Solvent-induced interactions of nanoparticles in colloidal solutions can substantially affect their physicochemical and transport properties. Predicting these interactions is challenging because the natural causes of the interactions are unclear. Here, we present a comprehensive experimental and theoretical study of the coagulation stability of the surfacted magnetic colloids. The magnetite nanoparticles stabilized by erucic acid were dispersed in 19 different good solvents. The colloidal stability was reduced by the gradual addition of a precipitant. As a precipitant, 19 other liquids were used. We show that coagulation is not associated with either dispersion or magnetic interactions. The coagulation mechanism is due to the osmotic attraction of nanoparticles induced by a specific local distribution of precipitant molecules. The precipitant molecules are repelled from the hydrophobic tails of the surfactant and form a depleted zone inside the surfactant layer leading to the appearance of the osmotic attraction between the nanoparticles and their subsequent coagulation when the critical concentration of the precipitant is reached. The quantitative description of the phenomenon is carried out within the framework of the generalized Asakura–Oosawa model of the attractive depletion forces between two adjacent particles and the Langmuir adsorption model for the equilibrium concentration of precipitant molecules in the surfactant layer of nanoparticles. The calculated precipitant critical concentrations, the coagulation curves of the polydisperse systems, and the variation of the coagulation criterion occurring upon changing the surfactant are in good agreement with the experimental data. The osmotic attraction mechanism is equally suitable for nanoparticles of any natureplasmonic, semiconductor, or magnetic. This is determined by the surfactant–solvent interactions and is generic for many solvent-mediated systems taken at arbitrary concentrations of precipitant.

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In order to expand the application range of carbon reinforced composite materials in shielding field, this paper aimed to modify the chopped carbon fiber (CF) by magnetite particles with ultrasonic treatment. The modification was carried out in three stages and the increasing amount of the cover was proved by TGA analyze. SEM analyze is showed the most part of CF surface was coated by magnetite particles Fe3O4. Also it was noticed some agglomeration on each stages what is possibly connected with high surface energy of these particles. The chemical compound of the coating corresponds to ferrous oxide (II and III) what is confirmed by Raman spectroscopy.

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