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)
where τλ(t) is the spectral bulk extinction factor of the medium [cm-1]; 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
medium [cm]; λ0 is the wavelength in air (λ01 = 0.6328×10-4 and λ02 = 0.4880×10-4 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
temperature of the medium; Rλ(d) is the scattering cross section [cm2]; 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 LGN302 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:
t* - induction period.
curve 1 - 30°C; 2 - 45°C; 3
concentration Csm =
2.7 mg/100 ml.
- 50°C.
3-13
When calculating RSCS εc(d) for the size of d particles to 0.5×10-4, 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
η(λ1, 17.5°C) = 1.8873, η(λ2, 17.5°C) = 1.9734, ∆η / ∆T ≈ −0.00033 °C-1.
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
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).
3-14
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
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,
whereas sulfur concentration in the critical point of supersaturation CS* ≈ 5.5×10-6 gatom / 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
CS* = 3.2×10-6 g-atom / l and C of saturated solution of sulphur in the water
CS* ≈ (1.2±0.1)×10-6 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
3-15
Csm = 2.7 mg / 100 ml) of growing nuclei N ≈ 7×1012 cm-3. 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
The current interest in monodispersed colloidal spheres is caused by their
applicability to the fabrication of photon crystals, computer chips and threedimensional 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.
3-16
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
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.
3-17
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
(a) ∆μ = 0.45; (b) ∆μ = 0.55. (1) layer-by-
10 different ways for the random addition
layer addition; (2) random addition of S8
of S8 ring molecules.
molecules.
a)
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
b)
add to coordination spheres.
Table 1. Parameters of critical nucleus at layer-by-layer and random additions of S8
molecules.
3-18
Layer-by-layer addition
∆μ, arb. un.
number S8, n
A, arb. un.
> 0.95
2
< 1.47
0.95-0.61
11
1.47-5.21
0.60-0.55
24
5.45-6.65
0.54-0.50
46
6.92-8.76
0.49-0.38
50
9.24-14.7
0.37-0.24
119
15.4-30.9
≤ 0.23
≥ 148
≥ 32.3
*) Only fragments of the table are shown.
∆μ, arb. un.
0.99-0.95
0.93-0.91
0.76-0.73
0.70-0.68
0.67-0.57
0.45-0.37
< 0.32
Random addition *
number S8, n
7
16
27
53
55
76
> 122
A, arb. un.
1.02-1.30
1.56-1.88
4.75-5.56
6.57-7.63
8.18-13.7
22.1-28.2
> 33.8
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
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.
3-19
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.
(a) Water absorption, in volume %
25
20
Untreated
15
10
Treated
5
0
Concrete rock
Concrete sand rock
3-20
(b) Strength to compression, MPa
90
80
70
60
Untreated
50
40
Treated
30
20
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
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.
3-21
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.
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
3-22
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
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.
3-23
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
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
3-24
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.
REFERENCES
1. V.K. La Mer. Ind. Eng. Chem. 1952. Vol. 44. No. 6. P. 1270-1277.
2. C.F. Bohren and D.R. Huffman, Absorption and scattering of light by small
particles, Wiley, N. Y., 1983.
3. H.C. van de Hulst, Light Scattering by small particles, Wiley, Chapman and Hall
Ltd, N. Y. - L., 1957.
4. I. Thormälen, J. Straub, U. Grigull J. Phys. Chem. Ref. Data. 1985. Vol. 14. No. 4.
P. 933-945.
5. F.Kh. Urakaev, L.Sh. Bazarov, I.N. Meshcheryakov, V.V. Feklistov, T.N.
Drebushchak, Yu.P. Savintsev, V.I. Gordeeva and V.S. Shevchenko. Journal of
Crystal Growth. 1999. Vol. 205. Iss. 1-2. P. 223-232.
6. Y. Xia, B. Gates, Y. Yin and Y. Lu. Advanced Materials. 2000. Vol. 12. No. 10.
P. 693-713.
7. S. Auer and D. Frenkel. Nature. 2001. Vol. 409. P. 1020-1023.
8. R. Steudel, T. Göbel and G. Holdt Steudel R., Göbel T., Holdt G. Zeitschr.
Naturforsch. 1988. Vol. 43b. P.203-218.
9. Elemental Sulfur. Chemistry and Physics, ed. B. Meyer, Intersci. Publ., New
York, 1965.
10. R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1983.
11. S.R. Elliot, Physics of Amorphous Materials, Longman, London, 1983.
12. L.Sh. Bazarov, T.N. Drebushchak, V.I. Gordeeva and F.Kh. Urakaev. Journal of
Crystal Growth. 1999. Vol. 206. Iss. 1-2. P. 75-80.
13. S.C. Abrahams. Acta Crystallogr. 1955. Vol.8. No. 11. P. 661-671.
14. H.E.A. Huitema, J.P. van der Erden, J.J.M. Janssen and H. Human. Physical
Review B. 2000. Vol. 62. No. 22. P. 14690-14702.
15. F.Kh. Urakaev, T.N. Drebushchak, Yu.P. Savintsev and V.A. Drebushchak.
Mendeleev Communications. 2003. Vol. 13. No. 1. P.37-39.
16. V.E. Zarko, V.N. Simonenko, A.V. Kiskin. Progress in Astronautics and
Aeronautics (eds. L. De Luca, E.W. Price, M. Summerfield) / Publ. AIAA,
Washington, 1992. Vol. 143. Ch. 10.
17. Yu.P. Savintsev, V.S. Shevchenko and F.Kh. Urakaev. Journal of Crystal Growth.
2005. Vol. 275. Iss. 1-2. P. e2345-e2350.
18. F.Kh Urakaev. Mendeleev Communications. 2005. Vol. 15. No. 3. P. 106-111.
19. F.Kh. Urakaev and V.S. Shevchenko. Russian Journal of Physical Chemistry.
2006. Vol. 80. No. 2. P. 157-163.
3-25