amorphous materials based on calcium phosphatesa maximum amount of amorphized substances is formed...

Vladimir A. Sinyayev, Elena S. Shystikova, Aigul A. Batyrbayeva, Kamalidin O. Sharipov

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AMORPHOUS MATERIALS BASED ON CALCIUM PHOSPHATES

Vladimir A. Sinyayev1, Elena S. Shystikova1, Aigul A. Batyrbayeva2, Kamalidin O. Sharipov3

ABSTRACT

The composition and structure of calcium phosphate substances formed from aqueous solutions as a result of the exchange reactions be tween sodium monohydrogen phosphate and calcium chloride in the presence of the inhibitor of hydroxуapatite crystallization – sodium diphosphate are studied. The behavior and the transformations of the materials under heating up to 1000oС are studied. The optimal conditions of amorphous materials formation are established.

A maximum amount of amorphized substances is formed in case calcium chloride is used together with solutions containing both monohydrogen phosphate and 20 % - 40 mol % of diphosphate at pH 7 - 8.

Keywords: hydroxyapatite, mono-calcium diphosphate, thermal analysis, amorphous materials.

Received 22 February 2018Accepted 20 December 2018

Journal of Chemical Technology and Metallurgy, 54, 3, 2019, 643-649

1 Scientific Center of Anti-Infective Drugs of the Ministry of Industry and New Technologies 71 Al-Farabi str., 050060, Almaty, Republic of Kazakhstan2 Al-Farabi Kazakh National University 71 Al-Farabi str., 050040, Almaty, Republic of Kazakhstan E-mail: [email protected] S.D. Asfendiyarov Kazakh National Medical University Biochemistry department, 94 Tole be str., 050050, Almaty, Republic of Kazakhstan

INTRODUCTION

The phosphoric acid salt of hydroxyapatite, Ca5(PO4)3

OH, is widely used in modern orthopedics and dentistry. Ceramic materials [1, 2], restorative means intended for the treatment of human bone and teeth diseases [3 - 5], carriers of medicines [6 - 8], and many other prepara-tions have been prepared on its basis. But, perhaps, more often than not, hydroxyapatite is used to apply metal implantable prostheses to the surface in order to make them biocompatible with the living tissues [9 - 11]. The crystalline salt is generally used for the purposes pointed above, although the amorphous form of this compound would be preferable because of its greater chemical activity and biodegradability due to its thermodynamic instability. Until recently, amorphous phases are found only in plasma sprayed on the metal surface with hydroxyapatite [12, 13]. But currently there have been evidences of the existence of amorphous hy-

droxyapatite obtained chemically [14-18]. The authors of the latest publications precipitate it from solutions of monophosphate in the presence of substances that inhibit the growth of apatite crystals. The results of the X-ray phase analysis and the neutron scattering spectroscopy serve as an evidence of the amorphous nature. Materi-als based on amorphous apatite are also of interest as a basis of bioceramics.

Calcium phosphate salts are of great interest for modern medicine, in particular for orthopedics and dentistry. Being similar in their chemical composition to natural bone tissues, they are included in the prepara-tions for the treatment of bone diseases, the manufacture of bone implants, as well as to impart biocompatibility to the metal implants and prostheses. There is a need to organize a large-scale production of ma terials based on nanocrystalline hydroxyapatite in the Republic of Kazakhstan, where the significant stocks of apatites are con centrated. The exchange reactions between soluble

Journal of Chemical Technology and Metallurgy, 54, 3, 2019

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monophosphates and calcium salts, which until currently have not been studied, refer to the chemical methods used for hydroxyapatite production.

At present, there is interest in calcium phosphate ma-terials based on nanocrystalline hydroxyapatite, which due to their thermodynamic instability are inclined to accelerated biodegradation and assimilation by a living organism. Amorphous calcium phosphate compounds are similar in their composition to that of natural bone tissues, i.e. hydroxyapatite should serve as an alternative to amorphous hydroxyapatite.

In view of the stated above, the aim of this work is to obtain materials including amorphous hydroxyapa-tite, to identify their nature, to determine the conditions under which they are formed, and to develop a method for their production.

EXPERIMENTAL

Sodium monohydrogen phosphate was synthesized by melting a dibasic acid phosphate, Na2HPO4•12H2O and an appropriate amount of alkali at 1000°C for 1 h in a platinum dish.

Sodium diphosphate was obtained by dehydration of disubstituted sodium phosphate, Na2HPO4•12H2O, within 2 h at 800°C.

The glass samples were prepared by fusing mix-tures of finely ground phosphates in an electric furnace at 900°C in platinum dishes. Then, the melts were quenched between two cooled massive copper plates. Thus, thin-layered pieces of glass were obtained from which were subsequently crushed to give grits of ap-proximate dimensions of 3mm´3mm´0,3mm with no crystalline inclusions and irregularities. The samples were stored in a dry atmosphere over phosphoric anhy-dride. The composition of the glasses was regulated by varying the number of the fused salts.

To characterize the composition of the glasses con-taining a metal of only one species, a molar ratio R = MxO/P2O5 or a related average degree of polymerization ñ = 2/(R-1) were used. It is worth adding that MxO stood for the amount of alkali metal oxide in the samples, x was the valence exhibited by the metal, while ñ was the average degree of polymerization.

Aqueous solutions of sodium diphosphate Na4P2O7 and sodium monohydrogen phosphate Na2НPO4 were used for synthesize the research objects.Aqueous

phosphate solutions were mixed with each other in the required proportions and treated with calcium chloride solution of the same concentration. The coprecipitation of the salts was carried out in presence of a double ex-cess of 10 % calcium chloride solution as compared to the stoichiometry.

At the same time, one of the reagents was slow ly added dropwise to the solution of the other one under intensive stirring. The acidic index of the medium was maintained at pH 8. The procedures were carried out at a room temperature. The precipitates in the form of aqueous suspension substances were subjected to re-peated washing with distilled water until the absence of Cl- ions in the wash waters. After that, the suspensions were settled for 1 day - 2 days, separated from water, first by decanting, and then on a glass porous filter. The substances were dried at 60oС - 80oС and then analyzed by X-ray phase anal ysis and IR spectroscopy.

A basic technological scheme for obtaining amor-phous calcium-phosphate materials was developed in view of the procedure applied to conduct the exchange reactions envisaged and the optimal conditions found favorable for precipitation of the target product (Fig. 1). In accordance with it 5 %-10 % solutions of the initial phosphate salts from reservoirs 1 - 4 subjected to additional purification using filters 5 - 8 are sent to intermediate tanks 9 - 12.

Fig. 1. The basic technological scheme of obtaining calcium-phosphate materials by the method of precipita-tion from aqueous solutions: 1-4 - bunkers with reagents; 5-8 - filters; 9-12 - intermediate tanks for reagent solutions; 13 - batcher; 14 - 16 - reactor; 17, 20 - sedimentation tanks; 18 - repulpator; 21 - the vacuum filter; 22 - drying.


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From there, the solutions are dispensed via dispens-ers to reactors 14 and 15, where they are mixed in the required proportions. Then the solutions of the salts, monophosphate and diphosphate or calcium chloride, enter the reactor 16 through the system of dispensers 13. There the ion exchange reaction is performed. The resulting substance suspensions are sent to settler 17 where the solid part is separated from the solutions containing unreacted reagents and sodium chloride. The precipitated material enters repulpator 18, where it disports after addition of water from the vessel 19. The resulting slurry settles in settler 20 and then it is sepa-rated from the moisture residues by vacuum filter 21. It is then sent for drying or used as a paste. The solutions containing sodium chloride (brines), obtained after the suspensions settling in sedimentation tanks 17 and 20 and the filtrate resulting from the vacuum filtration are sent for disposal.

The thermal analysis of the products obtained was carried out on the derivatograph of the system МОМ Q-1500. The diffractograms were recorded on a DRON-3 diffractometer using СuКa and СоКa-radiations.

31P NMR spectra were obtained on a WP-80 spec-trometer in aqueous solutions. The sodium content of the precipitated substances was determined by the method of flame photometry on “FLAPHO var” instrument. The investigation of the microstructure of the reaction prod-ucts was carried out on a JEOL Superprobe 733-JCXA electron probe microanalyzer. The bitmap pictures were shot in SEI and BEL-Compo modes.

RESULTS AND DISCUSSION

The exchange reactions of sodium monohydrogen phosphate and diphosphate with a solution of calcium chloride lead also to the formation of suspensions, which separate into two layers – a pure solvent and more con-centrated suspension. The substances ob tained from the solution of sodium monohydrogen phosphate in presence of sodium diphosphate precipitate much more slowly than the product formed in the solution containing only monohydrogen phosphate. Fig. 2 shows the change in the course of upholding the volume occupied by the water suspension of substances co-precipitated from aqueous solutions of sodium monohydrogen phosphate and sodium diphosphate. It can be seen from the Fig. 2 that the highest stratification rate refers to a substance

precipitated from a solution of sodium monohydrogen phosphate in absence of any impurities.

The X-ray diffraction analysis shows that crystalline hydroxoap atite is the product obtained from a solu tion containing only sodium monohydrogen phosphate.

Speaking about the amorphization of precipitated products, it should be taken into account that the char-acter of the diffractograms only does not provide stat-ing that the substance is really amorphous. The small particle size can be the reason for the low intensity and the broadening of the reflexes, because according to the well-known Scherer equation (19), the width of the lines in the diffractograms is related to the size of the microcrystal in accordance with the expression:

t = 0.9 λ / B cos qB

where t is the dimensions of the crystal, λ is the wave-length of the X-ray radiation, В is the line broadening, while qB is the Bragg angle.

It follows from the above formula that the broaden-ing of the lines is greater, the smaller the dimensions of the crystals are. To obtain additional information on the nature of the deposited substances, a high resolu-tion electron microscopy method is used. The results obtained show that hydroxyapatite is a quite typical crystalline formation, while the bulk of the products that

Fig. 2. Change in the course of upholding the volume occu-pied by the water suspension of substances co-precipitated from aqueous solutions of monohydrogen phosphate and sodium diphosphate: the designation of the curves is the relative content of sodium diphosphate in the initial phos-phate solutions,%: 1-0; 2-5; 3-10; 4-20; 5-30.


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are co-precipitated from a solution containing about 30 % sodium diphosphate (the rest is sodium monohydro-gen phosphate) is amorphous. True, there are particles of dimensions of 20nm-40nm, which can be considered nanocrystals. However, the content of the latter is insig-nificant, and it is possible that, in general, the substances are of an amorphous nature (Fig. 3).

In the presence of a small amount of sodium di-phosphate, partially amorphized substances form in the solutions. Only amorphous products are obtained in solutions richer in diphosphate, i.e. those with a molar ratio of Na4P2O7/ (Na4P2O7 + Na2HPO4) equal to 0.2 - 0.4.

Based on the synthesis conditions, it can be expected that hydroxyapatite and calcium diphosphate are the molecular basis of the precipitated amorphous products. The results of the IR spectroscopic study show that the content of hydroxyapatite, as an individual substance,

in products decreases rapidly with the increase of the amount of diphosphate in the initial solutions. One of the possible explana tions for this refers to the binding of the monophosphate groups PO4 of the apatite with the diphosphate anions P2O7

4- via Ca2+ cations. The screen-ing of the surface of small particles of hydroxyapatite with diphosphate molecules can be another one. Unfor-tunately, the final conclusion about the actual structure of the copre cipitation products can be made only after more detailed studies, for ex ample, using the 31P solid-state NMR method.

Fig. 4 illustrates the decrease of the weight of samples of precipitated calcium phosphates as a result of their heating to 500°C. The calcium phosphates de-scribed in the previous section, when heated above 40oC - 60оC lose moisture (Fig. 4). The process of dehydration is completed mainly at 400оC - 500оC. The amount of the water removed reaches 15 % - 17 % of the weight of the substance and depends little on the composition of the samples.

Conversions occur accompanied by a weak absorp-tion of heat by heating the substance precipitated from a solution of sodium monohydrogen phosphate at tem-peratures above 700оC. It is believed that hydroxyapatite is the substance reacting as it is known that precipitated hydroxoapatite is characterized by non-stoichiometry of the composition caused by deficiency of Ca2 + ions [19]. Furthermore, it is possible that the existence of a weak endothermic effect on the DTA curve is caused by the transformation of the non-stoichiometric part of the salt.

Fig. 3. TEM image and results of X-ray microanalysis hydroxyapatite precipitated in the presence of 30 % of sodium diphosphate.

Fig. 4. Reducing the weight of samples of precipitated calcium phosphates as a result of heating them to 500°C.


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Fig. 5 shows the dependence of the temperature (a) and the magnitude (b) of the exoeffects on the DTA curves of precipitated calcium phosphates on the sodium diphosphate content of the initial solutions.

The substances precipitated from a solution of monohydrogen phosphate in presence of 5 % sodium diphosphate under high temperature conditions behave differently than hydroxyapatite. Thus, a narrow and quite intense exoeffect appears on the correspond-ing DTA curve above 600оС. The latter indicates that transformations that are accompanied by the release of heat take place in the sample. In calcium phosphates precipitated in the presence of even more sodium diphos-phate, exothermic transformations are also observed. The temperature and the value of the thermal effects on the corresponding DTA curves depend on the samples composition. The exoeffects of the products precipitated from solutions in presence of 30%-40% sodium diphos-phate have maximum values of the thermal effect and the temperature (Fig. 5). Since both these parameters serve as a measure of disorder and thermodynamic instability of substances, it can be assumed that the corresponding phosphates are maximally amorphized.

The results of the X-ray analysis show that there are two-three phases in the samples heated above the thermal effects temperature: hydroxyapatite, tricalcium phosphate and calcium diphosphate. The first salt is a part of the products of the thermal transformations of the substances obtained from solutions of a sodium diphosphate content not exceeding 30 % (Fig. 6). The

proportion of hydroxyapatite in the transformation products decreases as the precipitated substances are enriched in sodium diphosphate .

Tricalcium phosphate is present in all samples apart from the precipitated amorphous calcium diphosphate formed as a result of the heating. This salt is also pre-sent in a significant amount among the products of the thermal transformations of hydroxyapatite, which may result from the transformations of the nonstoichiomet-ric part of the substance. As precipitated amorphous substances are enriched in calcium diphosphate, the

Fig. 5. Dependence of the temperature (a) and the magnitude (b) of the exoeffects on the DTA curves of precipitated calcium phosphates on the sodium diphosphate content in the initial solutions.

Fig. 6. The change in the relative intensity of the main reflections on the diffractograms of thermal conversion products of coprecipitated calcium phosphates as a func-tion of the content of diphosphate: notation of the curves: 1 - hydroxoapatite (d = 18.5 Å); 2 - Ca3(PO4)2 (d = 18.1Å); 3 - Ca2P2O7 (d = 17.2Å).


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proportion of Са3(PO4)2 in the thermal transformation products increases initially, reaches a maximum in the samples corresponding to the substances precipitated in presence of 15 % - 20 % sodium diphosphate, and then decreases. The amount of crystalline Ca2P2O7 increases only, which is quite natural.

The existence of exothermic effects on the DTA curves indicates that a complex sequence of transforma-tions occurs in the precipitated phosphates upon heat-ing. Their result is ordering, connected both with the consolidation of the same molecular forms into separate crystals, and the corresponding transport processes.

The presence of tricalcium phosphate transforma-tions among the products and the extreme nature of the curve corresponding to the formation of Са3(РО4)2 indi-cate that the interaction between the main components of the precipitated substances, in addition to transforma-tions of the non-stoichiometric part of hydroxyapatite, takes place according to the reaction:

2Са5(РО4)3ОН + Са2Р2О7 = 4Са3(РО4)2 + Н2О

The latter involves water allocation. However, no noticeable gravitational effects are observed near the temperatures corresponding to the exoeffects on the DTA curves. Therefore, it can be assumed that the interaction between hydroxyapatite and calcium diphosphate oc-curs earlier, possibly even during the dehydration, i.e. at temperatures below 400oC - 500оС.

CONCLUSIONS

It is found that diphosphate is an inhibitor of crystal-lization of HAP. The inhibitory properties of this system are mainly manifested when 20 mol % to 40 mol % of diphosphate are added to the initial solutions. This is due to two factors: the formation of non-stoichiometric hydro-oso-apatite and the inhibitory effect of diphos-phate on the process of hydro-oso-apatite crystallization.

The ideas obtained in the work will be useful in the creation of new amorphous materials based on calcium phos phates. The technology advanced can be practically realized for the production of amorphous apatites. The obtained materials can be used in medicine as restorative materials and precursors of bioceramics as well as sorbents.

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amorphous materials based on calcium phosphatesa maximum amount of amorphized substances is formed...

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