Langmuir 1993,9, 3357-3370
3357
Nature and Structure of Adsorption Layer on Apatite Contacted with Oleate Solutions. 2. In Situ and ex Situ Fourier Transform Infrared, NMR, and X-ray Photoelectron Spectroscopy Studies J. A. Mielczarski* and J. M. Cases Laboratoire "Environnement et Minkralurgie" UA 235 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-14s-Nancy Cedex, France
P. Tekely and D. Canet Laboratoire de Mhthodologie RMN, UA 406 CNRS (LESOC) Universitk de Nancy I, Vandoeuvre-14s-Nancy Cedex, France Received February 22,1993. I n Final Form: August 18, 1993" The chemicalcomposition,molecularstructure, and aggregationstate of oleate molecules in the adsorption layers on apatite were examined using Fourier transform infrared (FTIR) in situ and ex situ techniques, solid state 13C CP/MAS NMR, and X-ray photoelectron spectroscopy (XPS)methods. Two different samplesof oleate with different amounts of isomers and homologues("impurities")were used. The infrared reflection, solid-state 13C NMR, XPS, and X-ray diffraction characterization of sodium, calcium, and cadmium oleate precipitates were carried out in order to provide references for discussion of the spectra of the adsorbed oleate layers. For adsorption submonolayers fabricated with oleate I (lower "impurity" level), it was found that the chemisorbed oleate molecules prefer to assemble into well-organized closepacked patches with hydrocarbon chain directed toward the solution. Size of the patches increases with coverage. Oleate I1 (higher "impurity" level), on the other hand, maintains a partially organized state without a clear formation of patchwise structure. Highest ordered and packed structure of oleate were found at about 0.7 and 1.0 of statistical monolayer of oleate I and oleate 11, respectively. The presence of water molecules was found in the well-organizedstructures of the adsorbed layers. This water stabilizes the organized structures of chemisorbedoleate which are responsible for hydrophobicity of apatite. Higher coverages are formed by a three-dimensional condensation (surface precipitation) mechanism, where precipitation of calcium oleate takes place directly on surface of apatite or with the use of chemisorbed covered sites as nuclei. It was found that the structure of surface precipitated calcium oleate, for both oleate samples, shows uneven surface distribution and, in majority, amorphous structure. Furthermore, it was found that this adsorption layer contains other ions such as sodium and phosphate. Surfacestructure of the adsorption layer at different coverages for both types of the oleate samples used has been proposed.
Introduction In our previous work we postulated,l on the basis of adsorption and infrared diffuse reflectance studies, two mechanisms of oleate adsorption on apatite, two-dimensional (chemisorption) and three-dimensional (surface precipitation) condensations. Information about the nature of the adsorbed molecules and structure of the adsorption layer, which is crucial to understand and manipulate properties of the adsorption layer for different applications, is limited by the analytical method applied. In the case of infrared spectroscopy the interpretation of the recorded spectra weighted heavily on the shape and position of the carboxylate bands. Because of a previously existing controversy over assignment of the infrared absorbance bands and consequently over a composition of the adsorption layer of oleate" as well as common uncertainty associated with using only one technique (some
* Abatractpublishedin Advance ACSAbstracts, October 15,1993.
(1) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, 0.;Delon, J. F. Langmuir 1993, 9,2370. (2) Sivamohan, R.; de Donato, P.; Cases, J. M. Langmuir 1990,6,637. (3) Hanumantha b o ; Cases, J. M.; Forasberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 330. (4) Hanumanth b o ;Forasberg, K. S. E. Colloids Surf. 1991,54,161. (6) Hanumantha b o ; Cases, J. M.; de Donato, P.; Forasberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 314. (6) Wen Qi Gong; Parentich, A.; Little, L. H.; Warren, L. J. Langmuir 1992,8, 118.. (7) Kellar, J. J.; Young, C. A.; Knutaon, K.; Miller, J. D. J. Colloid Interface Sci. 1991, 144, 381.
limitation resulting from using only infrared diffuse relectance technique), additional studies were performed on surface characterization of apatite after contact with oleate solution. The aim of this paper is to characterize in detail, by different surface sensitive spectroscopic methods, the nature of the adsorbed species and the structure of the adsorbed layer of oleate on apatite at submonolayer and monolayers coverages. The in situ and ex situ infrared reflection techniques, and ex situ NMR and XPS methods have been applied to obtain information on chemical composition, molecular structure, and aggregation states of oleate molecules in the adsorption layers. Since significant amounts of detailed references were obtained, this approach allows a better understanding of the basic concepts sustaining the spectral interpretation and they should be broadly applicable to a variety of calciumcontaining substrates and carboxylate-type surfactants.
Experimental Section Materials. The synthetic hydroxyapatite fine crystalline materialused in this investigation was supplied by Stauffer,USA. Microscopicexaminations showedtheparticlesto be regular with a needle shape. X-raydiffractionconfiied the crystalstructure of hydroxyapatite, CadPOMOH)r, with the Ca/P molar ratio equal to 1.67. The BET surface area of the sample, determined by argon adsorption, is 68.8 male.
0743-7463/93/2409-3357$04.00/00 1993 American Chemical Society
3358 Langmuir, Vol. 9, No. 12, 1993 Oleic acid (>99% pure) and sodium oleate (>98% pure) (denoted oleate I) supplied by Aldrich-Chemie and pure sodium oleate from Prolabo (not labeled), France (oleate 111,were used. Solubility product determination and infrared and liquid state 13C NMR spectra of these sodium oleate samples suggest that the latter reagent contains a higher amount of different isomers and homologues. Other reagents used were all of analytical grade. Distilled water from a Millipore (Milli-Qplus) system was used throughout the experiments. Calcium and cadmium oleates were obtained by precipitation from sodium oleate and proper metal salt solution with pH adjusted to 10.0 and 5.0. It was found by solid-state 'H and I3C NMR that the samples prepared with oleate I show a high degree of crystallinity whereas those prepared using oleate I1 exhibit some degree of amorphous character. This suggests again that the latter reagent contains a higher amount of different isomers and homologues. More detailed information about the materials can be found elsewhere.' Adsorption Studies. Two sets of adsorption experiments were performed. In one of them, experiment A, 1 g of apatite was added to 60 mL of sodium oleate I1 solution with known concentration. After 24 h of vigorous shaking the suspension was centrifuged and the supernatant was additionally filtered through a Millipore filter with pore size of 0.22 pm. The filtrate was analyzed for oleate species. Because of very high surface area of the adsorbate, the equilibrating time of 24 h was chosen to avoid any diffusional effect on the adsorption process and to achieve lattice ion concentrations near the equilibrium. The second type of experiment, experiment B, differs from that described above by letting apatite equilibrate for 24 h in 40 mL of water before mixing with 20 mL of sodium oleate I solution. Adsorption and other treatments were performed in the same way as described above. The pH of solution was adjusted to a value of 10 by adding NaOH or HCl solution. The apatite samples after adsorption were divided into several parts which were immediately used in different spectroscopic studies in order to minimize data scattering which could result from separate preparation of samples. Ex situ surface characterization was performed on samples dried at room temperature. Infrared Analysis. The infrared spectra were recorded on a Bruker IFS88 FTIR spectrometer with an MCT detector. ATR spectra were obtained using an internal reflection attachment with a germanium reflection element (25 reflections at incident angle of 45O). A wire-grid polarizer was placed before the sample and provided the required polarized light. DRIFT spectra were recorded by means of a diffuse reflectance attachment equipped with a vacuum-controlled chamber. All these accessories were from Harrick Scientific Co. The spectrometer was purged with dry air (Balston Filter) to minimize the contribution of water vapor and carbon dioxide to the recorded spectra. The spectra were taken at 4-cm-l resolution by coadding up to 500 scans in the 4000-500 cm-1 region. The unit of intensity was defined as -log(R/Ro), where Ro and R are the reflectivities of the systems without and with investigated medium, respectively. For DRIFT analysis 50 mg of dry sample was dispersed in 350 mg of KBr. For in situ ATR studies the apatite samples in contact with oleate solution were placed on the germanium element. No noise reduction or smoothing algorithms were applied to any of the spectra shown in this paper. More detailed information about other experimental conditions can be found elsewhere.' NMR Studies. All experiments were carried out on a Bruker MSL 300 spectrometer. The operating frequencies were 300.13 MHz for lH and 75.47 MHz for 13C. The samples were studied at spinning frequency between 3.0 and 5.0 kHz in a zirconiarotor. The 13C cross-polarization/magic angle spinning (CP/MAS) spectra were recorded using a standard cross-polarization procedure with a contact time of 1 ms. The proton MAS spectra and carbon-13 MAS spectra without cross-polarization were obtained with the use of a single pulse sequence. NMR is not a surface sensitive method like XPS or infrared spectroscopyand requires significantdensity of studied molecules in the sample under investigation; nevertheless, NMR can provide valuable informationon the molecular dynamic and structure of the adsorbed surfactant.8 Recently it was demonstrated that 2HNMR spectroscopyis applicablefor investigation of molecular
Mielczarski et al. dynamic in the adsorbed layer.Sl0 The surface studies of small molecule surfactants have been mainly conducted on silica9J1 although other substrates were also used.1° In this work, we have applied the '3C NMR technique for describing the dynamic and structure of the adsorption layer of oleate mainly at submonolayer coverages. One of the advantage of this technique is the possibility of the use of simple nonlabeled surfactant molecules, contrary to the 2H NMR technique. Moreover, it is possible to explore the broader chemical shift range of carbon. The measurements of cross-polarizationrates between protons and carbons were performed with the use of the reverse crosspolarization evolution method by introducing a second variable contact time for proton phase inverted cross-po1arization.l2 Selective polarization inversion of the l3C resonance signal depends on the polarization transfer rates between protons and carbons. Therefore, the polarization of protonated carbons is inverted first before that of nonprotonated ones. Since molecular motions in general decrease the cross-polarization,methyl carbon is inverted more slowly than other protonated carbons. Consequently, by use of this procedure selective inversion or suppression of resonance signals is observed depending on the number and distance of surrounding protons from carbon atoms and on the molecular mobility of individual functional groups or sites in the sample. This method has been used for studying the dynamics of oleate molecules in the adsorption layer on apatite on the basis of changes in molecular mobility of individual groups along the hydrocarbon chain. This information could be also interpreted in terms of different conformationsof oleate molecules anchored on apatite surface or their surface distribution. XPS Measurements. The XPS spectra were collected on a Leybold-Herakus spectrometer with K a excitation from a aluminum anode operated at 12 kV and 10 mA. The pressure in the analyzer chamber was nearly 108Pa. The measurements were performed with a take-off angle close to 90°. The Ca2p, P2p, Nals, Ols, and Cls lines were fitted using a curve-fitting program with a Gaussian/Lorentian peak shape. A charging between 1and 2 eV was observed and the spectra were corrected by the use of the Cls line (284.8 eV). The pressure during the measurements of precipitates was somewhat higher, about 5 X 1o-BPa, indicating some desorption from the samples, which was probably mainly water.'
Results and Discussion Spectroscopic Characterization of the Structure of Precipitated Oleate Salts. The literature survey did not provide good spectral references since they do not exist or significant discrepancies between authors concerning the assignments of the observed spectroscopic peaks were reported. Therefore, a significant part of this work was devoted to the collection basic references needed for the explanation of the spectroscopicdata obtained from oleate adsorption studies on apatite. In our previous studies' we found that infrared reflection spectra of precipitated calcium oleate varied significantly with the crystalline structures of investigated samples. The asymmetric stretching vibration of the carboxyl group shows a well separated doublet with maxima at 1574 and 1539 cm-l or a broad band with a single maximum at about (8) See for example, Blum, F. D. In Application of NMR to New Materials; Webb, G. A., Ed.; Marcel Dekker, Inc.: New York, 1993, and selected references cited therein. (9) Zeigler, R. C.; Maciel, G. E. J. Am. Chem. SOC.1991, 113, 6349. Kang, H.-J.; Blum, F. D. J. Phys. Chem. 1991, 95,9391, and selected references cited therein. (10) Macdonald, P. M.; Yue, Y.; Rydall, J. R. Langmuir 1992,8,164. Yue,Y.; Rydall,J. R.; Macdonald,P. M. Langmuir 1992,8,390. Soderlind, E.; Blum, F. D. J. Colloid Interface Sci. 1993,157,172. Boddenberg, B.; Eltzner, K.Langmuir 1991,7,1498. Nadler, M. P.; Nissan, R. A,; Hollins, R. A. Appl. Spectrosc. 1988,42,634. (11) Zeigler, R. C.; Maciel,G. E. J . Phys. Chem. 1991,95,7363. Tuel, A.; Hommel, H.; Legrend, A. P.; Barald, H.; Papirer, E. Colloids Surf. 1991, 58, 17. (12) Zumbulyadis, N. J.Chem. Phys. 1987,86,1168. Wu, X.; Zhang, S.; Wu, X. J. Mngn. Reson. 1988, 77,343; Cory,D. G.; Ritchey, W. M. Macromolecules 1989,22, 1611. Tekely, P.; Delpuech, J. J. Fuel 1989, 68,947. Tekely, P.; Montigny, F.; Canet, D.; Delpiiech,J. d. Chem. Phys. Lett. 1990, 175, 401.
Nature of Apatite Adsorption Layer
1560 cm-l after different treatment of the sample before infrared studies. It has been proposed that the doublet is characteristic for two different polymorphic domains whereas the broad band is due to a mixture of a few different structural modifications or to a more amorphous structure. The presence of water, oleic acid, and other “impurities” plays an important role in the formation of the structural domains. A characteristic feature of unsaturated fatty acids in the crystalline state is polymorphism, which itself results from different modes of molecular conformation and crystal packing.’s-14 In this part of the study the highresolution solid-state 13C NMR and XPS spectroscopies in addition to ATR FTIR spectroscopy were applied to evaluate relationships between positions, intensities, and shapes of the bands, observed in the recorded spectra and the nature or structure of precipitated oleate salts. The calcium and cadmium oleates precipitated from basic and acidic solutions and sodium oleate and oleic acid “as received” were investigated. The calcium and sodium oleates as well as oleic acid are species which are expected to be formed as the adsorption products of oleate on apatite.’ Obviously, cadmium oleate species are not one of them, but these precipitates were also investigated since they are salts of divalent cation with lower charge to ion size ratio than that for calcium. We found that the spectroscopic data obtained for cadmium salt were very useful for the assingment of peaks in the infrared and NMR spectra of calcium oleate, and this will be exploited in this section. The cross-polarization magic-angle spinning 13CNMR spectra of these oleate compounds in the chemical shift range characteristic of carboxyl carbon are presented in Figure 1. The ATR infrared spectra of the same samples recorded for s-polarization are shown in Figure 2 for comparative purposes. Since the ATR technique does not require a special preparation of the samples before infrared studies (only simple contact with reflection element is needed), the recorded spectra are free from any alterations which can be caused by sample preparations. Oleic acid, which is aliquid at room temperature, shows, as expected, only a single peak at 180.45ppm in the liquidstate 13C NMR spectrum (Figure la). The infrared spectrum of this compound shows also a single strong absorbance band at 1710 cm-I due to the stretching vibration of the carboxyl group with a half width of 18 cm-I (Figure 2a). The low intensity shoulder at 1655cm-’ is due to the stretching vibration of the C=C group. Sodium oleate shows (Figure 2b) a single band at 1559 cm-l with some asymmetry on the lower frequency side which can be attributed to another conformation component.15 The 13C CP/MAS NMR spectrum of sodium oleate (Figure lb) shows also two components with a relative intensity ratio similar to that which can be estimated from the infrared spectra. A small chemical shift of 1.3 ppm between these two components may result from a conformational difference. The infrared and NMR spectra (Figures ICand 2c) of the calcium oleate precipitated from solution of pH 10.2 show the doublets which are separated by 36 cm-l and 5.7 ppm, respectively. The presence of the doublet also in (13)Sato, K.; Garti, N. In Crystallization and polymorphism of fats New York, andfattyacids;Garti, N.,Sato,K.,Eds.;MarcelDekker,Inc.: 1988; Chapter 1. (14)Hemqvist, L. In Crystallization and polymorphism of fats and fatty acids; Garti, N., Sato, K., Eds.; Marcel Dekker, Inc.: New York, 1988; Chapter 1. (15) Yang, P.W.;C a d , H. L.; Mantach, H.H.Appl. Spectrosc. 1987, 41, 320.
Langmuir, Vol. 9, No. 12,1993 3359
NMR spectra supports the early explanation’ that the carboxyl groups are present in two different structural forms. In order to investigate the origin of the two carboxyl peaks in NMR spectra and to evaluate whether they result from conformational or crystalline in type changes, the spectra of calcium oleate after precipitation were recorded at different temperatures, and the results obtained are presented in Figure 3. An increase of temperature from 297 to 373 K strongly increases a motion of all individual molecular groups in calcium oleate. Therefore, for recording good-quality 13CNMR spectra, an increase of the cross-polarization contact time to 5 ms was required. The X-ray pattern (not shown) recorded also at 373 K shows amorphous character of the investigated sample with some degree of a short range ordering. This agrees with results obtained by differential thermogravimetric analysis1where phase transition was observed at 358.6 K. Hence, the chemical shift to lower field of the doublet high field component observed a t 129 ppm at room temperature, the shift of the carboxyl doublet from the positions of 185.1and 179.4 ppm to 185.6 and 180.0 ppm, respectively, and the shifts to higher field in the methylene range indicate conformationally induced changes along the chain caused by transition from crystalline to amorphous structure. At temperatures higher than 373 K the peak at 180 ppm disappears first, which indicates that these two structural forms are perturbed differently and there is not close interaction between them. Owing to a possible analogy in structural properties of calcium oleate and oleic acid and the complex nature of the latter,14J6 separate studies, using a combination of X-ray diffraction with other methods, are required for better characterization of possible polymorphic forms. These studies are currently in progress. A significant difference in shape (half width) of the peaks in NMR spectrum (Figure IC)may indicate that one of the two structural forms represented by the peak at 185.1 ppm is more homogeneous than the other one which shows a larger distribution in isotropic chemical shift position. This difference could be related to the difference in half width values of the doublet components observed in the infrared spectra (Figure 24. The sharper band at 1537 cm-l with half width of 12 cm-I is probably associated with the sharper peak a t 185.1 ppm, whereas the broader band at 1573cm-l with half width of 14 cm-I may correlate with the broader peak at 179.4 ppm. For this sample the intensity ratios between these two structural forms estimated from the infrared (after taking into account the polarization effect) and NMR spectra are very similar, and these ratios were found to be almost equal (see the curves characterizing the integrated intensities in Figure 1). These results indicate that two structural forms ofthe carboxylgroup are present in the calcium oleate precipitate in an almost equal amount, and it may be suggested that one of them can be assigned to chelating (bidentate) bonding between the carboxyl group and calcium ion while another carboxyl group forms a more unidentate complex, similar to that found for the cu-Ca(HCOO)a compound by X-ray diffraction studies.” The observed doublet (Figure 2c) with bands at 1630 and 1619 cm-I is, as reported recently,l due to crystalline water. Removing this water involves dramatic changes in the crystalline structure of calcium oleate which results in an appearance of a broad band with a single maximum at about 1560 cm-l.’ Hence, (16) Kobayashi, M. In Crystallization and polymorphism of fats and f a t t y acids; Garti, N., Sato, K., Eds.; Marcel Dekker, Inc.: New York, 1988; Chapter 4. (17) Matsui, M.; Watanabe, T.;Kamijo, N.; Lapp, R. L.; Jacobson, R. A. Acta Crystallogr. 1980,836, 1081.
3360 Langmuir, Vol, 9, No. 12, 1993
Mielczarski et al.
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Figure 1. High-resolution 19c NMR spectra in the carboxyl region of chemical shift for oleate I salts: a, oleic acid; b, sodium oleate; c, calcium oleate precipitated at pH 10; d, d', calcium oleate precipitated at pH 5; e, cadmium oleate precipitated at pH 10; f, cadmium oleate precipitated at pH 5. Spectrum a is recorded in liquid state, the spectrum d' in a single pulse MAS experiment (repetitiontime of 300 s,u, = 4.2 kHz). The remaining spectra are recorded in cross-polarization(CP)experiments(CP = 1 ms, uI = 4.2 kHz). Multisteps curves show integrated intensities of the observed peaks.
the water plays an important role in the stabilization of the system. The calcium oleate precipitated from acidic solution of pH 5.0 contains oleic acid, which was disclosed by the infrared spectrum (Figure 2d) where the absorbance band at 1675 cm-I is observed. It is important to note that this band is strongly shifted from the position at 1710 cm-l which is typical for the carboxyl group in fatty acids (Figure 2a). The intensity of the band is very low compared with the intensity of the ionic form of the carboxyl group. The integrated intensity ratio is equal to 1:240. With the absorbance coefficient of the carboxylate bands for oleic acid and calcium oleate known, the ratio between these two components in the sample can be estimated. The required optical constants were estimated from the transmission spectra of calcium oleate in KBr and the ATR spectra of oleic acid. The estimated amount of oleic
Langmuir, Vol. 9, No. 12,1993 3361
Nature of Apatite Adsorption Layer
T=393K
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Figure 3. High-resolution solid-state CP/MAS NMR spectra of calcium oleate I precipitated at pH 10 recorded at different temperatures with a contact time of 5 ma (spectrum at 297 K was recorded with contact time of 1 ma) in three regions of chemical shift. The spectrum at 393 K was obtained in a single pulse experiment (repetition time of 5 a). Note that the individual regions are shown in different scale of intensity and frequency. The marked numbers in spectrum recorded at 297 K indicate intensity scale factors between particular regions but only at this temperature.
acid in this sample is about 2 '74. Therefore, one can expect only a very small intensity peak which could be assigned to acid species. However, the NMR spectrum (Figure Id) shows much stronger changes in the carboxyl carbon
chemical shift range than could be expected from composition of the sample determined on the basis of infrared results. New peaks with similar intensities appear at 182.4 and 186.5ppm with shapes very similar to those observed
3362 Langmuir, Vol. 9, No. 12, 1993 for the calcium oleate sample without oleic acid (Figure IC).The integrated intensity of these new peaks (Figure Id, the integration curve) is about 36% of the total integrated intensity observed for the peaks characteristic of the carboxyl group. Since the NMR spectra obtained in a cross-polarization experiment cannot be considered as a source of quantitative data, an experiment with a single pulse was also performed (Figure ld’). It can be seen that only small differences, less than l o % , in the relative intensity ratios are observed for the peaks obtained in these two experiments (parts d and d’ of Figure 1). These IR and NMR results are consistent if one assumed that the oleic acid does not form a separate phase. Oleic acid present in the sample is likely structurally incorporated within parts of the two structural forms characteristic of calcium oleate precipitate obtained at pH 10 and may perturb their structues forming two other distorted forms with characteristic peaks shifted to higher frequency. The differences between the 13Cresonances representing initial andnew structuresare 1.34and 3.1 ppm. These structural changes can be also noticed in the infrared spectra. Careful inspection of the infrared results in parts c and d of Figure 2 reveals an increase in the half width of the doublet components from 14 to 16 cm-l for the band at 1573 cm-’ and from 12 to 18 cm-l for the band at 1537 cm-l, for the sample containing oleic acid. Also a well marked shoulder at 1527 cm-l can be found (Figure 2d). Hence, it can be concluded that these data reflect the presence of four structural forms present in similar amount. The l3C NMR spectrum of cadmium oleate shows (Figure le) the three components separated by 0.8 and 0.7 ppm with similar intensities, whereas the infrared spectrum (Figure 2e) shows a broad absorbance band, with half width of 18 cm-l, with a low intensity shoulder at 1567 cm-l. These results suggest that cadmium oleate comprises a minimum of three forms with small structural differences. The cadmium oleate which was precipitated from the acidic solution (Figure 20 at the same pH as used for precipitation of calcium salt shows about a 6 times lower amount of oleic acid. Moreover, the carboxyl band of acid at 1682cm-l is shifted by 7 cm-l to higher frequency compared with the calcium salt (Figure 2d). These observations indicate a very weak interaction of acid molecules with cadmium oleate crystalline structures and an almost nondisturbed structure is expected. In fact, the NMR spectra (parts e and f of Figure 1) show only small differences between these two samples, and the infrared spectra (parts e and f of Figure 2) are almost exactly the same. The morphological difference between cadmium and calcium precipitates results from different properties of these metal ions in solution. The Ca2+ion has a high ionic character because of high charge to size ratio and the strength of the interaction between water and the ion is great. In contrast, cadmium shows a lower charge to ion size ratio and its association with water is not so strong as in the case of calcium. As a consequence, cadmium has a greater covalent character which would permit a stronger interaction with oleate while calcium interaction is mainly ionic in nature. This may explain differences between the close-packed structure of cadmium oleatel8 and, very sensitive to any “impurities”, the crystalline structure of calcium oleate in which the presence of water molecules plays a crucial role. These observations are consistent with the results of studies of the structure of fatty acid monolayers at the air-water interface (surface pressure) in the presence of various metal ions18J9 and with the finding of a high degree of Ca2+ ion hydration.20 (18)Yazdanian, M.; Yu,H.; Zografi, G.Langmuir 1990, 6, 1093.
Mielczarski et al.
The above discussion together with the observed differences between the calcium and cadmium oleates NMR spectra provides an alternative explanation for the observed differences between samples precipitated in basic and acidic conditions. The presence of a significant amount of water in the structure of calcium salt precipitates may likely involveformation of acidlike (more protonated) forms of calcium oleate at lower pH which are exhibited as new resonances at higher frequency. Similar differences in position of the carboxyl peaks were recently observed for histidine21prepared in solution a t different pH, though, in that case the shift observed with decreasing pH is reverse to that observed here for calcium oleate precipitates. The methylene region of chemical shift in the 13C CP/ MAS spectra yields also some information about local chain motions. First of all it is crucial for interpretation to assign the observed peaks to particular methylene groups. For this aim a two-dimensional 13C INADEQUATE experiment was carried out on liquid oleic acid. It was found that the chemical shift relationships obtained for oleic acid are the same as those observed for saturated fatty acids.22 This result allows unequivocal identification of all carbon positions in the spectrum of oleic acid and is useful for starting the assignment of peak resonances in the solid state 13C NMR spectra of different oleate salts (Figure 5a). As shown in Figure 4 significant differences are observed in the positions of individual peaks for different oleate salts. These peaks are shifted upon conformationallpolymorphic changes. In fact is has been recognized that the conformationally dependent 13C chemical shift can be as large as 12 ppm and it is related to a set of torsion angles of nearby single b0nd.~3 The conformational origin of differences observed in the methylene region for different salts with respect to the motioned averaged values in liquid state is shown in Figure 3 where a gradual shift to high field (in total about 2.5 ppm) of the central part of this region is observed as temperature increases from 297 to 393 K. The assignment of the individual peaks in these spectra can be done on the basis of differences in local molecular motion of methylene groups, depending on their position along the chain. For this purpose the cross-polarization inversion method12was applied which, as mentioned above, leads to selective inversion or suppression of individual methylene group depending on their local mobility. One of the examples of experimental data provided by this experiment is shown in Figure 5 for sodium oleate I1 sample obtained for different times of cross-polarization inversion. The whole set of changes in 13C intensity of individual peaks, for sodium oleate, in the cross-polarization inversion experiment is shown in Figure 6. A two-stage decay of carbon magnetization, an initial fast decay followed by a much slower one can be seen immediately. The initial stage results from cross-polarization transfer of magnetization between directly bound protons and carbons and from an energy equilibration in the proton system viaspin-diffusion processes.24@ The characteristic cross-polarization times of individual carbons are given in Table I. As expected the longest time constants, TCH, were observed for C1 (nonprotonated) and for Cla (rapidly rotating methyl) carbons. Large (19) Gordziel,S. A.; Flanagan, D. R.; Swarbrick,J. J.Colloidhterface Sci. 1982, 86, 178. (20) Pashley, R. M.; Izrealachvili, J. N. J . Colloid Interface Sci. 1984, 97. -.. (21jFrey, M. H.; Opella, S. J. J. Magn. Reson. 1986,66, 144. MR.
(22) Bengsch, E.; Perly, B.; Deleuze, C.; Valero, A. J. Magn. Reson. 1986,68, 1. (23) Saito, H. Magn. Reson. Chem. 1986,24, 835. (24) Wu,X.;Zhang, S.Chem. Phys. Lett. 1989, 156, 79. (25) Wu,X.;Zhang, S.;Wu,X. Phys. Rev. B 1988, 37, 9827.
Langmuir, Vol. 9, No. 12,1993 3363
Nature of Apatite Adsorption Layer
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PPM
Figure 4. High-resolution 13C NMR spectra in the methylene region of chemical shifts for oleate I salts recorded at the conditions aa shown in Figure 1: a, oleic acid; b, sodium oleate; c, calcium oleate precipitated at pH 10; d, calcium oleate precipitated at pH 5; e, cadmium oleate precipitated at pH 10; f, cadmium oleate precipitated at pH 5.
differences in the TCHare observed for the methyl-terminal fragment of hydrocarbon chain compared with carboxylterminal fragment. This proves a pronounced local molecular mobility of the chain tail and a lower mobility of carboxyl-terminal chain due to an anchoring effect of the ion-carboxyl head. The gradient of mobility along the chain is also reflected in the second-stage decay of cross-polarization inversion curves (Figure 6) for time longer than 0.5 ms. This stage is dominated by the proton and carbon spin-lattice relaxation in the rotating frame. This relaxation process is more rapid for amethyl-terminal fragment than for a carboxyl-terminal chain, which indicates a higher spectral density of molecular motion in the midkilohertz frequency region for the methylene groups placed in the methyl-terminal fragment.
Mielczarski et al.
3364 Langmuir, Vol. 9,No. 12, 1993
-
ul U U N
m
I
N
n
I
1
3000 28b0 YRVENUMEER CM-1
-1800
l6b0
I400
l2R0
Y R V E N U M B E R CH-I
Figure 7. In situ ATR spectra of the adsorption layer of oleate I1 on apatite obtained in experiment A at different adsorption density (pmol/m2): a, 2.10; b, 6.15; c, 7.66; d, 10.77.
The observed gradient of local mobility along the chain indicates some degree of amorphous character of the samples. The arrangement of the chains in ordered domains leads to more damped motional gradient and, in fact, it is observed for highly crystalline calcium oleate I samples as shown in Table I. Such sensitivity of the TCH time constant on a mutual arrangement of the hydrocarbon chain forms the basis for discussion of molecular dynamic of oleate within adsorption layer. These infrared and NMR results of characterization of structural forms of bulk oleate salts provide good references to discussion of spectra of structural changes and oleate molecular dynamics within the adsorption layer on apatite and other calcium-containing substrates. Spectroscopic Studies of the Adsorption Layer. Adsorption of Oleate I1 in Experiment A. In situ (ATR) reflection spectra of apatite after adsorption of oleate I1 in experiment A, where apatite was added to oleate solution, are shown in Figure 7. They are difference spectra obtained after subtraction of the ATR spectrum of a suspension of apatite in water, and therefore, they should represent solely a spectrum of the adsorption layer of oleate on apatite. Possible adsorption of oleate from solution directly on the germanium reflection element, which obviously might perturb the spectrum of the adsorption layer on apatite, was found in our preliminary experiments to be negligible. No adsorption of fatty acid on germanium at open circuit potential was reported recently.26 The recorded spectra (Figure 7) show some
negative bands which are caused by nonperfect subtraction of water. The spectra of apatite with the adsorption layer recorded in situ are dominated by very strong water absorbance bands. Our experience shows that the subtraction of the water bands in the entire frequency range from the spectra of the adsorption layers recorded in situ is usually unsuccessful. It is caused by a different amount of water in suspensions before and after adsorption which change the depth of penetration of reflected beam in investigated suspension with its density. As a consequence, it is very difficult, if not impossible, to find a reference spectrum for perfect water subtraction. Therefore, the changes in the spectral range of strong absorption of water will not be interpreted. Certainly, the use of a heavy water as a solvent allows this background correction problem to move to another frequency region. Nevertheless, it was found that uncertainties associated with subtraction of water have a very little influence on the shape of the ionic carboxylabsorbance bands and practically can be neglected since their positions are fortunately far enough from the absorbance band of water. The in situ spectra of the adsorption layer show (Figure 7) for the ,Y carboxylate vibration a broad absorbance band with position varying from 1549 to 1544 cm-l. The half width of the single band is equal to 50 cm-l. The shape and position of the bands due to adsorbed oleate are almost identical to those obtained by ex situ DRIFT technique1 for the apatite sample with similar adsorption density. Hence, these results show that preparation of the apatite sample after oleate adsorption (drying and mixing with KBr) does not alter structure of the adsorption layer. It was concluded recently' that some "impurities" (isomers, homologues) present in oleate I1are responsible for distortion of ordered structure of the adsorbed layers which is also the case observed here, where the reflection spectrum (Figure 7) exhibits a single carboxyl band at about 1545 cm-1. The critical importance of the position of the cis double bond in unsaturated fats on chain-chain (26) Hayashi, S.; Umemura, J. J . Chem. Phys. 1976, 63,1732.
Langmuir, Vol. 9, No. 12,1993 3366
Nature of Apatite Adsorption Layer
Table 11. Cross-Polarization Times. TCH(re) for the Apatite Samples after Adsorption of Oleate I1
adsorptiondensity 2.10 pmovm2 7.66pmovm2 10.77pmol/m2
CI CZ CM
Celo
c11-16
58 58 68
720 288 317
288 288 288
577 677
288 108 108
CI, CIS 288 288 288
"Estimated from the relation tl TCHIn 2 with corresponding to zero intensity of each signal.
-LPL---.l
iRD
I'IR
I an
-L 58
PPM
Figure 8. CP/MAS NMR spectra (CP = 1 ms, v, = 4.2kHz) of adsorption layer of oleate I1 on apatite obtained in experiment A at different adsorption density (pmol/m2): a, 2.10;b, 7.66;c,
10.77.
interaction in binary mixtures containing oleic acid was also reported re~ently.~' It is noteworthy that the intensities of the absorbance bands in the in situ spectra of the adsorbed layers (Figure 7) are proportional to the amount of oleate adsorbed determined on the basis of uptake of oleate from solution during adsorption. A similar relationship was also found on the basis of '3C NMR spectra results (Figure 8). This finding confirms the conclusion that, observed in the diffuse reflectance spectra,l lowering in intensity of the absorbance bands characteristic for the adsorbed oleate with increasing of adsorption density is due to optical effectsassociated with diffusereflectance techniques. More detailed studies of this phenomenon can be found elsewhereS28 (27) Yoehimoto,N.;Nakamura,T.;Suzuki,M.;Sato,K. J. Chem.Phys. 1991,96,3384.
tl
577 677
values
Three of the apatite samples with different thickness of the adsorption layer were investigated by means of the l3C CP/MAS NMR after in situ infrared studies, and the corresponding spectra are presented in Figure 8. The carboxylate peak at about 180 ppm is very broad for all investigated samples which indicates a multicomponent structure and/or different environment of the carboxylate groups in the adsorption layer. The observed very low intensity of the broad carboxyl peak for all coverages (Figure 8), compared with the spectra of precipitated calcium oleate (Figure 3 at 273 K), indicates a strong influence of apatite surface on adsorbed oleate molecules. The head of the oleate molecule is tightly bound to the surface with probably little water penetration. The broadening of the carboxyl peak can also result from heterogeneity of the apatite surface. These suggest that the adsorption takes place on the calcium site of apatite. The observed lowering of the intensity of the carboxyl peak can be also due to the smaller amount of water, which is present close to the carboxyl group, in the adsorption layer compared with calcium oleate precipitate. The presence of the water could significantlyincreasethe signal from the carboxyl group recorded in CP experiments. The latter hyphothesis may be excluded on the basis of single pulse experiments discussed in detail later in this section. The splitting of the methyl peak may also indicate the presence of different conformational forfns. Significant changes in the NMR spectra of the adsorption layer are observed in the methylene region (Figure 8). As discussed above, this part of the spectrum can provide useful structural information about the dynamics of molecules on the surface of apatite a t different adsorption densities. For this aim the cross-polarization inversion experiments were performed and the. TCHvalues of different parte of the aliphatic chain of oleate at different adsorption densities are given in Table 11. It can be seen immediately that for the lowest adsorption density of 2.10 pmol/m2 (0.26 of statistical monolayer, calculated with assumption of 20.5 A2/molecule)the adsorbed molecules show higher motion than that found for calcium oleate I1 precipitate. Moreover, beside the carboxyl adjacent carbon, different parts of the chain have the same mobility, which is characteristic for the overall motion of the chain. These suggest that the adsorbed molecules at this density are rather separated from each other on the apatite surface and, as concluded above, they interact with the surface through carboxyl group forming the chemisorption in type bonding. At close to statistical monolayer coverage (7.66 pmol/ m2)the cross-polarizationtimes, TCH, show slower motion for the carboxyl-terminal fragment and the C=C group compared with submonolayer coverage (Table 11). These may indicate more rigid structure with some lateral interaction between these parts of hydrocarbon chain and suggest a brushlike structure of the adsorbed layer. Nevertheless, though at this coverage the TCHvalues are low, indicating more rigid structure, they are still far away from those observed for calcium oleate I1 precipitate, indicating a high level of distortion of the organized (28) Mielczarski, J. A.; et al. Paper in preparation.
3366 Langmuir, Vol. 9, No. 12, 1993
Mielczarski et al.
Table 111. Binding Energies (eV) for the Characteristic Lines Observed in Spectra of Apatite Samples before and after Adsorption of Oleate I1 (fwhm values in parentheses) samples, absorption density C*lS Cbls Cbls Ca2p P2P Nals apatite, as received 284.8 (2.55) 289.3 (1.95) 530.8 350.4 346.9 133.3 apatite, 2.10 pmollm2 284.8 (2.09) 288.2 (1.95) 530.8 350.4 346.9 133.3 531.1 350.6 347.1 133.4 apatite, 7.66 pmol/m2 284.8 (1.84) 288.2 (1.95) 531.2 350.6 347.1 133.5 1071.9 apatite, 10.77 pmol/m2 284.8 (1.84) 288.3 (1.95) Ca(o1eate)zprecipitate
284.8 (1.84)
288.2 (1.95)
Table IV. Relative Intensities of the Observed Lines in XPS Spectra of Apatite Samples before and after Adsorption of Oleate I1 samples, adsorptiondensity Cnls Cbls 01s Ca2p P2p Nals 0.275 0.025 1.54 1 0.155 apatite, as received 11:l 1
0.156
0.670 0.027 1.40 25:l apatite, 10.77 pmol/m2 0.772 0.048 1.32 16:l
1
0.152
1
0.166
Ca(o1eate)zprecipitate 5.551 0.347 1.99 16:l 5.383 0.317 1.67 Ca(o1eate)p 17:l 1.09 apatiten
1
apatite, 2-10pmol/m2
0.238 0.022 1.43 11:l
apatite, 7.66 pmol/m2
a
0.08
1 1
0.148
Calculated from data of Wagner et a1.M
structure. Future increasmf surface density (10.77 pmol/ m2), somewhat above monolayer coverage,does not change significantly mobility along the hydrocarbon chain in the adsorbed molecules. Since the infrared and 13CNMR spectroscopic studies cannot provide complete information about chemical composition of the adsorbed layer, X-ray photoelectron spectroscopy was applied. Besides new surface information, the XPS results allow further clarification of a previous controversy2-' in the interpretation of the infrared reflection spectra of oleate on apatite and other calciumcontaining substrates. The XPS detailed studies of the adsorption layer of oleate on apatite are rather difficult especially at submonolayer coverages because the oleate consists of carbon and oxygen atoms which are always present at the surface of the substrate contacted with air and/or aqueous solution. Nevertheless, as is shown below, taking into account that carbon and oxygen contaminations are present at certain level, valuable information on the interaction of oleate with apatite can be obtained. In particular, this method gives direct information on the chemical composition of the surface; moreover, it is also possible in many cases to determine the orientation of the adsorbing species on The XPS results for apatite samples before and after adsorption of oleate I1 are presented in Tables I11 and IV. The apatite sample before adsorption shows broad components at 284.8 eV (fwhm = 2.55 eV) and 289.3 eV (fwhm = 1.95 eV) due to Cls emission of hydrocarbon contamination and carbonates,%respectively. The relative oxygen intensity is about 50 7% higher than that expected for pure (29) Mielczarski, J.; Werfel, F.; Suoninen, E. Appl. Surf. Sci. 1983,17, 160. (30) Mielczarski, J. A.; Yoon, R. H. Langmuir 1991, 7, 101. (31) Nakayama, Y.; Takahagi, T.; Soeda, F.; Ishitani, A,; Shimomura, M.; Kunitake, T. J. Colloid Interface Sci. 1988, 122, 464. (32) Nakayama, Y.;Takahagi, T.; Soeda, F.; Ishitani, A. J. Colloid Interface Sci. 1989, 131, 153. (33) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.;
Mudenburg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prarie, MN, 1979. (34) Wagner,C. D.;Davis, L. E.; Zeller, M. V.; Taylor, 3. A.;Raymond, R. M.; Gale, L. M. Surf. Interface Anal. 1981, 3, 211.
531.7
350.9
347.5
hydroxyapatite sample. These observations indicate preadsorption of speciessuch as hydrocarbons, carbonates, and water on the surface of apatite. These two latter surface species were also found in diffuse reflectance spectra of apatite before adsorption.' After adsorption of oleate on apatite at density of 2.10 pmol/m2 (about 0.26 of the statistical monolayer) the relative intensity ratios of the Cls and 01s lines to the Ca2p line significantly decrease (Table IV). Since this is contrary to what is expected in the case of simple oleate adsorption, these results suggest that the adsorption of oleate also involves removing of the preadsorbed species from the apatite surface. The significant narrowing of the (2% component from 2.55 to 2.09 eV (which additionally decreases to 1.84 eV at higher coverages),the shift of the second Cbls from the position of 289.3 eV characteristic of carbonate to the position at 288.2 eV characteristic of carboxylate group, and the lowering of the 01s relative intensity confirm the explanation about removing pteadsorbed species from apatite surface during chemisorption of oleate. These observations are in agreement with infrared results on apatite after adsorption' where also disappearance of the preadsorbed water and carbonates were clearly observed. The increase of the intensity of Cls line with increasing of the adsorption density was observed which correlates well with the results obtained from in situ ATR and 13C NMR studies. Careful inspection of the relative intensity ratio of Cls components (Table IV) discloses other interesting features. The intensity ratio of the Cals/Cbls (hydrocarbon carbons/carboxylcarbon) components reaches a maximum value of 25 for the sample with close to monolayer coverage. For the higher coverage this value is equal to 16 which is the same as that observed for precipitated calcium oleate. Applying the method based on measurement of the relative intensities of different components and including the effect of differences in attenuation of the photoelectron signals emitted by the adsorption layer depending on their point of origin in this it can be concluded that at the adsorption density of 7.66 pmol/m2,which is very close to statistical monolayer coverage, the highest fraction of the adsorbed oleate molecules are oriented with respect to the surface with aliphatic chains toward solution. This observation is consistent with the earlier results obtained from isotherm and hydrophobicity studies.' The adsorption layer with the highest adsorption density of 10.77 pmol/m2 is produced as was previously shown' by the surface precipitation mechanism which does not involve formation of well-oriented surface layer with high hydrophobic property. It is important to note here that only a small increase in the relative intensity of the Cals component is observed between samples with the adsorption densities of 7.66 and 10.77 pmol/m2. Moreover, the relative intensity ratio of the Cls line at 10.77 pmol/m2,the highest coverage, is still far away from the values found for calcium oleate precipitate. These indicate uneven distribution of surface precipitated calcium oleate on apatite surface compared with relatively well spread oleate at monolayer coverage.
Langmuir, Vol. 9,No. 12,1993 3367
Nature of Apatite Adsorption Layer
This also supports the conclusion that rather surprising relationship observed on the basis of the DRIFT spectra,l i.e. the lowering of the absorbance intensity for higher adsorption density, results from dramatic changes in the structure and distribution of the adsorbed oleate molecules on the surface of apatite. It seems that a similar explanation can be proposed for the observations made before2p3for other oleate-calcium mineral systems where similar decreases of absorbance bands were observed in DRIFT spectra of samples with higher adsorption density of oleate. At the higher adsorption densities, the integrated intensity (not shown in the tables) of the Cls line increases, whereas the integrated intensities of the other lines (Ols, Ca2p, and P2p) at first increase, then decrease a little, and again increase with increasing coverage. The measurement of the integrated intensities of the different emission lines is subject to several uncertainties and discussion of these results can be highly speculative if these results are considered alone. Nevertheless, some interesting observations could be made if these results are discussed when taking into account other observations made on the basis of the adsorption, infrared, and hydrophobicity studies.’ The integrated intensity of the Ca2p line increases after adsorption of oleate a t 2.10 pmol/ m2, reaches a minimum at 7.66 pmol,m2, and increases again at 10.77 pmol/m2. These observations support the conclusion that the adsorption of oleate removes preadsorbed species and the oriented layer if formed a t the adsorption density of 7.66 pmol/m2 where the strongest attenuation of the Ca2p emission is observed. The same applies to the interpretation of the changes of the integrated intensity of the 01s line. The observation made for the P2p line, generally, is in agreement with the explanation proposed for the Ca2p arid 01s lines except for the interesting fact that at the highest coveragethe relative intensity of the P2p line versus the Ca2p line increases. The formation of the adsorption layer of calcium oleate on apatite should involve, independently of its particular distribution, a decrease of the relative intensity of the P2p line. The observed increase cannot be explained by higher kinetic energy of emission of photoelectron for the P2p line compared with the Ca2p one. This probably indicates that P O r s ions are present in the structure of surface-precipitated calcium oleate. Moreover, for the same sample sodium ions are found. This indicates complex structure of the adsorbed layer formed by the surface precipitation mechanism. Based on above results it can be concluded that the adsorption layer of oleate which is formed by the surface precipitation mechanism has a complexstructure including beside the surface precipitated calcium oleate also the chemisorbed oleate and counterions as sodium and phosphate. This could explain alack of well-organizedstructure and hydrophilic character of this adsorption layer. Oleate I i n Experiment El. The in situ ATR spectra of the adsorption layers of oleate I on apatite produced in experiment B, when oleate was added to the suspension of apatite, are presented in Figure 9a,b. It can be seen immediately that for the lower coverage (4.43 pmol/m2) the doublet is found (Figure 9a) whereas for higher coverage (9.68 pmo1/m2,Figure 9b) a single maximum at 1541cm-1 is observed for the asymmetric stretching of the carboxyl vibration. In these experiments the oleate I, with low “impurity” level, was used. The results show that a t the lower coveiage, equal to 0.55 of the statistical monolayer (Figure 9a), the adsorptioii layer mainly consists of two organized polymorphic forms, whereas at the higher coverhge, equal to 1.20 monolayers (Figure 9b), exhibits rather amorphous structure. These in situ spectroscopic
YRVENUMBER CM-1
YRVENUMEER C M - 1
Figure 9. In situ ATR spectra of the adsorption layer of oleate I on apatite obtained in experiment B at different adsorption density (pmol/m2):a, 4.43; b, 9.68. Spectrum c (6.30 pmol/m2) represents the adsorption layer obtained in different adsorption conditions (for details see text).
results are similar to those obtained by DRIFT for apatite after adsorption in experiment of type A where apatite was added into oleate solution.’ This indicates that the order in which components are added to solution has not a crucial influenee on the structural formation of the adsorption layer of calcium oleate on apatite. Although the infrared spectra of the adsorbed layer (Figure 9) are very similar to the spectrum of bulk calcium oleate, and this may imply that the precipitated form of calcium oleate is formed also a t submonolayer coverages, the NMR spectrum of the same sample shown in Figure 10a does not support this suggestion. Again, as it was observed in experiment A (Figure 8), the intensities of the carboxyl peaks at about 180 and 186ppm are significantly lower and their positions are shifted to higher frequency in comparison with those observed for bulk precipitate (Figure 3 at 273 K). Although the intensities of the carboxyl peaks are rather low, they are still at sufficient signal/noise ratio to provide reasonable interpretation. This spectrum indicates that the oleate molecules are bound to the calcium atoms which belong also to crystalline structure of apatite. A similar explanation can be applied to the results obtained for higher coverage (Figure lob) with the meaningful difference that only a broad single peak a t 180 ppm is observed. This suggests that chemisorbed oleate molecules show wide distribution of structural forms. Hence, the spectrum of the thicker adsorption layer shows (Figure lob) features of a less organized layer. Comparison of the NMR spectra recorded with crosspolarization and single pulse sequences (Figures lOa,b,c and lOa’,b’,c’) provide additional information about the nature of molecular bonding of oleate to apatite. It can be seen immediately, for the sample at lower adsorption
Mielczarski et al.
3368 Langmuir, Vol. 9, No. 12,1993
Table V. Cross-Polarization Times. TCH(pa) for the Apatite Samples after Adsorption of Oleate I
b
3
.
h
u & L 111 IbY 118
apatite, 4.43 rtmol/m2 apatite; 9.68 bmol/m2 apatite,*6.30pmovm2
I
120
100
wn
80
61
II
C2
CH Ce-IOC11-16
72 433 108 288 1080 433 58 58 216 72
720 790 216
c17
Cle
216 289 577 720 144 216
OEstimated from the relation tl = TCHIn 2 with tl values corresponding to zero intensity of each signal. b Different adsorption conditions (see text).
I
1
3%
sample, adsorption density CI
21
Figure 10. 13CCP/MAS NMR spectra (CP = 1ms, v, = 4.2 W z ) of the adsorption layer of oleate I on apatite at the different adsorption density obtained in experimentsas shown in Figure 9. Spectraa’ and b’ were recorded in a single pulse lac experiment (repetitiontime of 5 8). For detail positions of carboxyl peaks see text. density, that the carboxyl doublet disappears when single pulse measurement was carried out indicating very long spin-lattice relaxation. The carboxyl doublet can be seen only after applying a relaxation delay time longer than 5 min. The close inspection of the NMR spectra of the sample with higher than monolayer coverage recorded with cross-polarizationand single pulse sequence (Figure 10b,b’) reveals smaller differences than those observed in the case of lower adsorption density. Here, the signal of the carboxyl group is also observed in the spectrum obtained in single pulse measurement (Figure lob’). These observations indicate that adsorbed molecuels are anchored by carboxyl group to the surface of apatite in the case of chemisorbed oleate molecules (Figure loa), whereas the carboxyl group-calcium ion complex formed at higher coverage (Figure lob) shows faster motion which supports the conclusion that this adsorption layer is formed by surface precipitation of calcium oleate. Moreover, the differences in the intensities of the resonance peaks in the methylene region prove high mobility of certain fragments of hydrocarbon chain, which are discriminated in the crosspolarization experiment, in the case when adsorption layer is formed by surface precipitation.
Table V shows the cross-polarization times, TCH, for these two samples of apatite after adsorption. As is expected on the basis of the infrared results and from the above discussion, the oleate molecules form more rigid structure at submonolayer coverage (4.43pmol/m2) than that observed at higher coverage (9.68 pmol/m2). It is interesting to observe that for the lower coverage the TCH values for the methyl-terminal fragment show relatively higher values while for other parts of adsorbed molecules including the methyl group they are similar to those found in the well crystalline structure of calcium oleate precipitated at pH 10 (Table I). Close inspection of the NMR results obtained for apatite samples after adsorption (Tables I1and V) and hydrophobicity determination (not shown here, ref 1)suggests the structure in which compact patches are surrounded by oleate molecules which form loops. For the adsorption density of 9.68 pmol/m2(Figure lob) calcium oleate molecules are very mobile which supports the conclusionabout the formation of amorphous phase by a three-dimensional condensation (surface precipitation) mechanism. It is also possible that micelles produced in these conditions are so large that they were centrifuged together with apatite particles, and although they are dry, they would participate in the observed increase of mobility. Equilibrium concentration for this experiment was 1.2 X 103 M, which is higher than that determined recently35for the cmc value of sodium oleate which is equal to 7 X 1V M. Since the samples were investigated directly after adsorption, the drying was only controlled by visual inspection, some water could be trapped inside “dry” micelles which could explain a tremendous increase in the observed very high mobility of all parts of the hydrocarbon chain. These conclusions are in good agreement with infrared results and the hydrophobicity data presented in part 1.’ Our preliminaryresults (not shown) obtained for values of pH other than 10 indicate that the structure of the layer produced by a three-dimensional condensation mechanism is more sensitive to changes (an increase or decrease) in pH of oleate solution than that found for the adsorption layer formed by the chemisorption mechanism. This suggests that besides the solution chemistry of oleate and solution chemistry of calcium,%other ions present in solution play an important role in the formation of the structure of the adsorption layer. Based on our previous experiences3’ that an adsorption performed from lower concentrated solution involves better organization of the adsorption layer than that observed for the layer obtained from higher concentrated solution, an additional experiment with several times higher solution/solid ratio and consequently with a lower initial oleate concentration was performed. The obtained sample of apatite shows the adsorption density of 6.30 pmol/m2, which is somewhat above half of the statistical monolayer. The in situ infrared spectrum (Figure 9c) ~~
~
(35) Mahieu, N.; Canet, D.; Cases, J. M.; Boubel, J. C. J.Phys. Chem. 1991,95,1844. (36) Ananthapadmanabhan, K.P.; Somasundaran, P. Miner. Metoll. Process. 1984,5, 36. (37) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649.
Nature of Apatite Adsorption Layer
Langmuir, Vol. 9,No. 12,1993 3369 \-
A
Figure 12. Proposed scheme of molecular arrangement in the adsorption layer of oleate close to the surface of apatite (note that only part of hydrocarbon chain is shown).
3580 3080 VRVENUKBER C H - I
“ 1008
1688
1488 1200 YRVCNUHBER C U - l
I880
Figure 11. DRIFT spectra of apatite sample after adsorption of oleate at pH 10, recorded at room temperature, and after different evacuation time: a, as received; b, 2 h at 2 x 10-1 Pa; c, contacted with water vapor for 10 min; d, 2 h at 2 x 10-1 Pa.
Spectra were recorded when sample was under controlled atmosphere.
shows a very well resolved doublet with maxima at 1573 and 1539 cm-l, indicating two well-defined structural forms, which is in good agreement with the NMR data (Figure 1Oc)where also a well-resolved doublet at 180 and 187ppm with very low intensity components was observed. The observed lowering in the intensity and the peak positions are similar to those found in the other NMR spectra of adsorbed oleate on apatite (Figure loa) and, as discussed above, these observations indicate chemisorption of oleate. The position of the asymmetric stretching vibration of the CH2 band shifts to the lower frequency, at value of 2920 cm-l (Figure 91, which indicates higher packing density38 and supports the conclusion about the high rigidity of this adsorption layer. The TCHvalues reported in Table V also indicate that this adsorbed layer represents the most rigid structure observed for the adsorption layer, very similar to the crystalline one, though only somewhat more than half of the surface is occupied by the adsorbed oleate molecules. Somewhat higher values of the TCHobserved for the methyl-terminal hydrocarbon chain than those for precipitatedcalcium oleate I and lower than those observed for the sample with the adsorption density of 4.43 pmol/m2 (about 0.55 of monolayer) suggest that the lower amount of the adsorbed oleate molecules forms loops on apatite surface for this sample. The sample of apatite after adsorption of oleate I, whose in situ ATR spectrum is shown in Figure 9c, was also investigated ex situ by the use of diffuse reflectance technique (Figure l l a ) after drying of the sample. The band positioned below 1200 cm-’ and the sharp band at 3570 cm-1 are due to vibration of P-0 and O-H groups, respectively, from apatite.39 Close examination of the spectrum (Figure l l a ) discloses a shoulder at 1629 cm-l which, as was previously assigned, in the case of prcipitated calcium oleate samples (Figure 2c,d), is due to the presence of water incorporated in crystalline structure of calcium oleate. This type of water could be removed from the (38) Snyder, R. G.; Hsu,5. L.;Krimm, S. Spectrochim. Acta, Part A 1978,34395. Snyder, R. G.; Straw, H. L.; Elliger, C. A. J . Phys. Chem. 1982,86, 5145. (39)Baddiel, C. B.; Berry, E. E. Spectrochim. Acta 1966,22, 1407.
precipitated calcium oleate sample by evacuation in vacuum at about 0.1 Pa.’ Figure l l b shows the spectrum of apatite sample with the adsorption layer conditioned in a vacuum of 0.2 P a at room temperature. It can be easily found that this treatment involvesdramatic changes. The doublet with maxima at 1573and 1538cm-l becomes a broad singlet with a maximum at 1557 cm-l. The asymmetric stretching vibration band of the CH2 group shifts from 2923 to 2927 cm-l and the scissoring mode of the same group shifts from 1468 to 1447 cm-l. These observations indicate that ordered structure of the adsorption layer is disturbed in high vacuum condition forming a randomlyoriented, more amorphous phase. This process is connected with removing of water (shoulder at 1630 cm-l disappears) during outgassing of the sample. Introduction of water vapor again causes significant changes; the recorded spectrum (Figure l l c ) shows all features of the initial sample before vacuum treatment, which is characteristic of a well-organized structure of the adsorption layer. Subsequent vacuum treatment involves again changes toward more amorphic structure. These results indicate an important role of the presence of water on the structure of the adsorption layer. It can be concluded that water molecules are incorporated in the organized structure of the adsorption layer of oleate on apatite. As a summary of the discussion described above, a model of the proposed structure of the chemisorption layer with incorporated water moleculesis shown in Figure 12. The role of water is to stabilize the well-ordered structure of the adsorbed oleate molecules and to lower repulsive interaction between negative charge of the oleate molecules adsorbed on apatite. Moreover, the water moleculesmay also interact with the surface P-OH groupa whose surface density is about two groups for a unit cell area of 64.8 A2,assuming the most developed crystalline surface of hydroxyapatite to be (1010) plane.41 Model of the Adsorption Structure of Oleate on Apatite. On the basis of the experimental resulta presented herein and in part 1of the paper,l it is possible to propose a description of the structure of the adsorption layer of oleate on apatite surface obtained from various concentrations of oleate solution (Figure 13). It was found that the structure depends strongly on the presence of “impurities” in oleate sample; therefore, they are presented separately. At lower concentration the adsorbed molecules slightly interact with each other and they are anchored to the surface by chemisorption bonding through the carboxyl group. In the case of oleate I some rigidity in the adsorption layer is observed which results from the formation of ~
(40) Ishikawa, T.; Wakamura, M.; Kondo, S. Langmuir 1989,5,140. (41) Kukura, M.;Bell,L. C.; Posner, A. M.; Quirk,J.P . J.Phys. Chem. 1972, 76, 900.
3370 Langmuir, Vol. 9,No. 12,1993 Oleate I
o 0
Oleate II
ca2+ Na+ Po:
Figure 13. Proposed structure of the adsorption layer of oleate depending on surface coverage and mechanism of formation of the adsorption layer. Two schemes are shown for oleate I (pure sample) and oleate I1 (with “impurity”).
patches, while for oleate I1 adsorbed molecules do not form a patchwise structure. At coverage close to half of the statistical monolayer of oleate I forms well-ordered close-packed patches on the apatite surface, while oleate I1produces a highly ordered and packed structure at only close to monolayer coverage. The hydrophobicity of apatite after adsorption is closely related to the structure ofthe adsorbed layer on the apatite surface. The maximum hydrophobicity is observed at about 0.7 monolayer coverage in the case of oleate I,l where the strong lateral interaction between the adsorbed molecules is found, and at about monolayer coverage for oleate II,l where the weaker interaction, caused by the presence of “impurity” species, is observed. A t higher concentration level (about 1order of magnitude lower for oleate I than for oleate 11), beside chemisorption of oleate ions from solution, also surface precipitation of calcium oleate takes place. In this case molecules are randomly oriented (amorphic-like surface phase) and unevenly distributed in the adsorption layer independentlyof the sample of oleate. Other species like sodium and phosphate ions are incorporated in the structure of this adsorbed layer. Adsorption of a positive calcium oleate complex from solution, e.g. (RCOOCa)+, may cause coadsorption of negative P04% ions which subsequentlyinvolves interaction with other positive ions, e.g. Na+,for balancing of charge. These processes produce a very inhomogeneous structure of the adsorption layer which shows hydrophilic properties.
Mielczarski et al.
Conclusions Oleate is adsorbed on apatite by (i) two-dimensional condensation (chemisorption)of oleate ions on the apatite surface at lower concentration and (ii) two- and threedimensional condensation of calcium oleate on surface apatite at higher concentration. At equilibrium concentration above the cmc value, in addition to mechanism ii, micelles which can be formed in solution can also interact with the surface modifying its properties. Data presented herein demonstrate that there are little or no differences in the structure of the adsorption layer of oleate on apatite produced in experiments with preequilibration of apatite in water or after adding apatite directly to oleate solution. Only the amount of calcium oleate observed on the apatite is different, and higher in the case when oleate is added to apatite suspension. The structure of the adsorption layer is strongly related to the amount of “impurity”(isomers,homologues) in surfactant used in the experiments. Relationships between the structures of the adsorption layers and their hydrophobicproperties were determined, indicating that the only ordered packed chemisorbed layer of oleate can produce hydrophobic properties of apatite. Full hydrophobic data have been presented in part l1of this paper. More rigid surface structure was obtained in the case of use of pure oleate reagent. This results in lower coverage required for producing the highest hydrophobicity. Water molecules are incorporated in the structure of chemisorbed oleate molecules and they play an important role in the formation of the well-organized structure of the adsorption layer which is responsible for hydrophobic properties of apatite. At higher coverages the adsorption layer formed by surface precipitation shows complex composition and uneven distribution. This adsorption layer does not provide high hydrophobic properties. Presented in this paper multimethod spectroscopic studies show that infrared studies themselves are an excellent source of informationon the nature and structure of the adsorbed layer of surfactant. At the chemisorption to surface precipitation transition, significant changes in intensity, frequency, and width of the absorbance bands are evident, indicative of the introduction of a high population of randomly oriented molecules, and an increased mobility upon formationof the conformationally disordered adsorption layer which was clearly verified by the other spectroscopic method applied. Acknowledgment. This research was supported by the Phygis Program (869096) from the MinistQrede la Recherche, France. J.A.M. thanks M. Alnot for his technical assistance during collection and plotting of the XPS experimental data.
