In order to obtain the relation between several factors affecting
the colloido-chemical properties of silver iodide in the heterogeneous
exchange processes, the following measurements were carried
out: determination of the adsorption capacity of silver iodide (radiometry)
tempered from 20 up to 400 °c, dry aged for 20 days to 5
years, pressed at 500 and 1500 kg cm-2 (on dried isoelectric precipitate);
determination of the relation between particle size (electron
microscopy) and crystallite size (X-ray diffraction analysis), and
of the adsorption capacity of AgI (formed by dilution of KI + AgI
complex solution) in the presence of sodium laurylsulphate (SLS);
determination of the surface area (BET) of the tempered isoelectric
dried silver iodide; determination of particle size and determination
of the volume fraction of cubic and hexagonal modifications
of AgI in the tempered samples. In conclusion, the effect of the inner
surface as a possible relevant factor for the elucidation of the
influence of surface active substances on acceleration of the
exchange rate is pointed out. Interactions with sodium laurylsulphate
are also discussed

R. Despotović

S. Popović

Z. Grabarić

English

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CCA-782 CR 0 AT IC A CHEMIC A A C TA 45 (1973) 441 541.182 Original Scientific Paper On Some Properties of Silver Iodide* R. Despotovic, Z. Grabaric, and S. Popovic »Ruder Boskovic« Institute, Zagreb, Croatia, Yugoslavia Received Jul y 20, 1972 In order to obtain the relation between several factors affecting the colloido-chemical properties of silver iodide in the heteroge-neous exchange processes, the following measurements were carried out : determination of the adsorption capacity of silver iodide (radio-metry) tempered from 20 up to 400 °c, dry aged for 20 days to 5 years, pressed at 500 and 1500 kg cm-2 (on dried isoelectric preci-pitate); determination of the relation between particle size (electron microscopy) and crystallite size (X-ray diffraction analysis), and of the adsorption capacity of AgI (formed by dilution of KI + AgI complex solution) in the presence of sodium laurylsulphate (SLS); determination of the surface area (BET) of the tempered isoelectric dried silver iodide; determination of particle size and determina-tion of the volume fraction of cubic and hexagonal modifications of AgI in the tempered samples. In conclusion, the effect of the inner surface as a possible relevant factor for the elucidation of the influence of surface active substances on acceleration of the exchange rate is pointed out. Interactions with sodium lauryl-sulphate are also discussed. INTRODUCTION The process of heterogeneous exchange of precipitates is convenient for obtaining information on the equilibration of the precipitates or solids with the supernatant liquor or the surrounding solutions. These investigations revealed several phenomena which are closely related to the processes in the boundary region of the solid phase1. In previous papers1•2 it has been shown that the heterogeneous AgI-I- exchange is predominantly characterized by the recrystallization process while AgI-Ag+ exchange is determined by auto-diffusion of the Ag+ ion in the crystal lattice of the precipitates. It seems that the crystal modification and the surface area are important factors in the process of heterogeneous exchange. Silver iodide in a suspension is poly-morphic and the fractions of cubic and hexagonal modifications of Agl and the adsorption capacity vary with experimental conditions2- 5• In order to elucidate these interrelations, the role of the crystal modification and the role of the surface area in the exchange processes have been examined and the results obtained are described in the present paper. * Partially reported at The 3rd Yugoslav Congress on Pure and Applied Che-mistry, Ljubljana (Slovenia), Yugoslavia 1972. 442 R. DESP:OTOVIC ET AL. EXPERIMENTAL Experiments were carried out by several different physical and chemical techniques. (i) In order to obtain data on the influence ·of dry aging on the adso.rption capacity, the isoelectric in vacuo dried silver iodide powder (prepared for the expe-riments as described beforee), was left in the dark in a brown bottle with a glass stopper. Dried silver iodide was aged for 20 days and for five years. The samples of AgI were separately put into the same NaI solutions at pI = 3 containing Eu(N03) 3 from 1.67 X 10-o to 3.33 x 10-4 M labelled with Eu-152, 154 at pH = 2.8 and at 20.0 ± 0.2 °c. After 1000 minutes adsorption-desorption equilibria were reached. The samples, decanted with care, washed, dried and weighed, were counted in a well type NaI(TlI) scintillator connected with an EKCO Electronic scaler. (ii) The same dried isoelectric silver iodide powder was tempered for 2500 minutes at various temperatures from 20 to 400 °c at normal atmosphere. After cooliing, by using the same .radioactive tracer techniques as before (i) , adsorption capacities were determined at 20.0 ± 0.2 °c in the systems with tempered AgI. By means of BET method the surface area of the investigated isoelectric dried silver iodide was determined (silver iodide was kept in vacuum for 2500 minutes at 20 °c and at 400 oc). The silver iodide prepared in the same thermal treatment was also used for X-ray diffraction analysis in order to obtain the volume fractions of cubic, C, and hexagonal, H, modifications of AgI, as well as the crystallite sizes, D, in the samples. To this end integrated intensities and diffraction broadening (corrected for the instrumental broadening) of several prominent reflections were measured.'3• Diffraction patterns inclu ding hexagonal reflections (100) and (101) and overlapping cubic (111) and hexagonal (002) reflections were recorded (at 20 °C) by a diffractometer with a scintillation counter and a .single channel pulse height analyser. Melted AgI was a1so analysed and the results are collected in Table I. TABLE I The Proportions of Cubic (C) and Hexagonal (H) Phases and the Crystallite Sizes (D) in Agl Samples: Q) I I I ...... Q. Thermal I s treatment C(%) I H(0/o) DH(lOOJhtm Dc(lll) /µm Drr(10l)him ro (/1 i 1 dried in vacuo 57 43 0.9 0.9 0.2 t = 20 °c 2 t = 120 °c 50 50 1.3 1.3 0.35 3 t=l70°C 44 56 1.4 1.5 0.60 4 t = 400 °c 15 85 1.5 1.5 1.4 5 melted ground 49 51 1.4 1.5 0.20 6 melted 40 60 1.4 1.5 0.02 (iii) X-ray diffraction technique was used to determine crystallite sizes, D of silver iodide sols prepared »in statu nascendi« with 1.0 X 10-5 M and/or 1.0 X 10-2 M sodium laurylsulphate, SLS. Particle sizes R in these systems, were determined by electronmicroscopy and their adsorption capacity was measured in the same manner as above. The results obtained are tabulated in Table II. The influence of pressure on the adsorption capacity of AgI was measured on silver iodides pressed for 1 minute under 500 and 1500 kg/cm2, respectively. Adsorption capacity was determined on plates and crushed plates by the standard radioactive tracer technique. PROPrERTJES OF SUJV1ER [0DJ[)E 443 TABLE II Relation between Particle Sizes (D), CrystaHite Sizes (R) and Adsorption Capacity of AgI in the Presence of Sodium LauryLsyLphate SLS, at 20.0 ± 0.2 °c, pl = 2 and 0.01 M AgI --'Y CsLs mmol (1/3 Eu3+) D/µm R/µm /mol (AgI) 1.0 X 10-s M I 0.20 ± 0.02 i 0.17 0.02-0.8 (mainly 0.4-0.8) I 1.0 X 10-2 M I 0.80 ± 0.05 0.04 0.07-1.3 (mainly > 1.0) RESULTS AND DISCUSSION Aging and Adsorption Capacity Dried isoelectric silver iodide precipitates obtained by »fast« precipitation2 dry aged for 5 years show an about 500/o smaller adsorption capacity (y = = 0.17 ± 0.02 mmol of 1/3 Eu3+ per mole of Agl) than the precipitates aged for 20 days only [y = 0.33 ± 0.03 mmol (1/3 Eu3+)/mol (Agl)]. This aging effect may be explained as follows. During fast precipitation the particles of silver iodide acquire larger adsorption capacity than those in slowly formed suspensions of Agl2. This difference of the adsorption capacity could be caused by different dispersity in suspension and by the formation of a bigger number of disorders in the solid Agl, which was precipitated faster2, 5,n 1. Tamman ripening, i.e. cold syntering on 5 year aged samples takes place in the aging of dried Agl. Therefore both the outer and the inner surface of Agl decrease by aging. With the decrease of the outer crystal surface the crystal disorders disappear too, i.e. the capacity of the Grimley-Mott double electrical double-layer also decreases. For this reason the adsorption capacity y diminished too. Crystallite Sizes, Cubic/Hexagonal Ratios, Surface Areas and Adsorption Capacities of Tempered Samples When the aging process is accelerated by heating of the silver iodide crystals, the adsorption capacity decreases with heating. For this reason the isoelectric silver iodide was tempered up to 400 °C and X-ray diffraction, BET and adsorption determinations were carried out. It can be seen from Table I that the proportion of cubic phase, C, in the tempered sample is somewhat smaller than in the untreated sample 1. All crystallite sizes shown in Table I increase with temperature. DH(ioo) is always very similar to Dc(lll )· For the sample 1 the c axis of the hexagonal crystallites is much shorter than the a axis, but for the sample 4 these two dimensions are even smaller. The growth rate of Du(ioi) is larger than the growth rate of DH(ioi)· This difference between the a and c dimensions of hexagonal crystallites is still larger in the samples 5 and 6. This has caused some difficulties in measuring the half maximum breadths of sharp C (111) lines, as they are superimposed on very broad H (002) lines. By tempering, the part of hexagonal Agl increased up to 800fo, and we may conclude that the crystallites of the silver iodide grew by tempering. At these conditions the mass of one crystallite increased by 444 R. DESPO'l\OVIC ET AL. a factor of about 11 and its surface by a factor of about 6. This increase was, of course, attended with an adequate decrease of dispersity. Meanwhile, BET analysis shows (P20 oc = 0.60 ± 0.15 m 2/g AgI and P 4 00 oc = 0.02 ± 0.01 m2/g Agl) that the total surface of the system decreased by tempering much more, which undoubtedly indicates that by tempering the cubic-hexagonal mixtures of AgI, cubic and hexagonal small crystallites grow into bigger hexagonal crystallites with a very small total surface. At the same time this indicates that the dependence of the adsorption capacity on tempering is the combined effect of the change of at least two variables [ y20 oc = 0.17 ± 0.03, y120 oc = 0.16 ± 0.01, Y110 °c = 0.12 ± 0.01 and y400 oc = 0.05 ± 0.01 mmol (1/3 Eu3+)/mol (AgI)]: the change of the C/H ratio and the change of the total surface of the solid phase. In this way the adsorption appears to be dependent only on crystallite size and crystalographic modification. Crystallite Sizes and Adsorption Capacities for Variously Pressed Samples and in Presence of SLS The role of the intrinsic area of crystallites in the exchange processes can be elucidated by examination of the role of the inner area in the adsorption phenomena. To this end the experiments were carried out on pressed AgI (small intrinsic area) and on the systems with sodium n-dodecyl sulphate (high dispersity). It is known14 that by pressing, the isoelectric cubic-hexagonal mixture can be entirely transformed into the cubic modification, which was achieved. At a lower pressure of the silver iodide, the plate has the structure of the pressed powder and at a higher pressure it looks like yellow transparent glass. In both cases cubic AgI with very small adsorption capacity was obtained. In the case of the yellow transparent glass plate the capacity was even smaller [at 500 kg cm-2 y = 0.08 ± O.Ql, at 1500 kg cm-2 y = 0.006 ± 0.003 mmol (1 /a Eu3+)/mol (AgI)]. By crushing the pressed silver iodide plates the ad-sorption capacity markedly increased [at 500 kg cm-2 y = 0.10 ± 0.01 , at 1500 kg cm-2 y = 0.03 ± 0.01 mmol (1/a Eu3+)/mol (AgI)]. We cannot determine crystallite sizes by X-ray technique because of the effects of preferred orien-tation and therefore we can only suppose that y is lower on the glassy plate because of the lower surfaces accessible to the electrolyte solution. From the results tabulated in Table II we can see that the intrinsic surface can also assume a determining role. In the presence of the surface active substances such as sodium laurylsulphate, SLS, by dilution of a AgI + KI complex solution, the silver iodide particles were formed with higher y for larger particles, because they were composed of many small crystallites. It seems that the inner surface of the silver iodide particles is accessible for communi-cation with the electrolyte, i. e. it is open for adsorption-desorption processes and for heterogeneous exchange processes too. The silver and iodide ions in the suspension in heterogeneous exchange processes are in this case in direct connection with the outer and inner surface of silver iodide particles, which is of great importance in the explanation of the influence of surface active substances on heterogeneous exchange rate in AgI-I- and AgI-Ag ' systems1,2 ,5,s. The results of Table II can be understood also in terms of adsorption of europium Eu3+ to Agl containing a high surface concentration of anionic SLS. This consideration is particularly important for the highest SLS concentrations where in addition to adsorption, the interaction of the PROPERTIES OF SILVER IODIDE 445 europium with LS has to be considered. According to the Herz's15 and Padday's16 concept of the adsorption on selected sites and similarly to Mirnik's concept of adsorption at discrete charged sites17, constitutive r- ions from the solution can be exchanged with the same constitutive ions from free crystal surfaces. CONCLUSION Conditions of precipitation and of aging (drying, tempering, pressing) as factors affecting crystallite or particle size, cubic/hexagonal ratio, surface area and adsorption capacity were investigated. Analysis of results shows mutual dependences of physicochemical properties of silver iodide on the conditions of precipitaion and aging. The results show that the surface of Agl decreases by aging dry and in the supernatant. It is supposed that crystal disorders disappear causing a decrease of the adsorption capacity y. X-ray diffraction analysis, BET and adsorption determination of the thermally treated isoelectric silver iodide indicate dependence of the adsorption capacity on crystallite sizes and on the crystallographic modification. Analysis of pressed AgI and the analysis in presence of SLS formed AgI show, in connection with other results, that in heterogeneous exchange processes the silver and iodide ions in the suspensions are in direct connection with outer and inner surface of silver iodide particles. This is of great importance in the explanation of the heterogeneous exchange rate in Agr- r- and AgI-Ag+ systems, parti-cularly in the systems containing surfactants. According to Tefak's concept the results obtained show that the processes (adsorption equilibria and heterogeneous exchange) in the methoric layer are the direct resultant of the equilibria of the processes in the solid and liquid phase. Acknowledgment. We are grateful to Mr. F. Matijevac for BET analysis. REFERENCES 1. R. D esp o to vi c, Discuss. Faraday Soc. 42 (1966) 208. 2. K. Yamada, Bull. Soc. Sci. Photogr. Jap. 11 (1961) 1 ; B. L. J . B yer 1 e y and H. Hirsch, J . Photogr. Sci. 18 (1970) 53; M. Mir n i k and R. D esp o to vi c, Kolloid Z. 180 (1962) 51; R. Despot o v i c, Croat. Chem. Acta 37 (1965) B 19. ; Tenside 7 (1970) 245. 3. R. D esp o to vi c and S. Pop o vi c, Croat Chem. Acta 38 (1966) 321; R. D esp o to vi c, Z. D es p o to v i c, M. J a j e tic, S. Pop o vi f:., and Z. Te 1 ism an, Kem. Ind. (Zagreb) 17 (1968) 197; R. D e s p o t o v i c, Z. D e s p o t o v i c, M. J a j e t i c, M. M i r n i k, S. P o-p o vi c, and Z. Te 1 ism an, Croat. Chem. Acta 42 (1970) 445 ; R. D e s p o t o v i c, Z. D e s p o t o v i c, M. M i r n i k, and B. S u b o t i c, ibid., 42 (1970) 557. 4. M. He r a k, Kolloid Z. Z. Polym. 232 (1969) 793. 5. R. D esp o to vi c and Z. G r ab a r i c, T enside 8 (1971) 313. 6. R. D esp o to vi c and V. K a r a iv a n ova, Proc. Vlth Internat ional Congress on Surface Active Substances, Zurich (1972) B 3, p. 27. 7. T. B. Grim 1 e y and N. F. Mott, Discuss. Faraday Soc. 1 (1947) 3; T. B. G rim 1 e y, Proc. Roy. Soc. London 201 (1950) 40. 8. C. R. Berry and D. C. Ski 11 man, J. Photogr. Sci. 16 (1968) 137. 9. B. Te z a k, Discuss. Faraday Soc. 42 (1966) 175 ; Croat. Chem. Acta 42 (1970) 81. 10. E. P. Honig and J . H. Th. Heng s t, J . Colloid Interface Sci. 31 (1969) 545. E. P. Honig, Trans. Faraday Soc. 65 (1969) 2248 ; Nature London 225 (1970) 537. 11. R. H. 0 t t e w i 11 and R. F. Woodbridge, J. Colloid Sci. 19 (1964) 606. 12. D. T e s 1 a - To km an o vs k i, M. J . Her a k, V. Pr av di c, and M. Mir n i k, Croat. Chem. Acta 37 (1965) 79. 446 R. DIESIPO'l'OVIC ET AL. 13. E. L. M a cko r, Rec. Trav . Chim. Pays -Bas 70 (1951) 763. 14. G. Bur 1 e y, Amer. Mineral. 48 (1963) 1266. 15. A. N. Herz and J. 0 . He 11 in g, Kolloi d z. 218 (1967) 157 ; A. H. Herz, R. B. Danner, and G. Janus on is, Advan. Chem. Ser. 78 (1967) 1. 16. J. F. Pad day, Proc. International. Symposium on Surface Area Determination Bristol 1969, p. 331. 17. M. Mir n i k, Croat. Chem. Acta 35 (1963) 217. IZVOD 0 nekim svojstvima srebrnog jodida R. Despotovic, Z. Grabaric i S. Popovic S ciljem utvrdivanja karakteristika koje utjecu na koloidno kemijska svojstva srebrnog jodida u procesima heterogene zamjene provedena su slijedeca mjerenja: odredivanje adsorpcionog kapaciteta srebrnog jodida grijanog od 20 do 400 °c (radio-metrija i termicki tretman), starenog kroz 20 dana do 5 godina i presanog na 500 i 1500 kg/cm2 (sve na susenom izoelektrienom precipitatu) ; odredivanje omjera velicina cestica (elektronska mikroskapija), velicine kristalita (rendgenska difrakcija) i adsorp-cionog kapaciteta srebrnog jodida u prisustvu natrijevog laurilsulfata; odredivanje vanjske povrsine (BET) termicki tretiranog izoelektrienog srebrnog jodida; odredi-vanje volumne frakcije kubicnog i heksagonalnog srebrnog jodida kao i velicine kristalita u termicki tretiranom srebrnom jodidu (rendgenska difrakcija). U dtskusiji rezultata pokazuje se interferencija kemijskih i karakteristika cvrstog stanja kao faktora koji utjeeu na koloidno kemijska svojstva srebrnog jodida. Pokazan je i utje-caj unutrafoje povrsine kao moguceg relevantnog fakto ra u obj asnjenju utjecaja povrsinski aktivnih supstanci na brzinu procesa heterogene zamjene. Interakcije sa SLS su takoder disku tirane. INSTITUT »RUE>ER BOSKOVH'.>< 41000 ZAGREB Primlj eno 20. srpnja 1972. 

On Some Properties of Silver Iodide

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