H. J. MODIAND D. W.FUERSTENAU
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STREAMING POTENTIAL STUDIES ON CORUNDTJM IN AQUEOUS SOLUTIONS OF INORGANIC ELECTROLYTES' BY 11. J. MODIAND D. W. FUERSTENAU~ Department of Metallurgy, Massachuselte Znrrlilule of Technology, Cambridge, Massachuaells Rscsivsd Dsambrr d. IPd8
Strcamin potential measurementa show that corundum is positively charged ill water and indicate that H+and OHare potenti&determining ione for oorundum. The ~ e r point o of oharge occurs a t pH 9.45. fJnder no conditions do monovalent ions, such as Na+, CI-and NOI-, change the sign of the r e t s potential, (, snd hence they muat oat as surface-inactive change the sign oounterions. Provided the surface is oppositely chargod multivalent iona,-auch &LI Be++, SO,' and ,'&S of t and must funotion a8 surface-active oounter ione. dnder oonditione of like oharge, however, these aune ions function as surface-Inaotive fone at the oorundum-solution interface.
Introduction Information concerning conditions which exist at solid-solution interfaces has ao far been principally derived from adsorption measurements, emloying electrochemical or radiotracer techniques. !'he electrokinetic method affords another sensitive approach to elucidating the structure of the electrical double layer, especially for oxide systems. By using streaming potential techniques to evaluate the electrokinetic or seta potential, t, the structure of the ne atively charged uartz-aolution interface recently &B been etudi e?itensively.*-6 However, quarts is negatively oharged except at low pH, and come uently an electrokinetic study of a positively c arged quarts surface is experimentally difficult. Corundum, on the other hand, has been re orted to be positively charged in water, but publis ed data on ita surface electrical properties are exceedingly cant.^^^ Accordin ly, the ob'ect of this paper is to present a stu y of the e ect of various inorganic electrolytes on the seta potential of corundum, and to postulate the function of different types of ions at the corundum-solution interface. Experimental Materials and Method
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Materials.-Since natural corundum givea reeulta that range from a high negative charge to a high positive charge in synthetic sapphire boules manufactured b Linde Air Produqta ,Company were used in this inyestfgation. This material ie extremely pure and has a E cific gravity of 3.98. A s ectrographic analysis indicaterthe presence of several oxiiea totalling lesa than 0.027' * Nan0 and SiO, Were present ,to the extent of O.OOr%. Yhe material waa crushed and eised into the two fractions used for the streaming potential studies, namely, a 48/66-rnesh fraction for the pluq and a 28/36-mesh fraction to cover the holes in the latinurn electrodes. A considerable portion of the iron pmpurity introduced during crushing was removed with a hand ma net and the laat traces were removed by treatment with hy%ocbloric acid. The material waa next washed (1) Condensed from a thesis submitted by H. J. Mod1 In partlal fulRllment of the requirements for the degree of Doctor of Saience at the Messaohusetts Inatltute of Technolopy. (2) Metals Reserrch Laboratories, Eleotro Metallurgloal Company, Nlagam Falla, New York. (3) A. M. Oaudin and D. W. Fuerstenau, Tronr. A.Z.M,E,, POI,
66 (1988).
(4) A. M. Oaudla and D. W. Fuersteaau. Tronr. A.I.M.B.. SOS, 968 (1966). (6) D. W. Fuerstenau, THls JOURNAb, 60, 981 (1966). (6) F. Hagel, ibid., 4% 409 (1938). (7) H. R. Kruyt, "Collold Science," Vol. I, Elaevler Publishing
N. Y., 1992, Chapters IV, V, and VIII. (8) D. J. O'Connor, N. Street and A. B. Buobsnan, Awlrolion J . Cham., 7, 246 (1964). (a) D. J. O'Connor, P. (3. johansen and A. 8. Buohansn, Trona. Faraday Sw., 89, 829 (1956), Co.. New York,
with dietilled water till free from chloride ion. Finally, i t WM wsshed eeveral times with conductivity wator and atored under the same til1 used. To avoid contamination of corundum with surface-active impurities, all eolutions were re ared with cbotiductivit water, which had been doubto-dh!led from a blook-tin stilf On1 water with a speciflo conductance of leas than 5 X 10-Toohrn-l cm." was wed. The water W M etored and dia ensed in an atmosphere free of carbon dioxide. Except for sodium hydroxide and hydroohloric acid, all Inorganic chemicals used in the experimente were of rea ent $ade and were uaed without further purification. 8odyn [droxide, free from sodium carbonate, wm prepared y t e standard methods and atored in a carbon dioxide-free atmoaphere. HydrochIoric acid eolutione of various normalitiee were prepared by dilution from a stock solution of constant boiling acid, A paratus and Procedure.-The apparatus uaed in this etu y consists easentially of the cell ssaembly, a flow system and the electrical measurin e uipment which have been described in detail reviou$!o The ap aratua h s been suitably modified overcome polarisatfk difflcufiies encountered with platinum electrodes for potential measuremente in concentrated solutions. Silver-eilver chloride electrodes, repared according to the method of Brown,ll werc substiruted for platinum electrodes and roved to be unpolarizable with respect to all electrolytee frweatigated. The deaign and conetructlon of these electrodes, as well as the manner of their introduction in the experimental setup has been described in a thesis by Modi.l* In dilute solutions, aurface conductance lays an important role in the evaluation of the zeta potentfal from streaming potential experimenta.loP18 Because of eurfare conductance, the calculated zeta potential in dilute solutions is depressed and it appeare to rise initially as the concentration of 1-1 valent electrolytes is increaaed although it may actually be decreaaing. In this work, the ex erimental values are strictly correct only if the ionic etreng% is greater than 4 X 104. If the effect of surface conductance is kept in mind, the data can still be interpreted satiefactorily
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Experimental Results Zeta potentials of corundum were determined in solutions of 1-1 valent, 1-2 valent, and 2-1 valent inorganic electrolytes. The data are presented graphically in the accompanying figures in plots of 5 agaiast: logarithm of the electrolyte concentration. Borne of the points have been omitted for the sake of clarity. For corundum in conducthit water, averaged +68.7 mv. The standard dyeviation basod on 32 values waB less than 1.6 mv. Monovalent Electrolytes.-Figure IL presents the eeta potentials of corundum in solutions of sodium chlonde, sodium nitrate, sodium hydroxide and hydychloric acid. { in conductivity water is (IO) D. W. Fueratenau, Yin. &no., 8, 834 (1968). ( 1 1 ) A. 8. Brown, J . Am. Chsm. Soc.. Be, 646 (1934). (12) €1. J. Modl, "Eleotroklnetio Properties and Flotatlon Behavior of Corundum," Sc.D. Thesls, Maseachuaetta Institute of Teohnology, 1956. (13) J. Tb. G. Overbeek and P. W. 0. WIiga, Re. brw. c h h . , 08, 566 (PJt6).
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STREAMING p0TE:NTfhLS OF CORUNDUM IN INOllGANIC ELECTltOLYTE SOLUTIONS
taken as the initial point for the various {-log C' curves. With sodium chloride and sodium nitrate, t increases in absolute value with electrolyte M ; thereconcentration up to about 4 X after, it undergoes a logarithmic decrease with concentration until 10-2 64 solutions are reached, the slope, d{/d(log C), being -17 mv. At still higher concent>rat,ions,{ approaches zero as a limiting value, hut does not change sign. Both the curves are very similar, with the sodium nitrate curve being about 5 mv. higher than the sodium chloride curve. In solutions of hydrochloric acid, ( markedly increases with concentration to reach a maximum of about +125 mv. a t 3 X 10-6 M. Further additions cause a, substantial reduction in its value till it finally drops to 45 mv. a t lo-' M . Since surface conductance can scarcely be ex ected to play a more significant role in hydroch oric acid solutions than in sodium chloride solutions, this observation suggests that the role of H + is distinctly different from that of Na+. Aqueous solutions of sodium hydroxide change { from positive to negative values at relatively low concentrations. The isoelectric point occurs a t a hydroxyl ion concentration of 2.8 X N . In other words, the value of { is zero a t pH 9.4,5. The magnitude of the negative { decreases somewhat beyond equivalent per liter. The striking difference between the curves for sodium hydroxide and sodium chloride once again points to a special function of OH- in the system as compared with C1-. Sodium Chloride at DBerent pH Values.-To ascertain the effect of ionic strength on the zeta potential of corundum when the solid is positively or negatively charged, t was measured as a function of the concentration of NaCl at pH 4, 0.5, 10 and 11. As can be seen from Fig. 2, sodium chloride has no effect on { until the ionic strength is changed. For a given p H , decreases in absolute value as a linear function of the ionic strength until the ionic strength exceeds approxiThe rate of decrease appears to inmately crease with increasing surface charge. Sodium Hydroxide and Hydrochloric Acid in lo-' M Sodium Chloride Solutions.-The effect of H+ and OH- on { indicates that these ions play a special role a t the corundum surface aa compared with Na+ and Cl-. T o confirm this belief, the effect of adding sodium hydroxide and hydroM sodium chloric acid to solutions containing chloride was investigated. Such experiments would indicate the effect of H + and OH- in the absence of surface conductance and initial change of ionic strength. Figure 3 shows that relatively small additions of sodium hydroxide and hydrochloric acid have a marked effect on t long before the ionic strength is changed (compare with Fig. 2). This indicates that the marked effect of H + and OH- is real. Effect of Ionic Strength and pH on {.--Since the zeta potential of corundum de ends on bot,h pH and the ionic strcngth of the so ution, the value of r is plotted as a function of p1-I for different ionic strengths in Fig. 4. The different ionic strengths
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Fig. 1.-Zeta potential of corundum in eolutione of various electrolytes.
Cmmtmtlon of Sodium CMorlde, Equlvlllmts per Liter.
Fig. 2.-Zeta potential a8 a function of the sodium chloride conrentration at different pH valuee.
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Fig. 3.--Zeta potential of corundum BE E function of the addition of sodium h droxide and hydroohlorin acid in conduativity water andr10-4 N sodium chloride eolutions.
and p H values were obtained by making solutions with sodium chloride, and hydrochloric acid or sodium hydroxide. Observation of, Fig. 4 shows that the only point which is common to all the curves is the point where 5 is eero namely, pH 9.45. These data indicate that { is determined by pH even though the ionic strength of the solution remains constant, whereas Fig. 2 shows that if pH remains constant, the addition of an inert electro-
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zero at a concentration nearly twenty times more dilute. This further indicates a special role of OHat the corundum surface. Barium Chloride and Sodium Sulfate at pH 10.The behavior of divalent ions at the solid-solution interface under condit,ionsof negative surface charge was further studied by determining f in aqueous solutions of barium chloride and sodium sulfate at pH 10. The experimental data are shown in Fig. 5, together with measurements for sodium chloride added for comparative purposes. When the corundum aurface is negative1 charged, ( is affected in an identical fashion by a ditione of sodium chloride and sodium sulfate. This simiIN. Fig. 4.--Zeta potontial of corundum as a function of pH for larity of behavior suggests that neither Na+, C1different tots1 ionic etrengthe. nor 804- have any special effect at the surface, Ba++, on the other hand, seem to have a special I I I I I I I 1 affinity for the negative surface. The zeta potent tial decreases in absolute value with increasing concentration of barium ions and final1 becomes t positive at about 10-0 M. h i s fact in icates that the behavior of Ba++ is remarkably different from that of Na+; barium chloride solutions can change the sign off at high concentrations, whereas sodium chloride solutions do not. In addition, it is possible to conclude that Ba++ act just the same at a negative aurface as SO,- or 8gOsmat a positive aurface. In brief, all these results seem to indicate that the counter ions charged opposite1 to the surface lay the determining role in the e ectrokinetic be Io-' lo-6 IO" lo-' IO" lo-* 10-1 gavior of corundum. It also appears that s ecific Conccntratlar of A d d d EkctWe, E@vaknt# per Liter. Fig. 6.--Zeta otentirtl of corundum in solutione of bar- adsorption of counter ions depends mainy on ium chloride, sodrum chloride snd 6odium sulfate at different valency. Thus, divalent counter ions reverse the pH valuee. sign of {, but monovalent counter ions do not. lyte like sodium chloride does not affect ( until the Dbcuasion of Results ionic strength is changed. The facts presented in In this paper the model of the double layer proFig. 4 firmly establish that H+ and OH- have a posed by will be used to interpret the exs ecial function at the corundum surface, distinctly perimental results. An excellent discussion of ifferent from that of either Na+ or C1-. Divalent Electrolytes.-Zeta potential-concentra- Stern's treatment of the double layer is presented tion curves for corundum in solutions of barium by Overbyek.' The total potential across the double layer, comchloride, sodium sulfate and sodium thiosulfate are presented in Fig. 1. These electrolytes scarcely monly referred to ae the surface potential, f i ~is~ show the effect of surface conductance, as evidenced determined only by the concentration of potentialby the absence of the hump. The reduction in f determining ions in solution. Indifferent electrobrought about by additions of barium chloride is lytes do not affect its value unless they have secondanalogoua to that by sodium chloride, exce t that ary effects. The zeta potential is more complex and affected it is achieved at lower concentrations; li ewise, the slope of the t-log C curve is -17 mv. The by ail electrolytes, the effect depending not only on value of ( approaches zero at high concentrations concentration but also on the valence and sign of but does not change sign. The observed ~imilarit~y charge of the ions. Increasing the concentration in the behavior of sodium and barium chlorides in- of indifferent electrolytes in solution reduces the dicates that C1- has no special effect at the inter- value of [ by comprassion of the double layer because more ions are forced into the Stern layer. face. In this work, two main types of ions are conThe experiments also show that sodium sulfate and sodium thiosulfate exert an identical influence sidered: ' (1) potentialdeterminin ions, and (2) on of corundum over the entire concentration indifferent ions, which are subdividged into surfacerange investigated. t i~ positive in dilute solutions active and surface-inactive indifferent ions. Each but negative in concentrated solutions. The re- of these t pea of ions can be characterized by its versal of sign occurs with ahout 3 X molar effect on t e zeta potential. Potential-determining Ioncs.-Results of streamsolutions. Thus, SO,' arid S2O3=behave differently from CI- trnd NOn- in that, tjtieyare ctipsl)le ing potential measurements indicate pH plays an of chtlngiiig t,he sigri of {, whereas the latter do not. exce tionttl role at the corundnm mrface aa nhowrl l'lveii though thew divalent8iorls revewe the sign of hy t e followirig observations. {, inoiiovtllerit OH - rt?ducen the setla potaiitial Lo
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STREAMING POTENTIALS OF CORUNDUM IN INORGANIC ELECTROI~YTE SOLUTIONS
(1) 11+ cause 5 to become more positive than any other cation tested (Fig. 1). (2) OH- change the sign of f a t pH 9.45, where C1- and NO3- merely reduce it to zero a t infinitely high coricent,rations (Fig. 1). (3) In solutions of surface-inactive indifferent rlrctrolytes at a given tnt,al ioiiic strength (sa,me thickness of the double layer), the value of is determined by pII (Figs. 2 and 4). (4) Na+ and C1- do not affect 5 until the ionic strengt,h of the solution is changed, but H + and OH- change 5 under conditions where the ionic strength remains constant (Figs. 2 and 3). , ( 5 ) { is zero only at pH 9.45 regardless of the ionic strength of the solution (Fig. 4). Because of the manner in which { is affected by the pH of the solution, it mu& be concluded that H + and OH- function as potentialdetermining ions for corundum, in agreement with the findings of Verwey.16 If H+ and OH- are potential-determining ions, they must enter into an electrolytic reaction a t the surface which leaves the surface with a net positive charge in water. Since corundum is uncharged at pH 9.45, the double layer ceases to exist at this pH and there is an equal number of anions and cations in the solution next to the surface. The total double layer potential #o may be calculated from the relation
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with pH 10. Qualitatively, the increased adaorpt,iori of counter ions with increasing H+ and OHconcentration can be explained by the increasing potential drop acrow the double layer. Surface-active Indifferent Ions. -If the counterions are attracted to the surface not only by simple electrostatic forces, but also by strong chemical or covalent forces they inay reverse the sign of l as in the case of ha++,90,- and Sj200- (Fi s. 1 and When potentialdeterniining ions, suc 88 OH-, ikange the sign of l , the charge a t the surface as well as q0 must change sign; whereas when a surface-active counter ion chan es the sign of t there must be a higher charge in t e Stern plane than a t the surface. This results in the formation of a triple layer.’ At pH 10, Ba++ must be surface active since f is reversed in molar solutions. However, this affinity of Ba++ for the surface is not apparent in neutral solutions, where merely approaches zero as a limiting value a t high concentrations. If Ba+f were specifically adsorbed, would have become more ositive on increasing the concentration of barium c loride, but this has not been observed. Similarly SO4- are also specifically adsorbed but only when the surface is ositively charged. The specifically adsor ed ions, Ba++, 804- and SaOa-, are all divalent; in contrast no monovalent ion was found to be specifioally adsorbed. It has been shown by Gaudin and Fuerstenau’ that Ba++ and Al+++ are specifically adsorbed by quartz wth a negative surface charge, but significantly no where k is the Boltzmann constant, T is the abso- monovalent inorganic ion was found t o be specifilute temperature, v is the valence of the ion in cally adsorbed. On the baais of these results, it question, e is the electronic charge, Ctl+ and COH- seems that multivalent but not monovalent counare the concentrations of the potential-determining ter ions can function as surface-active indifferent are their con- ions and reverse the sign of zeta potential. The ions in solution, and CH”-and COH~centrations at the zero point of charge. Since the specific adsorption probably results from a multizero point of charge for corundum occurs at p H valent ion sitting on a single site a t the surface. 9.45, in pure water the double layer potential is, Summary and Conclusions therefore, positive ($0 = +145 mv.). As menBy means of streaming potential techni ues, it tioned before, in dilute solutions, surface conductance lowers the experimental values of 5. I n the has been shown that corundum is positively c arged absence of surface conductance a near identity of in water. Hydrogen and hydroxyl ions are poand $o should, therefore, be expected. Conse- tential-determining ions for corundum, and the quently, the true zeta potential of corundum in zero point of charge occurs a t pH 9.45. Sodium, chloride and nitrate ions function as surface-inconductivity water should be about 125 mv. active indifferent ions under all conditions and, Surface-inactive Indifferent Ions.--Since surfaceinactive electrolytes reduce the value of by com- therefore, cannot change the sign of 5. Barium, pression of the double layer without changing its sulfate and thiosulfate ions function aa surfacesign, l approaches zero as a limiting value at higher active Counter ions, provided the surface is opconcentrations. Sodium chloride and sodium ni- positely’charged, and ar’e able t o change the sign of trate are electrolytes of this type as seen from Fig. r. Under conditions of like charge, however, these 1, C1- and NOa- functioning as counterions. The same ions function as surface-inactive indifferent difference between the curves, although small, is ions. Thus the reversal of zeta potential seems to believed to be due to the difference in anions, and be a characteristic property of multivalent inorpoints out that valency alone does not determine ganic counter ions. Since pN determines the nature of the surface the behavior of counterions at the interface. Experimentally, the results agree with the relative charge and also the magnitude of the surface popositions of C1- and NOI- in the lyotropic series. tential, $0, it will be expected to affect markedly As seen from Fig. 2, the reduction in potential by the adsorption of all other ions by corundum. Acknowledgments.--This research was made posthe addition of NaCl is more rapid at pH 4 than at pH 6.5. Similar is the case for pH 11 RS compared si ble through the financial support provided by the Aluminum Company of Atnerica. The authors (16) E, J. W. Verwey, In “Colloid Chemistry, Theoretical and wish to givr thank8 to Professors A. M. Gniitlin ant1 A l i i d i w l , ” 4. a. Alexander. Vol. 111, Rrinholrl Piibl. Corp., New Yiirk, N. Y.,I D A f ) , 1). 5fl. 1’. T,. ~ l t ~ l 3 1 * 1 1 . ~For 1 t t,lwir int,ct*tbst,iti this wciik.
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