THERMODYNAMIC OF OBTAINING OF MONODISPERSE PARTICLES SIO2 BY TETRAETHOXYSILANE HYDROLYSIS IN THE SI-O-H-C-N SYSTEM

І.Г. Каюн, О.П. Мисов. Термодинаміка отримання монодисперсних частинок SiO2 гідролізом тетраетоксисилану в системі Si-O-H-C-N. Висвітлено проблему синтезу монодисперсних частинок SiO2 гідролізом (C2H5O)4Si за методом Штобера. Метою дослідження є визначення умов проходження даної реакції у водно-аміачно-спиртовому середовищі при яких досягається максимальна концентрація твердої фази SiO2 та мінімальна концентрація іонних сполук силіцію в розчині. Шляхом термодинамічного моделювання досліджений склад системи Si-O-H-C-N при термодинамічній рівновазі для різних заданих умов. Отримання максимальної кількості твердої фази SiО2 при різних початкових концентраціях (C2H5O)4Si досягається при початковій концентрації C2H5OН більше 1,2 моль/л, концентрація твердої фази SiО2 пропорційна початковій концентрації (C2H5O)4Si. Термодинамічні дослідження показують, що зміна температури реакції від 1 до 70 С не впливає на концентрації іонних сполук силіцію в розчині та твердої фази SiO2. Отримані результати зменшують область пошуку оптимальних умов отримання монодисперсних частинок SiO2 та дають можливість більш глибоко зрозуміти процеси що протікають в системі Si-O-H-C-N. Ключові слова: термодинаміка, метод Штобера, гідроліз тетраетоксисилану

The described work only partially investigated the problem of synthesis of monodisperse particles SiO 2 . Restrict the search area of optimal conditions for the production of mono-dispersed SiO 2 particles is possible by thermodynamic studies. The thermodynamic study of tetraethoxysilane hydrolysis by the stoeber method has been given insufficient attention. There are studies that only partially describe the properties of the multi-component system Sі-O-H-C-N. Calculated and experimentally confirmed data of thermo chemical parameters for 47 molecules in the Si-O-H system [6]. The thermo chemistry of compounds that can exist in the early stages of the high-temperature decomposition of TEOS in the preparation of amorphous silicon dioxide (-SiO 2 ) nano particles [7] is investigated.
Thus, the Si-O-H-C-N system, represented by tetraethoxysilane in a water-alcohol-ammonia environment, is not investigated from thermodynamic positions. However, there is information on the thermodynamic parameters of the compounds that make up the investigated system and can be formed as a result of the reaction.
The purpose of the study is to determine the conditions for the hydrophilic reaction of the TEOS in aqueous ammonia-alcoholic medium, in which the maximum concentration of SiO 2 solid phase and the minimum concentration of ionic compounds of silicon in the solution is reached.
Presentation of the main material. Synthesis of monodisperse particles of SiO 2 by hydrolysis of TEOS by the Stoeber method is carried out by reaction (1) in a water-alcohol-ammonia environment.
(1) The reaction components (1) form a system of Si-O-H-C-N, the thermodynamic studies of which are well described by a mathematical model created on the basis of the "Selector" software complex. The principle embodied in the program is based on minimizing the isobaric-isothermal potential of Gibbs.
In thermodynamic calculations, the following assumptions were adopted: the Si-O-H-C-N system is at constant temperature and atmospheric pressure. In the thermodynamic model, the standard "Selector" databases are used: Yokokawa, sprons98, sprons07, dump. The model includes the following components:  4 Si=0.1 M, NH 4 OH=1.5 M and at a temperature T=25 C, the maximum solid phase of SiO 2 is formed at a concentration of C 2 H 5 ON greater than 0.9 M (Fig. 1). When the amount of alcohol from 0 M to 0.9 M changes, the concentration of H 2 SiO 4 -2 , H 3 SiO 4 -, HSiO 3 decreases with exponential dependence. A further increase in C 2 H 5 OН has little effect on their number. The concentrations of other compounds of silicon C are lower than those described above, so it is possible to assume that they will not affect the characteristics of the synthesized particles.
In order to obtain the maximum SiО 2 solid phase at different initial (C 2 H 5 O) 4 Si concentrations, we established the required concentration of C 2 H 5 OН (Fig. 2), which can be determined by equation (2). Thus under conditions of thermodynamic equilibrium for any initial concentrations (C 2 H 5 O) 4 Si the maximum amount of SiO 2 solid phase is reached at an initial concentration of C 2 H 5 OН of more than 1.2 mol/l.
In order to increase the SiO 2 solid phase, it is expedient to increase the initial number of TEOS. Concentrations of other compounds of silicon are practically unchanged. Simulation is limited by the maximum number of TEOS at which it is possible to obtain uniform spherical particles of size [3].
An increase in the initial concentration of NH 4 OH from 0 to 1.9 M leads to a decrease in the SiO 2 solid phase in the SiO-H-C-N system at 1е -5 М at a TEOS concentration of 0.2 M (Fig. 4)

) from the initial concentration of NH 4 OH at the thermodynamic equilibrium of the Si-O-H-C-N system
It is known that for the complete passage of reaction (1), the minimum concentration of H 2 O should be twice as high as the concentration of Si (OC 2 H 5 ) 4 . Given the experimental data, the indicated ratio should be greater [8].
Thermodynamic studies of the influence of the initial concentration of H 2 O are shown in Since the model shows the thermodynamic equilibrium of the system Si-O-H-C-N, which is not limited in time, H 2 O in the system is formed with NH 4 OH and C 2 H 5 ON and it is sufficient for the complete passage of the reaction (1). Taking into account the kinetic constraints on obtaining SiO 2 particles, it is expedient to provide an initial ratio of H 2 O/Si (OC 2 H 5 ) 4 >2 concentrations.
Investigated temperature range is due to boiling of the reaction mixture and its freezing is from 1 to 70 C. Table 1   The results of the thermodynamic calculations show that SiO 2 solids in the Si-O-H-C-N system are formed at a wide variation of the reaction temperature and the concentrations of reagents (Table 2). Table 2 The  (Fig. 6). In our opinion, the explanation for the difference in the results is the high concentration of NH 4 OH (a weak base) in the solution at the completion of the reaction. And as a consequence, the pH value is 10.8 and above, which leads to a higher concentration of water-dissolved silicon ions compared to the theoretically calculated value. In thermodynamic calculations, the pH ranges from 6 to 7, which is explained by the partial conversion of NH 4 OH into N 2 and NH 3 and their removal from the solution in the form of gas.

ranges of variations of the parameters of the system S-O-H-C-N, in which a solid SiO 2 phase is formed
Conclusions Under conditions of thermodynamic equilibrium, obtaining the maximum solids SiO 2 at different initial concentrations (C 2 H 5 O) 4 Si is achieved at an initial concentration of C 2 H 5 OH of more than 1 mol/l. The solid phase concentration of SiO 2 is proportional to the initial concentration of TEOS.
Thermodynamic studies show that SiO 2 can be obtained at an initial concentration of H 2 O equal to 0 M. The increase in the initial concentration of NH 4 OH from 0 to 1.9 M leads to a decrease in the SiO 2 solid phase in the SI-O-H-C-N system for 1e -5 M, regardless of the initial concentration of ISSN 2223-3814 (online) ХІМІЯ. ХІМТЕХНОЛОГІЯ 78 TEOS. The change in reaction temperature from 1 to 70 °C does not affect the concentration of ionic silicon compounds in the solution and solid SiO 2 phase.
Due to the kinetic constraints of the hydrolysis reaction of the TEOS, the practical yield of SiO 2 is less than the theoretical by 10…15 %, depending on the initial concentrations of reagents.
Further study of the problem of obtaining mono-dispersed particles SiO 2 should be directed to the experimental study of the SI-O-H-C-N system in order to detect the influence of technological parameters on the shape, average size and dispersion of the sizes of synthesized particles.