J . Phys. Chem. 1990, 94, 461 1-4617
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Measurement of the Interaction between Adsorbed Polyelectrolytes: Gelatin on Mica Surfaces Naoyuki Kawanishi: Hugo K. Christenson,* and Barry W. Ninham Department of Applied Mathematics, Research School of Physical Sciences, Institute of Advanced Studies, The Australian National University, GPO Box 4, Canberra ACT 2601, Australia (Received: August 18, 1989; In Final Form: December 12, 1989)
Direct measurements of the force between layers of gelatin adsorbed to mica surfaces have been carried out. Gelatin adsorbs to mica surfaces both above and below its isoelectric point (pH = 5 ) . At pH below the isoelectric point the gelatin appears to adsorb in a flat configuration, whereas at pH above the isoelectric point the adsorbed polymer adopts a more extended conformation, and the steric interactions are of longer range. The effect of electrolyte on measured forces is different for different pH values because the nature of the electrostatic interaction between the surface and the gelatin and between gelatin segments changes as the pH is varied with respect to the isoelectric point. The adhesion measured on separation of the surfaces shows a maximum at the isoelectric point. It is concluded that the adsorption of gelatin and the range of the interactions are governed by the relative magnitude of electrostatic surface-segment and segmentsegment interactions. The results can be interpreted by describing the structure of the adsorbed layer using a “three-layer model”, originally proposed by Norde and Lyklema.
Introduction The adsorption properties of biopolymers are fundamental to the understanding of their biological and medical uses. There are numerous examples where biopolymers are utilized in the manufacturing of commercial products. In the photographic industry, gelatin, a denaturated form of the naturally occurring fiber protein collagen, has been utilized since the very beginning as a protective agent in photographic emulsions. Even in recent years, when much progress has been made in the science of synthetic polymers, gelatin is still the sole ingredient for this purpose, the major reason being its distinct ability to control photographic properties. Impurities such as thiosulfates and nucleic acids and gelatin itself both act as agents to modify the growth of silver halide microcrystals, the formation of the latent image, and other important image reproduction processes. Because gelatin is provided via decomposition of collagen by acid or base treatment, it is not easy to regulate the quality in a large-scale manufacturing process. However, strict quality control can result in a very reliable outcome. Gelatin is a proteinaceous substance derived from the parent water-insoluble protein, collagen.’ The native collagen monomer has a three-chain helical structure in which the individual helical chains are stranded in a superhelix about the common molecular axis. When exposed to acids or bases, this secondary structure of collagen is destroyed. Because the breakage of the interstrand bonds is not uniform nor complete, the resultant product is not homogeneous. There are three primary compositions; the single-stranded a-component, the two-stranded &component, and the three-stranded y-component. Besides these primary compositions, there are some species formed by breaking of peptide linkages. When an acid- or alkali-treated collagen solution is heated to a temperature above 40 O C , these components can be extracted. By raising the temperature about 5-10 O C in each successive extraction, a range of gelatin extracts having different components can be produced. The first few extracts have a low molecular weight and a narrow molecular weight distribution. Gelatin is a versatile polyelectrolyte, having both positive and negative ionic group with different dissociation constants. Because the number of acidic and basic groups not only depends on the ionic groups present in the parent collagen but also on the breakage of amide bonds during the denaturing process, the isoelectric point of gelatin can vary from one sample to another. Depending on the pH and its relation to the isoelectric point, gelatin is an anionic, cationic, or neutral polyelectrolyte at all moderate pH values. To whom correspondence should be addressed. ‘On leave from Fuji Photo Film Co., Ltd., Kanagawa, Japan.
0022-3654/90/2094-4611$02.50/0
There are many studies on the adsorption of gelatin to silver halide surfaces. In 1952 Pouradier and Roman conducted experiments on various types of gelatin adsorbed to silver bromide surfaces.* Since then, numerous investigations have been performed, mainly in the 1960s. For example, Curme and Natale succeeded in determining the amount of gelatin adsorbed per unit area by employing the specific area of the adsorbate calculated from the occupancy of cyanine dyes.3 They concluded that gelatin must adsorb as random coils, close to the conformation in its aqueous solution state. Other studies to be noted are the desorption experiments by Kragh and Peacock4 and the heat of adsorption measurements by Berendsen and Borginon, which brought attention to the differences in the nature of adsorption above and below the isoelectric point of gelatine5 A more detailed picture of the adsorbed layer did not emerge until 1974, when Maternaghan and Ottewill first measured the adsorbed amount and the adsorbed layer thickness independently by ellipsometry! They adsorbed gelatin onto the (1 11) face of 1-2-cm-diameter silver bromide crystals and conducted a thorough study of pH effects on the adsorption, later extending the studies to the variations in solution properties such as pAg and electrolyte concentration and the effects of gelatin extraction number, molecular weight distribution, and degree of phthalation.’ Their conclusion was that in a bromide-rich environment, the surface of silver bromide crystals is negatively charged, so that for pH values below the isoelectric point, where gelatin is positively charged, it adsorbs in a flat configuration due to electrostatic attractions, whereas for pH values above the isoelectric point, adsorption occurs with the gelatin in a much more expanded state. Numerous direct force measurements have been carried out with adsorbed neutral polymers in both aqueous and nonaqueous systems.8-’2 Most of the work to date on charged polymers has been concerned with biopolymers like mucin,13 water-soluble ( I ) For example: Veis, A. The Macromolecular Chemistry of Gelatin; Academic Press: New York, 1964. (2) Pouradier, J.; Roman, J. Sci. Ind. fhorogr. 1952, 23, 4. (3) Curme, H. G.; Natale, C. C. J . f h y s . Chem. 1964, 68, 3009. (4) Kragh, A. M.; Peacock, R. J. Photogr. Sci. 1967, I S , 220. ( 5 ) Berendsen, R.; Borginon, H. J . Phorogr. Sci. 1968, 16, 194. (6) Maternaghan, T. J.; Ottewill, R. H. J . Photogr. Sci. 1974, 22, 279. (7) Maternaghan, T. J.; Banghan, 0. B.; Ottewill, R. H. J . Phorogr. Sci. 1980, 28, 1. (8) Israelachvili, J. N.; Tirrell, M.; Klein, J.; Almog, Y. Mucromolecules 1984, 17, 204. (9) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041. (IO) Luckham, P. F.; Klein, J. Macromolecules 1985, 18, 721. (1 I ) Hadziioannou. G.;Patel, S.; Granick, S.; Tirrell, M. J . Am. Chem. SOC.1986, 108, 2869. (12) Gotze, Th.; Sonntag, H. Colloids SurJ 1988, 31, 181.
0 I990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 TABLE III: Molecular Weight Distribution (wt %)
TABLE I: Physical Properties of Gelatin isoelectric point, pH units gel strength,'bloom in g viscosity,* m P conductivity,* wS/cm moisture, 7c
5.04
4 0 4
259 58 9.0 10.7
104-1 05
"Determined for 6.7% w / v gelatin at 10 OC. *Determined for 6.7% w / v gelatin at 40 "C.
TABLE 11: Ionic Impurities (ppm) Mg Fe cu 3.7 0. I 0.6
Kawanishi et al.
Na
Ca
2.8
0.5
collagen,14and poly-L-lysine.15 The only methodical study of the effects of pH, ionic strength, and charge on the interaction between adsorbed polyelectrolytes is that of Marra and Hair.I6 They studied the forces between mica surfaces bearing adsorbed layers of poly(2-vinylpyridine). This polymer is positively charged at low pH and becomes neutral at high pH. The results showed that a combination of strong adsorption affinity and intersegmental repulsion leads to a flat and extended adsorption at low pH. Only at high concentration of salt (0.1 M NaCI) does the screening of segment-surface attraction lead to a more extended adsorbed layer, and consequently steric forces begin to dominate the long-range interaction between surfaces. Those results are in accord with theoretical predictions" that polyelectrolytes carrying opposite charge to the surface adopt a much less extended configuration than neutral polymers, even up to very high ( - 1 M) salt concentrations. The adsorption behavior of gelatin is expected to be more complex. Not only does it show a net charge reversal at the isoelectric point, it also carries both negative and positive charges at pH values in the range 3-10. Nevertheless, an investigation of the effects of pH and ionic strength on the interaction between adsorbed layers of gelatin should provide information of fundamental interest in the study of adsorbed polyelectrolytes. In order to shed light on the colloid-protective nature of this biopolymer, the actual interaction of the adsorbed layers must also be investigated. In the present study, direct force measurements have been conducted between layers of gelatin adsorbed on mica surfaces using the method developed by Israelachvili and Adams.18 Mica surfaces acquire a negative charge when immersed in water for all pH's, with surface potentials in the vicinity of -100 mV. This allows a direct comparison between this system and the system with silver bromide surfaces, where for a pBr value of around 3, the surface potential is about -70 mV. Mica is chemically inert and its cleavage plane molecularly smooth, so that it provides an ideal substrate to work with. By varying the pH of the solution, one can explore different electrostatic affinities. Further, by addition of electrolytes, the screening effect on charges in both the polymer and the surface can be explored.
Experimental Section Materials. All water used in these experiments was first treated with an ion-exchange column, followed by a column of activated charcoal, and then distilled twice in an all-Pyrex still. After reaching equilibrium with the atmosphere, the water typically displayed a pH of 5.8 and a conductivity of about 3 pS/cm. The hydrochloric acid and the potassium hydroxide used to adjust the pH and the electrolyte, potassium chloride, were of Analar Grade. Gelatin was obtained from Nitta Gelatin Co. of Japan and was used as received. It was a low-ion-content sample with properties as shown in Table I. According to the manufacturer's analysis, it contains only minuscule amounts of ionic impurities (Table 11). (13) Perez, E.; Proust, J. E. J. Colloid Inierface Sci. 1987, 118, 182. (14) Klein, J. J. Chem. Soc., Faraday Trans. I 1983, 79, 99. ( I 5) Luckham, P. F.;Klein, J. J . Chem. Soc., Faraday Trans. I 1984,80, 865. (16) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92, 6044. (17) Papenhuijzen, J.; van der Schee, H. A,; Fleer, G. J. J . Colloid Interface Sci. 1985, 104, 540. (18) Israelachvili, J. N.; Adam. G.E. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 975.
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