Determination of the Porosity of Silicon Dioxide Microparticles by the Method of Refractive Index Matching
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ICAL OPTICS
Determination of the Porosity of Silicon Dioxide Microparticles by the Method of Refractive Index Matching A. A. Akhmadeeva, b, A. R. Gainutdinova, *, M. A. Khamadeeva, b, and M. Kh. Salakhova, b a Institute
of Physics, Kazan Federal University, Kazan, 420008 Russia Academy of Sciences, Kazan, 420111 Russia *e-mail: [email protected]
b Tatarstan
Received December 20, 2019; revised March 13, 2020; accepted May 4, 2020
Abstract—The porosity of silica microparticles synthesized by the Stöber–Fink–Bohn method was determined by the method of refractive index matching. The values of the total porosity of particles for different transmission radiation wavelengths are obtained. The limits of applicability of the method, which are associated with the ratio of the radiation wavelength to the size of microparticles, are analyzed. Keywords: porosity, refractive index, microparticles, dispersion, silicon dioxide DOI: 10.1134/S0030400X20090027
INTRODUCTION To date, micro- and nanoparticles are widely used in many practical applications [1–10]. Porous particles are of particular interest because they exhibit many unique properties, such as, for example, the specific kinetics of absorption and desorption of chemical compounds, the large specific surface area, the low density, etc. [11]. Porosity is the most important factor that has an effect on the properties of microparticles; therefore, it is necessary to determine and, if necessary, to control it [12, 13]. In addition, the porosity of the particles has an effect on the properties of artificial materials comprised of them. For example, the position of the energy gap a colloidal photonic crystal depends on the porosity of microparticles that form this crystal, since the effective refractive index of microparticles depends on it. However, this fact is usually ignored when analyzing the position of the energy gap by Wulff–Bragg’s law, which leads to erroneous values of the calculated volume fraction of filling of the photonic crystal [14]. Currently, there are several methods for measuring the porosity of particles. Each of them has its own advantages and disadvantages. Thus, for example, the method of saturation according to Preobrazhenskii [15] and the method of adsorption–desorption of nitrogen [16] do not allow one to take into account closed pores. In addition, the saturation method is not suitable for determining the porosity of submicrometer particles. Optical and photometric methods—in which the signal is formed by the entire particle volume and the refractive index is the main measured quantity used for the calculation of the porosity—are free from these disadvantages. The refractive index of
particles is calculated using, for example, the method of optical trapping [17, 18], video holographic microscopy [19], and the method of light-scattering indicatrix [20]. The disadvantage of these methods is high requirements to the hardware of the experiment. In this study, we propose a simple photometric method for measuring the total porosity (open poros
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