The density of the charges of (AgI)n is in direct relation with
the concentration of the potential determining ions (Ag+, i-),
reaching its maximum at 1.1 ions per nm2, and is in inverse
relation with the adsorption of neutral molecules (aliphatic alcohol).
The adsorption of aliphatic alcohol (which reaches its maximum
at 0.57 molecules per nm2 on the left side of the pzc) is in inverse
relation with the adsorption of the potential determining ions.
The adsorption of 1 molecule of alcohol is accompanied by desorption
of 2 ions and vice versa

C. Wasowski

M. Tschapek

English

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CROATICA CHEMICA ACTA CCACAA 47 (4) 577-585 (1975) CCA-902 YU ISSN 0011-1643 546.57 :541.183 Original Scientific Paper Competitive Adsorption of Potential-determining Ions and Neutral Molecules by (Agl)n M. Tschapek and C. Wasowski Casilla de Correo 30, Sucursal 19, Buenos Aires, Rep. Argentina Received April 9, 1975 The density of the charges of (AgI)n is in direct relation with the concentration of the potential determining ions (Ag+, i-), reaching its maximum at 1.1 ions per nm2, and is in inverse relation with the adsorption of neutral molecules (aliphatic alcohol). The adsorption of aliphatic alcohol (which reaches its maximum at 0.57 molecules per nm2 on the left side of the pzc) is in inverse relation with the adsorption of the potential determining ions. The adsorption of 1 molecule of alcohol is accompanied by des-orption of 2 ions and vice versa. INTRODUCTION The Agl sols have been the object of many studies. It would be no exag-geration to say that the electrochemistry of colloids has advanced ·On the basis of studies of (Agl)n: C-'Potential, coagulation, pzc, adsorption, etc. Nevertheless, one property ·of (AgI)n, namely its specific surface (a few m 2 g-1), is variable and difficult to determine and thi!s may constitute a serious handicap in electrochemical studies. It is a fact that determinations of specific surface, based on adsorption phenomena, are not precise1• A similar situation exists also with respect to the mean crystal size and density of charge. Lange and Be.rger2 assumed that the density of charge is 80 µC cm-2 or 5 X 1018 ions g-1• Later Lyklema et al. 3 gave values < 5 µC cm-2 or < 3.3 X 1017 ions g-1 for dried preparations. Mirnik4 gave values 5-8 X 1013 ions g-1 (after authors 2-3 gequiv. per 103 M of (Agl)) for coagulated by counter ions and aged for 16 hours, i.e. close to the value assumed by Lange and Berger2. Obviously, the discrepancy between the values of different authors is due to the incorrect determination of the crystal size and specific surface and to different modes of preparation of (Agl)n. Frumkin and Obrucheva5 were the first to establish that the maximum adsorption of neutral molecules (aliphatic acids: heptylic and caprylic) by (Agl)n from aqueous s·olution takes place at the negative side of pzc. Later, Lyklema et al.1 ,3 found the same phenomenon using as adsorbates aliphatic alcohols (butyl and propyl, etc.). Having available the specific surface values of (Agl)n these .authors established the relationship between medium and electrical double layer properties: density of charge, pzc, etc. 578 M. TS CHAPEK AND C . WASOWSKI The aim of this article is the quantitative study of the adsorption -desorption relations between 1-heptanol and the potential determining ions Ag+ and r-. MATERIALS AND METHODS All chemicals were of Analar grade and were used without further purification. The sediment was obtained by mixing of 50 cm3 1 M AgN03 with 51 cm3 of 1 M KI, and stirring. After being left in repose overnight, 2 cm3 of 1 M AgN03 were added to the sediment which was then washed with bidistilled water up to pAg = = 5.2 - 5.3. Then, the sediment was transfered to a measuring cylinder of 100 cm3 and the volume made up to 102 cm3 (2 om3 =volume occupied by 11.73 g of (AgI)0 with bidistilled water which contains determined quantities of alcohol and/ or AgN03 and KI, i. e. Ag! = 0.5 M. The supporting electrolyte (KN03) was added in a dry state. c b a Fig. 1 Illustration of determining of the pzc by the suspensions effect measurements. a) sedi-ment; b) decantate; c) bridges with 0.01 M KNOa; d) calomel electrodes. COMPETITIVE ADSORPTION 579' The changes of Ag• and r- concentrations were determined by the use of calibration curves with the silver electrode 242 of Metrohm, and silver - silver iodide electrode made after De Bruyn6 respectively. As a reference electrode the· KCl saturated calomel electrode 204 of Metrohm was used, connected to the solution by a 0.01 M KN03 salt bridge. The pzc of (AgI)n was determined by the suspension effect, measured with two calomel electrodes submerged, as shown in Fig. 1, in the sediment and the decantate obtained by centrifugation. The EMF was measured with a Keithley Digital Electrometer 616 whose input impedance is > 2.1014 ohms. The number of adsorbed ions per nm2 was calculated from the difference in amount of Ag• in the decantate and in blank solutions. The· specific surface, determined by Methylene Blue adsorption is A/Ag!= 3.6 m2 g-1, i.e. the surface of 11.73 g of (AgI)n is 42.2 m 2• The adsorption of 1-heptanol by (Ag!) Ag• and (Ag!) r- was determined by measurements of the surface tension (y) for decantate (Ydec) and blank solution (yb1) by a De Noi.iy tensiometer, at 25 °c. From the calibration curve y = f (n), where n is the amount of 1-heptanol in mg per 100 cm3, is found. The difference between the amount in the blank solution and in the decantate gave the amount of adsorbed alcohol, calculated in molecules per nm2• For a comparative study also the nonporous carbon black »Vulkan 3 - G (2700)" A/C = 71.3- 2.7 m 2 g-1, manufactured by Teddington Physical Laboratory (England), was used. RESULTS AND DISCUSSION In order to obtain the cross - sectional area of the 1-heptanol molecule-in the interface between the h y drophobic surface and water, it s adsorption on the !Ilonporous carbon black of known specific surface was studied. The curve-in Fig. 2 shows the number of 1-heptanol molecules adsorbed by nonporous carbon black per nm2, as a function of its concentration. 3 • • J 2 3 4 5 6 7 [7-Hept J x1a3 /mo/ dm3 Fig. 2 Adsorption of 1-heptanol b y carbon black as a funct ion of its concen t ra tion (mol/dm•)-Ord: r (1-hept)/nm• =number of adsorbed molecules per nm•; Absc: [1-hept] x 10•/mol dm-•. From the curve presented in Fig. 2 it can be concluded that the cross -sectional area of the 1-heptanol molecule in the interface between hydrophobic solid and water is 40 A2 (= 0.4 nm2) . Now, assuming that the cross-sectional 580 M. TS CHAPEK AND C. WASOWSKI area of the 1-heptanol molecule in the interface (AgI)n - water is the same as in the interface carbon-water, it is possible to determine the specific surface of (AgI)n· In Fig. 3 change of adsorption of 1-heptanol iby (AgI)0 , at pAg = 5.4, as a function of its concentration is given. 0 40 roo =0.40 ID> · -- · --.-- 1--:--•--:-·-~ t7) ...3... ~020 E . ....._ ~ 1= ~ 2 4 6 [7-Hept] x 10] mot dm3 Fig. 3. Adsorption of 1-heptanol by (Agl)n as a function of its concentration at pAg = 5.4. Ord: r (1-hept)/mg g·• =adsorbed 1-heptanol in mg per g-t; Absc: [1-hept] x 10•/mol ctm-•. As may be estimated, the maximum adsorpti.on, I' oo is 0.40 mg g-1, which at a cross - sectional area of the 1-heptanol molecule 40 A2 (= 0.4 nm2) gives the specific surface as 0.83 m 2 g-1• Tschapek and Caviglia7 for similar sediment of (AgI) Ag+ found I' oo = 0.21 mg g-1, at pAg = 3. The value 0.83 m 2 g-1 does not agree wi'th the parti"cle distribution curve of (AgI)n. Study of sedimentation of (AgI)n suspension, obtained by interaction of 1 mol-ar soluti·ons, shows that it contains 53.70/o of particles < 3 X 10-5 cm. Although the suspension contains a very low percentage of particles < 1 X 10-5 cm, it was evident that the specific surface must be > 0.83 m 2 g-1• In order to find a more exact specific surface value of (AgI)n, the adsorpt-ion of Methylene Blue (M. B.) was studied. In this case an attempt was made to compare data obtained for (AgI)n whith those for carhon black. The M.B. method has been used by many authors to determine the specific surface of disperse minerals1,s-12• Most people have accepted that the cross-sectional area was 132 A2• F rom the adsorption curve of M.B. by carbon - blask the cross - sectional area 132 A2 was found, i.e. close to the one accepted by many authors. The (AgI)n suspension at pAg = 5.4 adsorbed 1.46 mg g-1 of M.B., which gives a specific surface of 3.6 m 2 g-1, and this value was accepted for the present study. In F·ig. 4 the curve of Ag+ adsorption by (AgI)n as a function of its con-centration is given. As may be seen, the density of charge reaches 1.1 ions per nm2, i.e. 17.6 µC cm-2 or 4 X 1018 ions g-1• This V1alue agrees with that given by Mirnik4 for r coagulated on fresh (AgI)n W/(AgI) = 2 - 3 X 10-3), but is higher than that given by Lyklema et al.3 for dried (AgI)0 • 1.25 COMPETITIVE ADSORPTION la,: 7.1 -----------------:~ . 1.0 / • 2 3 4 5 6 . [ Ag+JX1lf mot dni 581 • 7 8 Fig. 4 Adsorption of Ag+ by (AgI)n as a function of its concentration. Or d : r (Ag+)/nm' = number of adsorbed Ag+ per nm'; Absc: (Ag+] X 10•/mol dm-s. In Fig. 5 the adsorption curve of Ag+ as a function of pAg 1n the presence of the supporting electrolyte (KN03 ) and 1-heptanol is given. As may be seen, adsorption of Ag• at high concentrations, in the presence of 1-heptanol increases only at high concentration of supporting electrolyte. At zero or at low concen-trations of KN03 , the surface of (AgI)n is occupied principally by 1-heptanol molecules. The studies of Frumkin and Obrucheva5 as well as those of Lyklema et az.1,3 show that maximum adsorption of neutral molecules takes place on the left side of pzc, which, as shown by Lyklema et al., shifts to the right. In order to establish the pzc of (AgI)n sediment in the presence of 3 X 10-3 M of 1-heptanol and 1 M 1-butanol, the pzc was determined by measurements of the suspension effect. In Fig. 6 the results of the study of pzc are given. As may be seen, the pzc shifts to the right from pAg = 5.57 to pAg = 5.30 (from 235 to 247 mV) in presence of 3 X 10-3 M of 1-heptanol and from pAg = = 5.57 to pAg = 3.62 in presence of 1 M of 1-butanol. The adsorption - deso,rption reactions between 1-heptanol and potential - determining ions were carried out as follows. T·o 11.73 g/100 cm3 of (AgI)n (= 42.2 m2), which contained constant quantities of 1-heptanol (3 X 10-s M) and supporting electrolyte ([KN03] = 0.5 M), increasing stepped quantities of AgN03 or KI were added. After 15 min. of stirring, the concentrations of 1-hep-tanol and Ag+ or r- were determined. In Tables I and II the results of these experiments are given. 582 M. TSCHAPEK AND C. WASOWSKI I / 3~ /. 1 I 2~ c ~~~ ~ ! ....... Oi ~ +Oi 1 G /·~ --· -· • 4 3 2 Fig. 5 Adsorption of Ag+ by (Agl)n as a function of pAg in presence of (1-heptanol] = 2.7 X 10-• mol am-• and supporting electrolyte [KNO,] = 1; 0.1 ; 0.001 mo! ctm·3, Ord : r (Ag+/ Agl)/nm' = =number of adsorbed Ag+ per nm2 ; Absc: pAg. +70 ./ ,,,?0~.,/ ~yo/ /• o+--------11'----+-----n---,1<-·~-------5 • / .. I ...... ;;;; °') ~ I.I)" "'i IJ'"i " +" + t:Ji ·~ CJ) '<f: '<f: Q Q +220 +240 +260 +-39J +400 EMF/mv Fig. 6 The pzc of (AgI)n in presence of alcohols . Ord: Esusp. effect . mV =EMF of cell: calomel electrode - calomel electrode ; Absc: EMF/mV of cell: calomel electrode - Ag+ electrode. As may be estimated from data presented in Tables I and II, the adsorption of Ag+ and r- from one side and alcohol molecules from the other side are incompatible. Adsorption of Ag• and 1-heptanol on (Agl)n Ag 1 dm3 contains: [Agl]n=0.5 M; A=42.4 m2; [KN03]=0.5 M; [1-Heptanol]=3X10-3 M 1-Heptanol by calibration curve: EMF-CAg+ 1-Heptanol by calibration curve: y-CHept c/mol dm-3 pAg adsorbed c/mg dm-3 adsorbed amount I with in 105 mol/42.2 m2 / ions/nm2 [ blank sol. / (A~)i:~g• mg/42.2 m2 / molec./nm2 blank sol. (Agl)n Ag• equilibrium pAg = 5.2 rpAg = 5.2 5.2 0 0 35.5 30.8 4.7 0.57 1.70 X 10-s 1.00 x 10-0 5.10 0.07 0.10 35.5 30.8 4.7 0.57 3.23 x 10-5 4.48 x 10-5 4.35 0.11 0.16 35.5 31.2 4.3 0.52 4.48 X 10-5 2.63 X 10-5 3.58 1.85 0.26 35.5 31.8 3.7 0.45 2.88 x 10-4 2.40 x 10-4 2.62 4.8 0.68 35.5 32.6 2.9 0.35 6.31 x 10-4 5.63 x 10-4 2.25 I 6.8 0.97 35.5 33.6 1.9 0.23 8.60 x 10-4 7.82 x 10-4 2.20 7.8 1.11 35.5 33.9 1.6 0.20 TABLE II Adsorption of r and 1-heptanol on (Agl)n I 1 dm3 contains; [Agl]n=0.5 M; A=42.4 m2; [KNOa]=0.5 M; [1-Heptanol]='3X10-a M I , by calibration curve: EMF-Cr 1-Heptanol by calibration curve: y-CHept ' c/mol dm-3 adsorbed amount c/mg dm-3 pl adsorbed amount blank sol. / with in 105 mol/42.2 m" I ions/nm2 blank sol. / with mg/42.2 m2 / molec./nm2 (Ag)n 1-equilibrium (Agl)n 1-pAg = 5.2 pAg = 5.2 10.6 0 0 33.7 29.0 4.7 0.57 5.62 X 10-s 1.00 X 10-a 5.0 0.46 0.06 33.7 30.0 3.7 0.45 3.23 x 10-• 1.10 X 10-5 4.96 2.13 0.32 33.7 30.6 3.1 0.37 2.34 x 10-4 1.90 x 10-4 2.7 4.40 0.62 33.7 31.6 2.1 0.25 8.91 x 10-4 8.13 x 10-4 2.1 7.80 1.11 33.7 32.4 1.3 0.15 (') ~ l%J >-3 ..... ::l < l%J :;.,. t:l Ul 0 !ll "d ..., ..... 0 z C,11 00 w 584 M. TS CHAPEK AND C. WASOWSKI Becuase from T'ables I and II it is not evident that maximum adsorption takes place on the left side of pzc, additional experiments were performed for the range 4 < pAg < 8 or 91<EMF/mV<323. To 11.73 g of (AgI)0 (= 42.2 m2) in 100 cm3 , at pAg = 4, were added 35 mg of 1-heptanol, 5 g of KN03 and increasing quantities of KI. After each addition of KI the sediment was shaken for 15 m1n. and there the EMF of the silver electrode - calomel elec-trode cell and the surface tension, were measured. The results of this experiment are given in Fig. 7. Ci 0.20 -1. 20 I ~ g 33 8 97 7 749 6 207 5 265 4 pAg 323mV Fig. 7 Adsorption, r of 1-heptanol by (Agl)n as a function of pAg and EMF of cell : calomel electrode - A g+ electrode. Ord: r (1-hept/Agl) mg/mol (Agl)n = adsorbed 1-heptanol in mg per mo! of Agl; r (1-hept/Agl)/nm' = number of adsorbed molecules per nm•; Absc: pAg (EM:r'). Curves: two reproduced experiments. This experiment shows that the maximum adsorption of 1-heptanol takes place at pAg = 6.9 - 7.0, i.e. on the left side ·of pzc, which in the presence of 1-heptanol is at 250 mV, i.e. at pAg = 5.3. It is interesting to point out that an inverse relation exists between ad-sorption - desorption of the potential determining ions and 1-heptanol mole-cules: desorption of two ions is accompanied by adsorption of one molecule and vice versa. Naturally,. it is not an equivalent exhange, but is like a com-petition for surface sites, although it is not clear why the competition takes place when sites are unlike and besides surface coverage is too low: the area occupied by one ion is = 0.9 nm2 and by one molecule = 1.8 nm2 (0.4 mg per g). The competition would be easy to understand if the surface coverage was higher. CONCLUSIONS 1. The coverage of (Agl)n surface by 1-heptanol even at pzc (by vertical oriientation of the molecule) is 0.3. 2. The maximum measured adsorption of the potential determining ions (Ag+ and r-) reaches 1.1 ions per nm2, i.e. 17.6 µC cm-2 or 4 X 1018 inos per g of (Agl)n or 1.57 ions per 1000 Agl, at pl = 2 and pAg = 2. COiv1PETITIVE ADSORPTION 585 3. The maximum measured adsorption of the 1-heptanol reaches 0.57 molecules per nm2, i.e. 0.78 X 10-3 mol per mol of (Agl)n, at pAg = 6.9 - 7.0 and pl = 8.9 - 8.8 on the left side of pzc, which in presence of 1-heptanol shifts from pAg = 5.571 to pAg = 5.3. 4. The adsorption - desorption relation between potential determining ions and 1-heptanol molecules is inverse: adsorption of one molecule provokes desorption of ::::::::; 2 ions and vice versa. Although it is not an equivalent exhange, the coefficient ::::::::; 2 is maintained. 5. The potential determin1ng ions compete with 1-heptanol only in the presence of supporting electrolyte. Acknowledgment. The authors thank the Consejo Nacional de Investigaciones Cientificas y Tecnicas for their support in the course of this study. REFERENCES 1. H. J . Van den Hu 1 and J . Ly k 1 em a, J. Amer. Chem. Soc. 90 (1968) 3010. 2. E. L a n g e and R. Berger, Z. Electrochem. 36 (1930) 171. 3. J. Ly k 1 em a and J. Th. G. 0 verb eek, J. Colloid Sci. 16 (1961) 595 ; B. H. B i j st er b o s ch and J. Ly k 1 em a, J . Colloid Interface Sci. 37 (1971) 171. 4. M. Mir n i k Croat. Chem. Acta 42 (1970) 161. 5. A. N. Frum kin and A. 0 b r u ch e v a, Biochem. Z . 182 (1926) 220. 6. H . D e B r u y n, Rec. Trav. Chim. Pays-Bas 61 (1942) 5. 7. M. Ts ch ape k and A. M. Ca v Lg 1 i a, J. Electroanal. Chem. 46 (1973) 131. 8. U. Hofmann and I. Damm 1 er, Chimia 23 (1969) 476. 9. J . J. Kip 1 in g and R. B. W i 1 s· on, J. Appl. Chem. 10 (1960) 109. 10. G. M. D arr and U. Lu d v ig, Mater. Constr. 6 (1973) 233. 11. C. S. Bro o cs, Koloid-Z Z. Polym. 199 (1964) 31. 12. P. T. Hang and G. W. B rind 1 e y, Clays and Clay Miner. 18 (1970) 203. SAZETAK Kompetitivna adsorpcija iona koji odreduju potencijal i neutralnih molekula pomoeu (Agl)0 M . Tschapek i C. Wasowski Gustoca naboja (AgI)n) u izravnoj je vezi s koncentracijom iona (Ag+, r-) koji odreduju potencijal, a najveca je kod 1,1 iona po nm2• Ona je obrnuto proporcionalna adsorpciji neutralnih molekula (alifatski alkohol). Ova posljednja postize maksimum kod 0,57 molekula po nm2 • Adsorpcija jedne molekule alkohola je popracena de-sorpcijom dvaju iona. CASILLA DE CORREO, 30, SUCURSAL 19, BUENOS AIRES ARGENTINA Prispjelo 9. travnja 1975. 

Competitive Adsorption of Potential-determining Ions and Neutral Molecules by (Agl)n

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