J. Phys. Chem. 1995,99, 14205-14206
14205
Temperature-Induced Heteroflocculation in Particulate Colloidal Dispersions A. M. Islam, B. Z. Chowdhry, and M. J. Snowden* School of Chemical & Life Sciences, University of Greenwich, Woolwich, London, SEI8 6PF U.K. Received: July 18, 1995@
Mixed dispersions of anionic polystyrene latex and cationic microgels of poly(N4sopropylacrylamide) coexist as colloidally stable mixed dispersions at 20 OC. However, at elevated temperatures (50 "C)a heteroflocculation of the dispersion takes place within certain concentration ranges of microgel particles. The system redisperses on cooling, but only at elevated pH values.
Introduction The stability of colloidal dispersions is of considerableinterest to both industrialists' and academics alike.2 The flocculation of like particles has recieved widespread attention in the scientific literature from both an experimental3 and theoretical point of view.4 Heteroflocculation, however, may be defined as the association of two different particulate species, a situation more representative of many industrially important systems, e.g., paints and inks, and has, therefore, been the subject of recent attention in the scientific l i t e r a t ~ r e . ~We . ~ now wish to report what we believe to be the fist example of a temperature-induced heteroflocculation, where a mixed dispersion of microgels and latex is destabilized following an increase in temperature. The dispersions very interestingly redisperse on cooling from solutions at alkaline pH thus offering potential for regeneration.
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Experimental Section Cationic poly(N-isopropylacrylamide) (NIPAM) microgel particles were prepared by the free-radical polymerization of NIPAM (5 gL) in water at 70 "C in the presence of N,N'methylenebisacrylamide [(CH2=CHCONH)2CH2, from Sigma Chemicals, 0.5 g/L] as a cross-linking agent. The initiator used was 2,2-azobis-(2-amidinopropane)dihydrochloride (0.5 gL), and the reaction was allowed to proceed at pH 3-6 hours under a nitrogen atmosphere with the dispersion being continually stirred. The microgels produced were monodisperse spheres and were found to shrink reversibly at 34 "C. This behavior is entirely consistent with that reported for other similar poly(NIPAM) microgels.' The microgels were then extensively dialyzed against distilled water and kept in plastic bottles until required. Particle microelectrophoresis (Malvem Instruments, Zetamaster plus) showed the microgels to have an electrophoretic mobility of f 1 . 9 x m2 s-' V-I in 1 x mol dm-3 NaCl solution at 25 "C. Anionic polystyrene latex particles were prepared by the free-radical polymerization of styrene, following the procedure described by Goodwin et using potassium persulfate as an initiator. The particles produced were extensively dialyzed against distilled water for 6 weeks to remove any unreacted monomer and were found to be monodisperse spheres by transmission electron microscopy, having a mean particle diameter of 825 f 22 nm. Particle microelectrophoresisshowed the latex to have an electrophoretic mobility of 3.3 x m2 s-l V-' in 1 x mol dm-3 NaCl solution at 25 "C. Stability experiments were carried out using dispersions containing variable concentrations of microgel and a fixed concentration (175 ppm) of polystyrene latex. The Abstract published in Aduance ACS Abstracrs, September 1, 1995.
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Figure 1. Plot of the dispersity n for a mixture of variable concentration of poly(N1PAM) and fixed concentration of polystyrene latex under a variety of solvent conditions: (A) in water at 20 O C ; (B) in water at 40 "C; (C) at pH 3 at 40 O C ; (D) at pH 10 at 40 "C; (E) in 0.005 mol dm-3 NaCl at 40 "C; (F) in 0.01 mol dm-3 at 40 OC.
samples were made up to a total volume of 4 cm3 at room temperature (18 f 2 "C) and allowed to stand overnight. The extent of any flocculation in the system was estimated from the wavelength (A) dependence of the absorbance (abs) of the dispersions9 where n = [d(log abs)/d log(A)]. The same experiment was repeated at 50 OC and also at pH 3 and 10 and in electrolyte solutions.
Results and Discussion Figure 1 illustrates the stability of the microgel as a function of concentration from 3 to 300 ppm. In water at neutral pH and at room temperature it can clearly be seen that the two particulate species coexist as a stable dispersion. This may be attributable to the very low charge density on the microgel at room temperature, thus making any electrostatic attraction very small. Further the microgels are swollen with water at room temperature and therefore have a very low Hamaker constant and hence a small van der Waals attractive force. On heating the dispersion, however, the diameter of the microgels decreases approximately 5-fold with a corresponding 25-fold decrease in surface area. This results in an increase in the charge density
0022-3654/95/2099-14205$09.00/0 0 1995 American Chemical Society
Letters
14206 J. Phys. Chem., Vol. 99, No. 39, 1995
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Figure 2. Transmission electron micrographs of a mixture of poly(NIPAM) and polystyrene latex.
on the microgel with the electrophoretic mobility increasing by an order of magnitude to +3.8 x lo-* m2 s-l V-' at 50 OC in 1 x loa4 mol dm3 NaCl. The electrostatic attraction between the microgel and the latex is now much greater and the particles undergo a clearly visible flocculation. As the concentration of microgel is increased so a second region of stability is observed (see Figure 1). Figure 2 shows a transmission electron micrograph of the mixed dispersion in this stability region (300 ppm) of microgel. It can clearly be seen that the latex particle is surrounded by adsorbed microgel particles which are conferring stability on the system, presumably by a combination of a repulsion of like cationic charges arising from the adsorbed microgel and some steric hinderance effects, Le., a loss of configurational entropy when the microgels become distorted at small interparticle separations. On cooling the particulate mixture from the flocculated concentration regimes, there is little evidence of any redispersion taking place at either neutral pH
or at pH 3. When the experiment is repeated however at pH 10 (Figure l), the critical flocculation concentration (CFC) of the microgel dispersion increases. This may be attributable to the high pH effectively neutralizing the cationic charge groups, hence decreasing the magnitude of the electrostatic attractive force between the latex and microgel. Interestingly however, the heteroflocculated dispersion does redisperse on cooling following gentle agitation with the value of n decreasing from -0.23 to -1.1. mol dm-3 NaCl the In the presence of electrolyte, 5 x, critical aggregation concentration of the microgel decreases slightly at 50 "C. This is probably attributable to the electrical double layer surroundingthe particles decreasing in length with increasing electrolyte concentration. Further the particles are allowed to interact more closely with the mean separation between adjacent particles in the dispersion decreasing. The particles do become dispersed at increased microgel concentramol dm-3 tions. At an electrolyte concentration of 1 x NaCl the dispersion is very readily flocculated, even at minimal additions of microgel; this again is the probable result of the electrical double layer being contracted. Interestingly, however, the system does not become redispersed at any concentration of microgel. This is due to the electrolyte present screening the charges within the system hence making any repulsive forces between adsorbed layers of microgel very small. These results conclusively illustrate how two oppositely charged colloids may coexist as stable dispersions and then, by means of gentle heating, undergo a destabilization making the heteroflocculated system readily filterable. Additionally at slightly elevated solution pH (typically pH 10 and above) the system will redisperse, thus offering a potential for the reuse of the microgel particles. Further work is currently ongoing within our laboratory to investigate the interaction of microgel particles with other colloidal dispersions as it is believed this may offer a potentially novel method of removing unwanted colloids from aqueous solutions and may therefore be of interest, e.g., to the water industry. We are also attempting to apply some of the current models used to predict colloid stability to these systems and hope to report these results in the near future.
Acknowledgment. The authors would like to thank Mr. A. Mandal for his assistance with the transmission electron microscopy experiments. References and Notes (1) Snowden, M. J.; Vincent, B.; Morgan, J. A novel method for the enhanced recovery of oil; U.K. Patent No. GB2262117A, 1993. (2) Hunter, R. J. Foundations ofColloid Science; Oxford Science Publications: Oxford, 1987. (3) Snowden, M. J.; Williams, P. A.; Garvey, M. J.; Robb, I. D. J. Colloid Intelface Sci. 1994, 166, 160. (4) Verwey, E. J. W.; Overbeek, J. Th. G. The Stability ofLyophobic Colloids; Elsevier: New York, 1948. ( 5 ) Marato, J. A.; de las Nieves, F. J. Colloids Surf. 1995, 96, 121. (6) Harley, S.; Thompson, D. W.; Vincent, B. Colloids Surf. 1992, 62, 163. (7) Murray, M.; Ranna, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. SOC., Chem. Commun. 1994, 1803. (8) Goodwin, J. W.; H e m , J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 461. (9) Long, J. A.; Osmond, D. W. J.; Vincent, B. J. Colloid Inte$uce Sei. 1973, 42, 545.
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