Kinetics and Mechanism of Coarsening for Nanoparticles of Sulfur and Alkaline Earth Metal Sulfates Coprecipitated from True Polysulfide Solutions

Abstract

Nanoparticles of alkaline earth metal sulfates (AEMSes) and nanocomposites of AEMSes with sulfur nanoparticles (nanosulfur) are synthesized from polysulfide aqueous solutions (PASes) of calcium, strontium, and barium AEMs (CaS_n, SrS_n, BaS_n; n > 1). The AEM PASes are prepared in an aqueous medium at temperatures of 70 and 90°C through the reactions between metal hydroxides and sulfur. It is found that using sulfur subjected to mechanical activation in a disintegrator for synthesis allows the formation of higher AEM PAS concentrations in shorter periods of time. To identify possible mechanisms of mechanochemical recrystallization in liquid media, kinetics of particle coarsening due to the reversible aggregation of sulfur nanoparticles and AEMSes is determined via static light scattering. It is found that particles with sizes around 30 nm form initially, and the particles coarsen to sizes of several tens of micrometers over time. The rate of particle coarsening (agglomeration) constants (Q) grow along with the concentration of acid, and the optimum concentration for implementing the Q-mechanism is 10%. It is found that using surfactants (neonol; concentration, 5%) results in a multiple reduction of Q. It is also revealed that the value of Q value rises along with temperature, and the energies of activation for the coarsening of S/MeSO₄ particles are determined for the optimum range of 300–350 K. Practical aspects of the results of this work are discussed with reference to using the synthesized samples for the germination of wheat seeds, and the hydrophobicity of sulfur-containing S/MeSO₄ samples is analyzed.