RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
The Rietveld Method as a Tool to Quantify the Amorphous Amount of Microcrystalline Cellulose LAYSA PIRES DE FIGUEIREDO, FABIO FURLAN FERREIRA Centro de Ciˆencias Naturais e Humanas (CCNH), Universidade Federal do ABC (UFABC), Santo Andr´e, SP CEP: 09210-580, Brazil Received 17 December 2013; revised 14 January 2014; accepted 31 January 2014 Published online 1 March 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.23909 ABSTRACT: With the use of X-ray powder diffraction data and the Rietveld method, we verified that both microcrystalline cellulose PH101 and PH-102 presented differences in the crystallinity degree. The results revealed these samples are semicrystalline in nature and via Rietveld refinements, it was possible, adding a known amount of an internal standard of corundum to the samples, to perform quantitative phase analyses, which allowed us to determine the amorphous amount of the studied samples: 63.7(11) wt % in microcrystalline cellulose PH-101 and 51.0(11) wt % in microcrystalline cellulose PH-102. An important contribution of this work refers to the attempt of using C 2014 Wiley Periodicals, Inc. and the this simple method, permitting to evaluate the degree of crystallinity of microcrystalline cellulose. American Pharmacists Association J Pharm Sci 103:1394–1399, 2014 Keywords: microcrystalline cellulose PH-101; microcrystalline cellulose PH-102; crystallinity; X-ray powder diffraction; Rietveld method; FTIR; dehydration; solid state
INTRODUCTION Many organic compounds are able to adopt one or more pure crystalline forms of well-defined and easily identifiable configuration, or an amorphous form without long-range order, depending on the conditions (temperature, solvent, time) under which crystallization is induced. It is in this context the term polymorphism arises, being defined as the ability of a compound to crystallize in two or more crystal structures with the same chemical composition, due to the differences in spatial arrangements and/or conformation of the molecules.1–4 It is noteworthy that polymorphism may cause a direct impact on several characteristics of pharmaceuticals, thus displaying unexpected results, which may compromise the efficacy of the medicinal product. As many pharmaceuticals, excipients can also present polymorphic transformations, being a worrying factor for the pharmaceutical industries.5,6 On this basis, the choice of excipients suitable for particular formulation is a key point to the therapeutic efficacy of the final product. This choice must be based on the characteristics of the substances contained in the formulation, as well as the possibility of interaction with the excipient. In this context, it is essential that during the research/development of formulations, one can be able to select the correct polymorphic form of an active pharmaceutical ingredient as well as excipients that do not suffer any kind of interaction, ensuring that both bioavailability and the pharmacological effect take place. Thus, the research of polymorphism in pharmaceuticals and excipients is one of the most important subjects to be considered prior to the production of medicaments, mainly because the lack of knowledge of different crystalline forms can influence the production procedure and may bring health problems to patients and financial losses to their manufacturers.7,8 Correspondence to: Fabio Furlan Ferreira (Telephone: +55-11-4996-8370; Fax: +55-11-4996-0090; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 103, 1394–1399 (2014) C 2014 Wiley Periodicals, Inc. and the American Pharmacists Association
1394
Herein, we report on the investigation of the structural characterization and quantitative phase analysis of microcrystalline cellulose, a purified and partially depolymerized cellulose prepared by high-quality treatment with mineral acid to hydrolyze cellulose wood pulp, in order to decrease the degree of polymerization. Figure 1 shows the molecular structure of cellulose as a carbohydrate polymer generated from repeating $D-glucopyranose molecules that are covalently linked through acetal functions between the equatorial OH group of C(4) and the C(1) carbon atoms ($-1,4-glucan), which is, in principle, the manner in which cellulose is biogenetically formed.9 As a result, cellulose is an extensive, linear-chain polymer with a large number of hydroxyl groups [three per anhydroglucose (AGU) unit] present in the thermodynamically preferred 4 C1 conformation. To accommodate the preferred bond angles of the acetal oxygen bridges, every second AGU ring is rotated 180◦ in the plane. In this manner, two adjacent structural units define the disaccharide cellobiose.10,11 As shown in the molecular structure represented in Figure 1, the hydroxyl groups of $-1,4-glucan cellulose are placed at positions C(2) and C(3) (secondary, equatorial) as well as C(6) (primary). The CH2 OH side group is arranged in a trans-gauche (tg) position relative to the O(5)–C(5) and C(4)–C(5) bonds.9 As a result, microcrystalline cellulose has a semicrystalline structure, that is, part of its structure is amorphous and other is crystalline (Fig. 2). The crystalline regions are grouped into nuclei called crystallites. These crystallites, in turn, are surrounded by amorphous regions.12 The proportion between amorphous and crystalline regions determines the degree of crystallinity, object of study in this work. Celluloses I␣ and I Currently, it is known that cellulose exists in more than one polymorphic form,14 that is, there is not a single unit cell dimension, showing six polymorphic forms for cellulose I, which also display polymorphism, and receive the following names: cellulose I, cellulose II, cellulose IIII , cellulose IIIII , cellulose
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
1395
Figure 1. Molecular structure of cellulose (n = DP, degree of polymerization).
by a pharmaceutical company that will not have its information disclosed. The techniques used for the studies were FTIR and XRPD together with the Rietveld method. Fourier Transform Infrared Spectroscopy Figure 2. Illustration containing crystalline regions interspersed by amorphous ones of cellulose. (Adapted from Ref. 13.)
IVI , and cellulose IVII , although the existence of cellulose IVI is controversial.15 Among those, cellulose I is receiving increased attention because of its potential use in bioenergy production, and according to the literature the existence of more than one polymorphic form of cellulose in native samples has been shown.16,17 Today, we know that cellulose I actually consisted of two polymorphic forms, called I" (one-chain triclinic structure) and I$ (two-chain monoclinic structure). In this scheme, native celluloses were all classified into I" -rich algal-bacterial type or I$ -dominant cottonramie type.18 The crystal structure and hydrogen-bonding system in cellulose I$ was elucidated by the combined use of synchrotron X-ray and neutron fiber diffraction.19 Oriented fibrous samples were prepared by aligning cellulose microcrystals from tunicin, reconstituted into oriented films. These samples diffracted ˚ resboth synchrotron X-rays and neutrons to better than 1 A olution, yielding more than 300 unique reflections and an unambiguous assignment of the monoclinic unit cell dimen˚ b = 8.201(8) A; ˚ c = 10.380(10) A; ˚ and sions: a = 7.784(8) A; ( = 96.55(5)◦ , in the space group P21 . The crystal and molecular structures of the cellulose I" have been established using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from the cell wall of fresh water alga Glaucocystis nostochinearum.20 Cellulose I" crystallizes in a tri˚ b = 6.717(6) A; ˚ c = 5.962(7) A; ˚ clinic unit cell: a = 10.400(10) A; " = 80.37(5)◦ ; $ = 118.08(5)◦ ; and ( = 114.80(5)◦ , having space group P1. It should be mentioned that the determination of the crystal structure of cellulose by X-ray diffraction played an important role in the polymer science and the concept of long-chain molecules. In order to verify such statements and information, which may serve as a subsidy for the pharmaceutical industries, quantitative phase analyses by means of X-ray powder diffraction (XRPD) data and the Rietveld method were used to evaluate the amorphous amount of both microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102. Also, FTIR was used to follow their possible differences in absorption bands.
Spectral data were recorded in the range from 4000 to 650 cm−1 with a resolution of 4 cm−1 , in a Fourier transform spectrometer, model 660-IR, from Varian (Palo Alto, CA, USA), equipped with an attenuated total reflection accessory and a ZnSe crystal. FTIR measurements were conducted at the Multiuser Experimental Centre/Universidade Federal do ABC (CEM/UFABC). XRPD and Rietveld Refinements X-ray powder diffraction data were collected on a STADI-P diffractometer, from Stoe (Darmstadt, Germany), operating in ˚ the transmission mode, using CuK"1 radiation (8 = 1.54056 A), an accelerating voltage of 40 kV and current of 40 mA, equipped with a primary beam monochromator [Ge (111) curved crystal], a 0.5-mm divergence slit, and a 0.3-mm circular scattering slit. The X-ray photons were detected with a linear detector (Mythen 1K, from Dectris(Baden, Switzerland)). The samples were loaded between two 0.014-mm thick cellulose-acetate foils, with density of 1.3 g cm−3 , and were spun during data collection. Measurements were performed in the angular range from 5◦ to 50◦ (2θ ), with step sizes of 0.015◦ and an integration time of 60 s at each 1.05◦ . These analyses were conducted in the Laboratory of Crystallography and Structural Characterization of Materials (LCCEM) of UFABC. For the identification of the crystal structures, both the Cambridge Structural Database (CSD) and Inorganic Crystal Structure Database (ICSD) were used; the structural characterization of samples and the evaluation of the amorphous amount were carried out by means of the Rietveld method21,22 using XRPD data and the Topas-Academic v. 523 software program. First, we refined the unit cell parameters and the zero point of the diffractometer. The background was fitted using a 12-term Chebyshev polynomial. The peak asymmetry was adjusted using the simple axial divergence model of Cheary and Coelho.24,25 The peak profiles were modeled using the fundamental-parameter approach25 and the preferred orientation of the crystals were corrected with a fourth-order spherical harmonics function. The isotropic atomic displacements (Biso ) of all nonhydrogen atoms were fixed to 2.8. For the hydrogen atoms, the Biso values were assumed to be 1.2 times larger than the values of the nonhydrogen atoms. R
R
RESULTS AND DISCUSSION Fourier Transform Infrared Spectroscopy
MATERIALS AND METHODS Powdered samples of microcrystalline cellulose PH-101 and PH-102 (Avicel from FMC BioPolymers) were kindly provided R
DOI 10.1002/jps.23909
The FTIR spectra for microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 samples are shown in Figure 3.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
1396
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
Figure 3. FTIR spectra for microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 samples, obtained with a resolution of 4 cm−1 in the region from 4000 to 650 cm−1 .
Figure 4. X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 collected in transmission mode. The X-ray diffractogram of microcrystalline cellulose PH-102 were offset for better visualization.
Table 1. Location of the Main Absorption Bands of Cellulose I (Native),26,27 Microcrystalline Cellulose PH-101 and Microcrystalline Cellulose PH-102 with Their Respective Chemical Bonds
Also, we can infer that both FTIR spectra are characteristic of cellulose I (native), as the absorption bands are similar to reported values in the literature27 for this polymorphic form of cellulose.
Cellulose I
X-Ray Powder Diffraction
3352 2901 1431 1373 1319 1282 1236 1202 1165 1032 983 897
101 L (cm−1 ) 3336 2900 1430 1372 1318 1289 1230 1206 1156 1031 984 897
102 Assignment 3339 2900 1430 1372 1314 1287 1232 1204 1161 1031 984 896
(OH (hydrogen bonded) (CH *CH2 (sym) at C-6 *CH *CH2 (wagging) at C-6 *CH *COH in plane at C-6 *COH in plane at C-6 *COC at $-glucosidic linkage (CO at C-6 (CO at C-6 (COC at $-glucosidic linkage, (COC, (CCO, and (CCH at C-5 and C-6
(, stretching; *, bending.
Table 1 displays the main absorption bands of cellulose I (native), microcrystalline cellulose PH-101 (named 101), and microcrystalline cellulose PH-102 (named 102) investigated in this study, with the assignment of the respective chemical bonds.26,27 Not all absorption bands were identified, considering that FTIR spectroscopic investigations were carried out with the aim of evidencing that an alteration of the crystalline organization leads to a significant simplification of the spectral shape by a reduction in intensity or even disappearance of the characteristic bands of the crystalline domains. It is worth mentioning that the band at 1430 cm−1 is known as the band of crystallinity, that is, a decrease in its intensity reflects in the reduction of the degree of crystallinity of the sample.26–28 As shown in the FTIR spectra in Figure 3, this band is more intense for microcrystalline cellulose PH-102, which gives us indication of the greater crystallinity for this sample.
The X-ray diffractograms for both samples of microcrystalline cellulose, collected in transmission mode, are shown in Figure 4. It can be seen that both diffractograms are characteristic of semicrystalline materials due to the presence of a hump, ascribed to the amorphous contribution, and some broad peaks, related to the crystalline part.12,29–31 Also, these patterns are very similar to the one displayed by cellulose I (native) with diffraction peaks at 14.7◦ , 16.3◦ , 20.8◦ , 22.5◦ , and 34.6◦ (2θ ),31–34 but four crystalline peaks (14.7◦ , 16.3◦ , 22.5◦ , and 34.6◦ ) have been assumed in other studies.35 However, to really confirm these samples belong to this polymorphic form of cellulose as well as to quantify the amorphous amount, Rietveld refinements using the available crystallographic information [monoclinic (I$ )] and the information provided by the manufacturer were performed. Quantitative Phases Analysis with the Addition of an Internal Standard—Determination of Amorphous Amount One of the most common and accurate ways to determine the crystallinity in semicrystalline polymers is the use of X-ray diffraction data. The study of crystallinity of cellulose, and consequently microcrystalline cellulose in the solid state, has been the subject of great interest of the scientific community.36 There is a common sense that the main variable in native cellulose crystallinity is crystallite size. However, a recent study,37 shows how the simple Segal crystallinity index can be used to evaluate crystallinity. As this method does not take into account the correction of many parameters, we decided not to make use of this procedure in our work even it has been largely employed. Keeping this in mind, XRPD data with the addition of an internal standard of corundum (SRM 676a), commercially distributed by the National Institute of Standards and Technology,38 were
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
DOI 10.1002/jps.23909
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
1397
Table 2. Crystallographic Data JINROO01 (CSD) and 88027 (ICSD) that were Used as the Input Files for the Rietveld Refinement Unit Cell Parameters ˚ a (A) ˚ b (A) ˚ c (A) " (◦ ) $ (◦ ) ( (◦ ) ˚ 3 V (A) Crystal structure Space group
JINROO01 (CSD) 101 and 102
88027 (ICSD) Al2 O3
7.784(8) 8.201(8) 10.38(1) 90 90 96.55(5) 662.6 Monoclinic P21
4.75919(1) 4.75919(1) 12.99183(1) 90.0 90.0 120.0 254.84 Hexagonal ¯ R3c
Table 3. Results of the Rietveld Refinements of Microcrystalline Cellulose PH-101 (Named 101) and Microcrystalline Cellulose PH-102 (Named 102) Figure 5. X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 with the addition of an internal standard (corundum), and the comparison with crystallographic data 88027 (corundum) retrieved from the ICSD.
used in conjunction with the Rietveld method in the quantification procedure of the amorphous amount of both microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102. Figure 5 presents the X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 with the addition of an internal standard of corundum used in this study as well as the calculated diffraction pattern of corundum (crystallographic data 88027)39 obtained from the ICSD. According to the visual inspection of the X-ray diffractograms, it should be noted that corundum was chosen as the internal standard because it does not present overlapping peaks with the most intense ones of the samples under study. In addition, the transmission geometry in which the samples were measured allowed us to prevent effects of preferred orientation of the crystallites, which may affect the quantitative phase analyses. As described in the literature, two polymorphic forms of native cellulose are known;19,20 thus, the Rietveld refinements were carried out without refining the structural coordinates, and they were compared with the results obtained with and without corrections for preferred orientation of the crystallites (four-term spherical harmonics function), using the two crystallographic information files available. The results of the Rietveld refinements were consistent with the information found in the literature that cellulose derived from wood pulp crystallizes under a monoclinic (cellulose I$ ) crystal system. Considering only a simple visual analysis of the Rietveld plots, one cannot distinguish between the fits what is better. Some statistical criteria were used to evaluate the quality of the refinements. Among them, the most important ones are RBragg , Rwp , Rexp , and P 2 .40 Taking into account that the values obtained for these parameters are considered good for the adjustments performed,40 we can see that the choice of the monoclinic crystal structure better describes the studied samples. In Table 2 are shown the crystallographic data of cards JINROO0119 (CSD) and 8802739 (ICSD) used as input files to perform the Rietveld refinements. It was verified that microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 crystallized under a monoDOI 10.1002/jps.23909
Unit Cell Parametersa ˚ a (A) ˚ b (A) ˚ c (A) " (◦ ) $ (◦ ) ( (◦ ) ˚ 3) V (A Crystal structure Space group
101
102
7.910(2) 8.245(7) 10.361(1) 90 90 95.83(12) 672.3(7) Monoclinic P21
7.909(1) 8.236(6) 10.360(1) 90 90 96.36(7) 670.8(5) Monoclinic P21
a The weighted Durbin–Watson statistics (d-DW) is quite low,41 indicating that the SDs are underestimated. There is no adequate physical model to correct the SD values in the Rietveld method, to make them representative of the repetition of the experiment.
clinic crystal system (space group P21 ) thus being attributed as cellulose I$ , as shown in Table 3. To perform quantitative phase analyses of both samples of microcrystalline cellulose, a known amount of corundum was added to the powders. Keeping in mind that also corundum presents a certified amount of amorphous material, which is equal to 1.11 wt %,38 this information was took into account and included in the refinement file into Topas-Academic v. 5. As the internal standard is introduced into a known quantity, the calculations are performed to provide the same amount at the end of each cycle of refinement. In other words, after each cycle, the ratio is determined and multiplied by a scaling factor to provide the same amount added. All other phases are corrected by the same scale factor. The sum of all phases refined, including the internal standard, should be less than 100%. The difference to 100% is the proportion of the amorphous materials. After deconvoluting the known amount of corundum added to the samples (11.00 wt % for microcrystalline PH-101 and 12.56 wt % microcrystalline PH-102), the amorphous quantification resulted in 63.7(11) wt % for microcrystalline cellulose PH-101 and 51.0(11) wt % for microcrystalline cellulose PH-102. The Rietveld graphs displayed in Figures 6 and 7 presented a good fit between the observed and calculated data. The statistical parameters40 also indicated the quality of the refinements. The knowledge of such information is important to the selection of candidate compounds for the formulation of new medicaments because the degree of crystallinity can influence various
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
1398
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
θ ° Figure 6. Rietveld graph of microcrystalline cellulose PH-101 with the addition of the internal standard (corundum). Goodness-of-fit indicator as well as R-factors40 were: P 2 = 1.08, Rwp = 2.89%, Rexp = 2.67%, and RBragg = 1.00.
Rietveld method, it was possible to identify the samples as cellulose I (native), which, in turn, has two polymorphic forms known as cellulose I" and I$ . However, after a careful analysis of the statistical indices from these refinements, one can see that microcrystalline cellulose PH-101 as well as microcrystalline cellulose PH-102 crystallized in a monoclinic crystal system (space group P21 ), being classified as cellulose I" . Quantitative phase analysis with the addition of an internal standard for determination of the amorphous amount is considered new in relation to what has been performed to date and constitutes an important tool to evaluate the degree of crystallinity, not just of microcrystalline cellulose, but of all samples that have some crystalline order. The FTIR results showed that it is possible to classify the samples as the polymorphic form I (native); however, only with this technique, it has not been possible to distinguish them between cellulose I" or cellulose I$ .
Acknowledgments ˜ Paulo ReThe authors thank the financial support from Sao search Foundation (FAPESP—proc. nr. 2008/10537-3), National Council for Scientific and Technological Development (CNPq—proc. nrs. 305186/2012-4 and 477296/2011-4), and UFABC for a scholarship (L.P.F.).
REFERENCES
θ ° Figure 7. Rietveld graph of microcrystalline cellulose PH-102 with the addition of the internal standard (corundum). Goodness-of-fit indicator as well as R-factors40 were: P 2 = 0.96, Rwp = 2.52%, Rexp = 2.62%, and RBragg = 0.51.
properties of pharmaceuticals and excipients, thus affecting their stability. The results obtained in the present work refer only to the specific samples studied. Some deviations from the values obtained may occur because of the conditions in which the experiments are carried out (controlled humidity conditions, for example).
CONCLUSIONS Through the X-ray diffractograms, it was possible to quantify the amorphous amount of microcrystalline cellulose PH101 and microcrystalline cellulose PH-102 via Rietveld refinements with the addition of an internal standard (Al2 O3 ), and we found that the microcrystalline cellulose PH-101 has lower crystallinity [63.7(11) wt %] when compared with microcrystalline cellulose PH-102 [51.0(11) wt %]. In addition, with the
1. Ansel HC, Popovich NG, Allen LV. 2000. Farmacot´ecnica: Formas ˜ de farmacos. ´ ˜ Paulo: farmacˆeuticas e sistemas de liberac¸ao 6th ed., Sao Premier: Cia. dos Livros, p. 568. 2. Vippagunta SR, Brittain HG, Grant DJW. 2001. Crystalline solids. Adv Drug Deliv Rev 48(1):3–26. 3. Rodriguez-Spong B, Price CP, Jayasankar A, Matzger AJ, RodriguezHornedo N. 2004. General principles of pharmaceutical solid polymorphism: A supramolecular perspective. Adv Drug Deliv Rev 56(3):241– 274. 4. Ritschel WA, Kearns GL. 2009. Handbook of basic pharmacokinetics: Including clinical applications. 7th ed., Washington, DC: American Pharmacists Association, p. 490. 5. Bernstein J. 2007. Polymorphism in molecular crystals. 1 ed. Oxford: Clarendon Press, p. 424. ´ 6. de Figueiredo LP. 2012. Analise de excipientes em comprimidos de ´ farmacos comercializados. Santo Andr´e, SP: Universidade Federal do ABC (UFABC), p. 171. 7. Est´evez-Carrizo FE. 2000. Estudio de bioequivalencia: Enfoque ´ metodol´ogico y aplicaciones practicas en la evaluaci´on de medicamentos gen´ericos; Bioequivalence studies: Methodological approach and practical applications for the assessment of generic drugs. Rev Med Uruguay 16(2):133–143. 8. Marzo A. 1997. Clinical pharmacokinetic registration file for NDA and ANDA procedures. Pharm Res 36(6):425–450. 9. Howard RL, Abotsi E, Jansen van Rensburg EL, Howard S. 2003. Lignocellulose biotechnology: Issues of bioconversion and enzyme production. African J Biotech 2(12):602–619. 10. Haward SJ, Sharma V, Butts CP, McKinley GH, Rahatekar SS. 2012. Shear and extensional rheology of cellulose/ionic liquid solutions. Biomacromolecules 13(5):1688–1699. 11. Klemm D, Heublein B, Fink HP, Bohn A. 2005. Cellulose: Fascinating biopolymer and sustainable raw material. Angew Chem Int Ed 44:3358–3393. 12. Ek R, Alderborn G, Nystr¨om C. 1994. Particle analysis of microcrystalline cellulose: Differentiation between individual particles and their agglomerates. Int J Pharm 111(1):43–50.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
DOI 10.1002/jps.23909
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
˜ 13. Quiroz-Castaneda RE, Folch-Mallol JL. 2013. Hydrolysis of biomass mediated by cellulases for the production of sugars. In Sustainable degradation of lignocellulosic biomass—Techniques, applications and commercialization; Chandel AK, da Silva SS, Eds., p. 275. 14. O’Sullivan AC. 1997. Cellulose: The structure slowly unravels. Cellulose 4(3):173–207. 15. Wada M, Heux L, Sugiyama J. 2004. Polymorphism of cellulose I family: Reinvestigation of cellulose IV(I). Biomacromolecules 5:1385– 1391. 16. Atalla RH, Vanderhart DL. 1984. Native cellulose: A composite of two distinct crystalline forms. Science 223(4633):283–285. 17. Sugiyama J, Persson J, Chanzy H. 1991. Combined infrared and electron diffraction study of the polymorphism of native celluloses. Macromolecules 24:2461–2466. 18. Wada M, Okano T, Sugiyama J. 1997. Synchrotron-radiated X-ray and neutron diffraction study of native cellulose. Cellulose 4:221–232. 19. Nishiyama Y, Langan P, Chanzy H. 2002. Crystal structure and hydrogen-bonding system in cellulose I$ from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 124(31):9074–9082. 20. Nishiyama Y, Sugiyama J, Chanzy H, Langan P. 2003. Crystal structure and hydrogen bonding system in cellulose i" from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 125(47):14300– 14306. 21. Rietveld HM. 1967. Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Crystallogr 22(1):151–152. 22. Rietveld HM. 1969. A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 2(2):65–71. 23. Coelho AA, Evans J, Evans I, Kern A, Parsons S. 2011. The TOPAS symbolic computation system. Powder Diffr 26:s22–s25. 24. Cheary RW, Coelho AA. 1998. Axial divergence in a conventional X-ray powder diffractometer. I. Theoretical foundations. J Appl Crystallogr 31(6):851–861. 25. Cheary RW, Coelho AA, 1998. Axial divergence in a conventional X-ray powder diffractometer. II. Realization and evaluation in a Fundamental-Parameter profile fitting procedure. J Appl Crystallogr 31(6):862–868. 26. Ciolacu D, Ciolacu F, Popa VI. 2011. Amorphous cellulose— Structure and characterization. Cellulose Chem Technol 45(1-2):13– 21. 27. Oh SY, Yoo DI, Shin Y, Kim HC, Kim HY, Chung YS, Park WH, Youk JH. 2005. Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy. Carbohydr Res 340(15):2376–2391.
DOI 10.1002/jps.23909
1399
28. Tomaz RMAG, Bittencourt E, Sabino NP, Kondo JI. 1994. ˜ dos ´ındices de cristalinidade de fibras celul´osicas. BraDeterminac¸ao gantia 53:121–126. 29. Park S, Baker JO, Himmel ME, Parilla PA, Johnson DK. 2010. Cellulose crystallinity index: Measurement techniques and their impact on interpreting cellulase performance. Biotechnol. Biofuels 3:1– 10. 30. Wang Z, McDonald AG, Westerhof RJ, Kersten SR, Cuba-Torres CM, Ha S, Pecha B, Garcia-Perez M. 2013. Effect of cellulose crystallinity on the formation of a liquid intermediate and on product distribution during pyrolysis. J Anal Appl Pyrol 100:56–66. 31. Garvey CJ, Parker IH, Simon GP. 2005. On the interpretation of X-ray diffraction powder patterns in terms of the nanostructure of cellulose I fibres. Macromol Chem Phys 206(15):1568–1575. 32. Suesat J, Suwan Ruji PJ. 2011. Preparation and properties of microcrystalline cellulose from corn residues. Adv Mater Res 332–334:1781– 1784. 33. Nelson ML, O’Connor RT, 1964. Relation of certain infrared bands to cellulose crystallinity and crystal lattice type. Part II. A new infrared ratio for estimation of crystallinity in celluloses I and II. J Appl Pol Sci 107:1325–1341. 34. Hult E-L, Iversen T, Sugiyama J. 2003. Characterization of the supermolecular structure of cellulose in wood pulp fibres. Cellulose 10:103–110. 35. He J, Cui S, Wang S-y. 2008. Preparation and crystalline analysis of high-grade bamboo dissolving pulp for cellulose acetate. J Appl Polym Sci 107(2):1029–1038. ˚ K. 2005. 36. Thygesen A, Oddershede J, Lilholt H, Thomsen AB, Stahl On the determination of crystallinity and cellulose content in plant fibres. Cellulose 12(6):563–576. 37. French AD, Cintr´on MS. 2013. Cellulose polymorphy, crystallite size, and the segal crystallinity index. Cellulose 20:583–588. 38. Certificate of Analysis. 2008. Standard Reference Material 676a. Alumina powder for quantitative analysis by X-ray diffraction. Gaithersburg, MD: National Institute of Standards & Technology— NIST. 39. Oetzel M, Heger G. 1999. Laboratory X-ray powder diffraction: A comparison of different geometries with special attention to the usage of the Cu K[alpha] doublet. J Appl Crystallogr 32(4):799– 807. 40. Toby BH. 2006. R factors in Rietveld analysis: How good is good enough? Powder Diffr 21:67–70. 41. Hill RJ, Flack HD. 1987. The use of the Durbin–Watson d statistic in Rietveld analysis. J Appl Crystallogr 20:356–361.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
The Rietveld Method as a Tool to Quantify the Amorphous Amount of Microcrystalline Cellulose LAYSA PIRES DE FIGUEIREDO, FABIO FURLAN FERREIRA Centro de Ciˆencias Naturais e Humanas (CCNH), Universidade Federal do ABC (UFABC), Santo Andr´e, SP CEP: 09210-580, Brazil Received 17 December 2013; revised 14 January 2014; accepted 31 January 2014 Published online 1 March 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.23909 ABSTRACT: With the use of X-ray powder diffraction data and the Rietveld method, we verified that both microcrystalline cellulose PH101 and PH-102 presented differences in the crystallinity degree. The results revealed these samples are semicrystalline in nature and via Rietveld refinements, it was possible, adding a known amount of an internal standard of corundum to the samples, to perform quantitative phase analyses, which allowed us to determine the amorphous amount of the studied samples: 63.7(11) wt % in microcrystalline cellulose PH-101 and 51.0(11) wt % in microcrystalline cellulose PH-102. An important contribution of this work refers to the attempt of using C 2014 Wiley Periodicals, Inc. and the this simple method, permitting to evaluate the degree of crystallinity of microcrystalline cellulose. American Pharmacists Association J Pharm Sci 103:1394–1399, 2014 Keywords: microcrystalline cellulose PH-101; microcrystalline cellulose PH-102; crystallinity; X-ray powder diffraction; Rietveld method; FTIR; dehydration; solid state
INTRODUCTION Many organic compounds are able to adopt one or more pure crystalline forms of well-defined and easily identifiable configuration, or an amorphous form without long-range order, depending on the conditions (temperature, solvent, time) under which crystallization is induced. It is in this context the term polymorphism arises, being defined as the ability of a compound to crystallize in two or more crystal structures with the same chemical composition, due to the differences in spatial arrangements and/or conformation of the molecules.1–4 It is noteworthy that polymorphism may cause a direct impact on several characteristics of pharmaceuticals, thus displaying unexpected results, which may compromise the efficacy of the medicinal product. As many pharmaceuticals, excipients can also present polymorphic transformations, being a worrying factor for the pharmaceutical industries.5,6 On this basis, the choice of excipients suitable for particular formulation is a key point to the therapeutic efficacy of the final product. This choice must be based on the characteristics of the substances contained in the formulation, as well as the possibility of interaction with the excipient. In this context, it is essential that during the research/development of formulations, one can be able to select the correct polymorphic form of an active pharmaceutical ingredient as well as excipients that do not suffer any kind of interaction, ensuring that both bioavailability and the pharmacological effect take place. Thus, the research of polymorphism in pharmaceuticals and excipients is one of the most important subjects to be considered prior to the production of medicaments, mainly because the lack of knowledge of different crystalline forms can influence the production procedure and may bring health problems to patients and financial losses to their manufacturers.7,8 Correspondence to: Fabio Furlan Ferreira (Telephone: +55-11-4996-8370; Fax: +55-11-4996-0090; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 103, 1394–1399 (2014) C 2014 Wiley Periodicals, Inc. and the American Pharmacists Association
1394
Herein, we report on the investigation of the structural characterization and quantitative phase analysis of microcrystalline cellulose, a purified and partially depolymerized cellulose prepared by high-quality treatment with mineral acid to hydrolyze cellulose wood pulp, in order to decrease the degree of polymerization. Figure 1 shows the molecular structure of cellulose as a carbohydrate polymer generated from repeating $D-glucopyranose molecules that are covalently linked through acetal functions between the equatorial OH group of C(4) and the C(1) carbon atoms ($-1,4-glucan), which is, in principle, the manner in which cellulose is biogenetically formed.9 As a result, cellulose is an extensive, linear-chain polymer with a large number of hydroxyl groups [three per anhydroglucose (AGU) unit] present in the thermodynamically preferred 4 C1 conformation. To accommodate the preferred bond angles of the acetal oxygen bridges, every second AGU ring is rotated 180◦ in the plane. In this manner, two adjacent structural units define the disaccharide cellobiose.10,11 As shown in the molecular structure represented in Figure 1, the hydroxyl groups of $-1,4-glucan cellulose are placed at positions C(2) and C(3) (secondary, equatorial) as well as C(6) (primary). The CH2 OH side group is arranged in a trans-gauche (tg) position relative to the O(5)–C(5) and C(4)–C(5) bonds.9 As a result, microcrystalline cellulose has a semicrystalline structure, that is, part of its structure is amorphous and other is crystalline (Fig. 2). The crystalline regions are grouped into nuclei called crystallites. These crystallites, in turn, are surrounded by amorphous regions.12 The proportion between amorphous and crystalline regions determines the degree of crystallinity, object of study in this work. Celluloses I␣ and I Currently, it is known that cellulose exists in more than one polymorphic form,14 that is, there is not a single unit cell dimension, showing six polymorphic forms for cellulose I, which also display polymorphism, and receive the following names: cellulose I, cellulose II, cellulose IIII , cellulose IIIII , cellulose
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
1395
Figure 1. Molecular structure of cellulose (n = DP, degree of polymerization).
by a pharmaceutical company that will not have its information disclosed. The techniques used for the studies were FTIR and XRPD together with the Rietveld method. Fourier Transform Infrared Spectroscopy Figure 2. Illustration containing crystalline regions interspersed by amorphous ones of cellulose. (Adapted from Ref. 13.)
IVI , and cellulose IVII , although the existence of cellulose IVI is controversial.15 Among those, cellulose I is receiving increased attention because of its potential use in bioenergy production, and according to the literature the existence of more than one polymorphic form of cellulose in native samples has been shown.16,17 Today, we know that cellulose I actually consisted of two polymorphic forms, called I" (one-chain triclinic structure) and I$ (two-chain monoclinic structure). In this scheme, native celluloses were all classified into I" -rich algal-bacterial type or I$ -dominant cottonramie type.18 The crystal structure and hydrogen-bonding system in cellulose I$ was elucidated by the combined use of synchrotron X-ray and neutron fiber diffraction.19 Oriented fibrous samples were prepared by aligning cellulose microcrystals from tunicin, reconstituted into oriented films. These samples diffracted ˚ resboth synchrotron X-rays and neutrons to better than 1 A olution, yielding more than 300 unique reflections and an unambiguous assignment of the monoclinic unit cell dimen˚ b = 8.201(8) A; ˚ c = 10.380(10) A; ˚ and sions: a = 7.784(8) A; ( = 96.55(5)◦ , in the space group P21 . The crystal and molecular structures of the cellulose I" have been established using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from the cell wall of fresh water alga Glaucocystis nostochinearum.20 Cellulose I" crystallizes in a tri˚ b = 6.717(6) A; ˚ c = 5.962(7) A; ˚ clinic unit cell: a = 10.400(10) A; " = 80.37(5)◦ ; $ = 118.08(5)◦ ; and ( = 114.80(5)◦ , having space group P1. It should be mentioned that the determination of the crystal structure of cellulose by X-ray diffraction played an important role in the polymer science and the concept of long-chain molecules. In order to verify such statements and information, which may serve as a subsidy for the pharmaceutical industries, quantitative phase analyses by means of X-ray powder diffraction (XRPD) data and the Rietveld method were used to evaluate the amorphous amount of both microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102. Also, FTIR was used to follow their possible differences in absorption bands.
Spectral data were recorded in the range from 4000 to 650 cm−1 with a resolution of 4 cm−1 , in a Fourier transform spectrometer, model 660-IR, from Varian (Palo Alto, CA, USA), equipped with an attenuated total reflection accessory and a ZnSe crystal. FTIR measurements were conducted at the Multiuser Experimental Centre/Universidade Federal do ABC (CEM/UFABC). XRPD and Rietveld Refinements X-ray powder diffraction data were collected on a STADI-P diffractometer, from Stoe (Darmstadt, Germany), operating in ˚ the transmission mode, using CuK"1 radiation (8 = 1.54056 A), an accelerating voltage of 40 kV and current of 40 mA, equipped with a primary beam monochromator [Ge (111) curved crystal], a 0.5-mm divergence slit, and a 0.3-mm circular scattering slit. The X-ray photons were detected with a linear detector (Mythen 1K, from Dectris(Baden, Switzerland)). The samples were loaded between two 0.014-mm thick cellulose-acetate foils, with density of 1.3 g cm−3 , and were spun during data collection. Measurements were performed in the angular range from 5◦ to 50◦ (2θ ), with step sizes of 0.015◦ and an integration time of 60 s at each 1.05◦ . These analyses were conducted in the Laboratory of Crystallography and Structural Characterization of Materials (LCCEM) of UFABC. For the identification of the crystal structures, both the Cambridge Structural Database (CSD) and Inorganic Crystal Structure Database (ICSD) were used; the structural characterization of samples and the evaluation of the amorphous amount were carried out by means of the Rietveld method21,22 using XRPD data and the Topas-Academic v. 523 software program. First, we refined the unit cell parameters and the zero point of the diffractometer. The background was fitted using a 12-term Chebyshev polynomial. The peak asymmetry was adjusted using the simple axial divergence model of Cheary and Coelho.24,25 The peak profiles were modeled using the fundamental-parameter approach25 and the preferred orientation of the crystals were corrected with a fourth-order spherical harmonics function. The isotropic atomic displacements (Biso ) of all nonhydrogen atoms were fixed to 2.8. For the hydrogen atoms, the Biso values were assumed to be 1.2 times larger than the values of the nonhydrogen atoms. R
R
RESULTS AND DISCUSSION Fourier Transform Infrared Spectroscopy
MATERIALS AND METHODS Powdered samples of microcrystalline cellulose PH-101 and PH-102 (Avicel from FMC BioPolymers) were kindly provided R
DOI 10.1002/jps.23909
The FTIR spectra for microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 samples are shown in Figure 3.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
1396
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
Figure 3. FTIR spectra for microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 samples, obtained with a resolution of 4 cm−1 in the region from 4000 to 650 cm−1 .
Figure 4. X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 collected in transmission mode. The X-ray diffractogram of microcrystalline cellulose PH-102 were offset for better visualization.
Table 1. Location of the Main Absorption Bands of Cellulose I (Native),26,27 Microcrystalline Cellulose PH-101 and Microcrystalline Cellulose PH-102 with Their Respective Chemical Bonds
Also, we can infer that both FTIR spectra are characteristic of cellulose I (native), as the absorption bands are similar to reported values in the literature27 for this polymorphic form of cellulose.
Cellulose I
X-Ray Powder Diffraction
3352 2901 1431 1373 1319 1282 1236 1202 1165 1032 983 897
101 L (cm−1 ) 3336 2900 1430 1372 1318 1289 1230 1206 1156 1031 984 897
102 Assignment 3339 2900 1430 1372 1314 1287 1232 1204 1161 1031 984 896
(OH (hydrogen bonded) (CH *CH2 (sym) at C-6 *CH *CH2 (wagging) at C-6 *CH *COH in plane at C-6 *COH in plane at C-6 *COC at $-glucosidic linkage (CO at C-6 (CO at C-6 (COC at $-glucosidic linkage, (COC, (CCO, and (CCH at C-5 and C-6
(, stretching; *, bending.
Table 1 displays the main absorption bands of cellulose I (native), microcrystalline cellulose PH-101 (named 101), and microcrystalline cellulose PH-102 (named 102) investigated in this study, with the assignment of the respective chemical bonds.26,27 Not all absorption bands were identified, considering that FTIR spectroscopic investigations were carried out with the aim of evidencing that an alteration of the crystalline organization leads to a significant simplification of the spectral shape by a reduction in intensity or even disappearance of the characteristic bands of the crystalline domains. It is worth mentioning that the band at 1430 cm−1 is known as the band of crystallinity, that is, a decrease in its intensity reflects in the reduction of the degree of crystallinity of the sample.26–28 As shown in the FTIR spectra in Figure 3, this band is more intense for microcrystalline cellulose PH-102, which gives us indication of the greater crystallinity for this sample.
The X-ray diffractograms for both samples of microcrystalline cellulose, collected in transmission mode, are shown in Figure 4. It can be seen that both diffractograms are characteristic of semicrystalline materials due to the presence of a hump, ascribed to the amorphous contribution, and some broad peaks, related to the crystalline part.12,29–31 Also, these patterns are very similar to the one displayed by cellulose I (native) with diffraction peaks at 14.7◦ , 16.3◦ , 20.8◦ , 22.5◦ , and 34.6◦ (2θ ),31–34 but four crystalline peaks (14.7◦ , 16.3◦ , 22.5◦ , and 34.6◦ ) have been assumed in other studies.35 However, to really confirm these samples belong to this polymorphic form of cellulose as well as to quantify the amorphous amount, Rietveld refinements using the available crystallographic information [monoclinic (I$ )] and the information provided by the manufacturer were performed. Quantitative Phases Analysis with the Addition of an Internal Standard—Determination of Amorphous Amount One of the most common and accurate ways to determine the crystallinity in semicrystalline polymers is the use of X-ray diffraction data. The study of crystallinity of cellulose, and consequently microcrystalline cellulose in the solid state, has been the subject of great interest of the scientific community.36 There is a common sense that the main variable in native cellulose crystallinity is crystallite size. However, a recent study,37 shows how the simple Segal crystallinity index can be used to evaluate crystallinity. As this method does not take into account the correction of many parameters, we decided not to make use of this procedure in our work even it has been largely employed. Keeping this in mind, XRPD data with the addition of an internal standard of corundum (SRM 676a), commercially distributed by the National Institute of Standards and Technology,38 were
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
DOI 10.1002/jps.23909
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
1397
Table 2. Crystallographic Data JINROO01 (CSD) and 88027 (ICSD) that were Used as the Input Files for the Rietveld Refinement Unit Cell Parameters ˚ a (A) ˚ b (A) ˚ c (A) " (◦ ) $ (◦ ) ( (◦ ) ˚ 3 V (A) Crystal structure Space group
JINROO01 (CSD) 101 and 102
88027 (ICSD) Al2 O3
7.784(8) 8.201(8) 10.38(1) 90 90 96.55(5) 662.6 Monoclinic P21
4.75919(1) 4.75919(1) 12.99183(1) 90.0 90.0 120.0 254.84 Hexagonal ¯ R3c
Table 3. Results of the Rietveld Refinements of Microcrystalline Cellulose PH-101 (Named 101) and Microcrystalline Cellulose PH-102 (Named 102) Figure 5. X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 with the addition of an internal standard (corundum), and the comparison with crystallographic data 88027 (corundum) retrieved from the ICSD.
used in conjunction with the Rietveld method in the quantification procedure of the amorphous amount of both microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102. Figure 5 presents the X-ray diffractograms of microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 with the addition of an internal standard of corundum used in this study as well as the calculated diffraction pattern of corundum (crystallographic data 88027)39 obtained from the ICSD. According to the visual inspection of the X-ray diffractograms, it should be noted that corundum was chosen as the internal standard because it does not present overlapping peaks with the most intense ones of the samples under study. In addition, the transmission geometry in which the samples were measured allowed us to prevent effects of preferred orientation of the crystallites, which may affect the quantitative phase analyses. As described in the literature, two polymorphic forms of native cellulose are known;19,20 thus, the Rietveld refinements were carried out without refining the structural coordinates, and they were compared with the results obtained with and without corrections for preferred orientation of the crystallites (four-term spherical harmonics function), using the two crystallographic information files available. The results of the Rietveld refinements were consistent with the information found in the literature that cellulose derived from wood pulp crystallizes under a monoclinic (cellulose I$ ) crystal system. Considering only a simple visual analysis of the Rietveld plots, one cannot distinguish between the fits what is better. Some statistical criteria were used to evaluate the quality of the refinements. Among them, the most important ones are RBragg , Rwp , Rexp , and P 2 .40 Taking into account that the values obtained for these parameters are considered good for the adjustments performed,40 we can see that the choice of the monoclinic crystal structure better describes the studied samples. In Table 2 are shown the crystallographic data of cards JINROO0119 (CSD) and 8802739 (ICSD) used as input files to perform the Rietveld refinements. It was verified that microcrystalline cellulose PH-101 and microcrystalline cellulose PH-102 crystallized under a monoDOI 10.1002/jps.23909
Unit Cell Parametersa ˚ a (A) ˚ b (A) ˚ c (A) " (◦ ) $ (◦ ) ( (◦ ) ˚ 3) V (A Crystal structure Space group
101
102
7.910(2) 8.245(7) 10.361(1) 90 90 95.83(12) 672.3(7) Monoclinic P21
7.909(1) 8.236(6) 10.360(1) 90 90 96.36(7) 670.8(5) Monoclinic P21
a The weighted Durbin–Watson statistics (d-DW) is quite low,41 indicating that the SDs are underestimated. There is no adequate physical model to correct the SD values in the Rietveld method, to make them representative of the repetition of the experiment.
clinic crystal system (space group P21 ) thus being attributed as cellulose I$ , as shown in Table 3. To perform quantitative phase analyses of both samples of microcrystalline cellulose, a known amount of corundum was added to the powders. Keeping in mind that also corundum presents a certified amount of amorphous material, which is equal to 1.11 wt %,38 this information was took into account and included in the refinement file into Topas-Academic v. 5. As the internal standard is introduced into a known quantity, the calculations are performed to provide the same amount at the end of each cycle of refinement. In other words, after each cycle, the ratio is determined and multiplied by a scaling factor to provide the same amount added. All other phases are corrected by the same scale factor. The sum of all phases refined, including the internal standard, should be less than 100%. The difference to 100% is the proportion of the amorphous materials. After deconvoluting the known amount of corundum added to the samples (11.00 wt % for microcrystalline PH-101 and 12.56 wt % microcrystalline PH-102), the amorphous quantification resulted in 63.7(11) wt % for microcrystalline cellulose PH-101 and 51.0(11) wt % for microcrystalline cellulose PH-102. The Rietveld graphs displayed in Figures 6 and 7 presented a good fit between the observed and calculated data. The statistical parameters40 also indicated the quality of the refinements. The knowledge of such information is important to the selection of candidate compounds for the formulation of new medicaments because the degree of crystallinity can influence various
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
1398
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
θ ° Figure 6. Rietveld graph of microcrystalline cellulose PH-101 with the addition of the internal standard (corundum). Goodness-of-fit indicator as well as R-factors40 were: P 2 = 1.08, Rwp = 2.89%, Rexp = 2.67%, and RBragg = 1.00.
Rietveld method, it was possible to identify the samples as cellulose I (native), which, in turn, has two polymorphic forms known as cellulose I" and I$ . However, after a careful analysis of the statistical indices from these refinements, one can see that microcrystalline cellulose PH-101 as well as microcrystalline cellulose PH-102 crystallized in a monoclinic crystal system (space group P21 ), being classified as cellulose I" . Quantitative phase analysis with the addition of an internal standard for determination of the amorphous amount is considered new in relation to what has been performed to date and constitutes an important tool to evaluate the degree of crystallinity, not just of microcrystalline cellulose, but of all samples that have some crystalline order. The FTIR results showed that it is possible to classify the samples as the polymorphic form I (native); however, only with this technique, it has not been possible to distinguish them between cellulose I" or cellulose I$ .
Acknowledgments ˜ Paulo ReThe authors thank the financial support from Sao search Foundation (FAPESP—proc. nr. 2008/10537-3), National Council for Scientific and Technological Development (CNPq—proc. nrs. 305186/2012-4 and 477296/2011-4), and UFABC for a scholarship (L.P.F.).
REFERENCES
θ ° Figure 7. Rietveld graph of microcrystalline cellulose PH-102 with the addition of the internal standard (corundum). Goodness-of-fit indicator as well as R-factors40 were: P 2 = 0.96, Rwp = 2.52%, Rexp = 2.62%, and RBragg = 0.51.
properties of pharmaceuticals and excipients, thus affecting their stability. The results obtained in the present work refer only to the specific samples studied. Some deviations from the values obtained may occur because of the conditions in which the experiments are carried out (controlled humidity conditions, for example).
CONCLUSIONS Through the X-ray diffractograms, it was possible to quantify the amorphous amount of microcrystalline cellulose PH101 and microcrystalline cellulose PH-102 via Rietveld refinements with the addition of an internal standard (Al2 O3 ), and we found that the microcrystalline cellulose PH-101 has lower crystallinity [63.7(11) wt %] when compared with microcrystalline cellulose PH-102 [51.0(11) wt %]. In addition, with the
1. Ansel HC, Popovich NG, Allen LV. 2000. Farmacot´ecnica: Formas ˜ de farmacos. ´ ˜ Paulo: farmacˆeuticas e sistemas de liberac¸ao 6th ed., Sao Premier: Cia. dos Livros, p. 568. 2. Vippagunta SR, Brittain HG, Grant DJW. 2001. Crystalline solids. Adv Drug Deliv Rev 48(1):3–26. 3. Rodriguez-Spong B, Price CP, Jayasankar A, Matzger AJ, RodriguezHornedo N. 2004. General principles of pharmaceutical solid polymorphism: A supramolecular perspective. Adv Drug Deliv Rev 56(3):241– 274. 4. Ritschel WA, Kearns GL. 2009. Handbook of basic pharmacokinetics: Including clinical applications. 7th ed., Washington, DC: American Pharmacists Association, p. 490. 5. Bernstein J. 2007. Polymorphism in molecular crystals. 1 ed. Oxford: Clarendon Press, p. 424. ´ 6. de Figueiredo LP. 2012. Analise de excipientes em comprimidos de ´ farmacos comercializados. Santo Andr´e, SP: Universidade Federal do ABC (UFABC), p. 171. 7. Est´evez-Carrizo FE. 2000. Estudio de bioequivalencia: Enfoque ´ metodol´ogico y aplicaciones practicas en la evaluaci´on de medicamentos gen´ericos; Bioequivalence studies: Methodological approach and practical applications for the assessment of generic drugs. Rev Med Uruguay 16(2):133–143. 8. Marzo A. 1997. Clinical pharmacokinetic registration file for NDA and ANDA procedures. Pharm Res 36(6):425–450. 9. Howard RL, Abotsi E, Jansen van Rensburg EL, Howard S. 2003. Lignocellulose biotechnology: Issues of bioconversion and enzyme production. African J Biotech 2(12):602–619. 10. Haward SJ, Sharma V, Butts CP, McKinley GH, Rahatekar SS. 2012. Shear and extensional rheology of cellulose/ionic liquid solutions. Biomacromolecules 13(5):1688–1699. 11. Klemm D, Heublein B, Fink HP, Bohn A. 2005. Cellulose: Fascinating biopolymer and sustainable raw material. Angew Chem Int Ed 44:3358–3393. 12. Ek R, Alderborn G, Nystr¨om C. 1994. Particle analysis of microcrystalline cellulose: Differentiation between individual particles and their agglomerates. Int J Pharm 111(1):43–50.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
DOI 10.1002/jps.23909
RESEARCH ARTICLE – Pharmaceutics, Drug Delivery and Pharmaceutical Technology
˜ 13. Quiroz-Castaneda RE, Folch-Mallol JL. 2013. Hydrolysis of biomass mediated by cellulases for the production of sugars. In Sustainable degradation of lignocellulosic biomass—Techniques, applications and commercialization; Chandel AK, da Silva SS, Eds., p. 275. 14. O’Sullivan AC. 1997. Cellulose: The structure slowly unravels. Cellulose 4(3):173–207. 15. Wada M, Heux L, Sugiyama J. 2004. Polymorphism of cellulose I family: Reinvestigation of cellulose IV(I). Biomacromolecules 5:1385– 1391. 16. Atalla RH, Vanderhart DL. 1984. Native cellulose: A composite of two distinct crystalline forms. Science 223(4633):283–285. 17. Sugiyama J, Persson J, Chanzy H. 1991. Combined infrared and electron diffraction study of the polymorphism of native celluloses. Macromolecules 24:2461–2466. 18. Wada M, Okano T, Sugiyama J. 1997. Synchrotron-radiated X-ray and neutron diffraction study of native cellulose. Cellulose 4:221–232. 19. Nishiyama Y, Langan P, Chanzy H. 2002. Crystal structure and hydrogen-bonding system in cellulose I$ from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 124(31):9074–9082. 20. Nishiyama Y, Sugiyama J, Chanzy H, Langan P. 2003. Crystal structure and hydrogen bonding system in cellulose i" from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 125(47):14300– 14306. 21. Rietveld HM. 1967. Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Crystallogr 22(1):151–152. 22. Rietveld HM. 1969. A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 2(2):65–71. 23. Coelho AA, Evans J, Evans I, Kern A, Parsons S. 2011. The TOPAS symbolic computation system. Powder Diffr 26:s22–s25. 24. Cheary RW, Coelho AA. 1998. Axial divergence in a conventional X-ray powder diffractometer. I. Theoretical foundations. J Appl Crystallogr 31(6):851–861. 25. Cheary RW, Coelho AA, 1998. Axial divergence in a conventional X-ray powder diffractometer. II. Realization and evaluation in a Fundamental-Parameter profile fitting procedure. J Appl Crystallogr 31(6):862–868. 26. Ciolacu D, Ciolacu F, Popa VI. 2011. Amorphous cellulose— Structure and characterization. Cellulose Chem Technol 45(1-2):13– 21. 27. Oh SY, Yoo DI, Shin Y, Kim HC, Kim HY, Chung YS, Park WH, Youk JH. 2005. Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy. Carbohydr Res 340(15):2376–2391.
DOI 10.1002/jps.23909
1399
28. Tomaz RMAG, Bittencourt E, Sabino NP, Kondo JI. 1994. ˜ dos ´ındices de cristalinidade de fibras celul´osicas. BraDeterminac¸ao gantia 53:121–126. 29. Park S, Baker JO, Himmel ME, Parilla PA, Johnson DK. 2010. Cellulose crystallinity index: Measurement techniques and their impact on interpreting cellulase performance. Biotechnol. Biofuels 3:1– 10. 30. Wang Z, McDonald AG, Westerhof RJ, Kersten SR, Cuba-Torres CM, Ha S, Pecha B, Garcia-Perez M. 2013. Effect of cellulose crystallinity on the formation of a liquid intermediate and on product distribution during pyrolysis. J Anal Appl Pyrol 100:56–66. 31. Garvey CJ, Parker IH, Simon GP. 2005. On the interpretation of X-ray diffraction powder patterns in terms of the nanostructure of cellulose I fibres. Macromol Chem Phys 206(15):1568–1575. 32. Suesat J, Suwan Ruji PJ. 2011. Preparation and properties of microcrystalline cellulose from corn residues. Adv Mater Res 332–334:1781– 1784. 33. Nelson ML, O’Connor RT, 1964. Relation of certain infrared bands to cellulose crystallinity and crystal lattice type. Part II. A new infrared ratio for estimation of crystallinity in celluloses I and II. J Appl Pol Sci 107:1325–1341. 34. Hult E-L, Iversen T, Sugiyama J. 2003. Characterization of the supermolecular structure of cellulose in wood pulp fibres. Cellulose 10:103–110. 35. He J, Cui S, Wang S-y. 2008. Preparation and crystalline analysis of high-grade bamboo dissolving pulp for cellulose acetate. J Appl Polym Sci 107(2):1029–1038. ˚ K. 2005. 36. Thygesen A, Oddershede J, Lilholt H, Thomsen AB, Stahl On the determination of crystallinity and cellulose content in plant fibres. Cellulose 12(6):563–576. 37. French AD, Cintr´on MS. 2013. Cellulose polymorphy, crystallite size, and the segal crystallinity index. Cellulose 20:583–588. 38. Certificate of Analysis. 2008. Standard Reference Material 676a. Alumina powder for quantitative analysis by X-ray diffraction. Gaithersburg, MD: National Institute of Standards & Technology— NIST. 39. Oetzel M, Heger G. 1999. Laboratory X-ray powder diffraction: A comparison of different geometries with special attention to the usage of the Cu K[alpha] doublet. J Appl Crystallogr 32(4):799– 807. 40. Toby BH. 2006. R factors in Rietveld analysis: How good is good enough? Powder Diffr 21:67–70. 41. Hill RJ, Flack HD. 1987. The use of the Durbin–Watson d statistic in Rietveld analysis. J Appl Crystallogr 20:356–361.
De Figueiredo and Ferreira, JOURNAL OF PHARMACEUTICAL SCIENCES 103:1394–1399, 2014
