THEADSOKPTIOR’ O F GELATIN TO
A
SILVERBROMIDE SOL
3009
The Adsorptiam of Gelatiin to a Silver Bromide Sol
by H. G. Curme and C. C. Natale Research Laboratories, Eastman K o d u k C o m p a n y , Rochester, ll’ew York
14660
(Receized June $8, 1963)
Adsorption of lime-processed, acid-processed, and acetylated gelatins to a silver bromide sol stabilized with excess bromide was studied. The specific area of the sol was determined by measuring the adsorption of a cyanine dye. Adsorption of dye in the presence of gelatin was the same as in its absence, but less gelatin was adsorbed in the presence of the dye than in its absence. Gelatin coverages of the order of 5-8 X lop4 mg./cm.* a t surface saturation mere observed, with no indication of multilayer adsorption, and liniiting adsorption mas proportional to the 0.23 power of weight-average molecular weight. These figures support a loop and bridge model for the adsorbed layer. Adsorbed molecules were found to occupy more or less space on the surface as pH and ionic strength were varied, and these variations appeared to be largely determined by electrostatic interactions. Adsorption at pH values well removed in either direction from the isoelectric point was less than a t the isoelectric point. Comparison with light-scattering dimensions in solution indicated that adsorbed molecules are compacted compared with their configuration in solution, or are elongated in a direction normal to the surface, or show both effects. Gelatins of diff erent chemical constitution showed similar adsorption behavior when coiiipared under conditions of similar net charge and molecular weight.
Introduction The adsorption of a series of gelatin fractions to a silver bromide precipitate has been studied by Pouradier and Roman.’ These authors measured the amount of gelatin left on silver bromide grains after the grains were washed with Lvater at 50”. Coverages a t apparent surface saturation amounted to layers which would be 25-40 A, thick if the gelatin were densely packed, as in the dry state Saturation adsorption was found to increase slightly with molecular weight and to increase somewhat with increasing pAg. Pouradier and Roman postulated that gelatin molecules are adsorbed by a few of their polar groups and that the reiiiainder of the molecules extend toward the solution. This model of adsorption at a few segments, with solvated loops and bridges in the polymer chain extending toward the solution, is similar t o one proposed by Jencliel and Rumbach2 for the adsorption of certain nonionic polymers out of organic solvents. The loop and bridge model seems to apply 1-0 the adsorption of other nonionic polyiiiers from organic solvents,:’-: and theoretical tre,xhents of this type of adsorption have been presented by Simha, Frisch, and Eirich6 and by Silk~erberg.~
Several papers dealing with the adsorption of charged synthetic polymers or proteins indicate that the relative net charges of the adsorbent and the adsorbate play an important role in adsorption, but are by no means the only determining factors. Certain synthetic polymers of fixed charge density show adsorption to oppositely charged mercury surfaces and no adsorption to mercury of the same charge.* The adsorption of gelating and of some synthetics carrying carboxyl groups, to near-neutral or negatively charged
(1) J. Pouradier and J. Roman, h i . Ind. Phot., 23, 4 (1952). (2) E. Jenckel and B. Rumbach, Z . Elektrochem., 5 5 , 612 (1951). (3) G. Kraus and J. Dugone, I n d . E n g . Chem., 47, 1809 (1955). (4) J. Koral, R Ullman, and F. R . Eirich, J . P h y s . C h e m . , 62, 541 (1958). (5) B. J. Fontana and J. R. Thomas, ibid., 65, 480 (1961). (6) (a) R. Simha, H. L. Frisch, and F. R . Eirich, ihid.. 57, 584 (1953); (b) H. L. Frisch and R. Simha, ibid., 58, 507 (1954); (c) H. L. Frisch, ibid.,59, 633 (1955): (d) H. L. Frisch and li. Simha, J . C h e m . P h y s . , 27, 702 (1957). (7) A. Silberberg, J . P h y s . Chem., 6 6 , 1872, 1884. (8) (a) I. R . Miller and D . C. Grahame, J . A m . C h e m . Soc., 79, 3006 (1957); (h) I . R . Miller, T r a n s . Faradau Soc., 5 7 , 301 (1961): (c) I. R. Miller and D. C . Grahame, J. Colloid Sei.. 16, 23 (19Gl). (9) A. &f. Kragh and W. B.Langston, ibid., 17, 101 (1962).
Volume 68, S u m b e , . 10 Octobei, 1~964
H. G. CURMEAND C. C. NATALE
3010
Table I : Adsorption and Solution Properties of Various Gelatins % Gelatin
l0SS
during clarificationQ
3 3.5
a
Mw
[580,000] [320,000]
0 5 0 0 2
[ 146,0001 263,000 450,000
0
117,000
80,000
[ E O , 0001
For methods of clarification see text.
mole/l.
dl./g.
4.7 4.7 3.6 6.9 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 3.6 8.6 8.6
0.0185 0.0635 0.0635 0.0635 0.0185 0.0185 0.0185 0.0185 0,0183 0.0185 0.0185 0.0635 0.0635 0,0185 0.0635
0.616 0.667 0,860 0,892 0.61 0,520 0,295 0.379 0,478 0.593 0,577 0.502 0,365 0.346 0.359
Ionic strength = 2 c t Z i 2 / 2 .
surfaces, has been shown to decrease as the negative charge on the polymer increased. Adsorption of certain proteins to negatively charged kaolinite12 has been found to be greater in pH regions near to, or somewhat on the acid side of, their isoelectric points than in p H regions either more acid or more basic. This behavior has been interpreted in terms of variations with pH of both the degree of ionization of the proteins and the number of ionized sites on the surface. I n the present work the influence of some of the ionizing groups (carboxyl and €-amino) on adsorption has been studied. This has been done by changing both the degree of ionization (through p H changes) of the groups, and by actual cheiiiical alterations. Since such changes are necessarily accompanied by changes in the size of the molecule in ad sorpt'on studies were accompanied by viscosity and light-scattering measurements,
Experimental Characterzxation of the Gelatins. Gelatins A, B, and C !',ere the first extractions of limed calfskins. D(30), D(43), D 50) and D(G0) were fractions of such a gelatin prepared by precipitation from aqueous solution by propanol. Gelatin C-acetyl was a saniple of gelatin C nhich was treated with 1Oyo by weight of acetic anhydride Titration data, using the criteria of Kenchington and showed that about 8 0 7 of the amino groups of the parent ge atiii were acetylated. Gelatin E(7) vias a fraction of ai1 acjd-processed pigskin gelatin prcparcd by precipitation by propanol. The Journal oJ Physical Cheniistrv
K,
hd
/Ab
PH
mg./cm.2
8.0
Y
ml./mg.
1.0
Charge, equiv./g.
x 10-4 6.9 5.3 6.1
29 23 45 35
$0.50
1.4 2.5 1.7
6.8 5.3 6.1 6.8 7.8 4.3 4.8 6.7 6.4 6.0
1.1 1.2 1.2 1.5 1.2 1.9 1.8 1.4 1.7 1.9
51
+0.50
33 29 37 28 51 43 32 30 42
Intrinsic viscosity.
x
10-4
$0.50 +6.15 -3,43 +0.50
-3.92 -4.21
+1.5 0 0
Limiting adsorption.
Viscosities of gelatin solutions were obtained in Cannon-Fenslie-Ostwald viscometers having flow times for water of about 200 sec. Solutions were filtered through Corning medium sintered glass filters (masimum pore size 15 p ) before viscosity measurements were made. This procedure removed less than 1% of the gelatin from solution. The adsorption from unfiltered solutions was found to be identical with that from solutions treated in this way in a number of tests, and adsorption measurements mere routinely made using unfiltered gelatin solutions. Intrinsic viscosities, [VI, listed in Table I, were obtained by measuring (qrel - l ) / c at concentrations of 0.2,0.4, and 0.8 g./lOO nil. and extrapolating the values to zero concentration. Gelatin w i g h t s mere corrected for moisture content by heating samples a t 105' for 15 hr. DXculty with extraneous suspended material is frequently encountered in making light-scattering measurements on commercial gelatins l6 l6 Treatments which have small effects on viscosity may produce substantial reductions in' weight-average molecular weights as calculated from light scattering. For this reason, more drastic means were taken to clarify (10) A. S. Michaels m d 0. Morelos, Ind. Eng. Chem., 47, 1801 (1955). (11) W.Schmidt and F R. Eirich, J . P h y s . Chem., 66, 1907 (1962).
(12) A. D. McLaren, i b i d . , 58, 129 (1954). (13) G. Strlinsby,
