Feb., 1959
DIFFUSION OF SILVER IN SILVER SULFIDE
( e ) The amplitude of t,he triangular wave was measured with a, DuMont volt.age cali1)rator in conjunction with the CRO. The maximum amplitude was 3 v. peak to peak; 2 v . is sufficient for most work in oscillographic polarography. (d) The a x . component of the d.c. power supply was found to be less than O.Olyo. (e) The frequency range of the triangular sweep, determined using a DuRIont signal generator and the CRO, was 1 to 200 c.p.s. The fine and coarse frequency controls were calibrated at this time. ( f ) .is a filial check, the instrument was used to obtain an oscillographic trace for a solution I inn1 in cadmium chloride nnd 1 Til in potassium chloride; a d.1n.e. was used with n nierc,ury pool reference e,lectrode. The result was viaunlly compared ivith those ol)tained by previous investigators. The strong 6O-cyc:le interferelice originally present wits largely eliminated by using shielded lends for a11 esternal connections; further reduct,ions were achieved by wrapping the mercury rescrvoir and hose with nluininuni foil connected t o ground and placing the entire appnrnt8uson a grounded copper sheet,. The slight amount of interference remailling did not hinder observation of the trace. Chemicals.-Oil-free air and oxygen-free nitrogen were obtained using customary gas purifying trains. Rtallinclirodtj ,4.R. grade potassium cliloride, potassium iodide iiiid cadmium chloride were used. Procedure.-Since the depressed B-cathode for lower output circuit (Fig. 5C) \i.ould not develop the sufficieiitly negntive starting potentials ( -0.5 v.) desired for the present esperinients, the conventional c:rthode follower out,put circuit \vas used. A niercuiy pool in the po1arogr:q)hic cell I)ot,tom served as reference electrode. The resistance of tho dyopping mercury electrode (d.ni.e.) \vas considerably lowered hg sealing a plnt~inuniwire into t,he cnpillayy tube about 0.5 inch from t,he tip; the resistance W:LR then about 10 ohnis. .4t the mercury column height of S i cin. used throughout, the drop-t,ime was 12.G see. and the rate of flow of mercury w:w 0.344 mg./sec. The tot,nl cell resist'ance I IiCl solution and the capill:ii,y was repentedly with 1 A measured, using 1000 c.p.s. and a Whe:rtst,one bridge; a v:ilne of 50 ohms was consistently obtnioed. In an espei,iment, the cell, maintained a t 25 + 1" and contniniiig the mercury pool, wis filled with the test solution. I n the oxygen runs, air was t,hen bubble,d t,lirough the solution for 5 miii. to saturate it; in t,he oxygen-free
223
runs, nitrogen was bubbled through for 20 miii. In the meantime, the appwatus was allowed to warm up. The triangular wave gener:ttor was set to the tlr~ired. frequency, iiiiti:il potential and potential span. The vertical and Iioiizontal centering and horizont:tl aniplifier controls of the CRO were then :tdjusted t o produce :L 2411. wide pattern in the center of the screen. With the vert8ic:rl CRO aniplificntion control ntlvnnced to n previously cdihrnted sensitivit.y of 2 to 6 mv./in., t8he pattern W:LS adjusted t80a height, of about, 2 in. using R. In this manner, the resistance, R, was kept t80a nlinimiun, and the trace W:LS Iai,ge enough for accurnte menaurenient, yet not so large as t o he distorted by the screen curvature. The trace was pliotogrnphicnlly rec.oi,detl Iiy niultiesposure on 35-mni. Eastman Kodnk Plus S film, using a DuhIont No. 2il-;\ oscil1ogr:iphic record cainer esposui'c nt f5.G. The p1iotogr:rph 01' tlie ti aln.xvs tttken 10 sec. :tfter t'lie tli,op hrgnn t80form, tinietl to i 0 . i sea. using n stopivntch. T o ol)tmaindetails of the t,rac,e against,the plnst,ic grid of the screen, islieproceduir! clrscl,ilied was followed. .4fter photogrnphilig t,lie tr:ire, t,lie ti,innguInr wave was teinpornrily shut, off, using switch 1 (Fig. 5B), iesulting in cessation of the voltnge swmp to the cell; a spot then nppe:wed on t,lie screeii, whose horiaoii tal position was indicative of the npplied potential re1:itive to the mercury pool. The lntter potent.iir1 was measured to f l m v . on a ~~ot~entionieter connected across t,he (:ell nnd an esposure of the spot taken. Thc applied potentinl then was chnnged by means of t'he init#inlpokntial regulat,or cotitt,ol ( R 4 , circuit 2, Fig. 5C), another re:itliiig made 011 the Tiot8entionietei,, and the new spot, photographed. Two poitits thus were 01)tninetl, from which the potential span niid the iiiitinl potc:utial of the t.rinngulnv smecp could \,e c:ilculated. S e x t , the spot. \vas deflected off t81irsci'ceii, t,he 1)right~nesstumed u p to give :t slight glolv :tiid the finnl esposure macle, recording tlie cnlibrntion lines 011 the grid.
Acknowledgments.-The authoix \\ish tmothniik the Atoniic Eiiergy Commission niitl the Office oi Na~valResearch for helping to support' t,liis inr-est'igatioii. Thaiilts arc: nlso due to Professoi, Charles 3'Inixh, Lj~hohelped desigii the electli*oiiiceqiiipnieiit used, mid to Professor John R. Ilnyes.
DIFFUSION OF SILVER I N SILVER SULFIDE' 'BYROYL. XLLEXAXD WALTERJ. TOO RE Clioiiicul Laboi,utot,,y,Indiuira Uniuoxily, Blooiiiinglon, Indiana Receiued
dugubt
6,105s
1)ifusioii of rndioxtivo hg-110 i i i silver sulfide has been measured i n pol>~cryst:rIlinespecinietis of tile lo\v tctml)cr:itrire (nionocliiiic) and high tempelntwe (cubic) forin. The diffusion coefficients %re: crihic Ag2S,D'g, = 2.8 X l o w 4cYp( -3450 c:il./RT) cm.2 see.-' and nionocliiiic Ag& D,ke = 1.4 X 10+ e,sp( -11,100 c n l . / R T ) to 2.5 X (!up( -9800/RT) S C C . - ~ The D A is ~ sensitive to coriditions of prep:irnt,ion but D is not. Neit81icrD , k g nor D'.kp npponrs to tlel)ciitl on the eRect,ivevapor pimswe of sulfur during preaniieding, :md thus tho diffusion is pob:tl)ly not c:iuml by defects duo t.o tlel)art,ure froin st,oicliionietry in the ilg?S structure.
Silver suuide (8iyst:Lllizes iii :L Ion, teiiiperntuit nionoclinic forin, tlie cry,stal stxucturc of \vhicli has aiid in n cnliic form recently been :ho\re t>lie trniiaitioii teinpernture of 1'7'7". The stmicture of the high tempernture forin appen,rn t'o consist of n t)ody-ceiitel.ed-c~il)ic:way of sulfur atonis \vit,h silver :Lt)oms raiidonily distrihuted :moiig the interstitial sites.3 Diff usioii of silver niid sulfur in t,he low teniperature forin \\'as iiieasured by Rlme. PechaiiskiJ by means of the ex(1) This work w a s supported by the U. S. Atotnio Energy Coniiiiissioii, Contract No. AT-(11-1)-250. Contribution No. S29 from t h e Chetnistry Delmrtiiient of Indians University. ( 2 ) A. H. FriicIi. 2. Kri,?t., 110, 2 (1958). (3) P. Rnlilfs, Z. plrusik C h e i n . , BS1, 1.77 (lU31i). (4) D. Pechanski, J . v l i i w . p h u s . , 4'7, 137 (193ti).
clixiige I)et>\veeii r:itlio:wt~i\~esill.ei- iiitixte :i,iid sodiuni sulfide ,qolut,ioiisniid fiiiely tli\.idecl cryst:illites of silver sulfide iii suspeusioii. This niethod is satisf:xct,ory, but liar? :i liniited :~c*curncyowiiig tJo the size clisti-ibut>ioiiof t'lie cxi.ystnllites :uid the necessity of npprosiiixtt iiig their geonietric. form hy spheres;. She found D S = 0.24 esp( --%.:< kc:il./RT) clii.' sec.-I niid D.k6: = i).:j X IO-' esp(--0.1 kc:Ll.,,BT). The diffusion of 8-35 ill cubic AgzS was reported5 t,o he 1 ) ' s = 2.4 x lo-* exp(-Z-l.O kcal.lRT). It is not easy to reconcile this result wit,h that for Ds. No ineasureiiieiits of diffusion of silver in cubic silver sulfide have beeit (.5) 11. I r l i i u i i r o 1:. Orln niid T. Bujitio. .llou. I i i s l . ,Sei, I i ! d t ~ s .Rcn., [ ' i i i w , , & i f ! / , 10, 1 (lV:,3).
Osaka
224
ROYL. ALLENAND WALTERJ. MOORE
crystalline specimens has been followed both above and below the transition temperature by means of penetration of the radioactive silver-110 into silver sulfide. Experimental Procedures
2.5 2.0 . 0 e
3*
.M
1.5
W
3
1.0
0.5
x * X'lO', cm.2 Fig. l.--Two m d a t 2W0 showing the anomalous Penetration Curves from a sulfided b ' e r of evaporated Ag-110 on A&.
50
100
Penetration, p. 150
200
2.0
1.8 c . '
'5 1.G .e U
s
8 1.4 d
1.2 1.o 2 3 4 5 lo4 z2,cm.2 Fig. 2.-Typical penetration curves i n nioiiocliiiic Ag2S in accord with ea. 1 after exchanee of Ae-110 with surface.
1
. . . "
Vol. 63
Penetration, mm. 1.0 1.5
2.0
3'o 1.8
1.2 1.0 2 3 4 lo* z2,cm.* Fig. 3.-Typical penetration curves in cubic Ag,S in accord with eq. 1 after exchange of Ag-110 with surface.
1
reported, although estimates of the diff usioii coefficient have been made.6 I n the present work, diffusion of silver i n poly(0) C. Wagner, J . CLem. P h g ~ . 21, , 1824 (1953).
Silver sulfide was prepared by the reaction between sulfur vapor and silver. A current of pure dry nitrogen was passed over sulfur at 200' and thence over silver grain a t 300'. The silver was a "Specpure" product from Johnson Matthey and Co. and the sulfur was a U.S.P. product recrystallized from carbon bisulfide. After the supply of sulfur was exhausted, the silver sulfide was heated in pure nitrogen for an additional hour a t 300' in order to remove excess sulfur. The composition of the silver sulfide, determined by weight gain, was always stoichiometric within the limits of error, &O.Ol%. The product was broken into small pieces and melted under vacuum in a sealed quartz tube. The tube was lowered slowly through a furnace maintained 100" above the melting point (825"), but attempts to prepare single crystals by this 2ethod were not successful because of the transition a t 177 Possibly single crystals of the cubic form could have been prepared if a method had been available for handling them at elevated temperatures. The grain size of the final product was about 0.01 mm. The surface of the rod of silver sulfide contained numerous small bubble holes. The rod was therefore machined until the diameter was reduced from 8.0 to about 6.5 mm., the surface then being free from imperfections. The rod was then cut into disks from 1 to 2 mm. in thickness for the studies on monoclinic Ag2S, or 7 to 8 mm. for those on cubic Ag2S. The disks usually were annealed for 140 to 150 hours a t 300' in sulfur vapor a t a pressure of 2.1 mm. In a few experiments the annealing was carried out at 400', but no difference in the subsequent diffusion behavior was noted. The radioactive tracer, Ag-110, was applied as an extremely thin layer to the ground surface of the disk. In the first experiments the silver was evaporated from a tantalum ribbon in v a c u o and the active layer sulfided by means of hydrogen sulfide or sulfur vapor. In later experiments, a drop of active silver nitrate solution and a drop of nitric acid were placed on the surface of the disk and left for 30 minutes. Partial exchange took place with the result that some Ag-110 was transferred to the silver sulfide. The escess solution then was removed and the surface washed and treated briefly with water saturated with hydrogen sulfide, washed again and dried a t room temperature. Autoradiographs of this surface revealed a uniform distribution of tracer. Preliminary experiments were carried out on the high temperature form at 200 to 400", with the specimens prepared by the evaporation method. The length of the diffusion anneal varied froin 0.3 to 10 hours in helium, vacuum and sulfur vapor. In Inter experiments the pellets were pieannealed a t the diffusion temperature and a diffusion anneal initiated by evaporation of the radioactive silver onto the pellet held a t the annealing temperature thus avoiding a heating and cooling cycle. The results of this first series of experiments were unusual. In all cases there was a high concentration c of Ag-110 on the surface, which decreased steeply until at approximately 50 p depth the rate of decrease became considerably less steep. A typical curve of log c us. z2 is shown in Fig. 1. Two lines can be drawn through the points, corresponding to a slow diffusion followed by some more rapid process. The cause of this anomaly is believed to he imperfect adherence of the active layer of silver sulfide. A microscopic examination of the evaporated layer of silver after sulfidation showed that in some cases it was formed of a mass of tiny crystals, which would make poor contact with the bulk of the disk. In two cases, whiskers of pure silver sulfide were observed growing outward from the surface. It appeared therefore that the standard method of applying the tracer by evaporation was not suitable in the present case because the silver sulfide formed from the active silver did not form a coherent thin layer. Probably the outward migration of the silver during the sulfidation process was too rapid. Therefore the second method of preparing the tracer layer was used in subsequent experiments, and no further difficulties due to silomalous penetration curves were encountered.
.
DIFFUSION OF SILVER IN SILVER SULFIDE
Feb., 1959
The diffusion coefficientsin both forms of silver sulfide are high, so that a special apparatus was constructed, adapted for short diffusion times and the use of thin specimens. The vessel holding the specimen was made of 0.010'' stainless steel, flanged at the open end and provided w t h a stainless st,eel lid. The vacuum seal was an aluminum gasket. A inch Kovar-Pyrex seal was brazed to the lid so that the vessel could be attached to a glass vacuum system. Two leads of a calibrated copper-constantan thermocouple were sealed through the glass. The specimen was placed in the diffusion vessel between two sheets of platinum foil and pressed tightly against the bottom by a steel spring. The thermocouple junction was soldered to another piece of platinum foil pressed. tightly against the upper platinum covering of the disk, which was therefore sandwiched between the thin bottom of the diffusion vessel and the thermocouple junction. The vessel was evacuated to less than 10-6 mm. and in most cases filled with pure helium, but in some cases an atmosphere of sulfur vapor or a vacuum was used in the diffusion anneal. A cylindrical bored aluminum block was mounted in a furnace cytrolled with a platinum 1,csistance thermometer to h O . 1 The block was filled with Wood's metal to accelerate heat transfer to the diffusion vessel. A t the beginning of the anneal the diffusion vessel was plunged into the Wood's metal. The thermovouple showed that the specimen was heated to the desired temperature in approximately 40 sec., and after 15 see. was only So below final temperature. Annealing times were usually 2000 see. Following the anneal the vessel was cooled rapidly to room temperature by immersion in cold running water. The specimens were sectioned on a prec,ision grinder.' This instrument also was used for the preliminary polishing of the circular disks t80a standard size oi 6.5 mm. by 1.5 mm. with parallel plane faces. The disk was attached to an aluminum mount by means of uncured Aralciite G000. Before sectioniiig, the outer edge of the disk was removed on a lathe to a depth of 1 nim., in order to eliminate possible effects of surface diffusion. Silicon-carbide paper of grades 2/0 to 4/0 was used for the sectioning and the surface so produced was planar Lo better than 0.1 p . After each section was removed the abrasive paper was folded into a tube and counted iu a well-type sciiitillation counter with a y-ray spectrometer. The counting was done on the peak a t 0.9 niev. The mass of each sect,ion was determined by weighing the mounted disk on a Mettler single-pan microlialance, which afforded accuracy of A0.002 mg. The thickness of the section was calculated from the expeyimentally measured density of the silver sulfide. From 10 to 30 ~.rwas removed in each section.
.
Results of Dsusion Experiments The solution of the diffusion equation for the case of a plane source on a semi-infinite solid is given by c = co(nDt)'/z exp( - x 2 / S D t )
(1)
where c is the activity of the tracer at time t and depth x, co is the initial activity and D is the diffusion coefficient. Thus D can be calculated from the slope of a plot of log c us. x2. Typical experiinentsl curves are shown in Fig. 2 for the low temperature silver sulfide and in Fig. 3 for the high temperature material. The excellent liuearity of these plots indicates that there is good agreeinent with the diffusion model assumed. I t also tends to indicate that grain boundary diffusion does not play a major role in the processes studied. In Fig. 4 we have plotted the diffusion coefficients It will be noted in the form of log D against ?''-I. that the coefficients for cubic silver sulfide fall on a single straight line and evidently minor changes in the preparative procedure and the annealing have little effect on this coefficient. In the case of the monoclinic silver sulfide, runs with any given preparation fall on a straight line, but there
225 I 4
4.0
f
0' 3.0
CUBIC
MONOCLINIC
-
M
s 2.0
1.0
L! 1.8
,
I
I '
,
!
,
,X!
2.0
2.2 2.4 2.6 100Q/T. Fig. 4.-Diffusion coefficients of Ag-110 in AggS ahove and below transition temperature: 0 , preannealed in S (2.1 mm.) for about 150 hr. a t 300 to 400". Run in He; 6, preannealed in He for 150 hr. a t 375'. Run in He; 9 , preannealed in S (48 mm.) for 150 hr. at 375". Run in S.
are different lines for different preparations. Only two extreme lines are shown; there were two other lines falling between them. In neither case, however, does there appear to be an appreciable dependence of the diffusion coefficient on the preannealing treatment of the silver sulfide. This independence is shown by the sets of experiments in which the specimens annealed in sulfur vapor ai e compared with the specimens annealed in helium. We may conclude that the diffusion coefficients do not depend on any noli-stoichiometric defects in the structure of silver sulfide, in contrast with the usual situation in metallic oxides. The least-squares vstlues for the Arrhenius plots are represented by Monoclinic Ag,S D A= ~ 1.4 X 10+ exp( -1100O~cal./RT) to 2.7 X exp( -9800 cal./RT) Cubic Ag9S D ' A ~= 0.28 X exp( -3450 cal./RT)
The diffusion coefficient of silwr in cubic sjlver sulfide is extraordinarily high, .being nbou t the same as that calculated for a gas of silver atoms a t the same temperature and density. The transition temperature of 177" therefore represents a, point at which the silver atoms in the structure lose the rigidity characteristic of' the solid state, while the sulfur atoms retain a rigid structure. The activation entropies for the diffusion processes can be calculated from the ecyuations D = ( k T / h ) d zexp(AS*/R) exp( -AH*/RT) (2) if me assume a model for the jmnp distance d. Typical results a t the transition temperature, 177", are
4.
hfonoclinic d = 1.52 Cubic d = 1.22 A.
AS* = -1.5 to +1.7 cal. dep.-1 AS* = -0.7 cnl. deg.-'
These values of AS$ close to zero are corisistmt with models in which the silver is able to move without creating any additioiial disorder iii the structures. Our general conclusion is that diffusion of silver
(7) H. Letaw, L. h1. Slifkin and W. M. Portnoy, Rev. Sci. I n e t . , 26, 8G5 (19.54).
(8) H. Eyring and W. Wynne-Jones. J . Chern. P h y n . , 3, 492 (1935).
HURLEY D. COOKAND HERMAN E. RIES,JR. in silver sulfide is not determined by defects due to deviations from stoichiometry. All the silver atonis may be mobile in cubic AgZS whereas mo-
Vol. 63
bility in monoclinic Ag2Smay require displacement of a normal lattice silver into an interstitial position (Frenkel defect).
ADSORPTION OF RADIOSTEARIC ACID AND RADIOSTEARYL ALCOHOL FROM n-HEXADECANE ONTO SOLID SURFACES’ BY HURLEY D. COOKAND HERMAN E. RIES,JR. Research Department, Standard Oil Company (Indiana), Whiting, Indiana Received August 11, 1968
The rate, extent and strength of adsorption of radiolabeled polar compounds from a hydrocarbon solution were measured directly on the window of a specially designed Geiger tube. Radiostearic acid and radiostearyl alcohol were adsorbed from dilute n-hexadecane solutions to form mixed films with the solvent on mica, and on iron and gold deposited on mica by thermal evaporation. Within a few minutes after contact between window and solution, 0.2 t o 0.3 of a monolayer of polar compound was adsorbed; the ratio of polar compound to n-hexadecane in the first layer is, therefore, about 1:2. With the more inert surfaces of mica and gold, this ratio remained nearly constant for as long as one week of continuous contact. In the same length of time, iron surfaces adsorbed the equivalent of almost two monolayers of stearic acid but only one of radiostearyl alcohol. Strength of adsorption was studied by rinsing with solvents a thicker film that had been deposited from the vapor phase. Exhaustive treatment with benzene reduced such a stearic acid film to about 0.2 of a monolayer. Furthermore, mixed monolayers of stearic acid and n-hexadecane exert surface pressures on the film balance when only one-third of the area is occupied by stearic acid. This fraction is similar both to that observed initially in monolayers formed by adsorption from solution and to that remaining after exteiisive desorption. Possible st,ructures of stable monolayers of stearic acid and n-hexadecane are schematically presented. Suggested distributions of vertically oriented molecules in a hexagonal close-packed configuration are consistent with both kinetic and equilibrium data.
Introduction In studies of the adsorption of polar organic molecules from hydrocarbon solvents onto solid surfaces, the rate, extent and strength of adsorption are of primary interest. The monolayer, or first layer of adsorbed molecules, deserves special study; it is the only layer that can react chemically wiCh the surface. Succeeding layers are held by much weaker physical forces. Little is known about how adsorbed films on solid surfaces compare with those formed under controlled conditions on the water surface of the film balance. Three techniques in the present study give iiiforniatioii 011 rate, extent and strength of adsorption and provide information on the structure of monolayers. Because radiolabeled polar molecules are uniquely suited to studies of adsorption, this area of research is growing r a p i t I l ~ . ~ - ’ In ~ the preseiit study, a specially constructed Geiger tube with a metalcoated window was brought into direct contact with a hydrocarbon solution of radiolabeled polar mole(1) Presented in part a t the ACS Meeting, Miami, Florida, April 7-12, 1957. (2) F.P . Bowden and D. Tabor, “The Friction and Lubrication of Solids,” Oxford University Press, Oxford, England, 1950. (3) D. E . Beischer, THISJOURNAL, 67, 134 (1953). (4) J. E. Willard, ibid., 67, 129 (1953). (5) N. Hackerman and R. A. Powers, ibid., 67, 139 (1953). (13) H. A. Sinith and K. A. Allen, ibid., 58, 449 (1954). (7) E. Rideal and J. Tadayon, Proc. Rou. Soe. (London),A2Z6, 346 (1954). (8) J. E. Young, Austr. J . Chem., 8, 173 (1955). (9) J. W. Sliepard and J. P. Ryan, THISJOURNAL, 60, 127 (195G). (10) H. D. Cook, Rev. Sei. Instr., 27, 1081 (1956). (11) H. D. Cook and H. E. Ries, Jr., THISJOURNAL, 60, 1583 (1956). (12) H. E . Ries, Jr., H. D. Cook and C. M. Loane, in “Symposium on Steam Turbine Oils,” ASTM Special Technical Publication No. 211, Philadelphia, Pa., 1957, p. 55. (13) 0. Levine and W. .4. Zisman, presented before the Division of Colloid Chemistry, A.C.S. ineeting, New York, N. Y., September 8, 1957 (preprint). (14) H. A. Smith and T. Fort, ref. 13. (15) H. Sobotka, ref. 13.
cules. The rate and extent of adsorption could thus be measured continuously.1° Strength of adsorption is related to resistance of the adsorbed material on the window surface to various solvent treatments. Information on the structure of the adsorbed films was obtained by spreading the same solutions used in the adsorption studies on the mater surface of a film balance. The packing of polar and solvent molecules produced a mixed film in which the relative ratio of polar molecules to solvent molecules could be calculated. Monolayers consisting of two or more different kinds of molecules have been studied previously, but most investigations have been concerned primarily with mixtures of polar molecules, rather than of polar and non-polar molecules.16-20 The present study was focussed on the adsorption of a long-chain fn,tty acid and the corresponding a1coho1 onto solid surfaces of widely different reactivity. Stearic acid and stearyl alcohol mere chosen because they have the same hydrocarbon chain attached to a relatively strong and a relatively weak polar group. Stearic acid is a classical compound iii monolayer studies and is representative of many types of lubricant additives. The solid surfaces, iron and gold, represent the reactive and non-reactive metals. The reactivity of iron is reflecked in the many corrosion and wear problems found in industry; gold exhibits very low reactivity-it is unique in t,hat it does not chemisorb (16) N. I
