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Bridging and depletion flocculation of synthetic latices induced by polyelectrolytes
Colloids and Surfaces A: Physiochemical and Engineering Aspects 153 Ž1999. 575]581
Bridging and depletion flocculation of synthetic latices induced by polyelectrolytes Kunio Furusawaa,U , Miki Uedaa , Takeshi Nashimab a b
Department of Chemistry, The Uni¨ ersity of Tsukuba, Tsukuba, Ibaraki 305, Japan National Research Laboratory of Metrology, Umezono, Tsukuba, Ibaraki 305, Japan
Abstract The flocculation and stabilization behaviors of concentrated anionic latex dispersion Ž5 ; 15 wt.%. induced by addition of cationic polyelectrolytes Žpoly-L-lysine; PLL. have been evaluated by rheological measurements. It was found that in the PLL concentration region lower than 0.10 wt.%, the latex dispersion was flocculated by the bridging effect of the adsorbed PLL layer. In the region of 0.1]0.3 wt.% of PLL, the system was stabilized extensively by the steric effect of the adsorbed layer, and finally it was destabilized again by the depletion effect of free PLL molecules after saturated adsorption, where the surface charge of the particles has been reversed from a negative to a positive sign and compact aggregates have been formed. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Depletion effect; Bridging flocculation; Poly-L-lysine; Rheological measurements
1. Introduction The stability of dispersion is greatly affected by the addition of polymer w1x. Polymers which have an adsorbing nature onto particle surfaces stabilize or destabilize the dispersion by their steric effect or bridging effects w2,3x of adsorbed polymer layers. Recently, the effect of polymers which do not adsorb onto the surface has attracted much attention. When the free polymers are eliminated from the interparticle region in which polymer molecules cannot exist without deformation, the U
Corresponding author.
depletion effect is induced by their osmotic pressure, due to the difference of polymer concentration between the inside and outside regions, and acts to draw the particles together. The depletion effect was first proposed by Asakura and Oosawa w4x and this effect was treated first for spherical particle systems. According to their analysis, the depletion attractive potential is added to the usual D.L.V.O. interaction potential and brings about a shallow secondary minimum in the resulted total potential curve w5,6x. Therefore it is expected that the flocculation based on the depletion effect will be a weak and a reversible character. Depletion flocculation has been observed in many systems, both aqueous and non-aqueous w7,8x. Also, in the case where polymer adsorbs onto particles, the
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 4 7 9 - 8
576
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
depletion effect operates after saturation adsorption by the excess polymer molecules w9x. In this paper, the depletion flocculation of an anionic latex dispersion induced by the cationic polyelectrolytes Žpoly-L-lysine., has been investigated by using rheological measurements. Anionic latices used here were synthesized by copolymerization of sodium styrenesulfonate into a polystyrene chain without any surface active agents w10x. Furthermore, from the measurements of z-potential of the latex samples and sedimentation volume of the floc formed under the different conditions, the mechanism of the flocculation]re-stabilization process has been investigated. 2. Experimental 2.1. Materials Anionic latex samples were prepared by the surfactant-free emulsion polymerization method w10,11x. Two latex samples with different diameters Ž D s 435 nm and 480 nm. were prepared without using any surface active agents by incorporating a small amount of ionic comonomer, sodium styrenesulfonate ŽNaSS., into the polystyrene chain according to Juang and Krieger w11x, and were composed of monodispersed spherical particles Ž DwrDn ) 1.05.. The particle diameters were determined by the electron microscopic observation. These latex samples were dialyzed well and were purified by the ion exchange treatment using the mixed ion exchange resin just before using as the sample. Then the solid contents Ž f . were controlled at a fixed value Ž5 ; 15 wt.%. by the ultra-filtration technique. Poly-L-lysine ŽPLL. is a typical cationic polypeptide and three PLL samples of commercial grade were purchased from Sigma Chemical Ltd ŽUSA.. The molecular weights Žmol wt.. being 193 700, 21 700 and 3970 were determined by the ultracentrifuge analyses. In the pH region lower than pH 8, PLL takes on an extended conformation by the inner electrostatic repulsion between the dissociated ammonium groups. The other reagents were commercially available and of analytical grade. All the solutions of these materials
were made with de-ionized and distilled water, all prepared using Pyrex apparatus. 2.2. Rheological measurement A new coaxial rotational viscometer w12x was used because of the high accuracy Žapprox. 10y7 N m. and the ability to make low viscosity measurements. The size of the inner cylinder was 40 mm in length and 9 mm in radius. The gap between the cylinders was 1 mm. Before starting a measurement, the sample, which was set into the rheometer, was subjected to high shear stirring to attain a uniform state of dispersion. A series of measurements was ordinarily begun from a low shear rate of 0.8 sy1 increasing up to a high shear of 320 sy1 with exponentially divided points in each decade and ended after measurements returned to the point of 1 sy1 . A temperature of 208C, a latex content of 10 wt.% and a salt addition of 10y4 mol dmy3 NaCl under pH 3, where the PLL molecules dissociated to the positive form, were maintained as common experimental conditions throughout the rheological measurements. 2.3. Electrophoresis Electrophoretic mobilities of the bare and the PLL-coated latex particles were determined at the various PLL concentrations under the diluted latex dispersion at 258C. The measurements of mobility were performed using the Rank Brothers Microelectrophoresis Apparatus ŽMK-2. using a rectangular glass cell. The z-potential was derived using the equation of Henry. 2.4. Measurement of sedimentation ¨ olume To obtain some knowledge on the structure of flocs induced by PLL, the sedimentation volume of the floc was measured under the different PLL conditions. In this study, the latex samples Ž f s 10 wt.%. treated under the different PLL concentrations put into the graduated cylinders Ž2 ml volume. and were allowed to stand for at least 1 month. After each time-lapse, the sedimentation boundary between the clear solution and the sedi-
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
577
ments was measured using the scale on the cylinder. 3. Results and discussion 3.1. PLL concentration and rheological beha¨ iors of latex dispersions Fig. 1 shows some typical plots of time dependency of dispersion viscosity Ž f s 10 wt.%. and in Fig. 2 the viscosity-shear rate profiles after 30 min under the different PLL concentration range Ž0.01; 0.07 wt.%.. It can be seen that the viscosity in a low shear rate Ž1 sy1 . is tremendously high and the time-dependency is hardly recognized except for a few initial minutes over the whole PLL concentration range. The shear-rate dependency was influenced strongly by repetition of the process as shown in Fig. 2, which means that the extended structure of flocs produced at the low shear rate will be destroyed by the high shear force. Furthermore, we can detect an interesting behavior from Fig. 2, i.e. the reproducibility of the shear rate dependency is not complete and the viscosity value at the successive second or third runs decreased to the half value of the respective first run. Such a tendency will imply that the flocculation is an irreversible process and the extended open structure of the original flocculant hardly recovered again after once they were broken.
Fig. 1. Typical plots of time dependency of dispersion viscosity under the different PLL Žmol wt.s 193 700. concentration range Ž208C, f s 15 wt.%..
To certify the phenomena, the viscosities in the low shear rate Ž1 sy1 . after 1 h were plotted against the PLL Žmol wt.s 193 700. concentration and the obtained results are given for the three kinds of latex concentrations Ž f s 5 wt.%, 7.5 wt.% and 10 wt.%. in Fig. 3. For all latex concentration systems, an extensive maximum of viscosity is initially observed and after attaining a minimum value for some concentration range of PLL, the viscosity increases gradually again beyond some critical polymer concentration. The polymer concentration at the maximum viscosity is dependent on the latex concentration of the system. The maximum range becomes broader and the maximum region extends to the higher po-
Fig. 2. The viscosity-shear rate profiles after 30 min under the different PLL Žmol wt.s 193 700. concentration.
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K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
Fig. 3. Viscosity variations of latex dispersion with PLL Žmol wt.s 193 700. concentration Žshear rate; 1 sy1 ..
lymer concentration with increasing latex concentration. Furthermore, according to the electrophoretic measurements for the latex samples treated under the different PLL concentrations, the z-potential of latex sample becomes zero at the concentration of PLL where the maximum viscosity of system was induced ŽFig. 4.. All these results indicate that the anionic latex dispersion has been flocculated effectively by adsorption of a small amount of cationic polyelectrolyte. The flocculation will be based on the bridging effect of adsorbed PLL molecules or the charge neutralization effect from the adsorbed cationic charges of PLL molecules. Perhaps the bridging effect will influence predominantly on their flocculation, because fairly extended open structures of flocs have been formed in the respective systems. However, it is noteworthy that the z-potential of latex samples shows a zero value at the maximum viscosity, indicating that the charge neutralization effect of adsorbed PLL will also play a role in the flocculation of dispersed systems. Furthermore, for all systems, the viscosities reach minimum values in the concentration range from 0.1 wt.% to 0.3 wt.% PLL and the minimum range in the viscosity curves becomes sharper with increasing latex concentration of the sample. This stabilization of the system will be explained by the steric and electrostatic repulsive energies Želectrosteric repulsion. due to the adsorbed cationic polymers. Also, it is confirmed that the low system viscosity can hold for a long period Ža
Fig. 4. z-Potential of latex particles under the different PLL Žmol wt.s 193 700. concentrations.
few months. with the same stability state. 3.2. Depletion flocculation by the free PLL after saturated adsorption Finally, we would like to pay attention to the higher PLL concentration region than 0.3 wt.%. According to Fig. 3, the system viscosity increases gradually with increasing PLL concentrations, which means that the system destabilized again under these conditions. This tendency is dependent on the molecular weight of PLL, as shown in Fig. 5. In the systems with low molecular weight PLL species ŽFig. 5., the increasing tendency of viscosity was scarcely observed at the high con-
Fig. 5. Viscosity variation of latex dispersion with PLL Žmol wt.s 21 700, 3970. concentration Žshear rate; 1 sy1 , f s 10 wt.%..
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
centration of PLL or not observed at any concentration range of PLL. From these results, it is realized that the increasing tendency of viscosity at the high PLL concentration range, is a characteristic behavior observed only in the system including a high molecular weight PLL. Also, it is apparent from Fig. 3 that this tendency is more pronounced in the high latex concentration system. To confirm that the slow increase of system viscosity will be based on weak flocculation due to the depletion effect, the variation of viscosity by the shear rate and their reversibilities of behavior, were investigated. Here, all the measurements have been conducted after stirring for 1 h at 1 sy1 . Fig. 6 shows the repeated results which indicate the shear rate dependence of system viscosity at the different PLL concentrations Ž0.2]0.75 wt.%.. The sample with 0.2 wt.% PLL Žmol wt.s 193 700. is a Newtonian fluid, indicating that there is no serious flocculation. From the sample of 0.3 wt.% PLL, rheological behavior becomes non-Newtonian indicating the flocculation occurs. At a low shear rate, enhanced viscosity is found, while small increases corresponding to the medium viscosity are found at a high shear rate. It seems that the high shear viscosities correspond to the dispersed state of the samples. This implies that the flocculation formed at the low shear rate can be re-dispersed by the high shear rate. Furthermore, the same measurements have been conducted repeatedly for the same
579
sample after waiting for 1 h. Fig. 6 indicates the same viscosity-shear rate profiles in the different PLL concentrations. As seen from the figure, the complete reversible character of the flocculation process have been recognised Žreversible shear thinning.. Such a character of flocculation is a specific properties of the flocculation due to the depletion effect. In the depletion flocculation, the attractive interaction energy between the particles is induced by the osmotic pressure, due to the difference of polymer concentration between the inside and outside of the depletion zone. So in a low molecular weight PLL system ŽFig. 5. or in a low particle concentration dispersion ŽFig. 3., the osmotic pressure becomes too weak and a pronounced effect can’t be realized. To clarify the correlation between the amounts of PLL and the stability of latex dispersion, the z-potential of particles has been measured. The result is shown in Fig. 4 where the relation between the z-potentials of the latex particles and PLL concentration in the medium after adsorption are plotted. It is clear that the z-potentials turn from negative to positive value and attain a plateau Ž; q30 mV. with increasing PLL concentration. This indicates that the negative charges on latex particle were neutralized by adsorption of cationic PLL molecules and turned to the positive charges on the latex surface by excess adsorption of PLL molecules. Therefore it is suggested that the further increase of positive PLL molecule after saturated adsorption brings about
Fig. 6. The viscosity-shear rate profiles at different PLL Žmol wt.s 193 700. concentrations.
580
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
an increase of free PLL molecules in cationic latex dispersion. The situation is identical with anionic latex dispersion, including free PSSNa molecules with negative charges, where a strong depletion flocculation of the particles has been recognized w12x. To obtain some knowledge on the nature of the interparticle forces involved, the sedimentation volume of the flocs induced under the different PLL concentration region has been measured. Fig. 7 shows the results for the sedimentation volume after 1 month. The sol concentration Ž10 wt.%. and the time of standing are arbitrarily chosen. Therefore the results of the flocculation volume test have relative significance only. As seen from Fig. 7, the flocculation volume under 0.75 wt.% PLL is fairly small compared with the sediment volumes produced under the other low PLL concentration region where the interparticle bridging effect will play predominantly. This result indicates that the flocculation induced in 0.75 wt.% PLL concentration will be based on the depletion effect and the flocculation mechanism will be different with the flocculants formed in the 0]0.1-wt.% PLL region, because the depletion flocculation is induced by the secondary shal-
Fig. 8. A schematic representation to explain the effect of PLL on the flocculation behavior of latex dispersion.
low potential minimum between the particles which results to a more compact floc than the extended floc formed by the bridging effect by the adsorbed PLL layers. All these results indicate surely that the gradual increase of dispersion viscosity in the 0.1]0.75-wt.% region of PLL with a high molecular weight, will be based on the depletion effect of the free PLL molecules after saturated adsorption on the latex surface. 4. Conclusion Fig. 7. Sedimentation volume of latex dispersions at different PLL concentrations after 1 month.
In Fig. 8, a schematic representation is given to
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
explain the effects of PLL concentration on the observed flocculation behavior. The factors determining the stability of the latex dispersion, if the particles carry an adsorbed PLL layer on the surface, are the bridging attraction energy Ž V brig ., the steric repulsion energy Ž Vstric ., and the depletion attraction energy Ž Vdepl . due to the free polymers. These are in addition to the classical van der Waals attraction Ž Va . and electric double layer repulsion terms Ž Vel .. It is usually considered that over the whole range of interparticle distance in the present system, Va is small in comparison to the other energy contributions. In contrast, Vel , especially in the present system, plays a significant role in controlling the system stability. 1. In the concentration range lower than 0.1 wt.% PLL, a pronounced maximum of system viscosity was observed at the low shear rate. This flocculation will be based mainly on the bridging effect of the adsorbed PLL layer. However, the charge neutralization effect of adsorbed polyelectrolytes will also play a role in the flocculation. 2. In the region of 0.1]0.3 wt.% PLL, the system was stabilized strongly mainly by the steric effect of the adsorbed PLL layer. In this case, an expanded conformation of adsorbed layer due to the electrostatic force will play a role to strengthen the repulsive force between the particles.
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3. In the PLL region above 0.3 wt.%, the system was destabilized again by the depletion effect of excess free PLL molecules, where the surface charge on the particles has been reversed from a negative to a positive sign. The flocculation process is reversible and weak, compact aggregates have been formed. These properties are typical of the aggregate formed by the depletion effect.
References w1x D.H. Napper, Polymer Stabilization of Colloid Dispersion, Academic Press, New York, 1983, p. 413. w2x F.Th Hesselink, A. Vrij, J.Th.G. Overbeek, J. Phys. Chem. 75 Ž1971. 2094. w3x G.J. Fleer, J. Lyklema, J. Colloid Interface Sci. 46 Ž1974. 1. w4x S. Asakura, F. Oosawa, J. Chem. Phys. 22 Ž1954. 1255. w5x G.J. Fleer, J.M.H.M. Scheutjens, J. Colloid Interface Sci. 111 Ž1986. 504. w6x H. Dehek, A. Vrij, J. Colloid Interface Sci. 84 Ž1981. 409. w7x P.R. Sperry, H.B. Hopfenberg, N.L. Thomas, J. Colloid Interface Sci. 82 Ž1981. 62. w8x B. Vincent, J. Edwards, S. Emmett, A. Jones, Colloid Surf. 18 Ž1986. 261. w9x T. Nashima, K. Furusawa, Colloids Surf. 55 Ž1991. 149. w10x K. Furusawa, W. Norde, J. Lyklema, Kolloid Z.u.Z. Polymere 25 Ž1972. 908. w11x M.S. Juang, I.M. Krieger, J. Polymer Sci. 14 Ž1976. 2089. w12x T. Nashima, Thesis, Osaka University, 1995.
Bridging and depletion flocculation of synthetic latices induced by polyelectrolytes Kunio Furusawaa,U , Miki Uedaa , Takeshi Nashimab a b
Department of Chemistry, The Uni¨ ersity of Tsukuba, Tsukuba, Ibaraki 305, Japan National Research Laboratory of Metrology, Umezono, Tsukuba, Ibaraki 305, Japan
Abstract The flocculation and stabilization behaviors of concentrated anionic latex dispersion Ž5 ; 15 wt.%. induced by addition of cationic polyelectrolytes Žpoly-L-lysine; PLL. have been evaluated by rheological measurements. It was found that in the PLL concentration region lower than 0.10 wt.%, the latex dispersion was flocculated by the bridging effect of the adsorbed PLL layer. In the region of 0.1]0.3 wt.% of PLL, the system was stabilized extensively by the steric effect of the adsorbed layer, and finally it was destabilized again by the depletion effect of free PLL molecules after saturated adsorption, where the surface charge of the particles has been reversed from a negative to a positive sign and compact aggregates have been formed. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Depletion effect; Bridging flocculation; Poly-L-lysine; Rheological measurements
1. Introduction The stability of dispersion is greatly affected by the addition of polymer w1x. Polymers which have an adsorbing nature onto particle surfaces stabilize or destabilize the dispersion by their steric effect or bridging effects w2,3x of adsorbed polymer layers. Recently, the effect of polymers which do not adsorb onto the surface has attracted much attention. When the free polymers are eliminated from the interparticle region in which polymer molecules cannot exist without deformation, the U
Corresponding author.
depletion effect is induced by their osmotic pressure, due to the difference of polymer concentration between the inside and outside regions, and acts to draw the particles together. The depletion effect was first proposed by Asakura and Oosawa w4x and this effect was treated first for spherical particle systems. According to their analysis, the depletion attractive potential is added to the usual D.L.V.O. interaction potential and brings about a shallow secondary minimum in the resulted total potential curve w5,6x. Therefore it is expected that the flocculation based on the depletion effect will be a weak and a reversible character. Depletion flocculation has been observed in many systems, both aqueous and non-aqueous w7,8x. Also, in the case where polymer adsorbs onto particles, the
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 4 7 9 - 8
576
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
depletion effect operates after saturation adsorption by the excess polymer molecules w9x. In this paper, the depletion flocculation of an anionic latex dispersion induced by the cationic polyelectrolytes Žpoly-L-lysine., has been investigated by using rheological measurements. Anionic latices used here were synthesized by copolymerization of sodium styrenesulfonate into a polystyrene chain without any surface active agents w10x. Furthermore, from the measurements of z-potential of the latex samples and sedimentation volume of the floc formed under the different conditions, the mechanism of the flocculation]re-stabilization process has been investigated. 2. Experimental 2.1. Materials Anionic latex samples were prepared by the surfactant-free emulsion polymerization method w10,11x. Two latex samples with different diameters Ž D s 435 nm and 480 nm. were prepared without using any surface active agents by incorporating a small amount of ionic comonomer, sodium styrenesulfonate ŽNaSS., into the polystyrene chain according to Juang and Krieger w11x, and were composed of monodispersed spherical particles Ž DwrDn ) 1.05.. The particle diameters were determined by the electron microscopic observation. These latex samples were dialyzed well and were purified by the ion exchange treatment using the mixed ion exchange resin just before using as the sample. Then the solid contents Ž f . were controlled at a fixed value Ž5 ; 15 wt.%. by the ultra-filtration technique. Poly-L-lysine ŽPLL. is a typical cationic polypeptide and three PLL samples of commercial grade were purchased from Sigma Chemical Ltd ŽUSA.. The molecular weights Žmol wt.. being 193 700, 21 700 and 3970 were determined by the ultracentrifuge analyses. In the pH region lower than pH 8, PLL takes on an extended conformation by the inner electrostatic repulsion between the dissociated ammonium groups. The other reagents were commercially available and of analytical grade. All the solutions of these materials
were made with de-ionized and distilled water, all prepared using Pyrex apparatus. 2.2. Rheological measurement A new coaxial rotational viscometer w12x was used because of the high accuracy Žapprox. 10y7 N m. and the ability to make low viscosity measurements. The size of the inner cylinder was 40 mm in length and 9 mm in radius. The gap between the cylinders was 1 mm. Before starting a measurement, the sample, which was set into the rheometer, was subjected to high shear stirring to attain a uniform state of dispersion. A series of measurements was ordinarily begun from a low shear rate of 0.8 sy1 increasing up to a high shear of 320 sy1 with exponentially divided points in each decade and ended after measurements returned to the point of 1 sy1 . A temperature of 208C, a latex content of 10 wt.% and a salt addition of 10y4 mol dmy3 NaCl under pH 3, where the PLL molecules dissociated to the positive form, were maintained as common experimental conditions throughout the rheological measurements. 2.3. Electrophoresis Electrophoretic mobilities of the bare and the PLL-coated latex particles were determined at the various PLL concentrations under the diluted latex dispersion at 258C. The measurements of mobility were performed using the Rank Brothers Microelectrophoresis Apparatus ŽMK-2. using a rectangular glass cell. The z-potential was derived using the equation of Henry. 2.4. Measurement of sedimentation ¨ olume To obtain some knowledge on the structure of flocs induced by PLL, the sedimentation volume of the floc was measured under the different PLL conditions. In this study, the latex samples Ž f s 10 wt.%. treated under the different PLL concentrations put into the graduated cylinders Ž2 ml volume. and were allowed to stand for at least 1 month. After each time-lapse, the sedimentation boundary between the clear solution and the sedi-
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
577
ments was measured using the scale on the cylinder. 3. Results and discussion 3.1. PLL concentration and rheological beha¨ iors of latex dispersions Fig. 1 shows some typical plots of time dependency of dispersion viscosity Ž f s 10 wt.%. and in Fig. 2 the viscosity-shear rate profiles after 30 min under the different PLL concentration range Ž0.01; 0.07 wt.%.. It can be seen that the viscosity in a low shear rate Ž1 sy1 . is tremendously high and the time-dependency is hardly recognized except for a few initial minutes over the whole PLL concentration range. The shear-rate dependency was influenced strongly by repetition of the process as shown in Fig. 2, which means that the extended structure of flocs produced at the low shear rate will be destroyed by the high shear force. Furthermore, we can detect an interesting behavior from Fig. 2, i.e. the reproducibility of the shear rate dependency is not complete and the viscosity value at the successive second or third runs decreased to the half value of the respective first run. Such a tendency will imply that the flocculation is an irreversible process and the extended open structure of the original flocculant hardly recovered again after once they were broken.
Fig. 1. Typical plots of time dependency of dispersion viscosity under the different PLL Žmol wt.s 193 700. concentration range Ž208C, f s 15 wt.%..
To certify the phenomena, the viscosities in the low shear rate Ž1 sy1 . after 1 h were plotted against the PLL Žmol wt.s 193 700. concentration and the obtained results are given for the three kinds of latex concentrations Ž f s 5 wt.%, 7.5 wt.% and 10 wt.%. in Fig. 3. For all latex concentration systems, an extensive maximum of viscosity is initially observed and after attaining a minimum value for some concentration range of PLL, the viscosity increases gradually again beyond some critical polymer concentration. The polymer concentration at the maximum viscosity is dependent on the latex concentration of the system. The maximum range becomes broader and the maximum region extends to the higher po-
Fig. 2. The viscosity-shear rate profiles after 30 min under the different PLL Žmol wt.s 193 700. concentration.
578
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
Fig. 3. Viscosity variations of latex dispersion with PLL Žmol wt.s 193 700. concentration Žshear rate; 1 sy1 ..
lymer concentration with increasing latex concentration. Furthermore, according to the electrophoretic measurements for the latex samples treated under the different PLL concentrations, the z-potential of latex sample becomes zero at the concentration of PLL where the maximum viscosity of system was induced ŽFig. 4.. All these results indicate that the anionic latex dispersion has been flocculated effectively by adsorption of a small amount of cationic polyelectrolyte. The flocculation will be based on the bridging effect of adsorbed PLL molecules or the charge neutralization effect from the adsorbed cationic charges of PLL molecules. Perhaps the bridging effect will influence predominantly on their flocculation, because fairly extended open structures of flocs have been formed in the respective systems. However, it is noteworthy that the z-potential of latex samples shows a zero value at the maximum viscosity, indicating that the charge neutralization effect of adsorbed PLL will also play a role in the flocculation of dispersed systems. Furthermore, for all systems, the viscosities reach minimum values in the concentration range from 0.1 wt.% to 0.3 wt.% PLL and the minimum range in the viscosity curves becomes sharper with increasing latex concentration of the sample. This stabilization of the system will be explained by the steric and electrostatic repulsive energies Želectrosteric repulsion. due to the adsorbed cationic polymers. Also, it is confirmed that the low system viscosity can hold for a long period Ža
Fig. 4. z-Potential of latex particles under the different PLL Žmol wt.s 193 700. concentrations.
few months. with the same stability state. 3.2. Depletion flocculation by the free PLL after saturated adsorption Finally, we would like to pay attention to the higher PLL concentration region than 0.3 wt.%. According to Fig. 3, the system viscosity increases gradually with increasing PLL concentrations, which means that the system destabilized again under these conditions. This tendency is dependent on the molecular weight of PLL, as shown in Fig. 5. In the systems with low molecular weight PLL species ŽFig. 5., the increasing tendency of viscosity was scarcely observed at the high con-
Fig. 5. Viscosity variation of latex dispersion with PLL Žmol wt.s 21 700, 3970. concentration Žshear rate; 1 sy1 , f s 10 wt.%..
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
centration of PLL or not observed at any concentration range of PLL. From these results, it is realized that the increasing tendency of viscosity at the high PLL concentration range, is a characteristic behavior observed only in the system including a high molecular weight PLL. Also, it is apparent from Fig. 3 that this tendency is more pronounced in the high latex concentration system. To confirm that the slow increase of system viscosity will be based on weak flocculation due to the depletion effect, the variation of viscosity by the shear rate and their reversibilities of behavior, were investigated. Here, all the measurements have been conducted after stirring for 1 h at 1 sy1 . Fig. 6 shows the repeated results which indicate the shear rate dependence of system viscosity at the different PLL concentrations Ž0.2]0.75 wt.%.. The sample with 0.2 wt.% PLL Žmol wt.s 193 700. is a Newtonian fluid, indicating that there is no serious flocculation. From the sample of 0.3 wt.% PLL, rheological behavior becomes non-Newtonian indicating the flocculation occurs. At a low shear rate, enhanced viscosity is found, while small increases corresponding to the medium viscosity are found at a high shear rate. It seems that the high shear viscosities correspond to the dispersed state of the samples. This implies that the flocculation formed at the low shear rate can be re-dispersed by the high shear rate. Furthermore, the same measurements have been conducted repeatedly for the same
579
sample after waiting for 1 h. Fig. 6 indicates the same viscosity-shear rate profiles in the different PLL concentrations. As seen from the figure, the complete reversible character of the flocculation process have been recognised Žreversible shear thinning.. Such a character of flocculation is a specific properties of the flocculation due to the depletion effect. In the depletion flocculation, the attractive interaction energy between the particles is induced by the osmotic pressure, due to the difference of polymer concentration between the inside and outside of the depletion zone. So in a low molecular weight PLL system ŽFig. 5. or in a low particle concentration dispersion ŽFig. 3., the osmotic pressure becomes too weak and a pronounced effect can’t be realized. To clarify the correlation between the amounts of PLL and the stability of latex dispersion, the z-potential of particles has been measured. The result is shown in Fig. 4 where the relation between the z-potentials of the latex particles and PLL concentration in the medium after adsorption are plotted. It is clear that the z-potentials turn from negative to positive value and attain a plateau Ž; q30 mV. with increasing PLL concentration. This indicates that the negative charges on latex particle were neutralized by adsorption of cationic PLL molecules and turned to the positive charges on the latex surface by excess adsorption of PLL molecules. Therefore it is suggested that the further increase of positive PLL molecule after saturated adsorption brings about
Fig. 6. The viscosity-shear rate profiles at different PLL Žmol wt.s 193 700. concentrations.
580
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
an increase of free PLL molecules in cationic latex dispersion. The situation is identical with anionic latex dispersion, including free PSSNa molecules with negative charges, where a strong depletion flocculation of the particles has been recognized w12x. To obtain some knowledge on the nature of the interparticle forces involved, the sedimentation volume of the flocs induced under the different PLL concentration region has been measured. Fig. 7 shows the results for the sedimentation volume after 1 month. The sol concentration Ž10 wt.%. and the time of standing are arbitrarily chosen. Therefore the results of the flocculation volume test have relative significance only. As seen from Fig. 7, the flocculation volume under 0.75 wt.% PLL is fairly small compared with the sediment volumes produced under the other low PLL concentration region where the interparticle bridging effect will play predominantly. This result indicates that the flocculation induced in 0.75 wt.% PLL concentration will be based on the depletion effect and the flocculation mechanism will be different with the flocculants formed in the 0]0.1-wt.% PLL region, because the depletion flocculation is induced by the secondary shal-
Fig. 8. A schematic representation to explain the effect of PLL on the flocculation behavior of latex dispersion.
low potential minimum between the particles which results to a more compact floc than the extended floc formed by the bridging effect by the adsorbed PLL layers. All these results indicate surely that the gradual increase of dispersion viscosity in the 0.1]0.75-wt.% region of PLL with a high molecular weight, will be based on the depletion effect of the free PLL molecules after saturated adsorption on the latex surface. 4. Conclusion Fig. 7. Sedimentation volume of latex dispersions at different PLL concentrations after 1 month.
In Fig. 8, a schematic representation is given to
K. Furusawa et al. r Colloids Surfaces A: Phyicochem. Eng. Aspects 153 (1999) 575]581
explain the effects of PLL concentration on the observed flocculation behavior. The factors determining the stability of the latex dispersion, if the particles carry an adsorbed PLL layer on the surface, are the bridging attraction energy Ž V brig ., the steric repulsion energy Ž Vstric ., and the depletion attraction energy Ž Vdepl . due to the free polymers. These are in addition to the classical van der Waals attraction Ž Va . and electric double layer repulsion terms Ž Vel .. It is usually considered that over the whole range of interparticle distance in the present system, Va is small in comparison to the other energy contributions. In contrast, Vel , especially in the present system, plays a significant role in controlling the system stability. 1. In the concentration range lower than 0.1 wt.% PLL, a pronounced maximum of system viscosity was observed at the low shear rate. This flocculation will be based mainly on the bridging effect of the adsorbed PLL layer. However, the charge neutralization effect of adsorbed polyelectrolytes will also play a role in the flocculation. 2. In the region of 0.1]0.3 wt.% PLL, the system was stabilized strongly mainly by the steric effect of the adsorbed PLL layer. In this case, an expanded conformation of adsorbed layer due to the electrostatic force will play a role to strengthen the repulsive force between the particles.
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3. In the PLL region above 0.3 wt.%, the system was destabilized again by the depletion effect of excess free PLL molecules, where the surface charge on the particles has been reversed from a negative to a positive sign. The flocculation process is reversible and weak, compact aggregates have been formed. These properties are typical of the aggregate formed by the depletion effect.
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