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Langmuir 1997, 13, 876-877
Comments Comment on “In Situ and Ex Situ Infrared Studies of Nature and Structure of Thiol Monolayers Adsorbed on Cuprous Sulfide at Controlled Potential. Simulation and Experimental Results”
In a recent paper,1 Mielczarski et al. conclude that the results of their FTIR investigations “do not support the previously proposed explanation that xanthate can be adsorbed on a cuprous sulfide surface at a very low potential ... which is more than 200 mV below the potential calculated from thermodynamic data”. From consideration of previous ex-situ FTIR spectroscopy2,3 they agree that underpotential adsorption of xanthate does occur on chalcocite, but only at potentials e60 mV below the reversible potential for the formation of copper xanthate. These authors found no spectra below the reversible potential with in situ FTIR spectroscopy but considered this was due to insufficient sensitivity in comparison to that operative in the previous ex situ measurements. Thus, their conclusions regarding spectra derived from surfaces held at potentials below those at which copper xanthate is thermodynamically possible are based on the ex situ studies. At greater underpotentials than 60 mV, they contend that “xanthate molecules undergo decomposition by cleavage of one C-S bond”. The probable products were considered to be “adsorbed sulfur” and “a species with a COCS group”. The latter species was considered to be responsible for the appearance of a band at 1225 cm-1 in FTIR spectra. These authors also considered that analogous bands observed for the interaction of xanthate with copper, silver, and gold substrates are “due to the stretching vibration of a COC group in the species which is formed during the decomposition of the first molecules which are adsorbed”. In this Comment, we present evidence from studies using a variety of techniques that contradicts such a conclusion. Rather, the evidence we present strongly endorses the judgement of other authors4-6 who assigned the dominant band appearing near 1200 cm-1 for xanthate adsorption on copper, silver, and gold to chemisorbed xanthate. These include in situ studies6 of the silver/xanthate system in which the dominant band was clearly distinguished in the underpotential region. The reason the other bands characteristic of silver xanthate do not appear when xanthate is chemisorbed was considered by Sundholm and Talonen6 to be “probably due to an orientation in which the S-C-S group of ethyl xanthate is parallel to and the C-O-C group is perpendicular to the silver surface”. Mielczarski et al. cite7 ex situ FTIR work carried out at the Virginia Center for Coal and Mineral Processing (VCCMP) to support their contention that the species formed at underpotentials on the metal surfaces is a decomposition product. They state that “the single band at about 1225 cm-1 was observed also on silver and gold at a very low reducing potential”, i.e., below the region in (1) Mielczarski, J. A.; Mielczarski, E.; Zachweija, J.; Cases, J. M. Langmuir 1995, 11, 2787. (2) Mielczarski, J. A.; Zachweija, J.; Yoon, R.-H. 119th Annual SME Meeting, Salt Lake City, Utah, February 26-March 1, 1990. (3) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649. (4) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733. (5) Ihs, A.; Liedberg, B. Langmuir 1994, 10, 734. (6) Sundholm, G.; Talonen, P. J. Electroanal. Chem. 1995, 380, 261. (7) Leppinen, J. O.; Yoon, R.-H.; Mielczarski, J. A. Colloids Surf. 1991, 61, 189.
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which voltammetric studies show the prewave to commence. More recent studies8 at the VCCMP on the ethyl xanthate/silver system have demonstrated, however, that the band is absent at such potentials if care is taken to remove xanthate from the surface before exposure to the atmosphere. Thus, when the electrode surface was rinsed with deoxygenated xanthate-free solution immediately following opening of the circuit, the potential dependence of the intensity of the band closely paralleled that of voltammetric currents. Furthermore, UV-vis spectroscopy carried out on the solution phase with a high surface area silver electrode demonstrated that xanthate was abstracted from the solution when the potential was taken into the chemisorption region and released again when the potential was returned to lower values. A similar finding was obtained9 for the copper/xanthate system. Earlier studies of the chalcocite/xanthate system employing a particulate mineral bed electrode showed10 that xanthate was abstracted from solution as the potential was taken in 100 mV steps through the prewave and copper xanthate formation regions and returned to solution when the procedure was reversed. It was reported that 70% of the xanthate was recovered when the potential was taken from -0.45 V vs SHE to 0.15 V and back to -0.45 V. Higher recoveries were observed in other studies11 at the U.S. Bureau of Mines Avondale Research Center when the potential excursion was restricted to lower upper-potential limits, an observation that suggests the loss of xanthate is associated with metal xanthate formation rather than the process responsible for the prewave. This conclusion was confirmed8 with the silver/ xanthate system; close to 100% of the xanthate was recovered when the potential excursion was restricted to the prewave region, whereas this fell to 95% when some silver xanthate was also formed. It was considered by the present authors and their co-workers8 that these UVvis spectroscopic results “establish confidently that the prewave is associated with the attachment of xanthate to the surface without any change in molecular composition”. In research carried out at the VCCMP and at CSIRO in Australia, stripping voltammetry has been employed to determine the potential dependence of the equilibrium coverage of thiol collector molecules adsorbed on metal and sulfide mineral surfaces. Coverage as a function of potential has been reported for the ethyl xanthate/ chalcocite,12 ethyl xanthate/copper,12 ethyl xanthate/ silver,8 ethyl xanthate/silver-gold alloy,13 ethyl xanthate/ galena,14 ethyl xanthate/lead,15 and diethyl dithiophosphate/chalcocite16 systems. All these systems display voltammetric prewaves at potentials below that for metal thiol compound formation, with the prewave commencing at underpotentials of around 200 mV, and the voltam(8) Woods, R.; Basilio, C. I.; Kim, D. S.; Yoon, R.-H. J. Electroanal. Chem. 1992, 328, 179. (9) Woods, R.; Kim, D. S.; Basilio, C. I.; Yoon, R.-H. Int. J. Miner. Process. 1994, 42, 215. (10) Richardson, P. E.; Stout, J. V., III; Proctor, C. L.; Walker, G. W. Int. J. Miner. Process. 1984, 12, 73. (11) Richardson, P. E. Private communication. (12) Woods, R.; Young, C. A.; Yoon, R.-H. Int. J. Miner. Process. 1990, 30, 17. (13) Woods, R.; Basilio, C. I.; Kim, D. S.; Yoon, R.-H. Colloids Surf. 1994, 83, 1. (14) Buckley, A. N.; Woods, R. Colloids Surf. 1994, 89, 71. (15) Woods, R.; Chen, S.; Yoon, R.-H. Int. J. Miner. Process., in press. (16) Buckley, A. N.; Woods, R. J. Electroanal. Chem. 1993, 357, 387.
© 1997 American Chemical Society
Comments
mograms exhibit a high degree of electrochemical reversibility in the prewave region. In each case, the coverage data obtained for 10-2 to 10-5 mol dm-3 thiol concentrations was found to fit the Frumkin adsorption isotherm. Multilayer deposition was observed for all these systems at potentials above the reversible value for formation of the metal thiol compound, with the extent of the thiolate layer at constant potential increasing with time. It should be pointed out that compound formation is expected to occur by a nucleation and growth mechanism and deposition should result in islands expanding in three dimensions rather than by layer-on-layer development. Thus, the thickness of such multilayers is not a singular quantity but will vary significantly from place to place across the sample. For all the above-cited substrate/thiol collector systems, other than that of ethyl xanthate/lead, the current/ potential relationships have been compared with the potential dependence of wettability, either through determination of contact angle or flotation recovery. Similar correlations were obtained for each of the systems investigated, flotation being found to commence at potentials corresponding to very small coverages and maximum efficiency being found to occur at about half coverage. Thus, flotation reached its maximum efficiency at underpotentials much greater than 60 mV, the value considered by Mielczarski et al. to pertain to the initiation of xanthate adsorption on chalcocite. With regard to the ethyl xanthate/chalcocite system, our results confirmed earlier studies17,18 in which the flotation recovery of chalcocite with ethyl xanthate as collector was shown to be initiated in the potential region of the first voltammetric peak.
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We consider the above findings to provide compelling evidence that the prewave arises from the charge transfer chemisorption of the thiol onto metal sites in the substrate surface and that the chemisorbed thiol is responsible for establishing a finite contact angle and inducing flotation. If the contrary view expressed by Mielczarski et al.1 were to be accepted, then it would be necessary to conclude that a decomposition product of xanthate, rather than the xanthate molecule itself, is responsible for rendering the mineral surface hydrophobic and hence for the collector action of ethyl xanthate. Furthermore, decomposition to S0 and the COCS species would have to be reversible in both the chemical and electrochemical sense. It is considered that such characteristics would be exceedingly difficult to justify. Thus, the interpretation made by Mielczarski et al. of the spectral features observed needs to be reconsidered. R. Woods* and R.-H. Yoon
Virginia Center for Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0239 Received December 14, 1995 LA951539R (17) O’Dell, C. S.; Walker, G. W.; Richardson, P. E. J. Appl. Electrochem. 1986, 16, 544. (18) Basilio, C. I.; Pritzker, C. I.; Yoon, R.-H. 114th AIME Annual Meeting, New York, NY, Preprint No. 85-86.
