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Collaborative studies of zeta-potential measurements and electrophoretic measurements using reference sample
Colloids and Surfaces A: Physicochemical and Engineering Aspects 140 (1998) 217–226
Collaborative studies of zeta-potential measurements and electrophoretic measurements using reference sample K. Furusawa *, K. Uchiyama Department of Chemistry, University of Tsukuba, 1-1-1 Ten-nodai, Ibaraki 305, Japan Received 10 February 1997; accepted 9 April 1997
Abstract Simultaneous measurements of zeta-potential for two standard latex suspensions were carried out so as to assess the reliability of each of these measurement techniques and find means for their improvement. Furthermore, syntheses of a reference particle dispersion stabilized sterically in an aqueous medium without any electrostatic effects and measurements of zeta-potential using the reference dispersion as a standard were performed under various experimental conditions. It became apparent that the dense adsorption layer of hydroxypropylcellulose (HPC ), with a lower critical solution temperature (LCST ), formed on latex particles with a low surface charge density at temperatures higher than the LCST, plays a role in completely shielding the electrostatic effect arising from the surface charge on the bare particles. Such reference particles with zero zeta-potential allow us to determine the electrophoretic mobility of unknown samples at the one-half depth in the electrophoretic cell by subtracting the mobility of the reference sample at the same level. Furthermore, the zeta-potential of the cell wall can be easily determined from the mobility of the reference sample, because the apparent velocity profile of the reference sample indicates the liquid flow velocity in the cell. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Electroosmosis; Electrophoretic mobility; Hydroxypropylcellulose; Zeta potential
1. Introduction The zeta(f)-potential is the potential at the slipping plane in the electrical double layer and its value is not precisely the same as that of the surface potential. In spite of the uncertainty of its meaning and ambiguous character, the f-potential has frequently been used for the discussion of colloid stability and its value is still considered useful in relation to the electrical double layer. The most familiar method for determining the f-potential is electrophoresis, which consists of setting up a potential gradient in the solutions. * Corresponding author. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 28 0 - X
The electrophoretic migration of colloid particles is always superimposed on the electroosmotic liquid flow from the cell wall and the apparent particle velocity observed coincides with the true electrophoretic mobility only at two special depths, known as the stationary levels. Unfortunately, however, the velocity gradient of the liquid at the stationary levels is usually large, and thus the observed velocity of the particles changes rapidly with cell depth so that errors in electrophoretic mobility may be substantial. However, if the electrophoretic measurements will be carried out by using some reference sample as a standard, the electrophoretic mobility of the unknown sample can be determined at any cell depth by subtracting the mobility of the reference
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particles at the same level, because the liquid flow velocity induced by the electroosmotic effect of the cell wall becomes to same value under the same experimental condition. So, if the electrophoretic mobility would be detected at the one half depth in the cell, the real electrophoretic mobility is also given at the maximum of the parabolic velocity as the function of the cell depth [1]. In that time, slight errors in focusing are less important than the usual measurements at stationary level and the rotational effect of the particles is minimized, since the velocity gradient is small at the cell center. In this work, we first report the results of the collaborative studies of zeta potential measurements to seek a suitable standard sample, and secondly a new electrophoretic technique using a reference sample as a standard is introduced. We also show that this new technique using a reference sample is also convenient to determine the fpotential of the cell wall. In the final part of this work, we report on the syntheses of a new reference dispersion stabilized sterically in an aqueous medium without any electrostatic effects and the f-potential measurements using the new dispersion as a standard [2]. Special attention has been given to synthesis of the reference particles which are covered with the polymer layer as densely as possible and display a zero f-potential under the variety of solution conditions.
2. Collaborative study of zeta-potential using standard sample In 1970, a group of Japanese surface and colloid chemists who had engaged in the study of electrokinetic phenomena and/or colloid stability, formed a committee under the Division of Surface Chemistry in the Japan Oil Chemist’s Society. This group measured, compared and discussed zetapotentials from such samples as titanium oxide, microcapsule, silica and some polymer latices or silver iodide [3]. Table 1 indicates some examples of simultaneous measurements of zeta-potential for four samples. All these measurements have been carried out by the micro-electrophoretic technique using the respective electrophoretic apparatus belonging to each member laboratory. Usually,
the mobility measurements were performed at a constant field strength of 4–5 V cm−1 using a rectangular glass (or quartz) cell and the f-potential was calculated by the Smoluchowski equation. As can be seen from this table, the collaborative results for titanium oxide and microcapsule dispersions do not agree with each other and especially, the data for the microcapsule dispersion indicated a large deviation within some members. On the other hand, the f-potential measured for polystyrene latices which were prepared under the surfactant-free system by the Kotera–Furusawa–Takeda method [4] and silver iodide sol prepared by the usual way, were very similar among those tested and were reproducible. As the next stage of collaborative study, the simultaneous measurements of f-potential and critical flocculation concentration (c.f.c.) for two standard latex suspensions (samples 1 and 2) have been conducted [5] so as to assess the reliability of each of these measurement techniques and find means for their improvement. Sample 1 is a negative styrene/styrene sulphonate copolymer latices which were prepared according to the Krieger method [6 ]. Sample 2 is the amphoteric latices which were synthesized by the Homola method [7]. These latex samples were employed after fairly purification by the ion exchange treatment and dialysis against distilled water. The f-potential measurements were carried out at nine laboratories using different electrophoretic apparatus, which included the Rank Brother M-2, PEN KEM-501, PEN KEM-3000 and Malvern Zetasizer-F. As can be seen from Fig. 1 and Table 2, the data from all the laboratories showed fairly good agreement and displayed possible means for their improvement. (1) Elevated potential supply (PEN KEM-3000 and Malvern Zetasizer-F ) was found to increase the slope of the f versus pH curve near the isoelectric point of the sample 2 (2) Differences between the f-potential values reported from each laboratory indicate a constant deviation over the whole pH range, i.e. data G shows relatively high values, while data A indicates low values over the whole pH region. This tendency suggests that the difference between data G and A will be based
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Table 1 Results of collaborative studies of f-potential measurements for various samples (1) Titanium oxide Laboratory Method f (mV ) (2) Polystyrene latices ( EP) Laboratory f (mV ) (3) Microcapsule (EP) Laboratory f (mV ) (4) AgI sol (EP) Laboratory f (mV ) (pAg3) f (mV ) (pI4) (5) Agl sol(EP) Laboratory U (mm cm−1 V−1 s−1) f (mV )
(A) EP −24.8
(B) SP −14.8
(C ) SP −8
(D) SP −21.0
(A) −48
(B) −42.5–46
(C ) −47
(D) −42.3
(A) −27.0
(B) −26.1
(C ) −70
(D) −35.4
(A) +50
(B)
(C )
−47.5–51
−45
(D) +66.4
(B) −3.8 −46
(C ) −3.67 −44.4
(A) −3.675 −44.5
(E ) EP −35.72
(F) SP −25
on the same reason, which can be eliminated completely by using a definite standard sample. The c.f.c. of KNO , Mg(NO ) and La(NO ) 3 32 33 for sample 1, were determined simultaneously by a static method and the results were compared with those determined by a kinetic method and dynamic light scattering (L.S.). These results for the critical flocculation concentration and some related data are indicated in Table 3 [5]. The results obtained from the four laboratories by a static method agreed fairly well and the order of the magnitude of the c.f.c. values determined by L.S. technique and kinetic method can be expressed as follows: static
3. Electrophoretic measurements using PSSNalatices as a standard
Fig. 1. f-potential versus pH curves for samples 1 and 2. (A) Rank brother M-2; (B) PEN KEM-500; (C ) Rank Brother M-2; (D,E ) Laser Doppler method; ( F ) PEN KEM-3000; (H ) Malvern Zetasizer-F.
From the above collaborative measurements on the zeta-potential, it was realized that in determination of the f-potential from an unknown sample, electrophoretic measurements using a reference sample as a standard is very useful to determine the reliable data. Next, f-potential measurements
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Table 2 f-potentials of sample 1 and isoelectric points (pH0) of sample 2 Laboratory Sample 1 (f, mV ) Sample 2
pH=4 pH=10 pH0
A
B
C
D
E
F
G
H
I
−44.0 −44.0 8.0
−45.0 −45.0 6.6
−48.0 −48.0 7.8
−57.7 −60.5 6.8
−58.0 −58.0 7.1
−53.0 −54.0 7.6
−62.0 −65.0 8.2
−59.0 −62.0 7.8
−44.5 −46.0 6.5
Table 3 Critical flocculation concentration (c.f.c) for sample 1 and some related data Laboratory
A B C D A* D*
KNO 3 c.f.c (M )
f* (mV )
1.1×10−1 1.01×10−1 2.0×10−1 0.7×10−1
−40 −39.4 −28 −41
Mg(NO ) 32 c.f.c (M ) 4.2×10−3 4.6×10−3 2.0×10−2 5.4×10−3 5.6×10−2 1.05×10−3
f* (mV ) −35 −20.9 −23 −35 −15
La(NO ) 33 c.f.c (M )
f* (mV )
1.8×10−4 2.2×10−4 3.0×10−4 1.9×10−4
−19 −3.7 −11 −14
f*—Zeta potential at c.f.c. C—Data measured by diluted dispersed system. A*—Data measured by kinetic method. D*—Data measured by dynamic light scattering technique.
using PSSNa latices as a standard have been conducted. The electrophoretic mobility measurements in this work were carried out by using a Zeecom IP-120B f-potential analyzer (Japan). The apparatus performs automatic tracking of the particles in the center of a 10 cm long electrophoretic cell. The particle velocities were measured by frequently changing the direction of the field to minimize possible errors from cell leakage. At least 50 different particles were counted in each measurement. The apparent electrophoretic mobility (U ) of app an unknown colloid sample is always the sum of two contributions, one of which is the real electrophoretic mobility (U ) and the other the liquid el flow velocity induced by the electroosmosic effect (U ) of the cell wall, which changes as a parabolic osm function of the cell depth: =U +U , el osm U 3h2 −1 , U = 0 osm 2 b2 U
app
A
B
(1) (2)
where b is the half-thickness of the cell and U is 0 the electroosmotic flow at the cell wall (h=b). Similarly, the apparent velocity of the reference sample (U∞ ) was also indicated by the sum of app the real electrophoretic mobility (U∞ ) and the el electroosmotic flow velocity (U∞ ), i.e. osm U∞ =U∞ +U∞ . app el osm
(3)
Under the same experimental condition using a finite electrophoretic cell (U =U∞ ) the osm osm following relation has been held from Eqs. (2) and (3): U −U∞ =U −U∞ . el el app app
(4)
Eq. (4) indicates that if U∞ is known exactly, el the U -value of unknown sample can be deterel mined from difference between the two apparent mobilities at any cell depth. So, if the particle mobility of the unknown sample is determined at the one-half depth in the cell, the real electrophoretic mobility is given at the maximum of the
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particle velocity as the function of the cell depth. U −U∞ =U (maximum)−U∞ (maximum). el el app app (5) Fig. 2 shows an example indicating the electrophoretic mobility profiles obtained experimentally for the reference sample (PSSNa latices) and an unknown sample (SM latices) along the cell depth in 1×10−3 M KCl solution at 25°C. SM latices employed as an unknown sample were prepared by copolymerization of styrene with 5% methacrylic acid at 70°C. It is apparent that both profiles indicate reasonable parabolic curves and the curve for the reference latices shows a constant mobility at the two stationary levels. Furthermore, the difference between the two apparent mobilities at the cell center agrees well with the velocity of the SM latices at the stationary level. Fig. 3 shows the f-potential versus pH curves for the SM latices which have been determined from the maximum mobilities using the PSSNa latices as a standard. In Fig. 3, the same relation
Fig. 2. Examples of electrophoretic mobility profiles of PSSNa latices (U∞) and SM latices (U ). (h*) stationary level; (%) PSSNa latices; (#) SM latices, (1×10−3M KCl, 25°C ).
Fig. 3. f-potential versus pH curves of unknown sample (SM latices) determined by the maximum velocity of reference sample ($) and the usual method (#).
obtained from the velocities of the SM latices at the stationary level are also indicated. As can be seen, both curves agree fairly well over the whole pH range. All of these results indicate that if we have a reliable colloid sample whose f-potential is exactly determined, the f-potential of unknown sample can be determined precisely from the measurements of apparent electrophoretic mobility at the cell center. In that case, slight errors in focusing, i.e. errors due to the depth of view field are less important, since the velocity gradient near the level of observation is very small. According to Eq. (2), the f-potential of cell wall is also determined by means of the plane interface technique, which involves by establishing the liquid flow velocity-depth profile using a reference sample. The electroosmotic velocity (U ) obtained 0 by extrapolation of the velocity profile to the cell wall permits determination of the f-potential of the cell wall–solution interface, and the f-potential measurement of various solid–solution interfaces [8] including the dissimilar cell system [9] has been conducted extensively. Here, we would like to emphasize again that the determination of f-potential of the cell wall can be also possible from the maximum velocity of reference sample, instead of the tedious plane interface procedure. According to Eq. (1), U∞ =U∞ −U∞ /2 at h=0, i.e. from the measured app el 0 apparent velocity of the reference sample at the cell center (U∞ /2), the f-potential of the cell wall 0
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extrapolation of the liquid flow velocity at the cell wall. It was found that both f-potential series determined by the two methods agree well with each other over a wide range of pH, and realized that new procedure using U∞ is also useful to 0 determine the f-potential of solid–solution interface.
4. Synthesis of a new reference sample without any electrostatic effect
Fig. 4. Apparent flow velocity profile of standard latex sample at various pH: ($) pH=10.24; (6) pH=8.04; (%) pH=6.47; (&) pH=5.00; (#) pH=4.01 (1×10−3 M KCl, 25°C ).
can be quickly determined, if the U∞ is preel viously known. Fig. 4 shows some examples of apparent flow velocity profile of standard latex sample (PSSNa latices) at various pH values in which the both boundaries refer to the glass–solution interface. A symmetrical parabola was given at all pH conditions where the surface charge of glass is consistent with both sides. In Fig. 5, open circles refer to f-potential of the cell wall–solution interfaces which were determined from the maximum velocity of reference sample. On the other hand, full circles in the same figure indicate the results which were obtained by
Fig. 5. f-potential versus pH curves of cell wall–solution interface: (#) determined by the maximum velocity of PSSNA latices; ($) determined by the plane interface technique.
It will be realized easily that if the electrophoretic measurement would be carried out by using a reference sample without any electrostatic effect as a standard, the electrophoretic mobility of unknown sample can be determined by just subtracting the mobility of the new reference sample at the cell center. Additionally, the fpotential of the cell wall can be quickly determined from the measured electrophoretic mobility at the cell center, because U of this sample is zero. el As our final goal, syntheses of new reference particles which are stabilized sterically in an aqueous medium without any electrostatic effect have been studied under the various experimental conditions. It has been reported that adsorption behavior of hydroxypropylcellulose (HPC ) with a lower critical solution temperature (LCST ) depends significantly on the adsorption temperature [10]. In Fig. 6 the molecular structure of HPC and its molecular weights of HPC-H, HPC-M,and HPC-L are indicated. The maximum adsorption (A ) of m HPC at LCST becomes a few times as large as the values at room temperature. Furthermore, the high A values obtained at the LCST have been m
Fig. 6. Molecular structure of hydroxypropylcellulose (HPC ).
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maintained for a long period of time at room temperature, and the dense adsorption layer of HPC shows a strong protective action against the flocculation of the particles [11]. These results suggest that the dense (or thick) adsorption layer of HPC plays a role in the synthesis of a new reference sample for determination of the f-potential of other colloid systems. Special attention has been given to the synthesis of reference particles which are covered with the HPC layer as densely as possible and display a zero f-potential under a variety of solution conditions. Concerning the adsorption behavior of HPC to the latex surface, it has been established that the behavior is influenced remarkably by the surface nature of the original latices used: the lower the surface charge density of the particles (i.e. the stronger the hyrdrophobic nature of the surface), the higher the adsorption amount [12]. The same trend can be detected in Fig. 7, where the amounts of K S O ( KPS) used as an initiator in the latex 2 2 8 polymerization are plotted against the saturated amounts of adsorbed HPC for the respective latex surfaces. The surface charge density (s ) becomes 0 higher with increasing KPS concentration under the same polymerization conditions. The adsorption treatments have been carried out as follows. The fresh polystyrene latices prepared by the Kotera–Furusawa–Takeda method [4] were treated by HPC-M (or -H and -L) solutions (0.05–0.08 wt.%) at the LCST (45–50°C ) for 2 h and the amounts of HPC adsorption on the latex surfaces were detennined calorimetrically
Fig. 7. Relation between the concentration of KPS and the saturated amount of HPC-H at the LCST.
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from depletion of the solution [13]. Then, to complete the HPC adsorption, one portion of the dispersion was heated in an oil bath held at 50–80°C by rotating the adsorption tubes for another 20 h. After that, the residual HPC remaining in the medium was removed completely by repeating the ‘‘centrifugation–decantation– redispersion’’ process several times. In Fig. 8, the experimental results of the second adsorption (heating) process under the different temperature regimes are indicated, where the residual amounts of HPC dissolved in each solution are plotted against the duration of the heating process as the temperature was raised to different upper limits (50–80°C ) and subsequently reduced to room temperature (25°C ). As can be seen from Fig. 8, as the temperature is raised the residual concentration of HPC gradually decreases with the lapse of time and attains final equilibrium values. Furthermore, the final concentrations of HPC maintain the same values, respectively, after the temperature in each system is reduced to 25°C. These results indicate that raising the medium temperature contributes to a reduction in the concentration of HPC remaining in the solution, and that the thick (or dense) adsorbed layers of HPC built up at elevated temperatures are maintained on the latex surface after reducing the temperatures to 25°C. Fig. 9 shows the relationship between the fpotentials and the adsorption amounts of the HPC-coated latices, which were picked out at different time intervals from the adsorption vessels in the heating process. It is apparent that the negative f-potential of the latex suspension decreases with increasing HPC adsorption amounts and finally attains a real zero when the adsorption amount exceeds 3.0 mg m−2 as seen in Fig. 9. Furthermore, Fig. 9 indicates that such a high adsorption amount can be achieved easily with a high medium temperature, which will be based on the solvency of the medium, i.e. a reduction in the solvency leads to an increased adsorption on the latex surface. It is expected that the adsorption layer of HPC formed in the high medium temperature is so dense that the permittivity of the layer will be nearly zero and brings about a zero f-potential of the composite [14].
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Fig. 8. Residual concentrations of HPC (solid lines) and temperatures of the medium (dotted lines) as a function of time lapsed during temperature raising and reducing. Heating temperature of the medium: ($) 50°C; (c) 60°C; (#) 70°C; (%) 80°C. Latex sample, diameter=401 nm, s =0.6 mC cm−2. 0
Now, according to Eqs. (2) and (4), the apparent electrophoretic velocity of the new reference sample directly indicates the liquid flow velocity (U =U ), because the reference particles are ref osm suspended without any electrostatic effects. This fact allows us to determine the U of the unknown el sample by just subtracting the mobility of reference
Fig. 9. Relationship between the f-potential of HPC-coated latices and the amount of adsorption of HPC. Adsorption temperature: ($) 50°C; (6) 60°C; (#) 70°C; (%) 80°C. Latex sample, Diameter=380 nm, s =0.6 mC cm−2. 0
Fig. 10. Electrophoretic mobility profile obtained by using the new reference sample (%) and unknown sample (#) (1×10−3 M KCl, 25°C ).
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particles from the U of unknown sample at the app same level, i.e. U =U (maximum)−U (maximum). (6) el app ref Fig. 10 shows an example indicating the electrophoretic mobility profile obtained by using the new reference sample and unknown sample (SM latices) along the cell depth in a 1×10−3 M KCl solution. It is apparent that both profiles indicate parabolic curves and the curve for the new reference sample shows zero mobility at the two stationary levels. Furthermore, the difference between the two apparent mobilities at the cell center agrees with the velocity of the SM latices at the stationary levels. All these results indicate that the new reference sample synthesized here is suspended without any electrostatic effects, and serves as a good standard in the determination of the f-potential of the other colloid systems. Figs. 11 and 12 show the results of the f-potential for amphoteric latices and negative AgI sol determined by using the new reference sample and by the usual technique. It was found that both fpotential series determined by the two methods agree very well with each other over the wide range of pH and KI concentrations. For extensive application of the reference
Fig. 11. Determination of the f-potential of amphoteric latices in 1×10−4 M KCl: (#) new technique, (6) usual technique.
Fig. 12. Determination of the f-potential of AgI sol in various KI concentrations: (#) new technique, (6) usual technique.
Fig. 13. Electrophoretic mobilities at stationary level of the new reference samples treated by the different HPCs: (A) electrophoretic mobilities versus KCI concentration curves; (B) electrophoretic mobilities versus pH curves: ($) HPC-H, (%) HPC-M, (6) HPC-L.
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sample, the stability of the particles is manifested in a few specific examples. In Fig. 13, the mobility of the reference samples at the stationary level is plotted against pH and electrolyte concentration, where the each sample has been incubated in the respective conditions for 24 h at 25°C. It is evident that over the entire range all series show nearly zero mobility within experimental errors and the zero mobility is held, especially in the case of reference sample treated by HPC-M. These results convince us to utilize the reference sample in many fields of colloid science and technology, because the f-potential in the new technique can be given at the maximum of the parabolic velocity, as a function of the cell depth. In that way, errors in determining f-potential are minimized and the measurements of f-potential can do exactly over the wide fields of colloid systems.
The real advantage of new method may lie in the determination of the electrical properties of a solid surface. According to Eq. (1), U (=U )= osm ref −U /2 at h=0; i.e. from the apparent velocity of 0 the new reference sample measured at the cell center, the f-potential of cell wall can be directly determined from the U . Fig. 14 shows the f0 potential versus KCl (conc.) and pH curves of the cell wall measured directly by the usual plane interface technique and our new technique using the electrically neutral sample. As can be seen, both f-potential series determined by the two methods agree very well with each other. Our new method will be more convenient than those by the conventional plane interface technique or streaming potential measurements.
References [1] K. Furusawa, Q. Chen, N. Tobori, J. Colloid Interface Sci. 137 (1990) 456. [2] A. Kitahara, A. Watanabe, Electrical Phenomena at Interface, Dekker, New York, 1984, p. 185. [3] Japanese Surface and Colloid Chemist Group (Japan Oil Chemist’s Society), Yukagaku, 25 (1976) 239. [4] A. Kotera, K. Furusawa, Y. Takeda, Kolloid Z. u. Z. Polymere 239 (1970) 677. [5] K. Furusawa, S. Usui, M. Ozaki, K. Konno, A. Kitahara, Yukagaku 37 (1988) 632. [6 ] M.S. Juang, I.M. Kriegcr, J. Polym. Sci. 14 (1976) 2089. [7] A. Homola, R.O. James, J. Colloid Interface Sci. 59 (1977) 123. [8] H. Sasaki, A. Muramatsu, H. Arakatsu, S. Usui, J. Colloid Interface Sci. 142 (1991) 266. [9] S. Usui, Y. Imamura, H. Sasaki, J. Colloid Interface Sci. 118 (1987) 335. [10] K. Furusawa, Y. Kimura, T. Tagawa, in ACS Symposium Series No. 240, E.D. Goddard, B. Vincent ( Eds.), American Chemistry Society, New York, 1984, p. 131. [11] T. Tagawa, S. Yamashita, K. Furusawa, Kobunshi Ronbunshu 40 (1983) 273. [12] K. Furusawa, T. Tagawa, Colloid Polym. Sci. 263 (1985) 1. [13] M. Dubois, K.E. Gillcs, J.K. Hamilton, P.A. Smith, F. Smith, Anal. Chem. 28 (1956) 350. [14] H. Ohshima, T. Kondo, J. Colloid Interface Sci. 116 (1990) 456.
Fig. 14. f-potential versus KCI concentration (A) and pH (B) curves of cell wall–solution interface: (#) determined by maximum velocity of HPC coated-latices; (6) determined by the plane interface technique.
Collaborative studies of zeta-potential measurements and electrophoretic measurements using reference sample K. Furusawa *, K. Uchiyama Department of Chemistry, University of Tsukuba, 1-1-1 Ten-nodai, Ibaraki 305, Japan Received 10 February 1997; accepted 9 April 1997
Abstract Simultaneous measurements of zeta-potential for two standard latex suspensions were carried out so as to assess the reliability of each of these measurement techniques and find means for their improvement. Furthermore, syntheses of a reference particle dispersion stabilized sterically in an aqueous medium without any electrostatic effects and measurements of zeta-potential using the reference dispersion as a standard were performed under various experimental conditions. It became apparent that the dense adsorption layer of hydroxypropylcellulose (HPC ), with a lower critical solution temperature (LCST ), formed on latex particles with a low surface charge density at temperatures higher than the LCST, plays a role in completely shielding the electrostatic effect arising from the surface charge on the bare particles. Such reference particles with zero zeta-potential allow us to determine the electrophoretic mobility of unknown samples at the one-half depth in the electrophoretic cell by subtracting the mobility of the reference sample at the same level. Furthermore, the zeta-potential of the cell wall can be easily determined from the mobility of the reference sample, because the apparent velocity profile of the reference sample indicates the liquid flow velocity in the cell. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Electroosmosis; Electrophoretic mobility; Hydroxypropylcellulose; Zeta potential
1. Introduction The zeta(f)-potential is the potential at the slipping plane in the electrical double layer and its value is not precisely the same as that of the surface potential. In spite of the uncertainty of its meaning and ambiguous character, the f-potential has frequently been used for the discussion of colloid stability and its value is still considered useful in relation to the electrical double layer. The most familiar method for determining the f-potential is electrophoresis, which consists of setting up a potential gradient in the solutions. * Corresponding author. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 28 0 - X
The electrophoretic migration of colloid particles is always superimposed on the electroosmotic liquid flow from the cell wall and the apparent particle velocity observed coincides with the true electrophoretic mobility only at two special depths, known as the stationary levels. Unfortunately, however, the velocity gradient of the liquid at the stationary levels is usually large, and thus the observed velocity of the particles changes rapidly with cell depth so that errors in electrophoretic mobility may be substantial. However, if the electrophoretic measurements will be carried out by using some reference sample as a standard, the electrophoretic mobility of the unknown sample can be determined at any cell depth by subtracting the mobility of the reference
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particles at the same level, because the liquid flow velocity induced by the electroosmotic effect of the cell wall becomes to same value under the same experimental condition. So, if the electrophoretic mobility would be detected at the one half depth in the cell, the real electrophoretic mobility is also given at the maximum of the parabolic velocity as the function of the cell depth [1]. In that time, slight errors in focusing are less important than the usual measurements at stationary level and the rotational effect of the particles is minimized, since the velocity gradient is small at the cell center. In this work, we first report the results of the collaborative studies of zeta potential measurements to seek a suitable standard sample, and secondly a new electrophoretic technique using a reference sample as a standard is introduced. We also show that this new technique using a reference sample is also convenient to determine the fpotential of the cell wall. In the final part of this work, we report on the syntheses of a new reference dispersion stabilized sterically in an aqueous medium without any electrostatic effects and the f-potential measurements using the new dispersion as a standard [2]. Special attention has been given to synthesis of the reference particles which are covered with the polymer layer as densely as possible and display a zero f-potential under the variety of solution conditions.
2. Collaborative study of zeta-potential using standard sample In 1970, a group of Japanese surface and colloid chemists who had engaged in the study of electrokinetic phenomena and/or colloid stability, formed a committee under the Division of Surface Chemistry in the Japan Oil Chemist’s Society. This group measured, compared and discussed zetapotentials from such samples as titanium oxide, microcapsule, silica and some polymer latices or silver iodide [3]. Table 1 indicates some examples of simultaneous measurements of zeta-potential for four samples. All these measurements have been carried out by the micro-electrophoretic technique using the respective electrophoretic apparatus belonging to each member laboratory. Usually,
the mobility measurements were performed at a constant field strength of 4–5 V cm−1 using a rectangular glass (or quartz) cell and the f-potential was calculated by the Smoluchowski equation. As can be seen from this table, the collaborative results for titanium oxide and microcapsule dispersions do not agree with each other and especially, the data for the microcapsule dispersion indicated a large deviation within some members. On the other hand, the f-potential measured for polystyrene latices which were prepared under the surfactant-free system by the Kotera–Furusawa–Takeda method [4] and silver iodide sol prepared by the usual way, were very similar among those tested and were reproducible. As the next stage of collaborative study, the simultaneous measurements of f-potential and critical flocculation concentration (c.f.c.) for two standard latex suspensions (samples 1 and 2) have been conducted [5] so as to assess the reliability of each of these measurement techniques and find means for their improvement. Sample 1 is a negative styrene/styrene sulphonate copolymer latices which were prepared according to the Krieger method [6 ]. Sample 2 is the amphoteric latices which were synthesized by the Homola method [7]. These latex samples were employed after fairly purification by the ion exchange treatment and dialysis against distilled water. The f-potential measurements were carried out at nine laboratories using different electrophoretic apparatus, which included the Rank Brother M-2, PEN KEM-501, PEN KEM-3000 and Malvern Zetasizer-F. As can be seen from Fig. 1 and Table 2, the data from all the laboratories showed fairly good agreement and displayed possible means for their improvement. (1) Elevated potential supply (PEN KEM-3000 and Malvern Zetasizer-F ) was found to increase the slope of the f versus pH curve near the isoelectric point of the sample 2 (2) Differences between the f-potential values reported from each laboratory indicate a constant deviation over the whole pH range, i.e. data G shows relatively high values, while data A indicates low values over the whole pH region. This tendency suggests that the difference between data G and A will be based
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Table 1 Results of collaborative studies of f-potential measurements for various samples (1) Titanium oxide Laboratory Method f (mV ) (2) Polystyrene latices ( EP) Laboratory f (mV ) (3) Microcapsule (EP) Laboratory f (mV ) (4) AgI sol (EP) Laboratory f (mV ) (pAg3) f (mV ) (pI4) (5) Agl sol(EP) Laboratory U (mm cm−1 V−1 s−1) f (mV )
(A) EP −24.8
(B) SP −14.8
(C ) SP −8
(D) SP −21.0
(A) −48
(B) −42.5–46
(C ) −47
(D) −42.3
(A) −27.0
(B) −26.1
(C ) −70
(D) −35.4
(A) +50
(B)
(C )
−47.5–51
−45
(D) +66.4
(B) −3.8 −46
(C ) −3.67 −44.4
(A) −3.675 −44.5
(E ) EP −35.72
(F) SP −25
on the same reason, which can be eliminated completely by using a definite standard sample. The c.f.c. of KNO , Mg(NO ) and La(NO ) 3 32 33 for sample 1, were determined simultaneously by a static method and the results were compared with those determined by a kinetic method and dynamic light scattering (L.S.). These results for the critical flocculation concentration and some related data are indicated in Table 3 [5]. The results obtained from the four laboratories by a static method agreed fairly well and the order of the magnitude of the c.f.c. values determined by L.S. technique and kinetic method can be expressed as follows: static
3. Electrophoretic measurements using PSSNalatices as a standard
Fig. 1. f-potential versus pH curves for samples 1 and 2. (A) Rank brother M-2; (B) PEN KEM-500; (C ) Rank Brother M-2; (D,E ) Laser Doppler method; ( F ) PEN KEM-3000; (H ) Malvern Zetasizer-F.
From the above collaborative measurements on the zeta-potential, it was realized that in determination of the f-potential from an unknown sample, electrophoretic measurements using a reference sample as a standard is very useful to determine the reliable data. Next, f-potential measurements
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Table 2 f-potentials of sample 1 and isoelectric points (pH0) of sample 2 Laboratory Sample 1 (f, mV ) Sample 2
pH=4 pH=10 pH0
A
B
C
D
E
F
G
H
I
−44.0 −44.0 8.0
−45.0 −45.0 6.6
−48.0 −48.0 7.8
−57.7 −60.5 6.8
−58.0 −58.0 7.1
−53.0 −54.0 7.6
−62.0 −65.0 8.2
−59.0 −62.0 7.8
−44.5 −46.0 6.5
Table 3 Critical flocculation concentration (c.f.c) for sample 1 and some related data Laboratory
A B C D A* D*
KNO 3 c.f.c (M )
f* (mV )
1.1×10−1 1.01×10−1 2.0×10−1 0.7×10−1
−40 −39.4 −28 −41
Mg(NO ) 32 c.f.c (M ) 4.2×10−3 4.6×10−3 2.0×10−2 5.4×10−3 5.6×10−2 1.05×10−3
f* (mV ) −35 −20.9 −23 −35 −15
La(NO ) 33 c.f.c (M )
f* (mV )
1.8×10−4 2.2×10−4 3.0×10−4 1.9×10−4
−19 −3.7 −11 −14
f*—Zeta potential at c.f.c. C—Data measured by diluted dispersed system. A*—Data measured by kinetic method. D*—Data measured by dynamic light scattering technique.
using PSSNa latices as a standard have been conducted. The electrophoretic mobility measurements in this work were carried out by using a Zeecom IP-120B f-potential analyzer (Japan). The apparatus performs automatic tracking of the particles in the center of a 10 cm long electrophoretic cell. The particle velocities were measured by frequently changing the direction of the field to minimize possible errors from cell leakage. At least 50 different particles were counted in each measurement. The apparent electrophoretic mobility (U ) of app an unknown colloid sample is always the sum of two contributions, one of which is the real electrophoretic mobility (U ) and the other the liquid el flow velocity induced by the electroosmosic effect (U ) of the cell wall, which changes as a parabolic osm function of the cell depth: =U +U , el osm U 3h2 −1 , U = 0 osm 2 b2 U
app
A
B
(1) (2)
where b is the half-thickness of the cell and U is 0 the electroosmotic flow at the cell wall (h=b). Similarly, the apparent velocity of the reference sample (U∞ ) was also indicated by the sum of app the real electrophoretic mobility (U∞ ) and the el electroosmotic flow velocity (U∞ ), i.e. osm U∞ =U∞ +U∞ . app el osm
(3)
Under the same experimental condition using a finite electrophoretic cell (U =U∞ ) the osm osm following relation has been held from Eqs. (2) and (3): U −U∞ =U −U∞ . el el app app
(4)
Eq. (4) indicates that if U∞ is known exactly, el the U -value of unknown sample can be deterel mined from difference between the two apparent mobilities at any cell depth. So, if the particle mobility of the unknown sample is determined at the one-half depth in the cell, the real electrophoretic mobility is given at the maximum of the
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particle velocity as the function of the cell depth. U −U∞ =U (maximum)−U∞ (maximum). el el app app (5) Fig. 2 shows an example indicating the electrophoretic mobility profiles obtained experimentally for the reference sample (PSSNa latices) and an unknown sample (SM latices) along the cell depth in 1×10−3 M KCl solution at 25°C. SM latices employed as an unknown sample were prepared by copolymerization of styrene with 5% methacrylic acid at 70°C. It is apparent that both profiles indicate reasonable parabolic curves and the curve for the reference latices shows a constant mobility at the two stationary levels. Furthermore, the difference between the two apparent mobilities at the cell center agrees well with the velocity of the SM latices at the stationary level. Fig. 3 shows the f-potential versus pH curves for the SM latices which have been determined from the maximum mobilities using the PSSNa latices as a standard. In Fig. 3, the same relation
Fig. 2. Examples of electrophoretic mobility profiles of PSSNa latices (U∞) and SM latices (U ). (h*) stationary level; (%) PSSNa latices; (#) SM latices, (1×10−3M KCl, 25°C ).
Fig. 3. f-potential versus pH curves of unknown sample (SM latices) determined by the maximum velocity of reference sample ($) and the usual method (#).
obtained from the velocities of the SM latices at the stationary level are also indicated. As can be seen, both curves agree fairly well over the whole pH range. All of these results indicate that if we have a reliable colloid sample whose f-potential is exactly determined, the f-potential of unknown sample can be determined precisely from the measurements of apparent electrophoretic mobility at the cell center. In that case, slight errors in focusing, i.e. errors due to the depth of view field are less important, since the velocity gradient near the level of observation is very small. According to Eq. (2), the f-potential of cell wall is also determined by means of the plane interface technique, which involves by establishing the liquid flow velocity-depth profile using a reference sample. The electroosmotic velocity (U ) obtained 0 by extrapolation of the velocity profile to the cell wall permits determination of the f-potential of the cell wall–solution interface, and the f-potential measurement of various solid–solution interfaces [8] including the dissimilar cell system [9] has been conducted extensively. Here, we would like to emphasize again that the determination of f-potential of the cell wall can be also possible from the maximum velocity of reference sample, instead of the tedious plane interface procedure. According to Eq. (1), U∞ =U∞ −U∞ /2 at h=0, i.e. from the measured app el 0 apparent velocity of the reference sample at the cell center (U∞ /2), the f-potential of the cell wall 0
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extrapolation of the liquid flow velocity at the cell wall. It was found that both f-potential series determined by the two methods agree well with each other over a wide range of pH, and realized that new procedure using U∞ is also useful to 0 determine the f-potential of solid–solution interface.
4. Synthesis of a new reference sample without any electrostatic effect
Fig. 4. Apparent flow velocity profile of standard latex sample at various pH: ($) pH=10.24; (6) pH=8.04; (%) pH=6.47; (&) pH=5.00; (#) pH=4.01 (1×10−3 M KCl, 25°C ).
can be quickly determined, if the U∞ is preel viously known. Fig. 4 shows some examples of apparent flow velocity profile of standard latex sample (PSSNa latices) at various pH values in which the both boundaries refer to the glass–solution interface. A symmetrical parabola was given at all pH conditions where the surface charge of glass is consistent with both sides. In Fig. 5, open circles refer to f-potential of the cell wall–solution interfaces which were determined from the maximum velocity of reference sample. On the other hand, full circles in the same figure indicate the results which were obtained by
Fig. 5. f-potential versus pH curves of cell wall–solution interface: (#) determined by the maximum velocity of PSSNA latices; ($) determined by the plane interface technique.
It will be realized easily that if the electrophoretic measurement would be carried out by using a reference sample without any electrostatic effect as a standard, the electrophoretic mobility of unknown sample can be determined by just subtracting the mobility of the new reference sample at the cell center. Additionally, the fpotential of the cell wall can be quickly determined from the measured electrophoretic mobility at the cell center, because U of this sample is zero. el As our final goal, syntheses of new reference particles which are stabilized sterically in an aqueous medium without any electrostatic effect have been studied under the various experimental conditions. It has been reported that adsorption behavior of hydroxypropylcellulose (HPC ) with a lower critical solution temperature (LCST ) depends significantly on the adsorption temperature [10]. In Fig. 6 the molecular structure of HPC and its molecular weights of HPC-H, HPC-M,and HPC-L are indicated. The maximum adsorption (A ) of m HPC at LCST becomes a few times as large as the values at room temperature. Furthermore, the high A values obtained at the LCST have been m
Fig. 6. Molecular structure of hydroxypropylcellulose (HPC ).
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maintained for a long period of time at room temperature, and the dense adsorption layer of HPC shows a strong protective action against the flocculation of the particles [11]. These results suggest that the dense (or thick) adsorption layer of HPC plays a role in the synthesis of a new reference sample for determination of the f-potential of other colloid systems. Special attention has been given to the synthesis of reference particles which are covered with the HPC layer as densely as possible and display a zero f-potential under a variety of solution conditions. Concerning the adsorption behavior of HPC to the latex surface, it has been established that the behavior is influenced remarkably by the surface nature of the original latices used: the lower the surface charge density of the particles (i.e. the stronger the hyrdrophobic nature of the surface), the higher the adsorption amount [12]. The same trend can be detected in Fig. 7, where the amounts of K S O ( KPS) used as an initiator in the latex 2 2 8 polymerization are plotted against the saturated amounts of adsorbed HPC for the respective latex surfaces. The surface charge density (s ) becomes 0 higher with increasing KPS concentration under the same polymerization conditions. The adsorption treatments have been carried out as follows. The fresh polystyrene latices prepared by the Kotera–Furusawa–Takeda method [4] were treated by HPC-M (or -H and -L) solutions (0.05–0.08 wt.%) at the LCST (45–50°C ) for 2 h and the amounts of HPC adsorption on the latex surfaces were detennined calorimetrically
Fig. 7. Relation between the concentration of KPS and the saturated amount of HPC-H at the LCST.
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from depletion of the solution [13]. Then, to complete the HPC adsorption, one portion of the dispersion was heated in an oil bath held at 50–80°C by rotating the adsorption tubes for another 20 h. After that, the residual HPC remaining in the medium was removed completely by repeating the ‘‘centrifugation–decantation– redispersion’’ process several times. In Fig. 8, the experimental results of the second adsorption (heating) process under the different temperature regimes are indicated, where the residual amounts of HPC dissolved in each solution are plotted against the duration of the heating process as the temperature was raised to different upper limits (50–80°C ) and subsequently reduced to room temperature (25°C ). As can be seen from Fig. 8, as the temperature is raised the residual concentration of HPC gradually decreases with the lapse of time and attains final equilibrium values. Furthermore, the final concentrations of HPC maintain the same values, respectively, after the temperature in each system is reduced to 25°C. These results indicate that raising the medium temperature contributes to a reduction in the concentration of HPC remaining in the solution, and that the thick (or dense) adsorbed layers of HPC built up at elevated temperatures are maintained on the latex surface after reducing the temperatures to 25°C. Fig. 9 shows the relationship between the fpotentials and the adsorption amounts of the HPC-coated latices, which were picked out at different time intervals from the adsorption vessels in the heating process. It is apparent that the negative f-potential of the latex suspension decreases with increasing HPC adsorption amounts and finally attains a real zero when the adsorption amount exceeds 3.0 mg m−2 as seen in Fig. 9. Furthermore, Fig. 9 indicates that such a high adsorption amount can be achieved easily with a high medium temperature, which will be based on the solvency of the medium, i.e. a reduction in the solvency leads to an increased adsorption on the latex surface. It is expected that the adsorption layer of HPC formed in the high medium temperature is so dense that the permittivity of the layer will be nearly zero and brings about a zero f-potential of the composite [14].
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Fig. 8. Residual concentrations of HPC (solid lines) and temperatures of the medium (dotted lines) as a function of time lapsed during temperature raising and reducing. Heating temperature of the medium: ($) 50°C; (c) 60°C; (#) 70°C; (%) 80°C. Latex sample, diameter=401 nm, s =0.6 mC cm−2. 0
Now, according to Eqs. (2) and (4), the apparent electrophoretic velocity of the new reference sample directly indicates the liquid flow velocity (U =U ), because the reference particles are ref osm suspended without any electrostatic effects. This fact allows us to determine the U of the unknown el sample by just subtracting the mobility of reference
Fig. 9. Relationship between the f-potential of HPC-coated latices and the amount of adsorption of HPC. Adsorption temperature: ($) 50°C; (6) 60°C; (#) 70°C; (%) 80°C. Latex sample, Diameter=380 nm, s =0.6 mC cm−2. 0
Fig. 10. Electrophoretic mobility profile obtained by using the new reference sample (%) and unknown sample (#) (1×10−3 M KCl, 25°C ).
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particles from the U of unknown sample at the app same level, i.e. U =U (maximum)−U (maximum). (6) el app ref Fig. 10 shows an example indicating the electrophoretic mobility profile obtained by using the new reference sample and unknown sample (SM latices) along the cell depth in a 1×10−3 M KCl solution. It is apparent that both profiles indicate parabolic curves and the curve for the new reference sample shows zero mobility at the two stationary levels. Furthermore, the difference between the two apparent mobilities at the cell center agrees with the velocity of the SM latices at the stationary levels. All these results indicate that the new reference sample synthesized here is suspended without any electrostatic effects, and serves as a good standard in the determination of the f-potential of the other colloid systems. Figs. 11 and 12 show the results of the f-potential for amphoteric latices and negative AgI sol determined by using the new reference sample and by the usual technique. It was found that both fpotential series determined by the two methods agree very well with each other over the wide range of pH and KI concentrations. For extensive application of the reference
Fig. 11. Determination of the f-potential of amphoteric latices in 1×10−4 M KCl: (#) new technique, (6) usual technique.
Fig. 12. Determination of the f-potential of AgI sol in various KI concentrations: (#) new technique, (6) usual technique.
Fig. 13. Electrophoretic mobilities at stationary level of the new reference samples treated by the different HPCs: (A) electrophoretic mobilities versus KCI concentration curves; (B) electrophoretic mobilities versus pH curves: ($) HPC-H, (%) HPC-M, (6) HPC-L.
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sample, the stability of the particles is manifested in a few specific examples. In Fig. 13, the mobility of the reference samples at the stationary level is plotted against pH and electrolyte concentration, where the each sample has been incubated in the respective conditions for 24 h at 25°C. It is evident that over the entire range all series show nearly zero mobility within experimental errors and the zero mobility is held, especially in the case of reference sample treated by HPC-M. These results convince us to utilize the reference sample in many fields of colloid science and technology, because the f-potential in the new technique can be given at the maximum of the parabolic velocity, as a function of the cell depth. In that way, errors in determining f-potential are minimized and the measurements of f-potential can do exactly over the wide fields of colloid systems.
The real advantage of new method may lie in the determination of the electrical properties of a solid surface. According to Eq. (1), U (=U )= osm ref −U /2 at h=0; i.e. from the apparent velocity of 0 the new reference sample measured at the cell center, the f-potential of cell wall can be directly determined from the U . Fig. 14 shows the f0 potential versus KCl (conc.) and pH curves of the cell wall measured directly by the usual plane interface technique and our new technique using the electrically neutral sample. As can be seen, both f-potential series determined by the two methods agree very well with each other. Our new method will be more convenient than those by the conventional plane interface technique or streaming potential measurements.
References [1] K. Furusawa, Q. Chen, N. Tobori, J. Colloid Interface Sci. 137 (1990) 456. [2] A. Kitahara, A. Watanabe, Electrical Phenomena at Interface, Dekker, New York, 1984, p. 185. [3] Japanese Surface and Colloid Chemist Group (Japan Oil Chemist’s Society), Yukagaku, 25 (1976) 239. [4] A. Kotera, K. Furusawa, Y. Takeda, Kolloid Z. u. Z. Polymere 239 (1970) 677. [5] K. Furusawa, S. Usui, M. Ozaki, K. Konno, A. Kitahara, Yukagaku 37 (1988) 632. [6 ] M.S. Juang, I.M. Kriegcr, J. Polym. Sci. 14 (1976) 2089. [7] A. Homola, R.O. James, J. Colloid Interface Sci. 59 (1977) 123. [8] H. Sasaki, A. Muramatsu, H. Arakatsu, S. Usui, J. Colloid Interface Sci. 142 (1991) 266. [9] S. Usui, Y. Imamura, H. Sasaki, J. Colloid Interface Sci. 118 (1987) 335. [10] K. Furusawa, Y. Kimura, T. Tagawa, in ACS Symposium Series No. 240, E.D. Goddard, B. Vincent ( Eds.), American Chemistry Society, New York, 1984, p. 131. [11] T. Tagawa, S. Yamashita, K. Furusawa, Kobunshi Ronbunshu 40 (1983) 273. [12] K. Furusawa, T. Tagawa, Colloid Polym. Sci. 263 (1985) 1. [13] M. Dubois, K.E. Gillcs, J.K. Hamilton, P.A. Smith, F. Smith, Anal. Chem. 28 (1956) 350. [14] H. Ohshima, T. Kondo, J. Colloid Interface Sci. 116 (1990) 456.
Fig. 14. f-potential versus KCI concentration (A) and pH (B) curves of cell wall–solution interface: (#) determined by maximum velocity of HPC coated-latices; (6) determined by the plane interface technique.