Adsorption of curcumin on the surface of fumed silica from water-ethanol solutions

Adsorption of curcumin on the fumed silica from water/ethanol solutions has been studied using UV–Vis spectroscopy and advanced quantum-chemical calculations. The adsorption value of curcumin increases with increasing the water content in ethanol solutions and linearly correlates with their dielectric constant. According to quantum chemical calculations, an increase in the polarity of the solvent leads to an increase in its polarizing effect on the curcumin molecule and, consequently, to a redistribution of the electron density. As a result, the electron-donating ability of the oxygen atom of the carbonyl group increases, which leads to a proportional increase in curcumin adsorption on the surface of fumed silica. The adsorption isotherm of curcumin belongs to the L4-type, which characterizes the transition from monomolecular to polymolecular adsorption, in this case, to the adsorption of curcumin aggregates formed in solutions at a concentration of С_Сur > 1.5 × 10⁻⁴ М. The value of the conditional molar extinction coefficient of adsorbed curcumin was determined (ε^S_{434 nm} = 3.44 × 10⁵ g mol⁻¹ cm⁻¹). The found spectral characteristics of adsorbed (λ_max = 432–434 nm) and precipitated (λ_max = 500 nm) curcumin were used as a criterion for distinguishing two types of curcumin in the solid phase of the sorbent. The revealed patterns of adsorption are important in the preparation of nanocomposites based on silica and curcumin.