Oct., 1963
XAPL’THATE ADSORPTION ON VACUUMDEPOSITED FILMSOF LEADSULFIDE
2121
INFRARED STUDY OF XANTHATE ADSORPTION O N VACUUM DEPOSITED FILMS OF LEAD SULFIDE AXD METALLIC COPPER UNDER CONDITIONS OF CONTROLLED OXIDATION BY
G.
w.POLIKG AND J. LEJA
Department of M i n i n g dl. Metallurgg, University of Alberta, Edmonton, Canada Received March 11, 1963 Infrared multiple reflectance spectroscopy has been adapted to study the effects produced on the surface of vacuum deposited films by the adsorption of gases and the subsequent adsorption from xanthate solutions. A special vacuum cell (10- mm.) has been constructed, incorporating a multiple reflectanre attachment; the cell is equipped with accessories t o enable the following sequences of operations: vacuum deposition of adsorbent films (either transparent or opaque to infrared); recording of their reflectance spectra in pure argon; exposure of the films to controlled additions of reactive gases, followed by recording of the resulting spectra; subsequent exposure of the films to xanthate solutions (under inert gas) and recording of spectra; finally, washing of the adsorbed xanthate films with various organic solvents and recording the resulting spectra. In absence of oxygen in solution, xanthate anions do not adsorb on either copper or lead sulfide even though the adsorbent surfaces are covered by oxygen (presumably a monolayer, insufficient to give a detectable spectrum of surface oxidation product). Under similar conditions, dixanthogen (oxidation product of xanthate) does adsorb from deoxygenated solutions. The adsorbed xanthate radical is coordinated with one metal atom regardless of the valency of the metal in the substrate. Abstraction of dixanthogen and nonabstraction of xanthate anions are confirmed by ultraviolet spectrophotometric analyses of solutions. Admission of oxygen t o the system resultri in the formation of multilayers of metal xanthates, proportional to the amount of oxidation occurring a t the solid-liquid interface.
Introduction Infrared studies of adsorption a t the solid-liquid interface are somewhat more difficult than similar studies of adsorption from the gaseous phase.’ For studying solid-gas interactions the adsorbent can be prepared in situ in a special infrared vacuum transmittance cell. The solid surface can be maintained free of contaniination (before its exposure to the pure gaseous adsorbate) by employing high vacuum teohniques. Infrared studies of adsorption from aqueous solutions are fraught with the difficulties in maintaining both the solid adsorbent surfaces and the adsorbate solutions free of contaminations, e.g., oxygen or other highly reactive gases. Coadsorption of the solvent also can introduce additional complications in these systems. Hence, applications of infrared spectroscopy to study adsorption a t solid-liquid interfaces have been relatively rare.2 I n flotation theory, the mechanisms operative in S the adsorption of xanthates (R-0-C
//
, R =
\
S--M
alkyl group, RI = metal) on initially hydrophilic sulfide surfaces (in order to render them partially hydrophobic) have long been debated. The two theories presently in vogue propose (a) an ionic exchange mechanismS involving replacement of previously adsorbed inorganic anions by xanthate anions from aqueous solutions and (b) a hydrolytic or free acid mechanism4 involving hydrolysis of the xanthate anions to xanthic acid which then can adsorb more effectively as a neutral molecular species. Most researchers now agree lhat the xanthate species adsorbed on a heavy metal or heavy metal sulfide consists of a heavy metal xanthate surface compound. However, (1) R. P. Cischens a n d ‘8. A. Pliskin, “Advances in CatalyBis,” Vol. IX. Academic Press, Inc., New York. N. Y., 1957,p. 662; Vol. X, 1958,p. 1. 12) E. M.Eyring and &I.E. Wadsworth, Mznsng Eng., 5, 531 (1956). (J) IC. L. Sutherland and I. W. Wark, “Principles of Flotation,” Aust.
Inst. hIin. a n d Met., nlelbowne, 1956. (4) M.A. Cook and J. C. Nixon, J . Phys. Collozd Chsm., 54, 445 (1950).
some proponents of the hydrolytic theory assume the first layer of the adsorbed xanthate to be xanthic acid15 while others cannot agree whether the actual bonding is highly ionic or covalent. It has been founds,’ that if oxygen is eliminated from a flotation system, xanthate collectors (added as highly soluble short chain alkali metal xanthates) are incapable of rendering sulfide surfaces hydrophobic or floatable. Thus, it appears that a preliminary alteration of the adsorbent surface to a n oxidation product is a prerequisite of xanthate adsorption. Since these oxidation products are more soluble than the parent sulfide mineral, metal cations are thereby made more readily available for interaction with xanthate species. The Russian scientists, Plaksin6 and Kakovsky,7 maintain that oxygen, present in the aqueous phase, is strongly adsorbed by sulfide minerals and markedly reduces hydration of their surfaces. Penetration of the xanthate species to the solid surface where it can chemisorb is thereby facilitated by this dehydration. I n 1934, Gaudin and his collaborators* recognized that xanthate oxidation in solution may play a crucial role in the chemisorption mechanism. They envisaged that xanthate anions may become oxidized to the neutral molecule, dixanthogen,
s
(R-0-C
//
\
s-s
s
\\
C-0-R)
which then acts as
/
the effective adsorbing species. This postulate has since been largely ignored but recently Nixong has revived it as a possible explanation of the xanthateoxygen-sulfide interaction. (5) H.Hagihara, J . Phgs. Chem., 56, 610 (1952). (6) I. N. Plaksin, Trans. AIiME’, 214, 319 (1959). (7) 1. A. Kakovsky, Akad. Nauk SSSR, 106 (1950). (8) A. M. Gaudin, F. Dewey, W. E. Duncan, R. A. Johnson, a n d 0. F. Tangel, Trans. A I M E , 112, 319 (1934). (9) J. C. Nixon, “Proc. I I n d Int. Congr. of Surface .4ctivity,” Vol. 111, Butterworths, London, 1967, p. 369.
G. W. POLING AND J. LEJA
2122
Vol. 67
In addition, xanthate adsorbate solutions could be maintained in a deoxygenated condition (under 1 atm. of pure argon) during the adsorption treatment in the cell. The mechanism involved in xanthate chemisorption and the crucial role that oxygen plays in this interaction have been elucidated for the systems studied. Experimental
L
L
I
Fig. 1.-Optical arrangement for reflection spectral studies.
Before infrared spectroscopy could be applied to study the adsorption of xanthates on metals and sulfide minerals, the spectra of numerous pure xanthates and related compounds were recorded, and absorption band assignments were made.lOsll Subsequent infrared studies of xanthate adsorption on copper,12 lead sulfide113 and nickel14 substrates, from aqueous solut'ions exposed to air, showed the following results. (i) I n the presence of oxygen in solution, xanthate species adsorbed on these surfaces as multilayer coatings of the corresponding heavy metal xanthates, irrespective of whether the surfaces were previously purposely oxidized or sulfidized. (ii) Treatment of the substrates in dixanthogen alone, either in the vapor phase or as an aqueous emulsion, led to dissociative chemisorption again forming multilayers of the corresponding heavy metal xanthates. Excess dixanthogen from the emulsion phase was physically coadsorbed on the underlying chemisorbed metal xanthate film. (iii) Most organic solvents, which were capable of dissolving bulk heavy metal xanthate compounds, could wash off the majority of the adsorbed xanthate multilayers but very thin xanthate films of near monolayer dimensions were left tenaciously adsorbed on the substrates. This paper describes some results obtained from infrared studies of xanthate adsorption on vacuum deposited films of copper and lead sulphide using a novel infrared high vacuum ~ell.~5,16The cell was designed to enable the elimination of contaminations on the adsorbent surfaces and in the adsorbate solutions. It was believed that if this condition were achieved, xanthate adsorption would be limited to approximately monomolecular coverages and characterized by their infrared spectra. Although oxygen could not be completely eliminated from this system, calculations indicate that the adsorbent surfaces could not have been covered by more than a monolayer of chemisorbed oxygen prior to their treatment in adsorbate solutions. (IO) L. H. Little, G. W.Poling, and J. Leja, C a n . J. Chem., 39, 745 (1961). (11) L. H. Little, G. W. Poling, a n d J. Leja, ibid., 39, 1783 (1961). (12) J. Leja, L. H. Little, and G. 7.V. Poling, Trans. Inst. Min. and dbfet., 72, 407 (1963). (13) J. Leja, L. H. Little, a n d G. W. Poling, ibid., 72, 414 (1963). (14) G. W. Poling and J. Leja, Can. M e t . Quart., 1, No. 2, 109 (1962). (15) G. W. Poling, Doctoral Dissertation, University of Alberta, Feb., 1963. (16) G. W. Poling a n d J. Leja, presented a t the conference on "Sorption properties of yacuum deposited metal films," T h e Institute of Physics end Physical Society, in Liverpool, England, April, 1963.
Potassium ethyl xanthate and ethyl dixanthogen, used as adsorbates, were high purity compounds (99%+) prepared in this Laboratory.10 Infrared spectra were recorded on a Perkin-Elmer 221G double beam infrared spectrophotometer. Spectra of bulk samples of pure adsorbates and heavy metal xanthates were recorded as Nujol mulls or liquid films using conventional cells. A multiple reflectance optical accessory was employed to record the spectra of vacuum deposited substrate films of copper and lead sulfide prior to and following the adsorption treatments. Figure 1 shows a schematic drawing of this accessory. On employing six reflections from the two identically prepared substrate sample-mirrors, M,, and using ordinate scale expansion facilities of 1OX or Z O X , adsorption coverages approaching monolayer dimensions could be detected. Infrared studies of xanthate adsorption, under controlled gaseous environments, were conducted in a novel high vacuum incorporating the multiple reflectance accessory. An exploded view of this cell is shown in Fig. 2. In operating position, trhe base plate of the cell rests on the sturdy cover of the spectrophotometer source housing. The "sample mirror well," seen projecting below the base plate in Fig. 2, occupies the sample mirror area (shown enclosed by dashed lines in Fig. 1) of the multiple reflectance accessory. The main feature of the vacuum cell is that the two sample mirrors, M,, can be externally manipulated into various positions within the cell to enable the following sequence of operations. (i) Cleaning of the backing plates (onto which the films are to be deposited) by argon ionic bombardment. (ii) Deposition of adsorbent films of pure copper or lead sulfide on these plates by vacuum evaporation a t total pressures of ca. mm. A zirconium evaporator-getter pump, located near the vapor source, was used to reduce further the partial pressure of oxygen t o less than 10-8 mm. (iii) Recording the infrared spectra of the freshly deposited adsorbent films by positioning the two mirrors in a spring-loaded mirror mount in the "sample mirror well" (ITspositions shown in Fig. 1). This step was carried out with an atmosphere of pure argon gas in the cell. (iv) Treatment of the adsorbent filmsin deoxygenated aqueous adsorbate solutions for periods of 1 min. to 1 hr. with the mirrors placed in one of the beakers which are located on the base plate. The adsorbate solution is delivered into the glass beaker through the portal, PI, in the top flange while a steady flow of argon is maintained through the cell to eliminate ingress of air. Following the removal of the films from the solution and the drying in the argon, their spectrum was again recorded as above. ( v ) The treated adsorbent films could then be subjected to solvent washings, high vacuum, or ionic bombardment desorption treatments and their spectrum again recorded. On dismantling the cell, the hydrophobic character of the films was determined by captive bubble contact angle measurements in distilled water. Changes in the adsorbate solution were concurrently studied by ultraviolet spectroscopic analyses17 prior t o and following the adsorption treatment. Adsorbent Films.-Copper adsorbent films were prepared bJr evaporation of spectroscopic grade pure copper (99.999%) from a molybdenum boat-filament onto the glass-flat backing plates. These films were deposited within a 1-2 min. period and were specular reflecting, with a density of ca. 1 x 10-4 g./cm.2. According to data of Allen, Evans, and hIitchell18 these films would have an actual surface area about 10 times their geometrical area. Lead sulfide films were vacuum deposited onto front-surface aluminum mirrors from a small vitreosil crucible, radiation heated by a tungsten coil-filament which dipped into the PbS ponTder (17) 4 . Pomianowski a n d J. Lefa, t o be published. (18) J. A. Allen, C. C. Evans, and J. W. Mitchell, "Structure and Properties of Thin Films," John Wiley a n d Sons, New York. N. Y., 1959, PP. 46-52
Oct., 1963
XANTHATE ADSORPTION ON VACUUM DEPOSITED FILMS OF LEADSULFIDE
charge. The l a d sulfide source material consisted of a powder freshly ground from a 1a:rgesingle crystal (-1 in. diam.) of natural galena or AnalaR grade reagent PbS. Thin lead sulfide films are highly transparent to infrared radiation below about 3500 cm. -1. Lead sulfide surfaces are poorly reflecting (:yers. When PbS films were first purposely allowed t o oxidize, thick PbSzOs oxidation product formed on their surfaces.13; on subsequent treatment in xanthate solutions, the PbSzO? was completely replaced by adsorbed Pb(EtX)2 multilayers. When the PbS films, covered by adsorbed multilayers of Pb(EtX)2, Fig. 5b, subsequently were subjected to high vacuum (10-6-10-7 mm.) for periods of a few days, most of the xanthate desorbed from the surfaces. The remaining xanthate was estimated to be present in near monolayer thickness and exhibited a definite shift in thle high frequency C-0-C band from about 1210 to 1195 cm.-l (Fig. 5c). I n addition, a weak band appeared a t about 1265 em.-' which may have been due to the appearance of some dixanthogen oxidation product on the surface (see Fig. 46). When PbS films, covered by multilayers of Pb(EtX)Z, were washed in diethyl ether (which readily dissolves bulk Pb(EtX)z) the resulting spectrum was the same as that shown in Fig. 5c except that there was DO band at 1265 crn.-l. As with copper, treatment of freshly deposited PbS films in deoxygenated solutions of KEtX (3 X AI) did not result in any adsorption of xanthate and the films remained completely hydrophilic. Treatment in deoxygenated salutions containing ethyl dixanthogen and ethyl xanthate anions resulted in the adsorption of a near monomolecular coating of lead ethyl xanthate. The high frequency C-0-C band in the spectrum was again located a t ca. 3195 cm.-l instead of ca. 1210 cin.-l as in the spectrum of bulk Pb(EtX)z. I n the light of systematic infrared studiesT5of several different mono- and divalent metal (27) A. M. Gaudin and G S. Preller, Trans. A I M E , 169, 248 (1946).
'5
..
I
I
I
(t) -K
'(t )t 0
Zn
1'5
TI
N!
1
Pb 1'0
1
C;;t,
Ag
\mc-o-c
So
db
I
0
Electronegativity difference between sulfur and metal.
Fig. B.--Jnfrared
group frequency shifts in mono- and divalent metal n-butyl xanthates.
xanthate compounds, Fig. 6, this shift in C-0-C band frequency has been interpreted as indicating a 1:1 coordination between the surface metal atom and the xanthate radical in the first monolayer, Le., Pb-EtX. This is contrasted with 1:2 coordination between Pb xanthate in the multilayer coatings and in bulk lead xanthate precipitate, Pb(EtX)z. The above interpretation agrees with Hagihara's6 proposal of a 1:1 coordination in the first xanthate monolayer derived from electron diffraction studies. Ultraviolet analyses again showed that dixanthogen was abstracted from the adsorbate solution while the ethyl xanthate anion coiicentration increased slightly. Calculations showed that the amount of dixanthogen abstracted by the PbS films corresponded within an order of magnitude to that required for covering the available PBS surface area by an adsorbed xanthate monolayer. Conclusions Ethyl xanthate anions have been found to be incapable of adsorbing on copper or lead sulfide surfaces from deoxygenated solutions even though these surfaces were previously covered by approximately one monolayer of Chemisorbed oxygen. Under similar conditions, ethyl dixanthogen did adsorb on copper and lead sulfide surfaces to form a nearly monomolecular layer of the respective metal xanthate surface compound. Therefore, the crucial role of oxygen in the xanthate adsorption mechanisin cannot be explained solely by tlie pre-adsorption of oxygen on these surfaces. The data strongly indicate that under normal conditions, with adsorbate solutions saturated with air, the dissolved oxygen oxidizes xanthate anions in solution to the neutral molecule, dixanthogen, which becomes the active adsorbate. Since most metal and metal sulfide surfaces develop negative surface potentials on immersion in water, it is to be expected that a neutral molecule such as dixanthogen should possess higher activity toward these negatively charged surfaces than the anionic species. Having penetrated through the electrical double layer, the dixanthogen molecules dissociate at the surface and chemisorb as the corresponding metal xanthate surface compound. The shift in the C-0-C band, observed in the spectra of xanthates adsorbed in near-monolayer thickness on divalent metal substrates (like PbS or metallic nickelL4)has been interpreted as indicating a 1:l coordination between the surface metal and the xanthate
2126
D. N.
S I T H A R A M A R A O AND
radical. In overlaying multilayers of adsorbed xanthate, the 1 :2 coordination is retained. The close similarity between the spectra of the first monolayer and the covalently bonded metal xanthate compounds indicates that the chemisorption bonding is also highly covalent. If the bonding in the first monolayer had been ionic, the infrared spectrum would have more closely resembled that of the ionic metal xanthates, like potassium ethyl xanthate, Fig. 4a.
J. F. DUNCAN
Vol. 67
The multilayers of xanthate form by replacing the oxidized metal compounds present on the surface of a metal or metallic sulfide; the concurrent oxidizing reactions a t the solid-liquid interface and in the bulk of solution, saturating the solution with dixanthogen, also contribute to multilayer formation. Acknowledgment.-Support of this work by funds from the National Research Council of Canada is gratefully acknowledged.
MOLECULAR EXCITATION OF WATER BY r-IRRADIATIOS BY D. h'. S I T H A R S M A R A O
AN)
J. F. DUNCAN'
Chemistry Department, University of Melbourne, Australia Received November 9, 1968
A cadmium sulfide crystal counter has been used t o investigate the thermoluminescence of r-irradiated ice The results afford evidence for the formaand the fluorescence from ice and water during C060 +radiation. tion of OH radicals in the (%) state in both ice and water, some of which lose their energy by radiation emission state. In ice, there appears to be a large number of electron traps of varying depths, to form the ground (TI) approximately 2.4 e.v. above the ground rstate.
I. Introduction The work hitherto reported on the radiation chemistry of water has been mainly concerned with identifying the free-radicals formed in liquid and solid states by electron paramagnetic resonance and electron spin reeonanre methods. Remarkably little work has utilized the visible radiation emission produced from a y-irradiated substrate for diagnostic purposes. Spectrophotonietric methods have been employed3 to measure the total emission during and after irradiation of ice, but a detailed spectral analysis has not been attempted. Likewise pho to-multiplier methods have been used for similar studies on a- and r-irradiated t ~ a t e r . ~Such investigations _are difficult on account of the simultaneously emitted Cerenkov radiations, and indeed Greenfield, et U Z . , ~ attributed the total emission to this caus?. Brown and Miller6 also mere unable to detect non-Cerenkov emission and concluded that radiations characteristic of molecular species were not present, although they may have been undetected for instrumental reasons. We have re-investigated these systems using a cadmium sulfide crystal counter,' which device has a much higher radiation sensitivity than the devices previously used.
11. Experimental A. Apparatus.-The assembly used in this work is shown diagrammatically in Fig. 1. The specimen subjected t o yirradiation was contained in a silica spectrophotometric cell ( A ) , 0.8 cm. square and 8 cm. long. It was positioned a t the center of the all-copper den-ar flask (B) by means of a wire frame (C) attached to an 18-cm. (close-fitting) lead collimator (D) with a 0.5-cm. aperture. During irradiation, a 200-mc. Co60 (1) Chemistry Department, Victoria University of Wellington, Wellington, Piew Zealand. (2) F. S. Dainton, Brit. J . Eadiobiology, 31, 645 (1959); E. J. Hart, J . A m . Chem. Soc., 81, 6085 (1959); K. Miller, Rev. Pure A p p l . Chem., 7 , 123 (1957). (3) (a) E. C . Avery and L. I. G r o s s ~ e i n e rJ, . Chem. Phys., 21, 372 (1063); (b) L. I. Grossweiner and M. 6. hlatheson, ibid., 22, 1514 (1954). (4) J. A. Ghormley. ibid., 24, 1111 (1956). (5) ill. 8 . Greenfield, A . Norman, A. 13. Drowdy, and P. XI, Dratz, J . Opt. SOC.A m . , 43 [l],42 (1953). ( 6 ) L. 0.Brown and N. Miller, Trans. Faraday Soc., 61, 1623 (1955). (7) J. F. Dunca,n and n. T.Sitharamwao. Brit. .I. A p ~ 2 Phys., . 12, 511 (191; 1 ).
source was surrounded by a 10-cm. lead cylinder (E) 6 cm. in radius. The source and its shield could be instantaneously removed when required, in a second lead castle by means of a hoist. Measurements were made a t several temperatures by filling the annular space ( F ) with mixtures of organic liquids which %'ere cooled until the solid and the liquid were in equilibrium a t the melting point. The outer annulus (G) of the dewar vessel was evacuated to limit radiation losses. The main dewar vessel was allomred t o remain filled with air a t atmospheric pressure to maintain a good temperature control of the irradiated specimen. The temperature variations of the assembly were explored by means of a thermocouple (H) and the cell temperature was always found to be less than 2" different from the temperature of the thermostatic fluid. During irradiation, the cell temperature was measured by placing the thermocouple as close to the surface of the cell as possible without interfering with the measurements. The thermocouple was insulated from the surroundings by means of ebonite. The secondary radiation emitted during -/-irradiation was detected by means of a cadmium sulfide crystal counter described earlier.? The crystal ( N ) was mounted on a glass slide by indium-soldering the ends to platinum contacts from which electrical leads were taken. The output pulses obtained with 750 v./cm. applied to the ends of the crystal were amplified (EKCO amplifier, Type 1003B) and fed into an oscilloscope. The spectral analyses were carried out on the photographically recorded pulses. Since the spectral response of the detector was not uniquely determined by the energy of the radiation, proportionality of quantum energy and pulse amplitude could only be assumed over a certain spectral region. It was therefore necessary t o introduce filters between the irradiated water and the detector so that the different regions of the spectrum could be studied separately. Corrections were made for the lowering of intensity by absorption, from a knowledge of the absorption coefficients. The following organic liquids and solutions of inorganic salts tvere used as filters Filter
Glyoxal Benzene Copper sulfate (0.05 A [ )
Range,
A.
4280- 5200 2200- 2600 5500-12500
These filter solutions were mounted between two optically flat silica plates (S) which were held in position with a movable ring (J) fitting over the ebonite collimator. The latter had an aperture of 0.5 cm. and was 15 em. long. The end was 3 cm. closer to the dewar flask than the silica plates, and the whole assembly was fitted over B metal tube (K), protruding from the dewar flask. To distinguish between Cerenkov radiation (which is planepolarized) and the radiation due to the decay of molecular exci-
