Directly probing surfactant adsorption on nanoscopic trenches and pillars.

Confinement causes a change in the amount of surfactant adsorbed and in the adsorption morphology. Two 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. Trenches: Adsorbed surfactants films in 50 nm and 80 nm trenches showed the same break-through 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 extend 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.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.