Surface Properties of Silica-Based Biomaterials: Ca Species at the

J. Phys. Chem. C 2008, 112, 16879–16892

16879

Surface Properties of Silica-Based Biomaterials: Ca Species at the Surface of Amorphous Silica As Model Sites Vera Bolis,*,† Claudia Busco,† Valentina Aina,‡ Claudio Morterra,‡ and Piero Ugliengo‡ Department DiSCAFF, Eastern Piedmont UniVersity “A. AVogadro”, Largo G. Donegani 2, 28100 NoVara, Italy, Centre of Excellence NIS (Nanostructured Interfaces and Surfaces), INSTM (Italian National Consortium for Materials Science and Technology), UdR Piemonte Orientale; and Department of Chemistry IFM, UniVersity of Turin, Via P. Giuria 7, 10125 Torino, Italy, Centre of Excellence NIS (Nanostructured Interfaces and Surfaces), INSTM (Italian National Consortium for Materials Science and Technology), UdR UniVersita` di Torino ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: August 21, 2008

The adsorption of CH3OH vapor at the surface of Ca-modified silica was studied by means of the combined use of an experimental and a theoretical approach. Parallel IR spectroscopic and microcalorimetric experiments were performed to describe quantitatively and energetically the surface features of nanosized Ca-modified specimens (A200/Cax, activated in mild conditions) as a function of Ca loading and in comparison with the unmodified parent silica (Aerosil 200). The presence of Ca species at the surface enhances the adsorption capacity with respect to the unmodified parent silica and creates a rather complex reactivity. Ab initio simulation provided microscopic information on the energetic of coordinated CH3OH adducts formed at the Ca sites (BE ) 104 kJ/mol vs qdiff ∼ 100 kJ/mol) and on the possible reaction path toward products. The methoxylation of the surface, yielding Si-OCH3 and Ca-OH species (and not Ca-OCH3 and Si-OH) occurs only to a limited extent (30-40% of the total methanol uptake) and depends on both CH3OH pressure and time contact, according to the activated reaction pathway, as provided by ab initio simulation. Data from both volumetric-calorimetric and IR spectroscopic experiments indicated, in good agreement with ab initio simulation results, that the overall interaction involves both chemical and physical adsorption processes which, to a large extent, occur simultaneously, the relevant energy transfers being very similar. 1. Introduction It is generally accepted that a bioactive material implanted in the body elicits a specific biological response at the implant-tissue interface that involves a number of biophysical/ chemical interactions at the surface of the solid material in contact with physiological fluids, but the molecular details of the interface reactions/interactions are still largely unknown. The adsorption of (bio)molecules at the solid surface is recognized to be the initial step of the chain of molecular events leading to the so-called biological fixation,1 and so the knowledge of the surface properties of the biomaterials is expected to contribute to a better understanding of the interface processes.2,3 The mechanism of such biological fixation in the case of bioactive silica-based materials (e.g., glasses, which are used mainly in maxillofacial reconstruction and dental regeneration)4,5 does involve a sequence of events at the biomaterial/physiological fluid interface which eventually lead to cells’ adhesion and proliferation, as was first proposed by Hench,6 and subsequently experimentally confirmed by other researchers.7-10 The first steps of the proposed mechanism involve a number of intrinsically “inorganic” processes; namely, (i) solubilization of surface layers of the glass; (ii) deposition of an amorphous layer of silica gel and of a mixed Ca-P oxide; and (iii) subsequent crystallization of the Ca-P oxide, leading to the formation a biomineralized hydroxyl-carbonate-apatite (HCA), * To whom correspondence should be addressed. Phone: +39-0321-375840. Fax: +39-0321-375-821. E-mail: [email protected]. † Eastern Piedmont University. ‡ University of Turin.

the composition and morphology of which is similar to that of the natural bone.11 This bonelike apatite layer is, in fact, the biologically active component of the (implanted) solid material. Still, it has been demonstrated12,13 that the formation of this Ca-P-rich layer at the surface of the pristine material can be reproduced when the material is immersed in acellular simulated body fluids (SBF) mimicking the ionic inorganic composition of the human blood plasma. The inorganic processes leading to apatite layer formation, which is responsible for the chemical interfacial bonding, are expected to strongly depend upon compositional, structural, and morphological properties of the solid,4,5 so an accurate knowledge of all these properties is required to design new efficient biomaterials. Unfortunately, the dimension of the implant-tissue interface (hundreds of micrometers) and the scale of the molecular events occurring at the surface of the solid (nanometric) are so dramatically different that the results obtained are not always straightforwardly comparable. In fact, the investigation and thorough characterization of phenomena occurring at the nanometric interfacial zone of the clean surface of the (bio)material requires high vacuum conditions and very simple model systems, as is typical of a surface chemistry approach.2,3,14-18 Bioactive glasses are silica-based materials1,4,19 obtained either by fusion of a mixture of SiO2, CaO, Na2O, and P2O5, or by sol-gel synthesis methodology, this latter yielding high-surfacearea materials. All the former glasses descend from Bioglass 45S5, discovered by Hench in the 1970s,20 and sometimes may also contain other oxides,21-23 whereas sol-gel bioactive glasses represent a second generation of bioactive materials.19,24,25 Such

10.1021/jp805206z CCC: $40.75  2008 American Chemical Society Published on Web 10/03/2008

16880 J. Phys. Chem. C, Vol. 112, No. 43, 2008 nanosized materials, thanks to their high surface area, interact quickly and efficiently with the biological fluids and allow a rapid growth of HCA.26 Their composition can deviate dramatically from the Bioglass-45S5 composition, in that sol-gel glasses are typically sodium-free and can be very rich in silica (up to ≈80% weight SiO2).27,28 Still, binary sol-gel glasses [without P2O5 and of general formula (CaO)x(SiO2)1-x] have been found to be efficient in generating an HCA bioactive layer at the interface.29,30 The present work is part of a research project aimed at better understanding the role played by heteroatoms (in particular, Ca and P) in modifying the surface properties and reactivity of silica-based materials of interest in biomedical applications. In fact, the role played by Ca and P species in the partial dissolution of the bioactive glass is not yet well-understood in molecular detail. At first, our interest has been focused on the surface chemistry of Ca species in a silica matrix, taken as (oversimplified) model sites for the investigation of the surface properties of bioactive sol-gel glasses. Skipper et al.29 used neutron diffraction with isotopic substitution to gain new insights into the structure of the environment of Ca species in binary bioactive sol-gel glasses of the general formula (CaO)x(SiO2)1-x. They also used high-energy X-ray total diffraction to probe the nature of the processes occurring at the interface when bioactive glass is immersed in simulated body fluid. The data obtained point to a complex Ca environment in which calcium is loosely bound and distributed within the glass network. In our case, a series of Ca-modified silica systems containing controlled amounts of Ca selectively introduced at the surface of an amorphous, nonporous silica (Aerosil 200, Degussa) were prepared ad hoc, in analogy to what previously done,14,15,31 to investigate the coordination chemistry and the reactivity of coordinatively unsaturated (cus) Ca2+ cations. The adsorption of CH3OH vapor at the surface of the nanosized materials of interest, activated in mild conditions, was studied by means of the combined use of an experimental (adsorption microcalorimetry and IR spectroscopy) and a theoretical approach (ab initio calculations). Parallel IR-spectroscopic and microcalorimetric experiments were performed to obtain both molecular (nature of surface sites and of adsorbed species) and molar (quantitative and energetic) information on the surface features of Ca-modified silica in comparison with the parent unmodified Aerosil. Ab initio simulation provided microscopic information on the energetic of the methanol adducts formed at the Ca sites, as well as on the possible reaction products. CH3OH is a small molecule widely used in surface chemistry studies,32-43 in that it possesses different functionalities suitable for the interaction with the surface terminations typical of oxidic systems (OH groups, coordinatively unsaturated cations, etc.). First of all, CH3OH is a Lewis base of medium strength. Its gas phase proton affinity (PA) is 775 kJ/mol, close to that of CH3CN (PA ) 779 kJ/mol) and slightly higher than that of H2O (PA ) 697 kJ/mol). Both CH3CN,15 and H2O,14 have been previously used to probe the surface chemical properties of Ca-SiO2 model systems and have put in evidence an increased adsorption capacity of these latter with respect to the parent Aerosil, as well as the onset of a specific reactivity of the Camodified surface. CH3OH has been chosen (i) to probe the capability of Camodified silica to form H-bonded adducts in comparison with the plain (still hydrated) silica surface, the interaction of which with CH3OH is largely known; (ii) to assess the Lewis acidic strength of cus Ca2+ cations anchored at the modified silica

Bolis et al. surface; and (iii) to evaluate their propensity to expand their coordination and to react. As for the first point, CH3OH is certainly a good probe to assess the H-bonding capability of the surface, in that its -OH termination interacts via H bonding with the (acidic) Si-OH species exposed at the surface of any silica-based material. In extreme cases, such as in Brønsted acidic microporous and mesoporous aluminosilicates, the interaction of methanol with the surface sites can lead to the proton extraction.33,44-46 The complex CH3OH/SiO2 system has been long investigated in most of its experimental and theoretical aspects, as witnessed by a huge amount of work done over the years and present in the literature.35,40,47 The initial heat of adsorption of CH3OH on silica (78 kJ/mol) reported by Natal-Santiago and Dumesic,38 is consistent with the formation of H-bonded adducts that can be removed by evacuation at relatively low temperature. In the case of microporous silica-based materials, it has been observed that methanol molecules tend to form stable, ringlike H-bonded structures.36 As for the second point, the electron lone-pair donor function makes CH3OH a Lewis base of medium strength that can interact by acid-base coordination through the O atom with cus cations exposed at the surface of oxidic systems, such as the charge-balancing cations in zeolites36,37 or Ti4+ in rutile TiO2.34 In the present case, the cus Ca2+ cations anchored at the silica surface were found to form stable adducts with coordinated CH3OH, only a fraction of which reacted through a complex reaction path successfully simulated by our ab initio study. The investigation of the propensity of the surface Ca-modified silica to react with molecules is expected to be of some interest in order to understand the role played by Ca species in processes leading to the solubilization of bioactive glasses in SBF solutions. 2. Experimental Section 2.1. Materials. The following materials have been employed: (1) Aerosil 200 (A200), amorphous nonporous silica, obtained by SiCl4 flame pyrolysis (Degussa, Frankfurt A.M., Germany; lot.: 1490), and kindly supplied by Eigenmann & Veronelli SpA (Milano, Italy). (2) Ca-modified silica specimens (A200/Cax), obtained by adding dosed amounts of an aqueous solution of Ca(NO3)2 · 4H2O (Sigma Aldrich 18620CB) to the dry silica powder (A200) using the impregnation to incipient-wetness technique. This technique is based on the gradual wetting of the powder by little amounts of a dosed aqueous solution, taking care not to overwet it. The added Ca species are expected to remain localized at the surface of the materials. Three different Camodified samples, containing 4% mol CaO (A200/Ca4), 8% mol CaO (A200/Ca8), and 16% mol CaO (A200/Ca16),14 were obtained by contacting 10 g of A200 with aqueous solutions of Ca(NO3)2 · 4H2O (50 mL) 0.14, 0.31, and 0.68 M, respectively. The wet powders were dried in air at 373 K (24 h) and then calcined at 873 K (2 h) to get rid of nitrates. Chemical and morphological/textural features of the A200/Cax samples are summarized in Table 1SM of the Supporting Information. (3) CH3OH vapor, employed as a molecular probe in adsorption experiments. Liquid CH3OH (Sigma-Aldrich 360465IL) was distilled in vacuo and rendered gas-free by several “freeze-pump-thaw” cycles. The vapor pressure of CH3OH at 303 K is 164 Torr (1 Torr ) 133.3 Pa), and the standard molar enthalpy of liquefaction is -∆LH0 ) 38 kJ/mol (which

Surface Properties of Silica-Based Biomaterials

Figure 1. BET surface area of Ca-modified silica (A200/Ca4, A200/ Ca8, A200/Ca16) as a function of surface Ca loading in comparison with the parent plain silica specimen, A200.

in the present paper will be referred to as the latent heat of liquefaction, qL).48 The surface area of all samples, preliminarily outgassed at 423 K for 2 h under a residual pressure of ∼10-3 Torr, was determined by N2 adsorption at 77 K using an automatic apparatus (ASAP 2010, Micromeritics). Data were analyzed with the BET model for specific surface area determination.49 As for the porosity, only minor amounts of macropores are introduced into the nonporous texture of Aerosil by the presence of Ca species and will not be considered in any detail. Prior to any adsorption/desorption measurement, the samples were vacuum-activated at 423 K for either 1 h (IR spectroscopy; p ∼ 10-5 Torr) or 2 h (microcalorimetry; p e 10-5 Torr) to get rid of physically adsorbed water but without inducing any appreciable surface dehydroxylation. The choice of a vacuum activation temperature only slightly higher than room temperature (RT) was determined by the need to study the surface properties of still highly hydrated samples;14 that is, of solids taken under conditions not too far from those experienced by biomaterials in contact with the biological medium.15,3 A reference methoxylated silica specimen (A200/M) was prepared ad hoc as follows: a portion of unmodified silica (A200, preliminarily outgassed at 673 K for 1 h) was kept in contact at 673 K with a CH3OH pressure (∼20 Torr) for 4 h to allow the surface to react and become thoroughly methoxylated.43 The sample was then outgassed again (T ) 673 K, 1 h), and the cycle of treatments was repeated four times to ensure maximum surface methoxylation. In Figure 1, the BET surface area (m2/g) of Ca-modified Aerosil specimens is compared with that of the unmodified parent Aerosil. The presence of Ca species induces significant changes at the silica surface, as witnessed by the fact that as the Ca loading increases, the surface area of the specimen decreases with respect to the A200 value. This indicates that the presence of Ca terminations at the surface allows the primary silica particles (which are supposed to stay intact during the incipient-wetness impregnation) to aggregate more tightly than in the case of the unmodified material. 2.2. Methods. IR Spectroscopy. In situ IR spectra were recorded on a FTIR spectrometer (Bruker IFS 113v, equipped with MCT cryodetector) in the 4000-400 cm-1 interval at a temperature that will be referred to as beam temperature (BT); that is, the temperature reached by samples in the IR beam. For insulating white samples, BT is estimated to be some 30 K higher than RT. The homemade quartz infrared cell, characterized by a very small optical path,50 was connected to a conventional high-

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16881 vacuum line. This setting allowed it to perform under strictly in situ conditions both sample thermal treatments, and adsorption-desorption-adsorption (ads-des-ads) of the molecular probe measurements. In all cases, the sample powder was compressed in the form of self-supporting pellets of ∼10 mg cm-2 thickness, but a second portion of the A200/Ca8 preparation was especially prepared in the form of a thicker pellet (∼20 mg cm-2). The first “light” portion of A200/Ca8 will be referred to as A200/Ca8-1, and the “heavy” one, A200/ Ca8-2. After a vacuum thermal activation at 423 K, the IR spectrum of each sample was recorded to compare the surface features of Ca-modified systems with those of the unmodified parent silica. Every activated sample was first contacted at BT with increasing doses of CH3OH vapor, up to an equilibrium pressure of ∼80 Torr (first adsorption run; ads I). Then after desorbing the reversibly adsorbed phase by outgassing at the adsorption temperature (des I), a second adsorption run was carried out (ads II). At any step of the ads-des-ads set of experiments, the IR spectrum was recorded. Only in the case of a fresh A200/Ca8-2 sample, after the standard BT ads/des I run, was the sample further outgassed stepwise at temperatures up to 973 K, and at each step, the IR spectrum was recorded. In parallel on another fresh A200/Ca8-2 sample, several consecutive ads-des-ads cycles were performed, and the experimental conditions of this nonstandard ads/des experiment are reported in the text and in the relevant figure caption. In all IR spectroscopy experiments, the adsorptive pressure was monitored by a conventional Hg manometer. IR spectra were normalized to both BET surface area and sample weight (i.e., to the total surface area exposed). A selection of spectra of adsorbed methanol is reported in differential form, as obtained by subtracting the starting baresample spectrum from the spectrum of the sample either in the presence of a CH3OH pressure (∼20 Torr) or after BT outgassing. Adsorption Microcalorimetry. Heats of adsorption of CH3OH vapor were measured at 303 K by means of a heatflow microcalorimeter (Calvet C80, Setaram, France) connected to a high-vacuum gas-volumetric glass apparatus. A wellestablished stepwise procedure was followed39,51-54 that allows one to determine during the same experiment both integral heats evolved (∆Qint) and adsorbed amounts (∆nads) for small increments of the adsorptive (in the present case, up to a final equilibrium pressure of ∼80 Torr). Thanks to the differential construction of the apparatus, all parasite effects (i.e., all effects other than the one due to the interaction(s) of the gas with the solid surface) were compensated. The adsorptive pressure was monitored by means of a transducer gauge (Ceramicell 0-100 Torr, Varian). Adsorbed amounts (nads ) Σ∆nads, µmol/m2) and integral heats evolved (Qint ) Σ∆Qint, J/m2) were normalized to the unit surface area and will be plotted in the form of volumetric and calorimetric isotherms, respectively. Evolved heats were also normalized to the adsorbed amounts and referred to as integral molar heats of adsorption [qmol]p ) [(Qint/nads)p].51 Differential heats of adsorption, which represent the enthalpy changes, qdiff ) -∆adsH, associated with the process, are properly defined as the first derivative (qdiff ) δQint/δnads) of the Qint ) f(nads) function which best fits the Qint vs nads equilibrium data.55 An alternative route for evaluating qdiff has been followed in the present paper, as already described elsewhere.15,39,51 The qdiff vs nads curves shown in the present paper are the functions that best fit the experimental points obtained by taking the middle point of the partial molar heats (∆Qint/∆nads, kJ/mol) vs nads histogram. [(∆Qint/∆nads, kJ/mol)

16882 J. Phys. Chem. C, Vol. 112, No. 43, 2008 quantities correspond to the adsorbed-amount-normalized heats evolved during the adsorption of the individual doses, prepared as small as possible, according to the mentioned stepwise procedure.] In harmony with parallel IR-spectroscopic experiments, the reversibility/irreversibility of methanol adsorption was investigated through ads-des-ads measurements. After the ads I run, the sample was outgassed (overnight) at the adsorption temperature, and a subsequent ads II run was performed. Also in the microcalorimetry study, an extra set of ads/des measurements (for a total of up to six isotherms) were performed on a fresh sample of the preparation called A200/Ca8-2, and the experimental conditions of this nonstandard ads/des experiment are reported in the text and in the relevant figure captions. Molecular Modeling. Geometry optimization was run at the ab initio level using the B3-LYP/6-31+G(d,p) model chemistry56 on three Ca-containing molecular clusters modeling a variety of Ca sites (i.e., coordinatively unsaturated cus Ca2+ cations) that are expected to be present at the surface of experimental Ca-modified silica specimens. The molecular adsorption of one CH3OH molecule was simulated on all considered models, as well as the possible evolution of the coordinated species toward a dissociative adsorption. The full set of B3LYP harmonic frequencies was computed to characterize structures as minima or transition state. To facilitate comparison with experimental IR spectra in the 2600-3050 cm-1 range (C-H stretching modes), the as-computed B3LYP frequencies were (i) scaled by the factor 0.947, which brings in coincidence the B3LYP CH set of stretching frequencies computed for the tetrasilasesquioxane cluster Si4O6H3-OCH3 (adopted as a methoxylated silica model) with those measured for a methoxylated silica specimen (A200/M) prepared ad hoc as a reference material; and (ii) modified by associating a Lorentzian function (width ) 10 cm-1) to each harmonic B3LYP value whose height corresponded to the computed IR intensity. On the B3LYP optimized geometries, a single MP2/ 6-31+G(d,p) energy point calculation was run to increase the accuracy of the energetic comparison (hereafter referred to as MP2//B3LYP). The Gibbs free energy and the corresponding equilibrium constants were computed correcting the MP2// B3LYP energies with the thermal and entropic contributions computed at the B3LYP level. The interaction energies of CH3OH molecules coordinated (up to two) to the Ca-containing clusters were corrected for the basis set superposition error using the standard Boys-Bernardi counterpoise method.57 3. Results and Discussion Bare-Samples IR Spectra. In Figure 2, the IR spectra of unmodified (A200) and Ca-modified silica (A200/Cax) are compared in the OH stretching spectral region (4000-2500 cm-1, section A)58 and in the region of carbonate-like species vibrations (2200-1300 cm-1, section B).59 In section A, a sharp peak near 3750 cm-1, typical of isolated Si-OH species ubiquitous in silica-containing materials,58,60-62 is observed in all cases examined. In the case of Ca-modified materials, the spectral position of this band is shifted to frequencies slightly lower than that of A200 (3746 cm-1 for A200/Ca4, 3743 cm-1 for A200/Ca8 and A200/Ca16), indicating that the acidity of isolated silanols increases slightly with the presence and amount of surface Ca species. Still in Figure 2A, at lower frequency with respect to the isolated ν(OH) peak, a broad band typical of H-bonded Si-OH pairs (interacting silanols)58 is observed in all investigated materials, the intensity of which slightly increases with surface Ca loading. No peaks to be specifically

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Figure 2. IR spectra of Ca-modified silica in comparison with that of unmodified parent silica. All samples were outgassed at T ) 423 K. (a) A200, (b) A200/Ca4, (c) A200/Ca8, and (d) A200/Ca16. Section A: the OH and CH stretching spectral region (4000-2500 cm-1). Section B: the spectral region (2200-1300 cm-1) of silica overtone modes and of surface carbonate-like species.

ascribed to Ca-OH species (for instance, a band at ∼3715 cm-1 is observed in the bare-sample spectrum of CaO preparations14) could ever be discriminated within the broad absorption due to interacting silanols. In Figure 2B it is rather evident that (i) increasing Ca amounts (spectra b-d) progressively reduce in mass-normalized spectra the relative intensity of the “structural” overtone and combination bands (2100-1600 cm-1) typical of the pure silica network (spectrum a); and (ii) the presence of surface Ca species strongly modifies the affinity of silica toward atmospheric CO2, because in the spectra of Ca-modified silicas a doublet at 1496 and 1419 cm-1 is present, the intensity of which increases with Ca content. These bands are typical of ionic carbonate-like surface species (of aragonite rather than calcite type16,63) which are formed as a consequence of the exposure of the samples to the (wet) atmosphere after their preparation. Such carbonate species, totally absent in the unmodified A200, cannot be eliminated by a mere thermal activation at temperatures up to 423 K, and consequently, the accessibility of surface Ca2+ ions should be expected to be somehow hindered, at least for soft molecular ligands, such as methanol. Still, the intensity of the carbonatelike bands was found to depend also on the aging of samples, and specifically checked, this time-dependent intensity turned out not to affect to an appreciable extent the samples’ activity toward CH3OH. Adsorption Microcalorimetry. Figure 3 reports the adsorption isotherms of CH3OH vapor on plain and Ca-modified silica. Ads I and ads II volumetric isotherms are shown in sections a and c, respectively; the corresponding calorimetric isotherms, in sections b and d, respectively. It is evident that the surface affinity of A200/Cax systems toward CH3OH is enhanced with respect to that of the parent silica, whereas it is only slightly affected by the extent of Ca loading. Both amounts adsorbed and integral heats evolved in the low-pressure range of ads I vary with Ca-loading in the order A200 < A200/Ca4 < A200/ Ca8-1 ≈ A200/Ca16. At high coverage, the volumetric isotherms of A200/Ca8 and A200/Ca16 are almost coincident and lie above the volumetric isotherm of A200/Ca4, which gradually approaches that of the parent silica. As for the corresponding calorimetric isotherms, they remain distinct (and virtually parallel) in the whole pressures interval examined. For all A200/ Cax systems, the early stages of the adsorption isotherms are


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Figure 3. Adsorption at 303 K of CH3OH on Ca-modified silica A200/Ca4 (up triangle), A200/Ca8-1 (square), and A200/Ca16 (down triangle), in comparison with the unmodified parent A200 (circle). Sections a, c: volumetric isotherms (nads vs peq). Sections b, d: calorimetric isotherms (Qint vs peq). Solid symbols: ads I. Open symbols: ads II.

steeper (i.e., occur at lower equilibrium pressures) than for plain silica, suggesting that Ca sites interact more promptly and specifically with the probe than do silica terminations. In ads II isotherms, reported in sections c and d of Figure 3, there are no differences among the three A200/Cax specimens, and the differences between unmodified and Ca-modified silicas are definitely lower than in ads I. By comparing ads I and ads II isotherms, it is observed that (i) in the case of pure A200, the two curves differ by less than 5% of the total amounts. This indicates a substantial reversibility of the methanol-silica interaction, as previously reported;35,38,39,41 and (ii) for all A200/Cax specimens, both volumetric and calorimetric ads II isotherms lie significantly below the corresponding ads I curves, indicating the presence of irreversible phenomena. The irreversible adsorption component was quantified by subtracting ads II from ads I curves at two equilibrium pressures (pCH3OH ) 10 and 80 Torr, respectively). Note that, while the (ads I - ads II) differences are hereafter referred to as the irreVersibly adsorbed component, the ads II curves will not be referred to as the reVersibly adsorbed component as is usually done,51,54 because in the present case, irreversible processes are far from being completed within the ads II run (vide infra). Volumetric and calorimetric data, including the integral molar heats of adsorption ([qmol int ]p ) [(Qint/nads)p]), relative to the ads I, ads II, and (ads I - ads II) processes and evaluated at the two selected equilibrium pressures are summarized in Table 2SM, available in the Supporting Information. As expected, the irreversible component increases with Ca loading: at low pCH3OH, it varies from 1.42 µmol/m2 (A200/ Ca4) to 1.98 (A200/Ca8-1) and to 2.37 (A200/Ca16), figures that correspond to 28.2, 34.9 and 39.7% of the total uptake, respectively. A further increase of the irreversible component was recorded at high pCH3OH: 1.77 (A200/Ca4), 2.59 (A200/ Ca8-1), and 3.15 µmol/m2 (A200/Ca16), corresponding to 18.7, 24.3, and 29.1% of the total uptake, respectively. The continuous increase in the irreversible component with methanol pressure

is the first indication that in the presence of surface Ca species, slow reactive processes are occurring at the solid/gas interface. This fact is generally assumed to monitor the onset of a dissociative chemisorption, facilitated by the preliminary adsorption of a molecular precursor.64 In the present case, irreversibly adsorbed methanol can be thought to be due, at least in part, to a slow methoxylation reaction, and some CH3OH preliminarily coordinated on cus Ca2+ cations is expected to be the molecular precursor that can speed up the methoxylation reaction. Modeling results obtained in the present work (vide infra) will confirm this hypothesis. Integral molar heats relative to ads I were found to be similar for all A200/Cax systems: [qmol int ]p ∼ 60 and ∼ 50 kJ/mol at low and high pressure, respectively. Surprisingly, such values are only some 10% larger than those obtained for the parent A200 (∼55 and ∼45 kJ/mol at low and high pressure, respectively), despite the fact that in the presence of Ca sites, CH3OH undergoes specific adsorption interactions, possibly leading to irreversible reactive phenomena. Most likely, all specific and more energetic processes occurring at Ca sites are intermingled with other reversible and less energetic processes, which are typical of the silica matrix surface. So the thermal contribution due to specific and, possibly, reactive interactions at Ca sites cannot be singled out, because it is diluted (and somehow hidden) within the overall integral molar heat quantities deriving from the thermal response of the surface as a whole. At low pressure, the ads II [qmol int ]p values for A200/Cax turn out to be still as high as in ads I (∼60 kJ/mol), indicating that a fraction of the strong component of the ads I process was not completely quenched within the first run. In addition, for the parent A200 system, the low-pressure [qmol int ]p value is still relatively large (∼50 kJ/mol), probably as a consequence of the presence of multiple H-bonding interactions between CH3OH and surface Si-OH groups.47,50 Conversely, the high-pressure [qmol int ]p ads II values are typical of low-energy processes for both plain and Ca-modified silica (∼45 kJ/mol).

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Figure 4. Differential heat of adsorption (qdiff) vs adsorbed amounts (nads) of CH3OH adsorbed at 303 K on Ca-modified silica (A200/Ca4, up triangle; A200/Ca8-1, square; A200/Ca16, down triangle) to be compared with the unmodified parent silica (A200, circle). Solid symbols: ads I, section a. Open symbols: ads II, section b.

To describe in molecular detail the energetic of CH3OH interaction with plain and Ca-modified silica, it is convenient to consider adsorption differential heats, as shown in the plots of Figure 4 [ads I in section a, ads II in section b]. It is recalled that adsorption differential heats [qdiff ) -∆adsH] represent, at any specific adsorbate coverage, a reasonable measure of the energy of interaction of the probe with individual sites. As a consequence, the shape of (qdiff vs nads) plots depends on and actually describes surface heterogeneity. The extrapolation of qdiff plots to vanishing coverage [q0 ) -(∆adsH)0] yields the enthalpy changes associated with uptake on the most energetic sites, expected to be active in the earliest stages of the adsorption process. The q0 extrapolated quantities of experimental origin can be conveniently compared with the interaction energy (binding energy, BE) of one methanol molecule with an individual model site, as obtained through ab initio calculations. These comparisons will be illustrated below. Plots relative to ads I, reported in Figure 4a, indicate that (i) The differential thermal response of the three Ca-modified surfaces is virtually the same (all experimental points lie on the same curve in the whole coverage interval examined), confirming that the nature of the interaction between cus Ca2+ ions and CH3OH depends only little, if at all, on Ca-loading. (ii) The qdiff values for Ca-modified systems are definitely larger than those for unmodified silica. This makes evident the presence, at the A200/Cax surface, of sites leading to interactions with the probe more specific than plain H-bonding. (iii) A rather strong interaction occurs in the early stages of the process on A200/Cax systems, as witnessed by a relatively large zerocoverage adsorption heat (q0 ∼ 100 kJ/mol). This value, significantly larger than that obtained for A200 (q0 ∼ 70 kJ/ mol), is compatible with either a strong CH3OH coordination onto cus Ca2+ ions acting as Lewis acid sites36,37 or with moderately exothermic reactive phenomena or, most likely, with a combination of the two. (iv) All qdiff plots are typical of highly heterogeneous surfaces. In fact, heat values progressively decrease with increasing coverage (exponentially in the case of A200/Cax and linearly in the case of A200) and tend to values that (as often observed with H-bonding adsorption processes)47 are even lower than the latent heat of methanol liquefaction (qL ) 38 kJ/mol). Figure 4b reports the qdiff plots relative to ads II. The shape of the curves is still typical of heterogeneous surfaces, and there are still no significant differences among the three Ca-modified specimens. By comparing ads II and ads I qdiff curves for A200/ Cax systems, it turns out that (i) The former curve lies below the latter one in the whole coverage range examined. This clearly indicates that irreversible phenomena occurring during the ads I run are not limited to (and completed in) the early stage(s) of the process. (ii) The ads II q0 value (∼85 kJ/mol) is significantly

Figure 5. Absorbance differential IR spectra (OH and CH stretching region, 4000-2500 cm-1) relative to the BT adsorption of CH3OH on unmodified and Ca-modified silica: A200 (a), A200/Ca4 (b), A200/ Ca8-1 (c), and A200/Ca16 (d). Differential spectra were obtained with respect to the various bare-sample spectra (i.e., spectra of samples vacuum-activated at 423 K). Dashed-line spectra: samples in equilibrium with ∼20 Torr of CH3OH. Solid-line spectra: samples after evacuation at BT (1 h). [Bands relative to species that formed during the ads/des run point up, bands relative to species that were eliminated point down. The horizontal line in spectral set b shows, as an example, the differential spectrum that would be obtained in the case of no adsorption or in the case of complete reversibility of the adsorbed species].

lower than that obtained for ads I, although still larger than that of plain silica (q0 ∼ 70 kJ/mol, value virtually coincident with the ads I one). At higher coverage, however, the ads II qdiff curve for Ca-modified systems lies surprisingly below that for A200 and decreases sharply to very low values. This suggests that the first contact of the probe with A200/Cax caused not only the irreversible saturation of a significant fraction of specific Ca sites but also an irreversible modification of the surface. In fact, the methanol-reacted surface resulted quite differently from both parent unmodified and pristine Ca-modified silica surfaces. To elucidate the reasons for this modification, the nature and stability of methanol species irreversibly adsorbed on Camodified silica has been investigated by in situ IR spectroscopy. IR Spectra of Adsorbed CH3OH. Figure 5 reports the 4000-2500 cm-1 IR spectral range (i.e., the O-H and C-H stretching region) for the three A200/Cax specimens contacted with CH3OH vapor in comparison with the unmodified parent silica. The differential IR spectra were obtained by subtracting the spectrum of the bare activated samples from (i) the spectrum

Surface Properties of Silica-Based Biomaterials of the samples in equilibrium, at BT, with ∼20 Torr of CH3OH (dashed lines) and (ii) the spectrum of the methanol-contacted samples after evacuation (BT, 1 h) of the reversibly adsorbed phase (solid lines). Dashed-line spectra clearly indicate that, as expected, all samples interact abundantly with CH3OH, giving rise in the 3500-2800 cm-1 region to the complex spectroscopic response typical of adsorbed CH3OH.35,40 Major differences between plain and Ca-modified silica are observed when the adsorbed phase is outgassed at BT. No residual IR bands are observed for A200 (trace a), confirming the reversibility of plain H-bonding interactions. The solid-line differential spectrum in spectral set a is very similar to the horizontal straight line, shown as an example in spectral set b, which would result in the case of 100% reproduction of the starting bare-sample spectrum. For all A200/Cax systems (traces b-d), the solid-line spectra are very far from the zero horizontal line and represent the methanol component irreversibly adsorbed at BT. They contain evidence for an abundant residual Hbonding interaction (note that virtually all of the perturbation produced by methanol uptake on the free OH peak at ∼3750 cm-1 is not recovered) and a complex C-H absorption with two maxima located at ∼2960 and 2855 cm-1, respectively. The spectral position of the high-ν band (antisymmetric CH3 stretching) is slightly dependent on the Ca content as it moves from 2960 to 2956 cm-1 with increasing Ca loading, whereas the spectral position of the low-ν component (symmetric CH3 stretching) remains almost constant at 2855 cm-1 for all A200/ Cax samples. The irreversible component may be ascribed either to CH3OH strongly coordinated on cus Ca2+ cations via the oxygen lone pair, to strongly held surface methoxy species formed through a dissociative chemical reaction, or perhaps, to both of them. The actual chemical nature of the irreversible methanol species cannot be simply inferred by the analysis of the spectra of Figure 5 in that methoxy groups belonging either to coordinated methanol or to surface species originated by dissociative methanol chemisorption give rise in the ν(C-H) region to substantially the same bands, as indicated by Pelmenschikov et al.40 and confirmed by the present work ab initio calculations, to be reported ahead. To assess the extent to which the irreversibly held component is due to strongly coordinated or dissociatively adsorbed methanol, we investigated the thermal stability of the species responsible for the two ν(CH3) bands at ∼2960 and 2855 cm-1. A portion of the A200/Ca8 preparation (the sample termed A200/Ca8-2) was contacted with methanol vapor (up to 80 Torr), evacuated at BT, and subsequently outgassed at stepwise increasing temperatures (up to 973 K) to assess at which temperature(s) the BT-irreversible methanol component(s) could be eliminated in vacuo. A thick pellet of the specimen was employed in this case to maximize the spectroscopic response in the region of interest. The results obtained in the 3900-2700 cm-1 IR spectral range are shown in Figure 6. Note that the bare-sample spectrum a in Figure 6A, corresponding to the starting A200/Ca8 sample after the vacuum activation at 423K, contains a spectral component due to hydrocarbonaceous residues with band maxima at 2928 and 2853 cm-1. This is a well-known phenomenon due to atmospheric contamination and common to all high-area oxidic systems that were not activated and oxidized at temperatures g723 K. As a consequence, the spectral features in the ν(C-H) region of nondifferential spectra, as are the spectra reported in Figure 6A, are partly artifact due to the presence of the spurious CH-containing bare-sample component. Only in the case of spectrum b, corresponding to the maximum amount of irreversibly adsorbed methanol, was


Figure 6. Section A: absorbance IR spectra in the 3900-2700 cm-1 range (OH and CH stretching region) of A200/Ca8-2 specimen first vacuum activated at 423 K (a, bare-sample spectrum), then contacted with ∼80 Torr of CH3OH and subsequently stepwise evacuated 1 h at BT (b), 423 K (c), 523 K (d), 573 K (e), 773 K (f), 873 K (g), and 973 K (h). The bare-sample spectrum (a) contains weak ν(C-H) bands due to atmospheric hydrocarbonaceous contaminants (see text). The brokenline trace in the upper part of the figure, termed (b - a), is an example of spectrum “cleaned” by bare-sample subtraction, and shows the actual ν(C-H) bands of BT irreversibly adsorbed methanol present in spectrum b. Section B: absorbance differential spectra in the 3900-2700 cm-1 range (OH and CH stretching region) showing the spectral features of species eliminated/formed during the vacuum thermal treatments. In all differential traces, the bands of species that were eliminated in the corresponding temperature intervals are pointing up, whereas the bands of species that formed in the corresponding temperature intervals are pointing down.

the real absorption in the ν(C-H) region obtained by a limitedrange subtraction of the bare-sample spectrum, reported in the figure, as an example, in the form of the upper broken-line trace termed b - a. In the case of CH3OH strongly coordinated to cus surface cations (Ca2+ cations in the present case), the adsorbed phase should be eliminated in vacuo at relatively low temperatures

16886 J. Phys. Chem. C, Vol. 112, No. 43, 2008 (T e 473K). This latter is the desorption temperature of molecular H2O coordinated on cus Ti4+ cations acting as strong Lewis acid sites, as reported in the literature.65,66 It is worth noting that the heat of adsorption of H2O coordinated on such a kind of site is qdiff ∼ 90 kJ/mol,54 which is close to the value measured in the present work for CH3OH adsorbed on Camodified silica. Conversely, the desorption of CH3OH from weak Lewis acid sites, such as alkali metal charge-balancing cations in zeolites, has been reported to occur at a temperature as low as 308 K.36 In the case of surface methoxy species formed through a dissociative chemical reaction, the IR bands should disappear upon outgassing at definitely higher temperatures, capable of inducing either the desorption through the recomposition of a methanol molecule or a pyrolytic degradation of the surface alkoxy groups. It is worth noting that in the case of CH3O-Si species at the surface of highly dehydrated Aerosil,50 vacuum thermal treatments lead only to the pyrolytic degradation, starting at temperatures as high as ∼900 K. By inspection of the spectra shown in Figure 6A, it is quite evident that, in large proportion, the ν(CH3) bands at ∼2960 and 2855 cm-1 resist a vacuum thermal treatment at temperatures up to at least 573 K. In some more detail, the spectral pattern reported in Figure 6A and corresponding to the stepwise vacuum thermal treatment leading to the complete elimination of all BT-irreversible methoxy species can be grossly divided in four steps: (i) At 423 K, a first fraction of the ν(CH3) bands is eliminated. The differential spectrum (b - c) in Figure 6B shows that the desorbed species possessed also a broad and strong band (centered at ∼3490 cm-1) due to strongly H-bonded OH groups, and this indicates that it was CH3OH adsorbed in a molecular form. (ii) In the temperature interval up to 523 K, nothing is desorbed, and in fact, the differential spectrum (c d) in Figure 6B is a flat and almost horizontal line. (iii) In the temperature interval up to 573 K (actually, up to 673 K, but the spectrum corresponding to the evacuation at T ) 673 K, quite similar to that at T ) 573 K in the ν(C-H) region, was not inserted in Figure 6A so as not to overcrowd it), a large amount of OH-containing species (band centered at ∼3490 cm-1, already present in the bare-sample spectrum a) is eliminated. This corresponds to a mere dehydration step of the surface, as the differential spectrum (c - e) in Figure 6B contains evidence for H-bonded OH species but does not contain any component corresponding to ν(CH3) absorptions. (iv) At T g 773K, and within T ) 973 K, there is the complete elimination of all -OCH3 species, as indicated by the spectra (f, g, and h) in Figure 6A and confirmed by the differential spectra (e - f) and (e - h) in Figure 6B. The simultaneous occurrence of steps of the surface dehydration process makes it difficult to understand if the surface -OCH3 species are eliminated totally, or only in part, through a pyrolytic process. Still, the concomitant elimination of the ν(CH) bands of hydrocarbonaceous contaminants present in the bare-sample spectrum suggests that pyrolytic mechanisms are indeed important in the elimination of -OCH3 species from the Camodified surface, as they are in the case of unmodified silica. No matter what the mechanism of -OCH3 species elimination is, what is reported in Figure 6 definitely indicates that methylcontaining species irreversibly held at the A200/Cax surface are, in large proportion, ascribable to the products of a chemical reaction; that is, methoxylation. Conversely, the fraction of BTirreversible methanol which is vacuum-desorbed at 423 K can be confidently ascribed to the coordination of CH3OH onto cus Ca2+ sites. After a complete BT ads/des cycle, this molecularly

Bolis et al.

Figure 7. Middle column: Ca1, Ca2, and Ca3 cluster models adopted to mimic cus Ca2+ ions exposed at the surface of Ca-modified silica. Left column: silica surface termination models, mimicking geminal Si-OH sites (SHG), vicinal Si-OH sites (SHV), and an H-bonding interacting Si-OH pair (SHI), to which the Ca atoms have been anchored. Right column: Ca1-C, Ca2-C and Ca3-C models with one coordinated CH3OH molecule. Free numbers refer to the MP2// B3LYP enthalpy of CH3OH adsorption [-∆adsH°(0), kJ/mol] on the selected models.

adsorbed component was found to correspond to ∼20% of the total irreversibly adsorbed phase on all Ca-modified silica specimens examined. To confirm the proposed scenario and to clarify the nature and mechanism of the reactive phenomena occurring at the Camodified silica/CH3OH interface, the interaction between Ca species anchored at a silica matrix and one CH3OH molecule has been investigated by ab initio simulation. Ab Initio Calculations of CH3OH Adsorption. Due to the large uncertainty of the local structure around Ca atoms in the real material (in which a heterogeneous distribution of sites has been indicated by the shape of the qdiff vs nads plot; Figure 4), three different cluster models (Ca1, Ca2, and Ca3) have been adopted to mimic cus Ca2+ cations exposed at the A200/Cax surface, as shown in Figure 7 (middle side column). The left column shows the models of silica surface terminations, mimicking geminal (SHG), vicinal (SHV), and H-bonding interacting Si-OH sites (SHI) to which the Ca atoms have been anchored. Taking into account that the proportion of polar terminations at the silica surface varies from sample to sample according to the origin and thermal history of the material,39,41,54,61,62,67-70 H-bonding interacting SHI sites, together with a few free Si-OH species, are expected to be the most abundant Si-OH species in Aerosil 200 vacuum-activated under mild conditions and, therefore, still highly hydrated,71 as confirmed by spectrum a of Figure 2A, so the Ca3 cluster model is expected to be the most representative of the Ca sites exposed at the surface of A200/Cax materials. It is worth noting that in Ca3 model, Ca2+ cations are coordinated more efficiently than in Ca2 and Ca1 models, because a lower geometrical strain is imposed by the larger silica ring surrounding the Ca atom. Conversely, the Ca1 model, in which the highest geometrical strain is imposed by the smallest silica ring, is expected to represent the most reactive sites. Still, such reactive sites are expected not to be easily accessible to methanol in the real systems of interest, in that they have most likely been already modified by reacting with wet CO2 atmosphere, giving rise to stable surface carbonates (vide supra, IR spectra in Figure 2B), which cannot be eliminated by simple evacuation at 423 K.


Figure 8. Possible reactive processes starting from CH3OH coordinated to different Cax (x ) 1, 2, 3) models (Cax-C, middle column): Cax-M1 products, that is, Ca-OCH3 and Si-OH species (left column); Cax-M2 products, that is, Si-OCH3 and Ca-OH species (right column). Numbers refer to the thermodynamic MP2//B3LYP equilibrium constant.

In any case, the coordination (C) of one CH3OH molecule was modeled on all Ca model sites to calculate a range of BE values. The structure of Ca1-C, Ca2-C, and Ca3-C adducts are shown in the right column of Figure 7. The MP2//B3LYP [-∆adsH°(0)] adsorption enthalpies are 124, 106, and 104 kJ/ mol for Ca1, Ca2, and Ca3 models, respectively. The range of energies computed for the coordinative (through O atom) bond of CH3OH on Ca sites reflects the degree of coordinative unsaturation of the Ca2+ cation (and, consequently, its Lewis acidic strength), which is bound to vary in the Ca1 > Ca2 ≈ Ca3 sequence according to the geometrical strain imposed by the size of the silica ring surrounding the Ca atom site. In particular, the value of 104 kJ/mol, computed for the Ca3 model site (which is expected to be the most representative), is in fairly good agreement both with literature data on other oxidic systems, such as TiO2,34 and with the experimental zerocoverage adsorption enthalpy [q0 ) (-∆adsH)0], which was assessed at ∼100 kJ/mol for all investigated A200/Cax systems (Figure 4). This confirms the validity of the adopted molecular models. Figure 8 shows, for the three Cax-C (x ) 1, 2, 3) models (middle column), the evolution of CH3OH coordinated at the Ca sites toward two possible dissociative adsorption products, Cax-M1 and Cax-M2, respectively. In the Cax-M1 case (left column structures), a Ca-OCH3 and a new Si-OH species are formed; in the Cax-M2 case (right column), a Si-OCH3 and a Ca-OH. Numbers reported in the scheme refer to the thermodynamic MP2//B3LYP equilibrium constants. In all models, the corresponding Cax-M1 and Cax-M2 products exhibit similar stabilities, as indicated by the closeness of the two MP2//B3LYP equilibrium constant values (but for a slightly larger value for Cax-M2 species). It is quite evident that the Ca1 model site is extremely reactive, as expected, but such species are thought to represent only a minor feature, if at all, of the real systems dealt with here, because they are more likely to be engaged in the formation of stable surface carbonates. Conversely, the Ca3 model site is much less reactive, as witnessed by the equilibrium constants lower than 1 and indicating that both dissociative adsorption paths are less favored with respect to the molecular adsorption (Ca3-C). The Ca2


Figure 9. Calculated and experimental spectra in the ν(C-H) region of some methanol-derived species. The bottom side of the figure (traces a-e) reports the B3LYP simulated vibrational spectra for (a) Si-OCH3 species in plain silica, mimicking methoxylated species; (b) CH3OH H-bonded to two vicinal Si-OH groups (SHV), mimicking physisorbed methanol; (c) Ca-OCH3 (see Ca3-M1 model); (d) CH3OH coordinated on the Ca3 site (see Ca3-C model); and (e) Si-OCH3 (see Ca3-M2 model). In the top side of the figure (traces f-j), the corresponding experimental vibrational spectra are reported: (f) Si-OCH3 groups (see A200/M, methoxylated silica preparation); (g) CH3OH physisorbed on silica (see A200, Figure 5); (h) Ca-OCH3 [Raman spectrum of Ca dimethoxide Ca(CH3O)2]; (i) CH3OH coordinated to cus Ca2+ cations [CH3OH f Ca2+; see difference spectrum (b - c) in Figure 6B]; (j) Si-OCH3 species [see difference spectrum (e - h) in Figure 6, methanol dissociatively adsorbed on A200/Ca8-2].

model site shows, as expected, a somewhat intermediate reactivity, though more similar to that of Ca3. By considering the results obtained for Ca3 model site, from the simulation standpoint, it can be inferred that the methoxylation reaction (i) occurs only to a rather limited extent, and (ii) two different reaction products (Ca3-M1 and Ca3-M2) can be obtained. From an experimental standpoint, let us discuss first the last point and try to assess the actual chemical nature of the surface species originated by methanol dissociation. The formation, in the real systems, of Si-OCH3 species (and thus, of Ca3-M2like products) can be easily demonstrated by examining the ν(C-H) spectral region of a selection of experimental systems, reported in the upper part of Figure 9 (in which experimental spectra are compared with the corresponding simulated ones). The spectroscopic starting point is the spectrum of methoxylated silica A200/M (trace f), in which there is no doubt that the ν(C-H) vibrations belong to Si-OCH3 groups. That -OCH3 stretching vibrational spectrum turns out to be virtually identical to that of methanol dissociatively chemisorbed at the surface of all A200/Cax systems (trace j) and is slightly different (it has sharper peaks shifted toward higher wavenumbers) from that of Ca-coordinated (trace i) and that of H-bonded (trace g) CH3OH. On the contrary, if dissociative chemisorption leads also to the formation of CH3O-Ca species (and thus, of Ca3-M1-like products), very different spectral features characterized by bands possibly broader and definitely shifted to lower wavenumbers, as indicated by the Raman spectrum of Ca dimethoxide (trace h), should also be observed. Let us consider now the simulated IR spectra (bottom section of Figure 9). It is quite evident that computed ν(C-H) vibrational bands present the same trend exhibited by the experimental ones: stretching modes of CH3O-Ca species (spectrum c) are heavily red-shifted with respect to the spectral


Figure 10. Reactive MP2//B3LYP free energy profile (kJ/mol) for the evolution of Ca3-C adduct toward Ca3-M1 and Ca3-M2 products of methoxylation. TS, TS1, and TS2 are transition state structures. Arrows show the penta-coordinated Si atom. I* is a Si-penta-coordinated stable intermediate.

position of gaseous CH3OH (2977, 2844 cm-1,72 the computed spectrum of which is not shown) and of liquidlike physisorbed CH3OH (spectrum b), whereas both CH3O-Si species (spectra a and e) and CH3OH coordinated on Ca3 sites (spectrum d) are blue-shifted. It is so evident, also from computational data, that a dramatic difference must exist between the spectroscopic response of the two possible products of methoxylation, termed C3-M1 and C3-M2, respectively. If the two products were actually formed at the real surface, we should be able to have clear and well-resolved spectroscopic evidence for both of them. It can be thus concluded that in real Ca-modified systems, the simulated dissociative reaction leading to Ca3-M2 products is favored with respect to that leading to Ca3-M1. Unfortunately, the possible formation of Ca-OH species (i.e., the other species making up the Ca3-M2 moiety) could not be verified because Ca-OH vibrations should be located within the broadband due to H-bond interacting Si-OH species, and were not discernible in our IR spectra. Experimental data reported and discussed above have indicated that (i) methoxylation reaction in Ca-modified silica, which occurs on Si atoms near a Ca site, initially involves only a fraction of the total uptake (∼30%, as reported in Tables 2SM in the Supporting Information), and (ii) reactive phenomena, which continue along the whole coverage interval examined, are not accomplished within the ads I run. Both of these points are in agreement with the indication coming from the computational study that methanol dissociation occurs only to a limited extent, as is not favored with respect to coordination on strong Lewis acid sites. Still, an important question remains open. Even if, from a thermodynamic point of view, the simulation indicates that Ca3-M2 products are slightly favored (and the same holds also for the less probable Ca1 and Ca2 model sites), from a kinetic point of view, it is quite surprising that Ca3-M1 products do not form at all in the real systems. Indeed, the rupture/formation of chemical bonds for the Ca3-C f Ca3-CM1 reaction path should be expected to occur more easily than for the Ca3-C f Ca3-CM2 one. For this reason and to justify the preferential formation of Si-OCH3 and Ca-OH species over Ca-OCH3 and Si-OH ones, a possible methoxylation reaction path has been simulated ab initio. Dissociative Adsorption of CH3OH at the Ca Sites: Ab Initio Simulation of the Reaction Profile. Figure 10 illustrates

Bolis et al. the reactive MP2//B3LYP free energy profile (in kJ/mol) for the reaction of the Ca3 model with methanol. Let us start from the Ca3-C adduct (the zero-energy level), in which one CH3OH molecule is strongly coordinated to the Ca3 site. TS is the transition state for the formation of Ca3-M1 products. The energy barrier (39 kJ/mol) is not excessively high with respect to the energy released during the coordination of the molecule (-∆H°(0) ) 104 kJ/mol for Ca3-C, in agreement with the experimental q0 ∼ 100 kJ/mol). It follows that Ca3-M1 species, which are less stable than the starting Ca3-C adduct by 27 kJ/mol, can be both easily formed and easily destroyed. Further, as illustrated in the right-side part of the free energy profile, a different fate can also be envisaged for Ca3-M1 species. TS1 is now the transition state for the formation of an intermediate I*, leading to Ca-M2 species through another transition state (the TS2 structure). The energy barrier for the formation of I*, which implies the expansion of the coordination sphere of a Si atom to five, is quite high (121 kJ/mol) but still compatible with the energy locally released during CH3OH coordination. The Ca3-M2 eventually formed is still less stable, by 20 kJ/mol, than the pristine Ca3-C methanol adduct, but by comparing this figure with the corresponding Ca3-M1 one (27 kJ/mol), it turns out that there is a tiny difference in stability (∆ ) 7 kJ/ mol) in favor of Ca3-M2 products. It is worth noting that the reverse path Ca3-M2 f I* f Ca3-CM1 is unlikely, owing to the high barrier (83 kJ/mol) to reform the I* intermediate, so Si-OCH3 species (the only species experimentally detected) are not easily formed, but once formed, they remain tightly bound to the surface. By contrast, the fate of Ca3-M1 products is either to go back to the Ca3-C coordinated state or to follow reacting. To confirm experimentally what is suggested by the simulated reaction study, the mechanism of irreversible CH3OH adsorption was further investigated through parallel IR spectroscopic and volumetric-calorimetric experiments on a new portion of the A200/Ca8-2 specimen. Influence of CH3OH Contact Time on Dissociative Adsorption. Figure 11 reports, in the 3100-2800 cm-1 spectral range, some absorbance differential IR spectra of irreversibly adsorbed methanol, normalized with respect to the starting baresample spectrum (so that spurious spectral contributions deriving from CH-containing contaminants present in the bare-sample spectrum are eliminated). Each spectrum corresponds to an individual ads/des step (out of five totally performed) on/from the sample A200/Ca8-2. Note that after the third ads/des run (ads/des III), the sample was kept in prolonged contact with CH3OH vapor (ads IV, 60 h contact at RT) before being evacuated, as usual, at BT (1 h). Then the ads/des V step was carried out, following again the standard procedure adopted in steps I, II, and III. The purpose of these ads/des steps, characterized by somewhat different adsorption times, was to confirm the invariant spectral features of the BT-irreversible methanol phase and to assess the influence of methanol contact time on the reactivity of the Ca-modified surface. The intensity of the two invariant ∼2960 and 2855 cm-1 bands resisting outgassing at BT and comprehensive of both strongly coordinated CH3OH and Si-OCH3 species was observed to increase at any ads/des step, as indicated by the numerals reported on the spectra. [In Supporting Information Table 3SM is reported both the integrated absorbance values of the complex ν(CH3) absorption and the increment observed at each step]. As expected, the integrated absorbance (A) of the ads/des II spectrum (curve b) is larger than that of the ads/des I (curve a) [∆A ) A(ads/des II) - A(ads/des I) ) 4.92 cm-1],


Figure 11. Absorbance IR differential spectra, normalized with respect to the starting bare-sample spectrum, of BT irreversibly adsorbed CH3OH. Each spectrum corresponds to an individual ads/des step on/ from A200/Ca8-2: (a) ads/des I; (b) ads/des II; and (c) ads/des III. In a-c, the spectra were recorded after 1 h contact at BT with ∼80 Torr CH3OH and subsequent 1 h evacuation at p ∼ 10-5 Torr. (d) ads/des IV. As for ads/des IV, the sample was evacuated 1 h after 60 h of RT contact with CH3OH vapor (∼80 Torr). (e) ads/des V. In trace e, the IR spectrum was recorded following the same protocol as in a-c. Numerals on the right-hand side of the 3100-2880 cm-1 spectral segments correspond to the integrated absorbance (A, cm-1) of each segment.

but a further slight increase was recorded also in the ads/des III step (curve c) [∆A ) A(ads/des III) - A(ads/des II) ) 1.52 cm-1]. The further increase of A at any further ads/des step, though with increments per unit contact time that are progressively lower, indicates that reactive irreversible processes are not concluded, even within the first five adsorption runs. In particular, it is worth stressing that the largest increase in the band intensity was recorded in the ads/des IV step (curve d; with respect to the previous step, ∆A ) 7.07 cm-1). This confirms that a prolonged contact of the surface with probe vapors keeps favoring the slow reactive phenomena. Some extra data relative to a similar experiment are reported in Figure 2SM and Table 6SM of the Supporting Information. The only difference is that at each BT ads/des cycle and before the subsequent BT ads/des one, an evacuation at 423 K was carried out to single out, within each irreversibly adsorbed amount, the fraction due to CH3OH coordinated to cus Ca2+ cations. The latter species was found to represent, in three consecutive and time-equivalent ads/des runs, ∼20% of the overall BT-irreversible phase, as previously observed for a single ads/des run (vide supra, Figure 6). In parallel with the IR experiments described above, an extra set of volumetric-calorimetric measurements were also performed on another fresh portion of the A200/Ca8-2 specimen. The standard ads-des-ads protocol described in the Experimental Section was followed until ads IV. Then the sample (still in the calorimeter at 303 K) was kept in contact with ∼80 Torr of CH3OH for 3 days before being evacuated overnight. Afterward, a fifth run was performed (ads V), as well as an additional sixth one (ads VI), returning to the standard protocol. In Figure 12, volumetric isotherms (section a) and qdiff vs nads plots (section b) are shown for the whole set of ads I-VI measurements. A large difference between ads I and ads II curves is well evident in both kinds of plots, as already observed

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16889 and discussed (Figures 3 and 4). At both low and high pressure, the irreversible component for the A200/Ca8-2 specimen represents over 30% of the total uptake, confirming the (ads II - ads I) datum previously obtained for A200/Ca8. In Table 4SM of the Supporting Information, the data for the six adsorption isotherms are summarized, whereas in Table 5SM the [ads N - ads (N + 1)] data are reported, which quantify the irreversibly adsorbed component at any ads/des step (N ) I, II,..., VI). It was so confirmed that the processes originating irreversibly adsorbed species are not accomplished within ads II, but continue in all subsequent adsorption steps, though representing a progressively decreasing component of the overall uptake, at least for ads III and IV runs. The irreversible component formed during ads III accounts for ∼10% of the total uptake at both low and high pressure, whereas the one formed during ads IV is almost negligible (less than 5% of the total uptake) erroneously suggesting that the reactivity potential of the surface was quenched. The scenario dramatically changes with the sample kept in prolonged contact with a high CH3OH pressure (ads V). Indeed, the irreversible component for this run (∼14% of total uptake in the early stage of the process) increases with respect not only to ads IV (where it was under the limits of detection), but also with respect to ads III. This confirms that the prolonged contact of Ca sites with a large CH3OH pressure allows (some of the) coordinated species to react, giving rise to an increased amount of surface Si-OCH3 and Ca-OH species. It is also evident from Figure 12a that the component irreversibly adsorbed during ads VI is lower than that adsorbed during ads V, but still significantly larger (at both low and high pressure) than that adsorbed in ads III and IV. It is worth noting that q0 is still ∼100 kJ/mol for ads I (confirming the results obtained for A200/ Ca8-1 and the other A200/Cax specimens, Figure 4) and is close to ∼80 kJ/mol for all the other five adsorption runs. All qdiff vs nads plots are typical of a highly heterogeneous surface, in that heat values progressively decrease with increasing coverage. A difference among the shape of ads II-IV curves on one hand, and the ads V and ads VI on the other hand, deserves a comment. As expected, the ads II, III, and IV curves lie below the ads I curve in the whole coverage range examined and turn out to be not so different. Conversely, the ads V and VI curves, even starting from the same q0 value, decrease (together) much more sharply and tend steeply to values lower than qL ) 38 kJ/mol. This datum is a further indication of the influence of the prolonged RT contact with methanol on the reactivity of the Ca-modified silica surface. Still, the virtually completed quenching of the reactivity of the Ca-modified surface was confirmed by the lack of any further irreversible component in the adsorption of CH3OH on the sample kept in contact with the probe (at RT) during three extra months after ads VI. In summary, both IR spectroscopic and volumetric-calorimetric data confirm the unicity of the reaction product (stable Si-OCH3 species) and the long-lasting nature of the reaction mechanism proposed by simulation and illustrated in Figure 10. Finally, taking into account the indication from the experimental data that the amount of methanol adsorbed irreversibly on A200/Cax samples increases with increasing adsorptive pressure, the coordination of a second CH3OH molecule in the Ca3-C adduct has been simulated ab initio. In other words, we tried to demonstrate that the equilibrium described by equation 1

Ca3-C(surf)+CH3OH(gas) a Ca3-M2(surf) is shifted to the right at increasing methanol pressure.

(1)


Bolis et al.

Figure 12. Adsorption at 303 K of CH3OH vapor on A200/Ca8-2 outgassed at 423 K. Section a: volumetric isotherms (nads vs peq); section b: differential heats of adsorption (qdiff) vs adsorbed amounts (nads). Ads I: square, solid symbols. Ads II: square, open symbols. Ads III: square, crossed (+) symbols. Ads IV: square, crossed (×) symbols. Ads V (run after evacuating overnight the sample previously kept 3 days in contact with ∼80 Torr of CH3OH vapor): diamond, solid symbols. Ads VI: diamond, open symbols.

words, the presence of a second molecule in the starting methanol adduct gives an extra stabilization to Ca3-2-M2 moiety, the only one experimentally observed (vide supra IR spectra results). It is thus reasonable to infer that such a difference in stability is bound to increase as the number of adsorbed molecules increases, as happens when a high CH3OH pressure is present in the real system experiment. 4. Conclusions

Figure 13. Enthalpy profile of Ca3-M1 and Ca3-M2 reaction products with respect to coordinated methanol (Ca3-C). Top side: Ca3-C model (one CH3OH molecule coordinated to a Ca3 site) as reference zero energy. Bottom side: Ca3-C-2 model (two CH3OH molecules coordinated on the same Ca site) as the new reference zero energy. Left side: coordination of a second molecule on Ca3-C model, giving rise to Ca3-C-2 adduct. MP2//B3LYP enthalpy of adsorption [-∆adsH°(0)] in kJ/mol.

Figure 13 shows the enthalpy profile of the two Ca3-M1 and Ca3-M2 reaction products, starting from the Ca3-C model (one molecule coordinated on the Ca3 site) taken as reference zero-enthalpy level (top side of the figure). This profile has been contrasted with the one for the Ca3-C-2 model (two molecules coordinated on the same Ca site) taken as the new reference zero-enthalpy (bottom side of the figure). In the left side of the figure, the MP2//B3LYP enthalpy of adsorption of a second molecule on the Ca3 site is -∆adsH°(0) ) 93 kJ/mol, which is very close to the value computed for the coordination of the first molecule [-∆adsH°(0) ) 104 kJ/mol], as was reported in Figure 7, bottom line. This datum indicates that the coordinative capacity of cus Ca2+ cations is not limited to the first adsorbed molecule and accounts for the relatively high q0 values obtained for ads II-VI runs (∼80 kJ/mol in all cases, as shown in Figure 12b). As for the methoxylation reaction, in Figure 13, the relative MP2//B3LYP enthalpy values for the two possible (M1 and M2) products are shown with respect to the Ca3-C and Ca3-C-2 zero enthalpy references. For the one-molecule adduct (Ca3-C, top side of the figure) the Ca3-M2 product is 12 kJ/mol more stable than Ca3-M1 [see the (32 - 20) kJ/mol difference], whereas for the two-molecule adduct (Ca3-C-2, bottom side of the figure) the Ca3-2-M2 product is 20 kJ/mol more stable than Ca3-2-M1 [see the (34 - 14) kJ/mol difference]. In other

The presence of Ca species at the surface dramatically modifies the surface features of amorphous silica, as revealed by the use of CH3OH as a molecular probe. Data from volumetric-calorimetric and IR spectroscopic adsorption experiments indicated, in fairly good agreement with ab initio simulation results, that the overall interaction of CH3OH with Ca-modified silica involves both chemical and physical adsorption processes, which to a large extent occur simultaneously, the energy transfer being very similar. The processes occurring at the interface are (in order of increasing strength of the interaction) (i) H-bonding interactions (the only ones occurring at the surface of plain silica), which originate rather labile adducts on silica polar terminations and are eliminated by pumping off at the adsorption temperature; (ii) a molecular adsorption at surface cus Ca2+ cations (acting as Lewis acidic sites), giving rise to coordinated CH3OH f Ca species, a fraction of which is eliminated by evacuation at the adsorption temperature, whereas another fraction is eliminated only by outgassing at temperatures up to 423K, according to the relatively high energy of interaction (q0 ∼ 100 kJ/mol); and (iii) a dissociatiVe reaction at the Ca sites, which is slow, not much more exothermic than coordination and dependent on both CH3OH pressure and contact time. The experimental evidence of a partial methoxylation of the silica surface in the neighborhood of Ca sites, yielding to Si-OCH3 and Ca-OH species (and not to Ca-OCH3 and Si-OH species) was confirmed by simulating an ab initio possible reaction path. Indeed, it was calculated that coordination of CH3OH at the Ca sites is thermodynamically favored with respect to the methoxylation reaction, in agreement with the experimental results that showed that only 30-40% of the total uptake was irreversibly bound to the surface. Still, the ab initio simulation showed that the Si-OCH3 species are not easily formed, but once formed, they remain tightly bound to the surface together with simultaneously formed Ca-OH species. Conversely, the (Ca-OCH3 + Si-OH) products, the fate of which is either to go back to the coordinated state or to proceed reacting, could not be detected experimentally. In summary, the composition of the reacted Ca-modified silica surface is made up of strongly coordinated CH3OH adducts as

Surface Properties of Silica-Based Biomaterials well as of methoxylated species. The methoxylation products comprise hydrophobic (and scarcely reactive) Si-OCH3 species and hydrophilic and reactive Ca-OH terminations. Remarkably, the presence of the latter species renders the reacted surface still available for new reactive phenomena. Indeed, it has been found that the Ca-modified surface follows reacting upon contact with molecules, and its reactivity is quenched only after prolonged contact with a high methanol pressure (conditions that allowed a larger fraction of coordinated CH3OH to react, in agreement with the simulated reaction profile). The fact that the surface reactivity of Ca-modified silica is not easily quenched at the first contact with molecules is likely to have an implication in the complex chain of events occurring at the interface between a silica-based material and physiological fluids. Acknowledgment. This work has been financially supported by the Italian Ministry MIUR (Project COFIN-2006, Prot. 2006032335, Interface Phenomena in Silica-Based Nanostructured Biocompatible Materials Contacted with Biological Systems) and by Regione Piemonte-Italy (Project CIPE-2004, Nanotechnologies and Nanosciences, Nanostructured Materials Biocompatible for Biomedical Applications), both coordinated by Prof. C. Morterra University of Torino. The contribution of E. Diana, A. Rimola, and V. Crocella` (Department of Chemistry IFM, University of Torino, Italy) is gratefully acknowledged, respectively for (i) the Raman spectrum reported in Figure 9 (E.D.); (ii) fruitful discussions in the modeling part of the work, especially referred to the reaction profile (A.R.); and (iii) for recording some of the IR spectra (V.C.). Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hench, L. L.; Wilson, J., Introduction to Bioceramics; World Scientific: Singapore, 1993. (2) Kasemo, B. Curr. Opin. Solid State Mater. Sci. 1998, 3, 451. (3) Kasemo, B. Surf. Sci. 2002, 500, 656–677. (4) Hench, L. L. J. Am. Ceram. Soc. 1998, 81, 1705. (5) Vallet-Regi, M.; Ragel, C. V.; Salinas, A. J. Eur. J. Inorg. Chem. 2003, 6, 1029. (6) Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 47 1976, 10, 161. (7) Cao, W.; Hench, L. L. Ceram. Int. 1996, 22, 493. (8) Xynos, I. D.; Edgar, A. J.; Buttery, L. D. K.; Hench, L. L.; Polak, J. M. J. Biomed. Mater. Res. 2000, 155 (2), 151. (9) Saravanapavan, P.; Verrier, S.; Jones, J. R.; Beilby, R.; Shirtliff, V. J.; Hench, L. L.; Polak, J. M. Bio-Med. Mater. Eng. 2004, 14, 467. (10) Cerruti, M.; Greenspan, D.; Powers, K. Biomaterials 2005, 26, 1665. (11) Roveri, N.; Palazzo, B., Nanotechnologies for the life Science. In Tissue, Cell and Organ Engineering; Kumar, C. S. S. R., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006; Vol. 9, p 283. (12) Kokubo, T.; Kushitani, H.; Sakka, S. A.-W. J. Biomed. Mater. Res. 1990, 24, 721. (13) Kokubo, T. Thermochim. Acta 1996, 280/281, 479. (14) Cerruti, M.; Magnacca, G.; Bolis, V.; Morterra, C. J. Mater. Chem. 2003, 13, 1279. (15) Cerruti, M.; Bolis, V.; Magnacca, G.; Morterra, C. Phys. Chem. Chem. Phys. 2004, 6, 2468. (16) Cerruti, M.; Morterra, C. Langmuir 2004, 20, 6382. (17) Cerruti, M.; Morterra, C.; Ugliengo, P. J. Mater. Chem. 2004, 14, 3364. (18) Bertinetti, L.; Tampieri, A.; Landi, E.; Ducati, C.; Midgley, P. A.; Coluccia, S.; Martra, G. J. Phys. Chem. C 2007, 111, 4027. (19) Hench, L. L. J. Biomed. Mater. Res. 47 1998, 41, 511. (20) Hench, L. L.; Splinter, R. J.; Allen, W. C.; Greenlee, T. K. J. Biomed. Mater. Res. Symp. 1971, 2 (Part I), 117. (21) Ebisawa, Y.; Kokubo, T.; Ohura, K.; Yamamuro, T. J. Mater. Sci.: Mater. Med. 1990, 1, 239.

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