Protonation of silica particles in the presence of a strong cationic polyelectrolyte

Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 20–25

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Protonation of silica particles in the presence of a strong cationic polyelectrolyte a,∗ ˇ Duˇsko Cakara , Motoyoshi Kobayashi b , Michal Skarba a , Michal Borkovec a a b

Department of Inorganic, Analytical, and Applied Chemistry, University of Geneva, Quai Ernest-Ansermet 30, 1211 Geneva 4, Switzerland Faculty of Agriculture, Iwate University, Ueda 3-18-8, Morioka, Iwate 020-8550, Japan

a r t i c l e

i n f o

Article history: Received 26 June 2008 Received in revised form 30 December 2008 Accepted 15 January 2009 Available online 23 January 2009 Keywords: Adsorption Silica pDADMAC DADMAC Surface charge Point of zero charge PZC

a b s t r a c t Potentiometric titrations and electrophoresis techniques were used to study the charging behavior of aqueous silica particle suspensions in the presence of poly(N,N-diallyldimethylammonium chloride) (DADMAC). The main finding is that the particles undergo charge reversal with increasing pH, which can be directly seen by electrophoresis and as a common crossing point in the titration curves at different salt levels. These features resemble the charging behavior of an amphoteric oxide surface. With increasing polymer dose, the charge reversal point shifts towards higher pH. These trends are well reproduced by a modified Stern model. The main effect of the adsorbed cationic polyelectrolyte is to further deprotonate the surface silanol groups, and thereby to induce additional negative charge on the surface. © 2009 Elsevier B.V. All rights reserved.

1. Introduction During the last decades, we have witnessed renewed interest to study properties of colloidal particle suspensions in the presence of oppositely charged polyelectrolytes [1–8]. This development was largely driven by various technological applications of polyelectrolytes, such as, for example, flocculating agents, adhesion modifiers, drug carriers, or food additives. One of the main findings of these studies is that the amount of the adsorbed polyelectrolyte determines the overall particle charge, and that the latter parameter can be correlated to many suspension properties, such as colloidal stability and particle adhesion characteristics. The particle charge in the presence of oppositely charged polyelectrolyte is primarily determined by the amount of the adsorbed polyelectrolyte, whereby the surface charge is initially neutralized, and reversed subsequently [1,5–8]. This process is also referred to as overcharging. While many aspects of this charge reversal mechanism are now understood, there is increasing evidence that co-adsorbed ions strongly modify the charge balance. Kleimann et al. [8] have shown that neutralization of the surface charge by polyelectrolytes is normally accompanied by adsorption of their

∗ Corresponding author at: Department of Biotechnology, University of Rijeka, ´ Maˇzuranic´ 10, 51000 Rijeka, Croatia. Tel.: +385 51 651 274. Trg brace ˇ E-mail address: [email protected] (D. Cakara). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.01.011

counterions, leading to a so-called superstoichiometric charge neutralization. The role of co-adsorbed ions in determining the charge balance of an adsorbed polyelectrolyte is certainly best documented in the case of protons [9–15]. As an example, consider the charge reversal of sulfate latex particles by poly(vinyl amine)(PVA) [13]. Since this poylelectrolyte is a weak polybase, it is fully protonated at low pH. Under these conditions, the charge reversal can be already induced by a small polymer dose. At higher pH, on the other hand, the degree of protonation is smaller, and thus more PVA is needed to neutralize the charge. Similar effects have been observed in a system containing silica particles and cationic strong polyelectrolyte poly(N,N-diallyldimethylammonium chloride) (DADMAC) [15]. These authors have reported that with increasing pH, an increasing polymer dose is needed to reach the charge reversal point. This behavior can be understood since the magnitude of the negative silica surface charge increases with increasing pH, and therefore a higher dose of the polymer is necessary to neutralize a higher charge at higher pH. Adsorption of DADMAC on planar silica substrates was further studied with sum frequency spectroscopy [16] and reflectivity techniques [17]. In systems, where the surface charge as well as the polyelectrolyte charge depend on pH, the situation is more complicated as upon adsorption, both the polyelectrolyte as well as the surface modify their protonation state. This situation was studied for iron oxide particles in the presence of humic and fulvic acids in substantial detail [11,18]. The latter study reports that adsorption of fulvic acids is largely reversible. On the

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other hand, adsorption of polyelectrolytes of higher molecular mass is close to irreversible [8], and for this reason these systems cannot be easily compared. Potentiometric titrations is the classical technique to study such surface induced protonation reactions [9–12,14]. The data interpretation is simplified if one of the components is non-titratable, namely either if one deals with strong polyelectrolyte or with particle with a fully ionized surface. In either case, the titratable charge can be clearly attributed to the titratable component, and one can obtain information on how the titration behavior is influenced by the adsorption process. Cakara et al. [14] have recently adopted this approach, and reported data on the titration behavior of carboxylated latex particles in the presence of DADMAC. They observed that adsorption of the strong cationic polyelectrolyte leads to a further deprotonation of the particle surface. The present study focuses on silica particles in the presence of DADMAC. While titration data on a similar system was already reported [9,10], these investigations focused on a weakly charged polyacrylamide added in excess. In the latter case, one is far beyond the charge reversal point. On the other hand, the present study reports titration data near the charge reversal point. Under these conditions, one observes interesting ionic strength dependence of the charging curves, which reminds of amphoteric water–oxide interfaces. Fig. 1. The charge density (upper part) and the electrostatic potential (lower part) profiles for the modified Stern model.

2. Charging model Surface complexation models are commonly used to model charging behavior of amphoteric surfaces. The basic Stern model was particularly successful to capture many features of the charging characteristics of water–oxide interfaces [19–21]. Similar approach has been proposed to describe the charging behavior of titratable surfaces in the presence of adsorbed polyelectrolytes [14]. However, the basic Stern model fails to describe the charging behavior of such systems. The main deficiency seems to be that the charges originating from the adsorbed polyelectrolyte cannot be placed in the same plane as the charged groups at the surface (0-plane) [14]. Better description of the charging data can be obtained with the modified Stern model, whereby the charged sites originating from the polyelectrolyte are placed at the origin of the diffuse layer (d-plane). The charge density in the latter model is depicted in Fig. 1. In this article, this model will be used to describe experimental adsorption of a strong cationic polyelectrolyte to a negatively charged silica surface. The charging of the silica surface can be well rationalized in terms of the basic Stern model [22–24]. The surface charge originates from the second step dissociation of the singly coordinated silanol groups SiOH  SiO− + H+

(1)

while we neglect the binding of a second proton to the singly coordinated oxygens and the protonation of doubly coordinated oxygens [22,23]. The equilibrium constant for the above surface reaction can be written as K=

SiO− aH+ exp(−e SiOH

0 /kT )

(2)

where aH+ is the bulk proton activity, i the surface concentration of species i, 0 the electrostatic surface potential (0-plane), e the elementary charge, and k T the thermal energy. As usual, we define pH = − log aH+

(3)

pK = − log K

(4)

The total concentration of the surface sites 0 is given by the sum of the protonated and deprotonated sites 0 = SiO− + SiOH

(5)

The surface charge density is given by the surface concentration of dissociated silanol groups 0 = −eSiO−

(6)

The charge of the diffuse layer is given by the classical Grahame equation [19,21] 2kT 0  d = − sinh e



e d 2kT



(7)

where d is the diffuse layer potential, 0 is the dielectric permittivity of vacuum,  the dielectric constant of water, and the Debye length −1 is given by relation 2 =

2e2 cNA kT 0 

(8)

where c is the molar concentration of the monovalent salt and NA the Avogadro’s number. The drop of the surface potential close to the interface is modeled with the so-called Stern layer of a constant capacitance CS =

0

0 −

(9) d

In the basic Stern model, the diffuse layer charge d just compensates the surface charge 0 . In the modified Stern model discussed here, we further consider the surface charge due to the adsorbed polyelectrolyte, which is assumed to be located at the origin of the diffuse layer (d-plane). In this case, the charge balance reads 0 + eN,s + d = 0

(10)

where N,s is the surface concentration of the positively charged surface-bound polyelectrolyte sites. The charging behavior of the surface is obtained by solving the above equations simultaneously. In the absence of the adsorbed polyelectrolyte (i.e., N,s = 0) the basic Stern model is recovered. The present model can be

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viewed as a simplified version of the ligand charge distribution model proposed to describe adsorption of fulvic acids to goethite [18]. 3. Experimental

and normalizing to the specific surface area of the particles. This protocol leads to good reversibility of the forward and backward titration curves. Slower titration rates have shown minor lack of reversibility, which could be due to diffusion into the micropores of silica or slow conformational rearrangements of the adsorbed polymer.

3.1. Materials Silica particles were kindly supplied by the Nippon Shokubai company (Japan). These particles were synthesized by a Stöber-like process. The particles were obtained as a dry powder, which was heated for 24 h in a furnace at 800 ◦ C, and then cooled to the room temperature in a desiccator with silica gel. The heat treatment eliminates the microporosity of the particles, which drastically increases the active surface and alters its charging behavior [24–26]. After the heat treatment, the silica sample remained in the state of a flowing powder. The particles were characterized by transmission electron microscopy (TEM), light scattering, and gas adsorption. The number-weighted particle radius obtained by TEM is 57 nm with a coefficient of variation of 0.05, indicating a low polydispersity. The surface area is 32 m2 /g and the radii as measured by static and dynamic light scattering are 68 and 67 nm, respectively. These results point towards a slight microporosity of these particles. The silica sample used here is identical to the KEP10H sample described by Kobayashi et al. [24], where further details on these particles can be found. The cationic polyelectrolyte poly(N,N-diallyldimethylammonium chloride) (DADMAC) was purchased from Aldrich as an aqueous solution of a concentration of 200 g/L. Its molecular mass is in the range of 100–200 kDa. The concentration was verified by means of the total carbon and nitrogen analysis (TOC-V, Shimadzu). All solutions were prepared with Milli-Q water, which was freed of CO2 by boiling, and cooled in a nitrogen atmosphere. Dilute HCl used was prepared from the Merck Titrisol, and the KOH solution was made from the Baker Dilut-it (CO2 -free) solutions. The ionic strength was adjusted with KCl (Merck, p.a.). All experiments were carried out at 25 ◦ C.

3.3. Electrophoretic mobility Samples were prepared in a similar fashion as for the titration experiments. First, a solution with desired DADMAC and silica concentrations was prepared from a silica suspension of 40 mg/L and DADMAC solution of 0.1 mg/L at pH 8.5. After shaking, the suspension was adjusted to pH 7 and to the desired ionic strength, and shaken again. Subsequently, the pH of the sample was adjusted with HCl or KOH, and the sample was split in two parts. The first part was used for the mobility measurement, while the second for pH monitoring. The measurements were performed within 20 min after sample preparation. Electrophoretic mobilities were measured with a laser doppler velocimeter setup (Malvern Zetasizer 2000). Cell potential was set to 75 V, except for the measurements at 0.01 M ionic strength, where it was increased to 100 V. 4. Results and discussion Titratable charge of silica is strongly influenced by adsorption of a strong cationic polyelectrolyte. Fig. 2 illustrates this point by comparing the charging curve of the silica particles in the presence of DADMAC with the charging curves of the pure components. The surface charge of silica is expressed with respect to its surface area in C m−2 , and the charge of the other components is converted to the same units.

3.2. Potentiometric titrations A stock suspension of silica particles of a concentration of 100 g/L was thoroughly sonicated. A DADMAC solution of 2 g/L was prepared by dilution, and the necessary amount was added to 10 mL of the silica suspension adjusted to pH 8.5. Subsequently, water was added to complete to 30 mL, and the pH was readjusted to 7.5. The titrations were carried out with a home-build Jonction titrator [27] using four burettes containing 0.25 M HCl, 0.25 M CO2 -free KOH, 3 M KCl, and water. The sample is thermostated and maintained in a flowing nitrogen atmosphere. The pH was measured with a glass electrode (Metrohm) and a separate Ag/AgCl reference electrode (Metrohm), which were previously calibrated with three standard buffers of pH 4.00, 7.00, and 10.00 (Merck). The electrode calibration was verified by measuring the known pK values of acetic acid and ethylene diamine [27]. The titration was started at pH 3 and an ionic strength 0.01 M. The suspension was then titrated with KOH up to pH 9 in steps of about 0.4 pH units, and titrated back to pH 3 with HCl. Each reading was recorded as soon as the drift was < 0.1 mV/min or the latest after 3 min. During the titration, the ionic strength was kept constant by additions of salt solution and water. After such a titration cycle, the ionic strength was increased by addition of salt, and the procedure was repeated. The ionic strengths of 0.01, 0.05, 0.10, 0.50 and 1.00 M were investigated, and the entire titration experiment, comprising all five ionic strengths, could be performed in less than 10 h. The charging curves were obtained from the difference between titration curves of the sample and the blank,

Fig. 2. Proton binding isotherms of the Stöber silica particles in the presence of adsorbed DADMAC, at a loading tot = 0.45 monomers per nm2 or equivalently 0.072 C m−2 . Closed symbols represent the data measured in the DADMAC/silica mixture. Open symbols represent the charge of the pure silica which were fitted by means of the basic Stern model (dash-dotted lines). The solid line in the upper part represents the charge of DADMAC, N,tot , which is divided into N,s and N,free . The mixing rule sum is represented by dashed lines.

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Table 1 DADMAC concentrations for the two studied systems (N,tot ), and partitioning between the proton-exchanging (N,s ) and non-exchanging (N,free ) amine charge Sample

N,tot (nm−2 )

N,s (nm−2 )

N,free (nm−2 )

PZC

1 2

0.45 0.68

0.27 0.53

0.18 0.15

6.7 7.6

The negative surface charge of the bare silica particles decreases with increasing pH and ionic strength (open symbols) and is well described by the basic Stern model (lines). The corresponding parameters are a silanol surface concentration of 0 = 8 nm−2 , a microscopic deprotonation constant of pK = 7.5, and a Stern capacitance of CS = 2.9 F m−2 as reported previously [22,24]. The solid line in the upper part of Fig. 2 represents the constant positive charge of the pure polyelectrolyte. The polymer concentration is expressed with respect to the surface area of the silica particles, and in this case it is 0.12 mg m−2 or equivalently 0.45 nm−2 or 0.07 C m−2 . The simplest estimate of the titratable charge of the mixed system is the sum of the charges of the two respective components, which corresponds to the pure silica data shifted vertically. This approximation is referred to as the “mixing rule” and can be interpreted as the charge of a mixed system, where DADMAC is not adsorbed to the silica surface, but dissolved in the bulk. The observed charging behavior (closed symbols) is substantially different from the simple mixing rule prediction, indicating that the adsorbed DADMAC strongly modifies the protonation state of the silica surface. In particular, the observed charging curves feature a distinct crossing point near pH 6.7. We interpret this crossing point as the point of zero charge (PZC). At this point, the surface charge of the silica is neutralized by the adsorbed polyelectrolyte. The observed inverse dependencies on the ionic strength below and above the PZC suggest that the polyelectrolyte-coated silica particles indeed undergo a charge reversal at the PZC. The titration curves were well reversible, indicating that neither significant desorption nor conformational rearrangements of the adsorbed polyelectrolyte occurred during the titration time window. We suspect, however, that only a certain part of the charged groups of the added polyelectrolyte contribute to the surface charge. This part of the groups will be referred to as surfacebound groups, and their surface concentration denoted as N,s . The remaining part of the charged groups will be denoted as free groups with a surface concentration N,free . The total polyelectrolyte charge added to the system corresponds to the sum of both quantities, namely N,tot = N,s + N,free . The experimental charging curves for two different polymer loadings are presented with open symbols in Fig. 3 a and b. Both charging curves have been shifted vertically such that the PZC corresponds to zero charge. The necessary shift is interpreted as the free polyelectrolyte charge, expressed as eN,free (cf. Fig. 2). The corresponding values are summarized in Table 1. The closed symbols represent points from the backward titration, indicating the good reversibility of the titration curves. One observes a common crossing point for both loadings, which shifts towards higher pH with increasing polymer concentration. This trend can be understood by realizing that a higher silica surface charge is needed to neutralize the positive charge originating from the polyelectrolyte. With increasing polymer dose, higher pH is necessary to reach the PZC. In both cases, the charging curves show the inverse ionic strength dependence below and above the PZC. This trend indicates that the particles are positive below the PZC and negative above. At high pH, however, the highly negative silica charge dominates the charging curve. Very similar trends were observed for the charging of carboxyl latex particles in the presence of DADMAC [14].

Fig. 3. Proton binding isotherms of Stöber silica particles in the presence of adsorbed DADMAC at two different surface concentrations N,tot . The open symbols represent data from forward titrations, while the closed symbols are selected data points from backwards titrations. The lines represent the fits with the modified Stern model.

Let us now compare the experimental data with the modified Stern model (lines, Fig. 3). The parameters used to describe the bare silica surface are 0 = 8 nm−2 and the pK = 7.5 as discussed above [22,24]. The amount of surface-bound polyelectrolyte charged groups N,s was obtained from the apparent charge at the PZC (see Table 1). The only remaining parameter is the Stern capacitance, and the charging curves for both loadings are well described by choosing CS = 1.2 F m−2 . The model predicts the charging behavior below the PZC quite well, while it works only in an approximate fashion above the PZC. The lower capacitance could be caused by a decrease of the dielectric constant due to the presence of the polyelectrolyte in the compact part of the layer, or by the fact that the positive polyelectrolyte charge is further away from the surface than the distance of closest approach of the salt ions. In the present case, the description is better than in the case of the carboxyl latex in the presence of DADMAC, where the same model consistently underestimated the net surface charge away from the PZC [14]. In the latter system, the Stern capacitance used was higher than in the present case. The necessity of a different Stern capacitance value in the presence of the polyelectrolyte than for the bare surface is a weakness of the present model. This difference probably results from the simplistic description of the charge distribution within the adsorption layer. At the expense of introducing additional param-

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Fig. 5. Induced charge for the Stöber silica particles in the presence of adsorbed DADMAC, normalized with the concentration of the surface-bound monomers, N,s , for different DADMAC loadings and ionic strengths. The lines represent the results of the modified Stern model.

Fig. 4. pH-dependence of the electrophoretic mobilities for the Stöber silica particles in the presence of adsorbed DADMAC, at varied ionic strengths. The surface loadings of DADMAC correspond to those in Fig. 3. Lines represent the standard electrokinetic model.

eters, the model agreement could be eventually further improved by including specific ion binding. Mobility measurements were carried out under similar conditions as the titration experiments. However, these measurements are subject to substantial scatter. The scattering is probably caused by solution aging and insufficient waiting time for the establishment of equilibrium. This problem can be reduced by introducing a longer waiting period prior to the mobility measurement, but in that case, the experimental conditions deviate from those used in potentiometric titrations. The experimental electrophoretic mobility data are shown in Fig. 4. They clearly indicate the charge reversal with increasing pH at the isoelectric point (IEP). The position of this point shifts towards higher pH with increasing polymer dose. Similar trends based on the electrophoresis data were observed for carboxyl latex particles in the presence of DADMAC [14], and for other polyelectrolytes in the presence of oppositely charged particles [5,7,13]. The potentiometric titration data discussed above reflect the similar trends. While the PZC agrees very well with the IEP at the lower polymer dose loading, for the higher dose the IEP lies at somewhat higher pH than the PZC. The different preparation protocol of both experiments and the resulting differences in the ageing is the most likely explanation for these deviations. We suspect that slow diffusion

of the polyelectrolyte into the micropores of the silica sample is responsible for this effect. Another explanation of this discrepancy could originate from a conformational rearrangement of the adsorbed surface film. Adsorption of DADMAC on a planar silica surface was studied with sum frequency spectroscopy, and based on these results a conformational transition from a loopy to a more compact film was suggested near pH 8 [16,28]. This transition may be equally responsible for the observed discrepancy, since the different conformations will be binding different amounts of counterions, and thus might lead to modification of the charge close to the surface. Nevertheless, we suspect that the explanation of this effect due to ageing is the more likely one. The solid lines shown in Fig. 4 were obtained with the modified Stern model. This model was used to estimate the potential at the shear plane (-potential). The position of this plane was assumed to be located at a fixed distance from the plane of origin of the diffuse layer. The resulting potentials were subsequently converted to electrophoretic mobilities by means of the standard electrokinetic model due to O’Brien and White [29]. By comparing with the experimental mobility data, a distance of 1.2 nm from the surface was estimated. This relatively large distance should be compared to the distance of 0.25 nm obtained for bare silica [24]. The larger distance is probably related to the larger thickness of the adsorbed polyelectrolyte layer. One observes that the model reproduces the data in an approximate fashion for the lower polymer dose (Fig. 4a), while at the higher polymer dose the agreement is only semi quantitative (Fig. 4b). The latter failure is related to the fact that the modified Stern model predicts a coincidence between the PZC and IEP, which is not observed experimentally at high polymer dose. Another way to interpret the present titration data is to consider the induced charge due to adsorbed polyelectrolyte, which corresponds to the difference between the actual titratable charge and the ideal mixing model (cf. Fig. 2). The induced charge, normalized to the surface bound polyelectrolyte charge e N,s , is presented in Fig. 5 for the two examined DADMAC loadings. This quantity corresponds to the number of charges induced by the adsorption of a single polyelectrolyte charge. A negative induced charge indicates enhanced deprotonation of the surface silanol groups relative to the protonation state in the absence of adsorption. Conversely, a positive induced charge indicates an enhanced protonation. At low

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ionic strengths, the induced charge is always negative in the whole pH range, meaning that the cationic charges originating from the adsorbed polyelectrolyte exchange with protons, and deprotonate the silanol groups. The magnitude of the induced charge is largest at lowest ionic strength, while the effect becomes less important with increasing ionic strength. At high ionic strength and high pH, the induced charge becomes positive, which suggests a coadsorption of protons. The modified Stern model predicts this behavior relatively well. Similar behavior was observed for carboxyl latex in the presence of DADMAC [14]. Fig. 5 indicates further that the induced charge is very similar for the different DADMAC loadings, when normalized to the surface bound polyelectrolyte charge, N,s . Only at highest ionic strength, a minor difference is observed above the PZC. This observation justifies that the suggested charge partitioning into N,free and N,s does not significantly vary with the protonation state of the surface. One should stress, however, that the free charged groups accounted in N,free do probably not correspond to any dissolved polyelectrolyte. Several authors have shown that near the charge reversal point polyelectrolytes adsorb fully on oppositely charged particles [8,7,15,30]. Therefore, such free charged groups likely belong to the adsorbed polyelectrolyte chains, but are located in extended loops and tails, thus sufficiently far from the surface in order not to contribute to the charging behavior of the surface [16,28]. On the other hand, the surface-bound groups accounted in N,s are tightly bound to the surface, probably as trains, and are directly responsible for the deprotonation of the surface silanol groups. Similar effect was discussed by Kleimann et al. [8]. These authors argue that near the IEP polyelectrolytes typically neutralize just a small fraction of the surface groups, while the larger fraction is neutralized by coadsorbed counterions of the polyelectrolyte. 5. Conclusion Charging behavior of aqueous silica particle suspensions in the presence of the cationic strong polyelectrolyte DADMAC was studied with potentiometric titrations and electrophoresis. The particles undergo charge reversal, which can be directly seen by electrophoresis and as a common crossing point in the titration data. The charge density observed potentiometrically at the point of zero charge can be attributed to the polyelectrolyte charge which is compensated by chloride ions. With increasing polymer dose, this charge reversal point shifts towards higher pH. These trends are well reproduced by a modified Stern model. We conclude that adsorption of the polyelectrolyte is so strong that the polyelectrolyte-coated surface behaves similarly to an amphoteric oxide surface. However, the adsorbed polyelectrolyte deprotonates the surface silanol groups, and thereby induces additional negative charge on the surface. Acknowledgment This research was supported by the Swiss National Science Foundation and the University of Geneva. M.K. is thankful to the support from the MEXT KAKENHI (18688013). References [1] R.S. Farinato, P.L. Dubin, Colloid–polymer Interactions: From Fundamentals to Practice, Wiley, New York, 1999. [2] A.Y. Grosberg, T.T. Nguyen, B.I. Shklovskii, Colloquium: the physics of charge inversion in chemical and biological systems, Rev. Mod. Phys. 74 (2002) 329–345.

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