Adsorção de surfactantes sobre superfícies homogêneas e heterogêneas: efeito da rugosidade e de centros hidrofóbicos na superfície

Abstract

In this Dissertation, we have used Langevin Dynamics simulation to investigate the ad sorption of nonionic surfactants on homogeneous and heterogeneous surfaces containing adsorption centers. Coarse grained methodology was employed to reduce the complexity of surfactants and adsorbing surfaces to their minimum constituents, using the free distri bution package ESPResSo. The hydrophobic effect, responsible for the micellar formation and adsorption of surfactant molecules on the surface, is represented in our model by a short-range Lennard-Jones attraction, since we do not represent explicitly the water present in the solution. We consider a totally hydrophobic homogeneous surface, flat or with a gaussian or with a harmonic modulation roughness, as well as a heterogeneous surface formed by hydrophobic adsorption stripes, flat or with a gaussian roughness. In the case of homogeneous surfaces, the presence of gaussian roughness produces trapping of the surfactant molecules adsorbed on the surface depressions. In the case of rough ness with harmonic modulation in one direction, the succession of peaks and valleys on the surface confines the surfactants on the valleys in the form of adsorption stripes. For harmonic modulation in two directions, on the other hand, the adsorption occurs in the form of trapping regions in the depressions present on the surface. For the heterogeneous surfaces, we find that the permanent adsorption is obtained from two hydrophobic stripes, and the aggregate produced along the stripes has a semi-cylindrical profile, with radius of curvature which decreases with increasing stripes width. When the stripes have gaussian roughness, on the other hand, the regions with different heights produce trapping regions for the adsorbed molecules. The aggregate formed on the stripes still is semi-cylindrical, with a radius of curvature greater than that with flat stripes.