Stabilization Mechanism of Colloidal Suspensions by Gum Tragacanth: The Influence of pH 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 11, 1987; accepted February 4, 1988 An acidic polyelectrolyte, gum tragacanth (GT), was found to stabilize polystyrene latex particles due to a steric stabilization mechanism. The stability depends significantly on the pH of the solution. To elucidate the stabilization mechanism, a number of parameters, such as the adsorption isotherm, the zeta potential, and the conformation were measured as a function of pH. The adsorption isotherms showed that pH has little effect on the amount of GT adsorbed on the surface. Photon correlation spectroscopy studies showed that the steric layer thickness increases as pH decreases, demonstrating that the pH dependence of stability is attributed to the conformational change of GT with pH. Finally, the degree of dissociation of carboxyl groups of GT molecules (i.e., pH) has a significant effect on the flocculation behavior of latex particles and can be modulated to bring about spontaneous deflocculation of latex particles. © 1988 AcademicPress, Inc. 1. INTRODUCTION (4). As a natural product, it is polydisperse, and has been used as a dispersing agent, an Recently, there has been a significant inter- emulsifier, and a coagulant in food indus- est in the use of hydrocolloids to stabilize col- try (5). loidal dispersions ranging from food stuffs to Previous workers have used natural poly- coal-water slurries (1). Some hydrocolloids mers and polyelectrolytes such as lignin sul- have the advantage that they can stabilize the phonates for similar studies (6) and since sul- colloidal system even at relatively low con- phonates are strong acids, the ionic strength centrations in the order of 10 ppm. Recently, of the medium was the sole property that could it has been shown that a complex formulation be adjusted to change the amount of charge containing xanthan gum (an acidic polysac- of these polyelectrolytes as a means of con- charide) as a key component was sufficient to trolling stability. Gum tragacanth, on the other prevent cake formation of a 60 wt% coal-water hand, has weakly dissociating surface acidic slurry (2). While there was no indication of groups, and solution pH was thought of as an how the coal particle interacts, an explanation ideal way to vary the amount of charge of GT of the mechanism may be suggested from our and to monitor the effect of such variations previous work on the stabilization of polysty- on the stability of polystyrene latices. rene latices by acidic and nonionic polysac- Therefore, as a result of these preliminary charides such as gum tragacanth (GT), pectic findings, a detailed study of the factors bring- acid, arabinogalactan, and dextran (3). ing about the stability was undertaken. As a Gum tragacanth was found to be quite an first step in the systematic investigation of the effective stabilizer at concentration in low ppm stabilization mechanism of GT, the effect of range, and this was suggestive of a possible pH on stability was studied. The amount of steric stabilization mechanism. Gum traga- GT adsorbed, electrostatic repulsive force, and canth is a dried gummy exudation of Astra- the conformation were measured as a function galus (family of Leguminosae) and an acidic of pH. Photon correlation spectroscopy (PCS) polysaccharide containing D-galacturonic acid was used to measure the steric layer thickness. 141 0021-9797/88 $3.00 Copyright© 1988 by AcademicPress, Inc. Journal of Colloid and Interface Science, Vol. 126, No. 1, November 1988 All rightsof reproduction in any form reserved. 142 YOKOYAMA, SRINIVASAN, AND FOGLER 2. EXPERIMENTAL the ellipsoidal axes of revolution for GT of 450 and 1.9 nm. 2.1. Materials 2.2. Measurement of Flocculation The monodisperse polystyrene latex (di- Rate Constant ameter: 0.33/~m) was prepared through a sur- factant free procedure. Gum tragacanth and The flocculation rate was measured by a poly (galacturonic acid), PGUA, were pur- turbidimetric technique (10). Polystyrene la- chased from Sigma Chemical Company. The tex spheres (0.33 #m) of the volume fraction chemical structures of GT and PGUA are of 2.6 X 10 -5 were incubated for 1 day in the shown in Figs. 1 and 2. Over half of carboxyl GT solution with the ionic strength of 0.085 groups of GT are known to be esterified (7). M at 293 K. After the incubation, the ionic GT is based on poly (galacturonic acid) strength was increased from 0.085 to 0.33 M which is a linear chain of 1,4 linked a-D-ga- by the addition of magnesium chloride solu- lacturonic acid. Three types of side chains are tion to induce flocculation. The turbidity was known, namely, single fi-D-xylopyranose, and measured as a function of time at the wave- disaccharide units of 2-O-a-L-fucopyranosyl- length of 500 nm with a DMS spectropho- D-xylopyranose and 2-O-fi,D-galactopyrano- tometer Model 200 (Varian Inc.). This incu- syl-D-xylopyranose (8). The GT solution was bation time (1 day) was found to be sufficient prepared by first dissolving weighed GT in since the dimensionless stability factor, W, the sodium chloride solution (0.085 M) then given as Eq. [1] remained the same after an filtering it to remove any aggregates. The mo- incubation time of 30 min or longer: lecular weight distribution of GT was obtained W = kwithout GT / kwith GT. [1] by gel permeation chromatography using Sepharose 4B/CL-4B (Pharmacia Inc.) as a The flocculation rate constant was obtained gel with the ionic strength of 0.085 M at pH from the slope of the initial 2 min of the time- 7.4 (Table I). The polydispersity of GT and turbidity curve, and the dimensionless stability PGUA was calculated to be 2.7. factor, W, was then calculated from Eq. [1]. Graten et al. (9) showed that GT molecules The complete details have been described are thread-like and calculated the values for elsewhere (10). c' OH Esterified carboxyl group (hydrophobic) o.°,, CO0 OH COOH ~O OH C 3 -'(3 O O FIG. 1. Chemical structure of GT. Journal of Colloid and Interface Science, Vol. 126, No. 1, November 1988 STABILIZATION BY GUM 143 COOH COOH OH OH o COOH COOH OH OH FIG. 2. Chemical structure of poly (galacturonic acid). The pH was adjusted with acetic acid-so- u = e~'/47r~/, [2] dium acetate buffer for pH 3.4 and 5.1, while where u is the mobility, e the dielectric con- a phosphate buffer was used for pH 7.4. The stant, ~" the zeta potential, and n the viscosity ionic strength was kept constant for different of the medium. pH solutions by adding the appropriate amounts of sodium chloride. 2.5. pK Measurement 2.3. Binding Studies GT was converted into its H-form by cation exchanger (Amberlite IRA-118H, Sigma Adsorption isotherms for the binding of GT Chemical Company) and its pK was measured to polystyrene latex spheres were obtained for by potentiometric titration at a constant ionic different pH solutions. This was achieved by strength of 0.33 M. first mixing GT and latex spheres for 1 day at 293 K followed by centrifugation at 11500 rpm 2.6. Measurement of Steric Layer Thickness for 30 min. The ionic strength was adjusted by Photon Correlation Spectroscopy (PCS) by adding sodium chloride. The concentration of GT in the solution was measured by phenol- PCS data were obtained using Lexel 2 W sulfuric acid method (11). Argon ion laser with an ITT FW-130 photo multiplier tube. The different angles (50 °- 2.4. Electrophoresis Measurements 100 ° ) and volume fractions of latex particles (0-1.9 X 10 -6) were employed to confirm that The electrophoretic mobility was deter- there is no effect of dust or particle-particle mined at 293 K with a Lazer-Zee Model 500 interactions. The steric layer thickness was (Pen Kern Inc.). Since the value for Ka is 330 then calculated as the difference in the radii (K, reciprocal of double layer thickness; a, of the bare and covered particles obtained from particle diameter), the zeta potential was cal- the diffusion coefficient using the Stokes-Ein- culated using the Smoluchowski equation, stein relation. 3. RESULTS AND DISCUSSION TABLE I 3.1. Effect of pH on Colloidal Molecular Weight Distribution of GT Samples as Stabilization with GT Determined by GPC The dimensionless stability factor, W, was Molecular weightrange Wt% measured as a function of GT concentration >600,000 25.3 for different pH solutions. Figure 3 shows that 600,000-500,000 21.7 the stability increases greatly as the pH de- 500,000-170,000 27.1 creases. 170,000-60,000 12.5 To find the degree of dissociation of car- <60,000 13.5 boxylic acid in GT at various pH and to study Journal of Colloid and Interface Science, Vol, 126, No. 1, November 1988 144 YOKOYAMA, SRINIVASAN, AND FOGLER 120 that proton-dissociation behavior of GT is the 100 same as that of poly (galacturonic acid). This similar proton-dissociation behavior of GT 80 and PGUA can also be suggested by the similar 60 effect of pH on the stability for PGUA as o m pH 2,8 * pH 4.8 shown in Fig. 4. 40 A pH 6.2 There are three main possibilities for the 20 variation in the dimensionless stability factor with pH at the same concentration of GT in O, -..~ 0 20 40 60 80 100 120 bulk: (A) the amount of GT adsorbed on par- Concentration of GT in Bulk [pprn] ticles varies with pH at the same concentration of GT in bulk, (B) the electrostatic repulsive FIG.