Langmuir 2000, 16, 8129-8133
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Xanthate Adsorbed on ZnS Studied by Polarized FTIR-ATR Spectroscopy M. L. Larsson,* A. Holmgren, and W. Forsling Division of Inorganic Chemistry, Luleå University of Technology, SE-971 87, Sweden Received March 24, 2000. In Final Form: June 26, 2000 The structure and orientation of heptylxanthate adsorbed on a ZnS surface have been studied by the FTIR-ATR technique. By using polarized light and the dichroic ratio, we found the average tilt angle to be approximately 44 degrees. The adsorbed layer studied was prepared by self-assembly from solution or by spraying the solution onto the surfaces of the ATR crystal for a short time and then rinsing with water. From the spectra we can conclude that there is a mixture of adsorbed heptylxanthate and formed diheptyldixanthogen on the ZnS surface. A bridging coordination of the adsorbed heptylxanthate is proposed.
1. Introduction Xanthates are the most commonly used collectors in the flotation of sulfide minerals such as galena, chalcocite, pyrite, sphalerite, etc. The flotation characteristics of these minerals have received considerable attention during the past decades in an effort to gain a better understanding of the factors influencing their flotation behavior.1,2 Thus, the effects of acyl chain length, activators, oxygen content, zeta potential, and pH on the flotation recovery have been investigated. It has been shown, for example that the formation of zinc xanthate on the sphalerite surface is necessary for its flotation in the absence of activators.3 In that investigation the precipitate was readily identified by infrared analysis, and no dixanthogen could be detected on the sphalerite surface. A number of investigations by vibrational spectroscopy have been performed to study adsorbed alkylxanthates on sphalerite, synthetic metal sulfides, or metal sulfide crystals.4-7 Most of these studies are concerned with whether the xanthate is precipitated on the surface. However, only a few studies have focused on the orientation of the adsorbate on the metal sulfide surface.7,8 The structural properties of the adsorption layer are important for the interaction forces between the air bubble and the particle to be floated, because they may affect the stability of the wetting film separating the two particles.9 In this study, which is part of a larger investigation on adsorption properties of mineral surfaces (e.g., refs 10, * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Fuerstenau, M. C.; Miller, J. D.; Kuhn, M. C. Chemistry of Flotation; Society of Mining Engineers, AIME Inc.: New York, 1985. (2) Forsberg, Eric K. S., Ed. Flotation of Sulfide Minerals 1990. J. Min. Proc. 1991, 33 (1-4). (3) Fuerstenau, M. C.; Clifford, K. L.; Kuhn, M. C. Int. J. Min. Proc. 1994, 1, 307. (4) Brienne, S. H. R.; Zhang, Q.; Butler, I. S.; Xu, Z.; Finch, J. A. Langmuir 1994, 10, 3582-3586. (5) Ga¨rd, R.; Sun, Z. X.; Forsling, W. Proceedings of the International Conference on Mineral Processings: Recent Advances and Future Trends, Kanpur, India, December 1995. (6) Valli, M. On the sorption of some Soft Ligands on Sulphide Mineral Surfaces. Doctoral thesis, Swedish University of Agricultural Sciences, Uppsala, Sweden, 1994. (7) Mielczarski, J. A.; Zachwieja, J.; Cases, J. M. Langmuir 1995, 11, 2787-2799. (8) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733. (9) Schulze, H. J. Physico Chemical Elementary Processes in Flotation; Elsevier: Amsterdam, 1983; pp 91-94.
Figure 1. Direction and polarization of the IR beam in the ATR setup.
11), we report on the orientation of heptylxanthate (RCS2-, R ) O-C7H15) on the surface of a ZnS crystal in the form of a trapezoidal attenuated total reflection (ATR) element. In addition to getting information about the preferred orientation of the adsorbate, the aim of the study was also to investigate possible formation of dixanthogen and/or precipitated zinc xanthate on the substrate surface. 2. Experimental Section 2.1. ATR Technique and the Dichroic Ratio. The geometry of an ATR setup can be seen in Figure 1. For a general description of the ATR technique, see ref 12. The x-, y-, and z-axes are those in the laboratory frame. To determine the molecular orientation on the surface, the dichroic ratio is used. The dichroic ratio, D, is defined as the ratio between absorbance of s-polarized light (normal to the plane of incidence) and of p-polarized light (parallel to the plane of incidence). The orientation of the molecule is determined by transforming the transition moment M from the molecular coordinate system into the laboratory frame and calculating the projection of the transition moment on the electrical vector, E, of the radiation. If we consider a specific transition moment making a known angle Θ to the molecular axis, we can transform this transition moment into the laboratory frame and calculate the tilt angle of the molecular axis. The molecular coordinate system and the laboratory coordinate system are shown in Figure 2. The z-axis is normal to the surface of the element, the x-axis is along the propagation direction of the IR beam, and the y-axis is along the direction of the s-polarization. The p-polarization is in the xz-plane. According to Figure 2, γ is the tilt angle of the molecular axis (c-axis) from the surface normal, φ is the angle between the c′-axis (the (10) Larsson, M. L.; Holmgren, A.; Forsling, W. Structure and Orientation of Collectors at the ZnS/Water interface. Presented at the Flotation 2000 conference, Adelaide. (11) Ivanov, A. V.; Antzutkin, O. N.; Larsson, A.-C.; Kritikos, M.; Forsling, W. Inorg. Chem. Submitted for publication. (12) Harric, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967; Chapter 2.
10.1021/la000454+ CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000
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Larsson et al. ratio is
D)
Ey2 As ) 2 Ap Ex + Ez2
(2)
For a perpendicular transition moment (e.g., symmetric and asymmetric stretching vibrations of CH2), the dichroic ratio is
D(γ0, Θ ) 90°) )
Figure 2. Laboratory axes (x, y, and z) and molecular axes (a, b, and c). Definition of angles γ, θ, φ, Θ. orthogonal projection of the c-axis in the xy-plane) and the x-axis, and θ is the angle between the a-axis and a line in the ozc′-plane. This line is also in the ab-plane. The transition moment M is parallel to the ac-plane and can be expressed as shown in note 13. To transform this vector into the laboratory frame, we need a transformation matrix. The whole transformation matrix is shown in note 14, which comes from ref 15. For a specific transition, Θ will be fixed and the components for the transition will be in the domain 0 e γ e π/2, 0 e θ e 2π, 0 e φ e 2π. The absorbance is a measure of the projection of M on the electrical field E and is given by Ai ) C∫γ∫φ∫θ(MiEi)2f1(θ)f2(φ)f3(λ) sin γdθdφdγ,16 where C is a normalization factor; i ) x, y, and z; and f1(θ), f2(φ), and f3(λ) are distribution functions. For a random distribution the distribution functions are f1(θ) ) f2(φ) ) f3(λ) ) 1, and for a self-assembled monolayer with a uniaxial distribution, the distribution functions are f1(θ) ) f2(φ) ) 1 and f3(λ) ) δ(γ-γ0), where γ0 is the preferred tilt angle and there are no preferred θ or φ angles. Distribution functions for other cases, and the whole theory for determining the tilt angle and how to choose orientation models, can be found in ref 16. The expressions for the electrical field amplitude Ei for a thin film on an ATR element with refractive index n1 can be found in ref 12. The dichroic ratio, in general, is
D(θ,φ,γ,Θ is fixed) )
As Ay(θ,φ,γ) ) ) Ap Ax(θ,φ,γ) + Az(θ,φ,γ)
Ey2(sin2γ0(2 - 3 sin2Θ) + 2 sin2Θ) (Ex + Ez )2 sin2Θ + (2 - 3 sin2Θ)(Ex2 sin2γ0 + 2Ez2 cos2γ0) (1) 2
2
For a random distribution of the adsorbed molecules, the dichroic (13)
(14)
Ey2(2 - sin2γ0) 2
Ex (2 - sin2γ0) + 2Ez2 sin2γ0
(3)
The measured absorbance always reflects the average tilt angle for all molecules or the average tilt angle for a transition moment on the surface. When γ0 ) 54.7°, the value of D is the same as for a totally random distribution of the sample. When Θ ) 54.7°, the dichroic ratio is independent of γ0. The transition moments most often used to determine the tilt angle are the symmetric and asymmetric stretching vibrations of CH2, which have Θ ) 90°. The asymmetric CH3 stretching vibration has Θ ) 54.7° and can be used to determine the refractive index of the film. The symmetric stretching vibration of CH3 has Θ ) 35.3°, but it could not be used in our calculations because of its low intensity. 2.2. Materials. The adsorbate used in this study was heptylxanthate synthesized from heptanol, KOH, and CS2. The product potassium heptylxanthate was first recrystallized from warm acetone/benzene and then from acetone/petroleum ether. After that, the potassium heptylxanthate was transferred to a vacuum desiccator containing a small beaker of concentrated sulfuric acid and heated active charcoal. The treatment in the vacuum desiccator was repeated twice, and then the product was stored under a vacuum. This method of purification of alkali metal xanthates is described in ref 17. The ZnS elements were delivered by Spectroscopy Central. The size of the trapezoidal element was 52 × 20 × 2 mm3, and the angle of the cut edges was 45° (R in Figure 1). This size allows approximately 25 internal reflections. One of the two ZnS elements was used as a reference to check the element used for adsorption, after cleaning. The spectrometer used was a Bruker IFS 113V (pressure ∼1 mm Hg), equipped with an MCT detector. 2.3. Preparation of Self-Assembled Layers. Xanthate solutions in the concentration range 0.01-0.1 M were prepared using Milli-Q water. The acidity of the solutions was measured to be in the pH range 8-9, depending on the concentration. Before each adsorption experiment, the element was washed with ethanol in an ultrasonication bath and subsequently washed with Milli-Q water. After being washed, the element was allowed to dry in a desiccator. Before scanning the background, the element was checked against the reference element. After 800 scans were added for each background of the different polarizations, the element was transferred to a beaker with freshly prepared xanthate solution; it remained there for 0.5 to 1.5 h. Next, the element was rinsed with Milli-Q water and dried in a stream of N2. The ATR element was then mounted in the sample holder and transferred to the spectrometer, which was evacuated, and 800 scans were accumulated at a resolution of 4 cm-1. Experiments in which the xanthate solution was sprayed on the elements and then sprayed with Milli-Q-water were performed as well.
3. Results and Discussion
The matrix is actually a product of three consecutive rotation matrices. The first rotation is -θ (positive direction is clockwise) about the c-axis, so that a will be parallel to the occ′-plane. The second rotation is -γ about the b-axis, so that the c-axis and z-axis will become parallel. The last rotation is φ about the c-axis, so that the a-axis and the x-axis become parallel. (15) Zbinden, R. Infrared Spectroscopy of High Polymers; Academic Press: New York and London, 1964; Chapter 4. (16) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952-9959.
3.1. Adsorbed Species. Figure 3 shows the ATR spectra of adsorbed heptylxanthate (heptylxanthate ) H15C7-O-CS2- ) HX) on the zinc sulfide surface. The conditioning time was 55 min and the concentration of the xanthate solution was 0.1 M. For comparison, the diffuse reflectance spectrum of potassium heptylxanthate (Figure 4), the spectrum of a thin film of diheptyldixanthogen (Figure 5), and the spectrum of precipitated Zn(HX)2 (Figure 6) are also shown. The assignments of (17) Rao, S. R. Xanthates and Related Compounds; Marcel Dekker: New York, 1971; Chapter 2.
Xanthate Adsorbed on ZnS
Figure 3. Infrared spectrum of adsorbed heptylxanthate on ZnS. In the left part of the figure, the upper spectrum is for s-polarized and the lower spectrum is for p-polarized radiation; in the right part of the figure, the upper spectrum is for p-polarized and the lower is for s-polarized radiation.
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Figure 5. Spectrum of a thin film of diheptyldixanthogen between KBr windows.
Figure 6. Diffuse reflectance spectrum of precipitated Zn(HX)2. Figure 4. Diffuse reflectance spectrum of potassium heptylxanthate, transformed using the Kubelka-Munk function.
infrared bands of metal xanthates and dixanthogen are not straightforward, and for a long time only empirical assignments were made. The absorption bands are highly coupled, and it is not possible to assign them to a specific vibration.6 We find the absorption bands originating from C-O-C and CS2 in the frequency region from 1270 to 1000 cm-1. Some theoretical studies of xanthates have been conducted,6,18,19 and Table 1 shows the assignments of solid potassium ethylxanthate. Empirical assignments of dixanthogen can be found in the literature, e.g., in ref 17. Of course, some ambiguity is involved in comparing absorption frequencies from adsorbed species with frequencies from the corresponding salt or a free molecule. However, only assignments that should be reliable are included in Table 1. The spectrum of adsorbed heptylxanthate shows a typical band from dixanthogen around 1257 cm-1 and also (18) Tossell, J. A. J. Colloid Interface Sci. 1993, 155, 98-107. (19) Colthup, N. B.; Powell, L. P. Spectrochim. Acta 1987, 43A (3), 317-322.
Table 1. Vibrations Giving Greatest Contribution to Absorption Bands in the Spectrum of Solid Potassium Heptylxanthate wavenumber, cm-1
assignment
reference
2956 2927, 2913 2874 2852 1470 1380 1098 1060
CH3 asymmetric stretching CH2 asymmetric stretching CH3 symmetric stretching CH2 symmetric stretching CH3 bending + CH2 bending CH3 bending +CH2 wagging CO stretching CS2 asymmetric stretching
19 19 19 19 19, 6 19a 18, 19 19, 6, 18
a This band is assigned to (CS + CO) symmetric stretching in 2 ref 6.
an absorption band at 1208 cm-1 (Figure 3). The infrared band at about 1042 cm-1 is due to an antisymmetric CS2 stretch mixed with the CH2 wagging mode.6 The intensity of the dixanthogen peak relative to other absorption bands in the spectrum is approximately the same for the experiments in which the heptylxanthate solution was sprayed on the ATR element as for those in which the ZnS crystal was immersed in the xanthate solution. Approximately the same amount of xanthate seems to be
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Table 2. Dichroic Ratio, Calculated Refractive Index, and Calculated Tilt Angles experimental settings
D(νa(CH3))
n2
D(νa(CH2))
γ
D(νs(CH2))
γ
t ) 55 min, C ) 0.1 M t ) 55 min, C ) 0.1 M t ) 50 min, C ) 0.01 M t ) 90 min, C ) 0.01 M sprayed, C ) 0.1 M sprayed, C ) 0.01 M
1.10 1.05 1.02 1.01 1.09 1.10 1.06 ( 0.04a
1.68 1.58 1.53 1.52 1.66 1.68 1.61 ( 0.07a
1.15 1.11 1.05 1.04 1.08 1.13 1.09 ( 0.04a
47° 47° 51° 52° 56° 51° 51° ( 3°a
1.14 1.15 1.07 1.07 1.05 1.16 1.11 ( 0.05a
49° 42° 49° 48° 60° 46° 49° ( 6°a
a
Mean value and standard deviation of the values in the column.
adsorbed on the surface in both cases. An estimation of the adsorption density, Γ, of xanthate is given in Table 3. The adsorption density was calculated by the use of the formula given in ref 20. The absorptivity 15 000 L/mol cm2, the mean value of the effective path lengths and the integrated absorbance of the asymmetric stretching of CH2, was used in the calculation. The density of zinc sites on a sphalerite surface is approximately 6.8/nm2, indicating roughly a monolayer coverage of the surface. The formation of dixanthogen on ZnS has not been observed in studies performed on natural or colloidal ZnS.5,6 The xanthates used in those studies were ethyl- and amylxanthate. In the spectrum of diheptyldixanthogen, there is also a strong absorption band at 1023 cm-1 which is not resolved in the spectrum of the adsorbed heptylxanthate, probably because of overlapping by other bands in the region. In solid Zn(HX)2, the coordination of xanthate is bridging between two different Zn2+ with a tetrahedral geometry around each zinc ion. The adsorption mechanism of xanthate on ZnS has been discussed previously, and a dissolution-precipitation mechanism was suggested,5,6 implying that zinc ions from the surface are dissolved and precipitated as Zn(HX)2 at the surface. This seems to be a plausible result at low pH values, but not at higher pH values, where the ZnS is very stable. The surface precipitate should exhibit nearly the same spectrum as the solid Zn(HX)2. However, the two spectra are rather different (Figures 3 and 6). In comparison with the spectrum of Zn(HX)2(s), some bands shift to higher frequency, others shift to lower frequency, and some appear as shoulders or are missing in the spectrum of heptylxanthate on the surface. This indicates the presence of adsorbed HX on the surface of the ATR element, in addition to observed dixanthogen. The shift of peaks around 1200 cm-1 has been taken as proof of chemisorbed species by some authors.5,8 Laajalehto21 has another explanation of the shifts for the peak(s) around 1200 cm-1, namely the degree of crystallinity in the sample. At high crystallinity, the peak would shift to a higher frequency and vice versa. In the present study, a peak position at 1208 cm-1 is observed for both adsorbed and solid Zn(HX)2, precipitated from solution. Valli and co-workers22 claim that two types of chemisorbed complexes are found on a ZnS surface treated with a xanthate solution. They suggest that if the infrared band at 1030-1040 cm-1 is shifted to a higher frequency, a monodentate coordination is more likely, and with no shift compared to the peak position in the spectrum of the solid Zn(HX)2, a bridging or bidentate coordination to the surface is dominating. According to our data, a bridging or bidentate coordination is the most probable, given that the infrared band at 1042 cm-1 has about the same position as in solid Zn(HX)2, (20) Lu, Y.; Drelich, J.; Miller, J. D. J. Colloid Interface Sci. 1998, 202, 462-476. (21) Laajalehto, K.; Nowak, P.; Suoninen, E. Int. J. Min. Proc. 1993, 37, 123-147. (22) Valli, M.; Persson, P.; Persson, I. Acta Chem. Scand. 1994, 48, 810.
although the absorption band of the latter species is split with peak positions at 1047 and 1035 cm-1. From the spectra of adsorbed heptylxanthate we therefore suggest that at least two types of species are present at the ZnS surface, one complex with adsorbed xanthate ions and diheptyldixanthogen. Experimental data supporting this suggestion are the peak positions at 1042, 1208, and 1257 cm-1. 3.2. Calculated Angles. Although at least two types of species are found at the ZnS surface, the dichroic ratio of a vibrational mode tells us about the average orientation of its transition moment. The calculated tilt angle between the normal and the molecular axis is a mean value of all the molecules on the surface. We determined the dichroic ratio by using the intensity of the peaks originating from the asymmetric stretching of CH2 and CH3 and the symmetric stretching of CH2. The peak height can be used because the shape of the peak is the same in the spectrum obtained from s-polarized light as in the spectrum obtained with p-polarized light. We calculated the refractive index of the film, n2, by using the dichroic ratio from the asymmetric stretching of CH3 (eq 1), the expressions for the electrical vectors, and n1 ) 2.25. When calculating the angles, we used the calculated refractive index of the film n2 and n1 ) 2.25 together with eq 3, and the values are presented in Table 2. Instead of calculating n2 from the dichroic ratio of the asymmetric stretching of CH3, we also calculated n2 from a randomly oriented film of diheptyldixanthogen. The result is D(νa(CH2)) ) 0.96 and D(νs(CH2)) ) 0.98. Equation 2, the expressions for the electrical vectors, and using D ) 0.97 and n1 ) 2.25 resulted in the refractive index n2 ) 1.46. In Table 3 the average tilt angle is determined by using n2 ) 1.5. This value of the refractive index is assumed to be typical for organic films and is close to the calculated value of the diheptyldixanthogen film. Tables 2 and 3 present the results from six different experiments. In the last row of the tables, the mean value and the standard deviation of the values in the column are shown. The standard deviation of the dichroic ratio reflects the precision of the measurements. Within experimental errors, all calculated tilt angles are equal, irrespective of experimental settings or estimated adsorption density. The infrared absorption from the symmetric stretching and the asymmetric stretching of CH2 should give the same value of the tilt angle. However, as is evident from Tables 2 and 3, the numerical values may differ by as much as 5°. One possible explanation could be that the peaks are overlapping in such a way that the measured peak heights do not reflect the absorbance from a single band. We tested this idea by performing a deconvolution of a few spectra and using the new peak heights obtained to calculate D, but no change of D was obtained. Haller and Rice23 obtained the same value of D for these two bands of an adsorbed stearate molecule and interpreted
Xanthate Adsorbed on ZnS
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Table 3. Dichroic Ratio, Calculated Tilt-Angles and an Approximation of the Adsorption Density, Γ. The Refractive Index n2 is Set Equal to 1.5.
a
experimental settings
D(νa(CH2))
γ
D(νs(CH2))
γ
Γ (molecules/nm2)
t ) 55 min, C ) 0.1 M t ) 55 min, C ) 0.1 M t ) 50 min, C ) 0.01 M t ) 90 min, C ) 0.01 M sprayed, C ) 0.1 M sprayed, C ) 0.01 M
1.15 1.11 1.05 1.04 1.08 1.13 1.09 ( 0.04a
38° 43° 49° 50° 46° 40° 44° ( 5° a
1.14 1.15 1.07 1.07 1.05 1.16 1.11 ( 0.05a
39° 38° 47° 47° 49° 37° 43° ( 5° a
5 4 5 2 4 5 4 ( 1a
Mean value and standard deviation of the values in the column.
this result as being caused by random orientation of the methylene groups. Accordingly, an explanation of the measured difference between D values obtained for the bands of the asymmetric and symmetric stretching of CH2 here could be that there is a preferred orientation in φ as well. To check this possibility, one needs an absorption band originating from a transition moment with a different Θ, and a biaxial orientation model.16 The symmetric stretching of CH3 could have been used, but this band is too weak and it is difficult to obtain reliable values of D. The numerical value of the calculated refractive index n2 also differs between experiments reported here (see Table 2). This difference may reflect the precision of the measurements in combination with a deviation from Θ ) 54.7° for the transition moment of the asymmetric CH3 mode, caused by deviation from the assumed all-trans configuration of the acyl chain. The most reliable values of D are obtained from the asymmetric stretchings of CH3 and CH2. The calculation model assumes that the molecular chain conformation is fully extended. The peak positions of the asymmetric and symmetric stretches of CH2 indicate the degree of this conformation.8 When the peak positions are 2926 and 2856 cm-1, respectively, the conformation is disordered, and when the frequencies are lower, viz. 2920 and 2851 cm-1, respectively, the conformation is fully extended. In the spectra from the six experiments reported here, the peak positions are 2922-2923 cm-1 and about 2853 cm-1. If we assume that we have a bridging coordination (Figure 7) and calculate the tilt angle from the normal of the surface by using the angles and bond lengths used in refs 6 and 19, the angle becomes 37°, to compare with the angle of 44° ( 5° determined from our experiments (Table 3, column 3). See Figure 7 for numerical values of angles and bond lengths used in the calculation. The shortest zinc-zinc distance in sphalerite is 3.82 Å, and the sulfursulfur distance in the CS2 entity is 3.01 Å in the configuration shown in Figure 7 (bond length of C-S used is 1.7 Å). The angle C-S-Zn would then be 127° (bond length of S-Zn is assumed to be 2.4 Å). In this context it may be mentioned that Ihs et al. calculated the angle between the normal of a gold surface and octylxanthate to 29°.8 Mielczarski et al. calculated the tilt angle for (23) Haller, G. L.; Rice, R. W. J. Phys. Chem. 1970, 74, 4368.
Figure 7. A heptylxanthate molecule on a surface with the molecular axis tilted 37° from the normal.
adsorbed ethylxanthate on a cuprous sulfide to 43°,7 but in that case the molecular axis was defined to go through the carbon in the CS2 entity and the first carbon in the ethyl group. In an in situ study of xanthate adsorbed on copper,24 it was found that the adsorbed xanthate was aligned perpendicular to the surface, but no calculation of the tilt angle was performed. 4. Conclusions The average angle between the surface normal and the molecular axis of the adsorbed complexes at the ZnS surface is approximately 44° ( 5° (based on the values obtained in Table 3 when using D(νa(CH2))). A bridging coordination of the adsorbed heptylxanthate is most likely. Diheptyldixanthogen is formed on the ZnS surface. Dixanthogen appeared even at the short conditioning times used for the spray tests (
