Aggregate structures of the sorbitan monooleate (SPAN80) surfactant at TiO$_{2}$(rutile)/water interfaces by computer simulations

Abstract

Molecular dynamics simulations were carried out to study the behavior of a nonionic surfactant close to TiO$_{2}$ surfaces at three different crystallographic orientations of rutile, (001), (100) and (110). Sorbitan Monooleate (SPAN80) molecule was used as nonionic surfactant and it was observed that these molecules seemed to aggregate in similar ways. Namely, the hydrocarbon chains of the surfactant molecules were attached at the solid surfaces. Structure of the molecules and surfactant adsorption on the surfaces were studied in terms of tails and head groups density profiles as well as surface coverage. From density profiles and angular distributions it was possible to determine the influence of the solid surface. For instance, on the three surfaces the surfactant molecules formed molecular layers parallel to the surface. Besides, it was found that in the solids (100) and (110), where there were oxygen atoms exposed on the surface, surfactants were attached to the surfaces along the sites between the lines of these oxygen atoms. Finally, diffusion coefficients for the aggregates were calculated in order to determine molecular mobility on the surfaces. These results were compared with those of the Sodium Dodecylsulfate (SDS) molecules on the same rutile surfaces and it was found that the SPAN80 molecules were more attached to surfaces than the SDS molecules. On the other hand the diffusion coefficients calculated in the present work were also compared with those obtained in a SDS/graphite system and we observed that SDS molecules on graphite showed a significant mobility compared with the same molecules on rutile.