Destabilization of Titania Nanosheet Suspensions by Inorganic Salts

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Destabilization of Titania Nanosheet Suspensions by Inorganic Salts: Hofmeister Series and Schulze-Hardy Rule Paul Rouster, Marko Pavlovic, and István Szilágyi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b04286 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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The Journal of Physical Chemistry

Destabilization of Titania Nanosheet Suspensions by Inorganic Salts: Hofmeister Series and SchulzeHardy Rule

Paul Rouster, Marko Pavlovic, and Istvan Szilagyi* Department of Inorganic and Analytical Chemistry, University of Geneva, CH-1205 Geneva, Switzerland

ACS Paragon Plus Environment

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ABSTRACT

Ion specific effects on colloidal stability of titania nanosheets (TNS) were investigated in aqueous suspensions. The charge of the particles was varied by the pH of the solutions, therefore, the influence of mono and multivalent anions on the charging and aggregation behavior could be studied, when they were present either as counter or coions in the systems. The aggregation processes in the presence of inorganic salts were mainly driven by interparticle forces of electrostatic origin, however, chemical interactions between more complex ions and the surface led to additional attractive forces. The adsorption of anions significantly changed the surface charge properties and hence, the resistance of the TNS against salt-induced aggregation. On the basis of their ability in destabilization of the dispersions, the monovalent ions could be ordered according to the Hofmeister series in acidic solutions, where they act as counterions. However, the behavior of the biphosphate anion was atypical and its adsorption induced charge reversal of the particles. The multivalent anions destabilized the oppositely charged TNS more effectively and the aggregation processes followed the Schulze-Hardy rule. Only weak or negligible interactions were observed between the anions and the particles in alkaline suspensions, where the TNS possessed negative charge.


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INTRODUCTION The influence of type and valence of dissolved salt constituents on fundamental phenomena such as conformation of polypeptides,1 interaction forces between surfaces2, diffusion of proteins,3 orientation of water molecules around ions,4 aggregation of particles,5 surface tension6 and charging7 or association8 of polyelectrolytes have been reported by numerous research groups in the past decades. Besides, the large number of recent reviews9-14 also indicates widespread contemporary interest in the topic of ion specificity. Concerning the colloidal stability of particle suspensions in the presence of electrolytes, two major theories have been developed, namely, the Hofmeister series15 and the Schulze-Hardy rule.16 As predicted by the classical theory developed by Derjaguin, Landau, Verwey and Overbeek (DLVO),17-19 charged colloidal particles suspended in monovalent salt (i.e., cations and anions are in 1:1 molar ratio) solutions are stable at low ionic strengths, while they rapidly aggregate at higher electrolyte concentrations. The transition between such slow and fast aggregation regimes occurs at the critical coagulation concentration (CCC). The DLVO theory predicts the same CCC for the particles irrespectively of the type or composition of the monovalent salts. However, aggregation studies revealed that the CCC shifts upon variation of the chemical composition of these electrolytes.5,20-23 Although this phenomenon cannot be explained by the DLVO theory, the order of CCCs for different ions usually follows the Hofmeister series.10 For monovalent anions, the sequence can be given as: H2PO4– > CH3COO– > HCO3– > F– > Cl– > Br– > NO3– > I– > ClO4– > SCN– To apply this tendency to colloidal stability of suspensions, the sign of the charge and hydrophobicity of the particles have to be considered.10,12 Accordingly, positively charged hydrophobic particles (e.g., layered double hydroxides22) follow the reversed Hofmeister series


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and the less hydrated anions on the right hand side of the series induce lower CCC. Moreover, the ions on the left side destabilize hydrophilic particles (e.g., chitosan nanocapsules24) at lower concentrations indicating the direct Hofmeister series. For negatively charged particles, however, the tendency is the opposite and the trend in the CCC of the hydrophobic particles (e.g., sulfonated latexes24) can be predicted by the direct series. The hydrophilic ones (e.g., proteincoated polystyrene particles24) follow the reserved order and hence, lower CCCs can be found in the presence of anions on the right hand side. The effect of anions is remarkable with negatively charged particles of lower surface charge, however, ion specificity was absent for highly charged particles of negative surface charge.20,21,25 Although certain multivalent anions (e.g., CO32– or HPO42–) have also been included in the traditional Hofmeister series, the effect of valence on particle aggregation has to be treated differently. The so-called Schulze-Hardy rule16,26 states that the CCC strongly depends on the valence ( z ) of the dissolved ions and the dependence can be quantified as: CCC ∝ z − n

(1)

The variation of the CCC by the valence can also be derived from the DLVO theory, however, the charge of the particles as well as the sign of the charge of the ions has to be considered. For asymmetric electrolytes containing multivalent counterions (i.e., ions of opposite sign of charge as the surface) the dependence can be quantified with n = 1.6 in eq 1 for particles of low surface charge, while a stronger dependence of n = 6.5 can be obtained for highly charged surfaces.25,27 A weak influence ( n = 1 ) of the CCC by the valence of coions (i.e., ions of the same sign of charge as the surface) has also been reported.28,29 Aggregation experiments carried out with latex,27,30,31 carbon derivative,32 clay22 and metal33 particles confirmed the significant shift of the CCCs towards lower concentrations by increasing the valence.


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Given their importance in consumer products34 as well as in biomedical35 and catalytic36 applications, colloidal stability of titania particles in different ionic environments has been in the focus of several research groups in the past. For instance, photocatalytic activity of titania nanoparticles has been assessed in various salt solutions.37 It was found that monovalent anions induced particle aggregation, while multivalent ions stabilized the particle dispersions via specific surface interactions. Such an adsorption process resulted in a competition for the active sites with contaminants and hydroxyl radicals and thus, significantly affected the catalytic activity of titania. Besides, ion specific effect on the structural properties of titania prepared by the sol-gel process has been reported.38 Accordingly, variation of the type of monovalent salts in the reaction mixture led to different surface area due to the control of the ionic medium on the oxide network growth during the synthesis. Moreover, the fundamental phenomena behind the aggregation of titania particles in the presence of mono or multivalent anions have been explored by various techniques. The charge of the particles was tuned by the pH, i.e., positively charged titania was studied below the point of zero charge (PZC) and negatively charged ones above the PZC. In the latter case, comparison of results from light scattering experiments and theoretical calculations revealed that aggregation processes in synthetic titania suspensions can be well-described by the DLVO theory.39 Besides, the CCC of positively charged titania particles was measured on the basis of the turbidity of the samples in the presence of different monovalent anions.40 The sequence in the CCCs of the asprepared particles followed the reversed Hofmeister series recommended for hydrophobic particles, however, most of the ion specific effect disappeared upon calcination of the titania due to the significant loss in the magnitude of the surface charge. In addition, the effect of monovalent and multivalent counterions on the stability of titania suspensions was studied above


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the PZC by photon correlation spectroscopy.41 It was found that the CCC decreased with increasing the valence in agreement with the Schulze-Hardy rule. Apart from these studies, no systematic investigation on the colloidal stability of titania in the presence of mono and multivalent anions (present as either counter or coions) has been published. More importantly, there is a lack of quantitative literature data of the aggregation rates and CCCs of titania particles of elongated structure such as rods, wires or sheets. The growing number of applications of the titania materials of non-spherical shape calls for comprehensive investigation of their suspension stability in different ionic environments. In the present work, charging and aggregation of synthetic titania nanosheets (TNS, structure is shown in Figure 1) were studied in the presence of various mono and multivalent anions by electrophoresis and light scattering. The charge of the particles was tuned by the pH. Accordingly, positively charged TNS were investigated in the presence of different counterions below the PZC, while the influence of the coions was probed in alkaline suspensions above the PZC. The results were interpreted by means of the DLVO theory in principle, however, the tendencies in the destabilization power of the different anions were compared with the ones predicted by the Hofmeister series for the monovalent and the Schulze-Hardy rule for multivalent ions.

EXPERIMENTAL METHODS Materials. TNS was synthesized hydrothermally through a fluorinated intermediate product, as detailed elsewhere.42 Analytical grade salts including KCl (Acros), KNO3 (Sigma), KSCN (Acros), KH2PO4 (Acros), K2SO4 (Fluka), K2HPO4 (Acros), K3Fe(CN)6 (Sigma) and K4Fe(CN)6 (Sigma) were used without further purification. The pH of the suspensions was adjusted by HCl


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(Sigma) and KOH (Sigma) and the measurements were carried out at 25 oC. High purity water (Millipore) was used for preparation of the samples, in which TNS concentration was kept always at 1 mg/L. Methods. Electrophoretic measurements were performed with a Zetasizer Nano ZS (Malvern) device to determine the electrophoretic mobilities ( u ), which were converted to electrokinetic potentials ( ζ ) using Smoluchowski’s model as:43

ζ =

uη

ε 0ε

(2)

where ε 0 is the dielectric permittivity of the vacuum, η is the dynamic viscosity and ε is the dielectric constant. The values of the two latter parameters in water at the temperature applied are 8.9×10–4 Pas and 78.5, respectively. The hydrodynamic radius ( Rh ) was obtained by dynamic light scattering (DLS) using a CGS-3 goniometer (ALV) at 90o scattering angle. Time-resolved measurements were carried out to determine the apparent aggregation rate coefficients ( k app ) as:44

kapp =

1  dRh (t )    Rh (0)  dt t →0

(3)

where t is the time of the experiment and Rh (0) is the hydrodynamic radius of TNS measured in stable suspensions. The measurements were run for 20-60 minutes depending on the speed of aggregation.

RESULTS Similar to other titania compounds,37,39,45-49 the TNS possess pH-dependent charge with a PZC of 5.2, as determined by electrophoresis (Figure 1).42 Therefore, the effect of mono and multivalent


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anions on the colloidal stability was investigated with positively and negatively charged nanosheets at pH 4 and 10, respectively. Surface charge properties were assessed by electrophoresis, while the aggregation processes were followed in time-resolved DLS experiments. Note that the experimental conditions (e.g., pH, salt and TNS concentration) in a given suspension were exactly the same in both types of measurements. The competitive adsorption of ions on titania surfaces has previously been reported45,46,50 and is expected in the present systems too. The influence of such processes on the colloidal stability will be clarified. Positively Charged TNS. The ionization constants of the anions used in the study are given in Table 1. Accordingly at pH 4, monovalent anions include Cl–, NO3–, SCN– and H2PO4–, while SO42– is divalent and Fe(CN)63– is trivalent. Note that 62% of the Fe(CN)64– ions are protonated at the respective pH.51 Note also that the anions are the counterions, since the TNS is positively charged under this experimental condition. Electrokinetic potentials measured at different ionic strengths are shown in Figure 2a. The shape of the plots was similar in the presence of Cl–, NO3– and SCN– anions and the potentials decrease with increasing the ionic strength. The values were the same within the experimental error for the first two anions, while slightly lower potentials were measured in the presence of the latter ion. Furthermore, the fact that the electrokinetic potentials turned to negative at high SCN– concentrations indicates significant adsorption of these anions on the oppositely charged TNS surfaces. The isoelectric point (IEP) is defined as the ionic strength, where the particles do not move in an electric field owing to the fact that their overall charge is zero. Therefore, one can conclude that the decreasing charge with increasing electrolyte level is due to two effects, namely, charge screening by the salt and anion adsorption. The first one is present in all three systems, while the second phenomenon is more pronounced for SCN– and exists in


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less extent in the other cases. Similar charging behavior was reported for positively charged titania particles in the presence of Cl– and NO3– anions,37,52 while comparison is difficult with previous results obtained with SCN– due to the lack of systematic literature data with titania in the presence of this ion. However, adsorption of SCN– on positively charged latex10,21 or clay22 particles and subsequent charge inversion was confirmed by electrophoresis. The charging behavior of the TNS was found to be completely different in the presence of H2PO4– counterions. The pK values of the PO43– (Table 1) indicate the presence of solely monovalent anions in the suspensions at pH 4, nevertheless, H2PO4– adsorption led to remarkable charge reversal of the particles, which showed the highest extent around 10 mM ionic strength. The potentials increased after this minimum due to the charge screening by the salt on the negatively charged nanosheets. Such a characteristic charge neutralization and inversion was also observed with spherical titania particles at low pH in the presence of H2PO4– ions.37,53 Experimental54 and theoretical55 studies carried out with titania surfaces revealed that H2PO4– binds as a bidental ligand to the titania surface atoms, i.e., through two oxygens coordinated by the titanium(IV) ions of the solid. This binding mode results in extremely strong adsorption on the TNS particles and in accumulation of negative charges on the surfaces. The tendencies of electrokinetic potentials in the biphosphate case were very similar to the ones in the presence of multivalent anions (Figure 2a). Namely, their adsorption decreased the TNS charge giving rise to charge neutralization at the IEP and subsequent charge reversal at higher ionic strengths. Towards very high salt concentrations, the potentials decreased, since the elevated level of the K+ counterions screen the surface charge and thus, the TNS move slower in the electric field. Comparing the potential data measured for the positively charged nanosheets in


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solutions of SO42–, Fe(CN)63– and Fe(CN)64– ions, two main features can be identified. First, the location of the IEP was shifted towards lower concentration by increasing the valence of the anions. Second, the extent of the charge reversal, i.e., the magnitude of the potentials at the maximum charge inversion, also increased with the valence of the counterions. These phenomena are due to the increasing affinity of the anions to the positively charged surface by increasing their valence. Similar correlation between the surface charge and the valence of the adsorbing counterions was discovered on the basis of electrophoretic measurements performed with colloidal particles22,25,30,31 including titania.46 In addition, Monte Carlo calculations were carried out to describe the distribution of multivalent ions around and on charged colloidal particles56 and planar surfaces.57 These results and others58-60 from model calculations also revealed that the strength of the counterion adsorption increases with the valence leading to a decreased effective charge and to charge reversal of the surfaces. Comparing the electrokinetic potentials in the presence of Fe(CN)63– and Fe(CN)64–, the higher affinity of the latter anion to the TNS is clearly visible. However, the ionization equilibria (Table 1) shows that only 38% of the ions are present in the Fe(CN)64– form. The remaining part is protonated and the majority of the species is trivalent.51,61 Given the significant difference of the charging properties of TNS in the presence of Fe(CN)63– and Fe(CN)64–, one can conclude that most of the ions close to the surface are tetravalent. Furthermore, the similar charging behavior of positively charged TNS in the presence of monovalent H2PO4– and multivalent Fe(CN)63– or Fe(CN)64– ions is owing to the specific adsorption of the H2PO4– via bidental coordination to the surface, which resulted in high surface density of the H2PO4– molecules and in a pronounced charge inversion. Deprotonation of these anions upon adsorption can also be feasible, however, no experimental evidences were obtained for this feature.


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The colloidal stability of the suspensions was assessed in time-resolved DLS measurements. The apparent rate constants (eq 3) were measured and the speed of aggregation was expressed in terms of stability ratio ( W ) as:44 W=

kapp (fast) kapp

(4)

where fast refers to fast or diffusion controlled aggregation of the particles. One can easily realize that W = 1 in the case of unstable suspensions and higher values indicate slower aggregation and thus, more stable samples. Stability ratios for positively charge TNS in the presence of mono and multivalent anions are presented in Figure 2b. Note that the experimental conditions (e.g., pH, ionic strength range and TNS concentration) were the same as in the electrophoretic studies, therefore, the corresponding charging and aggregation features will be discussed together later. For Cl–, NO3– and SCN– ions, the shapes of the stability ratio versus ionic strength curves were very similar and the trend followed the prediction by the DLVO theory.19 Accordingly, slow aggregation and stable samples were observed at low salt levels, while stability ratios close to unity indicated rapidly aggregating particles and unstable suspensions at high electrolyte concentrations. These regions are separated by the critical coagulation ionic strength (CCIS), which can be converted from the CCC using the ionic strength ( I ) as: I=

1 ci zi2 ∑ 2

(5)

where ci is the molar concentration of ion i with valence zi . It is obvious that CCC and CCIS are identical for monovalent salts, while CCIS is always higher than the CCC for multivalent electrolytes. In the case of Cl–, NO3– and SCN– ions and positively charged TNS, the destabilization occurred due to charge screening, which led to the weakening of the repulsive


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double layer forces and the predominance of the attractive van der Waals interactions. Similar aggregation behavior was reported for spherical39 or elongated62 titania particles below the PZC in the presence of monovalent salts. The trend in the stability ratios changed remarkably once the nanosheets were suspended in the H2PO4– solutions. The stability ratios decreased by increasing the ionic strength at low concentrations and the fast aggregation regime was reached. A further increase of the electrolyte level resulted in restabilization of the suspensions giving rise to extremely high or not even measurable stability ratios in this intermediate concentration regime. Finally, the dispersions were destabilized at higher ionic strengths and the stability ratios decreased to one and remained constant thereafter. Interpreting these data with the help of the results obtained by electrophoresis, one can easily see that the first fast aggregation regime is located around the IEP and hence, the destabilization is due to the lack of charge and repulsive double layer forces. Besides, the restabilization of TNS is the strongest near the minimum in the electrokinetic potentials indicating that the charge inversion resulted in the formation of an electrical double layer around the negatively charged particles, which was sufficiently strong to overcome the attractive forces giving rise to stable suspensions. Moreover, the destabilization at high ionic strengths is due to the screening of the surface charge by the K+ ions, which is also indicated by the decrease in the potentials in this concentration regime. The fact that the trend in the electrokinetic potentials is in good correlation with the stability ratios indicates that the major interparticle forces are of DLVO origin, while the adsorption process changes the extent of the repulsive double layer forces and hence, tunes the colloidal stability of the samples. Such a complex aggregation behavior of titania particles have not been published yet. However, HPO42–


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adsorption led to similar ionic strength dependent colloidal stability for layered double hydroxide particles for instance.22 Regarding the multivalent ions, the shapes of the stability ratio versus ionic strength plots obtained in the presence of SO42– and Fe(CN)63– are similar to the ones measured for the Cl–, NO3– and SCN– ions, however, the CCIS values decrease with increasing the valence. For the Fe(CN)64–, the tendency in the colloidal stability by varying the ionic strength is similar to the one discussed with the H2PO4– and significant restabilization was observed owing to the charge reversal phenomenon. Nevertheless, it is surprising that such a restabilization was not induced by Fe(CN)63– adsorption, albeit, the magnitude of the potentials in the charge inversion region are similar to the ones determined in the H2PO4– system. This fact indicates that the TNS stabilization occurred in the latter samples not solely by the electrical double layers. Indeed, measurements of interaction forces by the colloidal probe technique revealed that the accumulation of the H2PO4– anions on titania surface leads to an anionic layer of 3-4 nm thickness, which induces an additional short-ranged repulsion giving rise to an improved colloidal stability.48 On the other hand, this layer most likely does not form in the presence of the trivalent ions and the strength of the double layer forces is not high enough to overcome the van der Waals attraction and to slow down the TNS aggregation in the intermediate concentration regime. Negatively Charged TNS. In contrast to the above situation in acidic solutions, the deprotonation processes resulted in the formation of divalent HPO42– and tetravalent Fe(CN)64– ions at pH 10. The other anions kept the same valence as at pH 4 (Table 1). The TNS possess negative surface charge in alkaline suspensions (Figure 1), therefore, the anions are the coions in this case.


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In general, the electrokinetic potentials increased with the ionic strength in each case and remained negative in the overall salt concentration regime investigated (Figure 3a). The values measured for the Cl–, NO3– and SCN– ions were identical for the first two anions and slightly smaller for the last one especially at low concentration. Such a similarity for the data measured with Cl– and NO3– was also reported for spherical titania particles.52 For the multivalent ions, the magnitude of the potentials measured at the same ionic strength increased in the SO42– < Fe(CN)63– < HPO42– < Fe(CN)64– order indicating different affinity to the TNS surface of the same sign of charge. Similar ion specific adsorption of multivalent anions on like-charged titania surfaces was published earlier.37,46,53 The surface charge increased with the valence of the anions, with the exception of the HPO42–, which induced a more negative charge than the trivalent Fe(CN)63–. The stronger adsorption of the former ions is owing to the formation of the coordinative bonds54,55 between the HPO42– and the surface titanium(IV) ions leading to a more enhanced adsorption and to an increase in the magnitude of the surface charge density. For the other anions, the extent of the adsorption increased by increasing the valence, however, the driving force in the adsorption process is most likely the hydration level of the ions. Accordingly, the level of hydration decreases in the SO42– > Fe(CN)63– > Fe(CN)64– sequence and hence, the tetravalent anion tend to adsorb on the hydrophobic particle surface stronger than the more hydrated divalent one. Similar relation between the hydration of multivalent ions and the extent of their adsorption was also discovered with other ionic compounds and particles.25,30,31 The stability ratios of TNS measured in the presence of different anions at pH 10 are shown as a function of the ionic strength in Figure 3b. The tendencies can be well-described by the DLVO theory in each cases,17-19 and slow aggregation was observed at low concentrations and fast


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aggregation at high ionic strength. These regions were separated by the CCIS. Although these results indicate similar aggregation mechanism for all of the systems, the destabilization power of the mono and multivalent salts is different, as discussed later.

DISCUSSION Let us now interpret the above detailed experimental results concerning the influence of salt composition and valence of the anions on the colloidal stability of positively or negatively charged TNS. The most appropriate parameter to describe the suspension stability is the CCIS, therefore, its tendencies by variation of the type of electrolytes will be discussed and compared to the ones predicted by the Hofmeister series for anions and the Schulze-Hardy rule. Hofmeister Series. The effect of monovalent anions on the location of the CCIS is presented in Figure 4. The CCIS values were determined from the stability ratio versus ionic strength plots using the following formula:63  CCIS  W = 1+    I 

−β

(6)

where β can be calculated from the dependence of the stability ratios on the ionic strength in the slow aggregation regime as:

β=

d log W dI

(7)

In the case of positively charged particles studied at pH 4, the CCIS values decreased in the Cl– > NO3– > SCN– > H2PO4– order. For the first three ions, the sequence is in agreement with the reversed Hofmeister series recommended for positively charged hydrophobic surfaces.12 Such a trend can be explained by the fact that these anions adsorb on the oppositely charged TNS in different extent. Accordingly, less hydrated SCN– shows the highest affinity to the surface and


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the adsorption decreases the charge density leading to weaker double layer forces and thus, to lower salt concentration necessary to destabilize the dispersion. On the other hand, the wellhydrated Cl– prefers to stay in the bulk, therefore, the adsorption is not significant and the surface charge density of the particles remains high giving rise to stronger repulsion by the electrical double layers and to high CCIS. The same tendency was reported for the CCC of positively charged latex10,20,21 and layered double hydroxide22 particles as well as for titania spheres with Cl– and NO3– anions.40 Furthermore, the situation is more complicated in the H2PO4– samples. As discussed earlier, this anion adsorbs strongly on the positively charged nanosheets leading to charge neutralization and inversion. Such a high affinity led to the lowest CCIS among the monovalent salts under these experimental conditions. However, the reversed Hofmeister series predicts the highest CCIS for the H2PO4– ion. This atypical behavior is caused by the specific interaction between the anion and the TNS surface due to the bidental binding of the H2PO4– through coordinative bonds.37,54,55 Such a surface modification results in charge neutralization and the CCIS corresponds to this phenomenon rather than to the charge screening by the surrounding salt constituents. Similar charging and aggregation behavior and discrepancy from the Hofmeister series were also published with anionic clays.22 Let us now discuss the trend in the CCIS values measured with negatively charged TNS in the presence of the same anions at pH 10. Note that the H2PO4– deprotonates at this pH51 and the HPO42– is the major species in the samples (Table 1). As shown in Figure 4, the CCIS increase in the Cl– < NO3– < SCN– < HPO42– order. The position of the last anion is atypical since the traditional Hofmeister series predicts the lowest CCIS for this molecule, while the trend with first three ones agrees well with the sequence predicted by the direct series for negatively


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charged hydrophobic particles. The discrepancy with the HPO42– is again owing to the formation of the surface complexes, which leads to significant anion adsorption on the like-charged TNS and hence, to higher surface charge density and to the highest CCIS among the ions investigated. For the other ions, the poorly hydrated SCN– tends to adsorb on the hydrophobic nanosheets giving rise to higher surface charge and stronger double layer repulsions. On the contrary, the well-hydrated Cl– does not adsorb in considerable extent, therefore, the TNS are of smaller surface charge magnitude and thus, a lower CCIS was measured. Similar observations were reported for hydrophobic latexes of negative charge,23 however, such a coion effect was not detected for highly charged polymeric particles.20 These results show that the colloidal stability and corresponding sequence in the CCIS of TNS particles in the presence of KCl, KNO3 and KSCN salts can be predicted adequately with the direct or reversed Hofmeister series depending on the charge of the nanosheets. The dependence is stronger, when the anions are the counterions. Besides, the strong affinity of the H2PO4– or HPO42– anions to the TNS surface results in unusually low and high CCIS values under acidic or alkaline conditions, respectively.

Schulze-Hardy Rule. The effect of anion valence on the CCIS of the nanosheets is illustrated in Figure 5. It was shown regarding eq 1 that the trend in the CCC depends on the sign and the extent of the surface charge, therefore, the influence of co and counterions should be treated differently. Concerning the latter case, the relation of the CCIS of strongly charged particles with the valence of anions of asymmetric salts can be described by the DLVO theory as: CCIS ∝ z −4.9

(8)

For particles of low surface charge, the location of the CCIS does not depend on counterion valence.27 The CCIS data of positively charged TNS scatter for the monovalent anions due to ion


17


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specificity, as discussed in the previous section. Moreover, they decrease with the valence in general, but the dependence predicted by eq 8 was observed quantitatively only for the SO42– ions. For the tri and tetravalent anions, however, the experimental results show stronger dependence than the calculated one indicating that these ions interact with the oppositely charged surface specifically, most likely through chemical bonds, similarly to the case of H2PO4–. For the tetravalent case, 38% of the ions are present in the Fe(CN)64– form (Table 1), however, most likely these anions are located in the vicinity of the surface and hence, responsible for the charging properties. Therefore, we have presented Fe(CN)64– as tetravalent in Figure 5. Such an interaction results in nanosheets of somewhat different surface properties (e.g., charge heterogeneity and hydrophobicity) than in the case of other ions. Similar dependence in the CCC of functionalized polystyrene particles has already been reported in the presence of SO42–, Fe(CN)63– and Fe(CN)64– anions.31 Once the multivalent ions represent the coions (e.g., negatively charged TNS suspended in solutions of multivalent anions), the dependence given by the DLVO theory is:29

CCIS ∝ z + 1

(9)

As shown in Figure 5, this relation led to an increase of the CCIS of the negatively charged TNS by increasing the valence of the anions. Excellent agreement was found between the experimental and calculated CCIS values for the K2SO4, K3Fe(CN)6 and K4Fe(CN)6 salts, while the results with K2HPO4 showed stronger dependence as the predicted one. This is again due to the formation of surface complexes with the latter anions, as discussed before. This so-called inverse Schulze-Hardy rule (eq 9) has already been observed for both cationic and anionic latexes,29,64 however, no literature data are available for inorganic colloids including titania particles.


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To further explore the origin of the interparticle forces responsible for the colloidal stability of the suspensions, the DLVO theory was used to calculate the CCIS from the electrokinetic potentials measured at the destabilization point as:

72π  ε εζ 2  CCIS = 2  0  e LB  H 

2

(10)

where e is the base of the natural logarithm, LB is the Bjerrum length and H is the Hamaker coefficient. Their values used in the calculations were 2.72, 0.71 nm and 2.70×10–20 J, respectively. The experimental CCIS data agree with the calculated ones relatively well with the exception of the H2PO4–, Fe(CN)63– and Fe(CN)64– anions interacting with positively charged nanosheets (Figure 6). This finding indicates that the TNS aggregation is driven by DLVO type forces in the majority of the systems, however, the CCIS values vary with the potential owing to the adsorption of the ions especially in the case of positively charged nanosheets. Such an adsorption can change the charge densities in some extent, but the overall interparticle force remains the sum of the repulsive double layer and attractive van der Waals forces. Similar results were reported with latex particles on the basis of aggregation20 and colloidal probe2 measurements. Besides, in the presence of H2PO4–, Fe(CN)63– and Fe(CN)64– anions, additional forces also act between the particles giving rise to significant deviation between the measured and the calculated CCIS. Due to the fact that these ions interact strongly with the particle surfaces, such additional forces may originate from patch-charge interactions. Namely, the anions adsorb in the form of islands on the particle surfaces and the electrostatic attraction between those islands (patch) with the empty and oppositely charged place (charge) of the surface of another TNS leads to the rising of an additional attractive force. CCIS values lower than the calculated ones


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also confirm the presence of additional attractive forces, which were discovered also with positively charged latexes in the presence of Fe(CN)63– and Fe(CN)64– anions.31 In addition, for the H2PO4– system, the ionic layer forming upon anion adsorption48 may induce bridging interaction between the particles and thus, the presence of these attractive forces leads to lower CCIS than the one calculated by the DLVO theory. In the absence of chemical interaction between the anions and the surface and subsequent additional forces, however, the aggregation processes are mainly driven by DLVO-type double layer and van der Waals interactions, as in the case of the majority of the anions investigated in the present study.

CONCLUSIONS Ion specific effects on charging and aggregation of TNS were investigated in aqueous suspensions. Electrophoretic and light scattering experiments were performed in the presence of various mono and multivalent anions with either positively or negatively charged particles below or above the PZC, respectively. The majority of the monovalent ions destabilized the suspensions by charge screening, however, their different extent of adsorption led to variations in the CCIS values, which separate slow and fast aggregation regimes. The sequence in the CCIS was in good agreement with the direct or reversed Hofmeister series for anions depending on the charge of the TNS. Nevertheless, the CCIS in the presence of H2PO4– and HPO42– anions does not follow the trend predicted by the Hofmeister series and this atypical behavior is owing to the bidental coordination of these species to the TNS and subsequent accumulation of negative charges on the surface, which also leads to charge inversion of the nanosheets when used below their PZC.


20

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For positively charged TNS, the CCIS values decreased with the valence of the anions and pronounced charge reversal was reported for the di, tri and tetravalent ones due to their significant adsorption on the nanosheets. In the latter case, this phenomenon induced a restabilization of the particles due to the sufficiently high surface charge density upon anion adsorption. The CCIS values qualitatively followed the Schulze-Hardy rule by varying the valence of the anions, however, specific interactions with the surface gave rise to a stronger dependence for the Fe(CN)63– and Fe(CN)64– ions. For negatively charged particles, the inverse Schulze-Hardy rule applies to describe the change in the CCIS by the valence of the anions. Comparison of the experimental results with the ones obtained by calculations based on the DLVO theory revealed that repulsive electrical double layer and attractive van der Waals forces were responsible for particle aggregation in the majority of the systems, however, the CCIS values depended on the composition and the valence of the salts due to the different affinity of the anions to the surfaces. The presence of forces of non-DLVO origin were discovered only with anions (H2PO4–, HPO42–, Fe(CN)63– and Fe(CN)64–), which are in strong interaction (e.g., through primary chemical bonds) with the nanosheets. The present results shed light on the importance of ion specificity on colloidal stability of TNS dispersions. Since only limited data are available for spherical titania materials and no information was published with titania of elongated structure, these findings are of special importance for scientist working with titania nanowire, nanosheet or nanorod suspensions in the presence of mono or multivalent electrolytes or in their mixtures.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Phone: +41 22 3796031.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support by the Swiss Secretariat for Education, Research and Innovation (C15.0024), Swiss National Science Foundation (150162), COST Action CM1303 and University of Geneva is gratefully acknowledged. Special thanks to Professor Michal Borkovec for the possibility to use the facilities in his laboratory.

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Table 1. Ionization of the inorganic anions in acidic and alkaline solutions. Anions a

pK1 b

pK2 b

pK3 b

Valence c pH 4

pH 10

Cl–

-0.71

–

–

1.00

1.00

NO3–

-1.40

–

–

1.00

1.00

SCN–

-2.10

–

–

1.00

1.00

SO42–

1.98

-2.00

–

1.99

2.00

PO43–

12.35

7.21

2.14

0.99

2.00

Fe(CN)63–

–

–

–

3.00

3.00

Fe(CN)64–

4.20

2.00

-1.00

3.38 d

4.00

a

Composition of anions shown in the fully ionized stage. bThe protonation constants were taken from the Joint Expert Speciation System.51 cAverage valence calculated from the ionization equilibria at the respective pH. dThe solution contains 38% of Fe(CN)64– and 62% of HFe(CN)63– at pH 4.


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Figure 1. Schematic illustration of the structure of the TNS. The charging behavior is also presented by the electrokinetic potentials measured by electrophoresis in aqueous suspensions at different pHs and 1 mM ionic strength. Parameters d and h were measured by DLS and electron microscopy, respectively, while c and a were taken from the literature.65


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Figure 2. Electrokinetic potentials (a) and stability ratios (b) of TNS in the presence of monovalent (left column) and multivalent (right column) anions. The measurements were performed at pH 4 (positively charged particles) and 1 mg/L particle concentration. The solid lines in (a) serve to guide the eyes, while in (b) show the results of calculations using eq 6.


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Figure 3. Electrokinetic potentials (a) and stability ratios (b) of TNS in the presence of monovalent (left column) and multivalent (right column) anions. The measurements were performed at pH 10 (negatively charged particles) and 1 mg/L particle concentration. The solid lines in (a) serve to guide the eyes, while in (b) show the results of calculations using eq 6.


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Figure 4. CCIS values of TNS as a function of anions ordered in the Hofmeister series. Positively charged particles with different monovalent counterions (squares) were measured at pH 4, while TNS of negative charge with coions (circles) at pH 10. The results with the KCl, KNO3 and KSCN samples are enlarged in the inset. The solid lines are to guide the eyes.


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Figure 5. Relative CCIS values (normalized to the CCIS obtained in the presence of KCl in acidic suspension) as a function of valence of anions. Positively charged particles with different counterions (squares) were measured at pH 4, while TNS of negative charge with coions (circles) at pH 10. The solid lines indicate the traditional (eq 8) and the inverse (eq 9) Schulze-Hardy rule. Note that only 38% of the Fe(CN)64– ions are in the tetravalent form (Table 1), but these are the major anionic components close to the oppositely charged surface.


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Figure 6. CCIS values as a function of the electrokinetic potentials measured at the CCIS. Positively charged particles with different counterions (squares) were measured at pH 4, while TNS of negative charge with coions (circles) at pH 10. The solid line indicates the results of calculations using the DLVO theory (eq 10).


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Destabilization of Titania Nanosheet Suspensions by Inorganic Salts

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