(113a) Prediction of Nanoparticle Dispersibility Based on Its Surrounding Dielectric Environments

Metal oxide nanoparticles exhibit unique optical, electrical, magnetic, and catalytic properties that are different from those of bulk materials, so they have been utilized as fillers to functionalize polymer nanocomposite films. For efficient utilization of such nanoparticles, they must exhibit high dispersibility in media. However, most of the metal oxide nanoparticles possess hydrophilic surfaces, thus they are less dispersible in organic media than aqueous ones with high polarity. The surface modification of nanoparticles is a useful approach to improve the dispersibility of particles, and it should precisely be controlled to disperse the particles in desirable media.

To establish the method for performing the precise surface modification, it is necessary to evaluate the effect of surface modification on the surface properties of nanoparticles, and to quantitatively understand the behaviors of particles in the medium. One approach is to evaluate the Hansen solubility parameter (HSP), which is commonly used to understand the affinity between different substrates (e.g., polymer and solvent). In HSP theory, the cohesive energy density of a substance is divided into three components: dispersion, polar, and hydrogen bonding. Previous studies reported that the HSP values of surface-modified nanoparticles became closer to the ones of surface modifiers compared to unmodified nanoparticles [Ref 1]. However, the quantitative correlation between the amount of surface modifiers bound to the nanoparticles and each component of the HSP value of surface-modified nanoparticles is scarcely clarified. Furthermore, it is necessary to evaluate the dispersibility of nanoparticles in many solvents to calculate the HSP value, which requires lots of time and cost. Therefore, a novel method to evaluate the characteristics of particle surfaces should be established.

In this study, the surface-modified silica nanoparticles (SiO₂) were synthesized using octyltriethoxysilane (OTES) as a modifier. The surface modification ratio, θ, was defined as the ratio of the number of OTES molecules to that of silanol groups on the SiO₂. Then, the HSP values of surface-modified SiO₂ with different θ values were evaluated. In addition, the local polarity in the vicinity of nanoparticles was characterized based on the fluorescent spectroscopy analysis using 6-lauroyl-2-dimethylaminonaphthalene (Laurdan). Laurdan, a fluorescent dye that emits fluorescence depending on the polarity of the local environment, is utilized to evaluate the local polarity at the interface. The surface-modified SiO₂ with higher θ exhibited a high dispersibility in less polar solvents (e.g., n-hexane). It was also confirmed that such nanoparticles possessed a decreased surface polarity. Finally, the correlation between the surface properties and the dispersibility was investigated, which reveals that our characterization method can be used to predict the dispersibility of nanoparticles in the medium.

[Ref 1] N. Fujiwara et al., Mater. Chem. Phys., 229, 139–148 (2019)