442
13. P. GTAXI
appearance, but \\ ith a \-ariation in tint depending upon the surface-activc agent u.jed. K a t e r had an immediate coagiilating effect, and the addition of :dcohol, ethei. or acetone caused precipitrtion nithin L: -1ioit time. The also lieat sencitive. and congulntion h g n n after *ex cia1 niiiiutes' heciting at 70°C'. The organo*ols qmcldy piwpitateci at the hoilirLgpoint of xylene.
-4c1,w r p t I o I I The adsorption o determined by anal table 6 .
of s ii );face -(I c 1
me ot nioie succe~sfulburface-active agent., TI a i oi the .upernatant liquid. The results are 4ionn in ,St-lIlIARY
Dispersions of silica in xylene i:a~.eIiccn made in the presence of ii number of surface-active niaterizls. Thc- particlc-size distributions oi these dispersion5 sedimwtation annly ' -'.' 1lie moat successful .surfacehave heen determined activi. agents st-ere foiind to he the yariou? metal sulfo.succinats, metal naphthenatcs, and lecithin. l'lic~sc ageii~i:u'c tlioiiglit t o aid dispersion liy forcing aggregate, ~ w r efound to be strongly adsorbed on t lie silica. The silica had a surface (~1'11s iarc meter:: pei' gram. It TYL:. positively charged in tliil xylene wqxn.4on and \vas congulated by the addition of vater, alcohol. ncctone, and ether. Heating also produced coagulation. ILEFEREXCES (1) I).WERELL,, J-. It., A S D MATTSGS, R . : J. Phys. C'heix. 48,134-41 (1941). ( 2 ) D.~\IERELL, I*.Ile adsorption of carhon dioside on the qolicl takes place until enough prrssure is d e l eloped to force the gas molecules into the crystal lattice of calcium carbonate. The vhole piocess can be easily compared to the taking up of hydrogen chloride by ,iqueous silver nitrate solution. The Langmuir mechanism leaves unexplained the absence of the phase boundary belov the dissociation pressure during which stage niinute quantities of carbon dioxide
-150
B. P. GYrlNI
must be taken up. Incidentally, this line of thought neatly explains the specific character of chemical reactions where a solid separates, which comprises most cases of complex fomaticn; for specific chemical reactions must aliyays be identified with specific steric conci:!mitionG. The magnitude of energy chanpcs cannot be made the basis of distinction hetween true cheniical reactions on the one hand and adsorption 01- ordinary distribution processes on the other. The distinction lies in the fact that chemical reactions a1x-a)-s involve a distribution in spaces of zero dinlension, whereas the other processes i n r o k e spaces of one, t v o , or three dimensions. Even this distinction is lost in thzl type of adsorption vihich is governed b y equation 5 . X distinction of theoretical character still persists in the relative degree ot packing ( k ) n hich should. in general. be finite for cases of adsorption or ordinary distribution, but infinite for true chemical reactions where a new phase separates. This consideration requires that the lower part of the isotherm corresponding to equation 5 should coincide n-ith the zero pressure axis. Such cases have been strikingly realized in the adsorption iiieasurements of McBain and his students already referred to. Such behavior is certainly dificult t o explain on any of the current theories of adsorption. PCRIODIC ADsQRPTIOS
The phenomenon of linear adsorption has llcen srggestcd t o be the cause of certain periodicities in adiorption, first obser7 ed by Allrnand and Chaplin (1). They expressed the opinion tkat the primary adsorplion is on certain active spots around nhich concentiic ring., are afiernards built up (2). Benton and ’SThite (3) urge the reveise n’echanism, in hich primary adsorption is supposed t o occur along edge. oi microc tal. conztituting the solid adsorbent, which then spreads inivards in parallel YO!\ s. TVhaterer the detailed mec1ianis:n may be, the concept of linear adqorption seem-! t o IY an a:tracti:-e hypothosis lor the explanation of this type of adsorption. SURF.\CE CLTALYSIL:
Reactions of zero order are not infrequent, pariicuhrly in I)ioIogical p n x s s e s . The Langmuir theory requires that, such reactions shall occur only u-hen the catalyst surface is saturated with thc reactant, The present hypothesis provides this possiljilitg io;. those cn also \,:here the surface as a \Thole is riot fully covered, for it is enough if the ac,tive points or lines are satura:e$i. In the case of points. it is inimatcrial n-hether ali of them arc c3rerc:l or not, for the ap.ount of adwrption is independent of Cwcentxttion at any ?rage--a 1-ien-point particularly to be fa\-ore(! in I )iblogicnI reactiom. .-lcc3rilino; to Hogness and Johnson (IS), the rate oi tlecomposition of germaniuni hydride on metallic germanium is proportional i o the eu‘ue root of the pressure. This experiment is easily unciwstood ii decompositio:l occurs mainly among linearly adsorbed molecules. .Iceording t o Langinuir’s ideas, this case requires that the GeHa molecule I ~ Cadsorlied cn three elementary spaces in = 3 ) , each prod-
DISTRIBUTIOS LkM-, ADSORPTION, A S D CIIEMICAL RE.ICTIOS
451
uct of decomposition suhsequently occupying a unit space (m = 1). -1physical picture of this niechanism is not easily acceptable. The rate of decomposition of stibine on antiiiicny. rtudietl hy Bodenstein and Stocl, (i), varies as p0 6. This result agrees i th the 4nipie concept that decomposition occurs mainly an;ong mnleculrs ad5orl ed in a geneial v a y on the surf:xe Decomposition of arsine. phoqphinc. anti foiiric w i d 011 ccitain surface;: is apparently monot o the pas pressuie. This result is mdecular (I:), i e , the rate is pi~~portional pc ssible in rm-ious ~ i a y s Thus, the dccon-iposition may occur as a result of hi iary collisions betxveen inolecules from the gas phase and those adsorbed on ~c'rtair,actir e spots; it may a140 occur as a result of collision lieti) een molecules lit early adcorhed and tl:ov pexrally distributed on the surfacz; for the rate in tke latter cape: 7 . 1
For such cases, x i iiich arc of frequent occurrence, the Langmuir theory requires that tach d e c o r pcsition product occupy the same number of elementary spaces as it did nhen it rtas a part of the original molecule. The rate of dpccnipositicn of amnlonia on platinum is proportional to its pressure and inveisely proportional t o the pressure of hydrogen gas (33). Corresponding t o the t n o rr.echsnisnis for decomposition of arsine, the reaction may be written as or
SH3 i l n e
+ 3H3r,Jri,c.
=
H?
-t H2 surface
+ S*
I>"&
-t
N ? d re
(b)
The eficctire Concentration of ammonia molecules on the actir-e lines TI ill no longer be proportional to & & ? , -incc other kin& of molecules are also present. 1 7 , be replaced by the fractional concentration The actual quantity ki . p ~ ; i n1u.t
say. If the mechanism corresponding i o equation h is considered probahle, the i,:ite of reaction xi11 he gil-en I-,tl.r :&
=
const . P h e 'P s u r l a c e
If t h e ainmonia and iiitrogen terma in the cleno!rrinat-ors m a y be neglected in comparison Tr-it11 the H.: terms. we finally get
The most general velocity equation may evidently be written as :
452
B . P . GYANI
This consideration niay probably he helpful in clarifying the hitherto unexplained rate of decomposition above a pressure of 10 mm., which is reported to be governed b y
Finally, the idea of linear adsorption has been utilized else\\here (32) by Schwab and Pietsch to explain the complicated oxidation of d f u r dioside. SGJIhlARY
1. .lidsorption on solids has been regarded as a distribution of inolecules on isolated points. lines. patches, a i d the entire bulk of the adsorbent. These processes may occur singly, or in combination. 2. h simple interpretation of the claqsical adsorption equation ha- been indicated on this basis. 3. A fen- applications of the present hypothesis to some general facts of adsorption, chemical reaction\. and Qurfacecatalyhis have been discussed. 4. The distiiiction between chemical reaction 011 the one hand, and adsorption or ordinary distribution on tlw othci, ha. heen presmted from a new point of Tien..
In conclusion, the authoy niJie, t o thank the :iuthoritieq of Patna University for the grant of a postdoctornte Research Fellowship. He also records hi5 gratefulness t o Prof. Dr. P. €3. Ganguly and Principal 6. Prosad for their kind interest in this work anci for helpful criticisms, REFERESCXH
(1) ALLMAND, A . J . , ASD C H - ~ P L A I13.: S , Proc. R o y . Sor. ,Lontlonj 129A, 257 (1930). A S D CURR.IGE,I,. J . : Trans. Faraday SOC.28, 222
(2) ALLif.iND, A . J., CHAPLIS,R . . (1932).
( 3 ) RESTOS, *i.F., ASD WHITE,T. 1.: J . .%in. C'liem. SOC.53, 3301 (1031). (4) BODENSTEIN, hl.,A N D STOCK,-1.:Ber. 40, 570 (1907). (5) BREDIG,G . , ANI) STARK.h.:Z . Physik. Chein. B2, 282 11929). (6) COOLIDGE, A . S.: .J. h i . ('liem. Soc. 48, IT06 ( l 0 2 6 1 , figlire 1. (7) D.ivis, 0. C. 11.:J. ('hein. Soc. 91, 16G6 (190ii. 18) DCIN.c'. F.~ . n K o:l l o i d - % . 17, 123 (19151. (9) FELIIAX, J . . .is]) .%DIII#IBI. (2.: Z. pliysik. C'lieni. 131, 347 ( 1 9 % ) . (10) FREcsuLIcr~,13.: z. physili. ('hen?. 67, 444 i100Gi. (11) FRELXDLICH. 1.1.: Colloid m i d (,'a,p;ll//r!/ C'heuiistr!!. p . 120. I.ont1on (1923). (12) Reference 11, p . 122. (13) FREUSDLICH, TI.. .\si i k . Chem. 91, 1 (i916'1. Rei-. 59, 21.0 (1041 ) . (14) GOETZ.A , , . m i ) G O E T ZP.: , (15) GYASI,B. P . : J . Intli (16) HERZOG, R . 0 . : Z . pl (17) HINSHELWOOD, C. S . , H.-\RTLEI-. 1-1.. ASD TOPLET,B . : Proc. R o y . Soc. (London) A100, 575 (1922). (18) HOGSESS, T. R . , ASD JOHSSOS, JY. C . : J. - h i . Chem. SOC.54, 3 5 s (1932). (19) ISHIZAKA, 3.:Z. physik. Clirm. 83, 97 (1913). (20) I,.%XGMCIR. I . : +J. .\in. ('hem. Sect. 38, 2221 11916).
ALUMISO-SILICATES .4ND IROS-SILIC.1TES
-1-53
121) IASGMUIR, I.: J. Am. Chem. Soc. 39, 1848 11917).
(22) LISSER, E. R., ASD GORTSER, It. -1.: J . Ph;vs. Chem. 39, 35 (1935). (23) hICB.4IS, J. W.:The Sorptc'o~i(?/'Gaseshi/ ,'oli.Is. p . 5 . G . Routledge sild Sons, L t c l . , London (1832;. (24) Reference 2 3 , p . 13 et seq. ( 2 5 ) Reference 23, p . 1"s et seq. (28) MICHAELIS, L.,.\si)R0s.i. 13.: 13iociieiii. % . 15, 1% '190f). 127) PROSAD, IC.: S a t t i r e 127. Yo. 319.4. 90 (1931). (28) RICHARDSOX! J. R . : ,J. .Im.C'lieni. Soc. 39, 1842 i 1 9 1 i J . (29) RICH.4RDSOS, H. L . , .\XI)n O B E R ' r S O \ - i , 1'. \v.: J . Cheni. L?(>C. 127,553 (1925). (30) SCHAEFFER, v. J . : PliV.9. RtV. 62, 4% 11942). (31) SCHMIDT, G. C'.: Z . physii;. Clieni. 15, 56 i1894't. (32) SCHWAB, G . 3 3 . . A X D P m r s c a , E . : %. pliysik. C'htni. B1, 385 (1929). (33) SCHWAB, G.-11.. ASD SCHMIDT, 11.: Z.p h y s i k . C'lieni. B3, 3 3 i 11929). (34) SJfEK.IL, h.: %. Elektrorlieni. 35, 567 ! 1 9 2 9 i , (35) TITOFF, .I.:%. physik. Ctieni. 75, 611 (19101. (38) WILKER. J . , .%SI) . ~ P P L E Y . I R D , R . : ,J. C'hem. SOC.69, 1334 (1898). ( 3 i ) Z s I G l I o s D Y . I t . : Z . anorg. Cheni. 71, 3.56 11911).
ISI'RODL~C"~IG\~
I t is well kn0n.n that on mixing: t w o hydrophobic sols of opposite signs of charge, mutual adsorption of the oppositely chargecl prticles takes place with the lon-ering of the {-potential of the mixed particles (cf. Freundlich ( 5 ) ;Keiuer :md con-orkers (16-1 8)). IYeiser and con-orker.: have shon-n that the precipitating power of a positive sol for an oppositely charged colloid is determined by ( a ) the magnitude of the i--potential in the t\vo eok. ( h ) mutual adsorption of oppoeitely charged particles, (c) the presence of precipitating ions as impurities in the sol, and ( d ) the interaction hetn-een staliilizing ions in the sols. It appears, however, that there is no definite information in the literature regarding the chemical composition of the mut>ualcoagulum. Thus the precipitating colloids may lie side by side or definite chemical compounds may be formed. These alternative points of vien. have important hearings on the formation of clay complexes. The alumino-silicates of rocks are broken up by hydrolysis mostly into oxides of aluminum, silica, and other oxides which, when first formed, are present in the colloidal state. The colloidal sesquiosides are positively charged, ivhilst colloidal silica is negatively charged; mutual coagulat,ion takes place between
.
