Langmuir 1990, 6, 637-644
637
has shown that the surface diffusion process is promoted by the addition of alcohols. Octane, however, does not change the interface of a micellar aggregate.
gium Ministry of Scientific Programmation and the FKFO (Belgium) for continuing financial support.
Acknowledgment. H. Luo is indebted to K.U. h u ven for the Ph.D. grant. M. Van der Auweraer is a Research Associate of the N.F.W.O. S. Reekmans is a Research Assistant of the N.F.W.O. We thank the Bel-
21-8; c,,pyci, 2785-54-8; c,oH, 71-36-3; C,OH, 111-70-6; 3C,OH, 589-82-2; C,OH, 111-87-5;C,,OH, 112-30-1;~-c,oH, 62899-9; 1-methylpyrene,2381-21-7;octane, 111-65-9;dodecane, 112-
Registry No. SDS, 151-21-3;DTAC, 112-00-5;ClOPyCl,1609-
40-3.
Adsorption of Oleate Species at the Fluorite-Aqueous Solution Interface R. Sivamohan,+ P. de Donato,$ and J. M. Cases**$ Technical University of Luleii, S-951.87, Sweden, and Centre de Recherche sur la Valorisation des Minerais de 1'E.N.S.G. et U A 235, "Min6ralurgie" d u CNRS, BP 40, 54501 Vandoeuure, C6dex France Received July 13, 1989. In Final Form: October 26, 1989 Adsorption of oleate onto fluorite was measured as a function of the pH and residual concentration. Diffuse reflectance associated with the FTIR spectroscopy, { potential measured by electrophoresis, and turbidity have been used for the identification of adsorbed oleate at the fluorite surface. The adsorption isochrones are characterized by four distinct regions. At low oleate concentrations (region I), the oleate ions adsorb on the surface through Na+ ions or monocoordinated calcium ions and the adsorbed layer is filled with hemimicelles. At higher concentrations (region 11) and at pH 9.45, for instance, a surface calcium oleate precipitate region appears with a significantly higher adsorption density, reaching a mixed bilayer made of sodium oleate, calcium oleate, and carboxylic acid in a solid state. In these regions, absorbances of all the bands investigated (hydrocarbon chains and carboxylate groups) are linearly correlated with surface coverage. In regions I11 and IV, a change in absorbance (sharp or monotonic decrease according to the pH) indicates a change in the state of the adsorbed layer.
Introduction Characterization of the aqueous oleate/salt type mineral interface is a matter of controversy. At present, various mechanisms have been proposed, namely, physical or electrostatic interactions, chemisorption,'.' surface precipitation, chemisorption followed by surface recipitation, surface precipitation reaching multilayersJ4 and the involvement of dimers as well as ionomolecular comp l e x e ~ .A~ study to elucidate the involved mechanisms employing a direct surface-probing technique has been, therefore, carried out, using fluorite as a model system. A very early infrared study of the oleate adsorption/ abstraction on fluorite by Peck and Wadsworth was reported by Aplan and Fuerstenau.2 The infrared spectra of oleate-covered fluorite were claimed to provide the t
Technical University of Lulel.
t Centre de Recherche sur la Valorisation des Minerais de 1'E.N.S.G. (1)Peck, A. S. U.S.Bureau of Mines. Report of Investigation, 1963, 602,p 16. (2)Aplan, F. F.;Fuerstenau, D. W. Froth Flotation: 50th Anniuersary Volume;Fuerstenau, D. W., Ed.; A.I.M.E.:New York, 1962,p 215. (3)Hu, J. S.; Misra, M.; Miller, J. D. Int. J. Miner. Process. 1986, 18,57. (4)Hu, J. S.;Misra, M.; Miller, J. D. Int. J. Miner. Process. 1986, 18,73. ( 5 ) Somasundaran, P.; Ananthapadmanabhan, K. P. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979, p 17.
0743-7463/90/2406-0637$02.50/0
evidence for the presence of both oleic acid and calcium oleate for pH values less than 8. It was also claimed that the oleic acid on the fluorite surface disappeared, leaving only the calcium oleate, when the mineral was washed with acetone. Peck and Wadsworth also argued that the chemisorbed calcium oleate could be distinguished from the bulk calcium oleate by the differences in the infrared bands of bulk calcium oleate and the oleate attached to the surface. The depths of the infrared bands which were thought to characterize the physisorbed oleic acid and chemisorbed oleate were used to quantify the amount of oleate on the surface a t different pH values. It was concluded that physisorbed oleate did not aid flotation. Moreover, the amount of chemisorbed oleate passed through an apparent maximum at pH 9, and this was considered to result from interaction of fluorite with dissolved carbonate ions a t high pH values. More recently, using adsorption data, induction time, and contact angle to characterize the adsorbed oleate on fluorite surface, Hu et aL3 observed an adsorption isotherm characterized by three distinct regions: (1)a chemisorbed plateau region, (2) a surface calcium oleate precipitation with a significantly higher adsorption density reaching an adsorption maximum, and (3) a region which exhibits a decrease in adsorption density at higher equilibrium oleate concentrations. They thought that adsorption seems to involve a chemisorbed layer at low equilibrium oleate concentrations which can be distin0 1990 American Chemical Society
638 Langmuir, Vol. 6, No. 3, 1990
Sivamohan et al.
guished from a surface calcium oleate precipitate at M. In a equilibrium concentrations exceeding 5 X second paper,4 they used FTIR spectroscopy to elucidate an increase in the hydrophobic character of this system. They found that modest changes in the adsorption density at high temperature and high oxygen potential do not provide a satisfactory explanation for a significant increase in the hydrophobic character of the adsorbed layer. Their results indicate that the fluorite surface catalyzes an oxidation and/or a polymerization reaction between adsorbed oleyl groups. Apart from the above reported work, there does not seem to have been detailed work dealing with the direct probing of an aqueous oleate/ salt type mineral interface. The aim of our work is to fill this gap.
Experimental Section Materials. The fluorite used here was supplied by Indesko, Stockholm. The material contained the following (%): 97 (minimum) CaF,, 1.0 SiO,, 0.05 Pb, 0.02 S, 0.5 Fe,O, and A1,0,, as well as traces of CaCO,. Grinding was done in a ceramic ball mill, and the ground materials were dried a t =60 "C, kept in plastic bags, and stored in desiccators. The particle size distribution curve obtained by a Sedigraph particle size analyzer after dispersing in a 0.1 % sodium hexametaphosphate aqueous solution showed that 80% of the particles were less than 20 pm, 50% less than 10 pm, and 5% less than 1 pm. The specific surface area of the fluorite, determined by nitrogen adsorption using the single-point differential method according to Haul and Dumdgen (D.I.N. 66132) with a Area-Meter I1 from Strohlein, was found to be 1.14 m2 g-'. Sodium oleate (C17H3, COONa) from Prolabo, France, was used throughout. Sodium oleate solutions of required concentration were prepared, weekly, by the addition of sodium oleate to a hot (e60 "C) alkaline (=pH 9.45, adjusted by KOH) solution. The critical micellar concentration was found to be 1.67 X M at pH 9.5 and 22 O C 6 The deionized water used had a specific conductivity of 0.7 mS m-l a t 25 "C. All the experiments were carried out a t the CRVM/ CNRS laboratory of Nancy, France. Techniques. Adsorption/Abstraction Experiments. In these experiments, 3.75 g of fluorite particles was first conditioned for 2 h with deionized water in 50-mL adsorption bottles. The suspension was mixed with a magnetic stirrer a t low, constant speed. After 2 h of conditioning, the pH was adjusted to the required value. Isochrones were determined at two pH values-pH 9.45 0.1 and p H 10.5 0.1. Soon after pH adjustment, oleate was added to the suspension in such a concentration as to give a total aqueous volume of 50 mL. The concentration of oleate solutions were such that the initially added volumes of water were never less than 35 mL. The suspensions were conditioned for 1 min more after the oleate addition. The pH was checked and adjusted as necessary, and the conditioning was continued for another 5 min. Two experiments were carried out a t every oleate concentration and pH value. The purpose was to use one set for the oleate analysis as well as the FTIR study and to use the other set for turbidity and {potential measurements. The samples used for the oleate analysis and FTIR study were allowed to stand for a few minutes a t completion of conditioning. Afterward, the supernatant containing finer particles were transferred t o centrifuge flasks. These were later centrifuged with a Beckman L8 55 M apparatus a t 47 000 G in order to obtain the supernatants required for the oleate analysis. What was left in the adsorption bottles contained the settled particles with some amount of liquid; this was transferred into small glass bottles and was allowed to air-dry for the FTIR study. The oleate residual concentrations in the supernatant were determined by the spectrophotometric method of Gregory.' The oleate adsorbed/ abstracted was determined from the difference between the initial and residual oleate concentrations. The amounts adsorbed
*
Figure 1. Adsorption isochrone of oleate on fluorite at 25 "C and pH 9.45.
Figure 2. { potential and turbidity as a function of residual concentration for pH 9.45 and temperature 25 "C.
1
cs
1
t CMC
*
(6) Jacquier. P. These de docteur-ingbnieur INPL, Nancy, France, 1982, p 211. (7) Gregory, G. R. E. C. Analyst 1966, 251.
Figure 3. Adsorption isochrone of oleate on fluorite a t 25 "C and pH 10.5. (Qads) are plotted in Figures 1 and 3 either in pM.g-' or as surface coverage (e). It was assumed, in the latter case, that the molecular cross sectional area of oleate adsorbed on the surface is 20.5 (hydrated crystal state).' Calcium Concentration in the Supernatant. I t was necessary to determine the Ca ion content in different supernatants in absence of surfactant in order to know the theoretical (8) Cases, J. M.; Poirier, J. E.; Canet, D. Solid-liquid Interactions in Porous Media; Cases, J. M., Ed.; Technip: Paris, 1985, p 335. (9) Bellamy, L. J. The infrared Spectra of Complex Molecules; Chap man and Hall:London, 1975. (10) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: London, 1975.
Langmuir, Vol. 6, No. 3, 1990 639
Adsorption of Oleate Species Table I. Calcium Content in the Supernatants Obtained by Different Types of Conditioning ~~
~~
~~
conditions 1 2
3
4
1 g L-' (conditioned for 2 h,
open to atmosphere, conditioning pH was the natural pH) 75 g L-' (as in 1) 75 g L-' (as in 2, but pH was adjusted to 9.45 at the end of 2 h and conditioned for a further 5 min) 75 g L-' (as in 3 except for the adjusted pH of 10.5)
Ca content, M 1.125 X lo-' 2.300 x 10-4 1.600 x 10-4
c
-50
1.425 X low4
limits of calcium oleate precipitation. The results of such Ca analyses are given in Table I. The analyses were conducted by using flameless atomic absorption spectroscopy (Perkin Elmer Model 5100). Turbidity Measurements. The measurements were made on the supernatant of the fluorite suspensions within 10 min of decantation from the time of completion of the adsorption/ abstraction experiments. As soon as the turbidity measurements were completed, the suspensions in the turbidity bottles were returned to their respective adsorption bottles and taken for { potential measurements. The turbidity of the solution was measured with a Hach 2100 A turbidimeter. The results are expressed as formazine turbidity units (FTU). The turbidity for all four standards is based on formazine dilution. The standards are rated a t 0.61, 10, 100, and 1000 FTU and are contained in sealed glass tubes. The first is a liquid chlorobenzene solution, and the last three are liquid latex solutions. { Potential Measurements. The electrokinetic measurements were conducted with a Laser Zee Meter Model 501 equipped with a video system. FTIR Measurements. The FTIR spectra were obtained from the sample surface diffuse reflectance collected by a Harrick accessory. The spectra were analyzed with use of a Bruker IFS 88 Fourier transform infrared spectrometer equipped with a large-band mercury-cadmium-telluride (MCT) detector cooled to 77 K. The sample preparation for FTIR spectra involved mixing about 70 mg of the sample with a fixed quantity (370 mg) of KBr. The absorbance unit used corresponds to the decimal logarithm of the ratio of the reflectance of the pure finely powdered KBr, used as a reference, to that of the sample. Our attempt to quantatively estimate the oleate adsorbed on a sample in a Kubelka Munk unit was not successful. Therefore, we decided to express the absorbance as defined above. Each sample was scanned 200 times a t a resolution of 4 cm-' throughout the range 4000-600 cm-'. The influence of atmospheric water was always substracted.
Results The adsorption isochrones for oleate on fluorite at 25 "C are shown in Figures 1 and 3. The adsorption isochrones have four distinct adsorption regions irrespective of the pH of the solutions. An interesting feature of the isochrones is the presence of a plateau corresponding to about two monolayers, assuming that the adsorbed layer is in a hydrated crystal state. In region I at pH 9.45, the amount adsorbed increases linearly with the decimal logarithm of the residual concentration (C,),indicating that the filling of the adsorbed layer is due to the two-dimensional condensation on heterogeneous surfaces." In region 11, one observes a sharp increase in t h e a m o u n t adsorbed from a surface cover-
age value of 0.9 to the bilayer followed by a plateau (region 111) in the range 3 X C C, < 6 X (M) and a new increase in the adsorbed amount in region IV. Increase in surface coverage in regions I and I1 is followed by a decrease in the ( potential from -18 to -64 mV at 3 X lo-* M, which corresponds to a minimum in the i- poten(11) Cases, J. M. Bull. Mineral. 1979, 102, 684.
>
-10
'"
residuarconcentration(M.I-')
'-
Figure 4. { potential and turbidity as a function of residual concentration for pH 10.5 and temperature 25 "C.
tial values (Figure 2). The turbidity value (Figure 2) is about 350 FTU without surfactant. This is followed by deep drop in turbidity to 28 FTU, when the mineral surface is only slightly covered by the surfactant. Thereafter the turbidity increases constantly with increasing surfactant coverage. A close relationship is seen, in regions I and 11, not only between turbidity and surfactant coverage but also between turbidity and { potential at the respective surfactant coverages (Figures 1 and 2) or the residual concentrations. The surface coverage, the ( potential, and the turbidity values obtained at pH 10.5 (Figures 3 and 4) generally exibit the same pattern as that seen at pH 9.45. But the relationships between these parameters and the respective surfactant residual concentrations are much closer at pH 10.5 than at pH 9.45. Another striking feature is the vertical step seen in region I1 of the isochrone (Figure 3), between the surface coverage values 0.56 and 2.1. The diffuse reflectance spectra of sodium oleate, calcium oleate, fluorite, and calcite were recorded for band assignments. As an example, the spectrum of calcium oleate is presented in Figure 5. The bands in the region from 3020 to 2800 cm-' are related to the hydrocarbon chain^.^ The terminal methyl groups give rise to two characteristic bands at 2962 and 2873 cm-', corresponding to asymmetrical (Y,) and symmetrical (Y,) stretching modes. The CH, groups also give rise to two characteristic bands at 2926 and 2853 cm-' corresponding to the out-of-phase and in-phase stretches. Another band of significance is that at 3010 cm-' assigned to the stretching vibration of the CH group adjacent to the double bond, v(=CH). The bands in the region ranging from 1800 to 1400 cm-' are mainly related to the carboxylate groups, except for the CH, deformation band, which comes near 1463 cm-', and the asymmetric CH, deformation, which absorbs weakly near 1430 cm-'.l0 In saturated or unsaturated aliphatic carboxylic acids, the carbonyl frequency lies in the range 1725-1705 cm-', when examined in the solid or in the liquid state. When a salt is formed by a carboxylic acid, the C=O and C-0 are replaced by two equivalent carbon-oxygen bonds whose force constant is intermediate between those of C=O and C-0: -C
4O ' 0
9-
c )
-
--o
Or
2 0
-%
These two bonds and half oscilltors are strongly coupled, resulting in a strong asymmetricCO, stretching vibration at 1650-1540 cm-' and a somewhat weaker symmet-
640 Langmuir, Vol. 6, No. 3, 1990
Siuarnohan et al.
strong asymmetric CO, stretching vibration with peaks at 1577 and 1555cm-l and the symmetric stretching vibration a t 1426 cm-' (Figure 6b). This spectrum also shows the band at 1700 cm-', which is characteristic of the carboxylic acid. Calcium oleate (Figure 5) was found to show two distinct bands at 1576 and 1540 cm-l and also a small band for the weaker symmetrical stretching vibration at 1420 cm-'. The difference in wavenumber between the two peaks of the asymmetric stretching vibration irrespective of pH is 22 cm-l for sodium oleate from Prolabo and 34-37 cm-' for calcium oleate. The spectrum of fluorite used as reference presents a peak at 1417-1420 cm-' characteristic of the CaCO, impurity (CaCO, content is less than 0.3%) and a series of bands a t 1869,1792,1608,1510,1096,1058,799,780, and 695 cm-' assigned to quartz. Other anomalies are due to water (the broader band around 3200-3600 cm-') and atmospheric carbon dioxide (the split band at 2350 cm-' and the band at 667 cm-').
c c35,
C GC
J I
I
I
I
I
XC4
3000
l
l
I
I
2200 I800 WAVENUMBERS CM-I
I 1100
I
I I000
,
, 600
Figure 5. Diffuse reflectance infrared spectra of calcium oleate prepared by precipitation in stoichiometric conditions of sodium oleate and calcium chloride at 25 "C and pH 9.45 (a) and 10.5 (b).
800
i700
moo
mo
,400
A
NAVENUMBERS C M - I
Figure 6. Infrared spectra of sodium oleate: (a) diffuse reflectance spectrum of product in powder obtained from Ruedel and de Haen (dilution 290); (b)diffuse reflectance spectrum of product in powder obtained from Prolabo (dilution 2%); (c) transmission spectrum of sodium oleate solutions (4 X 10-1M) after subtraction of water-liquid film between two CaF, plates.
ric stretching vibration a t 1450-1360 cm".lo In fact, sodium oleate of Analar grade (Ruedel-De Haen) was found to show the COO- absorption at 1560 and 1425 cm-' (Figure 6a). On the other hand, the spectrum of the sodium oleate sample from Prolabo presents a slight split in the
Discussion For a better understanding of all the phenomena that can be involved in the adsorption/abstraction of oleate by fluorite, the possibility of surface precipitation of calcium oleate has to be deduced from solubility considerations. The pK values reported, by different authors, for calcium oleate precipitation are not the same. The values are 12.4,6,12,'314.13,14 14.85," 15.6,16and 15.71,, implying that the values of the solubility product can differ by as much as 3 orders of magnitude. The value of 12.4 has been chosen in this case because that value has been previously obtained by Jacquier' with sodium oleate of the same origin. Taking into account the results of Table I, the oleate concentration necessary for the onset of the precipitation of calcium oleate at pH 9.45 and 10.5 are 5.0 X lo-' and 5.29 X lo-' M, respectively. The limits of precipitation are marked on the isochrones in Figures 1 and 2 by an arrow noted Cs (saturated concentration of calcium oleate). If the largest pK value of 15.71 is considered, then the corresponding oleate concentrations for the onset of precipitation would be 1.10 X lo* and 1.17 X IO* M. This would mean that calcium oleate precipitation would be possible even at the lowest residual oleate concentrations in our investigation. But, both the fluorite/oleate isochrones at pH 9.45 and 10.5 show a discontinuity around a residual concentration value corresponding to 8 X M (borderline between regions I and 11). Thus, one can make the supposition that the sharp increase in surface coverage in region I1 is due to the surface precipitation of calcium oleate. In region 111, the surface coverage does not increase to form multilayers, because the amount of calcium precipitated with calcium oleate involving the dissolved Ca corresponds to only a part (maximum 66 % , minimum 45 % ) of the quantity required to form the second layer (example for pH 9.45). Indeed, the maximum total calcium content in a flask is 11.5 X lo* mol at pH 9.45, and the amount of oleate ions needed to form the second layer is 34.6 X mol. This means that the bilayer is filled by sodium oleate and calcium oleate. (12) Leja, J. Surface Chemistry of Froth Flotation; Plenum Press: New York, 1982. (13) Du Rietz, C. Progress in Mineral Dressing; Almquist and Wiksell: Stockholm, 1958. (14)Marinakis, K. I.; Shergold, H. L. Int. J . Miner. Process. 1985, 14, 161. (15) Du Rietz, C. In Proceedings of the I I t h International Mineral Processing Congress, Cagliari, 1975; Instituto di Arte Mineraria, Universita di Cagliari: Cagliari, Italy, 1976; p 375. (16) Fuerstenau, M. C.; Miller, J. D. Trans. A.I.M.E. 1967, 238, 153.
Langmuir, Vol. 6, No. 3, 1990 641
Adsorption of Oleate Species
9_
w
e - 205
e
-
1.99
e = 1.60 8-120
e
0.83
e = 0.61 e = 027
A I
IWO
1700
1600
I '500
NAVENUMBER
I
1400
I 1300
CM-1
I
3100
I
3000
I
2900
I 2800
e-0.10
27b0
WAVENUMBERS CM-1
Figure 7. Diffuse reflectance infrared spectra of fluorite after adsorption of oleate at pH 9.45 and at different surface coverages: (a, left) region 1300-1800 cm-'; (b, right) region 2700-3100 cm-'.
Region I. The monotonic increase of surface coverage with residual concentration can be explained by the theory of two-dimensional condensation on a heterogeneous surface."^'' The origins of the association of adsorbed ions forming two-dimensional aggregates called hemimicelles'8 are the strength of lateral bonds leading to two-dimensional condensation and the surface heterogeneity controlling the size of aggregates. But, what is the origin of the normal adsorbate/adsorbent bond in this region? Does region I consist of chemisorbed oleate bound to the lattice sites at the fluorite surface?'-3 In the region ranging from 1300 to 1800 cm-', the diffuse reflectance infrared spectra of fluorite show the following changes (Figures 7a and 8a): at low surface coverage (0.2 < e), the peak assigned to quartz at 1508 cm-' presents on its high-frequency side a shoulder at 1560-1550 cm-'. For the highest values of surface coverage (0.2 < 0 < l ) , the normal carbonyl absorption corresponding to the asymmetric mode occurs between 1560 and 1551cm-' (this mode is more of diagnostic value because the symetric mode, v,, is hidden by calcite). The measured widths of this band at the top of the peak never exceed 20 cm-'. This suggests that the carboxyl group, in the adsorbed layer, is in the same environment as sodium oleate in the solid state (Figure 6b). Fluorite presents a perfect cleavage plane (111) in which the mean molecular area per calcium ion is 21 A.2 This value is equal to that retained for the alkyl chain of sodium oleate in the (17) Cases, J. M.; Levitz, P.; Poirier, J. E.; Van Damme, H. Advances in Mineral Processing; Somasundaran, P., Ed.; S.M.E.: Littleton, 1986. (18) Gaudin, A. M.; Fuerstenau, D. W. T r a m . A.I.M.E. 1955, 202, 958.
hydrated crystal state (trans conformation of the C-C bonds). Two hypotheses can be proposed: either the ionized carboxyl groups form chemical bonds with the calcium sites on the surface after ion exchange with the OH groups, with the ions monocoordinated on calcium sites, or the carboxyl groups adsorb on the surface through the sodium counterions. This latter hypothesis could explain the decrease in { potential with increasing surface coverage (or residual concentration). Region 11. Referring to region I1 in Figures 1 and 3, the adsorption densities and the magnitude of the { potential in this region increase sharply up to the bilayer and residual concentrations of 3 X M at pH 9.45 and 1.3 X M at pH 10.5. The increase of the turbidity is due to the bilayer formation that enhances the stability of the solid suspension. In this range, the width of the band at the top of the peak, corresponding to the asymmetric mode of the carbonyl absorption of the ionized carboxyl groups, increases to 35 cm-l, and one observes (Figures 7a and 8a) two or three bands at 1540-1542, 1559-1561, and 1575-1577 cm-l. As predicted before by us from the solubility product of calcium oleate, surface precipitation contributes to adsorption phenomena in this region. The diffuse reflectance spectra when compared with the reference spectra presented in Figures 5 and 6 show that the bilayer is filled with calcium oleate, sodium oleate, and (at pH 9.45, as indicated by an anomaly at 1695 cm-') also with a small quantity of carboxylic acid. In order to establish the relationship between the amount of oleate adsorbed/abstracted and the absorbance corresponding to different bands of the hydrocarbon chains (3007, 2962, 2925, 2854, 1465 cm-') or the carboxylic or
642 Langmuir, Vol. 6, No. 3, 1990
Sivamohan et al.
e - 331
Y
9
4
a
'68
- . , =.".
P--
2 3 n
i
e
393
\
a
3 1 ~
5
152
00
e
293
-
243
0.225
\
8-093
1
8-074
9.056
9-052
e e
--
0.34 0.31
8-025
-
e-011 e 0.01
sw NAYENUMBERS
CU-I
WAVENUMBERS CM-1
Figure 8. Diffuse reflectance infrared spectra of fluorite after adsorption of oleate at pH 10.5 and at different surface coverage: (a, left) region 1300-1800 cm-'; (b, right) region 2700-3100 cm-l.
carboxylate groups (1700,1571,1561,1543cm-l), the base lines were drawn to be tangent to the base of the spectra at the two ends of 3050-2750 cm-' (for the region related to the hydrocarbon chains) and at 1750-1350 cm-' (for the carboxylate groups; Figure 7a,b, 0 = 1.99). The difference in absorbance between the peaks and the base line was measured. The carboxylate region needs some correction because of the quartz and calcite impurities. These corrections were calculated with the use of the following equation:
I ( 8%
,P=Y)&d
I
I
r
I
= I(8 = x ,V=Y)memd -
1(8=0,v=y) [I(O=X,V=I~OO cm-1)/I(0=0,v=1900cm-')I
where x is the considered spectrum for the indicated surface coverage and y the frequency of the band which is corrected. The frequency 1900 cm-' was chosen because the oleate spectrum presents no absorbance at this value. Absorbance and surface coverage in regions I and 11, i.e., 0 < B < 2.0, are linearly correlated for all frequencies (see Figures 9-12 and Table 11). I t is seen from Table I1 that the slopes corresponding to different band frequencies are independent of the pH of adsorption experiments. In fact, the intensities of the bands are directly related to the amounts of the various chemical groups (CH,, CH,, COOH, and CO-) that make up the adsorbed/ abstracted layers. Regions I11 and IV. In region 111, surface coverage remains constant. Turbidity increases or reaches a plateau due to the bilayer formation and the corresponding enhancement of the stability of the solid suspension. At higher oleate concentration (region IV), surface coverage increases but { potential and turbidity remain con-
surface coverage
Figure 9. Absorbance of the bands related to the hydrocarbon chains versus surface coverages at pH 9.45: 3007 cm-' (A); 2962 cm-' ( 0 ) ;2925 cm-' (+); 2854 cm-' (X); 1465 cm-I (0).
stant. That increase should be an artifact due to either bulk precipitation or micelle formation, these phases being centrifuged during the adsorption experiment and wrongly considered as adsorbed phases. The formation of stable micelles in solution should keep the density of adsorption constant and explain the constancy of turbidity and {potential. When the surface coverage is apparently higher than 2, the absorbance of all the bands investigated decreases either sharply at pH 9.45 (Figures 9 and 10) or pH 10.5 (Figure 12) or linearly with surface coverage
Langmuir, Vol. 6, No. 3, 1990 643
Adsorption of Oleate Species
surface coverage
Figure 10. Absorbance of bands related to the carboxylate groups versus surface coverage at pH 9.45: 1700 cm-' (+); 1571 cm-' (H); 1561 cm-' (a);1543 cm-' (0).
0,100
i
0.l00L
0.100
surface coverage
Figure 12. Absorbance of bands related to the carboxylategroups versus surface coverage at pH 10.5: 1571 cm-' (H); 1561 cm-' (a);1543 cm-' (0).
tion would decrease the cmc from 1.67 X to 6.70 X low4M, due to the adsorption of calcium ions on the micelles. Micelles cannot be present in the adsorbed layer, since this would have shifted the frequency characteristics of carbonyl absorption corresponding to the asymmetric mode to 1548 cm-' (Figure 6c).
I
Conclusions
surface coverage
Figure 11. Absorbance of bands related to the hydrocarbon chains versus surface coverage at pH 10.5: 3007 cm-' (A);2962 cm-' (0);2925 cm-' (+); 2854 cm-' (X); 1465 cm-' (0).
a t pH 10.5 (Figure 11). The region of the spectra related to the carboxylate groups presents mainly, at pH 10.5, either a narrow band at 1561 cm-' (Figures 7a and Ba), characteristic (as shown in Figure 6a) of a variety of sodium oleate in the solid state and in an ordered environment, or a broad band (width 37 cm-') made of one peak at 1560 cm-' and two weaker peaks at 1540 and 1577 cm-' characteristic of an adsorbed layer made with sodium (mainly) and calcium oleate in the solid state. The enrichment of the adsorbed phase in sodium oleate seems to be due to the release of calcium ions from the adsorbed layer. Since in these regions surface coverage remains constant or increases with residual concentration, it is impossible to ascribe the decrease in absorbance of all the bands investigated to an eventual decrease of the amount of oleate adsorbed. The sharp (at pH 9.45) or monotonic (at pH 10.5) decrease could correspond to a change in the state of the adsorbed layer. As the cmc will be very sensitive to the nature and charge of the counterions," the presence of calcium ions in the solu(19)Shinoda, K.; Tamamushi, B. I.; Nakagawa, T.; Isemura, T. Colloidal Surfactants;Academic Press: New York, 1963.
The major results of the investigation characterizing the fluorite/oleate interface in an aqueous environment can be summarized as follows: diffuse reflectance associated with FTIR spectroscopy is a quick, sensitive, and nondestructive technique of characterizing the nature of the adsorbed surfactant layers and, therefore, shows great potential in applied surface chemistry. The mechanism of oleate adsorption/abstraction on fluorite involves the following stages: (a) At low residual oleate concentrations, the adsorbed layer is filled with hemimicelles. These patches result from a two-dimensional condensation on a heterogeneous surface. The adsorbed layer resembles a solid state made of monocordinated sodium oleate or calcium oleate. (b) At higher oleate concentrations in regions I1 and 111, beyond the line of calcium oleate precipitation, calcium oleate precipitates on the surface. Dissolved Ca ions present in nearly equilibrated aqueous fluorite suspensions of very fine particles cannot cause precipitation of calcium oleate to the amount necessary to form multilayers. The precipitation of calcium oleate is not possible in the bulk either. The mineral surface is covered with only two monolayers of oleate. The bilayer has a lamellar structure. The hydrocarbon chains form the superstructure with the polar groups lying along the interface with the solid (first monolayer) and the water (second layer). The value of the oleate cross section area taken into account (20.5 A2)and the comparisons made between the diffuse reflectance spectra of the oleate coating the mineral surface and the reference spectra show that the bilayer is in a solid state (trans conformation of the C-C bonds). The arrangement is such that the hydrocarbon chains are perpendicular to the interface and the bilayer is approximately two hydrocarbon chain lengths thick.' The bilayer is a mixed layer made of calcium oleate, sodium oleate, and (at pH 9.45) a small quantity of
Langmuir 1990, 6, 644-649
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Table 11. Slope ( a ) ,Intersect at the Origin ( b ) ,and Correlation Coefficient ( p ) of the Different Linear Relationships Obtained between Absorbance and Surface Coverage pH 9.45, 0
