Interactions of fluorescent surfactant micelles in solution with colloidal silica nanoparticles and their penetrant diffusion in hydrated fumed silica films

Abstract

The preparation and properties of surfactant micelles labeled with the dye, Nile Red (NR), are described. Fluorescence emission and excitation spectroscopy establish the localization of NR in the hydrophobic core of nonionic Brij® S10, Brij® S20, and Triton X-100 micelles, and in ionic mixed micelles prepared from sodium dodecyl sulfate (SDS) and Triton X-100 mixtures of varying composition. Stokes diameter analysis of NR-micelles by fluorescence correlation spectroscopy (FCS) yields mean diameters ranging from 3.0 to 10.6 nm depending on the surfactant composition and its concentration used for micelle preparation. Adsorption of NR-micelles on a 20 nm, charged-stabilized, sol-gel silica nanoparticle is characterized by FCS. NR-micelle-nanoparticle solutions are analyzed as two-species mixtures of free and nanoparticle-bound NR-micelles allowing determinations of mean Stokes diameters and fractions of nanoparticle-bound NR-micelles. Incorporation of SDS into nonionic Triton X-100-NR-micelles progressively diminishes the adsorption of the mixed ionic NR-micelles on the nanoparticles with increasing SDS in the micelles. FCS analysis NR-micelle adsorption interactions with silica nanoparticles is shown to occur at the single NR-micelle level, providing a means for advancing the fundamental understanding of these interactions. The kinetics of a NR-micelle’s penetration into a hydrated fumed silica determined by FCS are shown to be a one-dimensional Fickian process and indicative of pores with low tortuosity. The use of NR-micelles as fluorescent diffusion probes in porous fumed silica films is concluded to be a viable general approach for the characterization of pore tortuosity in films composed of irregularly shaped (fractal) nanoparticles.