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As it was observed above, water density profiles showed strong peaks close to the Silicon Dioxide surface suggesting that thosemolecules might have some structure close to the solid. Therefore, studies of how water molecules were oriented in the solid were carried out. The analyses were conducted over the molecules in the adsorbed layers only. Angular distributions of water molecules are shown for the SDS and SPAN80 systems. The orientation of the water molecules was measured as the angle between the bisector vector of the OH bonds and the vector normal to the interface. In both systems, SDS and SPAN80, there was observed a privileged orientation of the water molecules, i.e., they were nearly perpendicular to the surface. In fact, there was noted a larger number of water molecules pointing perpendicular to the surface for the SPAN80 system than for the SDS system (small peak). Moreover, the water angle distribution presented a small peak when water interacted with SPAN80 and a wider distribution when water interacted with SDS. 

The positions of the silicon atoms of the Silicon Dioxide surface are also shown in the same directions. In all cases it was observed that water molecules(solid lines) were located only in specific sites. In fact, they were placed on the silicon atoms of the solid surface. The same plots for the SPAN80 system. From the above results we noted that water oxygens were adsorbed above silicon sites. In order to describe the attachment of the surfactant molecules to the solid surface, the diffusion coefficient of the molecular aggregates was also calculated. The diffusion coefficients were calculated in each direction by measuring the square mean displacements of the surfactant atoms and using the Einstein relation.

It was noted that both surfactants showed less affinity with the Silicon Dioxide surface than with other surfaces such as graphite and titanium dioxide. A series of Molecular Dynamics simulations were carried out in order to describe the behaviour of two different surfactant molecules interacting with a silicon dioxide solid surface. In the case of the anionic surfactant (SDS), there was observed a spherical micelle formation on a layer of water molecules previously adsorbed on the solid. Themicelle was described by density profiles and they showed a deformation of the micelle next to the adsorbed layer of water molecules. This deformation can be explained in terms of the SDS charged headgroups interactions with water molecules on the solid surface. On the other hand, the nonionic surfactant (SPAN80) did not showmuch influence by the solid surface. It formed a cilyndrical micelle next to the adsorbed water layer. In this case, there was observed a thicker water layer between the surfactant and the solid surface. The influence of the surfactant on the surface was characterized by water molecules in the surface. Dipole water orientation, in the solid surface, was more tilted for the SDS molecules than for the SPAN80 suggesting a stronger SDS-surface interaction and consequently more intensive adsorption of those molecules on the surface.


 

 

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