Langmuir 2008, 24, 6745-6753
6745
Chemical and Morphological Consequences of Acidification of Pure, Phosphated, and Phosphonated CaO: Influence of CO2 Adsorption Mohamed I. Zaki,*,† Helmut Kno¨zinger,‡ Bernd Tesche,§ Gamal A. H. Mekhemer,† and Hans-Josef Bongard§ Chemistry Department, Faculty of Science, Minia UniVersity, El-Minia 61519, Egypt, Department Chemie and Biochemie, UniVersta¨t Mu¨nchen, Butenandtstrasse 5-13, Haus E, D-81377 Mu¨nchen, Germany, and Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45466 Mu¨lheim/Ruhrr, Germany ReceiVed January 5, 2008. ReVised Manuscript ReceiVed March 12, 2008 In situ Fourier transform infrared (FTIR) spectroscopy was employed to characterize the adsorption behavior (as a function of pressure or time) and surface species of CO2 molecules on pure, phosphated, and phosphonated CaO. Carbonate and bicarbonate species were found to form on the pure oxide, whereas on the phosphated and phosphonated oxide samples the carbonate species were found to substitute favorably some of the OH- and PO43- groups thereon exposed, respectively. Before and after carbonation, the test samples were further examined by in situ FTIR spectroscopy of adsorbed pyridine species, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Then they were in situ acidified by exposure to a wet atmosphere of HCl vapor at 673 K for 10 min and re-examined similarly to reveal the influence of CO2 adsorption on the chemical and morphological consequences of acidification. The results obtained show the carbonate substitution of PO43- groups to enhance agglomeration of the otherwise fine, longitudinal material particles into much bulkier ones and to render the otherwise more stable phosphonate groups less stable to acid treatment than the phosphate groups. Moreover, the bulky particle agglomerates of the carbonated test samples were detectably eroded following the acid treatment.
1. Introduction In an effort to identify surface attributes of osteoporosis, meaning abnormalities in the amount and structure of bone that result in reduced bone strength and increased risk of fractures,1 two previous investigations were performed in these laboratories.2,3 In the first investigation,2 hydroxyapatite (HAP, Ca5(PO4)3(OH)), the major constituent material of bone substance, was subjected briefly to a wet atmosphere of HCl vapor at 673 K. Then chemical and morphological consequences of the acid treatment were characterized by spectroscopic and electron microscopic studies. The results obtained2 revealed removal of PO43- and OH groups, with formation of what appeared to be deep grooves. It has been shown,2 moreover, that thermal treatment of HAP in the ambient atmosphere leads to formation of surface (and bulk) calcium carbonate species, whose observed acid-induced decomposition into CaO has been tentatively suggested to facilitate the destabilization of HAP’s chemical integrity. Therefore, the second investigation3 was undertaken to characterize the influence of phosphate or phosphonate additives on the surface acid-base and morphological properties of CaO, being considered to simulate an osteoporotic bone material (i.e., phosphate-stripped bone via osteoclast activity3,4). The use of phosphonate additives was encouraged by the fact that recent medical treatments of osteoporotic bone favor the application of the methylene bisphosphonate (MBP) class of drugs.5,6 This has been attributed to the strong binding of MBP molecules to calcium * To whom correspondence should be addressed. Fax: +20 862360833. E-mail:
[email protected]. † Minia University. ‡ Universta¨t Mu¨nchen. § Max-Planck-Institut fu¨r Kohlenforschung. (1) Stevenson, J. C.; Lindsay, R. Osteoporosis; Chapman & Hall: Cambridge, U.K., 1988. (2) Zaki, M. I.; Kno¨zinger, H.; Tesche, B. Langmuir 2006, 22, 749. (3) Zaki, M. I.; Kno¨zinger, H.; Tesche, B.; Mekhemer, G. A. H. J. Colloid Interface Sci. 2006, 303, 9. (4) Kee, T.; Dixon, N. Chem. Br. 2001, 37, 38. (5) Parfitt, A. M. Am. J. Med. 1991, 91, 425.
sites exposed on the surfaces of HAP crystals in growing or regenerating bone.6 The fact that MBP molecules are pyrophosphate analogues in which the P-O-P bond is replaced by a P-C-P function renders them relatively more stable to enzymecatalyzed hydrolysis, but allows them to maintain a good deal of pyrophosphate character, especially in their ability to bind to HAP.6 The results obtained3 have shown phosphation and phosphonation, particularly the latter, to weaken the otherwise strong tendency of CaO toward rehydration and enhance its particle growth in a preferential direction. These chemical and morphological modifications of CaO, which are stable to hightemperature treatments under reduced pressures, have, moreover, been found to strengthen surface Lewis acid sites (Ca2+) slightly and weaken both the strength and reactivity of Lewis base sites (OH-, O2-). These results have been considered to underline surface chemical attributes for application of the MBP class of drugs to hamper acid-induced resorption of bone material. In the present paper we present and discuss the results of in situ infrared spectroscopy and electron microscopy investigations carried out on the same set of pure, phosphated, and phosphonated CaO samples that were examined in the previous investigation.3 These experiments were designed to reveal the influence of CO2 adsorption/absorption on the chemical and morphological consequences of the HCl treatment of the test samples, CO2 being released in increasing amounts in the surrounding atmosphere. Therefore, the test samples were acid-treated before and after exposure to a CO2 atmosphere. Subsequently, they were examined by infrared spectroscopy of adsorbed pyridine, scanning electron microscopy, and energy dispersive X-ray spectroscopy.
2. Experimental Section 2.1. Test and Reference Materials. The test materials used in the present investigation were those prepared and characterized (6) Flanagan, A. M.; Chambers, T. J. Bone Miner. 1989, 6, 33.
10.1021/la8000366 CCC: $40.75 2008 American Chemical Society Published on Web 05/30/2008
6746 Langmuir, Vol. 24, No. 13, 2008 previously.3 The pure calcium oxide (CaO; SBET ) 23 ( 2 m2 g-1) was synthesized by calcination at 973 K for 3 h of a homemade calcium hydroxide.3 Phosphated (P-CaO; SBET ) 18 ( 2 m2 g-1) and phosphonated (PN-CaO; SBET ) 15 ( 2 m2 g-1) calcium oxides were prepared by calcination at 973 K for 3 h of phosphoric acidimpregnated (H3PO4/CaO) and 1-hydroxyethane-1,1-diphosphonic acid-impregnated (HEDP/CaO) calcium oxides, respectively. The impregnation procedure and the chemicals used have been detailed previously.3 For control purposes, however, a commercial HAP (Ca5(PO4)3(OH); AR-grade, Aldrich) was used as supplied. A thorough characterization of these materials, using a range of bulk and surface analytical techniques, has revealed3 their strong tendency toward rehydration and carbonation when handled under ambient conditions. The CaO, which has been found to consist of cubic CaO microcrystallites,3 rehydrates almost quantitatively back to the parent hydroxide (CaO + H2O f Ca(OH)2) upon brief exposure to the ambient atmosphere. This strong rehydration tendency of the pure oxide has been shown3 to be considerably retrogressed upon phosphonation (PN-CaO) and, but to a lesser extent, phosphation (P-CaO). This has been attributed3 to replacement of P-O-P bridges in P-CaO by the much less hydrolyzable P-C-P bridges in PNCaO. It was not until the material was thermoevacuated at 973 K that carbonate surface species were completely eliminated from all test samples. 2.2. HCl Solution. HCl solution (36 wt %; d ) 1.18 g/mL), the vapor of which was used for the acid treatments, was a Prolabo product. It was used as supplied. 2.3. Gases and Probe Molecules. Carbon dioxide (CO2 gas; 99.99%, Merck) and pyridine (Py liquid, C5H6N; AR-grade, Aldrich) were used as adsorptive and infrared surface probe molecules. Oxygen (O2; 99%; ECIG, Hawamdyia, Egypt) was used for in situ pretreatments. The gases were used as supplied, whereas the Py liquid was thoroughly deaerated before application, using freezepump-thaw cycles. 2.4. Methods and Techniques. In situ infrared (IR) spectra were recorded from self-supporting wafers of test samples mounted in a purpose-made IR cell similar to that described by Peri and Hannan.7 Prior to the spectra being recorded, test wafers were subjected to a 1 h pretreatment in flowing oxygen at 723 K and a subsequent 30 min outgassing at the same temperature and ∼10-6 Torr (1 Torr ) 133.4 Pa) before being cooled to room temperature (rt) under a dynamic vacuum. The spectra (averaging 100 scans at a spectral resolution of 2 cm-1) were recorded before and after exposure of test sample wafers to CO2 gas or Py vapor (at rt). Spectra of the adsorbed species were obtained by absorption subtraction of background spectra. The IR cell was all-Pyrex glass, was equipped with CaF2 windows, and was connected to a vacuum-adsorption line with a residual pressure lower than 10-6 Torr. The frequency regime below ∼1000 cm-1 was not accessible because of strong absorptions of the window material. The spectra were measured using a GenesisII FT-IR Mattson spectrometer equipped with WinFIRST Lite v1.02 software for data acquisition and handling. The acid treatment was conducted by exposing wafers of test samples mounted inside the IR cell, following the oxygen treatment and the subsequent outgassing at 723 K, to a 10 Torr dose of moist HCl vapor (originating from a deaerated aliquot of the HCl solution) for 10 min at 673 K. This was followed by outgassing at the same temperature for 10 min and cooling to rt under dynamic vacuum. For simplicity, samples pretreated in the HCl vapor are referred to below as acidified, whereas those pretreated in the CO2 atmosphere are signified as carbonated. Scanning electron micrographs were recorded on a Hitachi S3500N scanning electron microscope equipped with an Oxford energy analytical attachment using Oxford-Inka spectrum processing software for energy dispersive X-ray (EDX) measurements. With only a few exceptions, test samples were investigated between 10 and 25 kV at a working distance ranging from 5 to 7 mm. The samples were mounted on an adhesive carbon support, which was carried by a metal disk. To ensure good contact with the adhesive (7) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1521.
Zaki et al.
Figure 1. IR ν(OCO) spectra of irreversibly adsorbed CO2 species on the indicated pure CaO, P-CaO, and PN-CaO samples as a function of the adsorptive gas pressure (P(CO2) ) 0.5-20 Torr).
surface and satisfactory electrical conductivity, the samples were coated with a gold film (approximately 8 nm thick) in a Balzers vacuum-coating unit. For EDX measurements, test samples were examined without gold coating.
3. Results and Discussion 3.1. CO2 Adsorption. 3.1.1. Pressure Dependence. IR spectra in the ν(OCO) frequency region (2000-1000 cm-1) of adsorbed species formed at rt on the CO2/CaO interfaces at varying CO2 gas pressures (0.5-20 Torr) are shown in Figure 1. For comparison purposes spectra obtained for CO2/P-CaO and CO2/ PN-CaO at a gas pressure of 20 Torr are also shown in Figure 1. The spectra are similar and display (i) a very weak absorption band at 1073 cm-1 overlapped by strong absorptions of ν(PO4) bond vibrations (at
