exactly equal distances, the polysoap can be regarded as being built of only one kind of soap molecule with only one critical micelle concentration for chloride counterions and one for bromide counterions. From the nature of the synthesis of our polysoap, we must assume that the distances will vary about an average value. This means that the polysoaps must be regarded as being built of a number of different kinds of soap molecules, earh with different critical micelle concentrations. The actual critical dodecyl group content for either the chloride or the bromide form is some kind of mean value. K i t h increasing dodecyl group content, the critical distance is not reached in all parts of the polymrr chains a t the same time. Consequently, the transition from the expanded form to the coiled form of polysoaps with chloride or bromide counteiions is spread out over a range of compoeitions. I n addition, the stiffness of the polymer chains may also contribute to the effect. This picture is further complicated by the fact that by changing the degree of quaternization of polyvinylpyridine with dodecyl groups, not only the distance betlveen the soap-like groups is changed, but also
the ratio of hydrophobic to hydrophilic groups. This ratio determines, in the case of ordinary soaps, the critical micelle concentration and the solubi1ity.l’ l 2 We must bear in mind that the critical micelle concentrations for ordinary soaps are influenced by the presence of added electrolyte.1° Accordingly, it is conceivable that the critical doderyl group contents and their difference for the chloride and bromide forms of the polysoaps are alsq influenced by the presence of added salt. If we plot an effective length of the chains of the polyelectrolyte and the different polysoaps, derived from thc intrinsic viscosity, against their dodecyl group coiltent, it can be seen immediately that the poly-soaps with a dodecyl group content in the critical rang(. represent a system converting chemical energy i i i t o mechanical by exchanging their bromide counttbrions with chloride and the reverse.13 l 4 (11) E. K. Goette. J. Cdlozd Scz., 4,467 (1949). (12) H. V. T a r t a r and K. A . K r i g h t , J . A m . Chem. Soc.. 61, 539 (1939). (13) V‘. Kuhn, A n g e x . Chem., 70,58 (1958). (14) D. FYoerniann, 2. physzb. Chem , Aeue F o l w , 19, 05 (19%).
THE ADSORPTTOS OF PHOTOGRAPHIC DET’EI,OPEKS BY n m T A i l i x SILVER BY R. J. NEWMILLER AND R. B. POXTICS Conim?cnicoiion AYo.2069 f r o m the Kodak Research Laboratories, Eastman Kodak Company, Rocheder, 1T. I. Received October 16. 1969
The adsorption by metallic silver of five common photographic developers has heen measured. Adsorption isotherms of the Lanymuir type have been obtained at 20 and 40” for 4-amino-3-methyl-~-ethyl-N-(~-methylsulfonamidoethyl)aniline (I), 4-amino-3-methyl-?J,?v’-diethylaniline (11) and p-methylaminophenol (Elon). Simple p-phenylenediamine was found to adsorb but did not follow a Langmuir-type isotherm. N o adsorption of hydroquinone by silver could be detected. Developers I and IT were found to adsorb with a heat of adsorption of 13 kcal. per mole. Elon and p-phenylenediamine were :ulsori)ed more weakly with heats of adsorption of 3 and 6 kcal. per mole, respectively.
Introduction veloper by silver has been discussed recently by It has long been realized that developer adsorp- Eggert5 and by Klein.6 tion plays an important part in the mechanism of photographic development. The adsorption may take place on silver, silver halide or both, depending upon the developer being used. This paper deals with the adsorption of developers by metallic silver. A study of developer adsorption by metallic silver is important for two reasons. First, in some theories of direct chemical development proposed in the literahre from time to time, adsorption of the developer by the lat,ent image has been regarded as a key step. 1--3 The adsorption of quinone by silver has been investigated by Staude and Brauer4 and the quest’ion of the part played in photographic development by the adsorption of de(1) A.J. Rabinovitch, s. S. Peissachovitch and L. Minaev, “Ber. I b e r
Second, in the study of physical development, knowledge of adsorption is important to the investigation of the catalytic effect exerted by silver, whereby the activation energy for the reduction of silver ion at the surface is lowered below that of the homogeneous, uncatalyzed solution reaction.’ Little work has been reported in the literature concerning the direct measurement of developer adsorption by silver. Rabinovitch2a3attempted to measure the adsorption of hydroquinone by colloidal silver, and reported that hydroquinone v a s strongly adsorbed. Later, Perry, Ballard and Sheppard,* using an improved method, attempted to measure the adsorption of hydroquinone and Elong to silver, but found no indication of adsorption with these developing agents.
d. VI11 Internat. Kongress f. wiss.. U. angew. Photographie,” Dresden, 1931,p. 188. (2) A. J. Rabinovitch, 2. wiss. Phot., 33, 57 (1934). ( 3 ) A. J. Rabinovitch and S. S. Peissachovitch, ibid., 33, 94 (1934); Acta Physicochim. U.R.S.S., 4, 70.5 (1936); Trans. Faraday SOC.,34, 920 (1938). (4) H. Staude and E. Rraner, Z.Elektrorhem., 58, 129 (1954).
(5) J. Eggert, Phot. Sci. Enp.. 1, 94 (1958). (6) E. Klein, 2. Elektrochem., 6%, 505 (1958). (7) S. E. Sheppard, Phot. J., 69, 135 (1919). (8) E. S. Perry, A . Ballard and S. E. Sheppard, J . A m . Chem. Soc., 63, 2357 (1941). (9) Trademark of N-methyl-paminophenol rulfate, sold by the Caytinan Kodah Company.
XU data havc f>vrrheen pu1)lished concerning the direct measurement of adsorption of a p-phenylenediamine-type color developer by silver. Kinetic evidence, however, has seemed to point to the fact that these developers are adsorbed quite strongly.lOJ1 In view of the conflicting results which have been obtained with black-and-white developers, it is clear that :my technique to be used to measure the adsorption must, be considerably more sensitive than those used previously. Two requirements are essential for the system. First, the silver surface to be used should be clean, free of oxide, and reproducible, and, second, the developer must be protected from oxidation a t all times. Thus, a method is needed i.:i which the entire process can be carried out in an enclosed system. Conway, Barradas and Zawidzki" have described a method for determining adsorption of organic materials t,o metals, employing an ultraviolet spectrophotometric method of analysis. The system to be described here utilizes R similar method of analysis.
b; :e;L1
Developer mixing chomber
Cel I
$Adsxption
Circulation pump
ves5el
i
Fig. 1.-Adsorption apparatus: 4 , developer miving chamber; B, developer inlet tube; C, bypass; I), adsorption vessel; E, Teflon gland; F, flow cell in spectrophotometer; G, circulation pump; H, direction of flow. All portions except pump and connecting leads thermostated ; thermostat not shown.
Experimental Apparatus.-A fine silver wire (0.001 inch in diameter), drawn by Engelhard Industries of Newark, New Jersey, from better than 99.98% pure silver (as verified by spectrographic analysis) was used as the adsorbent. The wire had a specific geometric surface of about 160 cm.*/g. Between 30 and 35 g. of silver was used in each experiment. Adsorption measurements were made in a flow system (Fig. I ) , utilizing a spectrophotometric analysis of the developer solution. A Beckman D U Spectrophotometer, equipped with dual thermospacers for better temperature control, was used in the system. A special silica flow cell for the spectrophotometer was constructed from a design by Sweet and Zehner.I3 All other glassware in the system was Pyrex. The silver chamber was a 24/40 standard taper joint which had been se:ded off a t the lower end and a small tube attached. The silver wire was unspooled, extracted in a Soxhlet extrxtor with anhydrous ether for 10 hr. and packed in the chamber. A Teflon gland with a glass tube through it was inserted in the top. Two short lengths of neoprene tubing were used to connect the silver chamber to the hydrogen inlet and exhaust. The lower portion of the chamber was inserted in a muffle furnace until all the silver was inside. Hydrogen was then slowly passed through the chamber and the temperature raised to 500'. After about 1 hr., the chamber was removed from the furnace and allowed to cool with the hydrogen still flowing. Pinch clamps then were placed on the short lengths of tubing and the hydrogen was disconnected. The chamber now was installed in the adsorption system, as shown in Fig. 1. No deleterious effects could be detected from the use of Pyrex in the 500' hydrogenation. The system used a Sigmamotor pump to circulate the solution. All connecting tubing was 1/8-inch i.d. neoprene. It was found that Tygon tubing gave off products at alkaline pH which interfered with the ultraviolet analysis of the developer, and therefore could not be used. Buffer.-Reagent grade sodium carbonate was used as a buffer in the experiments. The buffer solution was prepared by dissolving 1.68 g. of the monohydrate in a liter of water. In most experiments, 25 ml. of the buffer was used wit,h 25 ml. of the arid developer solution. This combination yielded a pI-1 of about 10.8 for the mixed developer. Slight variarions in p H had no efyect on the adsorption. Preparation of the Adsorption Apparatus.-The construction of the developer and silver chambers was such that they could be set up immersed in a water-bath to obtain the desired temperature control. The rest of the system waa connected as shown in the diagram. The amount of buffer -
F
Nltroqen Inlet
E
Fig. 2.-Enclosed system for preparation of developer: A, chamber for deaeration of distilled water; B, buret for measurement of water used; C, mixing vessel; D, pipet,; E, nitrogen inlet; F,F', nitrogen outlet.
necessary for the experiment was placed in the mixing chamber and pure nitrogen passed through the system. This was done in such a manner as to deaerate the buffer and flush the system free of oxygen. Preparation of the Developer.-The developer solution was prepared in the system shown in Fig. 2. Distilled water was deaerated in vessel A for about 1 hr. with pure nitrogen. The developer powder was weighed out on an analytical balance and placed in the three-necked flask C. Nitrogen then was passed through vessels B and C for 30 min. to free them of any air. The desired amount of water now was measured out in vessel B and run into C. When the developer had dissolved completely, the desired amount of solution was drawn into the pipet (D) and transferred to the developer inlet tube in the adsorption apparatus. The short contact with air, which the tip of the pipet encounters in transferring the solution, did not cause any oxidation of the developer, which, a t this stage, was still acidic. The solution was then allowed to come to temperature equilibrium before any measurements were made. Measurement of the Adsorption.-The pump was started 45, , 223 (1941): (b) A. Weisswith the bypass open and pinch clamps still in place on the (10) (a) T. E€. James, THISJ O U R N A L berger and D. S. Thorrias, Jr., J . Am. Chem. SOC.,64, 1561 (1942). short rubber tubes leading to the silver chamber. The system was run for about 3 min. to assure uniform tempera(11) G. I. P. Levenson a n d T. H. James, J . Phot. Sci., 2, 172 (19541. (12) B. E. Oonwap, R . G. Rarradaa and T. Zawidzki, Trim JOURNAL,ture and the optical density of the solution read a t a suitable 62, F7G (1958). wave length. The pump then was stopped, the bypass closed, anti the pinch clamps removed. The solution now ( I : ; ) T. R. ;i\vt,i,t :in11,J. Zi.linrsr, .-I /la?. C h e m . , 3 0 , 1713 (1958).
T'ol. 61
586 TABLEI
ADSORPTIONOF DEVELOPERS BY SILVER,PER CENT.COVERAGE A N D HEATOF ADSORPTION Calcd. amt. for monola er cover. molesJm.1
Structure
Developer
Actual amt. for monolayer cover. moles/cm.l
Isoteric heat of adsorption Coverage in kcal./ at 200 mole
%
SH1
4Amino-3-methyl-Nethyl-N-( 8-met hylsulfonamidoethyl janiline (Ij
VH3
10
x
10-11
x
100
12 t o 13
15.5 X 10-l1
100
11 to 12
x
23
10
10-11
I
4-Amino-3-methyl-N, S diethylaniline (11)
15 X lo-"
p-Methylaminophenol
20
x
10-11
4.6
10-11
(111) 23 X 10-11 30 plateau within present measurements
p-Phenylenediamine
(IV)
3
I
1
23
H O - ~ - O I i
Hydroquinone ( V )
4
5
2ev ert n solulion Moles/lder
6 )
i
7
IC4
Fig. 3.-Adsorption
isotherms for adsorption of developer 40': developer adsorbed in moles/ cm.2 x 10-11 (y-axis) us. developer remaining in solution in moles/l X 10+ (x-axis).
(I) to silver at 20 and
(
Dev left In solutton Moles/Ilter )
1
IO4
Fig. -I.---Adsorption isotherms for adsorption of developer (11) to silver a t 20 and 40": developer adsorbed in moles/ rm.2 X 10-11 (y-axis) us. developer remaining in solution in moles/l. X lo-' (x-axis). could flow through the bundle of silver wire. The pump was started again and the optical density determined at the same wave length until an equilibrium was attained. This usually required not more than 5 t o 10 min. Longer times did not change the values obtained. The developer adsorbed was assumed to be proportional to the change in the optical density of the solution. The solution optical densities were read in the ultraviolet around 300 mp, the exact wave length depending on the developer and concentration being used. Blank runs with no silver present showed no adsorption to the glass or tubing. Developer Purification .-Five developers were used in the experiments. The pphenylenediamine was Eastman Organic Chemical No. 207 White Label Grade. It wtls used without further purification. The Elon and hydroquinone were pure samples of standard Kodak developers.
x
IO-"
None
>30 Sone
Approx. 3
5 to 6
.....
Developer I was purified by converting Kodak Color Developing Agent, Type CD-3 (4-amino-3-methyl-N-ethylN-(P-methyl-sulfonamidoethy1)-aniline sesquisulfate monohydrate) to the free base, warming in activated carbon and filtering. The free base was extracted with chloroform and recrystallized by adding an equimolar quantity of dilute sulfuric acid, thus forming the monosulfate salt. This was then dried in a vacuum desiccator. Developer I1 was purified by recrystallizing Kodak Color Developing Agent, Type CD-2 (4-amino-3-methyl-N,h'diethyl aniline hydrochloride) in boiling ethyl alcohol and filtering. The solution then was cooled to 20' and the solid filtered off. The developer was washed in isopropyl alcohol, then acetone, and dried in a vacuum desiccator.
Results All five developers used showed some evidence of adsorption by silver, with the exception of hydroquinone. A number of runs with hydroquinone showed no detectable adsorption for various concentrations a t 20". The adsorption isotherms obtained for the remaining developers are shown in Figs. 3 to 7. I and I1 exhibit similar isotherms. Both developers were adsorbed strongly and showed strong dependence on temperature. Elon was adsorbed much more weakly and showed very little temperature - dependence. p - Phenylenediamine was adsorbed, but the adsorption isotherms are quite different in appearance from those of I and 11. No plateau was obtained within the sensitivity of present measurements and only a moderate dependence on temperature was seen. Data are listed in Table I for equilibrium coverage at 20" and for per cent. of surface covered. The flat, projected area of the developer molecules was computed from van der Waals radii and by using scale molecular models. The silver surface was determined by assuming the surface of the wire to be smooth and calculating the geometrical area. While it was realized that the developer may not always be adsorbed flat and the wire may not have a completely smooth surface, good agreement in the cases of I and I1 was obtained in this manner. It was considered, therefore, that this procedure
ADSORPTIOV OF PHOTOGRAPHIC DEVELOPRRS ns METALLI~ SILVER
May, 1960
gives a good comparison of the different developers tried. I n order to determine whether the adsorption isotherms were of the Langmuir type, which characteristically exhibit monomolecular coverage at equilibrium, the Langmuir equation was rearranged to the form14
-
P I =Y ki
2 f! 1
-
+ k*P
where P is the pressure of the adsorbate present, Y is the amount of material in the adsorbed phase, and kl and ICz are constants. It is apparent that the quantity corresponding to the pressure is the concentration of developer in the solution phase not adsorbed, while Y corresponds to the amount of material per unit area adsorbed. Both of these quantities can be taken from the adsorption isotherms. If the concentration of developer left in solution is plotted against the ratio of concentration left in solution to the amount adsorbed, a straight line should result, provided the adsorption is of the Langmuir type. Figure 7 shows this type of plot for the 20" isotherms. All developer data except those for p-phenylenediamine yield straight lines. Reproducibility of experimental values using the present method is about 4~5%. It is believed that this may be improved by refinements in technique. Isoteric heats of adsorption for the developers were calculated from the Clausius-Clapeyron equation, with data from the adsorption isotherms. The values given in Table I are an average of a number of values from 50 to S070 surface coverage. Time Required for Adsorption The present system was not constructed to measure rates of adsorption but some information can be obtained from the data. It was found that equilibrium adsorption occurred in less than 5 min. after contact with the silver. This represents the time required for the solutions in the apparatus to come to equilibrium. Solutions, which were allowed to circulate over the silver for as long as an hour, showed very little or no additional change in concentration. Discussion The primary object of the present experiments was to resolve some of the speculation concerning the mechanism of photographic development. The system which has been used does not actually measure adsorption by developed silver, but the clean, hydrogenated surface of the silver wire is believed to be similar to the surface of freshly reduced silver. I n photographic practice, the presence of a protective colloid and of silver ions may result in a competition with the developer molecules for adsorption sites. The relative amounts of the different species adsorbed must depend upon their heats of adsorption and relative concentrations. In the absence of direct measurements of the heat of adsorption of gelatin and of silver ion to silver, the effects of this competitive adsorption on photographic development cannot be assessed exactly. (14) J. H. deBoer, "The Dynamic81 Character of Adaorption," Oxford Univenity Press, New York, N. Y . , 1953, p. 58.
587
I
ZG * 40' 1
2
'
3
E
4
Dev e f t In solution Moles/ Iter
1:CL
Fig. 5.-Adsorption isotherms for adsorption of p-methyl0': developer adaminophenol (Elon) to silver at 20 and 4 sorbed in moles/cm.2 X (y-axis) vs. developer remaining in solution in moles/l. x 1 0 - 4 (z-axis).
(
De" left
in solution, Moles/liler
1 IO4.
Fig. 6.-Adsorption isotherms for adsorption of pphenylenediamine to silver a t 20 and 40": developer adsorbed in moles/cm.2 X (y-axis) us. developer remaining in solution in moles/l. X 10-4 (z-axis).
/
9
-
,
LL
-,4
5
6
-7
_.
Concentrotion of dev left in solution
Fig. 7.-Test for conformation of adsorption isotherms to Langmuir-type adsorption: concentration of developer remaining/concentration of developer adsorbed (y-axis) vs. concentration of developer remaining in solution (z-axis).
The data substantiate Perry, Ballard and Sheppard's8 criticism of the work of R a b i n ~ v i t c h ,who ~ concluded that hydroquinone is adsorbed by silver. Perry, Ballard and Sheppard may have been entirely correct in their conclusion that Elon is not adsorbed, since ionization of the developer may play a part in adsorption. At a pH of 9.0, used by these authors, Elon, which has a pK of 10.25,15is not ionized. Weak adsorption of Elon was found in the present experiments carried out a t the higher pH of 10.8, a t which value the Elon molecule is ionized. However, hydroquinone, which also is ionized at pH 10.8, showed no adsorption. The possibility that the adsorption of hydroquinone might require the prior adsorption of silver ions (15) R.W.Henn, P S A Journal ( P h o t . 8 ~ :Technique),lSB,90, (1952).
mas not investigated, since oxidation of the developer would interfere with the analysis. Evidently silver ions are, however, not necessary to effect the adsorption of the p-phenylenediamines and Elon. Levenson and James8-11 state that developers containing amino groups are able to be adsorbed by silver bromide because of their ability to form a covalent coniples with silver ion. Of the five developers studied, those that are adsorbed by silver do contain amino groups. The ability of the amino nitrogen to form a covalent complex with silver ion, and thus to promote adsorption by silver halide, is indicative of a property of the amino group which may or may not also be responsible for the adsorption of these developers by silver in the absence of silver ion. 'The heat of adsorption for I and I1 is too large for the adsorptive forces to be of the van der Waals type, and shows that some type of strong interaction between developer and metallic silver occurs. It is believed that the 3-methyl group on the I and I1 molecules16 increases the electron-releasing ability of the free amino group and this electromeric effect may contribute to the adsorptive forces. The weaker adsorption of simple p-phenylenediamine may be due to its symmetrical structure which causes a uniform distribution of charge over the molecule. Elon had a very weak adsorption which would be expected from an N-suhstitutet3-p-aminophenol. Hydroquinone, having no amino groups, is not adsorbed. In studying the rate of reduction of silver ions b y (16) R I.. Bent, et al., J .
A m . Chem. Soc., 73,3100 (1951).
hydroquinone and p-phenylenediamine in alkaline solution, ,James9statcs that the rate of reaction for hydroquinone is proportional to a fractional power of the silver-ion concentration. For p-phenylenediamine, however, the rate of reaction is dependent on the developer concentration to a fractional power. This is alw very strong evidence that silver-ion adsorption is necessary for hydroquinone to act a~ a reducing agent, while p-phenylenediamines may be adsorbed without silver ion on the surface. In studying postfixation physical development, Pontius and Thoinpson17 found, in the case of I, that the rate was proportional to the developed silver to the '/* power. This is consistent with ,James's postulates. Other give similar theories involving non-adsorption of black-andwhite developers and adsorption of p-phenylenediamine by silver. These measurements of adsorption of developers by qilver therefore confirm the evidence derived from the study of the kinetics of physical development. The method used for these measurements readily can be adapted to measurement of adsorption of other substances. Acknowledgment.-11-e wish to acknowledge the ~ror.1~ of Mr. A. Miller for the construction of the special qilica flow cell, and our indebtednes:, to Dr.. IY. R Riiby for many helpful discussions. (17) R. B. Pontius a n d J o a n S. Thompson, Phot. Set. E n g . , 1, 15 (1957). (18) T.H. James, J . Chem. Ed., 23, 595 ( 1 9 S ) (19) T. H. James a n d JY. Vanseloa, P h o t . Enganeerang, 6, 183 (1925).
T H E K ~ I O D T S A ~ I IPROPEIITIES C OF HIGHER FLUORIDES. I. THE H E i T CAPACITY, ENTROPY, ASD HEATS OF TlLlSSITIOS OF MOLTBDESURI HEXAFLUORIDE AND SIOBIUJI: PI?ST.IFI,T~ORIIDE1 BY A . P.BRADY, 0.E. A f Y E R S A N D I
