on gold became more viscoelastic as the concentration of AOT increased. Tween 20 reached the plateau adsorption on gold before its concentration reached the CMC, suggesting that the attraction between Tween 20 and gold is very strong. The Tween 20 adsorbed layer was rigid when compared to the AOT adsorbed layer, as indicated by low dissipation. The addition of Tween 20 to a surface covered by AOT resulted in an increase in adsorbed mass, suggestive of the insertion of Tween 20 into the AOT adsorbed layer as expected because Tween 20 is able to separate the repulsive headgroups of AOT. When AOT was added to a preformed Tween 20 layer, a drop in the adsorbed amount was found between 0 and 0.1 CMC, and then no change was observed until the CMC of AOT was reached; the adsorbed amount then increased, reaching a final adsorption greater than that of pure AOT. All data support the formation of mixed surfactant layers on the surface. Although a two-step model fit both AOT and Tween 20 adsorption kinetic data well, AOT was found to adsorb much more slowly than Tween 20.
Of these different surfactants, the tri-chain aromatic surfactant TC3Ph3 (sodium 1,5-dioxo-1,5-bis(3-phenylpropoxy)-3-((3phenylpropoxy)carbonyl) pentane-2-sulfonate) was shown to be highly graphene-compatible (nanocomposite electrical conductivity = 2.22 × 10 S cm), demonstrating enhanced electrical conductivity over nine orders of magnitude higher than neat natural rubber-latex matrix (1.51 × 10 S cm). Varying the number of aromatic moieties in the surfactants appears to cause significant differences to the final properties of the nanocomposites.
Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.
HypothesisConfinement causes a change in the amount of surfactant adsorbed and adsorption morphology. ExperimentsTwo cationic surfactants, tetradecyltrimethylammonium bromide (TTAB) and cetylpyridinium chloride (CPC) were adsorbed at the silica-water interface. Atomic force microscopy (AFM) force curves were measured on 50 nm and 80 nm wide trenches. Force curves were also measured on silica pillars, and the results were quantified based on distance from the edge. FindingsTrenches: Adsorbed surfactants films in 50 nm and 80 nm trenches showed the same breakthrough values. However, compared to unconfined values, TTAB in trenches had decreased break-through and adhesion forces while CPC in trenches had increased break-through and adhesion forces, indicating that surfactant identity varies the confinement effect.Pillars: Near the edge, few surfactants adsorb, and those that do stretch in the direction normal to the surface. While the experimental data agree qualitatively with previous coarse-grained molecular dynamic simulations, the length scales at which the phenomena are detected differ by ~ half-order of magnitude. Specifically, experimental data show measurable effects on adsorbed surfactant morphology at a distance from the edge 10-20 times the length of a surfactant molecule after accounting for the ~8 nm size of the probe.
The effects of temperature and surface roughness on the mass and viscoelasticity of an adsorbed surfactant layer were monitored using a quartz crystal microbalance with dissipation monitoring (QCM‐D). Adsorption isotherms at 30, 40, 50, and 60 °C and at two different roughnesses on gold were measured for cetyltrimethylammonium bromide (CTAB). All isotherms displayed an increase in mass and dissipation as surfactant concentration was increased to its critical micelle concentration (CMC). Above the CMC, adsorption reached a peak followed by a slight decrease to a plateau at the equilibrium adsorption value. As the temperature was increased, the adsorbed mass above the CMC decreased. The adsorbed mass decreased further by increasing substrate roughness, while the dissipation remained unchanged within experimental uncertainty. Dynamic adsorption experiments were also conducted at various temperatures for select concentrations above and below the CMC, providing evidence for the importance of different adsorption mechanisms as a function of both surfactant concentration and surface roughness.
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