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Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength
Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength Su-zhen Li a,b , Ren-kou Xu a,∗ a b
State Key Laboratory of Soils and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 19 January 2008 Received in revised form 17 April 2008 Accepted 18 May 2008 Available online 24 May 2008 Keywords: Electrical double layer Overlapping of diffuse layers Zeta potential Electrokinetic charge density Goethite Hematite Gibbsite ␥-Al2 O3 Ionic strength
a b s t r a c t Phyllosilicates with net negative surface charge and Fe/Al oxides with net positive surface charge co-exist in variable charge soils and the interaction between these oppositely charged particles affects the stability of mixed colloids, aggregation, and even the surface chemical properties of variable charge soils. The overlapping of the diffuse layers of electrical double layers between kaolinite and goethite, hematite, gibbsite and ␥-Al2 O3 was investigated in this article based on the change of zeta potential and electrokinetic charge density induced by the addition of Fe/Al oxides. The results indicated that in the range of pH from 3 to 7, the kaolinite is negatively charged and the Fe/Al oxides are positively charged, and the overlapping of the diffuse layers led to the increase in zeta potential and the decrease in surface charge density of binary-system containing kaolinite and Fe/Al oxides compared with the single kaolinite system. The presence of gibbsite resulted in the strongest interaction of electrical double layers between this and kaolinite, followed by goethite and hematite. The interaction extent of electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added. The interaction also resulted in the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. The increase in ionic strength led to the decrease in the thickness of colloid diffuse layer and thus resulted in the reduction of overlapping of the diffuse layers between kaolinite and Fe/Al oxides. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Colloid particles carry electrical charge, the requirement of overall electro-neutrality of the interfacial region results in the formation of a diffuse layer of oppositely charged counter-ion adjacent to the particle surface, and thus the formation of electrical double layers. Zeta potential is the most widely used experimental approach to study the charged surfaces, through the use of electrokinetic techniques. The electrical potential at the shear plane of colloid particles is defined as the electrokinetic potential, more commonly referred as the zeta potential, which can be deduced from electrokinetic measurements. Zeta potential can provide much valuable information about the structure and properties of the electrical double layers, although the exact location of the slip surface is perhaps not ascertained accurately. Clays possess amphoteric sites which are conditionally charged, either positive or negative charges depending on pH can develop on the O faces and at the edges by direct H+ /OH− transfer from
∗ Corresponding author. Tel.: +86 25 86881183; fax: +86 25 86881000. E-mail address: [email protected] (R.-k. Xu). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.05.023
aqueous phase. This surface charge heterogeneity of clay minerals presented originally by Van Olphen [1], then supported and elaborated further in many subsequent and recent investigations [2–7], which governs the particles’ interaction in clay mineral suspensions. Although the net particle charge is negative in general, both negatively and positively charged parts on the surface of clay mineral particles exist simultaneously under acidic conditions [8]. Therefore the particles’ interactions exist extensively, even in the single type of mineral system. The interaction of dissimilar double layers between the dissimilar colloidal particles as related to mutual coagulation of colloidal dispersions (HHF theory) has been developed by Hogg et al. [9]. The theory has been used to describe the potential energy of interaction between dissimilar spherical colloidal particles and evaluate the stability of binary colloids extensively. In recent time, some reports discussed the diffuse double-layer forces between dissimilar surfaces, such as the interaction between zirconia and ␣-alumina [10], the interaction between silica colloid sphere and an alumina flat crystal [11], and the interaction between a SiO2 glass sphere and a TiO2 crystal [12]. The findings suggest that zeta potential is in good agreement with the AFM force measurement. In authentic soil system, Qafoku and Sumner proposed the hypothesis of the diffuse double-layer interaction between the negatively charged
158
S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
phyllosilicates and the positively charged Fe/Al oxides to interpret the simultaneous adsorption of cations and anions by variable ´ charge soils in dilute electrolyte [13]. Tombacz et al. found, the presence of hematite or magnetite made the zeta potential of binary-system containing montmorillonite and hematite or magnetite shift towards the positive value direction [14]. A similar phenomenon was also observed by Hou et al. [15] in binary-system containing kaolinite and Fe/Al oxides. They interpreted, the overlapping of the diffuse layers of electrical double layers on kaolinite and Fe/Al oxides decreases the effective negative charge density on the kaolinite and thus increases the zeta potential of the binarysystem containing the kaolinite and Fe/Al oxides. According to the Gouy–Chapmann double-layer theory, the surface charge density can be calculated [16]. Based on the surface charge density it is possible to quantify the interaction between oppositely charged kaolinite and Fe/Al oxides. Kaolinite and goethite, hematite, gibbsite, ␥-Al2 O3 are chosen in this article to investigate the interaction of the electrical double layers on kaolinite and Fe/Al oxides because they are the main mineral constitutes in variable charge soils. The concentration of electrolyte solution is an important factor influencing zeta potential and structure of the electrical double layers, as was demonstrated earlier with pure oxides including hematite/yttrium oxide [17], ␣-alumina [18], ␥-Al2 O3 [19], TiO2 [20], quartz [21], and SnO2 [22]. The thickness of the diffuse layer of the electrical double layers on colloid particle changes with the change in ionic strength of bulk solution. The concentration of the background electrolyte could influence the electrical double-layer interaction between oppositely charged surfaces [8]. Therefore, the objectives of this article are: (1) to investigate the overlapping of diffuse layers of the electrical double layers between oppositely charged particles based on the change of surface charge density; (2) to study the effect of ionic strength on the interaction of electrical double layers between oppositely charged particles. 2. Materials and methods 2.1. Kaolinite and Fe/Al oxides Kaolinite and ␥-Al2 O3 are commercial clay minerals obtained respectively from Shanghai Reagent Company (Shanghai, China) and Dalian Luming Light Science and Technology Co., Ltd. (Dalian, China) with the diameter <2 m. Goethite was synthesized as the procedure reported earlier [23]. The pH of a 0.5 mol L−1 Fe(NO3 )3 solution was adjusted to 12 with NaOH with stirring, and then the precipitate was aged at 60 ◦ C in the oven for 24 h. The synthetic goethite was electro-dialyzed at a potential gradient of 15 V cm−1 until a constant specific electrical conductance was achieved. Finally, the treated sample was dried at 60 ◦ C using an IR lamp and ground to pass 100-mesh sieve. Gibbsite was prepared according to the method described by Kyle et al. [24]. A 4 mol L−1 NaOH was added slowly to 1.0 mol L−1 AlCl3 solution with stirring until the pH reached 4.6, and the suspension was allowed to stand for 2 h at 40 ◦ C. The product was electro-dialyzed as the method mentioned above. The treated sample was air-dried and ground to pass 100-mesh sieve. Similarly, the hematite was synthesized according to the reported method [25]. Two hundred millilitres of 3.3 mol L−1 FeCl3 solution was added drop wise to 2.50 L of boiling de-ionized water with stirring and then the product was purified with electrodialyzed method as mentioned above. Finally, the treated sample was dried at 60 ◦ C using an IR lamp and ground to pass 100-mesh sieve.
2.2. Preparation of colloid suspensions at different pH Sodium chloride of 0.1 mmol L−1 was used as a supporting electrolyte to maintain the ionic strength and the electrical double layer thickness to be constant. The colloid suspension containing 0.1 mmol L−1 NaCl was prepared with a concentration of kaolinite or oxides 0.25 g L−1 . The suspensions were dispersed ultrasonically at a frequency of 40 kHz and a power of 300 W for 1 h at 25 ◦ C. Finally, the suspension pH was adjusted to required values with dilute HCl or NaOH solution. Then the suspensions were shaken for 2 h at 25 ◦ C. pH was checked again and adjusted if necessary. Further, the suspensions were equilibrated for 24 h before electrokinetic potential was determined, and during this period the colloid suspensions were mixed manually at irregular intervals. For the binary-colloid suspensions, kaolinite and Fe/Al oxides were placed in the same container with different mass ratios of Fe/Al oxides to kaolinite, and the total concentration of the mixture was taken 0.25 g L−1 , the rest of the procedure was followed similar to that for the single colloid suspension as mentioned above. 2.3. Preparation of colloid suspension with different electrolyte concentration at different pH The colloid suspension containing 0.1 mmol L−1 and 10 mmol L−1 NaCl solution as background electrolyte was prepared with a concentration of kaolinite or oxides 0.25 g L−1 for single colloid system. When the binary-colloid suspension was prepared, kaolinite and Fe/Al oxides were taken in the same container according to their different quantitative ratio and 0.1 mmol L−1 or 10 mmol L−1 NaCl solution was added as background electrolyte. The rest of the procedure was followed similar to the method mentioned above. 2.4. The determination of electrokinetic potential The electrokinetic potential (zeta potential) was measured using the JS94G+ microelectro-phoresis apparatus made in China. The colloid suspension was agitated and transferred to the electrophoresis vessel after wetting the electrode to avoid any disturbance due to air bubble. An average of the electrophoretic velocity was obtained with the computer by timing 10 particles, first in one direction, and then upon reversal of the polarity of the applied electrical field in the opposite direction. The values of the zeta potential were calculated using computer with a specific software based on the Helmholtz–Smoluchowski equation: =
Ku ε
(1)
where u is the electrophoretic mobility, ε the permittivity of the medium and is the viscosity of liquid phase. K is a constant relative to the shape of the colloid particle. Under the condition of this investigation, the radii of colloidal particles (a) are much bigger than the thickness of electrical double layer (1/) and a 1, so K = 4. This step was repeated 10 times and an average value of zeta potential from 10 replicates was reported in this paper. The measurement error was found to be ±2 mV and at the temperature 25 ± 0.5 ◦ C. 3. Results and discussion 3.1. Electrokinetic charge density and the electrical double-layer interaction between kaolinite and Hematite According to the Gouy–Chapmann double-layer theory, the electrokinetic charge density on the shear plane of electrical double
S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
Fig. 1. Zeta potential of kaolinite or hematite and the binary-system containing kaolinite and hematite with different mass ratios in suspension systems (3:7, 1:4, 1:9 are the mass ratio of hematite to kaolinite).
layer can be calculated based on zeta potential () with the equation given below [16]: = (8εε0 ns KT )1/2 sinh
zF 2RT
(2)
where ε is the relative dielectric permittivity (ε = 78.5 for water at 25 ◦ C), ε0 the permittivity in vacuum (8.854 × 10−14 C V−1 cm−1 ), ns the number of ion pairs, K the Boltzmann constant, z the valence of ion, R the molar gas constant (8.314 J mol−1 K−1 ), F the Faraday constant (96,487 C mol−1 ), and T is the temperature (K). The zeta potentials () for the single system containing kaolinite and the binary-system containing kaolinite and hematite were presented graphically in Fig. 1. The results showed that the presence of hematite caused a shift of zeta potential towards a positive value for the binary-system containing kaolinite and hematite, which conferred the overlapping of diffuse layers on negatively charged kaolinite particles and on positively charged hematite particles. The results are in consistent to our previous observations [15]. The electrokinetic charge density () on the shear plane of the electrical double layers in the single system containing kaolinite and the binary-system containing kaolinite and hematite were calculated from the zeta potentials shown in Fig. 1 on the basis of the Gouy–Chapmann equation (2) and the results were presented in Fig. 2. From Fig. 2 it can be seen that the changing trends of the electrokinetic charge density with pH both in the single kaolinite and the binary-system containing kaolinite and hematite were similar to that for zeta potentials as shown in Fig. 1. Both zeta potential and surface charge density decreased with the increase of the suspension pH. This can be ascribed either due to the adsorption of OH− ions onto the positive charge centers of clay or due to the deprotonation of surface hydroxyl groups. Zeta potential of kaolinite was negative in the pH range of 3–7, on the contrary zeta potential of hematite is nearly all positive in the same pH range. Hence, it inferred the kaolinite carried net negative surface charge whereas hematite possessed net positive charge on their surfaces (Fig. 2). When kaolinite and hematite exist simultaneously in the same colloid suspension, the electrostatic attraction makes the particle of kaolinite close to the particle of hematite, and the diffuse layers on positively charged hematite and on negatively charged kaolinite can overlap to each other up to some extent. The interaction of the diffuse layers on oppositely charged particles decreased the value of effective charge density on kaolinite (Fig. 2). For example, at pH5.0 and when the percentage of hematite was 10%, 20% and 30%, the electrokinetic charge density for binary-system con-
159
Fig. 2. Electrokinetic charge density of kaolinite and hematite and the binarysystem containing kaolinite and hematite as calculated from the data of zeta potential presented in Fig. 1 (3:7, 1:4, 1:9 are the mass ratios of hematite to kaolinite).
taining kaolinite and hematite was −7.5 × 10−4 , −3.2 × 10−4 and −1.2 × 10−4 C m−2 , respectively. While the surface charge density in single kaolinite system was −9.5 × 10−4 C m−2 at pH5.0. Therefore, more the hematite addition led to greater decrease in effective surface charge density. Thus, the extent of the interaction between the negatively charged particles of kaolinite and positively charged particles of hematite increased with the increase in the content of hematite in mixed system. 3.2. Interaction of electrical double layers between kaolinite and goethite, gibbsite and -Al2 O3 The electrokinetic charge density for the single systems of goethite, gibbsite and ␥-Al2 O3 and the binary-system containing kaolinite and goethite, gibbsite or ␥-Al2 O3 was also calculated with Eq. (2) and the results were presented in Figs. 3–5. The changing patterns of the charge density in the binary-system containing kaolinite and goethite, gibbsite or ␥-Al2 O3 were observed similar to that of the binary-system containing kaolinite and hematite (Fig. 2). The presence of the goethite, gibbsite as well ␥-Al2 O3 decreased the effective surface charge density on kaolinite in the binary-system compared to the single kaolinite system. Therefore the overlapping of the diffuse layers also occurred between kaolinite and positively charged goethite, gibbsite and ␥-Al2 O3 . When different binary systems were compared, it was observed that the affecting extent of different oxides on the surface charge
Fig. 3. Electrokinetic charge density of kaolinite and goethite and the binary-system containing kaolinite and goethite with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratios of goethite to kaolinite).
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S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161 Table 1 Comparison of difference of charge density between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides Treatment
Goethite Hematite Gibbsite ␥-Al2 O3
Fig. 4. Electrokinetic charge density of kaolinite and gibbsite and the binary-system containing kaolinite and gibbsite with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratios of gibbsite to kaolinite).
Difference of charge density between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides (×10−3 C m−2 ) pH 4.0
pH 5.0
pH 6.0
0.62 0.69 2.05 0.93
1.0 0.83 2.40 1.04
1.51 1.20 2.75 1.27
to the stronger interaction of the electrical double layers between kaolinite and Fe/Al oxides. The effect of ␥-Al2 O3 on charge density was greater than goethite, but less than gibbsite at pH 4.0 and pH 5.0, while at pH 6 the effect of goethite was greater than ␥-Al2 O3 . The interaction extent of the electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added as shown in Figs. 2–5. With the increase in the content of Fe/Al oxides from 10% to 20% and 30% of total solid phase, the isoelectric point (IEP) of the binary-system containing kaolinite and goethite increased from 3.6 to 3.8 and 5.2, from 6.1 to 6.9 and 7.3 for the binary-system of kaolinite and gibbsite, and from 3.8 to 4.9 and 5.1 for the binary-system of kaolinite and ␥-Al2 O3 , respectively. Therefore, the presence of Fe/Al oxides and the interaction of the electrical double layers between kaolinite and Fe/Al oxides also caused the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. 3.3. The influence of ionic strength on the interaction of electrical double layers
Fig. 5. Electrokinetic charge density of kaolinite and ␥-Al2 O3 and the binary-system containing kaolinite and ␥-Al2 O3 with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratio of ␥-Al2 O3 to kaolinite).
density in the binary systems changed with the type of oxides present. For example, at the ratio of kaolinite to Fe/Al oxides 7:3, the difference of charge density between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides was estimated based on the data shown in Figs. 3–5 and the results were given in Table 1. The higher the difference of charge density, the stronger the interaction of electrical double layers between kaolinite and Fe/Al oxides. The gibbsite showed maximum effect on charge density, hence resulted in the strongest interaction of electrical double layers between the gibbsite and kaolinite, followed by goethite and hematite. This is also consistent with the values of charge density on the Fe/Al oxides. At pH 4.0, 5.0 and 6.0, the charge density is 1.65 × 10−3 , 1.38 × 10−3 and 1.10 × 10−3 C m−2 for gibbsite, 0.88 × 10−3 , 0.75 × 10−3 and 0.58 × 10−3 C m−2 for goethite and 0.85 × 10−3 , 0.67 × 10−3 and 0.42 × 10−3 C m−2 for hematite, respectively. The greater positive charge density on Fe/Al oxides led
In order to determine the influence of ionic strength on the electrical double-layer interaction, the difference in zeta potential () between the single kaolinite system and the binary-system containing kaolinite and Fe/Al oxides were compared in 0.1 mmol L−1 and 10 mmol L−1 NaCl (Table 2). The results indicated that in 0.1 mmol L−1 NaCl solution was much greater than that in 10 mmol L−1 solution for all goethite, gibbsite and ␥-Al2 O3 . For example, the is 22.63 mV higher in 0.1 mmol L−1 of NaCl than in 10 mmol L−1 of NaCl for gibbsite at pH 5.0. The corresponding value is 13.9 mV and 11.84 mV for goethite and ␥-Al2 O3 , respectively. These data further suggested, the interaction of the electrical double layers between kaolinite and Fe/Al oxides is stronger in 0.1 mmol L−1 NaCl solution than that in 10 mmol L−1 NaCl solution. It is well known that the thickness of diffuse layer of the electrical double layers on colloid particles decreased with the increase in the ionic strength of the medium. The increase in ionic strength and the decrease in the thickness of colloid diffuse layer caused the reduction of overlapping of the diffuse layers between kaolinite and goethite, gibbsite or ␥-Al2 O3 . Hence, it can be concluded that the electrical double layer interaction in 0.1 mmol L−1 NaCl is stronger than that in 10 mmol L−1 NaCl. This also gives a further evidence
Table 2 Comparison of the difference of zeta potential between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides at different pH and ionic strength Fe/Al oxides
Concentration of NaCl (mmol L−1 )
Difference of zeta potential between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides (mV) pH 4.0
pH 5.0
pH 6.0
pH 7.0
␥-Al2 O3
0.1 10.0
38.63 22.65
32.63 20.79
18.58 15.27
6.19 9.23
Gibbsite
0.1 10.0
68.14 47.46
75.36 52.73
71.89 52.41
59.62 43.98
Goethite
0.1 10.0
32.87 18.43
27.95 14.50
19.26 10.84
12.68 9.23
S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
to support the presence of the overlapping of the diffuse layers on oppositely charged colloid particles in the binary-suspension systems containing phyllosilicates and Fe/Al oxides. Acknowledgments This investigation was supported by the National Natural Science Foundation of China (40571076) and the Knowledge Innovation Program Foundation of the Chinese Academy of Sciences (ISSASIP0613). References [1] H. Van Olphen, An Introduction to Clay Colloid Chemistry, Interscience, New York, 1963. [2] H. Zhao, P.F. Low, J.M. Bradford, Soil Sci. 151 (1991) 196. [3] Z. Zhou, W.D. Gunter, Clays Clay Miner. 40 (1992) 356. [4] R. Keren, D.L. Sparks, Soil Sci. Soc. Am. J. 59 (1995) 430. [5] B.K. Schroth, G. Sposito, Clays Clay Miner. 45 (1997) 85. ´ ´ (Ed.), Adsorption: [6] E. Tombacz, Adsorption from electrolyte solutions, in: J. Toth Theory, Modeling, and Analysis, Marcel Dekker, New York, 2002, p. 711. ´ [7] E. Tombacz, Effect of environmental relevant organic complexants on the surface charge and the interaction of clay mineral and metal oxide particles, in: S.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24] [25]
161
´ any ´ Bar (Ed.), Role of Interfaces in Environmental Protection. NATO ASI Series, Kluwer Academic Publisher, Dordrecht, 2003, p. 397. ´ E. Tombacz, M. Szekeres, Appl. Clay Sci. 34 (2006) 105. R. Hogg, T.W. Healy, D.W. Fuerstenau, Trans. Faraday Soc. 62 (1966) 1638. A.E. Ghzaoui, J. Appl. Phys. 86 (1999) 5894. L. Larson, C.J. Drummond, D.Y.C. Chan, F. Greser, Langmuir 13 (1997) 2109. L. Larson, C.J. Drummond, D.Y.C. Chan, F. Greser, J. Phys. Chem. 99 (1995) 2114. N.P. Qafoku, M.E. Sumner, Soil Sci. Soc. Am. J. 66 (2002) 1231. ´ ´ A. Majzik, E. Klumpp, Org. Geochem. 35 (2004) E. Tombacz, Z. Libor, E. Illes, 257. T. Hou, R.K. Xu, A.Z. Zhao, Colloid Surf. A: Physicochem. Eng. Aspects 297 (2007) 91. W. Stumm, Chemistry of the Solid–Water Interface: Chapter 3 Surface Charge and the Electric Double Layer, Wiley–Interscience Publication, New York, 1992, pp. 43–86. ´ R.C. Plaza, F. Gonzalez-Caballero, A.V. Delgado, Colloid Polym. Sci. 279 (2001) 1206. J.W. Ntalikwa, R. Bryant, J.S.M. Zunzu, Colloid Polym. Sci. 279 (2001) 843. R. Sprycha, J. Colloid Interf. Sci. 127 (1989) 1. T. Preocanin, N. Kallay, Croatica Chem. Acta 79 (2006) 95. H.L. Michael, D.J.A. Williams, J. Electroanal. Chem. 179 (1984) 131. M.R. Houchin, L.J. Warren, J. Colloid Interf. Sci. 100 (1984) 278. R.J. Atkinson, A.M. Posner, T.P. Quirk, J. Phys. Chem. 71 (1967) 550. J.H. Kyle, A.M. Posner, J.P. Quirk, J. Soil Sci. 26 (1975) 32. M. Jarlbring, L. Gunneriusson, B. Hussmann, W. Forsling, J. Colloid Interf. Sci. 285 (2005) 212.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength Su-zhen Li a,b , Ren-kou Xu a,∗ a b
State Key Laboratory of Soils and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 19 January 2008 Received in revised form 17 April 2008 Accepted 18 May 2008 Available online 24 May 2008 Keywords: Electrical double layer Overlapping of diffuse layers Zeta potential Electrokinetic charge density Goethite Hematite Gibbsite ␥-Al2 O3 Ionic strength
a b s t r a c t Phyllosilicates with net negative surface charge and Fe/Al oxides with net positive surface charge co-exist in variable charge soils and the interaction between these oppositely charged particles affects the stability of mixed colloids, aggregation, and even the surface chemical properties of variable charge soils. The overlapping of the diffuse layers of electrical double layers between kaolinite and goethite, hematite, gibbsite and ␥-Al2 O3 was investigated in this article based on the change of zeta potential and electrokinetic charge density induced by the addition of Fe/Al oxides. The results indicated that in the range of pH from 3 to 7, the kaolinite is negatively charged and the Fe/Al oxides are positively charged, and the overlapping of the diffuse layers led to the increase in zeta potential and the decrease in surface charge density of binary-system containing kaolinite and Fe/Al oxides compared with the single kaolinite system. The presence of gibbsite resulted in the strongest interaction of electrical double layers between this and kaolinite, followed by goethite and hematite. The interaction extent of electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added. The interaction also resulted in the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. The increase in ionic strength led to the decrease in the thickness of colloid diffuse layer and thus resulted in the reduction of overlapping of the diffuse layers between kaolinite and Fe/Al oxides. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Colloid particles carry electrical charge, the requirement of overall electro-neutrality of the interfacial region results in the formation of a diffuse layer of oppositely charged counter-ion adjacent to the particle surface, and thus the formation of electrical double layers. Zeta potential is the most widely used experimental approach to study the charged surfaces, through the use of electrokinetic techniques. The electrical potential at the shear plane of colloid particles is defined as the electrokinetic potential, more commonly referred as the zeta potential, which can be deduced from electrokinetic measurements. Zeta potential can provide much valuable information about the structure and properties of the electrical double layers, although the exact location of the slip surface is perhaps not ascertained accurately. Clays possess amphoteric sites which are conditionally charged, either positive or negative charges depending on pH can develop on the O faces and at the edges by direct H+ /OH− transfer from
∗ Corresponding author. Tel.: +86 25 86881183; fax: +86 25 86881000. E-mail address: [email protected] (R.-k. Xu). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.05.023
aqueous phase. This surface charge heterogeneity of clay minerals presented originally by Van Olphen [1], then supported and elaborated further in many subsequent and recent investigations [2–7], which governs the particles’ interaction in clay mineral suspensions. Although the net particle charge is negative in general, both negatively and positively charged parts on the surface of clay mineral particles exist simultaneously under acidic conditions [8]. Therefore the particles’ interactions exist extensively, even in the single type of mineral system. The interaction of dissimilar double layers between the dissimilar colloidal particles as related to mutual coagulation of colloidal dispersions (HHF theory) has been developed by Hogg et al. [9]. The theory has been used to describe the potential energy of interaction between dissimilar spherical colloidal particles and evaluate the stability of binary colloids extensively. In recent time, some reports discussed the diffuse double-layer forces between dissimilar surfaces, such as the interaction between zirconia and ␣-alumina [10], the interaction between silica colloid sphere and an alumina flat crystal [11], and the interaction between a SiO2 glass sphere and a TiO2 crystal [12]. The findings suggest that zeta potential is in good agreement with the AFM force measurement. In authentic soil system, Qafoku and Sumner proposed the hypothesis of the diffuse double-layer interaction between the negatively charged
158
S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
phyllosilicates and the positively charged Fe/Al oxides to interpret the simultaneous adsorption of cations and anions by variable ´ charge soils in dilute electrolyte [13]. Tombacz et al. found, the presence of hematite or magnetite made the zeta potential of binary-system containing montmorillonite and hematite or magnetite shift towards the positive value direction [14]. A similar phenomenon was also observed by Hou et al. [15] in binary-system containing kaolinite and Fe/Al oxides. They interpreted, the overlapping of the diffuse layers of electrical double layers on kaolinite and Fe/Al oxides decreases the effective negative charge density on the kaolinite and thus increases the zeta potential of the binarysystem containing the kaolinite and Fe/Al oxides. According to the Gouy–Chapmann double-layer theory, the surface charge density can be calculated [16]. Based on the surface charge density it is possible to quantify the interaction between oppositely charged kaolinite and Fe/Al oxides. Kaolinite and goethite, hematite, gibbsite, ␥-Al2 O3 are chosen in this article to investigate the interaction of the electrical double layers on kaolinite and Fe/Al oxides because they are the main mineral constitutes in variable charge soils. The concentration of electrolyte solution is an important factor influencing zeta potential and structure of the electrical double layers, as was demonstrated earlier with pure oxides including hematite/yttrium oxide [17], ␣-alumina [18], ␥-Al2 O3 [19], TiO2 [20], quartz [21], and SnO2 [22]. The thickness of the diffuse layer of the electrical double layers on colloid particle changes with the change in ionic strength of bulk solution. The concentration of the background electrolyte could influence the electrical double-layer interaction between oppositely charged surfaces [8]. Therefore, the objectives of this article are: (1) to investigate the overlapping of diffuse layers of the electrical double layers between oppositely charged particles based on the change of surface charge density; (2) to study the effect of ionic strength on the interaction of electrical double layers between oppositely charged particles. 2. Materials and methods 2.1. Kaolinite and Fe/Al oxides Kaolinite and ␥-Al2 O3 are commercial clay minerals obtained respectively from Shanghai Reagent Company (Shanghai, China) and Dalian Luming Light Science and Technology Co., Ltd. (Dalian, China) with the diameter <2 m. Goethite was synthesized as the procedure reported earlier [23]. The pH of a 0.5 mol L−1 Fe(NO3 )3 solution was adjusted to 12 with NaOH with stirring, and then the precipitate was aged at 60 ◦ C in the oven for 24 h. The synthetic goethite was electro-dialyzed at a potential gradient of 15 V cm−1 until a constant specific electrical conductance was achieved. Finally, the treated sample was dried at 60 ◦ C using an IR lamp and ground to pass 100-mesh sieve. Gibbsite was prepared according to the method described by Kyle et al. [24]. A 4 mol L−1 NaOH was added slowly to 1.0 mol L−1 AlCl3 solution with stirring until the pH reached 4.6, and the suspension was allowed to stand for 2 h at 40 ◦ C. The product was electro-dialyzed as the method mentioned above. The treated sample was air-dried and ground to pass 100-mesh sieve. Similarly, the hematite was synthesized according to the reported method [25]. Two hundred millilitres of 3.3 mol L−1 FeCl3 solution was added drop wise to 2.50 L of boiling de-ionized water with stirring and then the product was purified with electrodialyzed method as mentioned above. Finally, the treated sample was dried at 60 ◦ C using an IR lamp and ground to pass 100-mesh sieve.
2.2. Preparation of colloid suspensions at different pH Sodium chloride of 0.1 mmol L−1 was used as a supporting electrolyte to maintain the ionic strength and the electrical double layer thickness to be constant. The colloid suspension containing 0.1 mmol L−1 NaCl was prepared with a concentration of kaolinite or oxides 0.25 g L−1 . The suspensions were dispersed ultrasonically at a frequency of 40 kHz and a power of 300 W for 1 h at 25 ◦ C. Finally, the suspension pH was adjusted to required values with dilute HCl or NaOH solution. Then the suspensions were shaken for 2 h at 25 ◦ C. pH was checked again and adjusted if necessary. Further, the suspensions were equilibrated for 24 h before electrokinetic potential was determined, and during this period the colloid suspensions were mixed manually at irregular intervals. For the binary-colloid suspensions, kaolinite and Fe/Al oxides were placed in the same container with different mass ratios of Fe/Al oxides to kaolinite, and the total concentration of the mixture was taken 0.25 g L−1 , the rest of the procedure was followed similar to that for the single colloid suspension as mentioned above. 2.3. Preparation of colloid suspension with different electrolyte concentration at different pH The colloid suspension containing 0.1 mmol L−1 and 10 mmol L−1 NaCl solution as background electrolyte was prepared with a concentration of kaolinite or oxides 0.25 g L−1 for single colloid system. When the binary-colloid suspension was prepared, kaolinite and Fe/Al oxides were taken in the same container according to their different quantitative ratio and 0.1 mmol L−1 or 10 mmol L−1 NaCl solution was added as background electrolyte. The rest of the procedure was followed similar to the method mentioned above. 2.4. The determination of electrokinetic potential The electrokinetic potential (zeta potential) was measured using the JS94G+ microelectro-phoresis apparatus made in China. The colloid suspension was agitated and transferred to the electrophoresis vessel after wetting the electrode to avoid any disturbance due to air bubble. An average of the electrophoretic velocity was obtained with the computer by timing 10 particles, first in one direction, and then upon reversal of the polarity of the applied electrical field in the opposite direction. The values of the zeta potential were calculated using computer with a specific software based on the Helmholtz–Smoluchowski equation: =
Ku ε
(1)
where u is the electrophoretic mobility, ε the permittivity of the medium and is the viscosity of liquid phase. K is a constant relative to the shape of the colloid particle. Under the condition of this investigation, the radii of colloidal particles (a) are much bigger than the thickness of electrical double layer (1/) and a 1, so K = 4. This step was repeated 10 times and an average value of zeta potential from 10 replicates was reported in this paper. The measurement error was found to be ±2 mV and at the temperature 25 ± 0.5 ◦ C. 3. Results and discussion 3.1. Electrokinetic charge density and the electrical double-layer interaction between kaolinite and Hematite According to the Gouy–Chapmann double-layer theory, the electrokinetic charge density on the shear plane of electrical double
S.-z. Li, R.-k. Xu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 157–161
Fig. 1. Zeta potential of kaolinite or hematite and the binary-system containing kaolinite and hematite with different mass ratios in suspension systems (3:7, 1:4, 1:9 are the mass ratio of hematite to kaolinite).
layer can be calculated based on zeta potential () with the equation given below [16]: = (8εε0 ns KT )1/2 sinh
zF 2RT
(2)
where ε is the relative dielectric permittivity (ε = 78.5 for water at 25 ◦ C), ε0 the permittivity in vacuum (8.854 × 10−14 C V−1 cm−1 ), ns the number of ion pairs, K the Boltzmann constant, z the valence of ion, R the molar gas constant (8.314 J mol−1 K−1 ), F the Faraday constant (96,487 C mol−1 ), and T is the temperature (K). The zeta potentials () for the single system containing kaolinite and the binary-system containing kaolinite and hematite were presented graphically in Fig. 1. The results showed that the presence of hematite caused a shift of zeta potential towards a positive value for the binary-system containing kaolinite and hematite, which conferred the overlapping of diffuse layers on negatively charged kaolinite particles and on positively charged hematite particles. The results are in consistent to our previous observations [15]. The electrokinetic charge density () on the shear plane of the electrical double layers in the single system containing kaolinite and the binary-system containing kaolinite and hematite were calculated from the zeta potentials shown in Fig. 1 on the basis of the Gouy–Chapmann equation (2) and the results were presented in Fig. 2. From Fig. 2 it can be seen that the changing trends of the electrokinetic charge density with pH both in the single kaolinite and the binary-system containing kaolinite and hematite were similar to that for zeta potentials as shown in Fig. 1. Both zeta potential and surface charge density decreased with the increase of the suspension pH. This can be ascribed either due to the adsorption of OH− ions onto the positive charge centers of clay or due to the deprotonation of surface hydroxyl groups. Zeta potential of kaolinite was negative in the pH range of 3–7, on the contrary zeta potential of hematite is nearly all positive in the same pH range. Hence, it inferred the kaolinite carried net negative surface charge whereas hematite possessed net positive charge on their surfaces (Fig. 2). When kaolinite and hematite exist simultaneously in the same colloid suspension, the electrostatic attraction makes the particle of kaolinite close to the particle of hematite, and the diffuse layers on positively charged hematite and on negatively charged kaolinite can overlap to each other up to some extent. The interaction of the diffuse layers on oppositely charged particles decreased the value of effective charge density on kaolinite (Fig. 2). For example, at pH5.0 and when the percentage of hematite was 10%, 20% and 30%, the electrokinetic charge density for binary-system con-
159
Fig. 2. Electrokinetic charge density of kaolinite and hematite and the binarysystem containing kaolinite and hematite as calculated from the data of zeta potential presented in Fig. 1 (3:7, 1:4, 1:9 are the mass ratios of hematite to kaolinite).
taining kaolinite and hematite was −7.5 × 10−4 , −3.2 × 10−4 and −1.2 × 10−4 C m−2 , respectively. While the surface charge density in single kaolinite system was −9.5 × 10−4 C m−2 at pH5.0. Therefore, more the hematite addition led to greater decrease in effective surface charge density. Thus, the extent of the interaction between the negatively charged particles of kaolinite and positively charged particles of hematite increased with the increase in the content of hematite in mixed system. 3.2. Interaction of electrical double layers between kaolinite and goethite, gibbsite and -Al2 O3 The electrokinetic charge density for the single systems of goethite, gibbsite and ␥-Al2 O3 and the binary-system containing kaolinite and goethite, gibbsite or ␥-Al2 O3 was also calculated with Eq. (2) and the results were presented in Figs. 3–5. The changing patterns of the charge density in the binary-system containing kaolinite and goethite, gibbsite or ␥-Al2 O3 were observed similar to that of the binary-system containing kaolinite and hematite (Fig. 2). The presence of the goethite, gibbsite as well ␥-Al2 O3 decreased the effective surface charge density on kaolinite in the binary-system compared to the single kaolinite system. Therefore the overlapping of the diffuse layers also occurred between kaolinite and positively charged goethite, gibbsite and ␥-Al2 O3 . When different binary systems were compared, it was observed that the affecting extent of different oxides on the surface charge
Fig. 3. Electrokinetic charge density of kaolinite and goethite and the binary-system containing kaolinite and goethite with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratios of goethite to kaolinite).
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Goethite Hematite Gibbsite ␥-Al2 O3
Fig. 4. Electrokinetic charge density of kaolinite and gibbsite and the binary-system containing kaolinite and gibbsite with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratios of gibbsite to kaolinite).
Difference of charge density between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides (×10−3 C m−2 ) pH 4.0
pH 5.0
pH 6.0
0.62 0.69 2.05 0.93
1.0 0.83 2.40 1.04
1.51 1.20 2.75 1.27
to the stronger interaction of the electrical double layers between kaolinite and Fe/Al oxides. The effect of ␥-Al2 O3 on charge density was greater than goethite, but less than gibbsite at pH 4.0 and pH 5.0, while at pH 6 the effect of goethite was greater than ␥-Al2 O3 . The interaction extent of the electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added as shown in Figs. 2–5. With the increase in the content of Fe/Al oxides from 10% to 20% and 30% of total solid phase, the isoelectric point (IEP) of the binary-system containing kaolinite and goethite increased from 3.6 to 3.8 and 5.2, from 6.1 to 6.9 and 7.3 for the binary-system of kaolinite and gibbsite, and from 3.8 to 4.9 and 5.1 for the binary-system of kaolinite and ␥-Al2 O3 , respectively. Therefore, the presence of Fe/Al oxides and the interaction of the electrical double layers between kaolinite and Fe/Al oxides also caused the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. 3.3. The influence of ionic strength on the interaction of electrical double layers
Fig. 5. Electrokinetic charge density of kaolinite and ␥-Al2 O3 and the binary-system containing kaolinite and ␥-Al2 O3 with different mass ratios in suspension systems as calculated from the data of zeta potential (3:7, 1:4, 1:9 are the mass ratio of ␥-Al2 O3 to kaolinite).
density in the binary systems changed with the type of oxides present. For example, at the ratio of kaolinite to Fe/Al oxides 7:3, the difference of charge density between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides was estimated based on the data shown in Figs. 3–5 and the results were given in Table 1. The higher the difference of charge density, the stronger the interaction of electrical double layers between kaolinite and Fe/Al oxides. The gibbsite showed maximum effect on charge density, hence resulted in the strongest interaction of electrical double layers between the gibbsite and kaolinite, followed by goethite and hematite. This is also consistent with the values of charge density on the Fe/Al oxides. At pH 4.0, 5.0 and 6.0, the charge density is 1.65 × 10−3 , 1.38 × 10−3 and 1.10 × 10−3 C m−2 for gibbsite, 0.88 × 10−3 , 0.75 × 10−3 and 0.58 × 10−3 C m−2 for goethite and 0.85 × 10−3 , 0.67 × 10−3 and 0.42 × 10−3 C m−2 for hematite, respectively. The greater positive charge density on Fe/Al oxides led
In order to determine the influence of ionic strength on the electrical double-layer interaction, the difference in zeta potential () between the single kaolinite system and the binary-system containing kaolinite and Fe/Al oxides were compared in 0.1 mmol L−1 and 10 mmol L−1 NaCl (Table 2). The results indicated that in 0.1 mmol L−1 NaCl solution was much greater than that in 10 mmol L−1 solution for all goethite, gibbsite and ␥-Al2 O3 . For example, the is 22.63 mV higher in 0.1 mmol L−1 of NaCl than in 10 mmol L−1 of NaCl for gibbsite at pH 5.0. The corresponding value is 13.9 mV and 11.84 mV for goethite and ␥-Al2 O3 , respectively. These data further suggested, the interaction of the electrical double layers between kaolinite and Fe/Al oxides is stronger in 0.1 mmol L−1 NaCl solution than that in 10 mmol L−1 NaCl solution. It is well known that the thickness of diffuse layer of the electrical double layers on colloid particles decreased with the increase in the ionic strength of the medium. The increase in ionic strength and the decrease in the thickness of colloid diffuse layer caused the reduction of overlapping of the diffuse layers between kaolinite and goethite, gibbsite or ␥-Al2 O3 . Hence, it can be concluded that the electrical double layer interaction in 0.1 mmol L−1 NaCl is stronger than that in 10 mmol L−1 NaCl. This also gives a further evidence
Table 2 Comparison of the difference of zeta potential between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides at different pH and ionic strength Fe/Al oxides
Concentration of NaCl (mmol L−1 )
Difference of zeta potential between single kaolinite system and binary-system containing kaolinite and Fe/Al oxides (mV) pH 4.0
pH 5.0
pH 6.0
pH 7.0
␥-Al2 O3
0.1 10.0
38.63 22.65
32.63 20.79
18.58 15.27
6.19 9.23
Gibbsite
0.1 10.0
68.14 47.46
75.36 52.73
71.89 52.41
59.62 43.98
Goethite
0.1 10.0
32.87 18.43
27.95 14.50
19.26 10.84
12.68 9.23
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to support the presence of the overlapping of the diffuse layers on oppositely charged colloid particles in the binary-suspension systems containing phyllosilicates and Fe/Al oxides. Acknowledgments This investigation was supported by the National Natural Science Foundation of China (40571076) and the Knowledge Innovation Program Foundation of the Chinese Academy of Sciences (ISSASIP0613). References [1] H. Van Olphen, An Introduction to Clay Colloid Chemistry, Interscience, New York, 1963. [2] H. Zhao, P.F. Low, J.M. Bradford, Soil Sci. 151 (1991) 196. [3] Z. Zhou, W.D. Gunter, Clays Clay Miner. 40 (1992) 356. [4] R. Keren, D.L. Sparks, Soil Sci. Soc. Am. J. 59 (1995) 430. [5] B.K. Schroth, G. Sposito, Clays Clay Miner. 45 (1997) 85. ´ ´ (Ed.), Adsorption: [6] E. Tombacz, Adsorption from electrolyte solutions, in: J. Toth Theory, Modeling, and Analysis, Marcel Dekker, New York, 2002, p. 711. ´ [7] E. Tombacz, Effect of environmental relevant organic complexants on the surface charge and the interaction of clay mineral and metal oxide particles, in: S.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24] [25]
161
´ any ´ Bar (Ed.), Role of Interfaces in Environmental Protection. NATO ASI Series, Kluwer Academic Publisher, Dordrecht, 2003, p. 397. ´ E. Tombacz, M. Szekeres, Appl. Clay Sci. 34 (2006) 105. R. Hogg, T.W. Healy, D.W. Fuerstenau, Trans. Faraday Soc. 62 (1966) 1638. A.E. Ghzaoui, J. Appl. Phys. 86 (1999) 5894. L. Larson, C.J. Drummond, D.Y.C. Chan, F. Greser, Langmuir 13 (1997) 2109. L. Larson, C.J. Drummond, D.Y.C. Chan, F. Greser, J. Phys. Chem. 99 (1995) 2114. N.P. Qafoku, M.E. Sumner, Soil Sci. Soc. Am. J. 66 (2002) 1231. ´ ´ A. Majzik, E. Klumpp, Org. Geochem. 35 (2004) E. Tombacz, Z. Libor, E. Illes, 257. T. Hou, R.K. Xu, A.Z. Zhao, Colloid Surf. A: Physicochem. Eng. Aspects 297 (2007) 91. W. Stumm, Chemistry of the Solid–Water Interface: Chapter 3 Surface Charge and the Electric Double Layer, Wiley–Interscience Publication, New York, 1992, pp. 43–86. ´ R.C. Plaza, F. Gonzalez-Caballero, A.V. Delgado, Colloid Polym. Sci. 279 (2001) 1206. J.W. Ntalikwa, R. Bryant, J.S.M. Zunzu, Colloid Polym. Sci. 279 (2001) 843. R. Sprycha, J. Colloid Interf. Sci. 127 (1989) 1. T. Preocanin, N. Kallay, Croatica Chem. Acta 79 (2006) 95. H.L. Michael, D.J.A. Williams, J. Electroanal. Chem. 179 (1984) 131. M.R. Houchin, L.J. Warren, J. Colloid Interf. Sci. 100 (1984) 278. R.J. Atkinson, A.M. Posner, T.P. Quirk, J. Phys. Chem. 71 (1967) 550. J.H. Kyle, A.M. Posner, J.P. Quirk, J. Soil Sci. 26 (1975) 32. M. Jarlbring, L. Gunneriusson, B. Hussmann, W. Forsling, J. Colloid Interf. Sci. 285 (2005) 212.