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THERMODYNAMIC OF OBTAINING OF MONODISPERSE PARTICLES SI[O.sub.2] BY TETRAETHOXYSILANE HYDROLYSIS IN THE SI-O-H-C-N SYSTEM.

UDC 661.682

Introduction. Actuality obtaining of mono-dispersed particles of Si[O.sub.2] due to their wide use in industry. This is the production of opals, photonic crystals, in the chemical industry as composite catalysts, etc.

Hydrolysis of tetraethoxysilane (TEOS) by the Stoeber method in a water-alcohol-ammonia medium allows for the obtaining of nano and submicron spherical particles of Si[O.sub.2] [1]. The size of Si[O.sub.2] particles synthesized by hydrolysis of TEOS largely depends on the concentration of TEOS, water, alcohol, ammonia and reaction temperature [2].

Due to the many components of the Si-O-H-C-N system, the development of technology for the synthesis of mono disperses spherical particles of Si[O.sub.2] requires numerous experiments. Thermodynamic studies will reduce their number, determine the conditions for which the maximum solid phase of Si[O.sub.2] will be obtained and determine the composition of the Si-O-H-C-N system, which greatly affects the characteristics of the synthesized particles [3].

Analysis of recent research and publications. The influence of water, ammonia and TEOS concentrations on the particle diameter and their uniformity in size was considered repeatedly [4]. The possibility to obtain homogeneous particle sizes at high concentrations of TEOS [5] was investigated.

The described work only partially investigated the problem of synthesis of monodisperse particles Si[O.sub.2]. Restrict the search area of optimal conditions for the production of mono-dispersed Si[O.sub.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 Si-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 ([alpha]-Si[O.sub.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 Si[O.sub.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 Si[O.sub.2] by hydrolysis of TEOS by the Stoeber method is carried out by reaction (1) in a water-alcohol-ammonia environment.

Si[(O[C.sub.2][H.sub.5]).sub.4] + 2[H.sub.2]O [right arrow] Si[O.sub.2] + 4[C.sub.2][H.sub.5]OH. (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:

Solid phase: N[H.sub.4]N[O.sub.3], SiC, Si, Si[O.sub.2];

Gas phase: N[H.sub.3], [C.sub.2][H.sub.4], C[O.sub.2], CO, [H.sub.2], C[H.sub.4], [N.sub.2], [C.sub.7][H.sub.8]O, [O.sub.2], [C.sub.6][H.sub.6]O, [H.sub.2]O;

Aqua solution: [H.sub.4]Si[O.sub.4], [H.sub.2]Si[O.sub.4.sup.-2], [H.sub.3]Si[O.sub.3.sup.+], [H.sub.3]Si[O.sub.4.sup.-], C[H.sub.3]CON[H.sub.2.sup.*], [C.sub.2][H.sub.3][O.sub.2.sup.-], [C.sub.2][H.sub.4][O.sub.2.sup.*], [C.sub.4][H.sub.7][O.sub.2.sup.-], C[O.sup.*], C[O.sub.2.sup.*], C[O.sub.3.sup.-2], [C.sub.2][H.sub.6.sup.*], [C.sub.2][H.sub.5]O[H.sup.*], HC[O.sub.2],[H.sub.2]C[O.sub.2.sup.*], [C.sub.2][H.sub.5]N[O.sub.2.sup.*], [C.sub.3][H.sub.3]O.sub.4.sup.-], [C.sub.2]H[O.sub.4.sup.-], [H.sub.2.sup.*], HC[O.sub.3.sup.-], HSi[O.sub.3.sup.-], [C.sub.3][H.sub.2][O.sub.4.sup.-2], C[H.sub.3]N[H.sub.2.sup.*], C[H.sub.3]O[H.sup.*], [N.sub.2.sup.*], N[H.sub.3.sup.*], N[H.sub.4]C[H.sub.3]CO[O.sup.*], N[H.sub.4][(C[H.sub.3]COO).sub.2.sup.-], N[H.sub.4.sup.+], OC[N.sup.-], [C.sub.2][O.sub.4.sup.-2], [C.sub.3][H.sub.8.sup.*], [C.sub.3][H.sub.5][O.sub.2.sup.-], [C.sub.3][H.sub.6][O.sub.2.sup.*], Si[O.sub.2.sup.*], [H.sub.2]NCON[H.sub.2.sup.*] O[H.sup.-], [H.sup.+], [H.sub.2]O.

The described thermodynamic model shows that the amount of solid phase Si[O.sub.2] depends on the concentration of [C.sub.2][H.sub.5]ON. At the initial concentrations of [H.sub.2]O=20 M, [([C.sub.2][H.sub.5]O).sub.4]Si=0.1 M, N[H.sub.4]OH=1.5 M and at a temperature 7=25[degrees]C, the maximum solid phase of Si[O.sub.2] is formed at a concentration of [C.sub.2][H.sub.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.sub.2]Si[O.sub.4.sup.-2], [H.sub.3]Si[O.sub.4.sup.-], HSi[O.sub.3.sup.-] decreases with exponential dependence. A further increase in [C.sub.2][H.sub.5]OH 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 Si[O.sub.2] solid phase at different initial [([C.sub.2][H.sub.5]O).sub.4]Si concentrations, we established the required concentration of [C.sub.2][H.sub.5]OH (Fig. 2), which can be determined by equation (2). Thus under conditions of thermodynamic equilibrium for any initial concentrations [([C.sub.2][H.sub.5]O).sub.4]Si the maximum amount of Si[O.sub.2] solid phase is reached at an initial concentration of [C.sub.2][H.sub.5]OH of more than 1.2 mol/l.

y = 1,2 - 4*[Si[([C.sub.2][H.sub.5]O).sub.4]]. (2)

In order to increase the Si[O.sub.2] solid phase, it is expedient to increase the initial number of TEOS. Fig. 3 shows the simulation results of the system Si-O-H-C-N at a temperature 7=25[degrees]C, which consisted of [H.sub.2]O=20 M, [C.sub.2][H.sub.5]OH=9 M, N[H.sub.4]OH=1.5 M. As can be seen from Fig. 3 concentrations of Si[O.sub.2] and TEOS are proportional.

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 N[H.sub.4]OH from 0 to 1.9 M leads to a decrease in the Si[O.sub.2] solid phase in the SiO-H-C-N system at [1e.sup.-5] M at a TEOS concentration of 0.2 M (Fig. 4). Subsequent studies have shown that the indicated change in the solid phase Si[O.sub.2] concentration does not depend on the concentration of TEOS.

It is known that for the complete passage of reaction (1), the minimum concentration of [H.sub.2]O should be twice as high as the concentration of Si [(O[C.sub.2][H.sub.5]).sub.4]. Given the experimental data, the indicated ratio should be greater [8].

Thermodynamic studies of the influence of the initial concentration of [H.sub.2]O are shown in Fig. 5 (initial modeling conditions: [N.sub.4]OH=1 M, [C.sub.2][H.sub.5]OH=9 M, [([C.sub.2][H.sub.5]O).sub.4]Si=0.2 M, T=25[degrees]C) show that Si[O.sub.2] can be obtained at an initial concentration of [H.sub.2]O equal to 0 M.

Since the model shows the thermodynamic equilibrium of the system Si-O-H-C-N, which is not limited in time, [H.sub.2]O in the system is formed with N[H.sub.4]OH and [C.sub.2][H.sub.5]ON and it is sufficient for the complete passage of the reaction (1). Taking into account the kinetic constraints on obtaining Si[O.sub.2] particles, it is expedient to provide an initial ratio of [H.sub.2]O/Si [(O[C.sub.2][H.sub.5]).sub.4]>2 concentrations.

Investigated temperature range is due to boiling of the reaction mixture and its freezing is from 1 to 70[degrees]C. Table 1 shows the simulation results of the system Si-O-H-C-N consisting of [H.sub.2]O=20 M, [C.sub.2][H.sub.5]OH = 9 M, [([C.sub.2][H.sub.5]O).sub.4]Si = 0.2 M, N[H.sub.4]OH =1.5 M. Thus, under thermodynamic equilibrium conditions, the temperature change of the system is not affects the concentration of compounds of silicon in the solution and solid Si[O.sub.2] phase.

The results of the thermodynamic calculations show that Si[O.sub.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).

The experimental verification of theoretical studies was carried out at a temperature of T=25[degrees]C and the concentrations of reagents [H.sub.2]O=20 M, [C.sub.2][H.sub.5]OH=10 M, [([C.sub.2][H.sub.5]O).sub.4]Si=0.2 M. Experimental studies show that the practical yield of Si[O.sub.2] is less than the theoretical by 10... 15 %, depending on the initial concentration of N[H.sub.4]OH (Fig. 6).

In our opinion, the explanation for the difference in the results is the high concentration of N[H.sub.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 N[H.sub.4]OH into [N.sub.2] and N[H.sub.3] and their removal from the solution in the form of gas.

Conclusions Under conditions of thermodynamic equilibrium, obtaining the maximum solids Si[O.sub.2] at different initial concentrations [([C.sub.2][H.sub.5]O).sub.4]Si is achieved at an initial concentration of [C.sub.2][H.sub.5]OH of more than 1 mol/l. The solid phase concentration of Si[O.sub.2] is proportional to the initial concentration of TEOS.

Thermodynamic studies show that Si[O.sub.2] can be obtained at an initial concentration of [H.sub.2]O equal to 0 M. The increase in the initial concentration of N[H.sub.4]OH from 0 to 1.9 M leads to a decrease in the Si[O.sub.2] solid phase in the SI-O-H-C-N system for [1e.sup.-5] M, regardless of the initial concentration of TEOS. The change in reaction temperature from 1 to 70[degrees]C does not affect the concentration of ionic silicon compounds in the solution and solid Si[O.sub.2] phase.

Due to the kinetic constraints of the hydrolysis reaction of the TEOS, the practical yield of Si[O.sub.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 Si[O.sub.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.

References

[1.] Zhokhov A.A., Masalov V.M., & Sukhinina N.S. (2015). Photonic crystal microspheres. Optical Materials, 49, 208-212.

[2.] Plumere N., Ruff A., & Speiser B. (2012). Stober silica particles as basis for redox modifications: particle shape, size, polydispersity, and porosity. Journal of Colloid and Interface Science, 368(1), 208-219.

[3.] Frolov Yu.G. (1979). Theoretical foundations for the synthesis of silica hydrosols. Preparation and use of silica hydrosols. Proceedings of Moscow. chem.-tech. institute of D.I. Mendeleyev, 107, 3-20.

[4.] Arantes T.M., Pintoa A.H., & Leitea E.R. (2012). Synthesis and optimization of colloidal silica nanoparticles and their functionalization with methacrylic acid. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 415, 209-217.

[5.] Tadanaga K., Morita K., Mori K., & Tatsumisago M. (2013). Synthesis of monodispersed silica nanoparticles with high concentration by the Stober process. Journal of Sol-Gel Science and Technology, 68(2), 341-345.

[6.] Mark D., Melius A.F., Melius C.F., Ho P., & Zachariah M.R. (1995). Theoretical Study of the Thermochemistry of Molecules in the Si-O-H

System. J. Phys. Chem., 99, 15285-15293.

[7.] Phadungsukanan W., Shekar S., & Shirley R. (2009). First-Principles Thermochemistry for Silicon Species in the Decomposition of Tetraethoxysilane. J. Phys. Chem. A., 13(31), 9041-9049.

[8.] Masalov V.M., Sukhinina N.S., & Omelchenko G.A. (2011). The nanostructure of silicon dioxide particles obtained by the multi-step Stober-Finqa-Bon method. Chemistry, physics and surface technology, 2(4), 373-384.

[phrase omitted]; Kayun Igor, ORCID: http://orcid.org/0000-0001-7175-7551

[phrase omitted]; Mysov Oleg, ORCID: http//orcid.org/0000-0003-2114-1382

Received March 13, 2018

Accepted March 23, 2018

I. Kayun, O. Musov, PhD, Assoc.Prof., Ukrainian State University of Chemical Technology, 8 Gagarin Ave., Dnipropetrovsk, Ukraine, 49005; e-mail: igorkayun@ukr.net

DOI: 10.15276/opu.1.54.2018.10

[Please note: Some non-Latin characters were omitted from this article]

Caption: Fig. 1. Dependence of the concentration of silicon compounds ([??]--[H.sub.4]Si[O.sub.4], [??]--[H.sub.2]Si[O.sub.4.sup.-2], [??]--[H.sub.3]Si[O.sub.4.sup.-], *--HSi[O.sub.3.sup.-], + - Si[O.sub.2.sup.*], [omicron] - Si[O.sub.2]) from the initial concentration of [C.sub.2][H.sub.5]OH at the thermodynamic equilibrium of the Si-O-H-C-N system

Caption: Fig. 2. The concentration of alcohol is required to obtain the maximum amount of Si[O.sub.2] solid phase at different initial concentrations Si[([C.sub.2][H.sub.5]O).sub.4]

Caption: Fig. 3. The dependence of the concentration of silicon compounds ([??]--[H.sub.4]Si[O.sub.4], [??]--[H.sub.3]Si[O.sub.3]+, [??]--[H.sub.3]Si[O.sub.4.sup.-], *---His[O.sub.3.sup.-], + - Si[O.sub.2.sup.*], [omicron]--Si[O.sub.2] from the initial concentration of TEOS at the thermodynamic equilibrium of the Si-O-H-C-N system

Caption: Fig. 4. The dependence of the concentration of silicon compounds ([??]--[H.sub.4]Si[O.sub.4], [??]--[H.sub.3]Si[O.sub.3.sup.+], [??]--[H.sub.3]Si[O.sub.4.sup.-], *--His[O.sub.3.sup.-] + - Si[O.sub.2.sup.*], [omicron]--Si[O.sub.2]) from the initial concentration of N[H.sub.4]OH at the thermodynamic equilibrium of the Si-O-H-C-N system

Caption: Fig. 5. Dependence of the concentration of silicon compounds ([??]--[H.sub.4]Si[O.sub.4], [??]--[H.sub.3]Si[O.sub.4.sup.-], *--HSi[O.sub.3.sup.-], + - Si[O.sub.2.sup.*], [omicron]--Si[O.sub.2]) from the initial concentration of [H.sub.2]O at the thermodynamic equilibrium of the Si-O-H-C-N system

Caption: Fig 6. Comparison of thermodynamic calculations of Si[O.sub.2] ([C.sub.c]) initial concentration at initial concentration change of N[H.sub.4]OH ([??]) and experimentally obtained concentration ([C.sub.e]) (*)
Table 1
Concentrations of compounds at the thermodynamic equilibrium of the
Si-O-H-C-N system in the temperature range from 1 to 70[degrees]C

Compound                 [H.sub.4]Si[O.sub.4]       [H.sub.2]Si
                                                  [O.sub.4.sup.-2]

Concentration, mol/l           2.85E-05               1.28E-13

Compound                     [H.sub.3]Si         [H.sub.3]Si
                           [O.sub.3.sup.+]      [O.sub.4.sup.-]

Concentration, mol/l           3.24E-10            8.80E-09

Compound                  His[O.sub.3.sup.-]    Si[O.sub.2.sup.*]

Concentration, mol/l           1.35E-08              9.52E-06

Compound                 Si[O.sub.2]

Concentration, mol/l       2.00E-01

Table 2 The ranges of variations of the parameters of the system
S-O-H-C-N, in which a solid Si[O.sub.2] phase is formed

Parameter        Concentration      Concentration    Concentration
                 Si[(O[C.sub.2]       [H.sub.2]O      N[H.sub.4]OH
               [H.sub.5]).sub.4]

Value range      0 ... 1.2 M          0 ... 19 M      0 ... 1.9 M

Parameter          Concentration          Temperature
                [C.sub.2][H.sub.5]OH

Value range       0 ... 10 M              1 ... 70[degrees]C


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Title Annotation:CHEMISTRY. CHEMICAL ENGINEERING
Author:Kayun, I.; Musov, O.
Publication:Odes'kyi Politechnichnyi Universytet. Pratsi
Article Type:Report
Date:Mar 1, 2018
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