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Energy & Fuels 2001, 15, 930-935
Averaged Structural Units in Bituminous Coals Studied by Means of Ultrasonic Wave Velocity Measurements Marta Krzesin´ska* Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-121 Gliwice, Poland Received January 19, 2001. Revised Manuscript Received May 16, 2001
The measurements of density and velocity of ultrasonic waves at frequency of 2.5 MHz were made for coals of various rank (81.6-89.65 wt % of carbon). The ultrasonic suspension method was used for the ultrasonic wave velocity measurements. The simple empirical relation between ultrasonic velocity, density, and the mean atomic weight found by Pinnov for a wide range of crystalline materials was applied for coals. It was found that the value of log(density/velocity) can be interpreted as a measure of molecular weight of averaged structural unit. The effect of HCl-demineralization procedure on the size of averaged structural units was also discussed.
Introduction Many structures have been proposed for bituminous coals.1 It is generally accepted that coals are macromolecular solids, although details of their network architecture are still lacking. Discussing the probability of the polymeric nature of coal, van Krevelen stated1 that “Since the average molecular mass of coals is not known (and perhaps nondeterminable) it is necessary to look for another basis of the molar functions. For linear polymers this is easy: one has to use the repeating polymeric unit. In coal, however, we do not know if repeating units actually exist”. Nevertheless, a polymeric structure of coal was proposed by numerous investigators, e.g., Larsen,2,3 van Krevelen,1,4 Shapiro and Alterman,5,6 Kasatochkin,7 et al. As a result of many studies on coal structure there is a generally accepted opinion that coal is a three-dimensional covalent crosslinked macromolecular network of aromatic and hydroaromatic clusters with cross-linking attributed to covalent bonds (etheric, methylenic, and sulfides), to hydrogen bonds or to entanglements between macromolecules. Such structures could arise from very large molecules which are inextricably entangled or from covalent cross-linked systems.8-17 * Fax: +48 (032) 231-28-31. E-mail:
[email protected]. (1) van Krevelen, D. W. In COAL Typology-Physics-ChemistryConstitution; Elsevier: Amsterdam, 1993; pp 345, 350, 570. (2) Larsen, J. W.; Kovac, J. Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symp. Series No. 71, Washington, DC, 1978; pp 36-49. (3) Kovac, J.; Larsen, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1977, 22, 181. (4) Van Krevelen, D. W.; Schors, A.; Bos, H.; Groenevege, M. P.; Westrik. R. Fuel 1956, 35, 230-243. (5) Shapiro, M. D.; Alterman, L. S. Chim. Tverdogo Topliva 1968, 4, 60-70. (6) Shapiro, M. D.; Alterman, L. S. Chim. Tverdogo Topliva 1971, 3, 12-17. (7) Kasatochkin, V. J. Chim. Tverdogo Topliva 1969, 4, 33-48. (8) Larsen, J. W. In Chemistry and Physics of Coal Utilization; Cooper, B. R., Petrakis, L., Eds.; Am. Inst. Phys.: New York, 1981; Chapter 1. (9) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 199-282. (10) Brenner, D. Fuel 1984, 63, 1324-1328.
Figure 1a,b shows two polymeric models of coal, i.e., the model of the polymeric structure of vitrinite, proposed by Kasatochkin7 and the two-phase model of low rank bituminous coals proposed by Marzec13 (see also ref 16). According to Kasatochkin, coal is a threedimensional polymer of irregular structure. The author distinguishes centers composed of polycyclic condensed aromatic rings connected with long aliphatic chains. These chains contain radicals and functional groups. Size of centers (number of aromatic rings) depends on coal rank. Kasatochkin is of the opinion that coal substance differs from typical polymers consisting of structural units of type of monomers in that coal structure is characteristic of irregularity of structural units in respect to both dimension of condensed aromatic center and length and composition of chains. It is correct to characterize only an averaged structural unit (ASU) and to describe only a general basic scheme of coal polymer structure. According to Marzec,13 coal organic matter consists of a three-dimensional, cross-linked macromolecular network and a complex mixture of relatively small moleculessmolecular phase. The components of molecular phase are assumed to be clathrated in pores of a macromolecular network by means of electron-donor-acceptor interactions. Some parts of the molecular phase may be not extractable due to restricted sizes of pores in a macromolecular network. Crosslinkages are supposed to be formed not only by single covalent bonds such as methylene bridges, ether, and thioether bonds, but also by larger fragments. Both models of polymeric coal, i.e., with compact ASU (Kasatochkin’s model) and less compact ASU, i.e., (11) Weller, M.; Wert, C. Fuel 1984, 63, 891-896. (12) Brenner, D. Fuel 1985, 64, 167-173. (13) Marzec. A. Fuel Process. Technol. 1986, 14, 39-46. (14) Gorbaty, M. L.; Mraw, S. C.; Gethner, J. S.; Brenner, D. Fuel Process. Technol. 1986, 12, 31-49. (15) Mackinnon, A. J.; Hall, P. J. Fuel 1996, 75, 85-88. (16) Jasien´ko, S. In Chemistry and Physics of Coal; OW Pl. Wr.: Wrocłw, 1995; Chapter 6. (17) Sanada, Y. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2000, 45, 216-220.
10.1021/ef0100101 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
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as a disturbance propagating in an elastic medium consisting in the transfer of energy by vibrated particles without change of their mean position.18 This disturbance produces instantaneous changes of the medium density. Every macroscopic body can be considered as a discrete arrangement consisted of many independent small bodies (material points) which act on each other by means of elastic forces. The simple model of such a body is presented by the arrangement consisting of the same masses connected with the springs having the same elasticity coefficients. Propagation of acoustic wave in the medium, e.g., velocity, is conditioned by two physical properties of the medium: its inertia represented by the mass of molecules and its elasticity arising as a result of the action of intermolecular forces. A qualitative understanding of the dependence of velocity of ultrasonic wave on microscopic parameters can be obtained by using a simple model of a linear array of mass points M with an equilibrium spacing a connected by equal springs of constant C. Accounting only for the nearest neighbor interactions, one finds the ultrasonic velocity as19
v ) (Ca2/M)1/2
Figure 1. Models of coal structure: (a) model of polymeric structure of vitrinite, according to Kasatochkin;7,16 (b) the twophase model of low rank bituminous coals, according to Marzec:13,16 shaded areassmacromolecules MM; unshaded areas with M insidesmolecules; (-b-b-) denotes crosslinkages between macromolecules; (- - -) denotes electrondonor-acceptor (EDA) interactions between molecules and macromolecular network. Aliphatic chains or cross-links connecting two neighboring ASU or macromolecular fragments can be considered as springs.
consisting of macromolecular fragments connected with cross-links (Marzec’s model) were chosen for further consideration. Information about the nature of the macromolecular network of coal can be achieved by studying mechanical properties and such features as the degree of swelling by solvents. Bulk coal is heterogeneous and imperfect so the bulk mechanical properties are often determined by macroscopic imperfections such as cracks parallel to the bedding plane. The mechanical properties can provide important clues to the molecular structure but only if one can measure the properties of the basic coal substance, the material between the cracks. The ultrasonic suspension method is a method which allows studying coals without cracks and macroscopic imperfections. Dynamic elastic modulus is determined from the measurements of the velocity of ultrasonic wave and density. The Empirical Relation between Ultrasonic Velocity and the Mean Atomic Weight for a Crystalline Substance. An ultrasonic wave can be considered
(1)
It is apparent from eq 1 that if a and C remain constant, the effect of increasing M is to decrease v, and this trend is found, in fact, to be qualitatively correct. Considerable information is available on the acoustic (ultrasonic) velocity in a wide variety of substances.20,21 From these data it was recognized, for example, for alkali halides, oxides, as well as groups IV, III-V, and II-VI compounds that the velocity is closely related to density and mean atomic weight. In dealing with the longitudinal velocity in a crystalline substance, it is convenient to consider only an average (scalar) velocity that would be observed if the material was a fine-grain polycrystalline aggregate. This scalar velocity can then be directly related to the scalar quantities of density and to the mean atomic weight. On the basis of the above statements, Pinnov21 found for a wide range of materials an empirical relation between ultrasonic velocity v, density F, and the mean atomic weight M h:
log(v/F) ) -b M h +d
(2)
where b and d are experimentally determined constants for a given type of a crystalline substance. The M h is the mean atomic weight defined as the total molecular weight divided by the number of atoms per molecule. As in a simple model of a linear array of mass points, an increase in the mean atomic weight M h generally changes the interatomic spacing as well as the effective spring constant that results in the change of the value of the ultrasonic velocity. Crystalline materials used in the Pinnov study were simple three-dimensional (18) Malecki, J. In Theory of Waves and Acoustic Systems (in Polish); PWN: Warsaw, 1964; p 32. (19) Kittel, C. In Introduction to Solid State Physics, 3rd ed., (translation to Polish); PWN: Warsaw, 1974; pp 106, 159. (20) Anderson, O. L. In Physical Acoustics; Mason, W. P., Ed.; Academic Press: New York, 1965; Vol. 3, Part B, pp 43-95. (21) Pinnov, D. A. IEEE J. Quantum Electronics 1970, QE-6, 223238.
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Krzesin´ ska
Table 1: Elementary Analysis and Maceral Content of Studied Coals Expressed in wt %a mineral matter (wt %) sample no.
source
Cdaf
WK-17 23 VI 21 2 17 5 7 6 1 13
Miechowice Sos´nica Nowa Ruda Moszczenica Nowa Ruda Thorez Wałbrzych3 Victoria Wałbrzych3 Victoria Victoria
81.6 83.66 84.89 85.63 85.99 86.78 88.20 88.75 88.96 89.25 89.65
a
VMdaf
ash
V
E
I
MM
32.3 32.28 21.15 24.21 20.79 20.55 13.28 12.35 14.09 7.35 6.87
7.0 6.68 18.01 3.53 10.93 8.07 4.74 6.12 6.01 6.39 6.73
47.4 56 93.1 50 70.0 83.3 88.5 81 79 82 67.8
10.8 10 0.2 10 3.0 1.9 0 1 1 0
40.2 34 6.6 40 26.0 11.6 7.7 18 20 18 20.2
1.6
clay
pyrite
carbonates
1.0 3.2 3.8
0.6 3.2 3.2
0.2 0 0.2
0.2 0 0.4
12.0
10.8
1.0
0.2
Quoted from refs 22 and 23.
networks consisting of material points ) atoms connected with bonds, which were considered as springs. According to Figure 1a,b, coal is characterized by the more complicated network consisting of two subnetworks. First, with material points being the averaged structure units (first-order structure) connected with long aliphatic chains (or cross-links) and, the second, centers built by aromatic rings or macromolecular fragments, containing atoms connected by strong covalent bonds (second-order structure). We believe that the first-order structure is more important for ultrasonic wave propagation, because of the more elastic bonds ) springs between material points, i.e., between ASU. Sanada17 suggested that the mechanical deformation occurring during hardness measurements is mostly governed by noncovalent bonding in the coal structure. Structure of crystalline substances is simpler than that of macromolecular coal, but it is possible to find a similarity between them if they are treated as arrangements of material bodies with the same averaged mass M h connected with the same springs, i.e., chemical bonds. For a crystalline substance, M h is a mean atomic weight of material points between crystalline bonds. For coal, M h ≡ M h ASU is a mean molecular weight of averaged structural unit between aliphatic chains (Figure 1a) or a mean molecular weight of molecular fragment between two adjacent cross-linking bonds or entanglements (Figure 1b). If we transform the fraction in eq 2, we will obtain M h ASU proportional to log(F/v):
M h ASU ) A log(F/v) + B
(3)
where A and B are constants equal to 1/b and d/b, respectively. The greater the log(F/v), the greater the mean molecular weight of ASU. Mass of the center composed of aromatic rings (Kasatochkin’s model) increases with increasing carbon content, while the coal rank dependence of mass of molecular fragment between cross-links (Marzec’s model) is characterized with maximum for C ≈ 87 wt %.1,8,17 Figure 2 (quoted from ref 17) shows the molecular weight per cross-linked unit Mc of coal estimated by applying the equation of FloryRehner with the results of the investigations of the swelling equilibrium of coal by pyridine at 25.0 °C. The data obtained by Sanada show that the value of Mc of coal over the range 65 to 80% carbon content is almost constant. For bituminous coal, Mc increases suddenly with rank to a maximum at about 85%, then decreases with increase of rank.
Figure 2. Relation between molecular weight per cross-linked unit and rank of coal, quoted from ref 17. The results of Sanada are different in comparison with the data of his previous original paper. They were corrected by the separate experiments, because the original paper contained miss calculations.
The aim of the present study was to apply the Pinnov’s formula to coals, i.e., to determine the value of log(density/velocity) for bituminous coals of rank within the range of 81.6 to 89.65 wt % and relate to the other properties, and to verify whether this logarithm may be interpreted as a molecular weight of averaged structural unit in coals. Experimental Section Eleven samples with carbon content within the range of about 82 to 90% collected from the mines situated in the Lower-Silesian Coal-Field and the Upper Silesian Coal-Field (Poland) were used in the study. The parameters of elementary analysis and petrographic content in (wt %) of these samples22,23 are presented in Table 1. Density and velocity of ultrasonic waves were determined for eleven raw coals and for four demineralized coals (samples 2, 5, 13, and 17). The demineralization procedure consisted of a treatment of coal samples with concentrated hydrochloric acid. The measurements of density were made using a helium pycnometer “Micromeritics” (U.S.A.). The velocities of ultrasonic waves of f ) 2.5 MHz in coal particles were determined by suspension method.24-27 Application of a method developed for suspensions of a dispersed phase of one material in a continuous second phase for coals was described in our previous papers.22,28-30 Coal samples of the same concentration, 3 wt %, were prepared as suspensions in glycol using an ultrasonic disintegrator for about 15 min. The measurements of the ultrasonic velocities (22) Krzesin´ska, M. Energy Fuels 1997, 11, 686-690. (23) Krzesin´ska, M. Int. J. Rock Mech. Min. Sci. 1997, 34, 167171. (24) Urick, R. J. J. Appl. Phys. 1947, 18, 983-987. (25) Krzesin´ska, M. Appl. Acoust. 1998, 55, 329-343. (26) Berdowski, J.; Krzesin´ska, M. Proc. Conf. GDRE Materiaux Carbones Fonctionnalises a Porosite Controlee (Zakopane, Poland) 1998; pp 94-98. (27) Krzesin´ska, M. Pol. J. Appl. Chem. 2000, 44, 127-138.
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Energy & Fuels, Vol. 15, No. 4, 2001 933
in pure liquid, i.e., glycol and coal suspensions were performed in the temperature range of 15-40 °C with SW-08 apparatus (TPNZ, Poland) based on the “sing-around” pulse method. This ultrasonic technique used for investigations of liquid coal products has been described in our previous papers.31-33 The ultrasonic velocities in glycol and suspensions at t ) 20 °C were calculated from the experimental dependencies of velocity on temperature. The ultrasonic wave velocities in coal particles v were calculated according to equation presented in refs 22 and 25. All samples were ground in an agate ball mill to obtain the size of particles of about 10 µm to fulfill the requirements of suspension method. All measurements, i.e., those of density, velocity, as well as demineralization procedure, were made using this granulation. The size of coal grains used is important, because the solvent action and the true density of coal after solvent action depend on the particle size.34
Results and Discussion Our recent papers22,28-30 reveal that the dynamic elastic moduli of studied coals, determined at the room temperature using the velocities of ultrasonic waves with the frequency of 2.5 MHz were contained within the range of 1-10 GPa. The analogous values measured for polymers are characteristic for their glassy state.35-38 In the glassy state every macromolecule is tied up in the stiff matrix formed with other molecules and practically, it is fixed. Similarly, as in a solid crystalline body, the thermal motions of atoms or fragments of molecules in glassy polymer are reduced to vibrations. Amplitude and dimension of vibrated fragment of a molecule depend on the temperature.39 The application of the empirical formula of Pinnov (originally developed for crystalline materials) to coals considered as polymers in the glassy state, seems to be valid. For crystalline substances studied by Pinnov,21 values of v/F were within the range of 0.19-5.15 in units of [kg s m-4], while for the studied coal samples they changed from 1.4 to 1.8, being within the above range. Figure 3 shows the plot of log(F/v) vs coal rank for raw, undemineralized samples. As it was shown in Table 1 samples differed in respect to ash content, i.e., to mineral matter content. Components of mineral matter are characterized by considerably higher densities, i.e., 2.5-5 g/cm3 in comparison with those for bituminous coals (1.2-1.4 g/cm3). Thus, undemineralized samples containing the higher concentration of mineral matter (or ash) were characterized by the more over high density than those with the smaller mineral (28) Krzesin´ska, M.; Pusz, S. Proc. 1997 Int. Conf. Coal Sci. 1997, 1, 465-468. (29) Pusz, S.; Nakonieczna, G.; Krzesin´ska, M.; Wanat, K. KarboEnergochem.-Ekol. 1994, 39, 3-7. (30) Krzesin´ska, M. Energy Fuels 2001, 15, 324-330. (31) Krzesin´ska, M. Fuel 1996, 75, 1267-1270. (32) Krzesin´ska, M. Fuel 1998, 77, 649-653. (33) Krzesin´ska, M. Fuel 2000, 79, 1907-1912. (34) Krzesin´ska, M. Erdo¨ l Erdgas Kohle 1998, 114, 388-389. (35) Perepechko, I. In Acoustic methods of investigating polymers (translated from the Russian); Mir Publishers: Moscow, 1975; pp 1418. (36) Perepechko, I. I. An Introduction to Polymer Physics; Mir Publishers: Moscow, 1981; Chapter 7. (37) Holliday, L. In Structure and Properties of Oriented Polymers; Ward, I. M., Ed.; Applied Science Publishers: London, 1975; pp 242246. (38) Treloar, L. R. G. In The Physics of Rubber Elasticity; Clarendon Press: Oxford, 1975; pp 142-143, 146. (39) Galina, H. In Physical Chemistry of Polymers; OW Pl. Rz.: Rzeszo´w, 1998; pp 104-105.
Figure 3. The plot of log(F/v) [kg s m-4] vs coal rank. The true density was corrected for ash content.
Figure 4. The plot of log(F/v) [kg s m-4] vs coal rank for samples: 2, 5, 13, and 17; (b) raw coals, (0) demineralized coals. The true density was uncorrected for ash content.
matter content. The mandatory correction for mineral matter, which converts the measured density to the density of the pure coal substance was made by means of equation presented in ref 1. Figure 3 shows that the maximum of the dependence of log(F/v) ≈ M h ASU on the carbon content occurs for C = 87-88%. Position of a maximum is almost the same as that of coal rank dependencies of molecular weight per cross-linked unit M h c determined using Flory-Rehner formula,39 presented by Larsen8 and Sanada.17 It can be seen from Figure 3, that in respect to the shape, the dependence of log(F/v) ≈M h ASU on the carbon content is also similar to those published by Larsen and Sanada. The shape of the plot of the log(F/v) versus coal rank suggests that it can be interpreted more as proportional to molecular weight per cross-linked unit than as proportional to molecular weight of Kasatochkin’s averaged structural unit, which increases with increasing carbon content. Figure 4 shows the plot of log(F/v) vs coal rank for raw and HCl-demineralized coals No. 2, 5, 13, and 17. It can be seen from Figure 4 that values of logarithm for deminearlized coals are considerably smaller than those obtained for raw coals. It suggests that demineralized samples contain smaller averaged structural units or that they are more cross-linked than raw, undemineralized samples.
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Figure 6. The plot of swelling ratio Q in MeOH, THF, EtDa, PY vs log(F/v) [kg s m-4] for coal No. 17; (b) - raw samples, (0) - demineralized samples.
Figure 5. The plot of the difference of [log(F/v)raw log(F/v)dem] [kg s m-4] vs coal rank for samples: 2, 5, 13, and 17.
Figure 5 shows the plot of the difference of [log(F/v)raw - log(F/v)dem] vs coal rank for samples No. 2, 5, 13, and 17. This difference can be considered as referred the effect of the demineralization process to mean molecular weight of unit through the effect on the log(F/v). It is clear that the greatest influence of demineralization on coal structure occurs for coals with carbon content about 87%. These coals are the lowest cross-linked.1,8,17 Some of the investigators tried to explain the effect of acids treatment on the cross-linking degree of coal. Milligan et al.40 suggested that acid washing removes the interacting cations (Ca2+, Na+, etc.), which may then allow the functional groups such as -COOH and -OH to form coal-coal bonding upon drying, effectively increasing the noncovalent cross-link density. Larsen et al.41 found that the demineralization significantly affects the dilatometric properties of coals. Residual acid in the coal may induce polymerization on heating, resulting in a loss of plastic properties and a decrease in dilation. Pearce and Hill42 stated that chlorine has been deposited in coal after the rank has been established (epigenesis) via hyper-saline strata waters. They found that chlorine is present in coal in one bonding form predominantly. The chlorine-coal bond is not covalent but is strongly ionic. The coal substance provides some holding mechanism for the chlorinated solvent. It is probable that chlorine from HCl acid also creates the same bonds increasing crosslinking degree in demineralized coals. For coal No. 17 the values of log(F/v) were determined also for samples subjected to an action of several solvents, i.e., methanol (MeOH), tetrahydrofuran (THF), pyridine (PY), and ethylenediamine (EtDa). It was found that values of log(F/v) for pyridine-, tetrahydrofuran-, and methanol-treated samples were greater for raw than those for demineralized coal. Thus, mean molecular weights between cross-links of samples obtained (40) Milligan, J. B.; Thomas, K. M.; Crelling, J. C. Energy Fuels 1997, 11, 364-371. (41) Larsen, J. W.; Pan, C.-S.; Shawver, S. Energy Fuels 1989, 3, 557-561. (42) Pearce, W. C.; Hill, J. W. F. Prog. Energy Combust. Bull. 1986, 12, 117-162.
from demineralized, then solvent-treated coal sample were smaller than for samples obtained from raw coal. It suggests that demineralized, then solvent-treated coal samples seem to be more cross-linked in comparison with the samples from undemineralized coal. The greatest magnitude of log(F/v) occurs for pyridine. According to eq 3 it means that mean molecular weight between cross-links is the greatest for pyridine-treated both raw and demineralized coal samples. According to ref 40 a strongly basic solvent such as pyridine disrupts a greater number of the cross-linking coal-coal hydrogen bonds followed by reorientation of the macromolecular clusters, resulting in the swelling of the coal. Hence, the macromolecular clusters gain a possibility of a higher mobility by removal of the hydrogen bonds. Thus, pyridine creates the greatest swelling, the greatest MASU, and consequently, the smallest elastic dynamic modulus of coal.30 Figure 6 shows the plot of swelling ratio Q vs log(F/v). It is known for natural rubber that the greater degree of cross-linking, the lower equilibrium swelling ratio.38 It means that the more cross-linked polymers exhibit worse swelling properties. It can be seen from Figure 6 that the greater log(F/v), i.e., the greater mean molecular mass of structural unit (i.e., the lower cross-linking degree), the higher is the swelling. Miligan et al.40 stated that for coals the low swelling ratios suggest higher apparent covalent cross-link densities. The data presented in Figure 6 show that swollen coals exhibit properties of cross-linked polymers. The results of the presented study are consistent with the studies of elastic moduli for raw and demineralized samples described in our previous papers.22,28,30 Elastic properties of HCl-demineralized coals differed considerably in comparison with those obtained for raw coals. Milligan et al.40 suggested that the demineralization process may modify the interactions within the coal structure; thus, swelling ratios and other parameters, including elastic modulus, obtained on demineralized samples should be viewed in this context. The value of log(F/v) is proportional to the mean molecular weight of the structural unit of coal. What is this structural unit: averaged structural unit of Kasatochkin’s model increasing with the carbon content or portion between cross-links showing maximum of the coal rank dependence for C = 87%? Our results show that the logarithm of (density/velocity) seems to be wellrelated to cross-linking degree.
Averaged Structural Units in Bituminous Coals
Conclusions It was found for raw, demineralized, and solventtreated coal that: 1. The value of log(density/velocity) is well-related to the swelling ratio and elastic properties of solventtreated undemineralized and HCl-demineralized coal and can be interpreted as a measure of mean molecular weight of portion between cross-links.
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2. Demineralized coal seems to be more cross-linked than undemineralized coal. Acknowledgment. The author gratefully acknowledges Mrs. Marianna Jasin´ska for fruitful laboratory cooperation. EF0100101
