Chemical and Morphological Consequences of Acidiﬁcation of Pure, Phosphated, and Phosphonated CaO: Inﬂuence of CO 2 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 CO 2 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 PO 4 3- 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 CO 2 adsorption on the chemical and morphological consequences of acidiﬁcation. The results obtained show the carbonate substitution of PO 4 3- 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 laborato- ries. 2,3 In the ﬁrst investigation, 2 hydroxyapatite (HAP, Ca 5 (PO 4 ) 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 obtained 2 revealed removal of PO 4 3- 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 investigation 3 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 activity 3,4 ). 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 sites exposed on the surfaces of HAP crystals in growing or regenerating bone. 6 The fact that MBP molecules are pyro- phosphate analogues in which the P-O-P bond is replaced by aP-C-P function renders them relatively more stable to enzyme- catalyzed hydrolysis, but allows them to maintain a good deal of pyrophosphate character, especially in their ability to bind to HAP. 6 The results obtained 3 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 high- temperature treatments under reduced pressures, have, moreover, been found to strengthen surface Lewis acid sites (Ca 2+ ) slightly and weaken both the strength and reactivity of Lewis base sites (OH - ,O 2- ). 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 CO 2 adsorption/absorption on the chemical and morphological consequences of the HCl treatment of the test samples, CO 2 being released in increasing amounts in the surrounding atmosphere. Therefore, the test samples were acid-treated before and after exposure to a CO 2 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 * To whom correspondence should be addressed. Fax: +20 862360833. E-mail: mizaki@link.net. † 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. (6) Flanagan, A. M.; Chambers, T. J. Bone Miner. 1989, 6, 33. ohio2/yla-yla/yla-yla/yla99907/yla4759d07z xppws 23:ver.6 5/9/08 16:53 Msc: la-2008-000366 TEID: jwp00 BATID: la6b68 A Langmuir XXXX, xx, 000-000 10.1021/la8000366 CCC: $40.75  XXXX American Chemical Society PAGE EST: 8.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80