Kallay, Nikola

Vuckovic, Smiljka

English

NOPR

Indian .1ournai of ChemistryVol. 14A, March 1976, pp. 156-159Influence of Potential Determining Ions on the Stability ofSilver Iodide Colloidal SystemvNIKOLA KALLAY & SMILJKA VUCKOVICDepartment of Physical Chemistry, Institute of Chemistry, University of Zagreb, Croatia, YugoslaviaReceived 1 July 1975; accepted 21 August 1975The influence of concentration of 1- ions on the stability of the AgI/I- colloidal system hasbeen investigated. The rate constant of coagulation is determined using tyndaltornetry. Theapplicability of the method is discussed. The rate constants of coagulation do not change signi-ficantly in the range of iodide ion cone. from 0·04 0101/013 to 2 0101/013 at 1 < W < 10,000.THE colloidal stabil ityi of lyophobic colloidsis due to electrical repulsion between chargedparticles--". The presence of counter ionsnear the particle surface decreases the effect ofcharge, thereby reducing the stability of suchsystems. Although many experiments have beenreported=w describing either the dependence ofcoagulation rate on the activity of counter ions fora certain activity of potential determining ions, orthe dependence of the C.C.C. of counter ions on theactivity of potential determining ions, no syste-matic investigation of the coagulation rate on boththe activity of potential determining and that ofcounter ions has yet been made .. We, therefore, decided to study the stability ofthe negatively charged silver iodide colloid as afunction of the potential determining iodide ion andMg2+ ion concentrations. The silver iodide hasbeen chosen because it is already a well investigatedmodel system and also because it is suitable forusing tyndallometry for the evaluation of the co-agulation rate constant. Magnesium ions havebeen chosen since they do not form any associateswith other ions in the systems.CalculationsThe rate constant (k) of coagulation is definedby the following equationv :dcjdt=-kc2 •• ,(1)where c is the (total) concentration of colloidalparticles. It was determined' by .measuring thechange of the intensity of scattered light with time.After Osterll the following equation was used asthe basis of the method:1rel=AI(Vt/Vs)2kt .. , (2)where 1rel is the relative intensity of scattered lightat an angle 7tj4 to the transmitted beam, A the pro-portionality constant of the Rayleigh equation,Vs the volume of the colloidal system, and Vt isthe total volume of all particles. For polydispersesystem A should be corrected for the effect of polydis-persity on light scattering by a factor I, which isequal to the intensity ratio of the light scattered156by the polydisperse system to that by the monodis-perse system of same total volume and concentra-tion of colloidal particles. The total volume ofcolloidal particles is equal to:Vt= nEME __1_P 1-¢ ... (3)where ne is the amount of entities (in the presentcase AgI) comprising the aggregate, ME the molarmass of this entity, P the density of primary particlesand ¢ the volume fraction of the intermicellar liquidin the aggregate. Since AgljI- was prepared bymixing of the precipitation components the initialamount of KI was greater than that of AgNOs, thetotal volume of colloidal particles is proportionalto the initial amount of AgN03• By using equations(2) and (3) the relative rate constant of coagulation(krel) can be obtained as follows:k I =kAI(M(AgI) _1_)2s mol- m?rei, P 1-¢=d1rel (Cin(AgNOa))( j••. (4)d t s) mol ne3where cin(AgNOs) stands for the initial concn. ofAgNOa. Eq. (4) was used to determine the krelunder the assumption that A, ¢ and I do not changesignificantly during the coagulation process. Thiswas used to calculate the stability coefficient by;W = krel, diff./krel ... (5)where the relative rate constant of coagulationcontrolled by diffusion krel. diff is assumed to bemaximum value of krel obtained by the additionof electrolyte. This applies for systems wheredkreddccounterion=0Materials and MethodsAll the chemicals used were analytically pure;AgN03 and KI were from Merck, and Mg(NOa)2from Kemika, Zagreb.The solutions in water were prepared by dissolv-ing the salts in redistilled water. The solutionswere standardized by potentiometric titration. Thecolloidal systems were prepared by direct mixingKALLAY & VUCKOVIC: STABILITY OF SILVER IOD,IDE COLLOIDof equal volumes of silver nitrate and potassiumiodide solutions. One minute after the preparationof colloid, the system was poured into magnesiumnitrate solution. This time was taken as the zerotime of coagulation. The components and thecolloidal system were kept at 293K. The relativeintensity of scattered light at the angle of 7tj4 tothe transmitted beam was measured on a Zeisstyndallometer, The Irel. values were plotted againsttime. Using Eq. (4) the relative rate constants ofcoagulation were determined from slopes of the linearplots. The wavelength of light was 527 nm.Results and DiscussionThe results are given in Table 1. The applicationof the light scattering (turbidity) method for therate of coagulation measurements is familiar incolloid chemistry, but there are a few querries whichneed to be answered before the results can be takenas valid. These questions involve the applicabilityof the Rayleigh theory, the constancy of the totalvolume of particles, and of the polydispersity effect,i.e. whether the value of A remains constant duringthe coagulation process. Assuming constant A,c/> and f the change in k in the coagulation can beinvestigated by plotting Irel. as a function of time.If a linear relationship is obtained k can be takenas independent of the size of aggregates. TheRayleigh theory is a basis of Eq. (2). For thewavelength of 527 nm the theory is applicable-e toparticles diameter smaller than 50 nm. Initially theAgIjI - system contained particles of diameters13,14in the range from 5 nm to 10 nm. During theexperiment IreI. increases to a value 10 times higherthan the initial. According to the Rayleigh equa-tion this increase corresponds to a change in theparticle diameter from 10 nm to approximately 20nm. The ratio of the intensities of scattered lightat angle 7tj4 to that at 37tj4was determinated andit was never found to be greater than 1·2, whichalso indicates that Rayleigh theory is applicable.After a certain stage of coagulation c/> and the totalvolume of particles can be assumed to be constant.At the same number and total volume of particlesTABLE 1 - VALUES OF THE VARIOUS PARAMETERSIrrtial concentration (Cin) (mol m-3) C (1-) st,« krel. log W(AgN03) KI Mg(N03).(mol m-3) d (t/s)0·10 0·14 0·80 0·04 2·1 10-5 2·1 10....'! 3-050-10 0·14 1-30 0-04 3·1 10-5 3-1 10....'! 2-880·10 0-14 1-50 0-04 8-0 10-5 8-0 10....'! 2·460-10 0-14 2-00 0-04 8-8 10-4 8·8 10-2 1'430·10 0-14 3-00 0-04 8-2 10....'! 8-2 10-1 0-460-10 0-14 4-00 0-04 1-3 10-' 1-3 0-270·10 0-20 1-00 0·10 3-0 10-5 3-0 10 ...• 2-890-10 0-20 1·30 0-10 4-0 10-5 4-0 10....'! 2·760-10 0-20 2-00 0·10 9·0 10-4 9-0 10-2 1-410-10 0-20 3-00 0-10 1-2 10-2 1-2 0-291-00 1-50 1-00 0-50 1-2 10-3 1·2 10....'! 3·310-40 0-90 1·00 0-50 4·1 10-4 2-6 10-" 2-961-00 1-50 1-30 0-50 8·8 10....'! 8-8 10"'" 2-431-00 1·50 1·50 0-50 5-0 10-2 5-0 10-2 1-670-40 0-90 2-00 0-50 4-1 10-4 2-3 10-1 1'001-00 2-00 0-60 1-00 1·2 10-4 1-2 10-4 4-29t·OO 2-00 0-80 1-00 4-5 10-4 4-5 10-4 3·710-40 1-40 1-00 1-00 2-2 10-4 1-3 10-3 3·''40-60 1-60 1-00 1-00 5-1 10-4 1·4 10....'! 3·220-80 1-80 1-00 1-00 8·8 10-4 1-4 10....'! 3·221-00 2-00 1-00 1-00 1·3 10-' 1-3 10....'! 3-240-15 HS 1-30 1-00 2-9 10-4 1·2 10-2 2-260-20 1-20 1-30 1-00 5-0 10-4 1-3 10-2 2-270-15 HS 1-50 1·00 9-3 10-4 4-1 10-' 1·750·20 1-20 1·50 1·00 1-6 10-3 4-0 10-2 1·770-04 1·04 2-00 1·00 2·6 10-4 1-6 10-1 H70-10 HO 2·00 1·00 1-9 10-3 1-9 10-1 1-090-40 1-40 2-00 1-00 2-6 10-' 1-6 10-1 1-160-10 HO 3·00 1·00 1-1 10-' H 0-330-04 1-04 4-00 1-00 3-3 10...."l 2-0 0-060-04 1-04 6-00 1-00 3-9 10...."l 2-4 -0-020-04 1-04 10-00 1-00 3·7 10...."l 2-3 0-010-04 1-04 67-00 1·00 3·7 10-3 2·3 0-010-40 2040 0·60 2-00 1·0 10-4 6·3 10-4 3·570-40 2-40 0-80 2·00 2-1 10-4 1-3 10....'! 3-21i0-40 2·40 1-00 2-00 3·9 10-4 2-4 10...."l 2-9~0'40 2·40 1·30 2-00 2·2 10...."l 1·3 10-' 2-240-40 2-40 2·00 2-00 1·9 10-' 1-2 10-1 1-300-40 2040 4·00 2·00 2·7 10-1 1·7 0-150-40 2·40 10·00 2-00 3·8 10-1 2-4 0-000-40 2·40 15·00 2'00 3·7 10-1 2-3 0-00157INDIAN J. CHEM., VOL. 14A, MARCH 19760:500 1000t.seeFig. 1 -- Influence of t~e initial co~centra:ion of AgNO~on the rate of coagulation of AgI/I at c(I ) = 1 molejmand C(Mg2+) = 1 mol/m3 [cin(AgNOa)/mol m-3: (A) 1,(B) 0'8. (C) 0'6. (D) 0·4]IIA BIIII<>o 500 1000 1500a t.sccFig. 2 -- Influence of the initial concentration of AgNOaon the rate of coagulation of AgI/I- at c(I-) = 1 molejrn"and C(MgH)= 2 mol/rn" [cin(Ag Oal/mol m-a: (A) 0·4.(B) 0'1. (C) 0'04]the intensity of scattered light increases with in-creasing polydispersity of the system. It ~asfound15-17 that coagulation is a "self preservingprocess" i.e. that polydispersity becomes constantafter a certain time and is practically independentof the initial condition so that constant kdifl and fcan be assumed. Linear relationship between Irel.and time are shown in Fig. 1 and 2. Such linearrelationships were obtained in all the experim~ntscarried out in this study. There are two possibleexplanations of these results. The first is, thatneither kdifI and W nor the A,], and c/> values chaI?-geduring the measurements. The second explanationcould be that the effects of change in k, A,f, and c/>values compensate each other in all the exami.nedsystems. However, this is rather improbable smcethe stability coefficient changes by four orders ofmagnitude. The first assumption is supported bythe fact that the slopes in Figs. 1 and 2 are p:opor-tional to the square of the initial concentration ofAgN03, in agreement with Eq. (4), Ha?n a,ndStummw have found similar linear relationships158c II-)1 mole m-3C> 2·00o 1·00e 0'50• 0·10o 0·04.4320L- ~ ~ ~o 2log [(MgH]/mol m-S)Fig. 3 -- Influence of the c(I-) and c(MgH) on the stabilityof AgI/I-for Ludox particles (initial diameter: 15 nm) aswell as the slopes proportional to the square of theinitial concentration of colloidal particles. Theinfluence of the total volume of colloidal particlesis taken into account in Eqs. (2) and (4) and itshould be included in the operational definition ofthe relative rate constant of coagulation.The effect of potential determining ions on thestability of lyophobic colloids was previously in-vestigated9,lO by examining the dependence of thec.c.c. of counterions on the activity of potentialdetermining ions. The C.C.c. was found to be con-stant in a wide range of activity. Such resultswere found for the AgBr/Br- and AgI/I- systems.This is in agreement with our results shown inFig. 3. We have found no significant change inthe stability coefficient in the iodide ion concentra-tion range from 0·04 rnoljrn" to 2 moljrn" at 1<W<10000. According to Reerink and Overbeck"this effect can be explained by the assumption thatthe potential in the Stern layer remains constantin spite of the change of surface potential which isproportional to the logarithm of the iodide ionactivity. The statement that the potential in Sternlayer does not depend on the activity of potentialdetermining ions in slow coagulation region shouldbe the subject of theoretical investigations whichshould lead to further refinement of a modern theoryof colloidal stability.References). Manual of symbols ar:d termii olog y for physicochemicalquantities and units Appendix II -- Terrnir-ologyand symbols in colloid and surface chemistry. Funappl. Chem .• 31 (1972). 577.2. KRUYT. H. R.. Colloid science. Vol. I. (Elsevier. Amster-dam) 1952.KALLAY & VUCKOVIC: STABILITY OF SILVER IODIDE COLLOID3. VERWEY, E. ]. W. & OVERBEEK, J. TH., Theory of the ~.stability of lyophobic colloids (Elsevier, Amsterdam) 1948. V4. REERINK, H. & OVERBEEK, ]. TH., Disc. Faraday Soc., 3.18 (1954), 74.TE!AK, B., Croat. Chem. Acta, 42 (1970), 81. 14-MIRNIK, M., Croat. Chem. Acta, 42 (1970), 49. v·LEPIN, L. & BROMBERG, A., Acta Physicochim., 10 (1939), -ys.83.HERMANS, ]. ]., Rec. trav. chim., 58 (1939), 139.TE!AK, B., J. phys. Chem., 57 (1953), 301.SCHULZ, K. F. & TEZAK, B., Archiu Kemi., 26 (1954), 187. j 7.OSTER, G., J. colloid su., 2 (1947), 291.Vs.v6.7.8.9.10.\ 11.KERKER, M., The scattering of light and other electromag-netic radiation (Academic Press, New York), 1969.OTTEWILL, R. H. & HORNE, R. W., Kolloid-Z., 149(1956), 122. vMIRNIK, M., STROHAL, P., \VRISCHER, M. & TEZAK, B.,icouoia-z.. 160 (1958), 146.SWIFT, D. L. & FRIEDLANDER S. K., J. colloid interfaceSci., 19 (1964), 621.FRIEDLANDER, S. K. & '.'YANG, C. S., J. colloid interfaceSci.,22 (1966), 126.LA!, F. S., FRIEDLANDER, S. K., PICH, 1. & HIDY, G. M.,J. colloid interface sa., 39 (1972), 395.159

Influence of Potential Determining Ions on the Stability of Silver Iodide Colloidal System

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Influence of Potential Determining Ions on the Stability of Silver Iodide Colloidal System

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