1122 ISSN 1027-4510, Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 2019, Vol. 13, No. 6, pp. 1122–1125. © Pleiades Publishing, Ltd., 2019. Russian Text © The Author(s), 2019, published in Poverkhnost’, 2019, No. 12, pp. 3–7. Features of Colloidal Aggregation in Tetraethoxysilane-Water- Ethanol Ternary Mixtures by Small-Angle Neutron Scattering O. V. Tomchuk a, b, *, M. V. Avdeev a, c, **, O. I. Ivankov a, d, e , L. A. Bulavin b, d , and V. L. Aksenov a, f a Joint Institute for Nuclear Research, Dubna, 141980 Russia b Taras Shevchenko National University of Kyiv, Kyiv, 01601 Ukraine c St. Petersburg University, St. Petersburg, 199034 Russia d Institute for Safety Problems of Nuclear Power Plants, Ukraine National Academy of Sciences, Chornobyl, 07270 Ukraine e Moscow Institute of Physics and Technology, Dolgoprudny, 141701 Russia f “Kurchatov Institute” National Research Center, Moscow, 123182 Russia *e-mail: tomchuk@jinr.ru **e-mail: avd@nf.jinr.ru Received January 20, 2019; revised March 18, 2019; accepted April 2, 2019 Abstract—The influence of the concentration of components in a tetraethyl orthosilicate (TEOS)–water– ethanol mixture on the growth of silicate aggregates in a basic environment is studied with small-angle neu- tron scattering. There is a general increase in the size of aggregates with an increase in the amount of both water and TEOS. At the same time, when the H 2 O : TEOS molar ratio is 2 : 1, the structure of the aggregates repeats, regardless of the TEOS concentration in the system. The scattering length density of the aggregates is found via hydrogen/deuterium isotopic substitution and contrast variation to analyze the possible inclusion of residual ethyl and hydroxyl groups into their structure. Keywords: small-angle neutron scattering, tetraethyl orthosilicate, sol-gel method, colloidal aggregation DOI: 10.1134/S1027451019060545 INTRODUCTION Tetraethyl orthosilicate Si(C 2 H 5 O) 4 (TEOS) is widely used in the synthesis of various organic materi- als, including catalysts, flavors, dyes, agrochemicals, and medicines [1–3]. Due to its ability to form branched silicon-containing polymers [4, 5], TEOS is an integral part of promising technologies in the ther- mocatalytic decomposition of methane for hydrogen production [6, 7] and in the manufacture of acid-base sensors [8, 9] and various membrane filters [10], including to protect the environment [11]. Varying the conditions of hydrolysis synthesis (acidity, temperature, and concentration of compo- nents) allows various structures to be obtained, from polymers with weak intermolecular bonds to dense colloidal particles [1, 12, 13]. Such structures are actively studied with a small-angle X-ray and neutron scattering (SAXS and SANS) [14], which make it pos- sible to obtain their structural characteristics (size, polydispersity, surface type, etc.) at the nanoscale (1– 100 nm). Indeed, previous small-angle scattering curves for silicate aggregates grown in TEOS water– alcohol (ethanol) solutions under basic conditions indicated that fractal structures consisting of silicate aggregates with a diffuse surface were formed [12, 13, 15, 16]. When the influence of the water–TEOS molar ratio (w) on the structure of aggregates was studied with SANS, the critical behavior of their structural parameters (w = 2) was found [15]. The surface prop- erties are independent of w for w > 2, which indicates a situation close to the complete saturation of Si–OH bonds in TEOS at w = 2. The internal structure of aggregates was studied with external and internal vari- ation of the SANS neutron contrast via isotopic hydrogen/deuterium substitution [16]. The authors showed that most hydrolyzed Si–OH bonds are not closed and participate in the condensation reaction to form Si–O–Si bonds. The aim of this work is to study the structure of such clusters with SANS (including contrast variation) depending on the total concentration of components in the system. Considering that the proportion of TEOS with those of other components is in certain ratios, the concentration of TEOS in the solution is chosen as the parameter determining the structure, which varies in our experiments. Experiments are also performed at earlier cluster growth stages in compari- son with previous studies [15, 16] to test the hypothesis about the inf luence of the amount of TEOS on a sol– gel polymerization processes during the active forma- tion of aggregates.