Effect of 1-1-charged ions on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide

Colloids and Surfaces A: Physicochem. Eng. Aspects 279 (2006) 10–19

Effect of 1-1-charged ions on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide Nataliya H. Tkachenko a,∗ , Zinoviy M. Yaremko a , Cornelia Bellmann b a

Faculty of Chemistry, Ivan Franko National University of L’viv, Kyryla & Mefodiya St. 6, UA-79005 L’viv, Ukraine b Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany Received 12 June 2005; received in revised form 24 September 2005; accepted 30 September 2005 Available online 23 March 2006

Abstract Dependencies of zeta potential of titanium dioxide particles and aggregative stability of its aqueous suspensions from pH in the solutions of alkaline metals halogenides: KF, KCl, KBr, LiF, LiCl and CsCl have been investigated. It has been shown that the dependence of zeta potential of titanium dioxide particles on the pH medium has a typical character, which is natural for the suspensions of amphoteric oxides of metals with isoelectric point of the medium at pH 3.60 for LiF, 6.00—LiCl, 6.10—KF, 6.21—CsCl, 6.35—KBr, 6.70—KCl. It has been established that displacement of isoelectric point depends on the relation of the hydrated ions charge to their radii. It has been found that aggregative stability of aqueous suspensions of titanium dioxide at isoelectric point is not the lowest, as it is to expect from the classical DLVO theory. Aggregative stability for KF and LiCl is even the highest at isoelectric point and commensurate with aggregative stability in the alkali region, where it is the best for all the salts in question. Aggregative stability at isoelectric point in solutions KCl, KBr and CsCl is somewhat lower compared to KF and LiCl, and the region of better stability is located left of isoelectric point, while for LiF it is located right of isoelectric point. These deviations of aggregative stability at isoelectric point from the classical DLVO theory are discussed taking into account the ability of background electrolyte ions to put into order or disorder the water structure. © 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Aggregative stability; Electrical surface properties; Alkali metals halogenides; Pigment

1. Introduction The problem of regulation of varnish and pain compositions properties still remains to be important despite the great number of investigations [1]. Scientific interest to above problem is supported by a lot of reasons, mainly by complication and interconditionality of processes that take place at making of varnish and paint compositions. A characteristic feature of these processes is that the equilibrium in the system is established in the result of their competitive course. If the number of components of varnish and paint compositions increases, the number of interactions increases as well and the systems in question become more complicated.

∗

Corresponding author. Tel.: +380 32 270 7557; fax: +380 32 294 8181. E-mail addresses: [email protected] (N.H. Tkachenko), [email protected] (Z.M. Yaremko), [email protected] (C. Bellmann). 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.09.037

Water, as the main component of aqueous suspensions, is one of the most complicated subjects of investigations [2]. Due to hydration of ions Essential changes in the structure of water have place due to hydration of ions into electrolyte solutions. These changes are viewed from the standpoint of both, thermodynamics and kinetics of physico-chemical processes. Thermodynamic sight at a problem studies interaction between the ion and the molecule of water and envisages creation of stable hydrate complexes, in which ion is strongly bonded with a certain number of molecules of water. Kinetic sight at a problem considers interaction of molecule of water—molecule of water in the presence of ions, and studies influence of ions on the translational movement of surrounding them molecules. Influence of the ion on the water structure is manifested in two effects [3]: • effect of disorder, in which exchange of water molecules around the ion occurs more often, than exchange of molecules in water itself, and hydration entropy on the boundary of divi-

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Nomenclature aY aX A131 ci C1 C0 d d0 da D e F H k K1 K2 KX KY n NA NS pH0 q R r ri rX rY T Uef VA VR z
S λ ϕ0 χ σ0 η ε ε0 ζ

anions activity cations activity constant of dispersion interaction of solid particles 1 through dispersion medium 3 concentration of monobasic acid or monoacid alkali capacity Helmholz’s external space volume concentration of electrolyte particle diameter diameter of primary particle diameter of aggregate diffusion coefficient charge of electron Faradey’s constant shortest distance between particles surfaces Boltzman’s constant constant of acidity for the surface hydroxyl groups constant of basicity for the surface hydroxyl groups constants which determines of anions constants which determines of anions quantity of primary particles in aggregate Avogadrov’s constant density of surface hydroxyl groups pH in isoelectric point ion charge universal gas constant particle radius radius of hydrated ion radius of cation radius of anion temperature electrophoretic mobility energy of attraction energy of repulsion valency of counterion change of hydration entropy descriptive wavelength surface potential converse thickness of electrical double layer specific surface electrical charge viscosity relative dielectric permittivity of the medium dielectric constant zeta potential

sion between ion and solvent increases (such hydration is called positive); • effect of order, in which exchange of water molecules around the ion occurs more seldom, than exchange of molecules in water itself, and hydration entropy on the boundary of division between ion and solvent reduces (the so-called negative hydration).

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Integral estimates of ion hydration are the number of hydration and the effective radius of hydrated ions. Processes of distribution and re-distribution of the molecules of water in electrolyte solutions are very complicated and depend on many factors: nature of anions and cations, their concentration, temperature, etc. Taking into account these processes it is possible to confirm, that there are no two similar electrolyte solutions with respect to the structure of water. Presence in researched systems the phases’ interface solid–liquid requires taking into consideration the influence of subsequent surface forces on water structure in upper layers [4]. Now nobody doubts that formation on the phases’ interface solid–water of upper layers has properties distinctive from the properties of volumetric water. Three-layered structure of surface layers, which consists of the first layer sorptionally (strongly) constrained water of thickness around 1 nm, the second layer of the structurally constrained water of 7.5–10 nm thickness forming in the result of directing influence of surface and the third layer of the osmotically constrained water of thickness of 15–20 nm, has been also proved [5]. Water structure in upper layers of phases’ interface solid–liquid depends upon surface nature [6]. Near hydrophobic surfaces water dipoles direct themselves parallel to surface but near hydrophilic-perpendicularly to it. Such direction of water molecules results appearance the structure forces in thin layers of water that cause attraction of hydrophobic and repulsing of hydrophilic surfaces. Degree of water structuring near hydrophobic and hydrophilic surfaces depends upon intensity of their interaction with the water molecules. Deryagin and Churayev [7] have shown that influence of surface on water structure in upper layers is inessential if equilibrium border angle is located within from 10 up to 50◦ . Besides, direction of water molecules near the surfaces is observed under certain conditions, namely: at low concentrations of electrolytes and/or at low temperature. With increasing of electrolytes concentration and/or temperature upper layers are gradually destroying and at concentration more than 0.1 mol/l and/or at temperature of 65–70 ◦ C there is no differences between structure of surface and volumetric water. Golikova et al. in their work [8] have shown that in isoelectric point structural forces and, obviously, the appropriate water structure in upper layers develop only at low degree of ionization of surface groups, namely, less than 0.01. Adsorption of ions n solutions of electrolytes has determining influence on aggregate stability and surface characteristics of water suspensions [9,10]. In this area important are researches of adsorption kinetics on the charged surface [11] and cationsalvation-anion interactions in double electric layer at high ionic strengths [12–14]. Interaction between the colloidal charged particles occurs through their double electric layer. Theoretical and experimental researches are concentrated on application the Poisson–Boltzmann theory to double electric layer and regulation of stability of colloid systems [15]. Last researches in the field of the electrokinetic phenomena and interparticle interactions from the point of view of dynamic interpretation are generalized in works Lyklema et al. [16,17].

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All the abovementioned processes, usually, proceed while manufacturing water dispersions with additives of low- and high-molecular substances, but degree of their development is different and inter caused. That is why, the properties of received dispersions are forming due to competitive development of parallel-series processes. Primary among them are those, which occur with higher rate and are connected with the formation of upper layer of water on oxides of metals and the formation of double electric layer.

- fixanal solution HCl (Fluka) to prepare 0.1 M solution. - 48% solution HF with density of 1.15 g/ml (Sigma–Aldrich) to prepare 0.1 M HF solution. - 47% solution HBr with density of 1.49 g/ml (Sigma–Aldrich) to prepare 0.1 M HBr solution. - 20% aqueous solution of cationic polyelectrolyte–polydiallyl-dimethyl-ammonium-chloride (poly-DADMAC) (Katpol Chemie Bitterfeld, Germany) to prepare 1 × 10−3 M solution. Molecular weight of Poly-DADMAC 35,000 g/mol.

2. Materials and experimental methods

Deionized water with specific electrical conductivity of 0.055 ␮S/cm, purified with the help of Millipore-Q instrument.

2.1. Materials and reagents description 2.2. Preparation of TiO2 suspensions The following materials and reagents were used for this analysis: Titanium dioxide powder of rutile type which surface is modified by inorganic oxides (4% Al2 O3 and 2% SiO2 ) and inoculated organic groups with aim to provide with necessary optical properties and regulation of hydrophobic-hydrophilic balance (DuPont, USA). The powder particles are of round shape with average diameter of 0.23 ␮m (Fig. 1). Specific surface of titanium dioxide powder, determined by BET method according to nitrogen adsorption, makes up 14.6 m2 /g. Calculated geometrical surface is 6 m2 /g. Maximum quantity of charge on surface of particles is equal to 3.86 C/g. Reagents: - solid substances: • KF (99.99%, Sigma–Aldrich), KBr (99.99%, Merck), LiCl (99.99%, Merck), CsCl (99.5%, Merck), LiF (99.9%, Merck) to prepare 1 × 10−3 M solutions. • LiOH (98%, Sigma–Aldrich), KOH (86%, Sigma– Aldrich), CsOH·H2 O (99.97%, Sigma–Aldrich), NaCl (99%, Sigma–Aldrich) to prepare 0.1 M solutions. - 0.1 M solution KCl (Fluka) – to prepare 1 × 10−2 to 1 × 10−5 M solutions.

For preparation of suspensions, 1 × 10−2 to 1 × 10−5 M solution of a certain salt was added to the volumetric flask of weigh out titanium dioxide, calculating so that concentration of TiO2 would be 0.03 g/l. The content of the flask was dispersed within 2 min in ultrasonic bath. The choice of concentration of solid phase in suspension was caused by technical chances of the instruments used for analysis. The analysis of pH medium influence on suspensions’ properties was conducted on a large scale of pH change, from 2 up to 12. The change of pH medium was executed by adding to suspensions the necessary quantity of 0.1 M solution of acid or alkali, which contained anion or cation in the composition of salt on which solution’s basis the suspensions were prepared. During experiments of determining particles diameter, zeta potential and suspensions’ conductivity pH was changed within one test, gradually increasing the quantity of 0.1 M acid solution or alkali in the suspensions received by the methods mentioned above. 2.3. Experimental methods 2.3.1. Determination of TiO2 specific surface Specific surface of titanium dioxide powder was determined by BET method according to nitrogen adsorption (Autosorc-1 instrument, Quantachrome, USA). Prior the analysis TiO2 powder was dried within 2 h at temperature of 373 K in the vacuum. The measurements were made at temperature of 77.4 K. The calculations were made in accordance to equation of BET polymolecular adsorption, which is used for adsorption isotherms of the second and the third type [18]. 2.3.2. Potentiometric titration The quantity of specific charge on TiO2 surface was defined by the method of potentiometric titration of titanium dioxide suspensions by cationic polyelectrolyte poly-DADMAC on instrument, which combined particle charge detector—M¨utek PCD 03-pH, and Mettler Titrator DL 21 (M¨utek Analytic GmbH) [19].

Fig. 1. SEM image of TiO2 particles.

2.3.3. Scanning electron microscopy Scanning electron microscopy (SEM) images were taken with Gemini microscope (Zeiss, Germany). Samples were

N.H. Tkachenko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 279 (2006) 10–19

prepared in the following manner. Suspensions were diluted with deionized water, dropped onto aluminium support and dried at room temperature. Samples were coated with thin Au/Pd layer to increase the contrast and quality of the images. Pictures were taken at voltage of 4 kV. 2.3.4. Electrokinetic analysis The electrokinetic measurements were made by the method of microelectrophoresis [20,21], using Zetasizer 3 (Malverm Instruments Ltd., UK) instrument. The rate of particles movement under the influence of external electrical field with voltage of 150 V (electrophoretic mobility, Uef ) was measured with the help of Doppler’s lazar anemometer where He–Ne lazar ray was used. The solution of the same salt on the basis of which suspension was prepared, became the solution for comparison. Zeta potential of particles was calculated under the condition that electrokinetic radii are greater 1, according to the Helmholz–Smolukhovskiy’s equation [20]: ζ=

ηUef , εε0

(1)

where η is the liquid viscosity; ε and ε0 , the relative dielectric permittivity of the medium and dielectric constant, correspondingly. The measurements of conductivity and pH suspensions were simultaneously conducted on the same instrument. The calibration of the instrument was executed by measuring of zeta potential of standard latex suspension and pH of standard buffer solutions from pH 4 and 7.

3. Results and discussions A number of works are devoted to the studies of electrical surface properties and aggregative stability of aqueous suspensions, but the analysis was conducted mainly in the solutions of low-molecular surfactants or polyelectrolytes. This work deals with discussion of results studies of dependencies of titanium dioxide zeta potential particles and the aggregative stability of its water suspensions on pH medium in the solutions of alkaline metals halogenides: KF, KCl, KBr, LiF, LiCl, CsCl. 3.1. Dependencies of titanium dioxide zeta potential particles on pH medium in the solutions of alkaline metals halogenides Electrical surface properties of titanium dioxide water suspensions was described as one of the integral estimates of electrical double layer by zeta potential particles. The surface charge of the metal oxides in water solutions of electrolytes are forming due to reaction of acid–basic interaction of solvent (aqua) with surface functional groups of acid and basic nature. In this case, the potentially determining are the ions H+ or OH− and, that is why, value and sign of the surface charge depends upon pH medium in conformity with chemical equilibrium, which are established with participation of the amphoteric surface hydroxyl groups ( MeOH): K1

+ MeOH+ 2(solid) ←→ MeOH(solid) + H(solution) , K2

2.3.5. Measurement of diameter of particles in suspensions In suspensions, researched on Zetasizer 3 instrument, the measurement of average size of the particles was simultaneously conducted on the Zetasizer 3000 (Malvern Instruments Ltd., UK) instrument in plastic cuvettes (Sardstedt AG). Since particles of suspensions subject to the thermal Brownian motion and the a rate of particles movement depends upon their size, it is possible on the basis of dynamic light scattering [22] of photon-correlation spectroscopy through coefficients of diffusion to determine directly the diameter of particles d from the following relation: d=

kT , 3πηD

(2)

where D is the diffusion coefficient; k, the Boltzman’s constant; T, the temperature; π = 3.14; η, the medium viscosity. The results of measurements were recorded as Gaussian distribution of particles according to their size and average diameter. During the experiments on Zetasizer 3 and Zetasizer 3000 instruments concentration of solid phase made up 0.03 g/l and temperature −25 ◦ C. The mean value from three measurements was accepted as a result.

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+ MeOH(solid) ←→ MeO− (solid) + H(solution) ,

(3)

where K1 and K2 are the constants which are the quantitative measure of acidity and basicity for the surface hydroxyl groups. One of the distinctive points of zeta potential dependency on pH medium is isoelectric point, which indicates pH medium, where zeta potential is equal to zero: pH0 = 0.5(pK1 + pK2 )

(4)

In the solutions of 1-1-charged electrolytes (XY), the ions different from H+ and OH− can also enter specific absorption interaction with the ionized surface hydroxyl groups and cause the following equilibrium: − + − MeOH+ 2(solid) + Y(solution) ←→ MeOH2 · · · Y(solid) , KY

+ − + MeO− (solid) + X(solution) ←→ MeO · · · X(solid) , KX

(5)

where KY and KX are the constants which determines the degree of anions and cations’ bonding of background electrolytes, correspondingly. The influence of background electrolyte on the position of isoelectric point has been theoretically studying in the works [23,24]. The following has been shown in the work [23]:

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Fig. 2. Dependency of zeta potential (a, c, e) and diameter (b, d, f) of particles TiO2 upon pH suspensions in solutions of 1-1-charged electrolytes with concentration 1 × 10−3 mol/l. Dotted line on b, d, f points out the location of isoelectric point.

pHζ=0 = 0.5(pK1 + pK2 ) −

(0.43 F 2 NS /C1 NA T ) (KY aY − KX aX ) , 2 + (K1 /K2 )1/2 + KY aY + KX aX

(6)

where F is the Faradey’s constant; NA , the Avogadrov’s constant; NS , the density of surface hydroxyl groups; C1 , the capacity Helmholz’s external space; aY , aX , the anions and cations activity, correspondingly. As it is shown in the Eq. (6), location of isoelectric point depends upon both nature and concentration of background electrolyte. Dependencies of zeta potential particles of titanium dioxide on pH medium in the solutions of alkaline metals halogenides KF, LiCl, KBr, KCl, CsCl and LiF are shown in Fig. 2a, c, e and the position of isoelectric point in Table 1.

If all ionized surface groups are uniform, it should be expected that the degree of ions bonding of background electrolyte will depend upon the ratio of charge to radius of hydrated ions q/ri where q, the ion charge; ri , the radius of Table 1 Position of isoelectric point in solutions of background electrolytes with concentration 1 × 10−3 M Electrolyte

pH0

LiF LiCl KFv CsCl KBr KCl

3.60 6.00 6.10 6.21 6.35 6.70

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Fig. 3. Dependency of
pH0 = pH0X2 − pH0X1 upon ratio (rX1 /rX2 ) for various cations at a common anion Cl− where X1 and X2 accordingly Li+ and K+ , Cs+ and K+ , Cs+ and Li+ (a) and
pH0 = pH0Y2 − pH0Y1 on ratio (rY1 /rY2 ) for various anions at a common cation K+ where Y1 and Y2 accordingly F− and Cl− , Br− and Cl− , Br− and F− (b).

hydrated ion. So, it should be expected that the higher the difference of ratio q/ri for ions will be, the higher will be the difference between the position of isoelectric point for various anions or cations at a common cation or anion, correspondingly. The Fig. 3a shows dependencies
pH0 = pH0X2 − pH0X1 from the ratio (rX1 /rX2 ) for various cations at a common anion Cl− where X1 and X2 accordingly Li+ and K+ , Cs+ and K+ , and Cs+ and Li+ , and Fig. 3b shows dependencies
pH0 = pH0Y2 − pH0Y1 on the ratio (rY1 /rY2 ) for various anions at a common cation K+ where Y1 and Y2 accordingly F− and Cl− , Br− and Cl− , and Br− and F− . The radii of hydrated ions are taken from the work [25]. As it is seen from this figure, while increasing the ratio of charge to radii of hydrated ions the difference between positions of isoelectric point increases as well. But expected difference between position of isoelectric point, i.e.
pH0 = 0 at (rX1 /rX2 ) = 1 or (rY1 /rY2 ) = 1 is not observed. Probably this effect is caused by interaction cation–salvation–anion in double electric layer [12]. It is obvious that the nature of common cation or anion also gives impact on change of the position of isoelectric point, namely: the lower the correlation q/ri of common

Fig. 4. Dependency of zeta potential (a); particles diameter (b) and electrical conductivity (c) of TiO2 suspensions upon pH suspensions at various concentration KCl, mol/l: (
) 1 × 10−5 ; () 1 × 10−4 ; () 1 × 10−3 ; (䊉) 1 × 10−2 .

ion is, the higher is the difference in the positions of isoelectric point. Comparison of the pair LiCl and KCl with common anion Cl− and the pair LiF and KF with common anion F− shows that the difference in the positions of isoelectric point increases from 0.7 for the first pair and up to 2.5 for the second one. Analogical comparison of the pair KF and KCl with common cation K+ and the pair LiF and LiCl with common cation Li+ also shows that difference in the positions of isoelectric point increases from 0.6 for the first pair and up to 2.4 for the second one. The change of concentration of background electrolyte KCl from 1 × 10−5 mol/l up to 1 × 10−2 mol/l practically does not changes the position of isoelectric point (Fig. 4a). It can be explained by the reason that the degree of bonding of both ions K+ and Cl− is according changing with the increase of their concentration in solution.

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3.2. Dependencies of aggregative stability of titanium dioxide suspensions on pH medium in solutions of alkaline metals halogenides The aggregative stability of titanium dioxide suspensions was estimated according to average diameter of particles. The investigated powder of titanium dioxide by the results of electronic microscopy has particles of average diameter 0.23 ␮m. Depending on the balance of subsequent surface forces, primary particles interacting between each other and forming aggregates, which sizes can be estimated according to the correlation [26]: da = n2/3 d0 ,

(7)

where da , d0 are the diameter of aggregates and primary particles, correspondingly; n, the quantity of primary particles in aggregates. The dependency of titanium dioxide particles on pH medium in solution of alkaline metals halogenides—KF, LiCl, KBr, KCl, CsCl and LiF is shown in Fig. 2b, d, f and in solution KCl at its various concentrations in Fig. 4b. As it is seen from the results provided above, for all electrolytes within the whole range of change of pH medium, average diameter of particles exceeds the diameter of primary particles. This certifies that primary particles of titanium dioxide in water suspensions are located in aggregative state. Even under the conditions of the best stabilization at pH medium higher 10 in accordance to the Eq. (7) there exist aggregates consisting of two to four particles. It should be noted that two processes take place during preparation of suspensions: the first one: re-dispersion, since high-dispersed powders in the air are in aggregative state [27] and while mixing them with water, they should be destroyed, but the conditions for their full destruction cannot always be reached and partial but not complete re-dispersion is taking place and aggregates of particles remain in suspensions [28]; the second one: stabilization of particles which left aggregates on the stage of re-dispersion.

In case of electrolytes’ solution, dispersion, ionic-electrostatical and structural constituents are taken into account. Firstly, have been conducted the analysis of received results according to the classical DLVO theory which considers energy of particles interaction as an amount of energy of attraction VA (H) and repulsion VR (H). The attraction energy of two identical spherical particles, taking into consideration electromagnetic delay, was approximated according the equation [30]: VA (H) = −

rA131 λ at H ≤ 15 nm 12H λ + 11.116H

(8)

and


rA131 2.45λ 0.59λ3 2.17λ2 VA (H) = − − − 12H 10πH 60π2 H 2 280π3 H 3 at H > 15 nm

(9)

where H is the shortest distance between particles surfaces; λ, the descriptive wavelength equal to 0.1 ␮m; A131 , the constant of dispersion interaction of solid particles 1 through dispersion medium 3; r, the particle radius. The repulsing energy of charged particles was calculated according to equation [31]:    ezϕ  exp(−χH) kT 0 VR (H) = 16εε0 th2 r2 , (10) ez kT 4 2r + H where ε is the relative dielectric permittivity of the medium; ε0 , the dielectric constant, k is the Boltzman’s constant; T, the temperature; e, the charge of electron; z, the valency of counterion; ϕ0 and χ, the surface potential and converse thickness of electrical double layer, correspondingly. While calculating particles radius was taken equal to radius of primary particles of titanium dioxide r = 0.115 ␮m, and constant of dispersion interaction as A131 = 10kT [32]. Surface potential of particles was calculated according to the equation [33]:  zeϕ  σ0 0 √ = sh 2kT NS e 8εε0 RTC0 [H+ ] − K1 K2 2

The dimension of particles diameter, fixed during analysis, depends upon both the degree of re-dispersion and the degree of stabilization [29]. Since, all suspensions were prepared on the basis of deionized water and then their pH have been changed, the conditions for re-dispersion for all suspensions were equal, that is why, the change of pH suspensions can influence only on the process of stabilization of particles. The degree of aggregative processes’ development or the level of suspensions stabilization depends upon the intensity of particles interaction. Today, it is known that in the thin layers between the dispersion particles, forces connected with five constituents of splitting pressure can appear [4]: dispersive, ionic-electrostatical, adsorptive, structural and sterical. The importance of each of these constituents in ensuring of stability of dispersion of particles in each concrete case can be determined by the effects of overlapping of proper areas of their existence.

=

, √ 2 (K1 [H+ ] + K1 K2 + [H+ ] )NS e 8εε0 RTC0 (11) where NS is the total amount of hydroxyl groups on a unit of metal oxides surface of proper chemical nature; R, the universal gas constant; C0 , the volume concentration of electrolyte; σ 0 , the specific surface electrical charge. The quantity conversed to thickness of electrical double layer was calculated according to the formula [20]:  e2 2i=1 (zi ci ) χ2 = , (12) εε0 kT where ci is the concentration of additives of monobasic acid or monoacid alkali at pH medium change (i = 1) and electrolyte (i = 2). The Fig. 5 shows typical energy curve of interaction of two disperses particles (a) and the dependency of energy barrier

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Table 2 Change of hydration entropy on the interface between ion in solution and the solvent,
S, which is connected with structural changes of water at ions hydration [3] Ion


S (J/mol K)

Li+

−55.6 +34.3 +59.0 −70.0 +6.30 +27.6

K+ Cs+ F− Cl− Br−

height Vmax /kT on pH dispersion medium for suspensions with various KCl concentrations (b) and for the suspensions with various electrolytes (c). The calculated energy curve of particles interaction does not correspond to changes of aggregative stability at pH medium change (Figs. 2b, d, f and 4b). The essence of disparity is that the lowest aggregative stability (maximum dimension of particles diameter) for ionic-electrostatical stabilized dispersion systems should be observed in isoelectric point where energy barrier between particles disappears (Fig. 5b and c), but experimental results (Figs. 2b, d, f and 4b) show that maximum on dependency of particles diameter on pH medium is somewhat shifted relative to isoelectric point. Deviation from classical DLVO theory in the area of isoelectric point can be explained by the availability of structural forces [8,34–36], which are caused by proper water structure in upper layers. As it is known, introduction of ions into water changes its structure. Both stabilization of water structure around ion in the result of strengthening of hydrogen bonds (ordering effect) and excitement of structure water around ion in the result of weakening of hydrogen bonds (disordering effect) are observed. Changes of hydration entropy on the interface between ion in solution and the solvent, shown in Table 2 [3], can be quantitative estimate of such processes of ordering and disordering in water structure around ions.

Fig. 5. Schematic image of the dependence of total energy of particle interaction versus distances between particles (a); dependence of height of energetic barrier Vmax /kT upon pH at various KCl concentrations, mol/l: () 1 × 10−5 ; (♦) 1 × 10−4 ; () 1 × 10−3 ; () 1 × 10−2 (b) and in presence of various electrolytes with concentration 1 × 10−3 mol/l: (×) LiF; (
) LiCl; (䊉) KF; (*) CsCl; () KBr; () KCl (c).

Fig. 6. Dependency of specific electrical conductivity of TiO2 suspensions upon pH suspensions in 1 × 10−3 mol/l salts solutions.

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Fig. 7. The schema of formation of two peaks of aggregate instability of suspensions on dependency of particles’ sizes upon suspensions pH.

Comparison of change of entropy with change of aggregative stability of suspensions around isoelectric point has allowed to expose the following effects: 1. If cations and anions, which possess disordering and ordering effect, namely: K+ and F− , and Li+ and Cl− , are the constituents of electrolyte, maximum aggregative stability is observed in isoelectric point, but while removing from isoelectric point to the both sides aggregative stability becomes worse at decreasing or increasing of medium pH relatively of pH0 (Fig. 2b). 2. If cations and anions, which possesses disordering effect only, namely K+ , Cs+ , Cl− and Br− , are the constituents of electrolyte, isoelectric point is located in the area between maximum and minimal aggregative stability of suspensions. At that if medium pH is less than pH0 , maximum of aggregative stability is observed, and if medium pH is higher than pH0 —minimum of aggregative stability (Fig. 2d). 3. If cations and anions, which possesses ordering effect only, namely Li+ and F− , are constituents of electrolyte, isoelectric point is also located in the area between maximum and minimal aggregative stability of suspensions, bat if medium pH is less than pH0 , minimum of aggregative stability is observed, and if pH medium is more than pH0 —maximum of aggregative stability (Fig. 2f) (Fig. 6).

taken into account, that under conditions of high concentration of electrolytes, the problem of ionic-electrostatic interaction of particles requires others quantitative researches [37]. Along wit it, it should be marked, that the influence of KCl concentration on aggregative stability is nicely described by classical DLVO theory, because while increasing KCl concentration in suspensions within the whole range of changes of medium pH, worsening of aggregative stability (Fig. 4b) and decreasing of energy barrier (Fig. 5b) are observed. 4. Conclusions Analysis of position of isoelectric points of titanium dioxide water suspensions in solutions of alkaline metals halogenides has shown that the degree of bonding of background electrolyte’s ions with ionized surface hydroxyl groups grows with the relation increase of ion charge to its radius. Relative shift of isoelectric point in solutions of electrolytes depends upon the degree of cation and anion bonding as well. It has been shown that the change of aggregative stability of water suspensions of titanium dioxide around isoelectric point depends upon ratio of ordering and disordering effects of water structure around ions of background electrolyte on solid–liquid interface. References

The presence of two peaks of aggregate instability in Fig. 2 b it is possible to explain by improved aggregate stability of suspensions in an isoelectric point, owing to presence of ions of background electrolyte which have various ability order and disorder structure of water. Two peaks arise owing to superposition of two counter effects near to an isoelectric point: • deterioration of aggregative stability owing to absence of ionelectrostatic pushing away; • improvement of aggregate stability owing to formation of fixed structure of water in surface layers. Schematically occurrence of two peaks of instability on the dependence particles sizes on medium pH is represented in Fig. 7. Stability of suspensions depends on the change of solutions ionic force. The abrupt increase of specific electrical conductivity of suspensions confirm the increase of ionic force in strong acid and strong alkaline (Figs. 4c and 6). Also, it should be

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