Mechanism of Adsorption of Anionic Surfactants on the Surface of Functionalized Nanoparticles

The mechanism of adsorption of amphiphillic molecules on the surface of Brownian particles is essential for understanding and controlling the stability of functionalized nanoparticles, which are nowadays used in a broad range of fields, such as semiconductor, drug delivery system and catalysis. In this work, we have used light scattering (LS) techniques combined with determination of adsorption isotherms, as developed previously in our lab1, to study the adsorption mechanism of anionic surfactants on the heterogeneous surface of particles consisting of a rubber core surrounded by a plastic shell. Particles with the same mass of the core but different masses of the shell have been used. The adsorption isotherms for the different particles have been obtained by surface tension measurements both before and after removal of the particles by centrifugation. At the same time, the shell thickness and the related change of the particle radius during the surfactant adsorption have been evaluated by LS. The evaluation of the radius of the particle, of the hydrodynamic radius, and of the gyration radius by LS (both static and dynamic) provided significant indications about the particle shape. Cryo-SEM images of the particles gave further evidence about their morphology. For pure rubber spheres, the information on the change of the particle radius obtained from LS confirmed the two-stage adsorption mechanism previously proposed for the adsorption on larger particles1 : at the beginning the surfactant molecules adsorb in a ?head to tail? way in order to maximize the hydrophobic interactions between the surface and the hydrocarbon tail; with the increase of the surfactant concentration on the surface, the molecules assume a ?tails on? disposition. However, for the particles with a partial polymeric shell surrounding the rubber core, the adsorption mechanism becomes more complicated. From the experimental data, the ratio of exposed surface of the rubber core to the total external surface has been calculated. A model has been developed, which is based on combining information about the adsorption of the same surfactant on the pure rubber core and pure shell materials measured indipendently, to describe the partitioning of the surfactant on different material surfaces. Starting from this clear picture of the adsorption mechanism, the colloidal stability of these particles both under shear and stagnant conditions has been investigated. For all the particles the turbulent coagulation was studied by shearing the dispersions in a micro channel at very high shear-rate and varying temperature and particle concentration. The conversion of the primary particles as well as the cluster size of the obtained gel were evaluated by Small Angle Light Scattering. The gel samples were also characterized in terms of rheological properties and by taking SEM pictures. Combination of several techniques allowed the understanding of the influence of the surface morphology on the particles interactions. References: [1] A. Zaccone, H. Wu, M. Lattuada and M. Morbidelli, Journal of Physical Chemistry B; 2008, 112, 1976