Fast coagulation of gold sols. Rate constants for dimerization of nanoparticles

By using the generalized Mie theory, the extinction cross sections for dimers of 46-nm gold particles immersed in water have been calculated. It has been found that, in the region of high-energy plasmon resonance band, the maximum value of normalized extinction cross section Qext1 gradually decreases as particles approach each other. The greatest changes in Qext1 are observed when interparticle distance h decreases from 10 to 3 nm. At shorter distances, Q_ext1 weakly depends on h. At the same time, the band position varies in a complicated manner; however, at h < 2.5 nm, it coincides with that for individual particles. The revealed properties of the high-energy plasmon resonance band for dimers have been used to determine the absolute rate constant for dimerization of 46-nm gold particles k₁₁. By spectrophotometry, we have investigated salt-induced coagulation of gold sols and have measured the rates of the decrease in optical density. Experimental and calculation data allowed us to establish, at initial stages of fast coagulation, when the distance between the surfaces of 46-nm gold particles is 1.3–2.0 nm (Dolinnyi, A.I., Colloid J., 2015, vol.77, p. 600.), the average value of k₁₁ is (9.20 ± 1.34) × 10^–12 cm³/s for sols with particle concentrations of (0.4–2.6) × 10₁₀ cm^–3.