Langmuir 1998, 14, 1739-1747
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Molecular Recognition Effect in Monolayer Formation of Oleate on Fluorite E. Mielczarski, J. A. Mielczarski,* and J. M. Cases Laboratoire “Environnement et Mine´ ralurgie” UA 235 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-le` s-Nancy, France Received November 11, 1997. In Final Form: January 29, 1998 Quantitative evaluation of self-assembled submonolayers (composition, orientation, and adsorbed amount) of oleate on polished and cleaved (111) fluorite surfaces has been carried out by means of infrared external reflection spectroscopy. We have found a preferential type interaction of the carboxyl group with calcium atoms on the cleaved fluorite surface with an orientation angle of 80° (angle between the asymmetric stretching vibration of the COO group and the surface normal), forming bidentate-like bonding. This form is characterized by the absorbance band at 1536 cm-1. It should be noticed that it is not a classical bidentate form with exactly the same O-Ca distances in the carboxylate bonding. There is still a difference in the distance between two oxygen atoms of the carboxylate group and the surface calcium. This surface product dominates (up to 75% of the total adsorbed molecules) on the fluorite surface prepared by cleavage at submonolayer coverages. This is the result of the molecular recognition of the surface calcium atoms on the (111) plane of fluorite by oleate ions from solution. The second calcium oleate surface form is characteristic of very different distances between oxygen and calcium atoms, i.e., the unidentate-like form with the absorbance band at 1575 cm-1. This form shows an average orientation angle to surface normal of about 60°. It has been found that the amount of this surface form is related to the amount of crystallographic defects present on the real cleavage plane of the mineral sample. The perturbation, found naturally or occurring during sample preparation (polishing), of the perfect cleavage plane results in the formation of both unidentate- and bidentate-like carboxylate surface complexes, as well as the intermediate forms. Other surface processes that could influence the molecular recognition interaction and the surface organization of the adsorbed molecules have been discussed.
Introduction Determination of the adsorbed layer composition and structure and understanding adsorption mechanisms and the role of adsorption conditions are essential to the preparation of surface layers with a specific function. The interaction of oleate aqueous solutions with calciumbearing semisoluble salt minerals such as apatite, calcite, and fluorite are important processes from fundamental and technological points of view. These minerals are produced on a large industrial scale as raw materials for various applications (fertilizers, cement, fillers, etc.). They are separated in the froth flotation process in the presence of surfactant, and the oleate is a major reagent used in this process that ensures selectivity. These calcium minerals are components of hard tissue, and unsaturated fatty acids play a crucial role in the functional activities of biological organisms; therefore, the interaction phenomena taking place in the system are very interesting from a biochemical point of view. The interaction of the first oleate molecules with the calcium mineral surface, i.e., the formation of the first monolayer, takes special attention. It was proposed during the sixties, and recently1-4 on the basis of infrared spectroscopic studies, that the formation of well-ordered chemisorbed carboxylate monolayer is characterized by a broad singlet at about 1550 cm-1, whereas a bulk- or (1) Finkelstein, N. P. Trans. Inst. Miner. Metall. 1989, 98, C157. (2) Miller, J. D.; Jang, W. H.; Kellar, J. J. Langmuir 1995, 11, 3272. (3) Sivamohan, R.; de Donato, P.; Cases, J. M. Langmuir 1990, 6, 637. Rao, K. H.; Cases, J. M.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 330. Rao, K. H.; Cases, J. M.; de Donato, P.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 314. (4) Free, M. L.; Miller, J. D. Int. J. Miner. Process 1996, 48, 197. Kellar, J. J.; Young, C. A.; Knutson, K.; Miller, J. D. J. Coll. Interface Sci. 1991, 144, 381.
surface-precipitated calcium dioleate is randomly organized and is characterized by a doublet at approximately 1575 and 1540 cm-1. It is suggested that the same mechanism of oleate adsorption takes place on fluorite, calcite, and apatite.2 A different concept of oleate adsorption on semisoluble calcium minerals was presented recently.5-8 It is proposed that the adsorbed molecules see each calcium on the mineral surface differently depending on stereochemical availability and surface distribution of calcium adsorption sites (molecular recognition), especially for the molecules adsorbed directly on the mineral surface (two-dimensional condensation) where the influence of the interface is the strongest. These molecular recognition mechanisms are based on the structural compatibility between the molecular stereochemistry of adsorbate and the structure of the mineral surface (distribution and availability of calcium adsorption sites). In other words, the number of carboxylate bands observed in the infrared spectrum of the adsorbed layer will be related to the number of different carboxylate group conformations produced at the interface (with different distances between oxygen and calcium atoms and angles of OCO group), which are related to the particular surface calcium sites. In recent papers where the adsorption studies of oleate on hydroxyapatite5,6 and fluorapatite7 were described, two narrow absorbance bands at 1576 and 1536 cm-1 were reported from monolayer to multilayer coverages. For a close to monolayer coverage, two well-separated types of (5) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delon, J. F. Langmuir 1993, 9, 2370. (6) Mielczarski, J. A.; Cases, J. M.; Tekely, P.; Canet, D. Langmuir 1993, 9, 3357. (7) Mielczarski, J. A.; Mielczarski, E. J. Phys. Chem. 1995, 99, 3206. (8) Mielczarski, J. A.; Cases, J. M. Langmuir 1995, 11, 3275.
S0743-7463(97)01229-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/12/1998
1740 Langmuir, Vol. 14, No. 7, 1998
conformations of carboxylate groups were found: (i) the band at 1576 cm-1 characteristic of a more unidentate form with the carboxylate group oriented at an angle of 62° versus the surface normal and (ii) the band at 1536 cm-1, more bidentate with an orientation angle of 83°. This observation was explained by the adsorption of oleate on two different types of calcium sites present at the (100) plane of the apatite sample. At a coverage close to two statistical monolayers, the molecules adsorbed on the top of the first well-ordered layer were randomly spread and oriented almost parallel to the interface. During the formation of multilayer coverage, the adsorbed molecules undergo surface reorganization and they produce a wellorganized structure with preferential orientation versus the mineral interface. The carboxylate groups in this multilayer surface structure adsorbed on apatite show the same conformations as in the calcium oleate precipitated from solution at pH 10. The precipitated calcium oleate shows usually the presence of two conformations that can be easily recognized by two well-distinguished carboxylate bands at 1576 and 1536 cm-1. The ATR spectra of the precipitate recorded for two polarizations5 show clearly the well-organized calcium oleate lamellar structure in which two carboxylate group conformations can be distinguished: unidentate assigned to the band at 1576 cm-1 and bidentate assigned to the band at 1536 cm-1. Fluorite with a (111) cleavage plane has a mineral surface that in ideal conditions shows only one type of calcium atom available for oleate adsorption. Hence, it is an interesting substrate for studying the molecular recognition interaction of oleate molecules with the mineral surface. For this substrate, one carboxylate group conformation (a narrow single absorbance band in the infrared spectrum) could be expected if molecular recognition plays the major role in oleate adsorption at submonolayer and monolayer coverages. In these studies two types of mineral samples: (i) with surfaces cut and polished and (ii) with surfaces immediately after cleavage, were contacted with oleate solutions and the composition and structure (orientation) of the produced surface layer were studied by the method recently described.7 Molecular recognition adsorption mechanisms of oleate on fluorite surfaces have been discussed. Experimental Section Materials. The natural mineral samples of fluorite with dimensions of about 13 × 20 mm2 or larger (cleavage plane) were used in this study. X-ray diffraction confirmed the crystal structure of fluorite, CaF2, with the (111) surface orientation for cleaved samples. In these experiments, two types of fluorite sample were employed: a sample cut and polished before use, showing a polycrystalline surface structure, and a sample with a cleavage plane, prepared immediately before the sample was contacted with oleate solutions. Fluorite was pure with traces of Cl, Si, and Mg at a few thousand parts per million, and Y, Nb, and Fe on a level of a hundred parts per million. More than 98% pure sodium oleate (cis-9-octadecenoic acid salt), supplied by Aldrich-Chemie, was used. Other reagents used were all of an analytical grade. Distilled water from the Millipore (Milli-Qplus) system was used throughout the experiments. Adsorption Studies. The mineral samples were polished with emery paper and alumina powder. The final polishing was made with the use of 0.03 µm alumina, and the polished sample was washed with water. The samples with a surface prepared by cleavage were contacted with solution instantly without any additional treatment. Typically, the mineral sample was immersed in 200 mL of oleate solution at pH 10.0 ( 0.2 for a few minutes. The solution concentration was about 3.3 × 10-5 M. Immediately after contact with oleate solution, the sample was
Mielczarski et al. immersed in water with a pH of 10 for about 1 s and then placed instantly in an FTIR spectrophotometer to record the reflection spectra. This procedure was applied in order to avoid deposition of oleate molecules from the thin film of solution remaining after the emersion of the sample from the adsorption solution. The mineral sample after contact with oleate solution becomes sufficiently hydrophobic to conclude that the possible surface deposition from solution (not adsorption) during the drying of the samples is negligible. A prolonged contact of the samples after oleate adsorption with water at the same pH as that of the oleate adsorption removes the adsorption layer. An evaluation of the adsorption layer’s resistance to this treatment will be reported in a future paper. Infrared Analysis. The infrared reflection spectra of slab samples were recorded on a Bruker IFS55 FTIR spectrometer equipped with an MCT detector and a reflection attachment (Seeguul). A wire-grid polarizer was placed before the sample and provided p- or s-polarized light. These accessories were from Harrick Scientific Co. The reflection spectra of the adsorption layers were obtained by the use of polarized light and different angles of incidence. Other details of experimental procedure could be found in our recent papers.7 Because of the use, in this study, of a new optical system (Bruker IFS55) and optimized optical reflection system, a much better sensitivity is obtained, of about 20% of a statistical monolayer (which is equivalent to a 0.4 nm thick uniform layer of calcium oleate), in comparison to the recently reported experimental conditions.7 The unit of intensity was defined as -log(R/Ro), where Ro and R are the reflectivities of the systems without and with the investigated medium, respectively. Both sample and reference spectra are averaged over the same numbers of scans, from 200 to 3000 scans, depending on energy throughput. X-ray Diffraction. A Jobin Yvon model Sigma 2080 diffractometer with a Cu KR1 source was used for diffraction studies of samples at room temperature. For investigation at high temperatures and under vacuum, a Jobin Yvon 2060 diffractometer was applied, equipped with a curved INEL detector and a controlled temperature vacuum chamber, and Co Kβ radiation was used.
Results and Discussion Optical Considerations. The recent instrumental development of infrared spectroscopy is contributing significantly to the increasing emphasis being placed on molecular level (monolayer and submonolayer coverages) surface characterization. To push the sensitivity limit even lower and to perform the proper interpretation of reflection spectra for a more detailed picture of the interfacial structure, it is vitally important to combine such spectroscopic measurements with a spectral simulation technique. The importance of such combination is reinforced by the anticipated sensitivity of surface infrared absorbances not only to surface concentration but also to adsorbate structure, molecular orientation, chain conformation, and so-called optical effects. As a consequence, the band intensities and positions do not display any simple relationship to the surface composition. Therefore, the acquisition of quantitative surface compositional and structural information necessitates a carefully designed experimental system. These were discussed in detail in a recent paper.7,9-16 These theoretical and experimental results show clearly that the optical consideration of the systems under investigation via simulation of various (9) Dluhy, R. A. J. Phys. Chem. 1986, 93, 1373. (10) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989, 93, 2034. (11) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (12) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649. (13) Mielczarski, J. A.; Mielczarski, E.; Zachwieja, J.; Cases, J. M. Langmuir 1995, 11, 2787. (14) Mielczarski, J. A.; Xu, Z.; Cases, J. M. J. Phys. Chem. 1996, 100, 7181. (15) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (16) Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Spectrosc. 1997, 51, 209.
Monolayer Formation of Oleate on Fluorite
Figure 1. Axis system and uniaxial orientation model of the transition moment. The plane of the incidence beam is the xz plane; the sample plane is the xy plane.
Figure 2. Optical constants: refractive index n, and absorption coefficient k, as functions of wavenumber for calcium oleate (solid line) and for fluorite (dashed line).
parameters provides an excellent basis for the detailed explanation of the experimental reflection spectra, as well as for optimization of the experimental conditions. Determination of Optical Constants of Mineral Samples. The optical properties of fluorite were determined from the reflection spectra of the same mineral samples that have been used in the adsorption experiments by applying the recently reported reflection method.17 In this method, the optical properties are determined from numerous infrared reflection spectra recorded at different angles of incidence and two polarizations. This procedure allows one to minimize the experimental error. Fluorite does not show any anisotropy in the xy plane (Figure 1) of the mineral sample, and the recorded spectra at two positions (orthogonal to each other) are the same. Isotropic optical properties in the x and z directions (the plane of the incident beam) are also expected. The isotropic optical constants determined for fluorite samples used in these studies are presented in Figure 2. The optical constants of the adsorption layer (Figure 2) were taken from a recent paper.7 It was assumed that (17) Mielczarski, J. A.; Milosevic, M.; Berets, S. L. Appl. Spectrosc. 1992, 46, 1040.
Langmuir, Vol. 14, No. 7, 1998 1741
the optical constants of the adsorption layers are very similar to those of the precipitated calcium oleate complex. This is obviously a crude assumption. Nevertheless, it is shown that the experimental spectra recorded with different incidence angles and polarizations can be accurately well reproduced by spectral simulation by using optical properties of precipitated calcium oleate. A discussion of this problem will be presented in the following sections. Spectral Simulation of the Adsorbed Layer. The spectral simulations were made with the use of the exact equations based on Hansen’s formulas18 for a multilayer system of isotropic and homogeneous phases with parallel interface boundaries. The calculations were performed using a three-phase model: phase 1, air with a refractive index of n1 ) 1.0 and an absorption coefficient of k1 ) 0; phase 2, the adsorption layer; and phase 3, the fluorite sample, with the optical constants as shown in Figure 2. On the basis of the theoretical simulation and consideration, similar to those presented recently for the oleateapatite system,7 and the practical limitations in optical geometry and angular dispersion of the incident beam in commercial spectrometers, the following experimental conditions are proposed for spectroscopic investigation of adsorbed layers of oleate on fluorite: for s-polarization, a 20° angle of incidence; (ii) for p-polarization, angles of 20°, 45°, 65°, and 70°. These spectral configurations are chosen for recording most informative (rich in all structural details) and high-quality (with a high signal-to-noise ratio) spectra of the adsorbed submonolayers. Simulated reflection spectra of a hypothetical monolayer of calcium oleate on fluorite, for the chosen optical conditions, are presented in Figure 3. Since these spectra are calculated for an isotropic layer of calcium oleate, all the observed variations are due to optical effects. Therefore, any difference found between the simulated spectra and the experimental spectra indicates orientation or/ and composition changes with regard to the assumed structure of the adsorbed layer. Quantitative Evaluation of the Adsorption Layer. The orientation of the transition moment of a particular molecular group of the adsorbed molecules at an angle φ from the surface normal, assuming uniaxial symmetry (Figure 1), can be determined from at least two reflection spectra recorded at two different optical conditions. Uniaxial symmetry of the system under investigation was confirmed experimentally by recording very similar spectra at 20° and for both s- and p-polarizations. When the spacial orientation of one or several molecular groups that constitute the adsorbed molecule are determined, it is possible in the next step to calculate the statistical coverage. A detailed discussion of the procedure is presented in a recent paper.7 To minimize the uncertainty of the performed calculation, the reflection spectra for a p-polarized beam with a minimum of three different incident angles (from four recorded) with the highest expected differences have been chosen for the calculations of the results presented here. Absorbance components in three directions, AX, AY, and AZ, determined for a hypothetical monolayer are used for the calculation of the orientation angles of the major molecular groups of oleate molecules, and thicknesses of the self-assembled layers on fluorite are presented in Table 1. Adsorption on Different Types of Fluorite Surfaces. Two different slab samples of fluorite were prepared: (i) slab sample cut and polished in the way that X-ray diffraction shows a polycrystalline surface (18) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380.
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Figure 3. Simulated reflection spectra (carboxylate and aliphatic region) of an isotropic 2.1 nm hypothetical monolayer of calcium oleate on fluorite for p-polarization at incident angles (a) 20°, (b) 45°, (c) 65° and (d) 70° for the two most characteristic wavenumber regions.
Mielczarski et al.
structure and (ii) sample with the surface obtained after cleavage without any additional preparation. These two types of samples were exposed to oleate solutions with an initial concentration of 3.3 × 10-5 M, at pH 10, for different times. On the first fluorite sample with polycrystalline surface structure (Figures 4 and 5) after 1 and 5 min of adsorption, the recorded spectrum shows very well separated absorbance bands at 1574 and 1536 cm-1 at each stage of monolayer formation by adsorbed oleate molecules. This indicates the formation of two types of surface calcium carboxylate species with two preferential conformations of carboxylate groups (band at 1574 cm-1, conformation I, more unidentate; band at 1536 cm-1, conformation II, more bidentate) similar to those reported for apatite7 and for a precipitated calcium oleate. It is estimated that the thickness of the adsorption layer presented in Figures 4a and 5a is about 0.2 of a statistical monolayer. The detailed quantitative analysis of the adsorption layer structure of the sample was not carried out because of a relatively low signal-to-noise ratio at this coverage which could result in high uncertainty in determined orientation angles. For the sample after 5 min of adsorption (Figure 4b and 5b) the performed calculation shows about 0.7 of a monolayer coverage, as presented in Table 2. Close inspection of the spectra recorded after 1 and 5 min of adsorption reveals very interesting features. After the shorter adsorption time there is almost no absorbance bands at about 2900 cm-1, characteristic for the stretching vibrations of the CH2 and CH3 of the aliphatic chain, whereas the absorbance bands of the asymmetric stretching vibration of the COO groups are clearly visible. This indicates that the adsorbed oleate molecules form a surface structure in which a different part of the aliphatic chain or/and different domains of adsorbed molecules have particular positions versus the interface. These positions cause the negative absorbance component, AX, and the positive absorbance component, AZ, to cancel each other, resulting in the recorded spectrum (Figure 5a) (for details see ref 7). This can happen only if the adsorbed molecules are organized in at least two types of conformations (domains) that are produced at the mineral surface. For a randomly oriented structure, negative absorbance bands with appropriate intensities related to the amount adsorbed, as is predicted for an isotropic surface layer (Figure 3), should be observed. Therefore, the formation of small and differently oriented patches at submonolayer coverage is suggested for the structure produced after 1 min of adsorption. This allows us to conclude that the coverage of 20% of monolayer means that 20% of the surface is covered by patches. Another explanation is the formation of the surface structure in which the aliphatic chains of the adsorbed molecules are almost perfectly parallel to the interface. The latter explanation does not have any future support from other observations (see next sections). The spectra of the adsorbed layer produced for 5 min of adsorption recorded at different angles and two polarizations are presented in Figures 6 and 7. In general, they are very close to those predicted by the simulation (Figure 3). However, there are also significant differences between the experimentally recorded spectra and the simulated spectra for a hypothetical isotropic monolayer. Close inspection of the experimental and simulated spectra demonstrates that the most striking differences occur for the absorbance band at about 1475 cm-1. This band, which is assigned to the symmetric vibration of the COO group, and the bending vibrations of the CH2 and CH3 groups show reverse intensity to that predicted by simulation. For example, for 70° and p-polarization (Figure 6) this
Monolayer Formation of Oleate on Fluorite
Langmuir, Vol. 14, No. 7, 1998 1743
Table 1. Absorbance Components (×103) Calculated for a Hypothetical Isotropic Monolayer (Thickness of 2.1 nm) of Calcium Oleate on Fluorite at Various Angles of Incidence 20°
45°
65°
70°
frequency, cm-1
assignment
AY
AX
AZ
AY
AX
AZ
AY
AX
AZ
AY
AX
AZ
2918 2852 1570 1536 1466
νas(CH2) νs(CH2) νas(COO-) νas(COO-) νs(COO-)a
-4.68 -2.98 -4.41 -5.96 -2.75
-5.35 -3.40 -5.04 -6.80 -3.14
0.209 0.118 0.195 0.164 0.094
-3.52 -2.24 -3.32 -4.48 -2.07
-10.4 -6.65 -10.3 -13.9 -6.46
2.18 1.37 2.16 1.82 1.06
-2.11 -1.34 -1.99 -2.68 -1.24
5.61 3.56 4.52 6.05 2.72
-2.43 -1.53 -2.02 -1.69 -0.952
-1.70 -1.08 -1.61 -2.17 -1.00
3.06 1.94 2.51 3.36 1.52
-1.50 -0.943 -1.28 -1.07 -0.606
a
Tentative assignment (see the text).
Figure 4. Reflection spectra (carboxylate group region), recorded for p-polarization at θ ) 45°, of the adsorbed layer of oleate on fluorite polished sample after different adsorption times: 1 min (a); 5 min (b).
Figure 5. Reflection spectra (aliphatic group region), recorded for p-polarization at θ ) 45°, of the adsorbed layer of oleate on fluorite polished sample after different adsorption times: 1 min (a); 5 min (b).
band is negative in the experimental spectrum whereas the simulation shows a positive band (Figure 3d, carboxylate region). This finding and the differences observed for other incident angles clearly indicate that the molecules in the adsorption layer are organized with the aliphatic chain toward the outside. The positions of the bands due to the CH2 stretching vibration at 2918 and 2850 cm-1 indicate19 the strong lateral interaction of aliphatic chains in the surface structure. This suggests the patch-like structure at the submonolayer coverage.
The results of quantitative calculation of the orientation of particular molecular groups of the adsorbed molecules and their adsorbed amount (in monolayers) after 5 min of adsorption (the reflection spectra shown in Figures 6 and 7) are presented in Table 2. The average orientation angle of the stretching vibration of the CH2 group is 57°. It is not surprising because oleate has the cis-double bond in the middle of its hydrocarbon chain; hence, even for the all-trans conformation, there are two parts of the hydrocarbon chain in each oleate molecule having different chain axes with an angle between them of 112°, as was determined for the crystalline structure of a low-melting form of oleic acid.20 Thus, even for a perfectly oriented structure of the adsorbed layer of oleate, the calculation of orientation of the aliphatic part will show an average value for their two differently oriented parts. For two carboxylate conformations, with bands at 1574 and 1536 cm-1, the orientation angles of the transition moments of the asymmetric stretching vibrations of the COO groups are close to 55°. Only the symmetric stretching vibration of the COO group shows a stand-up position (Table 2), as was already mentioned above. It is interesting to note that the two conformations of the carboxylate group, more unidentate with the band at 1574 cm-1 and more bidentate with the band at 1538 cm-1, show similar average orientation versus the surface. This could be explained by the perturbation of the surface structure of fluorite prepared by polishing and partially by the submonolayer coverage. Very interesting observations were made for the fluorite sample after cleavage (Figures 8 and 9). The spectra recorded after 2 min of adsorption show also a wellseparated doublet with band positions at 1574 cm-1 (more unidentate) and 1536 cm-1 (more bidentate), but the intensity ratio of the bands is very different from that previously observed (Figures 6 and 7). For example, in the spectrum recorded at 45° and at p-polarization, this ratio is only 0.29, whereas the value calculated for simulated or other experimental results obtained for cut and polished fluorite samples are usually close to 0.7. Two explanations could be proposed for this observation: (i) the amount of carboxylate groups with unidentate-like conformation is much lower than bidentate-like in the produced structure, (ii) the orientation of the asymmetric stretching of carboxylate groups with unidentate-like conformation is almost vertical to the interface. Both explanations exclude the interpretation that the produced surface calcium oleate complex is similar to that obtained by the precipitation, i.e., calcium dioleate product. The quantitative evaluation of the adsorbed layer on cleaved surface shows (Table 2) that the carboxylate group with the band at 1536 cm-1 has a position almost parallel to the interface (orientation angle 80°) and the conformation (19) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A. 1978, 34, 395. Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (20) Abrahamsson, S.; Ryderstedt-Nahringbauer, I. Acta Crystallogr. 1962, 15, 1261.
1744 Langmuir, Vol. 14, No. 7, 1998
Mielczarski et al.
Table 2. Recorded Absorbances (×103), Calculated Orientation Angles of the Major Molecular Group, and Thickness of the Self-Assembled Layers of Oleate on Fluorite from the Data Obtained for p-Polarization at Various Angles of Incidence sample polished, 5 min of adsorption
cleaved, 2 min of adsorption
frequency,a cm-1 2918 1574 1536 1465 2919 1574 1536 1475
assignment νas(CH2) νas(COO-) νas(COO-) νs(COO-)d νas(CH2) νas(COO-) νas(COO-) νs(COO-)d
20° -2.5 -2.5 -2.92 0.5 n.a.e n.a. -5.83 n.a.
absorbance 45° 65° -6.94 -6.39 -8.3 1.9 -5.83 -3.05 -10.56 0.50
1.94 1.41 2.5 -1.67 2.08 1.0 4.55 -1.25
70°
φ,b deg
thickness, in statistical monolayersc
0.97 0.83 1.4 -0.86 0.86 0.6 2.53 -1.39
57 54 55 20 53 56 80 20
0.7 0.7 0.7 1.2 0.7 0.35 0.7 0.7
a Average value from the spectra recorded at angles of incidence below θ . b Average value calculated from the combination of the B reflection spectra. c Thickness of the statistical monolayer was assumed to be 2.1 nm. d Tentative assignment (see the text). e n.a. ) not available, difficult to determine baseline.
Figure 6. Reflection spectra (carboxylate group region) of the adsorbed layer of oleate on polished fluorite after 5 min of adsorption recorded for s- and p-polarizations at chosen angles of incidence.
Figure 7. Reflection spectra (aliphatic group region) of the adsorbed layer of oleate on polished fluorite after 5 min of adsorption recorded for s- and p-polarizations at chosen angles of incidence.
characteristic of the band at 1575 cm-1 has a position at the interface that can be interpreted as randomly oriented (average orientation angle 56°). Therefore, it can be concluded that the carboxylate groups with bidentatelike conformation are preferentially formed on the fluorite (111) cleavage surface at the submonolayer coverage (Table 2), and they are not a part of the surface precipitation species. The conformation described as the bidentatelike bonding between the oleate carboxyl groups and surface calcium atoms from the (111) cleavage plane (Figure 10) is similar to that proposed for more available surface calcium atoms of apatite.7 The second conformation
with the absorbance band at the higher position is characteristic of a unidentate type of bonding. The quantitative evaluation of the amount of these two types of the adsorbed species, after taking into account the orientation effect, shows (Table 2) that about 70% of the adsorbed molecules at 70% of a monolayer coverage are bonded in bidentate-like form. These observations indicate clearly that the structure of the adsorption layer of oleate depends strongly on the structure of the mineral surface on an atomic scale (molecular recognition), in particular on the distribution and availability of calcium adsorption sites.
Monolayer Formation of Oleate on Fluorite
Figure 8. Reflection spectra (carboxylate group region) of the adsorbed layer of oleate on the cleavage plane of fluorite after 2 min of adsorption recorded for s- and p-polarizations at chosen angles of incidence.
In an ideal situation, the fluorite cleavage surface is the (111) plane with one type of Ca atom uniformly distributed with equal interatomic distances of 3.86 Å. This suggests that in the case of perfect conditions for a well-organized monolayer only one type of the carboxylate group, i.e., bidentate-like form with the absorbance band at 1536 cm-1, should be observed. During this work the adsorption layers were investigated on various samples after cleavage and the intensity ratio between the carboxylate doublet varied remarkably from sample to sample though the solution conditions were the same. This can be understood easily if the microscopic pictures taken from different surface regions are considered. Figure 11a shows more uniform and Figure 11b a significantly disturbed part of the cleavage plane of the same fluorite sample from which adsorption spectra are presented in Figures 8 and 9. The real surface of fluorite on a microscopic scale after cleavage is not perfect, showing local perturbations (presence of steps, dislocation, etc.), although the X-ray diffraction shows an ideal (111) cleavage plane. In the present spectroscopic experiments the size of the mineral surface interacting with the incident infrared beam varies from about 5 to 12 mm diameter depending on the angle of the incident beam. Hence, the recorded reflection spectrum carries out information from all oleate molecules adsorbed on different types of surface sites with different types of surface calcium atoms available for oleate adsorption. This could explain why it was not possible, in this study, to record spectra of the adsorbed oleate layer
Langmuir, Vol. 14, No. 7, 1998 1745
Figure 9. Reflection spectra (aliphatic group region) of the adsorbed layer of oleate on the cleavage plane of fluorite after 2 min of adsorption recorded for s- and p-polarizations at chosen angles of incidence.
Figure 10. Schematic representation of the orientation of the carboxylate group of adsorbed oleate on the cleavage plane of fluorite where 80° is the orientation angle of the νas of the OCO groups and 20° is the orientation angle of the νs of the same groups.
on fluorite with only one absorbance band of a carboxyl group at 1536 cm-1, indicating the bidentate-like form. The unidentate-like form was always observed with much lower intensity. There is also another reason that in real adsorption conditions it will be difficult to produce a perfect structure
1746 Langmuir, Vol. 14, No. 7, 1998
Figure 11. Optical microscope picture of fluorite after cleavage with more uniform (a) and more disturbed (b) surface structure. The scale bar applies to (a) and (b).
with only one, bidentate-like conformation of the carboxylate group in the first adsorbed layer on fluorite. Fluorite is a semisoluble mineral; therefore, the interaction with aqueous oleate solution is complex. Oleate ions from solution will interact directly with the calcium atoms located in the crystalline structure of fluorite but also with the calcium atoms that just left the fluorite structure in a dissolution process. The latter interaction could happen in the interfacial region as well as in bulk solution. This is the second and parallel possibility of how the adsorbed monolayer could be formed. As was already shown,21 the conditions of ionic concentrations in the interfacial region could satisfy the limit of solubility product before it is reached in the bulk solution. This so-called surface precipitation mechanism of the formation of the adsorption monolayer could also be responsible for the observed formation of the unidentate-like form observed in the spectra of the adsorbed layer on the cleaved plane. The participation of the surface precipitation mechanism in the formation of the structure of the adsorbed monolayer is associated with relative rates (kinetics) of oleate ion adsorption and fluorite dissolution. Another reason that the unidentate carboxylate conformation can be present in minor quantity even for an ideal cleavage plane could result from calcium distribution on the fluorite surface. One calcium atom occupies 12.8 Å2. The smallest oleate molecule projected area obtained for solid-state oleic acid is between 21 and 22 Å2.20 Other larger values including also the “liquid state” of adsorbed molecules are 25.8, 33.0, and 34.3 Å2.22 These experimental results (Figures 7 and 9) show a very strong lateral (21) Ananthapadmanabhan, K. P., Somasundaran, P., Colloids Surf. 1985, 13, 151.
Mielczarski et al.
interaction of aliphatic chains in the adsorbed layer, which excludes the highest attachment areas from this consideration. It is reasonable to propose that the adsorbed oleate molecule occupies an area equal to two calcium surface atoms. This gives 25.6 Å2 per oleate molecules on the fluorite surface. If the real occupied area of the oleate molecule will be lower, close to that determined for the solid state of oleic acid, this gives enough room for adsorption of oleate molecules in unidentate-like conformation, especially on the boundary between domains containing well-ordered bidentate-like bonded oleate molecules. These organized bidentate-like domains are produced in the growing process of monolayer formation from small patches; therefore, each domain has a different orientation in the xy plane. Another phenomena that could strongly influence the molecular recognition interaction of oleate molecules with the fluorite surface is the lateral interaction of the aliphatic chain of the adsorbed molecules. It was suggested about a decade ago23 that in the presence of oxygen a double bond of oleate can form ether type of cross-linking along the adsorbed molecules. This possibility was tested theoretically by molecular modeling.24 This cross-linking reaction required a particular orientation of the adsorbed molecules, which could modify the calcium-carboxylate bonding and influence the molecular recognition interaction of single oleate molecules in comparison to crosslinking moieties. The recently reported studies25 did not confirm the cross-linking phenomena at the fluorite interface. The results obtained in this work also do not support the suggestion of the formation of ether type of cross-linking bonding between the adsorbed oleate molecules; hence, this possible perturbation of molecular recognition interaction and orientation of the surface structure could be neglected. Conclusions Quantitative evaluation of the adsorbed submonolayers formed on the cleaved fluorite surface shows the molecular recognition type interaction of oleate molecules from solution with surface calcium atoms. There is a preferential type of interaction of the carboxyl group with the fluorite surface with the orientation angle of 80°, forming bidentate-like bonding. This form (conformation) is responsible for the presence of the with absorbance band at 1536 cm-1. It should be noticed that it is not a classical bidentate form with exactly the same O-Ca distances in carboxylate bonding. There is still a difference in the distances between calcium and the two oxygen atoms. Therefore, this form is called bidentate-like, and by analogy, another form is called unidentate-like. The bidentate-like surface product dominates on the fluorite surface prepared by cleavage (up to 75% of the adsorbed molecules) at below and about monolayer coverages. This is the product of the molecular recognition of the surface calcium atoms on the (111) plane of fluorite by oleate ions from solution. The patch-like structure is produced at submonolayer coverage. The second calcium oleate surface form is characteristic of a very different distance between oxygen and calcium atoms, i.e., unidentate-like form with the absorbance band at 1575 cm-1. This form shows an average orientation (22) Cases, J. M.; Pourier, J. E.; Canet, D. In Solid-liquid Interactions in Porous Media; Cases, J. M., Ed.; Tecnip: Paris, 1985; p 335. (23) Hu, J. S.; Misra, M.; Miller, J. D. Int. J. Miner. Process 1986, 18, 73. (24) Arad, D.; Kaftory, M.; Zolotoy, A. B.; Finkelstein, N. P.; Weissman, A. Langmuir 1993, 9, 1446. (25) Free, M. L. Ph.D. Dissertation, University of Utah, 1994.
Monolayer Formation of Oleate on Fluorite
angle to surface normal of about 60°. It has been found that the amount of this surface form is related to the amount of crystallographic defects present on the real cleavage plane of the mineral sample. The perturbation of the perfect cleavage plane of fluorite, found naturally or occurring during sample preparation (polishing) results in the formation of both forms: unidentate and bidentatelike carboxylate surface complexes. Other surface processes taking place during the adsorption of oleate from aqueous solution, like fluorite dissolution, the formation of a monolayer by association of small patches of initial
Langmuir, Vol. 14, No. 7, 1998 1747
adsorbed molecules, also could cause the perfectly uniform homogeneous bidentate-like calcium oleate layer to not be produced, even on the initially perfect cleavage plane of fluorite. Acknowledgment. The authors gratefully acknowledge the support of this work by the CEFIPRA (project 1315-1) and CNRS-NSF cooperation program (project 4606). LA9712294
