Langrnuir 1990, 6, 702-706
702
Stabilization of Colloidal Particles by Acidic Polysaccharides. Effect of Temperature on Stability A. Yokoyama,+ K. R. Srinivasan, and H. S. Fogler* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109 Received May 1, 1989. I n Final Form: October 2, 1989 The colloidal stability of polystyrene latices with adsorbed acidic polysaccharides was studied as a function of temperature by measuring the flocculation rate constant, the steric layer thickness, and the amount of adsorption. As the temperature increases, two competing effects arise and affect the colloidal stability: one effect is to decrease the amount of adsorption and the steric layer thickness and to suppress the stabilization; the other effect is to improve the solvent condition and to enhance the stabilization. These two competing effects were found to give a maximum stability at temperatures around 320-350 K.
Introduction Recently, there has been a significant interest in the use of biodegradable water-soluble polysaccharides as colloidal stabilizers, which can be used for various systems such as water-base paints, waste-water treatment, food stuff, coal-water slurries, and cell separations. For example, acidic polysaccharides such as gum tragacanth and poly(ga1acturonic acid) were found to be effective stabilizers at low concentrations on the order of 10 ppm,' and our previous showed that these acidic polysaccharides stabilize colloidal dispersions due to the steric stabilization mechanism. In addition, the colloidal stability imparted by these polysaccharides was found to increase with an increase in the steric layer thickness of the adsorbed polysaccharides, owing to a smaller van der Waals attractive force at the shallow m i n i m ~ m . ~ This effect of the steric layer thickness on colloidal stability suggests that colloidal stability can be controlled by temperature because the temperature influences the polymer conformation: the steric layer thickness. Therefore, this effect of temperature on colloidal stability was investigated in this study. The temperature has significant effects not only on the steric layer thickness but also on the solvent condition. This solvent condition is defined as the interaction between polymer molecules and solvent molecules and can be quantified in terms of the parameter x1 in the Flory-Huggins T h e ~ r y .The ~ parameter x1 is related to the second virial coefficient, B,, which describes the two-body interaction between polymer segments as B, = Y ~ VJO.5 ~ / - xl) (1) where u2 is the volume fraction of the polymer and V , the molar volume of the solvent [m3/mol]. When x1 >
* To whom all correspondence should
be addressed. Currently a t the Marshall R & D Laboratory, Du Pont & Co., Philadelphia, PA. (1) Bergenstahl, B.; Fogler, H. S.; Stenius, P. In Gums and Stabilizers for the Food Industry 3; Phillips, G. O., Wedloch, D. J., Williams, P. A,, Eds.; Elsevier: New York, 1985; p 285. (2) Yokoyama, A.; Srinivasan, K. R.; Fogler, H. S. J . Colloid Interface Sci. 1988, 126, 141. (3) Yokoyama, A,; Srinivasan, K. R.; Fogler, H. S. Langrnuir 1989,5, 534, (4) Yokoyama, A.; Srinivasan, K. R.; Fogler, H. S.; Tirrell, M.; Chen, Y.-L., manuscript in preparation. (5) Flory, P. J. J. Chern. Phys. 1949, 10, 51.
0743-7463/90/2406-0702$02.50/0
0.5, B, becomes negative, and the solvent is referred to as a poor solvent. This negative B, reflects the dynamic association of polymers, which reduces the effective number of discrete molecules and thus reduces the osmotic pressure. On the other hand, when x1 0.5, B, becomes positive, and the solvent is referred to as a good solvent. The parameter x1 is a function of temperature, as expressed in eq 2:5 = 0.5 - 91(1- 8 / T )
(2) where \kl is the dilution parameter of entropy, 8 the 0 temperature (the temperature at which x1 = 0.5) (K), and T the solution temperature (K). Although the values of 9,and 8 for the acidic polysaccharides in eq 2 are not available in literature, they can be approximated to be equal to those for a similar polyelectrolyte, poly(acrylic acid)? x1
(3) 8 = 287 K (4) Poly(acry1ic acid) is similar to the acidic polysaccharides since it is negatively charged due to carboxyl groups as the acidic polysaccharides. Note that this approximation is sufficient only for a qualitative analysis because the structure of poly(acry1ic acid) is different from that of the acidic polysaccharides: poly(acry1ic acid) is flexible while the acidic polysaccharides are semiflexible. By use of eq 3 and 4,x1 can be expressed 91 = 0.065
= 0.5 - 0.065(1 - 2 8 7 / T )
(5) Therefore, it is expected that x1 for the acidic polysaccharides decreases as the temperature, T,increases. This has been further confirmed by our experimental observation that solubility of the acidic polysaccharides increases with an increase in temperature. The good solvent condition is expected to enhance the steric stabilization because polymers are mutually repulsive in the good solvent condition, and particles with adsorbed polymers experience repulsion when they collide. This effect of the solvent condition on the steric stabilization can be expressed in eq 6, which correlates the free energy change occurring when two spheres XI
(6) Silberberg, A.; Eliassat, J.; Katachalsky, A. J . Polyrn. Sci. 1957, 23, 259.
0 1990 American Chemical Society
Stabilization of Colloidal Particles
Langmuir, Vol. 6, No. 3, 1990 703
approach each other, AG, to x1:7
ter solvent condition gave less stability because of a smaller steric layer thickness. This smaller stability with the smaller steric layer thickness results from the greater van der Waals attractive force4 and a smaller positive free energy change, AG. Equation 6 shows that for a given separation distance, H,, AG decreases when the steric layer thickness, 6, decreases. Therefore, an increase in temperature is expected not only to improve the solvent condition but also to decrease the steric layer thickness and, thus, to have complex effects on stability. These complex effects of temperature have not been studied, and the major focus of this study has been to examine these effects using negatively charged sulfate polystyrene latices with the adsorbed acidic polysaccharides. The flocculation rate constant, the steric layer thickness, and the amount of adsorption for these latices were measured as a function of temperature.
AG = 4*kT~'(0.5 - xJ(6 - Ho/2)2~/(~lp22) (6) where AG is the free energy change (J),c the concentration of segments in the steric layer (g/m3), Hothe minimum distance between the particles (m), 6 the steric layer thickness (m), a the radius of the particle (m), u1 the volume of the solvent molecule (m3), and pz the density of stabilizing moieties (g/m3). Equation 6 shows that the good solvent condition (xl < 0.5) gives a positive free energy change, leading to stabilization, while the poor solvent condition (xl > 0.5) gives a negative free energy change, leading to flocculation of the colloidal particles. Equation 6 could explain the effect of the solvent condition on the stability of the steric stabilization for many systems qualitatively-the better solvent condition leads to the greater stability (ref 8-l0)-although no quantitative arguments can be made because of many gross assumptions used in eq 6 and the discrepancy between eq 6 and the surface force experiments." These assumptions include (i) a constant segment density and (ii) a constant value of x1 irrespective of the polymer concentration and were found to be unrealistic for some case^.^^,^^ The discrepancy observed in the surface force experiments" was that even for the good solvent condition attractive forces were detected, although the qualitative effect of the solvent condition on stability was confirmed: a better solvent condition leads to greater stability. Therefore, in this study, these theories will be used only to discuss trends and results qualitatively. The effect of temperature on stability can be elucidated by combining eq 5 and 6: Equation 7 shows that greater stability results when the temperature is increased above 287 K due to a greater positive value for AG. In fact, the results on the poly(acrylic acid) have shown greater colloidal stability as the temperature increases above 287 K.14 There are competing effects which alter the colloidal stability of particles with adsorbed polymers as the temperature is increased. The temperature affects the amount of adsorbed polymers and the steric layer thickness as well as the solvent condition. The better solvent condition tends to decrease the amount of adsorption, as many researchers have shown.15*16The better solvent condition reflects favorable interactions between polymers and solvent, and thus, more polymers stay in a solution phase with solvent, rather than being adsorbed to colloidal particles. Due to the small amount of adsorption, the better solvent condition can lead to a smaller steric layer thickness, which results in less stability. For example, Furusawa et used a nonionic polysaccharide, (hydroxypropyl)cellulose, as a stabilizer and found that the bet(7)Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (8)Napper, D. H. Trans. Faraday SOC.1968,64,1701. (9)Dawkins, J. V.; Taylor, G. Colloid Polym. Sci. 1980,258,79. (10)Clarke, J.; Vincent, B. J. Chem. SOC.,Faraday Trans. 1 1981, 77. - . , 1881. (11)Israelachvili, J. N.;Tirrell, M.; Klein, J.; Almog, Y. Macromolecules 1984,17,204. (12)Doroszkowski, A.; Lambourne, R. J. Colloid Interface Sci. 1973, 4.-, .7 97 (13)Evans, R.;Napper, D. H. J. Chem. SOC.,Faraday Trans. 1 1977, 673,1377. (14)Buscall, R.J. Chem. Soc., Faraday Trans. 1 1981,77,909. (15)Koral, J.; Ullman, R.; Elrich, F. R. J.Phys. Chem. 1958,62,341. (16)van den Boomgaad, Th.; King, T. A.; Tadros, Th. F.; Tang, H.; Vincent, B. J. Colloid Interface Sci. 1978,66,68.
Experimental Section Materials. Monodisperse negatively charged sulfate polystyrene latex spheres supplied by Interfacial Dynamics Corp. were used as model colloids. These spheres have a diameter of 0.352 hm with a coefficient of variation of 4.87%. These latices are negatively charged due to sulfate groups, having the charge density of -0.42 pC/cm*. This charge density remains constant above pH 2, since pK, for sulfate groups is less than 2. Poly(galacturonic acid), PGUA, and gum tragacanth, GT, were supplied by Sigma Chemical Co. Both PGUA and GT are acidic polysaccharides with carboxyl groups and have a pK, of 2.9 and 3.0, respectively. The PGUA sample was fractionated by gel permeation chromatography using Sepharose 4B/CL-4B (Pharmacia Inc.) as a gel at an ionic strength of 0.085 M a t pH 7.4. The PGUA molecule is a linear homopolysaccharide of 1,4linked a-D-galacturonic acids. The PGUA sample used in the experiment had a molecular weight of 540 000 and a polydispersity of 1.08. G T was dissolved in a 0.085 M sodium chloride solution. Then, the G T solution was filtered through 0.22wm Millipore filters. The filtered GT sample had a weightaverage molecular weight of 560 000 and a polydispersity of 2.7. The GT molecule is a branched polysaccharide and is based on a linear chain of PGUA. It has three types of branches, namely, single P-D-xylopyranose and disaccharide units of 2-(O-a-~-fucopyranosy1)-D-xylopyranoseand 2- (0-/3-~-galactopyranosyl)-~xylopyranose.'E Measurements of Flocculation Rate Constants. Flocculation rate constants were measured by a turbidimetric technique." The polystyrene latices (volume fraction 2.6 X were incubated for 2 h in the polysaccharide solution (ca. 100 ppm) a t an ionic strength of 0.085 M a t various temperatures. The concentration of 100 ppm was found to correspond to the saturation point of the adsorption. The incubation time of 2 h was found to be sufficient since the dimensionless stability fac;tor, W , given in eq 8 remained constant after an incubation time of 1 h or longer. The incubation was carried out in temperature-controlled cell holders, which can control the temperature within 10.05 K W = k,/k, where k , is the flocculation rate constant in the absence of the polysaccharides (m3/s) and k , the flocculation rate constant in the presence of the polysaccharides (m3/s). After the incubation, the ionic strength was increased from 0.085 to 0.33 M by the addition of a sodium chloride solution to induce flocculation. The turbidity was continuously measured as a function of time a t a wavelength of 500 nm with DMS Spectrophotometer model 200 (Varian Inc.). The floccu(17)Furusawa, K.; Kimura, Y.; Tagawa, T. In Polymer Adsorption and Dispersion Stability; Goddard, E. D., Vincent, B., Eds.; Academic Press: Washington, DC, 1984;p 131. (18)Aspinall, G. 0.;Baillie, J. J . Chem. SOC.London 1963,318,1702. (19)Melik, D. H.;Fogler, H. S. J. Colloid Interface Sci. 1983,92, 161.
Yokoyama et al.
104 Langmuir, Vol. 6, No. 3, 1990
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PGUA (pH 4) PGUA (pH 8.2)
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Temperature [K)
Temperature [K]
Figure 1. Effect of temperature on the steric layer thickness.
Figure 2. Effect of temperature on the amount of adsorption.
lation rate constant was determined from the slope of the timeturbidity curve, and then the dimensionless stability factor, W , was calculated from eq 8. The complete details have been described e1~ewhere.l~ The pH was adjusted by adding a hydrochloric acid or a sodium hydroxide solution. Adsorption Isotherms. Adsorption isotherms for the binding of the polysaccharidesto the polystyrene latices were obtained for various temperatures. This was achieved by first mixing the polysaccharides and the latices for 2 h at a given temperature, followed by filtration through 0.22-gm Millipore filters. The volume fraction of the latices for the adsorption experiments was 1.15 X lo-'. The incubation time of 2 h was sufficient since preliminary experiments showed that equilibrium was attained within 1 h. The adsorption of the polysaccharides on the filters was negligible. The ionic strength (0.33 M) was adjusted by adding an appropriate amount of sodium chloride. The concentration of the polysaccharides in the solution was measured by the phenol-sulfuric acid method.20 Measurements of Steric Layer Thickness by Photon Correlation Spectroscopy (PCS). PCS data were obtained by using a Lexel2-W Argon ion laser with an ITT FW-130 photomultiplier tube. Different angles (5O0-1OO0) and volume fractions of latices ((0-1.9) x lo4) were used to confirm that there was no effect of dust or particle-particle interactions. Then, the steric layer thickness was calculated as the difference in the radii of bare and covered particles obtained from the diffusion coefficient by using the Stokes-Einstein relation. The ionic strength of the solution was 0.085 M.
higher temperatures are expected to result in a smaller adsorbed amount and a shorter layer thickness. This effect of the solvent condition on the steric layer thickness and the amount of adsorption has also been observed both experimentally with a nonionic polysaccharide, (hydroxypropyl)cellulosel' and theoretically with nonionic flexible polymers.21s22Consequently, the acidic polysaccharides (GT and PGUA), which are ionic and semiflexible molecules, behave in a manner similar to nonionic flexible polymers when the solvent condition is changed. The coiled structure of PGUA and GT a t lower temperatures is the result of the high ionic strength (0.085 M) used. Since the double-layer thickness of 1 nm is comparable to the distance between the carboxylate groups, coiling is possible due to reduced intramolecular repulsion. The effect of the solvent condition on the steric layer thickness is analogous to the effect of pH on the steric layer thickness a t a fixed t e m p e r a t ~ r e . ~At? ~higher pH, the polysaccharide molecule is extended due to a greater amount of charges and takes a flat conformation on the surface, while a t lower pH the polysaccharide molecule is coiled up and forms loops and tails on the surface. Under better solvent conditions, the acidic polysaccharides are expected to be in an extended conformation in solution, while under poorer solvent conditions they tend to form loops. It appears, therefore, that similar conformations also exist on the surface under these solvent conditions. Effect of Temperature on Flocculation Rate Constants. An alternative explanation for the observed results is that xs tends to a value below the critical (Le., x ~ ,at ~higher ~ ~temperatures. ~ ) Such an eventuality would result in the inability of the polysaccharides to adsorb onto polystyrene latices a t higher temperatures. Thus, the latices that are stabilized by steric interactions at low temperatures may be flocculated as the temperature is raised. This will come about due to the desorption of polysaccharides. If this happens, there will be little difference in the flocculation behavior of the bare vs polymer-treated particles at high temperatures. In order to test this idea, flocculation tests were carried out as described in the Experimental Section. The flocculation rate constants for bare particles and for particles with the adsorbed polysaccharides are shown as a function of temperature in Figures 3-5, while the dimensionless stability factor is shown in Figures 6-8.
Results and Discussion Effects of Temperature on the Amount of Adsorption and the Steric Layer Thickness. The steric layer thickness was measured by PCS as a function of temperature at the polysaccharide concentration of 100 ppm, which is the saturation point of adsorption. The solution pH for PGUA was varied (pH 4 and 8.2) in order to see if the effect of temperature is affected by the solution pH. Figure 1 shows that the steric layer thickness decreases as the temperature increases. This trend was observed irrespective of the solution pH. The smaller steric layer thickness at higher temperatures results from a better solvent condition, in which adsorbed molecules are stretched out and are laid down on the surface. When adsorbed molecules take this conformation on the surface, the area occupied by one molecule increases; thus, the amount of adsorption is expected to be small. Figure 2 shows that the amount of adsorption is smaller a t higher temperatures, as expected. We believe that the results shown in Figure 2 can be explained on the basis of the solvency of the medium. A t low temperatures, poor solvency leads to larger layer thickness as well as higher adsorbed amounts. Good solvent conditions at (20) Norris, J. R., Ribbons, D. W. Methods in Microbiology; Academic Press: New York, 1971; Vol. 5B, p 265.
(21) Fleer, G. J.; Lyklema, J. In Adsorption from Solution at the SolidlLiquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 153. (22) Cohen Stuart, M. A.; Waajen, F. H. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984, 17, 1825.
Langmuir, Vol. 6, No. 3, 1990 705
Stabilization of Colloidal Particles 10
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Figure 7. Dimensionless stability factor of PGUA (pH 4).
Figure 3. Flocculation rate constants of GT (pH 5).
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Figure 4. Flocculation rate constants of PGUA (pH 4). in.
Figure 8. Dimensionless stability factor of PGUA (pH 8.2).
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