RESUMEN
We study the nonequilibrium diffusive release of electroneutral molecular cargo encapsulated inside hollow hydrogel nanoparticles. We propose a theoretical model that includes osmotic, steric, and short-range polymer-cargo attractions to determine the effective cargo-hydrogel interaction, ueff*, and the effective diffusion coefficient of the cargo inside the polymer network, Deff*. Using dynamical density functional theory (DDFT), we investigate the scaling of the characteristic release time, τ1/2, with the key parameters involved in the process, namely, ueff*, Deff*, and the swelling ratio. This effort represents a full study of the problem, covering a broad range of cargo sizes and providing predictions for repulsive and attractive polymer shells. Our calculations show that the release time through repulsive polymer networks scales with q2eßueff*/Deff* for ßueff* â« 1. In this case, the cargo molecules are excluded from the shell of the hydrogel. For attractive shells, the polymer retains the cargo molecules on its internal surface and its interior, and the release time grows exponentially with the attraction strength. The DDFT calculations are compared to an analytical model for the mean first passage time, which provides an excellent quantitative description of the kinetics for both repulsive and attractive shells without fitting parameters. Finally, we apply the method to reproduce experimental results on the release of paclitaxel from hollow poly(4-vinylpyridine) nanoparticles and find that the slow release of the drug can be explained in terms of the strong binding attraction between the drug and the polymer.
RESUMEN
One of the most interesting properties of the surfactants is that they are able to alter the stability of colloidal dispersions. Despite its great industrial relevance, only a few works analyze the colloidal stability of these systems at high surfactant concentrations (well above the critical micelle concentration (CMC)). In the present work, the colloidal stability of polystyrene particles is studied under a wide range of ionic surfactant concentrations. The effects of the surface charge of the latex particles (evaluating both sign and value), and surfactant type (cationic or anionic) have been examined. Colloidal stability data have been gathered by monitoring aggregation using a nephelometric technique. As will be shown, it is possible to reach different stability regimes using the same colloidal system just by changing the surfactant concentration. Independently of the sign of both the surfactant and the surface, the destabilization of the system consistently takes place above certain surfactant concentration due to a depletion effect from non-adsorbed micelles. This destabilization can be predicted by adding to the DLVO interaction energy a new contribution addressing the force between two spherical particles in the presence of non-adsorbing spherical macromolecules.
