Langmuir 1992,8, 118-124
118
Adsorption of Oleate on Apatite Studied by Diffuse Reflectance Infrared Fourier Transform Spectroscopy Wen Qi Gong,+JA. Parentich,s L. H. Little,$ and L. J. Warren*$il The University of Western Australia, Perth, Western Australia 6009, Australia, Curtin University of Technology, Perth, Western Australia 6102, Australia, and CSIRO Mineral Products, Perth, Western Australia 6102, Australia Received September 11, 1990 The adsorption of oleate on apatite was studied a t pH values from 6 to 9.8 and oleate concentrations from 2 X 10-5 to 3 X mol L-l. Diffuse reflectance infrared Fourier transform spectroscopy has been found to be superior to the transmission infrared technique for detecting adsorbed oleate species. Confusion in previous studies is clarified and a better understanding of the adsorption mechanism obtained. Chemisorbed oleate corresponds to a single peak at 1550 cm-' and probably comprises one oleate ion bonding with one lattice calcium ion on the surface. Surface calcium oleate precipitate showing peaks a t 1574 and 1538 cm-I has a structure similar to that of bulk calcium oleate and probably adsorbs through ion-dipole interaction and hydrocarbon chain association. Oleic acid dimer and monomer adsorb via hydrocarbon chain association onto underlying chemisorbed oleate and correspond to a sharp peak a t 1713 cm-l and a shoulder a t 1732 cm-l, respectively. Chemisorption of oleate on apatite occurred under all conditions studied and was accompanied by physical adsorption of calcium oleate precipitate and/or oleic acid monomer and/or oleic acid dimer, depending on the pH and concentration of the solution.
Introduction Infrared spectroscopy has been found to be a most successful spectroscopic technique for determining t h e interactions between minerals and reagents.' T h e adsorption of oleate on calcium minerals has been widely studied with infrared spectroscopy over the last 30 y e a r ~ . ~ - I Nevertheless, there are still controversies with regard to t h e explanation of t h e spectra and t h e nature of t h e adsorbed oleate species. T h e spectra were obtained, in most cases, by t h e transmission K B r disk infrared spectroscopic technique. It has been suggested t h a t t h e sample may interact with t h e K B r matrix during t h e pressing of t h e disk8 although n o evidence for this has been reported. Due t o low sensitivity of t h e transmission infrared technique, t h e concentration of the oleate solution was, in most of t h e previous studies, at least a n order of magnitude higher, for example, 3 X to 2 X lo-' mol L-' in the study by Love11 e t al.,4t h a n t h a t used in practice in the flotation of calcium phosphate a n d fluorite (about 5 x mol L-I). At these higher concentrations there could be a change in t h e nature of t h e adsorbed oleate species. T h e aim of this study was t o solve t h e problems in the previous studies a n d t o achieve a better understanding of t h e adsorption mechanism of oleate on calcium minerals. Apatite was studied as a n example of a calcium mineral. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy is compared with the transmission infrared spectroscopic technique. t On leave from Wuhan University of Technology, Wuhan 430070, China. The University of Western Australia. Curtin University of Technology. 1 CSIRO Mineral Products. (1)Giesekke, E. W. Int. J . Miner. Process. 1983, 11, 19. (2) French,R. 0.; Wadsworth, M. E.; Cook, M. A.; Cutler, I. B. J.Phys.
*
Chem. 1954,58, 805. (3) Peck, A. S.Bur. Mines Rep. Invest. 1963, RI6202. (4) Lovel1,V. M.; Goold,L. A.;Finkelstein, N. P.1nt.J. Miner.Pr0ces.s. 1974, 1, 183. (5) Hu, J. S.; Misra, M.; Miller, J. D. Int. J . Miner. Process. 1986, 18,
73. (6) Yusupov, T. S.;Lapteva, E. S.; Gilinskaya, L. G. Dokl. Akad. Nauk USSR 1986,291, 693. (7) Sivamohan, R.; de Donato, P.; Cases, J. M. Langmuir 1990,6,637. (8) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227.
0743-746319212408-0118$03.00/0
Experimental Section Materials. A purified sample of apatite was prepared from a selected phosphate ore sample from the Mt. Weld phosphate deposit, Westem Australia,by gravityconcentration and magnetic separation. The chemical and X-ray diffraction analysis showed that this sample contained 98% hydroxyapatite. The sample was ground with a micronizing mill to a particle size of 50% less than 1.37 fim. The surface area of the sample was determined to be 9.85 m2 gl by the nitrogen adsorption method (BET). More than 99% pure oleic acid (Gold label) supplied by Almol L-'sodium oleate stock drich Chemical Co. was used. A solution was prepared by saponification of the oleic acid for 2 h at 60 O C at pH 11.6 adjusted with sodium hydroxide. Fresh solution was made up every 5 days. Other reagents used were all of analytical grade. Doubly distilled water was used throughout the experiments. Instrument and Method. The instrument was asingle-beam Fourier transform infrared spectrophotometer, Mattson FTIR Sirus 100, equipped with a liquid-nitrogen-cooled mercurycadmium-telluride (MCT) detector. Three hundred scans were coadded to obtain each spectrum with a resolution of two wavenumbers. The instrument was purged with dry Nz for 3 h to remove water vapor and carbon dioxide from the chamber of the instrument. Then a polyethylene glovebag was attached to the opening of the chamber to prevent air from entering the instrument when it was open for mounting the sample. A Mattson Instrument's diffuse reflectance accessorywas used, which was composed of a sample cup and a series of mirrors. The samples for recording the DRIFT spectra of adsorbed oleate species were prepared as follows: 0.1 g of sample was added into 50 mL of doubly distilled water and dispersed with ultrasonics. NaOH and HCl were used to adjust the pH of the suspension. Sodium oleate was added and the suspension conditioned by a magnetic stirrer for 10 min. Then the suspension was centrifuged and decanted. The sediment was washed with doubly distilled water adjusted to the same pH as the previous suspension and centrifuged and decanted again. Then the sediment was freezedried for 24 h. The washing step means that on drying there would be very low levels of dissolved solute and little chance of very fine emulsion, such as oleic acid, depositing on the particle surfaces. In order to help identify the adsorbed oleate species on the apatite surface, in some cases, extra washing steps were taken with the pH adjusted doubly distilled water in an ultrasonic bath, or with n-hexane, in an endeavor to remove adsorbed material from the surface. The DRIFT spectra were recorded according to the following procedure: 4 mg of sample was mixed with 200 mg of KBr powder
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 1, 1992 119
Adsorption of Oleate on Apatite 1425 1453 1
I
I
wavenumber (Cm.1)
Figure 1. DRIFT and transmission infrared spectra of apatite treated with lo4 mol L-' sodium oleate at pH 9.8.
~
~~~~
(9) Griffiths, P. R.; Fuller, M. P. In Advances in Infrared and Raman Spectroscopy; Clark,R. J. H., Hester, R. E., Ed.; Heyden & Son, Ltd.: London, 1982; Vol. 9, p 63. (10)Miller, R. G. J.; Stace, B. C. Laboratory Methods in Infrared Spectroscopy; Heyden: London, 1972.
1300
wavenumber ("11
Figure 2. DRIFT and transmission infrared spectra of apatite treated with 10"' mol L-l sodium oleate at pH 9.8 (a) KBr disk under pressure of 10 ton; (b) KBr disk under pressure of 8 ton; (c) Nujol mull; (d) DRIFT.
in a Specamill vibrating mixer. Then the diffuse reflectance sample cup was filled with the mixture and mounted in the position. Each DRIFT spectrum was taken with KBr powder as background. All DRIFT spectra were plotted by an interfaced computer as the Kubelka-Munk (KM) function, which might be expected to be quite similar to the absorbance spectra of the same samples by the transmission KBr disk t e ~ h n i q u e . ~ A reference spectrum was taken with the sample treated with doubly distilled water adjusted to the same pH as the oleate solution. The spectra of adsorbed species on the sample surface were obtained by subtracting the reference spectra from the spectra of samples treated with oleate solution. In most cases, the subtracting factors chosen were around unity. The transmission spectra were recorded using the conventional KBr disk or Nujol mull techniques.10
Results A comparison of the DRIFT and transmission infrared spectra of apatite after treatment with a mol L-' sodium oleate solution at pH 9.8 is shown in Figure 1. The two spectra are very similar except that the doublet peaks at 1574 and 1537 cm-' in the DRIFT spectrum are replaced by a single peak of relatively low intensity at 1560 cm-' in the transmission spectrum. At lower sodium oleate concentration, peaks in this region could not be seen in the transmission spectra but were clearly shown in the DRIFT spectra. Figure 2 gives the transmission spectrum of the sample in Nujol mull between two sodium chloride windows and the transmission spectra of the sample in the KBr disk prepared under reduced pressure (from 10 ton load to 8 ton load). The spectrum of the KBr disk prepared under the load of 10 ton has a single peak at 1560 cm-' and two shoulders at 1574 and 1537 cm-' (Figure 2a) whereas the spectrum of the disk prepared under 8 ton load shows an increase in the intensities of the two shoulders and a decrease in the intensity of the single peak (Figure 2b). The DRIFT spectrum and the Nujol mull spectrum are very similar, both with two peaks at 1574 and 1537 cm-I but no peak at 1560 cm-' (Figure 2c and d). The DRIFT spectra of adsorbed oleate species on apatite at various pH and oleate concentrations are shown in Figures 3-6. At high pH (9.8) doublet peaks were obtained at about 1574 and 1540 cm-'at all concentrations of sodium oleate (from 2 X to mol L-l) (Figure 3). A t pH 8, similar doublet peaks at about 1574 and 1540 cm-' were observed when the sodium oleate concentration
I
I
I 1500
1700
18w
2000
i538
1574
I
Figure 3. DRIFT spectra of adsorbed species on apatite treated with sodium oleate at pH 9.8. Sodium oleate concentrations were (a) 2 X 10-5 mol L-l, (b) 3 X 10-5mol L-l, (c) 5 X lo+ mol L-l, and (d) mol L-I. 1539
1574
I IMO
1
I
16w
14w WaYmYmtel (cm
1
L
'I
Figure 4. DRIFT spectra of adsorbed species on apatite treated with sodium oleate at pH 8.0. Sodium oleate concentrations were (a) 2 X mol L-l, (b) 5 X 10" mol L-I, and (c) lo4 mol L-1,
and was 5 X mol L-' (Figure 4b and c) whereas only a single peak was obtained at 1550 cm-l at the sodium mol L-l (Figure 4a). At oleate concentration of 2 X pH 6 only a single peak was obtained at 1550 cm-I a t all and 5 X concentrations of sodium oleate (5 X mol L-l) (Figure 5). However, at 5 X mol L-1 sodium oleate an additional strong band was present at 1713 cm-' (Figure 5c). This band became very weak at the sodium oleate concentration of mol L-' (Figure 5b) and disappeared at the sodium oleate concentration of 5X mol L-' (Figure 5a). The effect of pH on the spectra of the apatite/oleate mol L-' sodium system at flotation concentration (5 X oleate) is shown in Figure 6. At the higher pH (from 7.6 to 9.8), doublet peaks at about 1574 and 1540 cm-I were
120 Lnngmuir, Vol. 8, No, 1, 1992
Gong et al. 2x105moiL.' 1550
I
1500
i7 M
WIYmYmtnr
I
I 1600
1Bw
I
120.2
(LOO
1300 (cm 'I
Figure 5. DR!PT spect,raof adsorbed species on apatite treated with sodium oleate at pH 6.0. Sodium oleete concentrations were (a) 5 X 10-5 mol L-I, (b) 10-4mol JJ-l, and (c) 5 X IO-* mol L-1. 1539 1574
\
I
I
1Mo
15W
14w
-12w
wavsnumber 1"')
Figure 6. DRIFT spectra of adsorbed species on apatite treated with 5 X mol L-' sodium oleate: (a) pH 6.0; (b) pH 7.2; (c) pH 7.6; (d) pH 8.0; (e) pH 9.8.
obtained for the system, but at the lower pH (6.0 and 7.2) only a single peak at 1550 cm-1 was observed. The DRIFT spectra of adsorbed species on apatite treated with sodium oleate a t pH 8 and concentration of 2x to mol L-l and then washed with doubly distilled water in an ultrasonic bath for 10 min are shown in Figure 7. After ultrasonic washing, the doublet peaks at 1574and 1539 cm-l disappeared and one single peak at about 1550cm-' remained. Ultrasonic washing with water had little effect on the 1550-cm-l single peak, indicating that it was due to a strongly adsorbed species. The DRIFT spectra of apatite after treatment with 5 x mol L-' sodium oleate at pH 9.8 and 6.1 and then water washed in an ultrasonic bath for 1 or 10 min are shown in Figures 8 and 9. Again the doublet peaks at 1574 and 1539 cm-' disappeared and a single peak at 1550 cm-l remained even after further washing. Figure 10 shows the DRIFT spectra of apatite treated with 5 X mol L-l sodium oleate at pH 6 and then washed with water in an ultrasonic bath or with n-hexane. It can be seen that after ultrasonic water washing the intensity of the 1713-cm-l band was significantly reduced while the 1550-cm-l band remained the same. After washing with n-hexane, the 1713-cm-1band disappeared completely but the 1550-cm-' band only slightly decreased in intensity. Discussion Assignment of Peaks Due to the Adsorbed Oleate Species. The p a k s attributed to adsorbed oleate species were assigned to particular vibrations by comparison with data from literature on liquid oleic acid, solid sodium and calcium oleates, carboxylic acid in general, and on adsorption of carboxylic acids from nonaqueous solvents on various oxides. These peaks can be classified into four main regions:
Figure 7. DRIFT spectra of adsorbed species on apatite treated with sodium oleate of indicated concentration at pH 8.0 and then (a) without washing and (b) washed with water in an ultrasonic bath for 10 min.
1500
le00
1400
1 io0
wavenumber 1"')
Figure 8. DRIFT spectra of apatite treated with 5 X 10-5 mol L-I sodium oleate at pH 9.8 and then washed with water in an ultrasonic bath: (a) 1min washing; (b) 10min washing; (c) blank.
(i) Region 3100-2800 cm-l (carbon-hydrogen vibrations). The observed peak at 3007 cm-l is assigned to the asymmetric stretch of -CH= by comparison with data of Sinclair et al.ll Similarly, the observed peaks at 2959, 2922,2880, and 2852 cm-l (Figure 1)match within 3 cm-I to the peaks assigned by Sinclair et al." as arising from v,(-CH3, v,(-CHz-), vs(-CH3), and v,(-CHz-), respectively. (ii) Region 1800-1600 cm-l (carbonyl vibrations). It has been reported by a number of worker^'^-^^ that in the spectra of adsorbed carboxylic acid species on oxides, the (11)Sinclair, R. C.; McKay, A. F.; Norman Jones, R. J. A h . Chem. SOC.1952. 74. 2578. (12)Hasegawa, Masatomo; Low, M. J. D. J. Colloid Interface Sci. 1969, 29, 593.
(13)Hasegawa, Masatomo; Low, M. J. D. J. Colloid Interface Sci. 1969. - - , 30., 378. ~
~
~
(14)Buckland, A. D.; Rochester, C. H.; Topham, S. A. J.Chem. Soc., Faraday Trans. 1 1980, 76, 302. (15) Lorenzelli, V.; Busca, G.; Sheppard, N. J. Catal. 1980, 66, 28.
Langmuir, Vol. 8, No. 1, 1992 121
Adsorption of Oleate on Apatite 1453
A
'425
oleate. Peck and Wadsworth18also observed a single peak for this species but shifted to different frequency. More recently, Sivamohan et al.7 has considered that the peak at 1560-1550 cm-l is due either to the asymmetric carboxylate stretching vibration of the oleate ions chemisorbed onto the calcium sites on the surface or to the corresponding vibration of physically adsorbed sodium oleate. Therefore, although there is no doubt about the assignment of the observed peaks at 1574,1560,1550, and 1539 cm-l in the infrared spectra of oleate adsorbed on apatite (see, e.g., Figures 1-5) to the asymmetric carboxylate stretching vibration, whether this carboxylate group belongs to chemisorbed oleate or physically adsorbed oleate or both is debatable. As this problem is so important in understanding the mechanism of adsorption of oleate on calcium minerals, it will be discussed in more detail. (iv) Region 1500-1400 cm-l (symmetric carboxylate vibrations). The observed peaks at 1469-1456 cm-l in the DRIFT spectra of adsorbed oleate on apatite (see, e.g., Figures 3 and 4) may be assigned to the bending vibration of (-CH2-) by comparison with the data of Sinclair et al.11 Similarly, the observed peaks at 1445-1434 cm-l match with the peak at 1443 cm-' assigned by Sinclair et al.I1 as arising from v,(-COO-). The observed peak at 1406-1408 cm-l matches with the peak at 1405 cm-' assigned by the same authors to G(a-CH2). Nature of the Adsorbed Oleate Species on Apatite. One of the reasons for the incorrect information and controversy in the previous infrared studies in the region of asymmetric carboxylate vibrations could be due to the transmission KBr disk infrared technique used in most of the previous studies. In order to examine the effect of the possible interaction between the sample and KBr matrix, the spectra obtained by transmission KBr disk technique were compared with those by DRIFT and transmission Nujol mull techniques. DRIFT spectroscopy requires very simple preparation of the sample. No grinding or pressing between the sample and KBr powder is required and thus the possibility of the interaction between the sample and KBr matrix is minimized. The same may be said for the Nujol mull technique. Thus, the observations that (i) the DRIFT spectrum of oleate adsorbed on apatite (Figure 2d) showed the same peaks at 1574 and 1537 cm-l as those in the transmission Nujol mull spectrum (Figure 2c), whereas (ii) the KBr disks prepared from the same apatite/sodium oleate system but under different pressure (Figure 2a,b) gave rise to spectra different from each other and also different from those by the Nujol mull or DRIFT techniques, indicated that some interaction did occur during the pressing of the KBr disks and that this interaction depended on the magnitude of the pressure. The changes in the transmission spectra of the KBr disks can be well explained by the ion exchange reaction between the sample and the KBr matrix. KBr powder under the pressure usually employed to obtain a clear disk is actually mobile, increasing the contact and hence the ion exchange between the sample surfaces and KBr matrix. Such exchange could be represented by the equation
woo
1 m
1400
16W
Wavsnvmbr (cm-')
Figure 9. DRIFT spectra of apatite treated with 5 X mol L-1 sodium oleate at pH 6.1 and then washed with water in an ultrasonic bath (a) 1min washing; (b) 10min washing; (c) blank. 1552
A
1713
d
* 1 ~~
17w
1
I
I 1300
1Mo
W."e""mbl
(Em-')
Figure 10. DRIFT spectra of adsorbed specieson apatite treated mol L-' sodium oleate at pH 6.0 and then washed with 5 X with water in an ultrasonic bath or with n-hexane: (a) without washing; (b) 3 min ultrasonic washing; (c) 10 min ultrasonic washing; (d) 30 min ultrasonic washing; (e) n-hexane washing.
peak in the region 1700-1715~m-~ corresponds to the C=O stretching vibration of carboxylic acid dimer whereas the peak in the range 1730-1750 cm-' corresponds to the corresponding vibration of carboxylic acid monomer. Therefore, the sharp peak at 1713 cm-' in Figures 5 and 10 is assigned to the C=O stretching vibration in oleic acid dimer and the shoulder at 1732 cm-l in Figure 10 is assigned to the C=O streching vibration in oleic acid monomer. (iii) Region 1600-1500 cm-l (asymmetric carboxylate vibrations). This is the most important region as the characteristic peaks due to the asymmetric stretching vibration of carboxylate group in all oleates are in this region. However,this is also the region where considerable controversy still exists in correlating the type of adsorption and the observed peaks in the spectra of adsorbed oleate on calcium mineral surfaces. I t has been well established that the spectrum of calcium oleate in the bulk phase has two sharp peaks at about 1576 and 1540 cm-l which arise from the asymmetric carboxylate stretching vibration whereas for sodium or potassium oleate this vibration corresponds to a single peak at about 1560 cm-1.3-5 For oleate adsorbed on calcium minerals contradictory results about the characteristic carboxylate stretching peaks have been reported in the literature. Berger et a1.,16Moudgil and Chanchani,I7and Yusupov et aL6 observed peaks due to adsorbed oleate on calcium minerals at about the same frequencies as those due to bulk calcium oleate. Love11 et al.* observed a single peak for this species at the same frequency as that due to sodium (16) Berger, G. S.;Ishchenko, V. V.; Kiselev, L. M.; Turnsunova, S. A. Tsuetn. Met. (N.Y.) 1967, 9, 9. (17) Moudgil, B. M.; Chanchani, R. Miner. Metall. Process. 1985, 2, 13.
2KBr + Ca(oleate),(ads.)
-
2K(oleate)(ads.) + CaBr,
As a result the IR absorbing species in disks prepared for transmission spectroscopy is likely to be potassium oleate rather than calcium oleate. When the pressure for making the KBr disk is reduced from 10 ton to 8 ton, the extent (18)Peck, A. S.; Wadsworth, M. E. VIIth International Mineral Processing Congress; Gordon & Breach New York, 1964; Vol. 1, p 259.
122 Langmuir, Vol. 8, No. 1, 1992
of the ion exchange reaction should decrease. As would be expected, the intensity of the peaks at 1574 and 1537 cm-' due to the unchanged calcium oleate increased while that of the peak at 1560 cm-' due to potassium oleate decreased. Therefore, previous assignments of the 1560-cm-' peak by, for example, Love11 et al.? to the asymmetric carboxylate vibration in chemisorbed calcium oleate would appear to be an artifact of the transmission KBr technique. The sensitivity of DRIFT to species on surfaces is much greater than the transmission KBr disk technique so that more information is usually obtained. The principle of DRIFT spectroscopy is different from that of transmission KBr disk infrared spectroscopy in that the infrared radiation only penetrates a limited depth into the sample and undergoes reflection, refraction, multiple scattering, and absorption in varying degrees before reemitting at the sample surface. Since the reflected light has passed through only a portion of the bulk sample and has been reflected several times a t the particle surfaces, DRIFT technique should be more surface sensitive than the transmission KBr disk technique. This is shown in Figures 3-9, in which peaks due to the adsorbed oleate on apatite treated with sodium oleate solutions at or even below the flotation concentration are clearly shown. This sensitivity has not been achieved in the previous studies by transmission spectroscopy. Another cause of the incorrect information in previous studies is probably the effect due to the concentration of sodium oleate solution used. Most of the previous studies were carried out in sodium oleate solutions at least an order of magnitude higher in concentration than that in the actual flotation solution due to the sensitivity limit of the transmission KBr disk technique. Because the oleate species distribution in solution depends on the concentration of the oleate solution, the species and thus the spectra of the oleate adsorbed under sodium oleate concentrations much higher than the flotation concentration may be different from that under the flotation condition. This is clearly seen in Figure 4 which shows that the peaks at 1574 and 1539cm-' at concentrations of 5X mol L-' and above changed into a single peak at about 1550 cm-' at lower concentrations. The nature of the adsorbed oleate evidently changed with decreasing solution concentration from a species resembling bulk calcium oleate (-COO- vibrations at 1574 and 1539 cm-l) to another species whose carboxylate vibration frequency of 1550cm-' does not correspond with that of known bulk or solution species. The identification of the adsorbed oleate species was made by correlating the DRIFT spectra obtained a t different concentrations of sodium oleate solution with the corresponding region in Moudgil's adsorption isotherms of oleate on apatitelgand was assisted by the results of the washing testa given in Figures 7-10. The doublet peaks at 1574and 1538cm-l in the DRIFT spectra of apatite treated with sodium oleate of greater than 3 X mol L-l at pH 9.8 (Figures 3 b-d) arise from a bulk calcium oleate phase which is probably present as a thin layer on the apatite surface and may have been formed by surface or bulk precipitation. Moudgil et al.19 postulated the formation of surface or bulk precipitates in the corresponding region (region 111)of their adsorption isotherms. Moudgil et al.19also speculated that at oleate concentrations of 2 X 10-5 mol L-1 (region I1 in their isotherms) some chemisorbed oleate should occur along (19) Moudgil, B. M.; Vasudevan, T. V.; Blaakmeer, J. Miner. Metall. Process. 1987, 4, 50.
Gong et al.
with the surface precipitate of calcium oleate. Evidence for this mixture of chemisorbed oleate and bulk calcium oleate has been obtained from our DRIFT results of the washing tests shown in Figures 7-10. It is evident that after ultrasonic washing, the doublet peaks at 1574 and 1539 cm-' disappeared leaving a single peak at 1550 cm-' instead (Figures 7 and 8). The 1550-cm-' peak is therefore assigned to the asymmetric carboxylate vibration in chemisorbed oleate on apatite. Also, at low oleate concentration or low pH where the peak at 1550 cm-l was the only peak initially, it was almost not affected by the ultrasonic washing (Figures 7 and 9). This behavior is consistent with a weakly attached surface phase of calcium oleate (removed by ultrasonic treatment) and a strongly attached monolayer of chemisorbed oleate (not removed by ultrasonics). Therefore, at low pH and oleate concentrations, the surface species is primarily chemisorbed oleate. At high pH and oleate concentrations, chemisorbed oleate and bulk calcium oleate precipitatte coexist on the surface. All the results in Figures 7-10 are in good agreement with each other and confirm that the doublet peaks at 1574 and 1539 cm-' correspond to precipitated calcium oleate physically adsorbed, whereas the single peak at 1550cm-l corresponds to the chemisorbed oleate on apatite. There is another oleate species found on apatite under certain conditions and that is the oleic acid dimer, identified by the characteristic vibration of the >C=O bond at 1713 cm-'. The oleic acid dimer is probably physically adsorbed because the 1713-cm-' peak decreased in intensity as the ultrasonic washing in water continued (Figure lo), while the band at 1550 cm-' due to chemisorbed oleate was not affected. Similarly, after washing withn-hexane, the 1713-cm-' peakdisappeared completely whereas the 1550-cm-' peak was only slightly reduced in intensity. The proposition that the oleic acid dimer is physically adsorbed is confirmed by the similarity of its vibration frequency to that of liquid oleic acid, viz., 1710 cm-'. It has been shown (Figure 3) that as the concentration of oleate increased above the flotation concentration (5 X mol L-l) at pH values above 8, only the doublet peaks at 1576 and 1537 cm-' were observed and that the intensities of the doublet peaks increased continuously with the concentration. Previous studies" which attributed the doublet peaks to chemisorbed oleate were all carried out at concentrations much higher than the flotation concentration. Under such conditions the intensities of the doublet peaks were so high that the peak due to the chemisorbed oleate, which is weak and broad, would be masked. Therefore, the so-called "chemisorbed oleatewidentified according to the presence of the doublet peaks was in fact the physically adsorbed calcium oleate precipitate. The difference in the spectra of chemisorbed oleate and physically adsorbed calcium oleate precipitate can be explained by their different structures. Because of the relatively large size of the oleate ion (cross section area of 20.5 X m2),4it might be expected that each calcium ion on the mineral surface would bond with only one oleate ion. Dixit20 also indicated that when oleate chemisorbs on mineral surfaces, only one cation-oleate bond is formed for each cation on the surface. Therefore, the structure of chemisorbed oleate would be considerably different from that of the bulk calcium oleate. However, the physically adsorbed calcium oleate precipitate would (20) Dixit, S.G. Presented at the International Conference on Progress in Metallurgical Research: Fundamental and Applied Aspects, Feb 1115, India, 1985.
Langmuir, Vol. 8, No. 1, 1992 123
Adsorption of Oleate on Apatite
Table I. Concentration Product [Caz+][RCOO-]z in the Apatite/Oleate Solution System PH6 PH8 pH 10 total oleate, mol L-' [Ca2+l [RCOO-I [Ca2+l[RC00-12 [Ca2+l [RCOO-I [Ca2+l[RC00-]2 [Ca2+] [RCOO-] [Ca2+][RC00-]2 2 x 10-5 10-3.5 10-6.5 10-16.5 10-45 10-4.7 10-13.9 10-6.6 10-4.7 10-16.0 ~
7 x 10-5
3 x 10-4
10-3.5 10-3.5
10-6.5
10-8.5
10-16.5
10-16.5
10-4.5
10-45
10-13.5
10-4.5
10-4.0
10-12.5
retain the same structure as the bulk calcium oleate with two oleate ions bonding with each calcium ion. Therefore, the spectra of physically adsorbed calcium oleate precipitate and the bulk calcium oleate are very similar, all showing the double peaks at 1574 and 1537 cm-l, whereas that of the chemisorbed calcium oleate is considerably different, showing a single peak at 1550 cm-'. observed that the spectrum of chemisorbed Lovell et oleate on fluorite showed a single peak identical to the asymmetric -COO- stretch frequency of sodium or potassium oleate and believed that all materials having single metal-to-oleate bonds absorbed at the same frequency. However, Dixit20observed a variation of the asymmetric -COO- stretch frequency of the chemisorbed metal oleates with the cationic radius which is contradictory with the above conclusion by Lovell et al.4 Our DRIFT spectra of chemisorbed oleate on apatite also showed a shift of the asymmetric -COO- stretch frequency from that of sodium or potassium oleate. It is reasonable to assume for the single metal-to-oleate bonds of different cations to absorb at different frequencies. As already shown in Figure 2, the transmission KBr disk infrared technique, which was used by Lovell et al.,4would bring about an ion exchange reaction between the adsorbed surface species and the KBr matrix. Therefore, the species which absorbs at the same asymmetric -COOstretch frequency as that for sodium or potassium oleate and identified as chemisorbed calcium oleate would appear to be potassium oleate resulting from the ion exchange reaction. High sensitivity using DRIFT technique was also obtained by Sivamohan et al.' Unfortunately, the influence of the strongly infrared absorbing atmospheric water was not eliminated by purging the instrument but by subtracting, which resulted in uncompensated curious peaks in the region of asymmetric carboxylate vibrations (1500-1700 cm-'). According to the above discussion, the nature of the adsorbed species on apatite can be described as follows: The chemisorbed oleate corresponding to the single peak at 1550 cm-' in the DRIFT spectra probably comprises one oleate ion bonding with each lattice calcium ion on the surface. When a lattice calcium ion is dissolved into the solution, two oleate ions would bond with it to form calcium oleate precipitate. When the calcium oleate precipitate in the surface region is adsorbed, surface precipitate is formed. The positive end of the precipitate molecule may be preferentially attached to the phosphate anion on the apatite surface by ion-dipole interaction between the ions in the apatite lattice and the dipole of calcium oleate, given as 4.49 D.21 The nonpolar ends may be attached by hydrocarbon chain association to the nonpolar ends of the oleate already adsorbed. When more and more calcium oleate precipitate is adsorbed, multilayers are formed on the surface. The calcium oleate molecules are then attached to the adsorbed oleate layer via the hydrocarbon chain association only and orient randomly, forming bulk calcium oleate precipitate. (21)Leja, J. Surface Chemistry of Froth Flotation; Plenum: New York, 1982.
10-6.6 10-6.6
10-4.5 104.0
10-16.6 10-14.6
Mechanisms of Adsorption of Oleate on Apatite. According to Pugh and Stenius,22below pH 7 the major species will be liquid oleic acid, RCOOH, presumably as an emulsion, and this will account for at least 99% of the oleate. Most of the remainder is the singly charged monomeric carboxylate ion, RCOO-. Above pH 8.5 Pugh and Stenius' calculations22show that RCOO- is the dominant species, accounting for about 90% of the oleate at low concentrations (2 X mol L-l), but only about 33 % at higher concentrations (3 X 10" mol L-l), where the doubly charged dimeric carboxylate ion (RC00)22- accounts for about 67% of the oleate. In the region pH 7-8.5, both liquid RCOOH and RCOOand (RC00)22- coexist in significant amounts. Apatite is sufficiently soluble in the pH range 6-10 to allowthe possibility of reaction between Ca2+derived from the apatite and oleate species; if the solubility product of calcium oleate were exceeded, a precipitate would form. Somasundaran et al.23calculated the species distribution for the apatitetwater system, in the presence of atmospheric carbon dioxide. The major species at pH 6 is Ca2+ with a slightly lower concentration of H2PO4- and much lower levels of HzC03, HC03-, HP042-, and CaH2P04+. The concentration of Ca2+falls steadily until pH 8 and then more rapidly to pH 10, where the dominant species are HC03- and C032- followed by H2C03, HP0d2-, and Pod3-. From the data of Pugh and Stenius22and Somasundaran et al.23the concentration product Q = [Ca2+l[RC00-12 was calculated for the apatite/oleate solution system in the presence of atmospheric C02. Results are given in Table I for various pH values and oleate concentrations. Using the value of the solubility product of calcium oleate given by Fuerstenau and Palmer,24viz., pKsp = 15.6, it is seen that Q exceeds KBpat pH 8 but that at pH 10 the decrease in [Ca2+loutweighsthe increase in [RCOO-1 such that Q is less than Kspexcept at high [RCOO-I. In other words, precipitation of calcium oleate in solution at low levels of oleate is expected to occur only around pH 8. But at high concentrations, precipitation will occur over the range pH 8-10, No precipitation is expected at pH 6. In summary, the chemistry of the dissolved and suspended solution species suggests: (i) At pH 6, the major adsorbate will be un-ionized oleic acid and adsorption on the surface of apatite may occur during collisions between emulsion droplets and mineral particles. Alternatively, if the minor species RCOO- were more strongly adsorbed it could still be the major adsorbed species. (ii) At pH 8, there are four potential adsorbates, viz., liquid RCOOH, RCOO-, (RCOOZ)~-, and calcium oleate precipitate. Uptake of oleate by the mineral sufaces could occur during collisions between RCOOH droplets or Ca(OOCR)2precipitates and mineral particles or by diffusion of the ions RCOO- and (RC00h2-to the mineral surfaces. (22)Pugh, R.Stenius, P. Int. J. Miner. Process. 1985, 15, 193. (23)Somasundaran, P.; Amankonah, J. 0.;Ananthapadmabhan, K. P. Colloids Surf. 1985, 15, 309. (24)Fuerstenau, M. C.;Palmer, B. R. In Flotation, A.M. Caudin Memorial Volume; Fuerstenau, M. C . , Ed.; AIME: New York, 1976;Vol. 1, p 148.
124 Langmuir, Vol. 8, No. 1, 1992
(iii)At pH 10and at low [RCOO-I, the major adsorbates will be the ions RCOO-and (RC00)22-. At high [RCOO-I, a precipitate of calcium oleate will also be present. It is useful to compare the species in solution with the species adsorbed on surface as this can give clues to the adsorption mechanism. The value of the concentration product [Ca2+][RC00-I2 given in Table I indicate that calcium oleate precipitate will not occur in solution at pH 6 at all concentrations but will occur at pH 8 and 10 at concentrations greater than 2 X 10-5 mol L-l. This agrees with the adsorbed oleate species on apatite surface shown by the DRIFTspectra (Figures 3c,d, 4b,c, and 5). However, at oleate concentration 2 X mol L-l, calcium oleate precipitate occurs in solution at pH 8 but does not adsorb on the apatite surface (Figure 4a). Conversely, calcium oleate precipitate does not occur in solution at pH 10 at low oleate concentration but adsorbs on apatite (Figure 3a). This indicates that the speciesadsorbed are not always those occurring in the bulk solution, especially at low solution concentration when the driving force for the species to adsorb from solution onto the surface is small. Under such conditions, other factors must play an important role. As mentioned above, calcium oleate molecule prefers to adsorb with the positive end of its dipole toward the negative sites on apatite surface. At pH 8, the number of the negative surface sites on apatite is small, and this may explain why very little calcium oleate precipitate was adsorbed at the low concentration (2 X mol L-'). In summary, the mechanism of the adsorption of oleate depends not only on the species distribution in solution but also on the surface structure of the adsorbents. As a typical salt type mineral, apatite is characterized by fairly high solubility in water. Therefore, in addition to the chemisorption of oleate, the adsorption of calcium oleate precipitate plays an important role. The adsorption of calcium oleate surface precipitate is assisted by the iondipole interaction and the hydrocarbon chain association and is much stronger than the adsorption of oleic acid monomer and/or oleic acid dimer. Thus the adsorption of oleic acid was excluded by the adsorption of calcium oleate precipitate at pH 8. At pH 6, there is no calcium oleate precipitate in the bulk solution but there is substantial adsorption of oleic acid dimer and monomer, presumably onto the underlying chemisorbed oleate.
Conclusions DRIFT spectroscopy has been proved to be a better technique than the transmission infrared technique in the
Gong et al.
study of the adsorption of oleate on calcium minerals. Misinterpretation of the transmission infrared spectra of the adsorbed oleate on calcium minerals in previous studies was found due to the interaction between the sample and KBr matrix and the relative insensitivity of the transmission infrared technique. The confusion in the previous studies is clarified and a correct description of the nature of the adsorbed oleate species on calcium minerals and a better understanding of the adsorption mechanism obtained. Chemisorbed oleate corresponds to a single peak at 1550cm-' and probably comprises one oleate ion bonding with one lattice calcium ion on the surface. The structure of surface calcium oleate precipitate is similar to that of bulk calcium oleate, both comprising two oleate ions bonding with each calcium ion and showing double peaks at 1576 and 1539 cm-'. Oleic acid dimer and monomer adsorb via hydrocarbon chain association onto chemisorbed oleate species and correspond to a sharp peak at 1713cm-' and a shoulder at 1732 cm-l, respectively. The adsorption of surface calcium oleate precipitate is assisted by the ion-dipole interaction and hydrocarbon chain association and is much stronger than the adsorption of oleic acid dimer and monomer. Chemisorption of oleate on apatite occurred under all conditions studied and was accompanied by physical adsorption of calcium oleate precipitate andlor oleic acid monomer andlor oleic acid dimer, depending on the pH and concentration of the solution. At pH 6, the chemisorption was accompanied by the adsorption of oleic acid dimer and monomer, At pH 9.8, it was accompanied by the adsorption of calcium oleate precipitate. At pH 8, the adsorption of calcium oleate precipitate occurred only at high oleate concenmol L-l). tration (above 2 X
Acknowledgment. The financial support provided by the Minerals and Energy Research Institute of Western Australia (MERIWA), in conjunction with CSBP and Farmers Ltd., and the University Research Studentship awarded by the University of Western Australia are gratefully acknowledged. This work forms part of Gong Wen Qi's PhD thesis. Registry No. Apatite, 64476-38-6; sodium oleate, 143-19-1; calcium oleate, 142-17-6.
