Langmuir 1986, 2, 765-770 2). Subtle effects seem to be at work here, and perhaps further work will elucidate these. When MnO and Mn0, were used as supports, unusual effects were also noted. Since these materials are much lower in surface area than the A1203 or SiOz materials, reaction rates cannot be rationally compared. However, product distributions were changed significantly. The greater hydrogenation activities may be due to electron withdrawal of MnO or Mn02 from the Fe, Co, and Ni particles, thereby weakening the metal-C0 interaction and enhancing metal-H2 interactions (as expressed earlier by Sachtler and co-workers)." (4)Perhaps the most interesting behavior of Co and Fe SMAD catalysts is their selective formation of linear, terminal alkenes. There may be some connection between the facts that (a) adsorbed C, CH, and/or CH2 are likely intermediates in the F-T mechanism15aand (b) such species are already present on SMAD catalyst parti~1es.l~The presence of these species where methylene species are statistically a b ~ n d a n t ~has ~ . been ' ~ ~ demonstrated by earlier work3J5 and supported by the present work in that hydrogen, ethene, or propene exposures to the catalysts under mild conditions yielded CH, hydrogenation (CH,), CH2 oligomerization, and dimerization, as well as metathesis products.27
CHz=CHR
Fe
765
+ Fe=CH2 + CH2=C-CH3 I R
/CH2\
E Fe
\
/CH2\
/CHR CHZ
/
+
Fe\CHR/CH2
Fe=CHR Fe
+
+
CHz=CH2
or
CH,=CHCH,R
Sites favorable for CH2formation may be formed during SMAD catalyst preparation and perhaps such sites facilitate further CH2 formation and/or stzbilization during F-T catalysts. That insertion of such species into C=C bonds and rapid release of oligomerized product before isomerization take place would explain such selective linear, terminal alkene formation. A model for a SMAD catalyst particle must be presented at a later date, after surface spectroscopic studies have been completed.
Acknowledgment. The generous support of the National Science Foundation is acknowledged with gratitude. Registry No. Fe, 7439-89-6; Co, 7440-48-4; Ni, 7440-02-0; Mn, 7439-96-5; Cr, 7440-47-3; CO, 630-08-0; MnO, 1344-43-0. (27) Hugues, F.;Besson, B.; Basset, J. M. J . Chem. Soc., Chem. Commun. 1980,719.
Effect of Corona Discharge Treatment of Poly(ethy1ene terephthalate) on the Adsorption Characteristics of the Fluorosurfactant Zonyl FSC As Studied via ESCA and Surface Energy Measurements L. J. Gerenser,* J. M. Pochan, J. F. Elman, and M. G. Mason Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received May 2, 1986. I n Final Form: July 21, 1986 ESCA and surface energy measurements have been used to study the effect of corona discharge treatment (CDT) of poly(ethy1ene terephthalate) (PET)on the adsorption characteristics of the cationic fluorosurfactant Zonyl FSC. Oxidation of the surface via CDT causes the surfactant to adsorb uniformly and continuously over the surface, whereas on non-CDT PET the surfactant coverage is incomplete. Within a monolayer coverage regime, the surfactant molecules reorient as the coverage is increased. The ESCA data suggest that the surfactant molecule is chemisorbed to the CDT PET surface due to an ion-exchange process. Models and correlations are derived to relate dispersion energy measurements to surface coverage as determined by ESCA.
Introduction ESCA is an important analytical tool for ascertaining the surface composition of thin films.'P2 Surface energies obtained via contact angle measurements are used to describe the wettability and other surface characteristics of materialsS3p4 While contact angle measurements are (1)Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.; Lindgren I.; Lindberg, B. ESCA Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy; Almquest and Wiksells: Uppsala, 1967. (2) Clark, D.T.In Advances in Polymer Science; Springer-Verlag: New York, 1977; pp 126-187. (3) Wu, S. Polymer Interface and Adhesion; Marcel1 Dekker: New York, 1982. (4) Osipow, L. I. Surface Chemistry; Reinhold: New York, 1962.
0143-1463/S6/2402-0165$01.50/0
thought to be influenced by the first chemical layer, ESCA can detect the first 10-50 A of a substrate surface. We have been actively involved in understanding the role of surfactant and substrate structure on the adsorption characteristics of surfactant molecules. Recently, we presented a study of the adsorption characteristics of the fluorosurfactant Zonyl FSC on clean SiO, and clean poly(ethy1ene terephthalate) (PET).5 In that study ESCA was used to probe the structural characteristics of an adsorbed monolayer of FSC on each of the surfaces studied. On SiOz it was shown that FSC coverage is uniform and continuous at all coating thicknesses and that the sur(5) Gerenser, L. J.; Pochan, J. M.; Mason M. G.; Elman, J. F. Langmuir 1985, 1 , 305.
0 1986 American Chemical Society
766 Langmuir, Vol. 2, No. 6, 1986
factant molecules reorient within certain coating regimes. When monolayer coveage existed, good agreement was obtained between calculated surfactant coverages based on molecular space-filling models and actual values determined via ESCA. Surface projection models were derived that related observed dispersion energies with molecular orientations predicted from the ESCA studies. On PET, ESCA measurements indicated nonuniform surface coverage with possible aggregation even at surfactant coverages where the observed thickness is greater than that of monolayer coverage. In the previous study, the differences noted between the adsorption characteristics of FSC on Si02 and PET were associated with the differences in the SiOz and P E T surfaces. PET is an organic hydrocarbon with little or no hydrophilic surface moieties, whereas Si02is a much higher surface energy material containing charge sites and possible hydrogen bonding sites.3 We believe that the more uniform coverage on SiOz is due to the more uniform charge distribution of the SiOz surface. The effect of a corona discharge treatment (CDT) of PET on FSC adsorption was undertaken to test this hypothesis. We have previously shown that CDT treatment of clean polymer surfaces results in incorporation of at least five chemical species (C-0-H, -C(=O)-, C-0-0-H, -C- --C-, -C(=O)-OH).637 Such incorporation induces -0hydrophilicity in the surface of the polymer and provides a mechanism for charge exchange sites.
Gerenser et a1.
- .
e
....
... .
.
,
.
....
,:'I.I
':,
.
.
...
'.V'
. . . ...... I. ........ ....
.:'
I
. . . ....
.-
. .
... C
. .
.......
b
... ,.
...... .. *,....
'I
. . '.'*,._. .,.
.
......I_l.ZC _.:.
0
.,..(......* ...........
I
.
J
.%+.--.
Experimental Section ESCA. The ESCA spectra were obtained on a Hewlett-Packard 5950A ESCA spectrometer with a monochromatic A1 K a X-ray source (1486 eV). The use of the monochromatic source precludes sample radiation damage, which can be especially important in polymers. All samples used in this study were analyzed at ambient temperature and showed no evidence of X-ray damage during measurements. The pressure in the spectrometer during analysis was typically 5 X torr. The data were collected with a Hewlett-Packard 9836 computer and stored on disk. Angle-dependent depth profiling was done with a Surface Science Laboratory Model 259 angular-rotation probe. Typically, the fwhm for the individual components of the Cls spectrum of a clean P E T surface was 1.0 eV. All spectra were referenced to the C l s peak for neutral carbon which was assigned a value of 284.6 eV. Corona Treatment. The corona unit used in these experiments was a 3-kHz pillar system consisting of six glass-covered aluminum electrodes arranged in a hemispherical array equidistant from one another. The spacing from the glass surface to the sample surface was 0.026 in. Treatment power level was chosen so that maximum oxygen incorporation occurred without producing water-soluble ~ p e c i e s . ~The . ~ surface of the P E T was cleaned by rinsing consecutively for 30 s in pentane, methylene chloride, and ethyl alcohol and drying in a vacuum oven. The films were considered clean when the ESCA Cls and 01s spectra were consistent with P E T stoichiometry. The P E T sample was a biaxially oriented 4-mil sample of Kodak Estar film base. Surfactant Coating. Solutions of various concentrations of Zonyl FSC fluorosurfactant in distilled water were placed in a beaker in an Instron mechanical testing machine. CDT-treated samples of P E T were then dipped into the solutions, equilibrated for 1 min, and withdrawn from the solution at 0.51 cm/min. At this withdrawal speed excess solution was not retained on the surface of the test samples, and a drying front was observed -2 mm above the surface of the solution. The critical micelle concentration (cmc) for FSC (0.005 g/100 mL) exists within the experimental concentration range of the experiments.s (6) Gerenser, L. J.; Pochan, J. M.; Mason M. G.; Elman, J. F. Polymer 1985,26, 1192.
( 7 ) Pochan, J . M.; Gerenser, L. J.; Elman, J. F. Polymer, submitted for publication. ( 8 ) Du Pont product Booklet D5324-02, Zonyl fluorosurfactants.
I
I
290
299
J
281
Binding energy (eV1
Figure 1. Cls spectrum for (a) CDT PET, (b) 0.0002% FSC on CDT PET, (c) 0.005% FSC on CDT PET (d) 0.02% FSC on CDT PET, and (e) for pure Zonyl FSC fluorosurfactant.
Contact Angle Measurements. Contact angles were measured by standard procedure^.^ Hexadecane was the liquid of choice for dispersion energy calculations. Water, glycol, and polyglycol could not be used to obtain a polar interaction term because these liquids interact with the surfactant. The dispersion energy of the fluorosurfactant-treatedsurfaces was calculated from the hexadecane contact angle by rearranging the Good-Girafalco, Fowkes-Young e q u a t i ~ n ignoring ,~ the equilibrium spreading pressure terms,g (cos l#J YsD
=
+ 1)*yL2
(1)
47LD
where:y is the dispersion energy of the solid, l#J is the hexadecane contact angle, yL is the surface tension of hexadecane, and yLD is the dispersion energy contribution to the surface tension (in this case yL = yLD). The Zonyl FSC fluorosurfactant structure has been shown to be CF,(CF2),CH2CH2-S-CHz-CHz-N+(CH,),CH3S04with n = 7 the most prevalent ~ t r u c t u r e . ~
Results and Discussion The ESCA C l s spectra for the pure surfactant, the uncoated CDT PET surface, and several coverages of FSC on CDT PET are shown in Figure 1 along with the peak assignments for the various functional groups. As can be seen from Figure 1, the peaks due to the CF3 and CF2 groups of the surfactant and the O=C-0 peak of the substrate PET do not overlap. Therefore, the integrated area under these peaks can be used to determine surfactant (9) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302.
Langmuir, Vol. 2, No. 6, 1986 767
Effect of Corona Discharge Treatment on Zonyl FSC ,
2s
I
,
I I
1
, ,,
1 ,
1 I I,,
I
I
I
, I ,
0
20
F L
0
8
Figure 2. Plots of In [ R / K + 11 vs. 1/ sin 19 for various concentrations of FSC on CDT PET. (@) 0.02%, (A)0.01%,).( 0.005%, ( 6 ) 0.002%, (0) 0.001%, (A)0.0005%,
( 0 ) 0.0002%.
-1.0
-2.0 Log % FSC
-3.0
-4.0
[/sin
0.0
Figure 3. Overlayer thickness vs. coating concentration for FSC on CDT PET determined by the substrate attenuation method ).( and the angular-dependent method ( 0 ) .
thickness as a function of surfactant concentration in coating solution. The CF2 peak was chosen for the surfactant because of its greater intensity. The methods used to determine overlayer thickness of surfactant via ESCA have been discussed in detail previ~usly.~ The substrate attenuation method and angular-dependent methods were used.5 In the former case, the intensity (I)of a photoelectron peak arising from a component in the substrate will be attenuated by the presence of a thin overlayer, relative to the uncovered substrate, by a factor
L)
I,(covered) = exp( I,(uncovered A, sin 0
(2) 175
where t is the overlayer thickness, A, is the electron inelastic mean free path (IMFP) in the overlayer, and 6 is the electron take-off angle measured relative to the sample surface (38O is standard in the Hewlett-Packard spectrometer). Angular-dependent ESCA (variable take-off angle) is analyzed by a simplified expression given by FadleylO for the 0-dependent ratio of overlayer to substrate, overlayer substrate
Figure 4. S2p spectrum for 0.001 % FSC on (a) CDT PET and (b) untreated PET. T is an effective overlayer thickness given by t/hoverleyer where t is the actual overlayer thickness. When K is known, eq 3 can be converted to a form involving a linear relationship between experimental quantities,
overlayer
= K[exp(r/sin 0) - 11
(3)
where K is a function of atom density, instrument response function, the kinetic energies of the measured levels of the substrate and overlayer atoms, and the differential cross sections of the involved atoms. K has been discussed in depth p r e v i o ~ l y .Since ~ we monitored the same core-level line in both substrate and overlayer, Cls (see Figure 11, K is simply determined by the atom densities for the CF, carbon atoms in the surfactant and the O=C-0 carbon atoms in CDT PET. These were calculated to be 9.84 X 1021at/cm3 and 9.35 X 102l at/cm3, respectively. Note that these values are different than those determined for untreated PET.5 After CDT of PET, an increase in the O=C-0 carbon atoms with respect to the neutral carbon atoms is observed, giving rise to a larger value for the O=C-0 atom density. The reason for the larger value for the atom density of the CF2 group will be discussed later. Therefore, on the CDT PET substrate K = 9.84/9.35 = 1.05 (4) (10)Fadley, C. S. h o g . Solid State Chem. 1976,2, 265.
I55
Binding Energy (ev)
In
+
+
'1
=
&
(5)
A plot of In ( R / K 1) vs. l/sin 0 should provide a straight line of slope T passing through the origin.1° A plot of such data for various FSC coating solutions is shown in Figure 2 where it is shown that all coating solutions provide linear data passing through the origin, indicating continuous coverage of the CDT P E T surface for the coating solutions tested. These results are similar to those observed for SiO, but unlike the data for untreated PET, which showed noncontinuous coverage up to 0.02% FSC coating solution. A plot of the overlayer thicknesses determined by the two methods is shown in Figure 3 where it is shown that both methods provide comparable results, with the angular-dependent method always providing a slightly larger surfactant thickness. Although these results suggest that the adsorption of FSC on CDT P E T is similar to that on SOz, a surprising difference was observed. The FSC molecule contains two unique sulfur atoms, the thioether sulfur in the cation and the sulfate sulfur in the anion. S2p photoelectrons originating from thioether and sulfate-type sulfur atoms have binding energies of 163 and 168 eV, respectively. The S2p
Gerenser et al.
768 Langmuir, Vol. 2, No. 6, 1986
I
IS
0
I
2
3
-20a
I
4
n ( 1 0 molecules/cm2) '~
Figure 5. FIN ratio vs. surface population for FSC on Si02(O), PET (A). and CDT PET ( 0 ) . Figure 6. Orientational models for FSC on CDT PET. spectrum for FSC on PET a t a coating concentration of 0.001% is shown in Figure 4h. In this spectrum, two peaks are observed corresponding to the thioether and sulfatetype sulfur with a one-to-one ratio as expected. The same region of the spectrum for FSC on CDT P E T is shown in Figure 4a. The signal due to the sulfate-type sulfur is completely absent. This behavior was observed a t all coating concentrations in the monolayer coverage regime. Above monolayer coverage, two S2p peaks are observed. This suggests that the distinct chemical species created on the P E T surface via CDT caused the uniform distribution of FSC molecules by an ion exchange process. The ion exchange process may involve the replacement of hydrogen ions, associated with CDT-induced species on the surface of PET, with the cationic portion of the surfactant, while the anionic portion remains in the coating solution. The absence of the anion results in a larger value for the CF, atom density discussed earlier. The orientation of the surfactant can he determined hy measuring the intensity of a photoelectron peak for an atom on one end of the molecule relative to a peak for an atom on the opposite end. If there is a preferred orientation, one peak should he enhanced relative to the other for a given take-off angle. This was done by using the integrated areas under the F l s and Nls peaks and correcting for ionization cross section." Thus, the F I N atomic ratio can he calculated. On the basis of the stoichiometry of FSC, a randomly oriented FSC overlayer, or one in which all surfactant molecules are lying flat on the surface, should exhiihit a F I N ratio of 17. A value of 17 was found for the pure surfactant. For a close-packed FSC overlayer oriented with the fluorocarbon portion upward, the FIN ratio should increase. Plots of F/N ratios vs. the surface population of FSC are shown in Figure 5. Data are also included from our previous s t ~ d y The . ~ data on untreated PET indicate that the FIN ratio is constant and is not a function of surface coverage. For SiO, and CDT PET, the F I N ratio is -17 a t the lowest coverage and increases with the surface population to a maximum a t 0.005% FSC in coating solution and then decreases. The ESCA data in Figures 2, 3, and 5 suggest orientation regimes similar to that observed on (see Figure 6). In the low concentration regime the 5-b, layer thickness and the F I N ratio of 18 suggest that the FSC molecules are lying flat on the surface (Figure 6a). A t 0.005% FSC, the
-
(11) Scofield. J.
H.J. Ekctmn Speetroac. Relot. Phewm. 1976,8,129.
26
I
I .
r" 14
j
1 I
.
L
2l4IO -4 j
m,:,,;
-3
I
':
, , 1 , , , 1-2
8 8
. * . *m
, , * ,,,,,,I
m ,,,,
-4
LOG %FSC
1
-0 -0
Figure 7. Dispersion energy vs. percent FSC in the coating solution for FSC on SiOl ( 0 )and on CDT PET (m). F / N ratio maximizes a t a value of -39, suggesting that the FSC molecules are uniformly orientating their fluorocarbon portion upward away from the surface in a fashion similar to that shown in Figure 6c. However, the experimentally determined thickness of 12-13 b, is considerably less than the value of 20 8, that one would expect for a close-packed monolayer of FSC standing on end with the anion absent. If the thickness measurements are reasonable (i20%),then the molecules are either not close packed, or the molecules may not stand on end hut rather bend a t the thioether linkage as shown in Figure 6b. The projected surface area of both the horizontal and vertical components of the FSC molecule must he taken into account when calculating the thickness based on the orientations shown in Figure 6. Thus, a close-packed monolayer of FSC molecules with the configuration of Figure 6b would present an effective thickness of 12 A, consistent with the experimentally determined value. Above monolayer coverage the F I N ratio drops, probably due to random orientation of the subsequent overlayer. These data suggest that CDT treatment of P E T produces a surface similar to SiO, in that uniform surface coverage of surfactant is obtained with molecular reorientation occurring, hut with the anion absent. Dispersion EnergyISurface Concentration Correlation. A plot of the measured surface energy as a function
-
-
Langmuir, VoE. 2, No. 6, 1986 769
Effect of Corona Discharge Treatment on Zonyl FSC 36r
I -
24
E 22 ,
-
5
20-
z
1816-
1st
12 -
I4
+ 4 2
0
14 -
* 0
n ( 1 0molecules/cm') ~ ~
(0)
Tilted surfactant Model
n
I
1210-
3
Figure 8. Dispersion energy vs. the surface population of FSC on CDT PET. The tick marks at 2.88 X 1014mol/cm2and 0.87 x 1014mol/cm2 indicate the calculated coverages for a close-packed overlay of FSC molecules with the anion absent, standing on end, and lying flat, respectively.
i
' \
16-
I , 10
05
1
15
20
I ,
I
30
25
35
40
n (10I4m o I e c u l e s / (311121
Figure 10. Comparison of dispersion energy vs. surface population determined experimentally ( 0 )and calculated from the close-packed model of Figure 9b (0). by a vertical and horizontal component. A linear fit of the data in the changing portion of the curve in Figure 8 provides = (-11.21 x 10-14)n
+ 33.43
(6)
Let n =N ( b ) Bent Surfactant Model
H
+ Nv
(7)
and NvAv
+ NHAH = 1
(8)
where N H and Nv are the number of horizontal and vertical molecules in a unit area and AH and Av are the respective areas of each orientation. Then, assuming that the total surface dispersion energy is really an average of the individual components, i.e.,
Figure 9. Schematic representation of (a) "tilted" surfactant model and (b) bent surfactant model.
provides
of coating concentration for SiOz and CDT PET is shown in Figure 7. The data are superimposable, indicating the effective surfaces being measured are similar in similar concentration regimes and the absence of the anion has no measurable effect on the surface energy. A plot of y vs. n (FSC coverage, molecules/cm2) for FSC on CDT PET is shown in Figure 8. The data in Figures 7 and 8 indicate that as the coating concentration increases the FSC coverage increases and more fluorocarbon surface is exposed to the dispersion measuring liquid. At high coverages the surface appears as a saturated fluorocarbon. The data in Figure 8 suggest that the surface energy of the surfactant overlayer reaches the value for a saturated fluorocarbon at a much lower coverage than calculated for monolayer coverage of a close-packed space-filling model of FSC standing on end with the anion absent. Saturation occurs at -1.85 X 1014mol/cm2, reasonably close to the calculated value (2.13 X l O I 4 mol/cm2) for monolayer coverage of a close-packed space-filling model of FSC bent at the thioether linkage with the anion absent. Two models have been proposed to describe the correlation of y with n observed in Figure 8. They are shown in Figure 9. The first model (the molecular tilt model) provides that the tilted array can be reasonably described
YT =
(
nYvAv -
[YHAHAV - yvAv21
YHAH- ~ v A v
-
)+
AH
- AV
(10)
where the ys are the dispersion energies of the individual components. By calculation of AH and Av from spacefilling models (AH = 100 A2, Av = 46 A2) and assumption of yH= 24 dyn/cm and yv = 13 dyn/cm, based on the end points of the monolayer coverage regime, eq 10 becomes yT =
(-9.37 x 1044)n + 33.34
(11)
not an unreasonable fit of the experimental data (eq 6) considering that there are no adjustable parameters in the theory. An alternative model is shown in Figure 9b. It is a two-dimensional model that assumes van der Waals radial contact of the end of one molecule with the side of another. (This distance is held constant a t 5 A.) With this model, the surface energy is assumed to be controlled by the vertical exposure of surface active groups, i.e., YT
= CYiAi i
(12)
where the A irepresents the area of the individual components that can be observed from the vertical. With this
770
Langmuir 1986, 2, 770-773
model the fluorocarbon portion can shield the hydrocarbon portion due to overlap Since the sulfate ion has been removed, the calculation for the model in Figure 9b assumes the molecules are attached to the surface via the quaternary nitrogen and are bent near the thioether linkage. A plot of dispersion energy vs. n (calculated from the close-packed model of Figure 9b) is shown in Figure 10. The plot is similar to the experimental data and indicates that the model is not unrealistic. A detailed discussion of the model has been described previ~usly;~ however, a few comments are relevant at this time. (1) The model is calculated for tilt angles of 0-45". Beyond this value, the base of the molecule would become unshielded. Experimentally, this may not occur because the molecule is not as rigid as the model proposes but is free to move, creating a fluorocarbon umbrella that effectively shields the base of the molecule. ( 2 ) The model assumes the bent conformation, but there is no way of experimentally discerning this configuration vs. the one proposed in Figure 9a or other overlap con- formations.
Conclusions ESCA and surface energy measurements have been used to study the effect of CDT of PET on the adsorption characteristics of the cationc fluorosurfactant Zonyl FSC. Results indicate that oxidizing the surface via CDT causes the surfactant to uniformly and continuously distribute over the surface. Within a monolayer coverage regime, increasing the surfactant coverage causes reorientation of the surfactant molecules. On untreated PET, the surfactant coverage is incomplete. On CDT PET, the ESCA data suggest that the surfactant molecule is chemisorbed due to an ion exchange process. In the monolayer coverage regime two models have been proposed to predict the measured dispersion energy, Both models have no adjustable parameters and, in one case, a surface energy/surface population model is predicted that fits experimental data quite well. Results indicate that area average surface energies can be used to model observed effects. Registry No. PET, 25038-59-9; Zonyl FSC, 67479-85-0.
Investigation of Electrochemical Properties of Lead Ethyl Xanthate by Linear Potential Sweep Voltammetry I. C. Hamilton Chemistry Department, Footscray Institute of Technology, Footscray, Victoria 301 1, Australia
R. Woods* CSIRO Division of Mineral Chemistry, Port Melbourne, Victoria 3207, Australia Received May 6, 1986. I n Final Form: August 3, 1986 Electrochemical reactions of lead ethyl xanthate have been investigated by linear potential sweep voltammetry of the compound deposited on a gold electrode. Reduction resulted in the deposition of metallic lead with the release of xanthate ions. Oxidation resulted in the formation of dixanthogen with Pb2+or Pb(OH)2depending on solution pH. Lead xanthate hydrolyzed at pH values 110.2. The surface of the electrode was found to be hydrophobic when lead xanthate or its oxidation products were present. It is suggested that inhibition of galena flotation at high potentials with xanthate collectors results from surface oxidation of the mineral concurrent with oxidation of lead xanthate.
Introduction In the flotation of galena with ethyl xanthate as collector, lead ethyl xanthate is formed on the mineral surface as a result of the interaction between the collector, dissolved oxygen, and lead sulfide. As this process necessarily involves electrochemical steps, a knowledge of the electrochemical characteristics of lead xanthate is important in understanding the response of the galena/collector system to changes in the conditions of flotation. The reactions of alkali-metal xanthates have been studied at galena ele~trodes.l-~These investigations have identified lead xanthate, chemisorbed xanthate, and dixanthogen as oxidation products. However, on the basis of these measurements, it is difficult to characterize further (1) Gardner, J. R.; Woods, R. Aust. J. Chem. 1977, 30, 981. (2) Pritzker, M. D.; Yoon, R. H. In Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 26. (3) Lamache, M.; Lam, D.; Bauer, D. In Proceedings of the International Symposium on Electrochemistry i n Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R., Eds.; The Electrochemical Society: Pennington, NJ, 1984; p 54.
0743-7463/86/2402-0770$01.50/0
reactions of lead xanthate since they are not readily separated from other processes involving xanthate and from reaction of the sulfide mineral with the aqueous electrolyte. Guy and Trahar4 found the flotation of galena with ethyl xanthate to be inhibited at high potentials and considered this to arise from oxidation of lead xanthate on the mineral surface by the reaction Pb(CzHSOCSJ2
+
2H20 ---* HPbOZ-+ (CzH,OCSz)z+ 3H+ + 2e (1)
Inhibition of flotation occurred close to the reversible potential of reaction 1 and displayed a similar pH dependence. oxidation of lead xanthate to dixanthogen could also result in Pb2+or Pb(OH)2depending on solution PH. Pb(CzH50CSz)z Pb2++ (C2H50CSz)2+ 2e (2)
-
-
Pb(C2HsOCSZ)z + 2Hz0 Pb(OH), + (CzH,0CSz)2+ 2H+ + 2e (3) (4) Guy, P. J.; Trahar, W. J. Int. J. Miner. Process. 1984, 12, 15.
0 1986 American Chemical Society
