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Molecular dynamics simulations to study the behaviour of anionic (Sodium Dodecylsulfate, SDS) and nonionic (Monooleate of Sorbitan, SPAN80) surfactants close to a silicon dioxide (SiO2) surface were carried out. Simulations showed that a water layer was first adsorbed on the surface and then the surfactants were attached on that layer. Moreover, it was observed that water behaviour close to the surface influenced the surfactant adsorption since a semisphericalmicelle was formed on the SiO2 surface with SDS molecules whereas a cylindricalmicelle was formed with SPAN80 molecules. Adsorption of the micelles was conducted in terms of structural properties(density profiles and angular distributions) and dynamical behaviour (diffusion coefficients) of the systems. Finally, it was also shown that some water molecules moved inside the solid surface and located at specific sites of the solid surface.

Silicon dioxide surface were carried out. Adsorption of surfactant molecules at solid-liquid interfaces has been investigated for years not only for its relevance in science but also for its numerous industrial applications, such as detergency, crude oil refining, treatment of waste water, adsorption on activated charcoal and even in pharmaceutical preparations. In particular, self-assembly of surfactant molecules on solid surfaces has shown different issues from those observed at liquid/vapor and at liquid/liquid interfaces. For instance, it has been observed that interactions between hydrophobic tails, repulsions between headgroups and interactions between surfactant molecules with solid surface could change the isotherms at the criticmicellar concentration(CMC). Therefore, studies of surfactant aggregation will help us to obtain more physical insights of selfassembly phenomena.

The studies of surfactant aggregation on a hydrophilic silicon dioxide (SiO2) surface in order to compare the behaviour of surfactant molecules on different solid substrates. The surfactant adsorption on surfaces has been studied by different techniques, such as streaming potential methods, calorimetry, neutron reflection, ellipsometry, fluorescence spectroscopy and atomic force microscopy (AFM). In fact, AFM has proved to be a reliable technique to obtain information on the topology of surfactant aggregation since it allows us to observe how surfactants are formed on surfaces. For instance, CTAB arrays in parallel stripes on a graphite surface, SDS forms hemimicelles on a rough gold surface and similarmorphologies have been seen for other surfactants on hydrophobic surfaces.



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