COLLOIDS AND h

Colloids and Surfaces SURFACES E L S E V I E R A: Physicochemical and Engineering Aspects 137 (1998) 319-328

Spectroelectrochemical investigations of the interaction of ethyl xanthate with copper, silver and gold:

I. FT-Raman and N M R spectra of the xanthate compounds

Ronald Woods *, Gregory A. Hope Faculty of Science and Technology, Griffith University, Nathan Campus, Queensland4111, Australia

Received 4 September 1997; accepted 19 December 1997

Abstract

Raman and 13C NMR spectroscopies have been applied to characterise the ethyl xanthate compounds of potassium, sodium, copper, silver, and gold, and diethyl dixanthogen in order to provide a basis for investigating the interaction of ethyl xanthate with metal surfaces. Spectra for the various species in the solid state obtained by both techniques are interpreted in terms of molecular conformation and differences in atomic environments in the crystal structure. Spectra for potassium ethyl xanthate in aqueous solution provide evidence of hydrophobic interaction between the hydrocarbon moiety and water. Raman spectroscopy provides an effective means of distinguishing between dixanthogen and metal xanthates. © 1998 Elsevier Science B.V.

Keywords: Ethyl xanthate; Copper; Silver; Gold; NMR spectroscopy; Raman spectroscopy; Hydrophobicity

1. Introduction thate radical, formation of a xanthate compound with a metal component of the sulfide, and/or

Xanthates (alkyl dithiocarbonates, R O C S 2 ) a r e dimerization to form dixanthogen. The species applied widely as collectors in the froth flotation formed in each particular situation depends on the process for the concentration and separation of nature of the surface and the potential across the metal sulfides from their ores. The key chemical mineral-solution interface. Each of these species step in this process is the interaction of the collector types could play an important role in the flotation with the sulfide surface that renders the mineral process. Chemisorption is the thermodynamically hydrophobic. Xanthate-mineral interaction occurs favoured reaction [2] and thus this process would by an electrochemical mechanism in which anodic be expected to be the most important of the three oxidation of the collector is coupled with the possibilities in inducing the floatability of mineral cathodic reduction of oxygen (see Ref. [ 1 ] for a particles. Indeed, the potential at which the mineral review). The process is, therefore, amenable to begins to float for a number of systems has been study using electrochemical techniques, found [ 1,2] to correlate with the potential at which

It has been shown [ 1] that the anodic oxidation chemisorption of collector species on its surface of xanthate can result in chemisorption of a xan- occurs.

In order to provide detailed information on the chemical identity, structure, configuration, and ori-

* Corresponding author, entation of species formed at the mineral-solution

0927-7757/98/$19.00 © 1998 Elsevier Science BN. All rights reserved. PII S0927-7757 (98) 00225-8

320 R. Woods, G.A. Hope / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328

interface, a number of spectroscopic techniques by the oxidation of xanthate in the same potential have been employed to complement electrochemi- region as xanthate adsorption on this metal [9]. cal investigations [1]. The techniques that have The present paper also reports t3C nuclear mag- been applied most widely to mineral systems are netic resonance (NMR) spectra of the same ethyl X-ray photoelectron spectroscopy (XPS) and xanthate compounds. NMR provides another Fourier transform infra-red spectroscopy (FTIR). effective spectroscopic technique for characterising These techniques have provided a wealth of infor- the molecular composition and conformation of mation on surfaces under flotation-related condi- solid and solution species; such information is tions, but different interpretations have been put derived from the chemical shifts that arise from forward regarding the nature of the surface species changes in the screening of nuclei as a result of formed by the interaction of xanthates with metals differences in the chemical environment of the and sulfide minerals under chemisorption condi- carbon atoms in the molecule. 13C NMR has been tions, i.e. at underpotentials to the formation of applied to understanding the crystal structures of the bulk compounds [3,4]. some solid xanthate compounds [10], but these

Raman spectra provide information comple- did not include those of silver, copper, or gold. mentary to that from FTIR; both spectra result Theoretical studies of xanthates and related com- from transitions between the vibrational energy pounds [ 11] suggested that the 13C NMR spectro- levels of molecules but with different selection scopic technique could prove valuable for the rules. Raman spectroscopy has been applied much characterization of xanthates and showed that the less widely than FTIR to mineral processing sys- nearest neighbour geometry about C atoms could tems because of its lower inherent sensitivity. The be determined from the shielding tensor. Raman scattering signal can, however, be enhanced by a factor of 104-106 when adsorption occurs on copper, silver and gold surfaces, and 2. Experimental details this allows characterization of surface species at the sub-monolayer level. These metals are appro- Sodium and potassium ethyl xanthates were priate for studying xanthate adsorption since they prepared from ethanolic solutions of the corre- provide model surfaces for understanding thiol sponding alkali metal hydroxides and carbon disul- adsorption on metal and metal sulfide surfaces, fide [12] and were recrystallised from acetone by Surface Enhanced Raman Scattering (SERS) was the addition of diethyl ether. Silver ethyl xanthate recently applied to verify the molecular integrity was prepared by precipitation from an aqueous of ethyl xanthate chemisorbed on silver [5]. solution of potassium ethyl xanthate by the addi-

In order to interpret SERS spectra, it is neces- tion of silver nitrate. Copper and gold ethyl xan- sary to have a detailed knowledge of the Raman thates were prepared by reaction of aqueous spectra of the corresponding bulk compounds and solutions of potassium ethyl xanthate with the identity of the vibrations responsible for the copper(II) sulfate and gold(III) chloride, respec- individual bands. Raman spectra of potassium and tively, to form the univalent metal xanthate plus sodium xanthates have been reported by a number diethyl dixanthogen. The dixanthogen content of of authors [6-8] and assignments established. A the precipitate was removed by washing with FT-Raman spectrum of silver xanthate has been diethyl ether. Diethyl dixanthogen was prepared obtained [5] using low laser power to avoid photo- by reaction of potassium ethyl xanthate with induced decomposition. In this paper, the sodium persulfate and was extracted from the FT-Raman spectra of the ethyl xanthate corn- aqueous solution into ether. Evaporation of the pounds of potassium, sodium, copper, silver and ether resulted in dixanthogen in an oily form that gold are reported. The FT-Raman spectrum of remained liquid at ambient temperatures. Sig- diethyl dixanthogen has also been determined. This nificant supercooling was required to nucleate species is particularly important in understanding crystals of dixanthogen that remained as a solid the interaction of xanthate with gold; it is formed provided the melting point of 32°C [13] was not

R. Woods, G.A. Hope / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328 321

exceeded. Thus, both liquid and solid dixanthogen High resolution 13C N M R spectra of solutions could be investigated at the same ambient of potassium ethyl xanthate in D20 and temperature, acetone-d6 were obtained using a Varian Unity

Raman spectra for most species were collected 400 spectrometer operating at 100 MHz for on a Perkin Elmer System 2000 NIR FT-Raman carbon. The FID was recorded in 32k data points, spectrometer, equipped with a Spectron Laser spectral width of 22 kHz, 24 ~ts (90 °) pulse and a system SL301 Nd: YAG laser emitting at 1064 nm, cycle time of 30 s. a quartz beam splitter and an InGaAs detector operated at room temperature. Spectra were recorded with a total of 50-100 co-added scans at 3. Results and discussion a resolution of 2 cm- 1 or 4 cm- 1 and a laser power of 400 mW. All spectra were recorded at ambient 3.1. a3C N M R temperature.

Acceptable Raman spectra for copper and silver Fig. 1 presents 13C N M R spectra of (a) solid xanthates could not be obtained with the above sodium ethyl xanthate, (b) solid potassium ethyl system, even at low incident light intensities, owing xanthate, and (c) a concentrated potassium ethyl to laser-induced decomposition and photon xanthate solution. The chemical shifts observed absorption. Spectra for these xanthate species for the three carbon atoms in the ethyl xanthate were obtained with a Renishaw 1000 Raman molecule are similar to those reported previously Microprobe which includes a monochromator , for xanthate compounds [10]; the shifts increase Rayleigh filter, and charged coupled detector in the order C H 3 < C H 2 < O C S 2. It can be seen (CCD). The compounds were placed on a polished from Fig. 1 that there are two signals arising from metal surface on the stage of an Olympus CH3 groups in sodium ethyl xanthate with equal BH2-UMA microscope equipped with 10 ×, 20 ×, intensity. This results from the presence of two and 50 × objectives that were part of the spectrom- different conformers of this xanthate in the solid eter. Spectra were obtained by irradiating the compound; one has the CH 3 group trans, and the sample with 633 nm laser light from a Spectra other gauche, to the CS2 group [7]. The chemical Physics model 127 Helium Neon Laser. The laser shifts observed are presented in Table 1 together power was 20 mW and this could be attenuated with those for the other systems investigated. by a factor of 10 or 100 by the insertion of In contrast to sodium ethyl xanthate, there is appropriate filters, only one peak from the CH 3 group in the potas-

Solid state cross-polarization magic-angle spin- sium analogue (Fig. 1 (b)). This is consistent with ning (CP/MAS)13C N M R spectra were obtained the crystal structure of this compound [14,15] at ambient temperature on a Varian Unity-400 which has all the CH 3 groups in the trans configu- spectrometer at 100.593 MHz. Single contact times ration. The OCS2 group now gives rise to two of 1 ms were used with a 90 ° proton pulse (5.2 ~ts), equal resonances and this results from different a proton decoupling field of 62 kHz, and a recycle environments in the crystal. There are two non- delay time of 4 s. The field induction decay (FID) equivalent potassium atoms in the unit cell with was transformed with an experimental line broad- equal abundance; each potassium atom is com- ening value of 50 Hz. Chemical shift data were pletely surrounded by sulfur atoms, but one coor- referenced to the aromatic signals of hexamethyl- dinates with six sulfurs while the other coordinates benzene at 132.1 ppm. This is equivalent to using with seven [14, 15]. The splitting of the OCS2 peak tetramethylsilane referencing at 0 ppm. The 13C into a doublet is absent in aqueous media as is to N M R spectrum of diethyl dixanthogen in the be expected since each OCS~- group will now have liquid phase was obtained with the solid state an equivalent average environment. probe but with only dipolar coupling; there was There are other significant differences between no cross polarisation and the sample was spun the 13C N M R spectrum for potassium xanthate in at 200 Hz. its crystalline state and when dissolved in D20. In

322 R. Woods, GA. Hope / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328

(c) KEX (aq)

(c ~)

t . . . . I " ' q J ' " l " " t

14.5 13.5 12.5

(b) KEX (s)

(a) NaEX (s)

2dO 220 200 80 60 dO 20 0

ppm

Fig. 1. 13C N M R spectra recorded for (a) solid sodium ethyl xanthate (NaEX); (b) and (c) potassium ethyl xanthate (KEX); (b) solid and (c) a concentrated aqueous solution. Curve (c') was recorded at a higher sensitivity.

contrast to the solid state where molecular confor- Secondly, the resonance arising from the CH 3 mations are locked in, conformation changes in groups of potassium ethyl xanthate in D20 gives solution generally occur rapidly so that a molecular rise to two major peaks and this suggests that they moiety becomes characterised by a single chemical correspond to trans and gauche conformations. A shift. This is not the case with the CH 3 region of spectrum of the relevant region recorded at a the spectrum. Firstly, considerable line broadening higher sensitivity is included in Fig. 1 as curve (c'). is observed and this is indicative of inhibition of Close inspection of this spectrum shows that each motion within the xanthate molecule [ 16]. The peak consists of 4-6 components. The multiplicity unique ability of water to restrict rotation of bonds of the peaks is above the background-noise level within organic species has been shown in previous and indicates the presence of a range of discrete NMR studies of organic species [17]. It is a environments. In the concentrated solution, ion manifestation of the hydrophobic effect in which pair formation will occur between the xanthate the organic species occupies a cavity surrounded and potassium ions. It is possible that multiple ion by highly structured water molecules [16]. association also occurs, resulting in a range of

R. Woods, G.A. Hope/Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328 323

Table 1 CH2 peak for the copper compound is particularly 13C NMR chemical shifts found for the xanthate compounds broad. This can be explained by the intractable, studied polymeric structure of copper ethyl xanthate [21]; Compound 13C NMR chemical shift (ppm) there will be a range of environments for the

CH2 carbon as a result of different Van de Waals CH 3 CH z ocs2 contact between the associated oxygen atom and

KEX (s) 15.8 73.0 233.5, 231.9 copper atoms in the polymer. This particular struc- KEX (D20) 14.0, 13.4 70.3 232.7 ture could also account for the significantly lower KEX ((CD3)20) 14.3, 13.8 66.7 232.3 chemical shift for the OCS 2 group in copper ethyl NaEX(s) 15.6, 14.4 71.7 232.4 xanthate compared with the silver and gold CuEX (s) 16.0 70.9 2 1 8 . 7 compounds. AgEX (s) 15.9 71.3 229.5 Fig. 2(d) shows the 13C N M R spectra for liquid AuEX (s) 16.8 75.7 227.8 EX2 (s) 16.3 74.8 206.2 diethyl dixanthogen. As pointed out in Section 2, EXz(1) 14.9 72.6 207.4 diethyl dixanthogen requires significant super-

cooling to nucleate crystals and hence both liquid and solid forms of this compound can be investi-

CH 3 environments. It is known [ 18,19] that similar gated at ambient temperature. The liquid form thio-compounds exist in solution in organic appears to be the most relevant to the flotation solvents as multiples of the molecular formula, situation since nucleation is unlikely to occur when The formation of micelles with the short chain diethyl dixanthogen is formed at a mineral surface

and hence its spectrum is presented here. Similar xanthate seems unlikely [20]. Furthermore, the organization of the organic ions into micelles spectra were recorded for dixanthogen in the solid

state. The chemical shifts for this compound in would be expected to promote greater freedom of both phases are listed in Table 1. The three carbon

the CH3 groups since they would be in a hydro- atoms on either side of the S-S bond in diethyl

carbon environment and this should allow suffi- ciently rapid rotation around the C - C bond for dixanthogen are equivalent, and hence there are only a single chemical shift to be observed from only three carbon environments in this molecule,

as in the metal xanthates. There is only one this group, chemical shift for the CH3 groups in the solid

The lower chemical shift of the CH 3 group of phase because diethyl dixanthogen has a crystal potassium ethyl xanthate in water than in the solid structure in which these groups, like those in (Table l ) could also be due to hydrophobic inter- potassium ethyl xanthate, have an all trans config- actions. The corresponding decrease in the CH2 uration [21]. The OCS2 chemical shift is signifi- chemical shift is attributed to the changed environ- cantly less than that in the alkali metal xanthates ment of the associated oxygen atom which is close as a result of the strong covalent bonding between to a potassium atom in the crystal [ 14,15]. the two sulfur atoms binding the xanthate moieties.

Spectra were also recorded for potassium xan- The 13C NMR spectrum for liquid dixanthogen thate dissolved in acetone-d 6. The CH 3 chemical also shows a single CH 3 line but dixanthogen is shift was again a multiplet indicating restricted not considered to retain its trans configuration on rotation also occurs in this medium. The peak melting. Rather, rotation around the C - C bond is intensities were greater than those in D20 suggest- probably rapid enough in the liquid for time- ing greater freedom of motion. The two major averaging of all possible configurations to occur shifts of the methyl carbon atom are listed in within the resonance period (approximately Table 1 together with those for the other two 10 -8 s). This would also explain the CH 3 group carbons, having a lower chemical shift in the liquid state

Fig. 2 presents 13C N M R spectra for the solid (Table 1). ethyl xanthate compounds of(a)silver, (b)copper, The chemical shift found for the CH2 group is and (c) gold; the chemical shifts observed are listed less in the liquid state of diethyl dixanthogen than in Table 1. It can be seen from Fig. 2 that the in the solid phase, while that for the OCS2 is

324 R. Woods, G.A. Hope/Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998)319-328

(d) Ex~ (0

(c) AuEX (s)

(b) C~EX (s)

(a) AgEX (s)

2~0 220 200 0

ppm

Fig. 2. 13C NMR spectra recorded for the solid ethyl xanthates of (a) silver (AgEX), (b) copper (CuEX), (c) gold (AuEX) and (d) for liquid diethyl dixanthogen (EX2).

greater (Table 1). Again, this could result from in the alkali metal compounds (Table 1) as a result the changed environment of the carbon atoms on of the metal sulfur bond being covalent rather melting. Stacking of dixanthogen molecules in the than ionic. They are also significantly greater than crystal results in Van de Waals contact between the equivalent chemical shift in dixanthogen, which the carbon in a CH2 group and a carbon in a is indicative of a smaller degree of covalency than similar position in a neighbouring molecule as well in the disulfide. This difference in covalency could as one of the sulfurs in that molecule. Such struc- also explain the difference in hydrophobicity tural arrangements will be diminished in the between metal xanthates and dixanthogen. The liquid state, adsorption of xanthate on a range of minerals

The OCS2 chemical shifts found for the copper, gives rise to a contact angle of 60 ° whereas dixan- silver and gold xanthates are slightly less that those thogen gives a contact angle of 90 ° [22].

P~ Woods, G.A. Hope / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328 325

3.2. FT-Raman from both trans and gauche forms. The spectrum from the aqueous solution shows a decrease in the

Fig. 3(a) shows a Fourier transform (FT) CH3 stretch compared to the equivalent vibration Raman spectrum for sodium ethyl xanthate. The of the CHz group and the bands are broadened vibrations giving rise to the various scattering somewhat. In addition, one of the two bands bands, based on the comprehensive assignments arising from the CH3 antisymmetric deformation of Colthup and Powell [7], are presented in vibration is absent. These features could result Table 2. In addition to the assignments listed in from hydrophobic interaction with water as dis- the Table, there is a band at approximately cussed in the above section for N M R spectra of 3370 crn- 1 characteristic of water [23, 24]; this is aqueous potassium ethyl xanthate. There is a water as expected since sodium ethyl xanthate crystals band centred at approximately 3370 cm-1 which are known to be hydrated [7,15]. As discussed in is broader than that from the structured water of the previous section, ethyl xanthate is present in hydration of sodium ethyl xanthate. the sodium compound in both the trans and the The spectra from copper and silver ethyl xan- gauche conformation. This results in two peaks for thates (Fig. 4) and that for gold ethyl xanthate the CS2 symmetric stretch and COC deformation (Fig. 5(b)) show the presence of only the trans vibrations, conformer which indicates that the crystals of these

The FT-Raman spectra for a concentrated aque- species follow similar structural characteristics to ous solution of potassium ethyl xanthate and the those of potassium ethyl xanthate and diethyl solid compound are presented in Fig. 3(b) and dixanthogen. A small band at the wavenumber (c), respectively, and assignment of the bands is expected for the gauche conformer was observed presented in Table 2. The spectrum from the solid with silver ethyl xanthate at the higher laser inten- is consistent with the presence of only the trans sities investigated; it can be seen in Fig. 4 as a conformer [ 14,15]. The C - C bond in the dissolved shoulder on the low wavenumber side of the strong ion is free to rotate and hence bands are observed band that appears at 644 cm- 1. The small band

was not evident at the lowest laser intensity investi- gated and is considered to result from a temper- ature increase on irradiation rather than being characteristic of the crystal structure.

A decrease in the CS 2 in-phase stretching or the OCS2 out-of-plane wag vibrations is considered [6,7] to indicate an increase in the metal-sulfur bond strengths. These bands are at similar wave-

i numbers for the xanthates of ethyl copper, silver t - and gold suggesting that the metal-sulfur bond

strengths are of the same order of magnitude. The (b) KEX (aq) A ,/k, , COC and SCS vibrations, which are, to a large

/'k, /~ extent, coupled [25-27], also appear at similar wavenumbers in the FTIR spectra of copper [25],

(a) NaEX (s) silver [26], and gold [26,27] ethyl xanthates. The FT-Raman spectrum for liquid dixanthogen

is shown in Fig. 5(a) and the vibrations are listed ~ r~ . . . . . . in Table 2. The assignment of most of the bands

c m "~ is derived from those for the alkali metal xanthates.

Fig. 3. FT-Raman spectra recorded for (a) solid sodium ethyl Dixanthogen has an S-S linkage that is not present in metal xanthates and the strong band at xanthate (NaEX); (b) and (c) potassium ethyl xanthate

(KEX); (b) a concentrated aqueous solution and (c) the solid 498 cm- 1 is assigned to stretching of this bond compound. [28,29]. The CS 2 symmetric stretch appears at

326 R~ Woods, G.A. Hope/Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 319-328

Table 2 Characteristic Raman bands found for the xanthate compounds studied

Vibration Wavenumber (cm- 1)

NaEX KEX KEX CuEX AgEX AuEX EX 2 (s) (s) (aq) (s) (s) (s) (1)

CH3 antisymmetric stretch 2975 2961 2983 2971 2971 2987 2980 CH 2 symmetric stretch 2956 2935 2939 2926 2928 2941 2935 CH 3 symmetric stretch 2890 2868 2883 2880 2860 2880 2867 CH 3 antisymmetric deformation 1448 1461, 1445 1449 1448 1446 1453 1451 COCS2 antisymmetric stretch 1170 1146 1150 1194 1197 1186 1100 CH 3 rock; CH 2 rock 1086 1096 1095 1107 1111 1112 1092 CS2 antisyrnmetric stretch 1054 1051 1046 1030 1000 1021 1041 CCOC stretch 864 867 864 866 857 858 845 CS2 symmetric stretch trans 663 666 660 659 644 644 695 CS 2 symmetric stretch gauche 618 - - 615 - - - - - - 646 OCS 2 out of plane wag 555 582 556 549 556 544 528 SS stretch n.a. n.a. n.a. n.a. n.a. n.a. 498 COC deformation gauche 485 - - 493 - - 473 COC deformation trans 448 449 449 448 447 467 427 OCC deformation 396 402 399 404 409 430 378

n.a. not applicable

approximately 30 cm -1 higher wavenumber than xanthate molecule. The sulfurs are equivalent in the equivalent band for the corresponding metal the metal xanthates, both being situated close to xanthates (Table 2). This could be related to the the metal atom in the crystal. They are different different bonding of the two sulfur atoms in the in dixanthogen, one being bonded to a sulfur in

~ ~ (a) EX 2 (I)

J o o o o o o o o o o o o o o o o ~ o o o o o o o o o o o o o o o o

o 0 o o o 0 0 o o 0 0 o o o o 0 o o ~ 0 o 0 0 ~ 0 0 o 0 0 o 0 o o o o

c m t ~ ~

cm-1 Fig. 4. FT-Raman s ~ c t r a recorded ~ r solid samples of (a) copper ethyl xanthate (CuEX) and (b) silver ethyl xanthate Fig. 5. FT-Raman s p ~ t r a r~o rded ~ r (a) liqmd diethyl dixan- (AgEX). thogen and (b) solid gold ethyl xanthate (AuEX).

P~ Woods, G.A. Hope / Colloids Surfaces A. Physicochem. Eng. Aspects 137 (1998) 319-328 327

the other half of the dixanthogen molecule and scattering from each part of the xanthate molecule, the other having a double bond to the CS2 carbon based on work reported in the literature, provides atom. This shift allows the presence of a metal a basis for identifying the integrity of xanthate xanthate to be identified in the presence of dixan- molecules on metal surfaces. The band arising thogen. An analogous shift in the FTIR band from the S-S bond in diethyl dixanthogen allows owing to coupled COC and CS2 vibrations from this molecule to be detected in the presence of approximately 1200cm -1 for gold xanthate to metal xanthate species. approximately 1240cm -1 for dixanthogen has been used for this purpose [9,26]. The band at 498 cm-1 provides a means of identifying dixan- Acknowledgment thogen in the presence of gold xanthate.

The FT-Raman spectrum for dixanthogen The authors are grateful to Dr P.M. Fredericks (Fig. 5 (a), Table 2) exhibits vibrations arising from of the Centre for Instrumental and Developmental the gauche as well as the trans conformation and Chemistry for access to Raman instrumentation this is as expected for the liquid state since rotation and to Dr S. Perera for assistance with NMR is possible around the C-C bond. In contrast, only spectroscopy. one chemical shift was observed by 13C NMR spectroscopy. The detection of different conform- ers in Raman spectra but not by NMR can be References explained by the different response times of the two spectroscopic techniques. The response time [1] R. Woods, in: J.O'M. Bockris, B.E. Conway, R.E. White of NMR is approximately 10-as and rotation of (Eds.), Modem Aspects of Electrochemistry No. 29, the C-C bond can occur over this time frame. Plenum, New York, 1996, pp. 401~,53. However, vibrational spectroscopy involves sam- [2] A.N. Buckley, R. Woods, Int. J. Miner. Process. 51 piing in 10 -13 s and this will allow the two con- (1997) 15-26. formers to be discerned. [3] R. Woods, R.-H. Yoon, Langmuir 13 (1997) 876-877.

[4] J.A. Mielczarski, Langmuir 13 (1997) 878-880. [5] A.N. Buckley, T.J. Parks, A.M. Vassallo, R. Woods, Int.

J. Miner. Process. 51 (1997) 303-313. 4. Condudingremarks [6] R. Mattes, G. Pauleickhoff, Spectrochim. Acta 30A

(1974) 1339.

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Soc. Dalton Trans. 233-236 (1989). Broadening and multiplet splitting of the peak [11] J.A. Tossell, D.J. Vaughan, J. Colloid Interface Sci. 155 from CH 3 groups for ethyl xanthate in aqueous (1993) 98-107. solution is explained in terms of hydrophobic [12] A.I. Vogel, Textbook of Practical Organic Chemistry, 3rd interactions. Spectra from copper, silver and gold edn., Longmans, London, 1956, p. 499. ethyl xanthates and from diethyl dixanthogen indi- [13] R.D. Crozier, Flotation: Theory, Reagents and Ore

Testing, Pergamon Oxford UK p. 31. cate that only the trans conformer is present in [14] F. Mazzi, C. Tadini, Z. Krist. 118 (1963) 378-392. these solids. Broadening of the CH2 peak in the [15] S.R. Rao, Xanthates and Related Compounds, Marcel copper compound is consistent with its polymeric Dekker, New York, 1979. structure. [16]C. Tanford, The Hydrophobic Effect: Formation of

Interpretation of Raman spectra is consistent Micelles and Biological Membranes, Wiley-Interscience, New York, 1980.

with that from the 13C NMR regarding conforma- [17] A. Ben-Naim, Hydrophobic Interactions, Plenum, New tion of ethyl xanthate in the compounds investi- York, 1980, p. 111. gated. The assignment of the bands resulting from [18] D. Coucouvanis, Progr. Inorg. Chem. 11 (1970) 233-371.

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[19] C.N. Murphy, G. Winter, Aust. J. Chem. 26 (1973) [26] J.D. Leppinen, R.-H. Yoon, J.A. Mielczarski, Colloids 755-760. Surfaces A61 (1991) 189-203.

[20]I.C. Hamilton, R. Woods, Int. J. Miner. Process. 17 [27]A. Ihs, K. Uvdal, B. Liedberg, Langmuir9(1993)733-739. (1986) 113-120. [28] F.R. Dollish, W.G. Fateley, F.F. Bentley, Characteristic

[21] Y. Watanabe, Acta Cryst. B27 (1971) 644-649. Raman Frequencies of Organic Compounds, Wiley, New [22] K.L. Sutherland, I.W. Wark, Principles of Flotation, York, 1974.

Australian I.M.M., Melbourne, 1955. [29] E.J. Bastian Jr.,, R.B. Martin, J. Phys. Chem. 77 (1973) [23] W.A. Senior, W.K. Thompson, Nature 205 (1965) 120. 1129 1133. [24] G.E. Walrafen, J. Chem. Phys. 40 (1964) 3249. [25] J.A. Mielczarski, R.-H. Yoon, J. Phys. Chem. 93 (1989)

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