Studies on strontium succinate crystals

Transcription

1 Studies on strontium succinate crystals Binitha M.P Studies on growth and properties of some metal succinate crystals Thesis. Department of Physics, University of Calicut, 2014

2 Chapter 4 Studies on strontium succinate crystals 4.1 Introduction Alkaline earth metals are almost constantly found in an oxidized state as a component of metal-organic salts due to the highly reactive nature of such elements. Salts of these metal ions are widely distributed throughout nature and are used in a large number of industrial processes and in the production of food products, medical products, pharmaceutical ingredients, vitamins, other health related products and products for personal care. Strontium is an important candidate of alkaline earth metals, because of its favourable actions in biological systems [1]. Strontium stimulates the formation of bone volume and is the only trace metal of bone that can be positively correlated with compression strength of bone [2-3]. Various strontium compounds modulate bone loss in osteoporosis, when present at levels higher than those required for normal cell physiology [4]. Also, the binding modes of strontium cations with carboxylate and dicarboxylate anions have special interest because of the significance of such interactions in the blood and bone proteins [5]. S. Pors Nielsen has written a review on the biological relevance of strontium and reported that strontium forms divalent cations in biological fluids such as plasma and serum with different degrees of protein binding [6]. Strontium succinate and other strontium salts find immense applications in the medical field, especially for treatment of rheumatic and arthritic disorders[7]. Yannis Tsouderos et al. have reported that strontium, in the form of inorganic or 75

3 4.2. Growth of crystals organic salts like strontium succinate, has remarkable pharmacological properties and finds application in therapy which is completely beneficial in the prevention and treatment of arthrosis [8]. Also it is reported that strontium succinate has better pharmaceutical action compared to strontium ranelate, for the treatment of osteoporosis, in post menopausal women, to reduce the risk of fractures [8]. Many works have been reported on the synthesis and characterization of strontium carboxylates [9-11] and the pharmaceutical importance of strontium complexes motivated us to crystallize strontium succinate in the environment friendly growth medium and characterize the material for exploring its action in biological systems. 4.2 Growth of crystals The growth process of strontium succinate (abbreviated as SS) involves the diffusion of strontium chloride solution into the succinic acid impregnated silica gel. Silica gel was prepared by adding a solution of sodium meta silicate of specific gravity ranging from 1.02 to 1.06 to succinic acid of molarity ranging from 0.25 to 1.5M, slowly with continuous stirring to get a ph between 3 and 7. This gel solution was then transferred to several test tubes and after assuring proper gel setting, an aqueous solution of strontium chloride (0.25M-1.5M) was poured over the gel carefully along the walls of the test tube. The reaction between Sr 2+ and C 4 H 4 O 2 4 ions leads to the formation of strontium succinate crystal aggregates as shown in figure 4.1. In this case the growth has been studied by changing the parameters, viz. ph, gel density, gel ageing and concentration of reactants and a similar result as obtained in the case of calcium succinate crystals were observed here also, by the variation of ph, gel density and gel ageing. In all these experiments crystallization resulted such aggregate growth, but the size and number of crystals formed were highly dependent on the above discussed conditions. When the concentration of strontium chloride was increased from 0.25 to 1.5M, the number of crystals formed got increased, but their size is reduced comparatively. Similar variation is observed in the case of variation of concentration of succinic acid also. The optimum conditions for getting the best quality crystals of strontium succinate is shown in table 4.1 and the growth set up and grown crystals of strontium succinate are shown in figure 4.1. The reaction responsible for the 76

4 4.3. Structural characterization Table 4.1: Optimum conditions for the growth of SS crystals Parameters Value Gel density 1.03 ph 5 Gel set time 1 day Ageing time 1 day Concentration of succinic acid 1.25M Concentration of SrCl 2 1.5M Figure 4.1: The growth set up and grown crystals of SS growth of strontium succinate crystal is SrCl 2 +C 4 H 6 O 4 SrC 4 H 4 O 4 +2HCl (4.1) 4.3 Structural characterization Powder X-ray diffraction The fine crystalline specks from these aggregates were taken and used for the single crystal XRD studies, but could not make enough reflections for elucidating the structure of this crystal. Hence in this thesis we include only the powder X-ray diffraction studies for structure determination of this crystal. The most transparent crystalline aggregates that have been grown well inside the gel column were harvested and finely crushed and were used for the powder X-ray diffraction studies. The XRD spectrum of grown crystals of strontium succinate 77

5 4.4. Spectroscopic characterization is shown in figure 4.2 and this study shows that strontium succinate crystals have monoclinic structure with space group P2 1 /c. Lattice parameters are calculated by PDXL software and the planes are indexed by powderx software. The lattice parameters obtained are a = Å, b = Å, c = Å, α = β = 90 and γ = 91.8 which are in close agreement with ICDD data having card number Figure 4.2: Powder XRD spectrum of SS 4.4 Spectroscopic characterization FT-IR and FT-Raman studies FT-IR spectrum of strontium succinate crystal is shown in figure 4.3. Absence of any absorption peak in the O-H stretching region ( cm 1 ) in IR spectra confirms that grown crystal is anhydrous strontium succinate, complimenting the powder XRD results. The weak absorption peak at 2975 cm 1 represents asymmetric stretching of CH 2 group in compound and peak at 2933 cm 1 corresponds to symmetric stretching of CH 2 group. Asymmetric stretching of carboxylate group occurs at wavenumber 1552 cm 1 and symmetric stretching of COO group gives medium vibration at 1431 cm 1. The medium vibrational peak at 1470 cm 1 and strong peak at 1339 cm 1 are due to the bending of C-H 78

6 4.4. Spectroscopic characterization Figure 4.3: FT-IR spectrum of SS bond. Strongabsorptionbandsat1293, 1247cm 1 areduetoc-ostretchingand CH 2 twisting vibrations respectively. The medium peak at 1176 cm 1 and strong vibration at 1037 cm 1 are due to C-CO 2 stretching and the medium IR peaks at 976 and 937 cm 1 are due to central C-C stretching vibrations respectively. CO 2 symmetric bending gives strong vibrational peak at 663 cm 1. Bending and rocking vibrations of COO are observed at 696 and 498 cm 1 respectively [16-17]. FT-Raman spectrum of the crystal is given in figure 4.4. Spectrum exhibits a medium peak at 2974 cm 1 and a strong peak at 2933 cm 1, which are assigned to asymmetric and symmetric stretching vibrations of C-H bond. Strong band at 1441 cm 1 correspond to symmetric stretching vibrations of carboxylate group and medium band at 1396 cm 1 is ascribed to CH 2 wagging mode. The medium Raman peaks at 1468 and 1340 cm 1 correspond to bending of C-H bond. Weak band at 1295 cm 1 is due to C-O stretching vibration and medium bands at 1073, 975 and 943 cm 1 are due to C-CO 2 and central C-C stretching vibrations, respectively. Peaksat686and663cm 1 areassignedtobendingofco 2 bondand rocking vibrations of COO give rise to vibration at 498 cm 1. Larger masses of the metals give Raman line at a very low stretching frequency of 121 cm 1 corresponding to Sr-O stretching [18]. The observed IR and Raman frequencies 79

7 4.4. Spectroscopic characterization Table 4.2: FT-IR and FT-Raman band assignment of SS FT-IR(cm 1 ) FT-Raman(cm 1 ) Assignment 2975(w) 2974(m) ν as (C-H) 2933(w) 2933(s) ν s (C-H) 1552(s) ν as (OCO) 1470(m) 1468(w) δ(ch 2 ) 1431(m) 1441(s) ν s (OCO) 1399(m) 1406(w) ρ w (CH 2 ) 1339(s) 1340(m) δ(ch 2 ) 1293(s) 1295(w) ν(c-o) 1247(s) 1246(vw) ρ t (CH 2 ) 1176(m) 1172(vw) ν out (C-CO 2 ) 1037(s) 1073(m) ν in (C-CO 2 ) 976(m) 975(s) ν(c-c) 937(m) 943(w) ν(c-c) 804(s) 803(m) ρ r (CH 2 ) 696(m) 686(w) δ(oco) 663(vs) δ(oco) 498(s) 498(vw) ρ r (OCO) 157(m) ν(sr-o) 121(vs) ν(sr-o) ν as - asymmetric stretching; ν s - symmetric stretching; δ - bending; ρ w - wagging; ρ r - rocking; ρ t - twisting mode; vs - very strong; s - strong; m - medium; w - weak; vw - very weak 80

8 4.4. Spectroscopic characterization Figure 4.4: FT-Raman spectrum of SS and their assigned vibrations are depicted in table UV/Vis/NIR diffuse reflectance studies The finely ground powder of strontium succinate crystal is used for the optical measurement in the wavelength range of nm. The variation of diffuse reflectance for strontium succinate crystal at room temperature are shown in figure 4.5 and this is used for the determination of the approximate value of band gap of the material using Kubelka-Munk theory. From Kubelka-Munk function, the optical band gap of the material is determined by extrapolating linear portion of plot of [ k s hν]2 versus hν (figure 4.6) and the calculated band gap is 5.66 ev. 81

9 4.5. Thermal decomposition studies Figure 4.5: Diffuse reflectance spectrum of SS Figure 4.6: Plot of [ k s hν]2 versus hν of SS 4.5 Thermal decomposition studies Figure 4.7: TG curve of SS Figure 4.8: DTA curve of SS Thermal analysis of strontium succinate crystals were made in the temperature range 10 to 1000 C at heating rate 10 C/min. The decrease of weight with temperature is shown in the TG curve in figure 4.7, while the DTA results are given in figure 4.8 and DSC curve is shown in figure 4.9. From the TG curve it can be seen that strontium succinate crystal presents relatively good thermal stability, since no significant weight loss occurs until 540 C. This also confirms the anhydrous nature of the crystal, complimenting the X-ray diffraction studies and spectroscopic results. After this temperature, the curve describes a mass loss of 19.65% in the temperature range of 540 to 626 C. This mass loss is attributed to the decomposition of the sample to give SrCO 3 and residual carbon. 82

10 4.5. Thermal decomposition studies Figure 4.9: DSC curve of SS The SrCO 3 formed after decomposition is further decomposed to strontium oxide (SrO) in the temperature range of 763 and 942 C, the TG curve shows a mass loss of 30.7%. The observed mass loss is in close agreement with the calculated value (29.8%) for the formation of SrO as the final product. The DTG curve of strontium succinate showed two major peaks in the curve. The first peak corresponds to the endothermic decomposition of strontium succinate to SrCO 3 at C (588 C in DTA and 581 C in DSC). The second peak shows the decomposition of SrCO 3 to SrO at a temperature of C (853 C in DSC and 856 C in DTA curve). The details of thermal decomposition at all the stages of strontium succinate are given in table 4.3 Table 4.3: Thermal degradation details of SS Temp. range ( C) Weight loss (%) Loss of molecule Residue Observed Calculated CO+CH 4 SrCO 3 +C CO 2 SrO+C The Coats-Redfern plots for the thermal decompositions for the two stages are depicted in figure 4.10 and the evaluated kinetic and thermodynamic parameters are given in table 4.4. The higher value of activation energy (E) for stage I, compared to stage II suggests that formation of strontium carbonate from strontium succinate requires more energy than the formation of strontium oxide from strontium carbonate. Also the reaction to form strontium oxide takes 83

11 4.6. Dielectric studies Figure 4.10: Coats-Redfern plot for different stages for SS place faster since the values of E and H are low. The positive value for H suggests endothermic nature of reactions in stage I and stage II, as evident from DTA and DSC curves. 4.6 Dielectric studies Variation of dielectric constant as a function of frequency is shown in figure It is clear from the figure that dielectric constant of strontium succinate crystal decreases gradually with increasing frequency, which is a normal dielectric be- Table 4.4: The kinetic and thermal parameters of SS n E S log A H G r (kj mol 1 ) (J mol 1 ) (kj mol 1 ) (kj mol 1 ) n = order of reaction, E = activation energy, S = entropy, A = frequency factor, H = enthalpy, G = Gibb s free energy, r = regression coefficient 84

12 4.6. Dielectric studies haviour and can be explained on the basis of various mechanisms like electronic, ionic or atomic, dipolar or orientational and space charge or interfacial polarization. In the present case high value of dielectric constant at lower frequencies may be attributed to space charge polarization due to crystal lattice defects. From the plot it can be seen that dielectric constant decreases as frequency of applied field increases. Electronic exchange of the number of ions in crystals gives local displacement of electrons in the direction of the applied field, which in turn gives rise to polarization. As frequency increases, a point is reached where the space charge cannot sustain and comply with variation of external field and hence polarization decreases, which gives rise to diminishing values of dielectric constant. Also in this case, decrease in dielectric constant with temperature is due to effect of orientational polarization. The ac-conductivity of crystal is found to be increasing with frequency and decreasing with temperature as shown in figure When temperature is increased, thermal expansion reduces the density of crystal and this causes reduction in conductivity [19]. Figure 4.11: Variation of dielectric constant with frequency for SS Figure 4.12: Variation of acconductivity with frequency for SS The value of dielectric constant gets saturated at high frequencies and this value along with the structural data of strontium succinate crystal is used to determine the values of plasma energy ( ω p ), Penn gap (E P ), Fermi energy (E F ) and polarizability (α) for the grown crystal and the values of these parameters are depicted in table

13 4.7. Magnetic studies Table 4.5: Plasma energy, Penn gap, Fermi energy and polarizability for grown crystal of SS Parameters Value Plasma energy ( ω p ) ev Penn gap (E P ) 6.26 ev Fermi energy (E F ) ev Polarizability (α) From Penn analysis 3.11 X cm 3 From By Clausius-Mossotti 2.99 X cm Magnetic studies Figure 4.13: M-H curve of SS The variation of magnetic moment of powdered sample of strontium succinate crystal with external field is shown in figure From this curve, the weak ferromagnetic nature of the sample is evident. Such a magnetic behaviour is exhibited by other carboxylate bridged metal complexes such as strontium malonate and lead malonate [20-21] and can be explained as due to the ability of carboxylate group to induce magnetic interaction between the paramagnetic centres that it links. Depending on the confirmation of carboxylate bridge with the metal ion, the material can exhibit different magnetic properties as explained in chapter 3 of this thesis. The magnetization curve of strontium succinate suggest a syn-anti type of bridging of two independent strontium ions by the succinate 86

14 4.8. Conclusions ligand, in the form of Sr(s)-O(p)-Sr(s)-O(p) in its structure, as in the case of calcium succinate. The 2p orbital of oxygen and the s orbital of the strontium ion may overlap in some parts of the effective orbital and this exchange pathway may involve an equatorial position in one strontium atom and an axial position on the other strontium atom. Accordingly, through this exchange pathway, strontium succinate shows weak ferromagnetic exchange coupling. Saturation magnetization, remanent magnetization and coercivity associated with the hysteresis loop of strontium succinate crystal are given by Coercivity = G Saturation magnetization = X 10 3 emu/g Remanent magnetization = X 10 6 emu/g 4.8 Conclusions Strontium succinate crystal is grown by gel aided solution growth technique. Powder X-ray diffraction studies confirmed the monoclinic structure for the grown crystals. Different functional groups in the material were identified from FT-IR analysis and have been confirmed from Raman spectrum. Optical band gap of the material is found to be 5.66 ev, from UV/Vis/NIR spectral studies. From TG, DTA and DSC studies, a two stage decomposition pattern of the crystal is suggested, confirming the anhydrous nature of the crystal and the kinetic parameters are evaluated by Coats-Redfern method. As in the case of calcium succinate, the contribution of space charge polarization towards total polarization is confirmed at lower frequencies and the thermal variation of dielectric constant proved the contribution of orientational polarization. Dielectric parameters are also used to calculate the plasma energy and polarizability. From room temperature VSM studies, a weak ferromagnetism is found to exist in the crystal suggesting the possibility of syn-anti confirmation of carboxylate group with the metal atoms. Bibliography [1] S Christgau and J E T Andersen, U.S. Patent No A1 (2009). 87

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