Preparation, Simulation And Applications Of Monodisperse Spherical Sulfur Nanoparticles

PREPARATION, SIMULATION AND APPLICATIONS OF MONODISPERSE SPHERICAL SULFUR NANOPARTICLES

F. Kh. Urakaev, Yu. P. Savintsev

Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

The current interest in monodispersed hard spheres is caused by their applicability to the fabrication of photon crystals, three-dimensional nanoporous materials, repellents, etc. The nucleation and growth of sulfur particles in solutions are also of importance because it is known that monodisperse colloid spherulites of solution-stable amorphous sulfur form in acidified aqueous solutions of thiosulfates. Homogeneous nucleation and growth of monodisperse spherulites of sulfur was studied using an original optical method for measuring relative scattering coefficient, based on alternative application of coherent radiation of two lasers with different wavelengths. The model for homogeneous nucleation of sulfur crystals has been developed, which considers the nuclei ranging from n to n×102 atoms and uses the noncontinuous calculations. By the change pH water solutions of thiosulphates and polysulphides of alkaline and alkaline earth metals were obtained submicronic globular particles of sulfur. Various ways of obtaining polymeric composites on the basis of nanosized particles of sulfur are studied. They are submitted based on the decomposition of unstable compounds of sulfur and the nucleation process of sulfur from multicomponent systems of high-supersaturation. Various aspects of application of sulfur particles in the agriculture, construction, coatings and in high technologies are considered.

1. STUDY OF NUCLEATION AND GROWTH OF SULFUR PARTICLES IN SOLUTIONS

In this Section, on the basis of original coherent-optical procedure, we have studied some aspects of the kinetics of homogenous nucleation and growth of monodisperse spherulites of amorphous sulfur Sn in the Na2S2O3 + HCl + H2O system [1] in the range of temperatures 25−50°C. This system has a number of advantages which make it model for studying mass crystallization in solutions, these are: reproducibility of induction periods and supersaturation levels according to solid component of the system; a clear mechanism of reaction for the formation of free * = + atoms and sulfur associates up to critical content (Sn ) in the solution (SO3 + 2H ↔ * H2S2O3 → H2SO3 + S; S + S ↔ S2 + S... ↔ Sn-1 + S → Sn ); monodispersity and sphericity of sulphur particles formed; etc. To carry out the investigations, we have substantiated, created, and put into service a stand of coherent optics designed for studying mechanism and kinetics of mass crystallization of natural and synthetic minerals in solid and liquid media, see Fig. 1. The principle of simultaneous measurement of the number and size of new phase particles is based on an alternative use of coherent radiation of two lasers with different wavelengths during registration of changes in optical scattering coefficients (method of "relative scattering coefficient" - RSC). Below we give expressions needed for application of RSC:

3-11 τλ(t) = ln [I0λ / Isλ(t)] / L (1)

τλ(t) = N(t) Rλ[ηs, ηp, d(t)] (2)

ε(t) = τλ1(t)(t) / τλ2(t) = Rλ1[d(t)] / Rλ2[d(t)] (3)

εc(d) = Rλ1(d) / Rλ2(d) (4)

-1 where τλ(t) is the spectral bulk extinction factor of the medium [cm ]; L is the dish diameter [cm]; I0λ and Isλ are the radiation intensities [a. u.] registered by photomultiplier in the dish with a highly purified water and in the dish with investigated medium, respectively; λ = λ0 / η(ηs, ηp, λ, T) is the wave length in the -4 -4 medium [cm]; λ0 is the wavelength in air (λ01 = 0.6328×10 and λ02 = 0.4880×10 cm for radiation of lasers LGN 302 and LGN 402, respectively); η(ηs,ηp,λ,T) is the refractive index of the medium (ηs belongs to solution, ηp – to particle)]; T is the 2 temperature of the medium; Rλ(d) is the scattering cross section [cm ]; d(t) is the particle diameter [cm]; N(t) is the number of particles in a volume unit [cm-3]; t is the current time; ε(t) is the experimental RSC; εc(d) is the calculated relative scattering cross-section (RSCS). The Rλ1(d) and Rλ2(d) values for the medium containing only monodisperse and spherical particles were calculated by Mie's theory [2, 3]. We neglected the effects of multiple scattering in our systems for the reasons reported in [3].

Fig. 1. Principal scheme of coherent-optical stand: 1 - beams of lasers LGN- 302 and LGN-402; 2 - diaphragms; 3 – polarizing Glan-Tailor's filters; 4 - interference light filters; 5 - mirror; 6 - semi-transparent mirror; 7 - cube beam splitter; 8 - assembly of light filters; 9 - polarization plates; 10 - photomultiplier; 11 - quartz windows; 12 - cell with vacuum oil; 13 - measuring dish; 14 - assembly of diaphragms (2 - 0.05 mm in diameter; 15 - movable, 0.01 mm in diameter); 16 - amplifier-discriminator of one electron pulses; 17 - frequency meter; 18 - frequency meter for controlling check intensity in channel of comparison; 19 - CAMAC; 20 - computer; 21 - movable blind; 22 - thermostat.

3-12 Fig. 2. Calculated curve Fig. 3. Dependence of the Fig. 4. Change in the RSCS εc(d) versus transmittance κt(λ=300 nm) diameter d(t) of S- diameter d for the on time t of S-spherulites spherulites for the Csm = monodisperse S- formation at 25°C and Csm 2.7 mg/100 ml and spherulites for 25°C and = 2.7 mg/100 ml; temperatures: concentration Csm = 2.7 mg/100 ml. t* - induction period. curve 1 - 30°C; 2 - 45°C; 3 - 50°C.

-4 When calculating RSCS εc(d) for the size of d particles to 0.5×10 , we took a step 10-7, and in the range (0.5-2)×10-4 - step 10-6. In calculations, according to abundant reference data [4.5], we accepted the following values of refractive indexes η (to an accuracy to the fourth decimal place) for applied laser wavelengths and their temperature dependencies (to an accuracy for the fifth decimal place): - for the mixture of sodium thiosulfate and hydrochloric acid solutions both of 0.0025 normality (work on nucleation and growth of sulphur spherolites is performed predominantly at this concentration of agents, namely (0.02 g Na2S2O3 / 100 ml) + (0.01 g HCl / 100 ml) η(λ1,17.5°C) = 1.3324, η(λ2,17.5°C) = 1.3372; - for the solutions independent of the wavelength ∆η / ∆T ≈ −0.00014 °C-1; - for particles of the amorphous sulphur we used data for melted (glassy) sulfur -1 η(λ1, 17.5°C) = 1.8873, η(λ2, 17.5°C) = 1.9734, ∆η / ∆T ≈ −0.00033 °C . Results of calculation of RSCS εc(d) for the system of monodisperse spherical sulfur particles are graphically illustrated in Fig. 2. Fig. 1 shows the technique for determining spectral intensities of radiation Iλ1(t), Iλ2(t), and for calculating extinction factors τλ1(t), τλ2(t) and RSC ε(d) by Eqs. (1)-(3). Beam 1 of the first laser passes through projecting optical system 2-6 and falls on the path of the second laser beam. An alternative switching of the beam paths is carried out by a block of automatic control 21 of blind 21; beams 1 pass through the optical system – cell 12 thermostated by highly pure vacuum oil, with quartz windows 11, dish 13 with the system studied and further into registering optical-electronic system

3-13 10, 14-20; part of radiation is carried by a light splitting cube 7 to channel of comparison 8-10, 18; temperature in the cell is maintained by a liquid circulation thermostat 22 (accuracy of temperature regulation is ±0.05°C). To calibrate the RSC method we repeated the results of [1], using different spectrophotometers [5]. The way of successive use of lasers allows determination of the change in spectral radiation intensities with the time constant of transition from one wavelength to another (10−30 s). To reduce the random scattering in data, the values of intensities ISλ(t) were smoothed out by the third-degree polynomials in each point by eleven neighbouring points. The error of smoothing for all experiments was no more than ±2%. A parallel use of lasers is also possible but this complicates measurements (two photomultipliers and devices for separation of beams coming out from the cell are required). Separate use first of one laser and then other needs two experiments to be carried out. The above possibility hinder automatization of processing of data and, as a rule, does not assure correct results. The procedure for concurrent determination of the change in diameter d(t) and concentration N(t) of monodisperse spherical particles is as follows: - from Eq. (4) dependence RSCS ε(d) is determined, see Fig. 2; - from experimental data the function RSC ε(t) is determined; - numeral coincidence of the course of change in ordinate ε(t), within the totals of errors of measurement and disagreement of adopted reference optical parameters with real ones in the mediums under study, with the ordinate of theoretical curve ε(d) serves as the main criterion of the correspondence of experimental data to the theoretical calculation and unambiguous interpretation of the results obtained; - from Eq. (3) numerically or graphically (see Fig. 2), by comparing calculated ε(d) experimentally obtained ε(t) values, one can find the function of change in particle diameter over time d(t), and from Eq. (2) we find the function of change in the number of particles N in a volume unit over time (concentration of particles) - N(t). The starting solutions of hydrochloric acid and sodium thiosulphate of 2000 ml each were prepared from corresponding titrimetric standards in three-times distilled water and had a concentration 0.05 n. These solutions were purified from impurity particles through a system of filters in a closed volume. Titrimetric and conductometric analyses of final solutions confirmed correspondence of their concentrations to 0.05n with an accuracy of ±1%. Aliquots of 3.5 to 14 ml were selected from the solutions and were diluted to 70 ml in volume so that, when the starting equinormal solutions were mixed the total volume of the solution under study was 140 ml (working volume of thermostated dish). This, according to the equation of + = reaction 2H + S2O3 → H2SO3 + S, corresponds to a maximum possible concentration of produced sulfur particles in solution Csm =2.7−8.0 mg / 100 ml, and to concentration of initial reagents before mixing them 0.0025−0.01 n. Fig. 3 shows the dependence of the transmitance κt(t) = IS(t) / I0 on time t at wave length 300 nm at 25°C. A smooth decrease in κt on curve r, describing the occurrence of chemical reaction of formation of sulfur atoms and clusters to nucleation stage (to value 0.988 - point of the beginning of nucleation - n) takes place during ~34 min after mixing the starting solutions for the experiment with Csm =2.7 mg / 100 ml. Beginning with point n to point c against the general background of decreasing κt, a drastic unstable reduction of the κt at 300 nm during 1÷2 min is observed, and then a nearly linear (straight line g of the growth of "supercritical" sulfur nuclei) decrease in κt against time down to a value 0.975 in 90 min. The reproducibility of κt(t) curve in its kink n (field of unstable drop of κt) was ±1 min and coincided with the appearance of already discrete scattering particles (origin of the

3-14 faint blue Tindall beam). The concentration CS of sulfur atoms in the kink point n corresponds to the critical supersaturation point CS*, and time t* - to the duration of pre-nucleation induction period. With due regard for the measured value of the absorption coefficient of sulphur atoms and/or molecules 1460 cm2/g-atom [1,5] at a wave length 300 nm was obtained the experimental dependence of concentration C of dissolved sulfur CS(t) before the beginning of nucleation in the solution. It was established that t* depends on temperature T and concentration of starting reagents and decreases with their growth, -6 whereas sulfur concentration in the critical point of supersaturation CS* ≈ 5.5×10 g- atom / l remains practically a constant value and poorly depends on the initial T and C of reagents in the range Csm = 2.7−4.0 mg / 100 ml of solution. The values -6 CS* = 3.2×10 g-atom / l and C of saturated solution of sulphur in the water -6 CS* ≈ (1.2±0.1)×10 g-atom / l were determined. The monodispersity and sphericity of sulfur particles were controlled by crystallooptical investigation of the final products of experiments on microscope NU−2E. The values of deviations of sufhur spherulites from monodispersity, established by statistical analysis, for these experiments did not exceed ±5%. Further measurements of extinction coefficients on the stand were conducted for two wavelengths (488 and 633 nm) with an interval of 10−30 s (time resolution of RSC method), starting from the moment of nucleation to the end of experiments (180 min counting from the moment of mixing the initial solutions) with the aim to simultaneously determine the size d and the number of nuclei N in unit volume versus time t, and T−C dependencies of formation and growth of sulfur particles - d(t,T,C) and N(t,T,C). Numerous experiments at T = 25, 30, 45, and 50°C and different Csm values allowed us to establish that the least determinable size of spherical monodisperse sulfur particles is d ≈ 30 nm at concentration (for example, 25°C and 12 -3 Csm = 2.7 mg / 100 ml) of growing nuclei N ≈ 7×10 cm . As temperature grows, the initial processes of formation and growth are accelerated at the same number (under these conditions) of generated nuclei. The experimentally established dependencies d(t) of sulfur spherulites with time resolution 10−30 s are shown in Fig. 4. Their growth rates in the region of small sizes are high. Therefore, the first reliably established point at 25°C (correct measurement of the transmitances or RSC in point c for calculating sizes and number of sulfur spherulites after the beginning of nucleation in point n in Fig. 3) corresponds to d ≈ 30 nm. The total period of nucleation and growth of sulphur nuclei from critical size d* to d ≈ 30 nm is about 2 min at 25°C (Fig. 3). The curve at 25°C is not shown in Fig. 4. When the temperature of solutions higher, the duration of induction period t* decreases. At 25°C it is 34±1 min (Fig. 3). At higher temperatures, t* and values of initially measured diameters of sulfur spherolites might be roughly determined by extrapolation of curves d(t) to time axis and by the value of ordinates of the beginning of curves in Fig. 4, respectively. More accurate values: at 30°C (curve 1) - 26 min and ~40 nm; at 45°C (curve 2) - 19 min and ~200 nm; at 50°C (curve 3) - 15 min and ~260 nm. When the concentration Csm higher (4−8.0 mg / 100 ml), the concentration of growing nuclei changed over time, and solutions exhibited coagulation and precipitation of particles during the experiment. These results are a technologic aspect of formation and growth of sulfur nuclei.

2. MECHANISM AND MODELING OF FORMATION OF AMORPHOUS SULFUR NUCLEI

3-15 The current interest in monodispersed colloidal spheres is caused by their applicability to the fabrication of photon crystals, computer chips and three- dimensional nanoporous materials [6,7]. The nucleation and growth of sulfur particles in solutions are also of importance because it is known [1,5,8] that, under specific conditions, monodisperse colloid spherulites of solution-stable [8] amorphous sulfur form in acidified aqueous solutions of thiosulfates. Elemental sulfur has many unique properties [9], for example, a separate phase of amorphous sulfur particles during ageing gradually changes into α-sulfur. The complexity of the numerical modeling of the nucleation of amorphous particles is that the real structure of an amorphous state cannot be adequately described by theoretical and experimental methods. There are different kinds of amorphous states for particular types of substances [10,11]. Their common feature is the absence of a long-range order in the structure of an object. Earlier [12] we proposed a method for the estimation of parameters of crystal critical nuclei based on a discrete approach. The structure of a crystal phase is assumed to be known. It is supposed that the nucleus forms on the successive addition of molecules at the positions which they would occupy in an ideal crystal. A correction is made for the length and energy of the bonds of surface molecules whose nearest neighbours are fewer than those of molecules in the bulk. This method is used for calculating the characteristics of critical sulfur nuclei on the assumption that the growth of clusters proceeds by the addition of S8 ring molecules [9]. In the rhombic crystal of α-sulfur, each S8 molecule has two neighbour molecules at a distance of ~0.5 nm and two at ~0.6 nm [13]. The difference in the distances is related to different orientations of the rings. At a random orientation of S8 rings during rapid nucleation, an averaged distance can be used in the calculated model (Fig. 6). In this case, the long-range order is missing and an amorphous sulfur (S-spherulite) phase is formed. There are two approaches to the selection of the way: (1) the addition proceeds by the the coordination spheres (layer-by-layer growth), and (2) the positions of added molecules were chosen randomly, provided that the newly added molecule had at least one bond with the cluster. At the second stage of the chosen way of addition, the contribution of the cluster-infinite phase interface energy gsn is calculated [12]. At the third stage, the formation energy A is calculated for various values of the relative supersaturation ∆μ depending on the number n of molecules in a cluster: ∆μ = μ1 − μ0, where μ1 and μ0 are the chemical potentials of a supersaturated solution and a solution at the points of phase transition, respectively. Then, A = −n∆μ + gsn. All the energy values gsn, A, and ∆μ are measured in arbitrary units [12]. The intermolecular bond energy in an infinite crystal E∞ was taken to be unity; as a first approximation, this parameter can be estimated using the enthalpy of sulfur melting. The maximum value of A(n) is used to determine the number n in the critical nucleus, and n helps us to establish the shape of a nucleus at each step of changes in the ∆μ value. Random addition. Ten different ways of addition were generated for approach (2). The energy of formation A(n) was calculated for each of them. The results are shown in Fig. 7. For simplicity, only three versions of calculations among all the possible random ways of addition are shown. All the plots relate to the same supersaturation ∆μ. Fig. 8 shows the averaged plots of the energy of formation of a nucleus for three possible values of ∆μ. One can see that with the growth of ∆μ = kT ln γ/E∞ [14] or absolute supersaturation γ [at γ = 3−5 and T = 37°C, see above and [1,5], ∆μ ~ 0.5 corresponds to high γ values, if the enthalpy of sulfur melting ~12 kJ/mol [8] is taken

3-16 as E∞], the values of energy barrier A and n for critical nucleus decrease. These conclusions also correspond to the classic case. A more precise calculation of the characteristics of the critical nucleus with the changes of supersaturation is shown in Table 1. The calculation was carried out with a step of 0.01 by ∆μ. A stepwise behaviour of the number of molecules in a critical nucleus is explained by the fact that even an averaged curve is nonmonotonic but has local maximums and minimums. As a consequence, the size of a critical nucleus remains constant within a range of ∆μ. Table 1 indicates that the widest supersaturation range corresponds to the size of the nucleus comprising 55 molecules of S8.

Fig. 5. Mechanism of S-spherulite Fig. 6. The energies of the formation of formation. sulfur nuclei for different ways of the random addition of S8 molecules for the supersaturation ∆μ = 0.50. An averaged plot is shown with a bold line.

Fig. 7. Averaged energy of S-spherulite formation for different supersaturations ∆μ: (1) ∆μ = 0.55; (2) ∆μ = 0.50; (3) ∆μ = Fig. 8. Energy of S-spherulite formation: 0.45. The averaging was carried out over 10 different ways for the random addition (a) ∆μ = 0.45; (b) ∆μ = 0.55. (1) layer- by-layer addition; (2) random addition of of S8 ring molecules. S8 molecules.

Fig. 9. Energy of S-spherulite formation at different ∆μ: (1) ∆μ = 0.80; (2) ∆μ = 0.55; (3) ∆μ = 0.45. The molecules of S8

3-17 add to coordination spheres. b)

Table 1. Parameters of critical nucleus at layer-by-layer and random additions of S8 molecules.

Layer-by-layer addition Random addition *

∆μ, arb. un. number S8, n A, arb. un. ∆μ, arb. un. number S8, n A, arb. un. > 0.95 2 < 1.47 0.99-0.95 7 1.02-1.30 0.95-0.61 11 1.47-5.21 0.93-0.91 16 1.56-1.88 0.60-0.55 24 5.45-6.65 0.76-0.73 27 4.75-5.56 0.54-0.50 46 6.92-8.76 0.70-0.68 53 6.57-7.63 0.49-0.38 50 9.24-14.7 0.67-0.57 55 8.18-13.7 0.37-0.24 119 15.4-30.9 0.45-0.37 76 22.1-28.2 ≤ 0.23 ≥ 148 ≥ 32.3 < 0.32 > 122 > 33.8 *) Only fragments of the table are shown. Layer-by-layer growth. It is assumed in approach (1) that the molecules fill coordination spheres in turn. The plot of the energy of cluster formation versus the number of S8 molecules is shown in Fig. 8 (curves 1). For comparison, Fig. 8 shows averaged plots of A(n) for random addition (2) (curves 2); (a) and (b) correspond to different ∆μ. One can see that addition to coordination spheres or a nearly spherical shape of nucleus is more profitable from the viewpoint of passing over the energy barrier during nucleation. Fig. 9 shows the plots of A(n) for the addition of molecules to coordination spheres for different supersaturations. The number of energy barriers to be overcome can vary depending on supersaturation. The size of a critical nucleus remains constant within a wide supersaturation range (Table 1). The broadest range of supersaturation corresponds to the nucleus containing 50 molecules of S8. At a lower supersaturation, a sharp increase of the size up to 119 molecules is observed. A nucleus of amorphous sulfur containing 50 S8 molecules is 2.6 nm in diameter [5, 15]. The layer-by-layer addition of S8 ring molecules is more beneficial than the random addition. The nucleus has a near-spherical shape.

3. APPLICATION OF SUBMICRONIC PARTICLES OF SULFUR

Accumulation of sulfur at the enterprises of the petroleum and gas complex, and also such properties of sulfur as water repellency, bacterial action, low toxicity, etc., make this substance useful to practical applications. Scopes of sulfur can be expanded

3-18 by its obtaining in ultra disperse condition by the methods of mass nucleation from water solutions. In this Section the solutions of the polysulphides of alkaline earth = metals is applied. The way is based on property of S4 ion chip of sulfur atoms at pH reduction. It was established, that in process of solutions dilute of polysulphides there is a formation and growth polydisperse sulfur particles from 10 nm up to 300 nm. On this basis were developed effective hydrophobia compositions for building materials (concrete, brick etc.), allowed essentially to lower (in 5–7 times) water absorption, to increase on 40–70% mechanical durability and in 1.5–2 times frost-resistance. Researches have shown, that during impregnation sulfur in the molecular form in structure of the solution of calcium polysulfide (due to low viscosity) gets in the smallest pore materials, and at drying a solution on internal walls of pore (Figs. 10, 11) generates ultra disperse and waterproof layer of sulfur with high adhesion to a basis, interfering penetration of a moisture. After drainage the covering is not dissolved in water and the majority of other liquids, stable in relation to a number of aggressive liquid environments. Impregnation of different samples was carried out by their immersing in the bath with the solution and quotation during certain time in the conditions of atmospheric pressure and room temperature.

Fig. 10. The structure of porous building Fig. 11. The structure of treated by sulfur porous materials. building materials.

Fig. 12. The comparative indexes of concrete rock and concrete sand rock samples on: (a) - water absorption; (b) - strength to compression.

3-19 (a) Water absorption, in volume %

15 Untreated

10 Treated 5

0 Concrete rock Concrete sand rock

(b) Strength to compression, MPa 90 80 70 60 50 Untreated 40 30 20 Treated 10 0 Concrete rock Concrete sand rock

Comparative characteristics (water absorptions and strength) for the initial and impregnated samples within 2 hours from concrete rock and concrete sand rock solution are given durability on Fig. 12(а, b). Similar parameters are available for samples of the autoclave gas concrete. For wall materials the important parameter from the point of view of accumulation of the moisture in volume of a material and the subsequent evacuation on the mechanism of drainage is speed of water absorption within 1-3 hours from the beginning of influence. Change of sizes of water absorption for concrete, brick and autoclave gas concrete in conditions of frontal influence of water also were in details investigated (there are only quantitative differences from Fig. 12). The results given on Figs. 10–12 specify efficiency of the way of hydrophobia method of impregnation of materials with the solution on the basis of sulfur. Advantage of the offered method also is the opportunity of regulation of depth and a degree of impregnation of a material, changing frequency rate and duration of processing and density of a solution. Finally efficiency of the offered method hydrophobia building materials is provided with generation ultra disperse particles of sulfur in pore space in conditions of pH change. Tests of the composition containing of calcium polysulphide as means of protection of plants from fungoid diseases, have shown its essential advantages above

3-20 similar preparations (ground sulfur, etc.), caused by formation on the surface of a plants ultra disperse and it is very good hold of the sulfur layer.

4. MATERIALS ON THE BASIS OF SULFUR NANOPARTICLES IN POLYMERIC MATRIX

In the Sections 1 and 2 we determined the most probable form (sphere) and diameter (~2.6 nm) of critical amorphous germs of sulfur in the acidified solutions of sodium thiosulphate. Further this method of obtaining monodisperse spherulites of sulfur was used for production of composites on the basis of water-soluble polymers. On the basis of this approach, nanocomposites containing particles of sulfur in a matrix of the water-soluble polymers containing surfactants (surface-active agents, SAA) were produced by known methods [6]. However, there was a disadvantage. During drying the produced samples of nanocomposite salts of sodium were formed. It demands development of special methods of clearing of a composite from the salt ballast. In this Section, other ways of creation of the systems containing elementary sulfur in nanosized state, which do not demand making of the additional procedures of clearing of samples, is submitted. The lead concerned with the restriction of the growth of germs of sulfur formed as a result of their injection in the polymeric matrixes, including the matrixes containing surfactants was of interest for us. Thus such systems, which due to high supersaturation conditions for the predominant formation of germs of sulfur would be created, and not the ones for their growth in the course of process are only considered. The following systems which do not contain and do not form the ballast solid substances (salts and other compounds) which could participate in the formation of the nanocomposites and do not result in uncontrollable changes of its properties, are investigated: (1) Water systems where sulfur was injected as ions S= (hydrogen sulfide); (2) The same systems with the photochemical initiation of formation of elementary sulfur; (3) Solutions of sulfur in organic solvents beyond all bounds mixable with water and at the same time are dissolvent for used water-soluble polymers; (4) The systems formed at drying of an organic solution of sulfur, polymer and surfactant. In the preparation of composites, the following were applied: polyvinyl alcohol (PVA) from ZakŁady Chemiczne (Poland); elementary sulfur and acetone of high pure grade; paraffin, dimethyl sulphoxide (DMSO) and perchlorvinyl resin (PCVR) chemically pure and pure grade from Reachim (Russia); deionized water; and surfactants. Samples for research were prepared by mixing solutions of starting components with the helpof low speed mixers or ultrasound (the device UZDN-A from Sumy electronic work, Ukraine). For obtaining polymeric films, containing particles of sulfur, the produced mixtures were dried up on various polished substrates: microscope slides, polymethylmetacrylate, Teflon, polyethylenterephtalate, etc. A number of samples were produced on irradiation of sulfur-containing films by a light flux of xenon lamp with specific capacity 134 J/cm2∙s [16]. Produced composites (some cleavage surfaces of our composite films) are also investigated by the optical and scanning electron (SEM, LEO-1430 VP, cleavage planes were covered preliminarily with gold by evaporation in vacuo) microscopy methods.

3-21 Water-polymeric systems. For making nanoparticles of sulfur water solutions PVA (5 wt%), surfactant (neonol AF-12, ~0.05%) and hydrogen sulfide (~0.7%) were used. Solution of H2S was made by interaction of pure grade sulfur with paraffin at 170−200°C on passing the formed gas through water at 0°C within 30 min. On pouring together solutions of PVA, surfactant and H2S opalescence solutions formed (for acceleration of reaction of decomposition of H2S in the system H2O2 and HCl were added). On heating the solutions, the opalescence disappears and SO2 evolves. Studying of films, produced from solutions on the polished surface of polymethylmetacrylate has shown the presence of large (>10 μm) needle crystals of sulfur (crystal optics of sulfur is described well [9]). As in these systems it is not possible to inhibit the process of growth and crystallization of germs of sulfur, their studying with the use of high-energy influences is of interest. Photochemical formation of sulfur particles. One such way is the influence by the powerful unfiltered stream of radiation of xenon lamp with a power of 5 kJ/s on the polymeric matrix containing unstable substances on theirs decay nanoparticles of sulfur could be formed. We investigated samples (transparent film having size 10×1×0.1 mm), obtained in system PVA-SAA-H2S. Time of an exposition (6 s) of the beginnings of fusion of the polymer appropriate to temperature 230°C was experimentally determined. The further irradiation of the films was made at smaller expositions. The influence of hermetic sealing of polymer on process of formation of particles of sulfur was also investigated. A film was armored with the diluted solution of glue BF-2 in isopropyl alcohol with subsequent drying. Unfortunately these samples were unstable when observed in SEM. Some fragmentation of cleavage plane took place. Therefore, the direction of researches with the use of high-energy influences on the hermetically sealed systems is perspective [17]. Samples which use teragertz radiations of the laser on free electrons, created in the Siberian Branch of the RAS are now being investigated. Water-organic systems. Research of such systems was complicated with absence of information in the literature on the solvents having the required properties. It was known that PVA also dissolved well, except in water, in DMSO, which beyond all bounds mixes with water. We carried out studying solubility of sulfur and its positive temperature dependence (upto 100°C) in DMSO (the volatility of solvent thus is insignificant as the boiling temperature of DMSO is 189°C). Further, systems where all production solutions used before by us were water-soluble polymers and SAA (anionic surfactant – dodecylsulphates [17]), and also solutions of sulfur in DMSO, saturated both at room temperature (~20°C) and at 100°C were investigated. On pouring the solutions together, the biphase system is formed - on the top of the system there is an opalescence solution, and on the bottom there is a white gel. For the preparation of nanocomposite, the top layer was taken. Some cleavage surfaces of these composite films were investigated on SEM. At first, we find parts of the films with an optical microscope where luminous point in dark field illumination can be mainly seen [17]. At these parts, we cleave films with razor blade. Then we have deposition of gold on our cleavage planes. One of the typical images received on SEM is in Fig. 13. At first it is interesting that we can see a network formed with cracks of gold film. Evidently we have a decoration of boundary of grains (having in mind partial crystallinity of PVA). Then we can see particles of sulfur which are associated with boundary grains. The smallest one has a round shape. Their size ranges from 40 to 100 nm. It is known that boundary grains are a good place for adsorption of any molecules and for surfactants too. That is why we have the

3-22 favorable conditions for formation of nanoparticles at these places where evidently the concentration of surfactant is sufficient to prevent the growth of crystal.

Fig. 13. SEM image of the cleavage plane of nanocomposite film obtained in system H2O–S–DMSO–PVA–SAA.

Organic systems. The obvious interest presents the nanocomposites which can be used as a porous material after selective dissolution of one of the components. For sulfur-containing composites it can be an application of polymer, which is not soluble in the solvents taken out of the sulfur system (toluene, etc [17]) and having with sulfur the common organic solvent. Such a polymer is polyperchlorvinyl, well soluble in acetone. The practical importance of such a system arises because of the perchlorvinyl pitch which is an optimum matrix for particles of sulfur as it is not dissolved in aromatic hydrocarbons that is important for making experiments with selective dissolution of sulfur from a polymeric matrix. Solutions of sulfur in acetone, saturated at room temperature, were used. For removing the parasitic centres of crystallization the solution was heated up with a return condenser for 2 h at 50°C (below a boiling point of acetone) and 5% was diluted with acetone. The obtained solution mixed with the solutions of perchlorvinyl and neonol (AF-12) in acetone. On mixing slightly opalescent solutions were produced. A film was prepared on a surface of microscope slides for optical microscopy whose studies have shown the presence of pyramidal objects with optical anisotropy (1-5 μm), characteristic for crystals of sulfur, and formations as luminous points, distinct only with dark field condenser [17], as well as in case of systems on the basis of solutions of sulfur in DMSO.

CONCLUSION

3-23 Various ways of production of nanosize particles of sulfur in polymeric matrixes are investigated and compared. The method of optical microscopy of the produced samples establishes the presence of nanostructures in them. For manufacturing nanosystems not containing large crystals of sulfur creation of higher supersaturations or application of other nonconventional methods is necessary. A very perspective use of high-energy influences is presented, see also methods of mechanochemistry [18, 19]. This work was supported by the Integration grant of the SB RAS # 11.

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