1. Introduction In general, colloidal systems contain a large number of individual nanoparticles dispersed in some continuous medium moving by Brownian motion and colliding with one another so that i) they remain separated as individual nanoparticles or ii) form agglomerates by a processes called coagulation or focculation [1]. Colloidal systems are mostly stabilized by various capping (surface coating) additives providing their steric and/or electrostatic stabilization or by deposition on solid supports [2-4]. Nanoparticles have many unique properties, such as high surface area to volume ratio, large surface area and energy etc. [5]. ZnS nanoparticles have some important properties typical for other semiconductor nanoparticles, including a quantum size effect [6], which predetermine them for luminescence and photocatalytic applications [7]. Synthesis, characterization and applications of semiconductor nanoparticles were reviewed in detail in, e.g. a book edited by Smith [2]. In particular, precipitation of ZnS nanoparticles in the presence of cationic surfactants was reviewed in our recent papers [8,9]. The aim of this study was to investigate the agglomeration of ZnS nanoparticles without capping additives at low temperatures reducing their kinetic energy of Brown’s movement. These colloid dispersions were stabilized only by repulsive electrostatic interactions among ZnS nanoparticles. According to the von Weimarn law, colloidal dispersions can be obtained from very diluted or very concentrated solutions, therefore, ZnS nanoparticles were precipitated in diluted solutions of zinc and sulphide ions. Experimental techniques were Central European Journal of Chemistry Agglomeration of ZnS nanoparticles without capping additives at different temperatures * E-mail: petr. praus@vsb.cz Received 12 May 2013; Accepted 12 November 2013 Abstract: © Versita Sp. z o.o. Keywords: ZnS nanoparticles • Agglomeration • Low temperatures • Molecular modelling 1 Department of Analytical Chemistry and Material Testing, VŠB-Technical University of Ostrava, 708 33 Ostrava-Poruba, Czech Republic 2 Institute of Physics, VŠB-Technical University of Ostrava, 708 33 Ostrava-Poruba, Czech Republic 3 Department of Chemical Physics and Optics, Charles University in Prague, Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic Petr Praus 1* , Richard Dvorský 2 , Petr Kovář 3 , Ladislav Svoboda 1 Short Communication ZnS nanoparticles were precipitated in diluted aqueous solutions of zinc and sulphide ions without capping additives at a temperature interval of 0.5–20 o C. ZnS nanoparticles were arranged in large focs that were disaggregated into smaller agglomerates with hydrodynamic sizes of 70–150 nm depending on temperature. A linear relationship between hydrodynamic radius (R a ) and temperature (T) was theoretically derived as R a =652 – 2.11 T. The radii of 1.9–2.2 nm of individual ZnS nanoparticles were calculated on the basis of gap energies estimated from their UV absorption spectra. Low zeta potentials of these dispersions of -5.0 mV to -6.3 mV did not depend on temperature. Interactions between individual ZnS nanoparticles were modelled in the Material Studio environment. Water molecules were found to stabilize ZnS nanoparticles via electrostatic interactions. 312 Cent. Eur. J. Chem. • 12(3) • 2014 • 312-317 DOI: 10.2478/s11532-013-0385-2