Refractive index matching of large polydisperse silica spheres in aqueous suspensions

Standard synthesis protocols produce silicon dioxide (SiO2) particles with an inhomogeneous material distribution, which defines their optical properties. The inner structure and effective refractive index of the particles can be investigated by light scattering and optical contrast variation techniques. The standard analysis relies on scattering models for particles smaller than the wavelength of the scattering probe, thus restricting its use for a large variety of particle systems of academic and industrial interest. However, to overcome this difficulty should be possible by using the appropriate scattering model in the analysis. Here, SiO2 particles of size comparable to the wavelength of the illuminating source are dispersed at low particle number concentration in different mixtures of polar solvents to study the optical contrast. The angle-dependent scattered intensity from SiO2 particles suspended in different mixtures of water and DMSO are obtained by static light scattering (SLS) and the effective refractive index of the particles is estimated from refractometry experiments. The measured angle-dependent scattered intensity is well described by the Aden-Kerker theory for coated polydisperse spherical particles. Fitting of experimental data is performed by using a non-linear least squares routine assuming that the core-to-shell ratio remains constant from where the refractive index and radius of the particles shell are obtained. Additionally, we use the structural characterization to estimate the effective refractive index of SiO2 particles, which allows identification of the solvent composition that produces the minimal optical contrast between colloids and suspending medium. This is, to our knowledge, the first report describing the interplay between internal structure and optical properties of core-shell spherical colloids of size comparable to the wavelength of the scattering probe. Consequently, our study provides an experimental scheme that could be useful in the structural and optical characterization of large complex colloids.