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Molecular dynamics simulations of anionic and nonionic surfactant molecules at silicon dioxide surface (SiO2), in its Cristobalite form, were carried out for the present study. The surface was constructed using an atomistic model with a surface orientation (001). The parameters used for a solid  surface were taken from reference. For the anionic surfactant molecule, sodium dodecyl sulfate (SDS), amodel of 12 united carbon atoms attached to a headgroup (SO4) was simulated by using a force field already reported in the literature]. For the nonionic surfactant molecule, Monooleate of Sorbitan (SPAN80), there was used, a model with 17 united carbon atoms, three OH? and one ester groups in the head group. The force field reported in reference was used for this molecule. In the case of simulations with the anionic molecules, the initial configuration was prepared from a monolayer of 36 molecules in all-trans-configuration with the headgroups initially pointed to the solid surface. Then, 2535 water molecules were added (using the SPC model) to the system and 36 sodium cations (Na+). In the case of simulations with nonionic molecules, 25 molecules were used with the same number of water molecules.

Here is the computational method and model of  silicon dioxide surface (SiO2). A simulation box having dimensions X = Y = 43.7019 and Z = 150 ? was used with the usual periodic boundary conditions. The Z-dimension of the box was long enough to prevent the formation of a second water/solid interface due to the periodicity of the system. Instead, a liquid/vapor interface was present at one end of the box (z > 0) whereas at the other end of the box (z < 0) beyond the solid there was an empty space. All simulations were carried out in the NVT ensemble with a time step of 0.002 ps using DL-POLY package. Bond lengths were constrained using SHAKE algorithm with a tolerance of 10?4, and the temperature was controlled using the Hoover-Nose thermostat having a relaxation time of 0.2 ps at T = 298 K. Long-range electrostatic interactions were handled using the Particle Mesh Ewald method, and the Van der Waals interactions were cut off at 10 ?. Finally, the simulations were run up to 40 ns and configurational energy was monitored as a function of time in order to determine the moment the systems have reached equilibrium. Then, the last 2 ns were collected for analysis.

In this section we present calculations of the surfactant molecules at the silicon dioxide surface. Studies on the behaviour of the surfactant molecules and how they aggregate at the liquid/solid interface are discussed. In order to determine where the surfactant molecules arrayed in the system, mass Z-dependent density profiles for the headgroups and the tails were calculated, i.e., normal to the liquid/solid interface. We observed that water molecules were not only adsorbed but also absorbed by the solid surface which is located at a position of Z = ?23 ? in the figure. In fact, the first water profile peak (to the right of the surface) indicated strong adsorption, i.e., a water layer on the surface. The other water peaks (to the left of the surface) suggested that few particles were inside the solid surface. The presence of water molecules inside SiO2 surfaces has been also observed in real experiments.



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