J. 1’. URATOIWIL, RI. OILHANOVI~ AND E. MATIJEVI~
121(j
Vol. 64
COAGULATIOX OF LYOPHOBIC COLLOIDS IN MIXED SOLVEKTS. ISFLUEKCE OF THE DIELECTRIC CONSTANT BY J. P. I~RATOHVIL, ?vl. O R H A N O V-4ND I ~ E. MATIJEVIC~ Lnhorutory of Phycical Cheinistry, Faculty of Pcience, University of Zagreb; Department of Phzpical Chemzstrij, Institute “Rudjer BoSXotzC,” Zuyreb, Croutza, Yuyoslavza, and Clarkson College of Technologg, Potsdam, New York Received March 7, 1960
Krgat ivr d v r r bromide sols in statu nascendi were coagulated by potassium, barium and lanthanum nitrate in isodielectric mixtures of water n ith methanol, n- propanol and isopropyl alcohol, respectively. Dielectric constants varied from 80 t o 29. The results are compared with those for water-ethanol mixtures published previously. Coagulation concentrations decreased ~ i t ah lonering of the dielectric constant and with an increase i n the concentration of stabilizing ions (Br-). Coagulation concent1ations of individual counterions were approximately the same for a given concentration of the stabilizing ion anti mixtrircs of different solvents having thc same dielectric constant. The results are discussed in the light of the current theories of stability and coagulation of lyophobic colloids.
Introduction
In a series of previous papers2-+ numerous results of the influence of ethanol, acetone, dioxane and glycine in aqueous solut’ions on the electrolytic coagulation of negative silver halide sols in statu nascendi were described and compared. The change of the dielectric constant (DC) of medium in these invest’igations covered the range from 126 ( 2 131 glycine solutions) t,o 30 (87 1x4. % of ethanol). These result,s have shown several regularities in the behavior of silver halide sols in mixed solvents, the main one being a direct relationship between the coagulation values of a given counterion and the DC of the medium. Compared with the values for mater, the coagulation values of KN03, Ba(NO& and L:z(?;Oa)3 hai-e generally decreased in media of lower DC and increased in glycine solutions. However, a comparison of coagulation values of the same counterion in mixtures of different solvents with water (nater-ethanol, water-acetone or water--dioxane) having the same DC, showed appreciable differences indicating specific influences of t,he solvent used. In case of isodielectric mixtures, coagulation values of mono-, di- and trid e n t counterions were the highest iii ciioxa,rie mixt,ures, intermediate in acet’one, and the lomst, in ethanol solutions.6 The differences were partjiciilarly significant at greater conceiitrations of st.abilizing ions in solution and with sols of higher charge. In x-iew of t’his specific influence of solvents of various chemical composit,ion we decided t o undert,ake some additional investigations of t,he coagnlation of iiegntive silver bromide sol in statu nascendi, using a series of homologous solvents of similar chemical properties and structure. The solvent mixtures, used were water-methanol, mater-l-propanol and water-isopropyl alcohol in the range of DC from 80 to 39. The results are comparcd with those for n-atcr -ethanol mcdia obtained previously. Siucc current theories of thc stability a i d coaguI:Lt ion of lyophobic colloids include also the influence of DC of medium, the experiments described in ( 1 ) C’1:irkuin Collrge of Technology, Potsdnm, New I-ork. ~kn n i l .T. Kratoliril. Arhiu kern., 24. 1 ( 1 9 2 ) .
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I. We nish to emphasize some chaiac teristics oi our experimental in\ estigations: (a) uze of well-defined solq (at least iii regard to htoichiometri(* composition, exactiiess, ease and Icproducibility of their preparation) : (13) suitable aiid reliable method for follon iiig cmgulntioii processes; (c) iinambiguous criteria i n deterniinntion ( i f coagulation valiies, 1:uqed oil the aiidysii of Aiiictics oi cwapulatioii ; (d) x ery i t ide r a n g oi rompohitioii :tiid propertics of sol\wit Data from the literature. (lealing \\ itli :idciitix et, XIhich cnuw c+oiisideial)lechange of D e . :dl iiidicate tlic same general correlation bet i~ eeii cmigiilatioii value a i d DC a. predicted hy rongulatioii tlicories.3 I ? 16-19 l'rcmit results :ire alici i n qun11-
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7
E'ig. CriLic:il coagulatioii conccntrlitions of L:i( Xc'08)5 2's. log conccntrwtioii of Br- (from HBr). Full circles tlciioto tlie values for Al(Ii08)3.
flueiice as can be seen when Figs. 3 and 4 are coiiipared. Coagulation values were always smaller for acid solutions irrespective of t'he D C of the medium. Since lanthanum is a polyvaleiit ioii susceDtible t'o hvdrolvsis. the coaeulatioii values obtained in acidic so1;tions should-be accepted as rcpresentative for the unhydrolyzed ioii. In solutions of higher p H the hydrolytic products have a smnller
1-01. 64
(19)
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(19jOl
Sept., 1960
COAGULBTION
OF
LI-~PHOBIC COLLOIDS I N
1219
M I X E D SOLVENTS
tative agreenieiit with such theoretical predictions. The quantitative comparison of various theoretical requirements with our experimental resu1t.s in different' solvents was extensively treated in previous
BJERRUM'S DISTANCE, ,d
(d I
It has been shown6 that Ostwald's theory of constant activity coefficientsL2could not be successfully applied to our results. Since t3hemain postulate of Ostwald's theory may be directly derived from the Derjaguin-landau-T'erwey-Overbeek theory (D-L-Y-0),16,?0one could infer also the intLpplicability of the latter theory. In fact, neither exact nor approximate check of D-L-V-0 theory is possible. The coagulation T-alues should be proportional to the t'hird power of the DC of medium. However, the expression which defines the coagulaI ' tion value contains besides the DC two ot,her quanl L ~ _ ~ - _ L - ~ l t'it'ies which should change when the composition Fig. 5.-Critical coagulation coiicentratioris I'S. Bjcrriini's ai1d propert,ies of the solvent is changed. These for various values of the dielectric constant. Th(1 ;ire the attraction coilstant d of long-range van der distance dat,a for various isodielectric niixturcs fall n-ithin short W;itals-Loiidon forces and the Stern poteiitial of vertical lines. the double h-j-er, $ 6 . Sothing is known about the BJERRUM'S DISTANCE. cd (i 1. ~.aluesof constmt d when medium is changing (the values for v x u u n i or air as medium, measured OK 5 10 15 20 calculated, may differ as much as four orders of I I I I Inagiiitude21-24),while the Stern potential is not measurahle by :t direct experimental meth0d.~5 The evaluation of this potential by indirect methods, is n o t a very reliable procedure,1*,26 and to carry t.).iis out for :I large nrimber of solvent mixtures would be a foraiidablc t'ask, if possible at all. However, it shouhl be emphasized that the decrease of coagulation values with the increase of the concentration of stabilizing ions in solution, obserired in our experiments for all counterions and all solveiit mixtures uscd, is completely illconsistent' with the D--L-V--O theory. I n fact, the contrary iv:is t o be expected (cf. ref. 17)Fig. 31, p. 120). We have :iholvn following the reasoning by Teiak,"l9 t,hat there exists a good linear correlation betmen t-he logarithms of coagulation 1-duesarid t'lie crit8icaldistance for t'he formation of ioiiic pairs (I3jcvuni's dist'mee) as given by the exT, 21 and z2 denote pression cd = ~ 1 ~ 2 e 2 , ? D kwhere the ioiiic charge of the cougrilating and stabilizing ioils, respec:t'ivcly, e is the elemeiitary electric charge, II is the dielectric coiist'aiit of the medium, k Boltzmanu constant mid T absoliite temperatmure. Siiicae DC i? n-itlcly \ - a r i d in our experiments, the Fig. G.-Crit.ical coagulat,ion concentrations 2's. Bjrwnm'a r:ilculatecl R j c r r p ' s dist,aiicrs change considerably distance as they change x i t h the valency of counterions for (from 5.7 to 28 A , ) . A typical plot is represented several dielectric constants. The points denote the mean valin Pig. 5, w.?cre the c:oagiilat'ioii values at, one eoii- ues of congulat,ion concentrations for various isodielect,ric wiitnition c ~ &--f (0.000-4AT) me used. The linear mixtures. treiid is oii\-ious for all three couiit8criorisover the ~v1101crsngc of I)
