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Dynamics of Fluorite-Oleate Interactions J. A. Mielczarski,* E. Mielczarski, and J. M. Cases Laboratoire “Environnement et Mine´ ralurgie” UMR 7569 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-le` s-Nancy, France Received May 19, 1998. In Final Form: October 14, 1998 The nature, structure, and kinetics of formation of the adsorbed layers of oleate on fluorite in basic solutions have been examined by means of infrared external reflection spectroscopy. The adsorption of oleate on fluorite was found to be a very dynamic process which fails to reach equilibrium. Different steady states can be achieved. The surface composition and structure depend on competitive processes taking place at the interface, such as fluorite dissolution and interaction with water, and adsorption of oleate ions and other oleate aggregates from solution. Fluorite immersed in oleate basic solution shows surface phenomena dependent on an initial oleate concentration if other solution conditions (pH, agitation, etc.) are constant. Three characteristic regions of adsorption can be distinguished. In diluted solutions, below 10-5 M, a steady state is reached. The amount of calcium oleate surface species does not exceed 0.3 of a statistical monolayer. At concentrations between 10-5 and 10-4 M the most dynamic interactions between fluorite and oleate aqueous solutions were observed. The adsorbed amount shows a maximum. The level of the maximum (up to 20 statistical monolayers) and the adsorption kinetics are strongly related to the initial oleate concentration. There is clear evidence that the thicker coverage is produced by a nucleation and growth mechanism. First nuclei are formed on the fluorite surface through tridimensional condensation before the monolayer is completed. Dissolution of the produced surface calcium oleate with an increase of adsorption time is caused by the Ostwald ripening that transfers the calcium and oleate from the fluorite surface to solution, where fine particles of calcium oleate are formed. At higher than 10-4 M oleate concentration the total adsorbed amount of oleate does not exceed a monolayer coverage after a long adsorption time. A steady state is reached, and micelles are formed in solution, at concentrations lower than the cmc of sodium oleate due to the adsorption of calcium ions on the micelles, lowering significantly the calcium and oleate concentrations in solution. This prevents the formation of thick tridimensional patches on the fluorite surface. Adsorption kinetics is also very sensitive to other changes, such as solution hydrodynamics (agitation) or addition of external calcium (ions or/and solid calcium oleate) to the oleate solution.
Introduction The fluorite (CaF2)-oleate system, which is the subject of the studies reported here, has been studied intensively, and a review of the works until 1989 was published.1 The review excellently pointed out all the questions and discrepancies between the proposed explanations and the experimental results reported by different authors. Recently, scientists have made a new effort to understand the adsorption mechanism of oleate on fluorite. These results show new aspects of the influence of the adsorption conditions, such as the solid-liquid ratio, which changes the shape of the isotherm,2 and the amount of calcium ions added to solution,3 on the formation of the surface layer. Large differences in the experimental results obtained for the fine particles and the slab (reflection element) fluorite samples were recently pointed out.3 All these data suggest that the thermodynamic approach which dominated in the past studies4 is not sufficient to explain all the experimental observations. The fluorite-oleate aqueous solution interaction depends on how the components are brought into contact. It seems that the most important interaction is the beginning of the contact of fluorite with the oleate aqueous solution. Oleate ions from solution may interact directly with the calcium atoms located in the crystalline structure of fluorite but also with the calcium atoms which just left * To whom correspondence should be addressed. Fax: 33 3 83 57 54 04. E-mail:
[email protected]. (1) Finkelstein, N. P. Trans.sInst. Min. Metall. 1989, 98, C157. (2) Rao, K. H.; Cases, J. M.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 330. (3) Free, M. L.; Miller, J. D. Int. J. Miner. Process 1996, 48, 197. (4) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251.
the fluorite structure in a dissolution process. The latter interaction could happen in the interfacial region as well as in bulk solution. The concentrations of oleate and calcium ions in the interfacial region undergo important fluctuation with an increase of adsorption time, especially immediately after immersion of fluorite in oleate solution. As was already shown,2,5 the conditions of ionic concentrations in the interfacial region could satisfy the limit of the solubility product before it is reached in the bulk solution. There is also an interaction of fluorite with water which produces different types of surface hydroxide and carbonate groups with different densities depending on the solution compositions, pH, and CO2 content. This obviously modifies the fluorite surface which interacts with oleate ions from solution. The adsorbed oleate molecules inhibit significantly the diffusion of calcium and fluorine ions to solution. It seems that the relative kinetics of the fluorite dissolution (and other processes between fluorite and water) and the oleate interaction with calcium on the mineral surface and in solution are the key factors which govern the formation of the adsorbed layer. We are dealing with the very difficult problem of distinguishing the adsorption from tridimensional condensation on the substrate. Another interesting finding reported first by Gutie´rrez6,7 and later by other authors8-10 is that the adsorption density of oleate on fluorite attains a maximum at a (5) Ananthapadmanabhan, K. P.; Somasundaran, P. Colloids Surf. 1985, 13, 151. (6) Gutierrez, C. Proceedings of 7th international conference on surface active substances, Moscow, 1976; pp 638-47. (7) Gutierrez, C. Trans. Am. Inst. Min., Metall. Pet. Eng. 1979, 266, 1918.
10.1021/la980593f CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998
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concentration of about 3.5 × 10-5 M. The competitive character of the oleate interaction with fluorite versus precipitation of calcium oleate in solution,6,7 the presence of premicelles in oleate solutions,4,11,12 or the formation of other bigger aggregates such as oleic acid colloid species13 has already been suggested. It seems that the concentrations of both oleate and calcium ions in the interfacial region are responsible for the composition and structure of the adsorption layer on fluorite. As a result, the processes of dissolution, adsorption, and desorption govern the surface phenomena. The importance of these phenomena can be illustrated by the influence of the solid-solution ratio on adsorption density2 and probably on the structure of the adsorbed layer. Major differences are reported for the adsorption on fine particles and slab samples.3 In the latter case at least one parameter, namely the concentration of oleate in the bulk solution, could be assumed as constant along the adsorption time. In the case of fine particles (large surface area) the solution concentration varies dramatically during the adsorption. The differences in the adsorption density between the results obtained for fine particles (isotherm determined by depletion method) and slab samples3 are difficult to explain if it is assumed that they are representative of an adsorption equilibrium. This rather suggests that different steady states were reached in those experiments. In a recently presented concept of oleate adsorption on calcium minerals it is proposed that the mineral surface structure on an atomic scale and the availability of calcium atoms for oleate bonding will govern the structure of the adsorbed layer14-16 especially when the first monolayer is formed. This concept was discussed in the recent papers where detailed spectroscopic studies of the structure of the adsorbed layer on apatite14 and fluorite15 were reported. Molecular recognition phenomena of oleate adsorption on these minerals were described.16 The work presented in this paper is focused on both the nature of the adsorbed species and the kinetics of oleate adsorption on fluorite at different solution conditions (oleate concentration, pH). The previously reported studies of oleate adsorption did not pay particular attention to the kinetics of surface layer formation; the thermodynamic approach was rather dominating. To the best knowledge of the authors, there is only one report17 on the kinetics of oleate adsorption on fluorite. The study17 was carried out under very specific conditions, where fresh oleate solution flows through a cell with fluorite at different rates varying with adsorption time. Under those conditions ions released from the fluorite surface or aggregates formed in solution are immediately removed with flowing solution. Hence, the important influence of the solution chemistry on the adsorption of oleate was omitted in those studies. (8) Hu, J. S.; Misra, M.; Miller, J. D. Int. J. Miner. Process 1986, 18, 57. (9) Kellar, J. J.; Young, C. A.; Knutson, K.; Miller, J. D. J. Colloid Interface Sci. 1991, 144, 381. (10) Rao, K. H.; Cases, J. M.; de Donato, P.; Forssberg, K. S. E. J. Colloid Interface Sci. 1991, 145, 314. (11) Somasundaran, P.; Ananthapadmanabhan, K. P. Solution chemistry of surfactants and the role of it in the adsorption and froth flotation in mineral-water systems. Solution Chemistry of Surfactants; In Mittal, K. L. Ed.; Plenum Press: New York, 1979; Vol. 2, pp 17-38. (12) Sivamohan, R.; de Donato, P.; Cases, J. M.. Langmuir 1990, 6, 637. (13) Laskowski, J. S.; Nyamekye, G. A. Int. J. Miner. Process 1994, 40, 245. (14) Mielczarski, J. A.; Mielczarski, E. J. Phys. Chem. 1995, 99, 3206. (15) Mielczarski, J. A.; Cases, J. M. Langmuir 1995, 11, 3275. (16) Mielczarski, E.; Mielczarski, J. A.; Cases, J. M. Langmuir 1998, 14, 1739. (17) Free, M. L.; Miller, J. D. Langmuir 1997, 13, 4377.
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The simple Langmuir adsorption model was assumed17 in that consideration, and the study was limited to only one oleate concentration, 1 × 10-5 M. The results obtained in the present work suggest that the oleate interaction with fluorite is very dynamic with formation of various surface intermediates before one of the steady states is reached. Mechanisms of the interactions of fluorite with different oleate solution concentrations with a strong influence of solution chemistry, at basic pH, are proposed. Experimental Section Materials. The natural mineral sample of fluorite (procured from Ward’s) with dimensions of about of 13 × 20 mm2 was used in this study. X-ray diffraction confirmed the crystal structure of fluorite. Fluorite was pure with traces of Cl, Si, and Mg at a few thousand ppm and Y, Nb, and Fe on the level of a hundred ppm. 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 performed with the use of 0.05 mm alumina, and the polished sample was washed with water. Typically the mineral sample was immersed in 200 mL of oleate solution at pH 10.0 ( 0.1 for a period of 1 min to 20 h. The solution concentration varied from 2 × 10-6 to 5 × 10-4 M. Immediately after contact with oleate solution, the sample was 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. To distinguish the adsorbed species from those deposited on the mineral surface by gravitation, the sample was hung flat in the middle of the oleate solution and both “face up” and “face down” mineral sample surfaces were characterized spectroscopically. Other experimental details can be found in a previous paper.16 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. For each adsorption layer, reflection spectra, usually five, were recorded by the use of s- and p-polarized light at different angles of incidence, 20°, 45°, 65°, and 70°. Other details of the experimental procedure could be found in our recent paper.16 An optimized optical reflection system permits us to detect an adsorbed amount as low as about 20% of a statistical monolayer of calcium oleate on a fluorite surface, which is equivalent to a 0.4 nm thick uniform layer of calcium oleate. The unit of intensity was defined as -log(R/R0), where R0 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. Spectral Simulation. The recent instrumental development of infrared spectroscopy is contributing significantly to the increasing emphasis being placed on molecular level surface characterization. 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 a 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. These were discussed in detail in recent papers for oleate-fluorite16 and oleateapatite14 systems. The theoretical and experimental results show clearly that the optical consideration of the systems under investigation via simulation of various parameters provides an excellent basis for the detailed explanation of the experimental reflection spectra, as well as for optimizing the experimental conditions. Quantitative evaluation of the adsorbed layers was performed in the way described in the recent paper.16 For convenience the
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amount adsorbed is reported in statistical monolayers (as equivalent to a 21 Å thick homogeneous layer of calcium oleate), and it does not mean that the adsorption takes place regularly layer by layer.16 The quick and easy way to estimate the amount of adsorbed oleate on the mineral surface is to compare the observed absorbance in the region of aliphatic chain stretching vibrations, 2700-3100 cm-1, with those simulated for the hypothetical isotropic layer. As was already demonstrated14,16 the presence of the cis double bond in the middle of the oleate hydrocarbon chain causes the determined orientation angle of the aliphatic chain even for the all-trans conformation to be not very different from the value characteristic for the randomly oriented structure. This observation was also positively verified in the study during detailed quantitative characterization of the adsorbed layer applying the calculated orientation angles of the most characteristic molecular vibrations.
Results and Discussion The review1 shows clearly that no simple explanation of the surface phenomena between fluorite and oleate solution can be offered on the basis of the results already reported by different authors. The adsorption isotherms determined in various studies are very different; they seem to be inconsistent and illogical, indicating that the equilibrium was not reached under experimental conditions. In kinetics studies, which are the aim of this work, the complexity of the surface and solution phenomena will play an even more significant role. The competitiveness of the interface processes means that very small changes in solution composition, mineral surface structure (solubility), and adsorption hydrodynamics can produce different adsorbed surface layer compositions and structures. Therefore, special attention was paid to carry out adsorption experiments at controlled conditions as far as possible. Each of the presented experimental results is representative of at least two experiments performed at the same conditions. Adsorption Kinetics. The preliminary spectroscopic studies of fluorite after 5 min of adsorption from different concentrations of oleate solution reveal that the largest amount of oleate adsorbed can be observed at a concentration of about 3 × 10-5 M. Hence, there is an optimal oleate concentration at which the fastest adsorption rate is observed. The reflection spectra of the fluorite sample contacted with a 3.3 × 10-5 M oleate solution recorded at the same optical conditions, at an angle of incidence of 45° and p-polarization, after different adsorption times are presented in Figure 1. It can be seen that the amount of the adsorbed surface product, calcium oleate, at first increases and then decreases with an increase of adsorption time. Two absorbance regions, at 3200-2700 cm-1 characteristic for the stretching vibration of the aliphatic chain and at 1800-1300 cm-1 due to the stretching vibration of the carboxylate group and the bending vibration of the aliphatic group, are mainly discussed in this work. The amount adsorbed after 5 min is estimated, on the basis of comparison of the experimental with the simulated spectra (see ref 16), to reach about a statistical monolayer (Figure 1a). At this coverage two well-separated absorbance bands at 1575 and 1537 cm-1 due to the carboxylate stretching vibration are observed, indicating two types of conformations of the adsorbed molecules, unidentate-like and bidentate-like, respectively.16,18,19 Prolongation of the adsorption time results in a gradual increase of the adsorbed amount; after 6 h of adsorption the formation of about 20 monolayers was found (Figure (18) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delon, J. F. Langmuir 1993, 9, 2370. (19) Mielczarski, J. A.; Cases, J. M.; Tekely, P.; Canet, D. Langmuir 1993, 9, 3357.
Figure 1. Reflection spectra of the adsorption layer on fluorite from a 3.3 × 10-5 M oleate solution after different adsorption times: 5 min (a); 15 min (b); 1 h (c); 6 h (d); 8 h (e). Spectra recorded for p-polarization at an incident angle of q ) 45°.
1d) as the thickest layer observed at the concentration. After 8 h of adsorption the amount of calcium oleate on fluorite decreases tremendously to 8 monolayers (Figure 1e). The solutions in all experiments were transparent. Though the adsorbed amount is reported as a number of statistical monolayers, it does not mean that the adsorbed layer is uniform. On the contrary the oleate forms patches which are clearly visible on a microscopic picture (Figure 2). The dimensions and growth of the patches are related to the adsorbed amount of oleate and vary with solution concentration and adsorption time at the same hydrodynamics of solution. Two forms of adsorbed aggregates can be distinguished: (i) elongated and (ii) round patches. Long patches are an intermediate form. The aggregates are not uniformly distributed on the fluorite surface. A preliminary atomic force microscopy (AFM) study shows that the patches occupy only a small part of surface area and that their height changes from 10 to 600 nm depending on adsorption time. It is striking that the top surface of the patches is very flat and parallel to the mineral surface, indicating the formation of a lamellar structure of the produced surface calcium oleate. The microscopic pictures obtained for other samples are very similar. Only changes in the size of the produced patches and their surface density are observed. These results clearly show that the nucleation and growth mechanism is responsible for the formation of calcium oleate surface structure. Prolongation of contact of fluorite with oleate solution at 3.3 × 10-5 M results in dissolution of the surface species. At an oleate concentration of 9 × 10-6 M, the adsorption maximum was also observed (Figure 3). After 4 h of adsorption the adsorbed amount was about 4 statistical monolayers (Figure 3b). Microscopic studies show distributed surface aggregates in the form of threads and patches similar to those shown in Figure 2. After 19 h of adsorption the surface species undergo dissolution and
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Figure 2. Selected optical microscopic picture of fluorite after adsorption from oleate solution at the concentration 3.3 × 10-5 M. Scale 1 cm ) 15 µm.
Figure 3. Reflection spectra of fluorite after adsorption from a 9 × 10-6 M oleate solution, recorded for p-polarization at q ) 45°, after different adsorption times: 1 h (a); 4 h (b); 19 h (c).
the adsorbed amount is about 0.2 of a monolayer (Figure 3c). At this very low coverage a broad band centered at about 1560 cm-1 due to the asymmetric stretching vibration of the COO- group was observed. The large broadening of the band indicates a wide distribution of
different conformations of the carboxylate groups of the adsorbed oleate molecules. This is in agreement with the expected surface structure where separate oleate molecules as well as very small patches, with a high ratio between the number of external (less oriented) and internal (with high lateral interaction) molecules in the patches, are mainly present. At a diluted concentration of 5 × 10-6 M the adsorbed amount does not exceed about 0.25 of a monolayer after 2 and 6 h of adsorption and the recorded spectra are similar to that presented in Figure 3c. At a concentration about 10-4 M sodium oleate in solution, an additional intermediate adsorption product was observed. The growth of the adsorption layer from a 10-4 M solution with increasing time is illustrated by the reflection spectra presented in Figure 4. The intensities of the aliphatic stretching vibration bands in the spectra indicate the formation of about monolayer coverage after 30 min of adsorption. The carboxylate absorbance region shows a doublet at 1574 and 1537 cm-1, similar to those observed at lower oleate concentrations, and an additional band at 1563 cm-1 (Figure 4a). Prolongation of the adsorption time to 1 h results in disappearance of the additional band at 1563 cm-1 whereas the total amount of adsorbed oleate increases to about two statistical monolayers (Figure 4b). This is also the highest coverage which was found at this solution concentration. After 4 h of adsorption the reflection spectra (Figure 4c) are similar to those recorded after 1 h but with lower intensities, indicating a coverage of about 1.2 monolayer. The presence of the sharp band at 1563 cm-1 (fwhm 18 cm-1), which is partly overlapped with the band due to the surface calcium oleate unidentate-like form at 1574 cm-1 (Figure 4a), could indicate the presence of sodium oleate in the surface structure. The band at about 1560 cm-1 was already assigned2,20-21 to adsorbed sodium oleate by simple comparison with the spectrum of pure sodium oleate. The spectroscopic observations (Figure 4a) allow us to conclude
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Figure 4. Reflection spectra of the adsorption layer on fluorite from a 10-4 M oleate solution after different adsorption times: 30 min (a); 1 h (b); 4 h (c). Spectra recorded for p-polarization at q ) 45°.
Figure 5. Reflection spectra of the adsorption layer on fluorite from a 5 × 10-4 M oleate solution after different adsorption times: 20 min (a); 1 h (b); 4 h (c). Spectra recorded for p-polarization at q ) 45°.
that mixed calcium-sodium oleate surface species are produced on the mineral surface. It implies that at higher sodium oleate concentrations in solution sodium ions are trapped in the surface calcium oleate structure. Sodium ions which at first are electrostatically adsorbed on negatively charged fluorite surface are exchanged for calcium ions with increasing adsorption time. Finally the surface calcium oleate organized structure is the only adsorption product formed after extended adsorption time. Sodium atoms were previously found19 on the surface of apatite after adsorption from higher concentrations of sodium oleate solutions, at which mixed calcium-sodium oleate aggregates could also be formed. Microscopic studies show also a patchlike structure formed at this oleate concentration. Surface products formed at a solution concentration of 5 × 10-4 M were found (Figure 5) to be very similar to those formed at 1 × 10-4 M at a short adsorption time (Figure 4a). There are also three adsorption bands at 1574, 1561, and 1537 cm-1 which, as previously, are assigned to two conformations of calcium oleate and the calciumsodium oleate surface species. The difference is that at higher concentrations the same adsorption products are observed after a shorter adsorption time and also after longer adsorption times. After 4 h of contact with oleate solution there are still sodium ions in the surface structure (Figure 5c). The total amount of oleate adsorbed on the fluorite surface does not exceed a statistical monolayer coverage. The microscopic picture shows small tridimensional patches which are unevenly distributed on the
mineral surface. It can be proposed that at higher sodium oleate concentrations a steady state is reached with some level of sodium ions usually present in the surface layer. For the concentration the formation of mixed calciumsodium oleate micelles in solution is the fundamental issue. It was already suggested2 that the presence of calcium ions in solution will decrease the cmc of sodium oleate, which is about 7 × 10-4 M.22 An increase of the solubility of another surfactant, sodium dodecyl sulfate, in the presence of calcium ions was already quantitatively determined.23 This is in line with the present proposition of the formation of mixed calcium-sodium micelles in solution. Another parallel process which could also decrease the intensity of the observed oleate bands is the reorganization of the surface structure with time. If the growing of the large patches takes place at the expense of the small patches and the produced large patches are very thick (above 500 nm), the incident beam will see differently the outermost adsorbed molecules and those which are close to the fluorite surface. However, this phenomenon can only give a small decrease in the observed intensity of the adsorbed product and obviously does not explain the observed sharp decrease in the absorbance intensity. The kinetics of the adsorption of oleate and the desorption of the oleate surface species at different concentrations of oleate in solution is presented in Figure 6. This figure was drawn on the basis of the spectroscopic results presented in Figures 1 and 3-5, where the amount adsorbed is reported in statistical monolayers.
(20) Peck, A. S.; Wadsworth, M. E. In Proceedings of the VIIth International Mineral Congress; Arbiter, N. N., Ed.; Gordon and Breach: New York, 1965; p 259. (21) Lovell, V. M.; Goold, L. A.; Finkelstein, N. P. Int. J. Miner. Process 1974, 1, 183.
(22) Mahieu, N.; Canet, D.; Cases, J. M.; Boubel, J. C. J. Phys. Chem. 1991, 95, 1844. (23) Kallay, N.; Fan, X.-J.; Matijevic, E. Acta Chem. Scand. 1986, A40, 257.
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Figure 7. Reflection spectrum of the adsorption layer on fluorite from a 7.2 × 10-5 M oleate solution after 14 h of adsorption. The pH of the solution drifted from an initial 10.0 to 8.6. Spectrum recorded for p-polarization at q ) 45°.
Figure 6. Dynamics of oleate interaction with fluorite at different oleate concentrations in solution (marked at each curve) revealed by spectroscopic study of the mineral surface.
The adsorbed amount of oleate limited to nearly monolayer coverage was reported recently3,24 for a slab type fluorite sample for a wide range of oleate concentrations at pH 9.8. These authors did not find multilayer coverage in the concentration range from 10-6 to 3 × 10-4 M. This is not in accord with the work presented here; however, it could be explained by possible differences in the solution hydrodynamics between these and their experiments. The second possibility is that they simply missed the multilayer coverage because their studies were limited to long adsorption times. Another reason could be a different type of fluorite sample used in their studies (optical grade fluorite prepared as attenuated reflection element), but lack of reported analytical characterization data of the sample does not allow us to make any conclusion. The adsorbed multilayer of calcium oleate was reported3,24 only when external calcium ions were added to oleate solution at an initial concentration of 1 × 10-4 M. However, the external addition of calcium ions alters tremendously the very delicate balance between solution and surface compositions, and the solution conditions are very different from those when fluorite is contacted with sodium oleate solution (see the next sections). Nevertheless, their studies clearly demonstrate that external addition of calcium ions to solution has a great influence on the adsorption layer formation. During the extended contact of fluorite with oleate solution other observations were made. The pH of the oleate solution decreases gradually with an increase of time, indicating continuous dissolution of fluorite. If the pH of the solution is not maintained by addition of base, it decreases significantly and oleic acid is found as the adsorption product on the fluorite surface. Figure 7 shows the spectrum of fluorite after 14 h of adsorption from a 7.2 × 10-5 M oleate solution. The initial pH of the solution was 10, which fell to 8.6 at the end of adsorption. There (24) Free, M. L. Ph.D. Dissertation, University of Utah, 1994.
is no indication of calcium oleate formation on the fluorite surface. The only adsorption product is oleic acid in an amount higher than a statistical monolayer coverage. The carboxylate band at 1711 cm-1 and a broad band of the CH2 stretching vibration at 2923 cm-1 (refs 18 and 25) indicate that the adsorbed oleic acid is between liquidlike and solid-like states in the adsorption layer. It should be noted that at pH 8.6 oleic acid is not a stable form in aqueous solution; the ionic oleic monomers and dimers are postulated from the thermodynamic prediction.11,26 This suggests that oleic acid is formed directly on the fluorite surface, which under these conditions is covered by surface groups which are sources of hydrogen, for example the OH2+ groups. Significant changes of the baseline (maximum, minimum, or plateau), at frequencies about 1600-1700 and 2700-3600 cm-1, and some at 1400 cm-1, observed in all the recorded reflection spectra, depending on the solution conditions are probably the indication of variation in the relative density of the surface groups (OH, OH2+, HCO3-, CO32-, etc.). More work is necessary to prove the latter suggestion. Kinetics of Surface Layer Formation from Saturated Calcium Oleate Solutions. In these experiments oleate solutions were saturated with calcium oleate during 20 min before the fluorite sample was immersed in the solutions. All solutions were transparent before and after adsorption. The reflection spectra of fluorite after contact with 3.3 × 10-5 M solutions at various times are presented in Figure 8. The types of adsorption products are the same as those observed after adsorption from pure oleate solution (Figure 1). Similarly the amount adsorbed shows a maximum. However, the kinetics of the adsorption preceding the dissolution is much faster from the oleate solution saturated with calcium oleate. After 1 h of adsorption (Figure 8c) the coverage reaches about 12 monolayers. A prolongation of adsorption to 2 h results in a dissolution of the adsorbed amount to about 0.6 of a monolayer. The recorded spectrum (Figure 8d) shows in the carboxylate region a low-intensity broad band centered at about 1550 cm-1. Microscopic results show a fluorite surface covered by surface aggregates similar to those presented in Figure 2 at the multilayer coverage. Influence of External Calcium Ions on Adsorption of Oleate. Immediately after immersion of fluorite in oleate solution the dissolution of fluorite is the most (25) 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. (26) Pugh, R.; Stenius, P. Int. J. Miner. Process 1985, 15, 193.
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Figure 9. Reflection spectra of the adsorption layer on fluorite from a solution containing 3.3 × 10-5 M oleate and 2 × 10-5 M Ca2+ ions after 5 min of adsorption: (a) without calcium; (b) with calcium addition. Spectra recorded for p-polarization at q ) 45°.
Figure 8. Reflection spectra of the adsorption layer on fluorite from a 3.3 × 10-5 M oleate solution saturated with calcium oleate, after different adsorption times: 1 min (a); 15 min (b); 1 h (c); 2 h (d). Spectra recorded for p-polarization at q ) 45°.
extensive and is slowed with time by (i) the increase of the concentration of dissolved ions and (ii) the increase of adsorbed oleate molecules on the fluorite surface. In these experiments the effect of addition of external calcium ions to oleate solution on the adsorption layer formation was investigated. The influence of addition of calcium ions to oleate solution was already examined,3 showing no effect on adsorption layer formation at low calcium concentrations of about 10-5 M, a very little increase of the adsorbed oleate at 3 × 10-5 M, and an about four times increase in calcium oleate adsorption at 10-4 M in the narrow range of oleate concentrations from 5 × 10-5 to 10-4 M. Those adsorption experiments3 were carried out for 2 h. In the present studies the surface characterization was performed after adsorption from aqueous solution containing 3.3 × 10-5 M oleate and 2 × 10-5 M Ca2+ ions at pH 10. Immediately after mixing of the virgin solutions of the two components, fluorite was introduced in the solution for 5 min. The solution quickly turned nontransparent because the initial oleate and calcium concentrations exceeded significantly the solubility product of calcium oleate, that is, 1.48 × 10-16 mol-3 L-3 (ref 18) or 1.96 × 10-16 mol-3 L-3 (ref 8). The reflection spectra of the adsorption layers produced with and without external addition of calcium ions to solution are presented in Figure 9. In both cases, the surface product on fluorite is a calcium oleate surface complex in the amount of about 0.7 and 0.3 of a statistical monolayer, respectively. Though the solution conditions after calcium addition are in principle more favorable for the formation of an adsorption layer by a surface precipitation mechanism, the adsorbed amount of oleate on the fluorite surface was less than half of that adsorbed without any external addition of calcium. It is also interesting to note that the intensities of the absorbance bands at 2919 and 2850 cm-1 (Figure 9)
characteristic for the asymmetric and symmetric stretching vibrations of the CH2 groups of the aliphatic chain are very low, nearly noise level, in comparison to those predicted for an isotropic structure at this coverage.16 This indicates a specific organization of the adsorbed molecules. These results indicate that the addition of calcium ions to oleate solution decreases the adsorption kinetics of oleate on fluorite by lowering the effective concentration (activity) of oleate ions in solution. Moreover, these results show that tridimensional condensation (surface precipitation) is not a favorable adsorption mechanism when the surface is covered by a monolayer and when precipitation in solution takes place. If any surface species are produced through the precipitation mechanism in solution, they are very weakly adsorbed on the fluorite surface and are removed from the surface during emersion of fluorite from solution. Mechanisms of Interactions of Fluorite with Oleate Solution. On the basis of the experimental results, the detailed mechanisms of interaction of fluorite with oleate solution could be proposed (Figure 10). When the fluorite sample is immersed in a basic solution of oleate, two parallel processes take place: the dissolution of fluorite and the adsorption of oleate on the fluorite surface with the formation of a calcium oleate surface layer. The relative kinetics of these major processes determine the composition and structure of the adsorption layer. Hence, any changes of solution conditions, also those occurring during adsorption such as the concentrations of oleate and other ions (calcium, sodium), and hydrodynamic conditions (agitation, solution flow rate, solid/liquid ratio) would change significantly the relative adsorption kinetics (Figure 6) and in consequence modify the surface composition and structure. The surface phenomena are very complex, and different steady states could be observed for different solution conditions. For clarity as well as because of the lack of sufficient data, the interactions of fluorite with water and CO2 were not discussed. At very low concentrations of oleate, below 8 × 10-6 M (Figure 10A), the adsorbed amount does not exceed 0.20.3 of a monolayer. It should be kept in mind that oleate concentrations above 9 × 10-7 M are high enough for the formation of calcium oleate precipitate (the solubility
Dynamics of Fluorite-Oleate Interactions
Langmuir, Vol. 15, No. 2, 1999 507
Figure 10. Proposed mechanisms of interactions of fluorite with oleate solution at pH about 10 at different solution concentrations.
products of 1.48 × 10-16 mol-3 L-3), assuming the calcium ion concentration in solution of 2 × 10-4 M by fluorite dissolution. The real concentration of calcium ions in solution, though it is not expected to reach saturation, should be high enough to ensure precipitation in solution. In the presence of 5 × 10-6 M oleate, a calcium concentration of 5.92 × 10-6 M meets the solubility product conditions. Assuming that 10 times supersaturation is required to initiate nucleation of calcium oleate particles in solution, the calcium concentration 5.92 × 10-5 M should
be easy to reach and calcium oleate precipitation should occur. The formation of the precipitate lowers the oleate concentration to a level at which oleate adsorption on fluorite is limited to submonolayer coverage. It is important to notice that at an initial oleate concentration in solution of 5 × 10-6 M the precipitation of calcium oleate in solution is more favorable than tridimensional condensation on the fluorite surface. The precipitation of calcium oleate in the form of very small particles was reported at much higher oleate (5 × 10-4 M) and calcium
508 Langmuir, Vol. 15, No. 2, 1999
(6 × 10-5 M) concentrations in solution.27 Nevertheless, other turbidity and light-scattering data presented in those works27 strongly suggest that the microparticles could be present also in solution at much lower concentrations similar to that used in our work. Unusual adsorption behavior was found in the concentration range between 10-4 and 10-5 M (Figure 10B); the adsorbed amount of oleate at first increases to a multilayer coverage and then decreases with prolongation of adsorption. The level of the maximum of adsorption and the adsorption and desorption kinetics are strongly related to solution conditions. The highest coverage (about 20 monolayers) was monitored after 6 h of adsorption from a 3.3 × 10-5 M oleate solution. Similar estimation of the supersaturation ratio at a concentration of 3.3 × 10-5 M indicates that an abundant nucleation should occur in bulk solution. This phenomenon plays a vitally important role in adsorption layer formation. The previous works27 on the precipitation of calcium oleate from solution at various calcium and oleate ion concentrations show the formation of very small particles (diameter of 0.09-0.17 µm) of calcium oleate at a concentration nearly 10-4 M for each ion. These particles were found to be very stable in solution because of their negative charge resulting from oleate ion adsorption on their surface. Immediately after immersion of fluorite in oleate solution the concentration of calcium ions in solution is negligible and oleate is present in ionic form (pH 10). Hence, oleate ions diffuse quickly to the fluorite surface, producing surface calcium oleate species at multilayer coverage. Multilayer coverage produced by tridimensional condensation forms patches. It seems that even the first monolayer formation is not completed before thicker coverage is produced by the nucleation and growth mechanism. The adsorbed oleate lowers the kinetics of fluorite dissolution. However, the calcium concentration in solution increases with time sufficiently to produce very fine calcium oleate particles in solution. The continuous release of calcium ions from the fluorite surface ensures an increase of the particle size in solution. When the average size of calcium oleate particles in solution significantly exceeds the size of surface patches, dissolution of the adsorbed layer takes place. The observed adsorption maximum could be explained by the Ostwald ripening of the metastable surface layer, which indicates its pure kinetic origin. The observed effect of an addition of external calcium in ionic form or/and as a solid calcium oleate on the kinetics of oleate adsorption and dissolution is in line with the proposed mechanism where the nucleation and growth of very fine calcium oleate particles in solution is the fundamental issue. At higher than 10-4 M oleate concentration (Figure 10C), from the beginning of adsorption, the fluorite dissolution releases to solution enough calcium ions, which at this high oleate concentration could form mixed calciumsodium oleate micelles. No tridimensional precipitation takes place in solution because of the adsorption of calcium ions on the micelles. High oleate concentration means also high sodium ion concentration in solution. Under these conditions sodium ions could adsorb electrostatically directly on the negatively charged fluorite surface. They are trapped in the surface structure of adsorbed oleate species; however, with an increase of adsorption time they are replaced by calcium atoms. Calcium oleate is formed as the most stable surface product. At the highest sodium oleate concentrations, this replacement is less favorable. (27) Matijevic, E.; Leja, J.; Nemeth, R. J. Colloid Interface Sci. 1966, 22, 419. Nemeth, R.; Matijevic, E. Kolloid-Z. 1971, 245, 497.
Mielczarski et al.
The formation of the mixed calcium-sodium oleate micelles results in a significant lowering of the real concentration (activity) of oleate ions. The adsorbed amount does not exceed a statistical monolayer coverage even after a long adsorption time. Conclusion The presented results of fluorite-aqueous oleate solution interactions show clearly that the fluorite-oleate system behaves differently from a well-described classical surfactant adsorption on oxides. This allows us to understand better the conclusion presented in Finkelstein’s review1 that the previous published results for this system seem inconsistent and illogical. The reason is that the surface interactions in the system are very complex and dynamic. Equilibrium is not reached under experimental conditions, whereas different steady states can be achieved. The surface composition and structure depend on competitive processes taking place at the interface, such as fluorite dissolution, interaction with water or CO2, or adsorption of oleate ions and other aggregates from solution. If the fluorite is immersed in a pure oleate alkaline solution, the observed surface phenomena depend on the initial oleate concentration if other solution conditions (pH, agitation, etc.) are constant. Three characteristic regions of adsorption can be distinguished for which the formation of very fine calcium oleate particles or mixed calcium-sodium oleate micelles in solution is vitally important. If the other solution conditions are changed, such as solution hydrodynamics (agitation), an addition of external calcium (ions or/and solid calcium oleate) to oleate solution, they change tremendously the adsorption kinetics. The observations underline again the importance of the solution chemistry of calcium/sodium oleate (with fluorite as the source of calcium ion) on the composition, structure, and stability of the oleate adsorbed layer on fluorite. Three distinguished adsorption products were observed on the fluorite surface: calcium oleate with two different conformations, bidentate-like (the absorbance band at 1536 cm-1) and unidentate-like (the absorbance band at 1576 cm-1), and mixed calcium-sodium oleate surface species (the narrow absorbance band at about 1560 cm-1). The sharp band at about 1560 cm-1 (fwhm about 18 cm-1) is different from the broad band centered at about 1550 cm-1 (fwhm about 50 cm-1), which was recently interpreted28 as the proof of a chemisorbed layer of oleate on fluorite as well as other calcium minerals. The band at about 1550 cm-1 (fwhm about 45 cm-1) also observed in the study at submonolayer coverages is assigned to complex structural features of the adsorbed carboxylate groups, indicating isolated oleate molecules on the fluorite surface with various intermediate states between unidentate and bidentate. The results obtained show that the study of oleate adsorption on fluorite is very challenging considering the many factors that could affect the final results and that should be kept under control. Acknowledgment. We thank Dr. J. J. Ehrhardt and J. Menaucourt for their helpful assistance with the AFM experiments. We gratefully acknowledge the support of this work by the CEFIPRA (project 1315-1). LA980593F (28) Miller, J. D.; Jang, W. H.; Kellar, J. J. Langmuir 1995, 11, 3272.
