N. SARKAR AND A. M. GAUDIN
2512
Hysteresis of Contact Angle in the Galena-Water-Nitrogen System
by N. Sarkar Chemicals Department, Research Laboratory, The Dow Chemical Company, Midland, Michigan
and A. M. Gaudin Department of Metallurgy, Massachusetts Institute of Technology, Cambridge, Massachusetts (Received January 28, 1966)
The hysteresis of contact angle in the system galena-water-nitrogen was studied using potassium ethyl xanthate and hexyl mercaptan as surfactants a t various pH values and surfactant concentrations. Contact angles were found to increase with increasing surfactant concentrations. The advancing contact angle was found to remain constant for a wide range of pH values, whereas the receding contact angle changed with change in pH. Adsorption of ethyl xanthate a t the galena-water interface was measured at different xanthate concentrations and a t different pH values. At pH 5.5 the maximum adsorption density was found to be equivalent to a monolayer of xanthate on galena surface. The adsorption density was found to decrease with increasing pH. Oxygen was found to be one of the factors influencing adsorption; the adsorption increased with an increase in the oxygen level of the system.
Introduction From the significant; amount of work that has been reported on the hysteresis of contact angle, it is clear that the phenomenon is complicated by a large number of factors. Several explanations to interpret the causes and effects of hysteresis and its incompatibility with the concept of an equilibrium contact angle have been published. These explanations include differential adsorption at solid-fluid interfaces, l p 2 surface roughness, a-7 surface inhomogeneities,s* kinetics and nature of adsorption, and related chemical and physical factor~.'O-'~ Further exploration of this phenomenon in a practical system is necessary with the point of view of the interdependence of the several chemical and physical factors, such as adsorption of the surfactant, interfacial tension, pH, etc. The present work has been instigated by the pioneer work done by Wark and Cox in the field of flotation, where hysteresis was ignored. Hysteresis of contact angle is investigated in the present work in the galena-water-nitrogen system on addition of minor quantities of potassium ethyl xanthate and of hexyl mercaptan. The Journal of Physical Chemistry
Experimental Section Materials. For contact angle measurements, galena blocks about 1.5 X 1 X 0.5 cm were cleaved from a large chunk under water at natural cleavage planes and (1) A. B. D. Cassie, Discussions Faraday Soc., 3 , 11 (1948). (2) G. MacDougall and C. Ockrent, Proc. Roy. SOC. (London), A180, 151 (1942). (3) Ya. B. Aron and Ya. I. Frenkel, Zh. Eksperim. i Teor. Fiz., 20, 453 (1950). (4) N. K.Adam and G. Jessop, J . Chem. SOC.,127, 1863 (1925). (5) B. R. Ray and F. E. Bartell, J . Colloid Sci., 8 , 214 (1953). (6) F. E. Bartell and J. E. Shepard, J. Phys. Chem., 5 7 , 211 (1953). (7) R. Shuttleworth and G. L. J. Bailey, Discussions Faraday SOC., 3, 16 (1948). (8) D. C. Pease, J . Phys. Chem., 49, 107 (1945). (9) R. E. Johnson, Jr., and R. H. Dettre, ibid., 6 8 , 1744 (1964). (10) S. J. Gregg, J . Chem. Phys., 16, 549 (1948). (11) N. K.Adam, Discussions Faraday Soc., 3 , 5 (1948). (12) I. Langmuir, Trans. Faraday SOC.,15, 62 (1920). (13) R. C. Lumis, Thesis, Massachusetts Institute of Technology, 1958. (14) A. M. Gaudin, L. B. Bangs, and A. F. Wilt, "Hysteresis of Contact Angles in the System Benzene-Water-Quartz,'' 7th International Mineral Processing Congress, New York, N. Y ., 1964. (15) L. R.Sonders, D. P. Enright, and W. A. Weyl, J . A p p l . Phys., 21, 334 (1950).
HYSTERESIS OF CONTACT ANGLEIN
THE
GALENA-WATER-NITROGEN SYSTEM
then polished under water on fiberglass cloth. The galena was then treated with hot concentrated sodium hydroxide solution for about 10 min, then with sodium sulfide solution, and then polished again under water. It was then washed thoroughly with distilled water and tested for lack of contact with a nitrogen bubble under water in the absence of intentionally added surfactant. If there was any contact, the piece was repolished after treatment with sodium sulfide and washing until no contact was obtained. This uncontaminated galena cube was then placed in surfactant solution for contact angle measurements. For adsorption measurements, galena lumps were crushed on rolls, jigged to eliminate low-density impurities, ground wet in a porcelain ball mill, and finally screened under water to collect all the -150 mesh particles. This -150 fraction was then deslimed by sedimentation to collect the -150, +600 mesh fraction. This was stored under water deoxygenated by constantly bubbling nitrogen. Analysis of this galena showed 86.7y0 lead and 13.23% sulfur as compared with the theoretical values of 86.60% lead and 13.40% sulfur for pure PbS. Early adsorption experiments indicated that even though extreme precautions had been taken to prevent aerial oxidation of galena, stock galena samples were oxidized enough to show multilayer adsorption of potassium ethyl xanthate. For all adsorption experiments, the samples were sulfidized with sodium sulfide solution prior to adsorption tests. The surface area of the sulfidized samples determined by the BET method gave an average of 867 cm2/g. Potassium ethyl xanthate was purified according to the method of Goldstick,16employing acetone dissolution and petroleum ether reprecipitation. Hexyl mercaptan was purified by double distillation under reduced pressure. Apparatus and Procedure. Hysteresis of contact angles was measured using the sliding captive bubble method.” The procedure was to hold a nitrogen bubble, squeezed out of a vertically held capillary glass holder, on the polished surface of galena immersed in surfactant solution. A steady lateral motion was then imparted to the galena which resulted in distortion of the bubble, and later a sliding of the bubble over the galena surface. Motion pictures of the magnified image of the bubble in relative movement were taken with a movie or a time-lapse camera. The contact angles were measured on the film through the water phase. The advancing contact angle was measured under conditions where the solid-gas interface was shrinking and the solid-water interface was gaining. It was always larger than the receding angle. For adsorption measurements a special apparatus was
2513
made which allowed batch determination of the adsorption density of xanthate on galena in a controlled atmosphere. About 10 g of galena was placed in the cell which was then filled with 100 mg/l. aqueous solution of sodium sulfide. It was agitated by rotation at about 40 rpm for 1.5hr. The solution was then filtered out and the solid washed with deoxygenated water four times. Every washing and filtering was done by displacing the water of the cell by deoxygenated nitrogen. It was observed that a t the end of the fourth wash all the sulfide ions were washed away (as could be ascertained by testing the wash with lead ions). After that the cell was completely filled with deoxygenated xanthate solution and agitated for 1.5 hr. This long reaction time was assumed adequate for equilibration. After equilibration, the solution was filtered out, centrifuged, and analyzed for xanthate. Analysis for xanthate was made spectrophotometrically, using a Model DU spectrophotometer at a wavelength of 300 pm.18v19 Analysis for oxygen was done polarometrically, using a Beckman Model 777 laboratory oxygen analyzer.
Results Xanthate solutions, deoxygenated by constantly bubbling with nitrogen, showed that under neutral or alkaline conditions, xanthate is reasonably stable. At pH values lower than 7 , xanthate solutions were found to be unstable in the presence of air, whereas at alkaline pH values xanthate was found to be stable even in the presence of air. Ultraviolet spectra of decomposed xanthate solutions indicated the presence of carbon disulfide. Surface tension measurements, done by the pendant drop method at pH 6.5, indicated that the surface tension of water did not change appreciably with increase in xanthate concentration. The results of the adsorption of ethyl xanthate ions from aqueous solutions a t pH 5.5, 6.5, 8.0, 10.0, and 11.0 are shown in Figure 1. All of these experiments were done under the identical conditions of minimal oxygen content of the solution and in the same cell at room temperature. The solutions used for the washing and adsorption tests had an oxygen content of 0.05 i 0.01 ppm. It was observed that adsorption of xanthate on galena increased with decreasing pH values. If we (16) T. K. Goldstiok, Thesis, Massachusetts Institute of Technology, 1959. (17) A. M. Gaudin, A. F. Witt, and T. G. Decker, Trans. A I M E , 226, 107 (1963). (18) 0. Mellgren and M. G. Subba Rau, Bull. Inst. Mining Met., 676,425 (1963). (1s) A. Pomianowski and J. Leja, Can. J. Chem., 41, (9), 2219 (1963).
Volume 70,Number 8 August 1966
2514
N. SARUR AND A. M. GAUDIN
:I 20
0.01.
0.I I.o 10 EOUILIORIUY XANTHATE CONCENTRATION IN MQ./LlTER
100
Figure 1. Adsorption density of xanthate on galena as a function of xanthate concentration.
OD1
0.1 IO2 XANTHATE CONCENTRATION IN WQ. PER LITER
Id
Figure 3. Contact angle as a function of xanthate concentration at pH 6.5.
p'
loo
A. WITH S T W ( GALENA (SOYWHAT DXDi7.d 8. WTH SlOa GALENA AFTER WLFDIZINQ USINQ AN 411 SATURATED SOLUTION
C. WITH THC LOYLST OpCONTENT ATTAINABLE WITH THE PRESENT ECHNIOUE
80
I-/?' I
A
B.
c
~
60
:F-: I
I
I
I
40. k
1
-I
CONTACT ANQLE IN DEGREES
' O t
A
st---ow C.
* I
I
I
^ l
l
" :
"
I
I
OQ01
0.1
I
10
I d
ma
XANTHATE CONCENTRATION I N W . / L I T E R
I
Figure 4. Contact angle as a function of xanthate concentration at pH 9.8.
assume the parking area of xanthate to be 25 A2, the adsorption density for a complete monolayer is 6.7 X nio1e/cm2, which corresponds closely to the maximum adsorption noted at pH 5.5. These results are similar to those obtained by Mellgren and Subba Rau.'* The effect of oxygen on the adsorption of xanthate on galena at pH 6.5 is shown in Figure 2, in which xanthate abstraction is plotted as a function of equilibrium xanthate concentration. Curve C gives the adsorption isotherm at the minimum oxygen level attainable (see also Figure 1). Curve B gives the xanthate abstraction from the solution saturated with air, and curve A gives the xanthate abstraction by the stock galena sample which is oxidized. Results of the contact angle measurements as a function of xanthate concentration at pH values 6.5, 9.8, and 11.6 are shown in Figures 3, 4, and 5, respectively. All of these experiments were done with a speed The Journal of Physieal Chemistry
,,c CONTACT ANGLES
0.1
XANTHATE CONCENTRATION IN Ma/ LITER
Figure 5. Contact angle as a function of xanthate concentration a t pH 11.6.
of movement of the solid of 1.4 mm/min, and after an elapsed time of 1 hr. It was observed that over the entire range of concentration and pH values, the angles attained their steady-state values in a very few minutes after immersion of the galena piece in the bath. All of
HYSTERESIS OF CONTACT ANGLEIN
THE
GALENA-WATER-NITROGEN SYSTEM
these measurements show a high hysteresis and a hysteresis maximum at a concentration of about 0.35 mg/l. At all pH values both the advancing contact angle @A) and the receding contact angle (&) are found to increase gradually with increasing xanthate concentration, ultimately becoming constant. However, Figure 5 indicates that at extremely high xanthate concentration, i.e., about los mg/l., both BA and start to drop again. Figure 6 shows the results of the contact angles as a function of hexyl mercaptan concentration at pH 6.0 f 0.1. These results are similar to those observed with xanthate except that the receding contact angle, OR, is a little lower and the point of maximum hysteresis is at about 2 mg/l. Figure 7 shows the effect of pH on the contact angle at constant xanthate concentration of 25 mg/l. At pH 5.3 both the receding and advancing angles are maximum and hysteresis is minimum. Above and below pH 5.3, f?R decreases gradually becoming zero at about pH 1 and pH 12.5, respectively. 0.4 remains constant for a wide range of pH values (from about 2 to 12) above and below which it drops rapidly to zero. Similar general phenomena are observed at a lower xanthate concentration, e.g., at 0.35 mg/l. The results of the contact angle measurements as a function of pH at constant hexyl mercaptan concentration of 25 mg/l. are shown in Figure 8. It is observed that with increasing pH, the advancing angle remains constant until pH of about 13.5 after which it drops very rapidly, whereas the receding angle gradually decreases with increase in pH values.
loo
2515
I
I
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,
w
m
I 5 1 0
I
I
h Y L Y R U P T A N CWCENTRATWN ‘IIMG Limn
Figure 6. Contact angle aa a function of hexyl mercaptan concentration at pH 6.0.
pw
Figure 7. Contact angle as a function of pH a t an initial xanthate concentration of 25 mg/l.
Discussion In analyzing these results, it is important to consider the form in which the surfactant exists in each fluid phase. In experiments in which a mercaptan was used, the surfactant may have existed in the form of mercaptan molecules in either the aqueous or the gas phase. In the aqueous phase it may also have existed as mercaptide ion. Furthermore, it may also exist in the form of an oxidation product of mercaptan- as the disulfidein either phase. Since the mineral is a reducing agent, passage of the reagent from the aqueous phase to the solid-gas interface could take place, without a net over-all change in oxidation, by first oxidizing the mercaptan to disulfide at the solid-water interface or in the aqueous phase, then by passage of disulfide from the aqueous to the gas phase, and finally by reduction of disulfide to mercaptan at the solid-gas interface. Such a transfer niechanism is possible in addition to transfer by direct! passage of mercaptan molecules from
Figure 8. Contact angle as a function of pH a t a hexyl mercaptan concentration of 25 mg/l.
the aqueous to the gas phase, followed by adsorption to the solid-gas interface. In experiments in which a xanthate is used as surfactant, similar considerations prevail. The surfactant may have existed in the aqueous phase either as xanthate molecules, as xanthic acid, as xanthate ions, or as dixanthogen molecules (the results of temporary oxidaVolume 70, Number 8 Auguet 1066
N. SARKAR AND A. M. GAUDIN
2516
tion), but xanthates being salts of strong (though unstable) acids, xanthate molecules must be rare. Since xanthic acid is known to be unstable, the rate of decomposition being proportional to the hydrogen-ion concentration,20*21 we conclude that in the aqueous phase the surfactant is either in the ionic or in the dixanthogen form. In the gaseous phase we can have no xanthate since the salt is nonvolatile, and we can have but little acid, since the acid is unstable in the aqueous phase. However, we could have the corresponding oxidation product, dixanthogen, which could revert to xanthate at the gas-solid interface, just as disulfide could revert to mercaptan at the gas-solid interface of the mercaptan analog. The behavior of contact angle on changes in pH can be explained by taking into account the competition between hydroxyl ion and the surfactant. With increasing pH the competition by hydroxyl ions for the sites occupied by xanthate ions increases due to increase in the relative abundance of hydroxyl ions. Clearly, this should affect the receding angle before it affects an advancing angle, as the receding angle is concerned primarily with a wetted surface, while the advancing angle is concerned primarily with a dried surface. If we assume that the adsorption of xanthate on galena at the gas phase is larger than that at the water phase,22then when we move the bubble on the galena surface, the galena-water interface must change to a galena-gas interface and consequently become a surface of higher xanthate adsorption density than it had been. Conversely, on the other side of the bubble a galena-gas interface must change to a galena-water interface and become a surface of lower xanthate adsorption density than it had been. The receding angle corresponds to the first side of the bubble and the advancing angle corresponds to the second. The constancy of the advancing angle over a wide range of pH values must be related to the relative unimportance of pH on a kinetic basis at the advancing angle. Conversely, adsorption density at the watergalena interface is dependent on pH and with increasing pH, adsorption density decreases, as indicated in Figure 1. Consequently, with increasing pH the receding face of the bubble meets less and less densely coated surfaces, and lower and lower receding angles are noted. According to this interpretation, receding angle is primarily related to the adsorption density at the watergalena interface, whereas advancing angle is related primarily to adsorption at the galena-nitrogen interface. This postulation is well shown in Figure 8 with hexyl mercaptan. The time for wetting to occur after a bubble is brought in contact with a wet surface is defined as the The Journal of Physical Chemistry
induction time. Induction time in the present system was measured by pushing down the bubble in contact with the surface and then pulling up the bubble after a certain time to observe visually any sticking of the bubble to the galena surface. By this trial and error method, induction times longer than 5 sec could be estimated with an error of the order of 5 sec. The results are summarized in Table I, which shows that the induction time is highly dependent on pH and the concentration of xanthate in solution. Induction time generally increases with decreasing xanthate concentration or on increasing the pH. The important conclusion was that whenever the induction time was too short to
Table I: Effect of pH and Xanthate Concentration on Induction Time Induction
PH
Xanthate concn, mdl.
6.5 6.5 6.5
0.025 0.25 2 . 5 &up
About 15
9.85 9.85 9.85 9.85 9.85 9.85
0.004 0.04
45-60 15-30 5-10 1-2
11.62 11.62 11.62 11.62 11.62 2.03 3.05 5.9 6.35 9.5 10.0 11.02 12.03 9.8 11.45 12.0 12.25 12.55 12.9
0.10
0.4 2.5 25
0.04 0.10 0.4 4.0 25.0 0.352 0.352 0.352 0.352 0.352 0.352 0.352 0.352 25 25 25 25 25 25
time, aec
0
0
0 0
About 30 About 15 2-5 0
0
2-5 1-2 0 0
1-2 2-5 About 15 About 30 0 0
5-10 About 15 15-20 20-30
(20) C.du Rietr, Iva., 24, 257 (1953). (21) V. I. Klassen and V. A. Mokrousov, “An Introduction to the Theory of Flotation,” Butterworth and Co. Ltd., London, 1963, p 237. (22) P. L. de Bruyn, J. Th. G . Overbeek, and R. Schuhmann, Jr., Mining Eng., 6, 519 (1954).
GIBBS FREEENERGIES OF FORMATION OF THORIUM PHOSPHIDES
be measured, the receding contact angle was not nil, and conversely that when the induction time was measurnble, the receding contact angle was zero or very close to zero. For flotation to occur, the receding contact angle must not be nil. This conclusion arises from the very short time (of the order of a millisecond) that a bubble is in a position close enough to a suspended particle to make or not make adhesive contact. The contact angles were not observed to be dependent on the speed of the movement of the solid, neither on the
2517
bubble volume. No relaxation phenomenon was observed, and the system was not found to be completely reversible with respect to pH. The irreversibility of contact angles with respect to pH can be attributed to the extremely slow rate of desorption of xanthate ions from the water-galena interface.
Acknowledgment. The authors wish to record their appreciation for the fine support they received from the National Science Foundation for conducting the research on which this paper is based.
Gibbs Free Energies of Formation of Thorium Phosphides from Solid=StateElectromotive Force Measurements1
by K. A. Gingerich Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio
and S. Aronson Brookhaven National Laboratory, Upton, New York
(Received January 31, 1966)
Thermodynamic information in the thorium-phosphorus system has been obtained from measurements on solid electrochemical cells a t 800-950'. The following cells were employed: (I)Th, ThF41CaF21ThF4, ThP, Th3P4; and (11)Th, ThF41CaF21ThF4, Th,, ThPo.&. The measured emf values at 900" were 582.2 f 30 mv for cell I and 8.9 f 8 mv for cell 11. From the data on cell I, the following thermodynamic properties were calculated for the reaction Th ThaP4 = 4ThP at 1173'K: a free-energy value of -53.7 i 2.8 kcal/g-atom of Th, an entropy value of -9.3 f 3.7 cal/'K g-atom of Th, and an enthalpy value of - 64.6 f 7.1 kcal/g-atom of Th. Free energies of formation at 1173'K of ThP of -71.5 f 4.8 kcal and of Th3P4 of -232 15 kcal were calculated using the emf data and information on the PZpartial pressures over Th3P4. An enthalpy of formation of ThP of - 106 f 10 kcal a t 1173°K has also been estimated.
+
*
Introduction In the thorium-phosphorus system, two compounds have been reported by Strotzer, et a1.,2 Th34 and a subphosphide: Gingerich and Wilson3 have confirmed these findings and showed that the subphosphide is phosphorus-deficient monophosphide that . exists be-
tween the compositions ThPo.6s-ThPo.st, at lOOO', whereas ThsP4 was found to be stoichiometric within (1) This work was in part performed under the auspices of the U. S. Atomic Energy Commission. (2) E. F. Strotzer, W. Biltz, and K. Meisel, 2.A W Q . Allgem. Chem., 238, 69 (1938).
Volume 70, Number 8 August 1986
