The influence of ionic and nonionic surfactants on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide

Journal of Colloid and Interface Science 299 (2006) 686–695 www.elsevier.com/locate/jcis

The influence of ionic and nonionic surfactants on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide Nataliya H. Tkachenko a,∗ , Zinoviy M. Yaremko a , Cornelia Bellmann b , Mykhaylo M. Soltys a 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 St. 6, 01069 Dresden, Germany

Received 7 November 2005; accepted 5 March 2006 Available online 17 April 2006

Abstract The influence of concentration of nonionic TRITON X-100 and anionic ATLAS G-3300 surfactants, and pH of medium on the size and zetapotential of TiO2 particles in the water suspensions has been studied. Suspensions have been prepared by mixing of the titanium dioxide in the suitable mediums at 10 min and 6 h correspondingly. It was established, that the duration of mixing of the suspensions has an essential influence on the dependence of zeta-potential and size of particles versus concentration of surfactant. However, the duration of mixing does not influence the dependence of electrical conductivity and pH of the suspensions on concentration of surfactant. It is shown that anionic ATLAS G-3300 surfactant is more effective stabilizator of aqueous suspensions of titanium dioxide, than nonionic surfactants of TRITON X-100. It is found that hydrophobic interaction has important role in the processes of stabilization of suspensions for nonionic surfactant, and for anionic surfactant—moving of ψδ -planes into solution’s depth. © 2006 Elsevier Inc. All rights reserved. Keywords: Titanium dioxide; Aggregative stability; Electrical surface properties; ATLAS G-3300; TRITON X-100

1. Introduction The ionic and nonionic surfactants are very often used to regulate the colloidal and chemical properties of dispersed systems. The diphilic construction of their molecules and presence of ionic groups cause different types of strong and weak interactions in water solutions of surfactants and during adsorption on the solid surfaces. It creates favourable mode for the properties regulation of water suspensions of metal oxides. Among nonionic surfactants TRITON X-100 is widely used [1–11]. As aggregative stability and electro-surface properties of the colloid systems substantially depend on intermolecular interactions in water solutions of surfactants, influence of temperature, concentrations of electrolytes [9], pH of the medium [8], other surfactants [10,11] on micellar processes in solutions of TRITON X-100 have been investigated. The researches of TRITON X-100 adsorption on the silicon dioxide [4,5,7] have shown that the surface aggregates being on * Corresponding author. Fax: +380 32 294 8181.

E-mail address: [email protected] (N.H. Tkachenko). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.03.008

interface begin to form at lower concentrations if to compare with solutions of surfactants. Researches of electrokinetical mobility and aggregative stability of latexes [1–3], aggregative stability [4,5] and rheological properties [6] of the aerosil suspensions have shown that aggregative stability of the colloid systems in the presence of TRITON X-100 is provided both sterical [4,5] and electrosterical [1] repulsion. Anionic surfactants, owing to the presence of ionic groups, have greater possibilities in regulation of the colloid systems’ properties. One of such surfactants is ATLAS G-3300. Articles [12–18] studied its influence on aggregative stability and electrosuperficial properties of water suspensions of oxide of silicon [12,18] and titanium dioxide [13–15], rheological properties of suspensions of aerosil [16] and titanium dioxide [17]. It is shown that adsorption of ATLAS G-3300 is mainly predetermined by co-operation of anion group with a surface and forming of superficial aggregates [18]. Adsorption of ATLAS G-3300 conduces to improvement of aggregative stability of suspensions of silicon and titanium dioxide [15–17], and does not influence the efficiency of flocculation of silicon dispersions [12].

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Although the basic tendencies of influencing of surfactants on property of water suspensions of titanium dioxide are known [19,20], no doubt, features of influencing of adsorption of TRITON X-100 and ATLAS G-3300 on them in a wide changing of range of medium pH and concentration of surfactants have caused scientific interest. 2. Experimental 2.1. Materials and reagents The following have been used for investigation: • Titanium dioxide powder of rutile type the surface of which is modified by inorganic oxides (4% Al2 O3 and 2% SiO2 ) and inoculated organic groups with the aim to provide with necessary optical properties and regulation of hydrophobic–hydrophilic balance (DuPont, USA). The powder particles are of a round shape with the average diameter of 0.23 µm. 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. • ATLAS G-3300–anionic surfactant, produced by Atlas Chemical Co, USA. It is soluble in water and contains 90% of sodium dodecylbenzenesulfonate:

Mr = 348.05. It was known that CMC value is about 1.2 × 10−3 –1.5 × 10−3 mol/L [16,21]; its value, which was established experimentally in the 1 × 10−3 mol/L KCl environment, is equal to 1.57 × 10−3 mol/L. • TRITON X-100–low molecular nonionic surfactant, produced by Aldrich Co, The Netherlands. It is soluble in water, toluene, trichlorethylene, xylene, ethylene glycol, and alcohols. It contains more than 97% of 4-(1,1,3,3,-tetramethylbutyl)phenyl-polyethylene glycol:

and less than 3% polyethylene glycol. Mr = 646, HLB = 13.5, n = 1.473, ρ = 1.029 g/ml. The authors of [2,22] have shown that CMC is about 2.4 × 10−4 –5.1 × 10−4 mol/L. The CMC value, which was established experimentally in the 1 × 10−3 mol/L KCl environment, is equal to 1 × 10−3 mol/L. • Reagents: – KOH (86%, Sigma–Aldrich), – 0.1 mol/L solution KCl (Fluka), – fixanal solution HCl (Fluka).

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• Deionized water with specific electrical conductivity of 0.055 µS/cm, purified with the help of Millipore-Q instrument. 2.2. Preparation of solutions and suspensions 2.2.1. The solutions of KCl 1 × 10−3 mol/L KCl solution was prepared using standard KCl solution (0.1 mol/L) and deionised water. 2.2.2. Surfactant solutions The intermediate water solutions of ATLAS G-3300 (0.14 mol/L) and TRITON X-100 (0.08 mol/L) were prepared by means of weight method from the initial reagents without their additional purifying. During our investigation all solutions and suspensions with necessary surfactant concentration were prepared from the intermediate solutions by dilution of initial solutions. 2.2.3. TiO2 suspensions For preparation of the suspensions, TiO2 powder and 1 × 10−3 mol/L KCl solution were mixed together in the retort so that TiO2 concentration was equal to 0.03 g/L. After that the content of retort was dispersed during 2 min using ultrasound device “Ultrasonic.” 2.2.4. TiO2 suspensions with surfactants Suspensions of TiO2 with surfactant were prepared as follows. Initially, TiO2 suspensions were obtained by the methods described above. Then the necessary quantity of concentrated surfactant solution was added, and the suspensions were carefully mixed during 10 min or 6 h by using the mechanical mixer. Suspensions were prepared on the basis of surfactants solutions by 6 h mixing and concentration of titanium dioxide always was permanent and was evened at 0.03 g/L. After mixing for 10 min the concentrated solutions of surfactants were gradually added to the suspensions, which caused the gradual volume increase of suspensions and gradual concentration decrease of titanium dioxide from 0.03 to 0.025 g/L at maximal concentration of TRITON X-100 and to 0.021 g/L at maximal concentration of ATLAS G-3300. 2.2.5. Suspensions with different pH values The influence of medium pH on the suspensions properties has been studied in the pH range from 2 to 12. To investigate the influence of pH on the properties of suspensions containing surfactant, the suspensions that have been prepared by mixing of TiO2 powder with surfactant solution during 6 h, were used. The pH value was changed in the same sample by adding a suitable amount of 0.1 mol/L HCl or KOH to the solutions. By adding the solutions of HCl and KOH for the change of the medium pH, the volume of suspensions was gradually increased and the concentration of TiO2 was gradually decreased from 0.03 g/L in a neutral medium to 0.02 g/L in maximally sour or maximally alkaline mediums.

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2.3. Experimental procedure 2.3.1. Electrokinetic analysis The electrokinetic measurements were made by the method of microelectrophoresis [23,24], using Zetasizer 3 (Malverm Instruments Ltd., UK) instrument. The rate of particles movement under the influence of external electrical field of 150 V voltage (electrophoretic mobility Uef ) was measured with the help of Doppler’s laser anemometer where He–Ne laser ray was used. The solution of the same salt on the basis of which the suspension was prepared, became the solution for comparison. Zeta-potential of particles was calculated under the condition that electrokinetic radii are greater than 1, according to the Helmholtz–Smoluchovski’s equation [23]: ζ = ηUef /εε0 ,

(1)

where η is the liquid viscosity, ε and ε0 are the relative dielectric permittivity of the medium and dielectric constant, correspondingly. The measurements of conductivity and pH of suspensions were simultaneously conducted by 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 with pH 4 and 7. 2.3.2. Measurement of diameter of particles in suspensions In suspensions that were researched using Zetasizer 3 instrument, simultaneously was conducted the measurement of an average size of the particles by using Zetasizer 3000 (Malvern Instruments Ltd., UK) instrument in plastic cuvettes (Sardstedt AG). As particles of suspensions are subject to the thermal Brownian motion and a rate of particles’ movement depends upon their size, so, on the basis of dynamic light scattering [25] of photoncorrelation spectroscopy through coefficients of diffusion, it is possible to determine directly the diameter of particles (d) by using the following relation: d = kT /3πηD,

presence of the 1 × 10−3 mol/L KCl solution at 25 ◦ C. The mean value from the three measurements of surface tension was accepted as a result. 2.3.4. Adsorption investigations To investigate surfactants adsorption in dependence upon concentration in suspensions and with the aim of taking into consideration all possible side influences, their actual concentration in solutions prepared without solid phase was determined. Afterwards, suspensions of titanium dioxide with certain surfactant content was prepared and mixed on mechanical stirrer for 6 h to reach adsorption equilibrium. Then suspensions were centrifuged at 5000 rpm within 10 min and sediment was separated by decantation. Having defined surfactant quantity, which remains after adsorption in the solution, adsorption value was calculated as follows: As [mg/m2 ] = (C0 − Ce )Ms × 103 /CTiO2 STiO2 ,

(3)

where Ms is the molecular weight of surfactants (g/mol), C0 and Ce are the output and equilibrium concentration of surfactants in suspensions (mol/L), CTiO2 is the concentration of TiO2 (g/L), STiO2 is the specific surface of TiO2 (m2 /g). To define the surfactant concentration in solution the determination method of general Carbon amount, using “Total Carbon Analyser” (TOC-5 000) (Shimadzu Europa GmbH) device was applied. To calculate a surfactant concentration in solution (mol/L), using measured carbon quantity (C, g/L) following equation was used: Cs = C/Mc ,

(4)

where Mc is the carbon mass in mol of surfactant (g/mol). 3. Results and discussion Adsorption isotherms are very similar for both types of surfactants, but quantitatively the adsorption of ATLAS G-3300 is

(2)

where D is the diffusion coefficient, k is the Boltzmann constant, T is the temperature, π = 3.14, η is the medium viscosity. The results of measurements were recorded as Gaussian distribution of particles according to their size and average diameter. During the experiments, using Zetasizer 3 and Zetasizer 3000 instruments, concentration of solid phase made up 0.03 g/L and temperature 25 ◦ C. The mean value from the three measurements was accepted as a result. 2.3.3. Measurements of a surface tension To define CMC the measurements of a surface tension of solutions containing ATLAS G-3300 were carried out by the method of Wilhelm, using “Tensiometer K 14” (Krüss) device. The measurement of a surface tension of solutions containing TRITON X-100 was carried out by the method of maximal pressure of bubble using “SITA online t 60” (SITA Messtechnik GmbH) device. These measurements were performed in the

Fig. 1. The dependence quantity of adsorption TRITON X-100 (2) or ATLAS G-3300 (") versus surfactant concentration in suspension. Concentration of TiO2 is equal 0.03 g/L. Dotted lines correspond to a critical micelle concentration of TRITON X-100 (1) and ATLAS G-3300 (!).

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much better than of TRITON X-100 (Fig. 1). There is a plateau in the region of CMC, and at the subsequent increase of concentration, adsorption of both surfactants grows consistently. By the form, received isotherms concern to step isotherms Langmuir L4 which are observed for many adsorption systems [26]. Usually, adsorption of surface-active substances on hard adsorbents is described by the isotherm of Langmuir L2, for which characteristically achievement of adsorption of satiation and regions of CMC surface-active substances in a suspension. Growth of adsorption of the explored surfactants on the surface of titanium dioxide at the concentrations higher CMC can be explained by influence of other processes which take place simultaneously with adsorption at the increase of concentration of surfactants in a suspension. Obviously, the amount of bounded molecules of surfactant depends on the state of amphoteric hydroxylic groups on the surface of an adsorbent. The equilibrium processes of co-operation of these groups with a proton in aqueous solutions are described by equations:

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(a)

K1

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

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

(5)

where K1 and K2 are the the constants which are the quantitative measure of acidity and basicity for the surface hydroxyl groups. The increase of concentration of both surfactants in higher CMC suspensions results in the decrease of medium’s pH (see Figs. 2c and 3c). The increase of concentration of ions of hydrogen in a suspension results in a displacement of balance to the left (Eq. (5)). Such displacement of balance to the left (5) is instrumental in binding by a solid surface as ionic surfactant ATLAS G-3300, so nonionic surfactant TRITON X-100. For ATLAS G-3300 this effect, usually, shows up considerably stronger, than for TRITON X-100. The adsorption of surfactant on the solid/solution interface causes the changes of its properties appearing in such macroeffects as changes of zeta-potential particles, aggregative and sedimentical stability of suspensions. The dependences of zeta-potential and diameter of TiO2 particles, electrical conductivity and pH of suspensions versus concentrations of TRITON X-100 and ATLAS G-3300 are shown in accordingly Figs. 2 and 3. The measurements were performed after 10 min or 6 h of mixing of TiO2 suspensions in solutions of surfactants. As one can see in Figs. 2c and 3c, the electrical conductivity and the suspensions’ pH for solutions of both surfactants remain the same both after 10 min and after 6 h of mixing. It means that equilibrium in the reactions between solid surface and components of medium, in which H+ and OH− ions are taking part, establishes very quickly. On the other hand, the diameter and zeta-potential of TiO2 particles in solutions of both surfactants depend on the duration of mixing (see Figs. 2a and 2b and 3a and 3b). The diameter and zeta-potential of particles in suspensions containing ATLAS G3300, essentially depend on the duration of mixing only up to

(b)

(c) Fig. 2. The dependence of zeta-potential (a), particle diameter (b), pH and electrical conductivity (c) of the TiO2 suspensions in the 1 × 10−3 mol/L KCl solution versus concentration of TRITON X-100. Opened symbols: suspension was mixed for 10 min; closed ones—for 6 h. The dotted vertical line corresponds to a critical micelle concentration, and horizontal line—for the value of zeta-potential and diameter of particles in water. Concentration of TiO2 is equal 0.03 g/L at 6 h mixing, and at mixing for 10 min it gradually decreases to 0.025 g/L at maximal concentration of TRITON X-100.

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(a)

(b)

(c) Fig. 3. The dependence of zeta-potential (a), particle diameter (b), pH and electrical conductivity (c) of the TiO2 suspensions in the 1 × 10−3 mol/L KCl solution versus concentration of ATLAS G-3300. Opened symbols: suspension was mixed for 10 min; closed ones—for 6 h. The dotted vertical line corresponds to a critical micelle concentration, and horizontal line—for the value of zeta-potential and diameter of particles in water. Concentration of TiO2 is equal 0.03 g/L at 6 h mixing, and at mixing for 10 min it gradually decreases to 0.02 g/L at maximal concentration of ATLAS G-3300.

the surfactant concentration 1 × 10−4 mol/L. At the higher concentrations the particle diameter and their zeta-potential do not differ much (Figs. 3a and 3b). The isoelectric point (ζ = 0) coincides with a maximum of aggregative instability in the suspensions containing ATLAS G-3300 that were mixed for 6 h. Moving away from the isoelectric point causes the increase of zeta-potential and improves aggregative stability. At 10 min mixing with the increase of concentration of ATLAS G-3300 the zeta-potential gradually grows, and aggregative stability gets better. Essential increasing of zetapotential at increasing of surfactant’s concentration is causing insignificant improving of aggregative stability. Such character of these dependences can indicate that the ion-electrostatic constituent of a splitting pressure has an influence on aggregative stability of such suspensions; however, an influence is not determinative. The comparison of dependences of zeta-potential from concentration of ATLAS G 3300, which have been obtained at mixing during 10 min and at mixing 6 h gives that at low concentrations, up to 1 × 10−4 mol/L, negative values of zeta-potential at mixing during 10 min are greater than at mixing during 6 h and on concentration dependence they look displaced aside smaller concentrations. If to accept that zeta-potential of particles of titanium dioxide simply depends on the size of adsorption of surfactants [27], it is plausible that in an initial moment (at 10 min) adsorption of ATLAS G-3300 is greater than at 6 h mixing. Further, as a result of superficial aggregative processes, which begin on the surface at considerably lower concentrations than CMC in solution [28], and as a result of forming the surface equilibrium structures, zeta-potential decreases. The same dependence has been observed in work [29]. At high concentrations of surfactants, where aggregative processes intensively take place already in the volume of solution, it is highly expected that on surface they pass more intensively and equilibrium is established faster. That is why noticeable difference in dependences of zeta-potential and diameter of particles at high concentrations of ATLAS G-3300 is not observed. In suspensions of TRITON X-100, where adsorption cooperation of molecules of surfactants with the surface of titanium dioxide is considerably weaker in comparison to suspensions of ATLAS G-3300, the time changes are more complicated and transition from dependences at mixing during 10 min to dependences at mixing during 6 h is less obvious. Comparison of dependences of zeta-potential and size of the particles, which have been received at mixing during 10 min and 6 h, does not allow to simply identify the mechanism of processes that take place in this system. Obviously, it is a result of a few simultaneous processes with weak effects. At least three simultaneous processes which take place in the adsorption system are accountable for the obtained dependences of zeta-potential on the concentration of surfactant: (a) equilibrium processes of co-operation of amphoteric hydroxylic groups with a proton, which are described by Eq. (5);

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(b) binding of molecules of surfactant with surface hydroxylic groups with formation of surface complexes: –MeOH+ 2(solid) + HO–E10 PhC8(solution) K3

←→ –MeOH+ 2 . . . HO–E10 PhC8(solid)

(6)

where E10 is the polyoxyethylene chain, PhC8 is the alkylphenyl hydrocarbonic chain; (c) lateral co-operations of alkylphenyl hydrocarbonic chains with formation of supramolecular structures of molecules of surfactant on a solid surface. A sign and amount of surface’s charge are determined by correlation of concentration of the charged groups –MeOH+ 2(solid) and –MeO− and effective charge of these groups: (solid) α=

+ [–MeOH+ 2(solid) ]β − [–MeO− (solid) ]e

,

(7)

− where β + is the effective charge of group –MeOH+ 2(solid) , e is the effective charge of group –MeO− (solid) . In the absence of adsorption on a solid surface, effective charges β + and e− , obviously, are identical in amount. Binding by the surface of any ionic or polar compounds draws the change of these effective charges. If α > 1, a surface has positive charge, and at α < 1 it is negatively charged. Intensity of processes (a)–(c) depends on the concentration of surfactant and time of contact of surfactant solution with a solid surface. In an initial moment at the low concen-

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trations of surfactant (less than CMC) a process (b) is most intensive—binding of molecules of surfactant with surface hydroxylic groups –MeOH+ 2(solid) . Although TRITON X-100 is the nonionic substance, its molecules as a result of division of electronic density are polar. At binding of polar molecules of TRITON X-100 with surface groups –MeOH+ 2(solid) , their effective charge β + partly decreases. It results in diminishing of correlation α. As in initial suspensions, zeta-potential of particles is small, α can purchase values less than 1 and to stipulate the recharge of particles. That is why with the increase of concentration of surfactant to CMC the negative value of zeta-potential increases. In an initial moment at the concentrations higher than CMC influencing of process (a) grows substantially, as at these concentrations of surfactant in suspensions medium’s pH decrease and as a result of displacement of balance to the left (5) the concentration of the surface charged groups –MeOH+ 2(solid) increases. It predetermines the increase of correlation α and decrease of negative value of zeta-potential up to a recharge. Experimentally found dependence of zeta-potential on the concentration of surfactant at time of contact 10 min fully confirms these conclusions (see Fig. 2a, opened symbols). With the increase of time of contact of surfactant solution an influence of the third process (c) grows—lateral cooperations of alkylphenyl hydrocarbonic chains. This process can be described by the chart presented in Fig. 4. As a result of this co-operation the surface supramolecular structures appear and change the effective charge β + of group –MeOH+ 2(solid) . By dark rectangles on the horizontal line of solid surface are

Fig. 4. Diagram of adsorption co-operation of molecules of surfactant with surface groups –MeOH+ 2(solid) . Surface groups are marked by dark rectangles the width of which specifies the value of effective charge β + .

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marked places of location of surface groups –MeOH+ 2(solid) and their effective charge β + (than more wide rectangle, the greater effective charge). As a result of binding by the solid surface of molecules of surfactant the effective charge of surface groups decreases. After formation of compact supramolecular structures as a result of lateral co-operations, a part of surface groups –MeOH+ 2(solid) are out of the influence of bound molecules of surfactant and their effective charge β + is increasing. Presence of maximum on dependence of zeta-potential on the concentration of surfactant in the positive region of values at the concentration 1 × 10−5 mol/L after 6 h contact (see Fig. 2a, closed symbols) is explained by the described above processes (b) and (c). At low surface concentration of bound molecules of surfactant as a result of their surface migration and forming of supramolecular structures, their influence on an effective charge β + is greater, that is why correlation α grows and the positive values of zeta-potential increase. At an increase of surface concentration of bound molecules of surfactant their surface migration diminishes and the changes in the values of effective charge β + are less. These processes predetermine appearance of maximum in the positive region of zetapotential. Appearance of maximum on dependence of zeta-potential on the concentration of surfactant in the negative region of values at the concentration 1 × 10−4 mol/L after 6 h contact is explained by the result of processes (b) and (a) (see Fig. 2a, closed symbols). As after achievement of CMC in solution, an influence of process (a) substantially grows through the change of medium’s pH, obviously, correlation α also will grow up to the recharge of surface. At the high concentrations of surfactant in a suspension the process of co-operation of amphoteric surface hydroxylic groups with a proton make a basic contribution to forming surface charge. To it testifies that in this area of concentration of surfactant (>0.01 mol/L) zeta-potential of particles almost does not depend upon time of contact. For the solutions of TRITON X-100 is expressly possible to claims about the important role of hydrophobic co-operation, as at all concentrations of surfactants, for both methods of suspensions’ preparation, the diameter of particles appears greater, than in water. It testifies to more intensive processes of aggregation of the titanium dioxide particles. As sizes of titanium dioxide particles in suspensions at mixing during 10 min for all concentrations of TRITON X-100 are greater, than at mixing during 6 h, it is possible to assume that the degree of surface hydrophobization at the beginning of adsorption (at 10 min) is higher than at 6 h. Such supposition is acceptable, because adsorption of any surfactants begins by forming of molecules monolayer that in the case of TRITON X-100 will assist in the hydrophobization of surface. Further, as a result of forming of surface at molecular structures including bilayer, the degree of hydrophobization is decreasing. Dependence of particles size with a maximum at 5.5 × 10−4 mol/L for mixing during 10 min from concentration of TRITON X-100 is also explained by similar suppositions, namely: at low concentrations of surfactants, as a result of molecules adsorption, the degree of hydrophobization is in-

creasing, and with the increase of surfactants concentration, at which superficial aggregative processes develop intensively— decreasing. Search of conditions for which it is possible to attain maximal aggregative stability of suspensions, researches of influencing of medium’s pH on their electrosurface properties and aggregative stability were caused. It is necessary to expect that by the change of medium’s pH it is possible more effectively to improve aggregative stability of those suspensions which in the initial state have relatively high aggregative stability. These conclusions stipulated the choice of concentrations of surfactant in suspensions. For TRITON X-100 maximal stability of suspensions is observed at two concentrations: 1 × 10−5 and 2 × 10−4 mol/L (see Fig. 2b). But for research it is more interesting a case at the concentration 1 × 10−5 mol/L, at which maximal aggregative stability of suspensions is at very low zeta-potential of particles. For ATLAS G-3300 the high aggregative stability of suspensions is achieved in wide, but near to CMC, range of the concentrations: from 1 × 10−4 to 1 × 10−2 mol/L (see Fig. 3b). For TRITON X-100 at the concentration 1 × 10−5 mol/L and at the conditions of monolayer structure of molecules of surfactant, obviously, not very much dense, pH of medium influences on zeta-potential of particles in accordance with processes (a)–(c) (Fig. 5) which have place in system. A process (a) is main, as character of dependence of zeta-potential from pH of suspensions does not almost change in comparison with such dependence for the aqueous suspensions of titanium dioxide (Fig. 5a). There is only a displacement of an isoelectric point toward lower value of suspension’s pH, that actually confirms the fact that part of surface groups –MeOH+ 2(solid) is related to the molecules of surfactant. For ATLAS G-3300 at the concentrations in solution, near to CMC, at the conditions of the fully formed bi-layer structures from the molecules of surfactant, influence of medium’s pH on electrosurface properties and aggregative stability is practically absent (Fig. 6). It also should be expected, taking into account possible processes (a)–(c). The aggregative stability of TiO2 suspensions, containing ATLAS G-3300, is much better in whole range of pH values than corresponding stability of water suspensions on the base only of 1 × 10−3 mol/L KCl. On the contrary, aggregative stability of the suspensions, containing TRITON X-100, is much lower than of water suspensions without surfactant in a whole pH range. For comparison of experimental results the calculations of particles interaction energy according to the DLVO theory (taking into account the attraction energy [30] and the energy of ion-electrostatic repulsion [31]) were performed, using equations: rA131 λ at H  15 nm VA (H ) = − (8) 12H λ + 11.116H and   rA131 2.45λ 0.59λ3 2.17λ2 − − VA (H ) = − 12H 10πH 60π 2 H 2 280π 3 H 3 at H > 15 nm (9)

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693

(a)

(a)

(b)

(b)

(c)

(c)

Fig. 5. The dependence of zeta-potential (a), particle diameter (b) electrical conductivity (c) of TiO2 suspensions in the 1 × 10−3 mol/L KCl solution versus pH of suspensions at different TRITON X-100 concentrations, mol/L: (") 4.5 × 10−4 , (a) 5.5 × 10−5 , (2) 0. The concentration of TiO2 gradually decreases from 0.03 g/L in a neutral area, to 0.02 g/L in sour and alkaline areas.

Fig. 6. The dependence of zeta-potential (a), particle diameter (b), electrical conductivity (c) of TiO2 suspensions in the 1 × 10−3 mol/L KCl solution versus pH of suspensions at different ATLAS G-3300 concentrations, mol/L: (F) 8.62 × 10−4 , (Q) 1.57 × 10−3 , (") 2.91 × 10−3 , () 2.22 × 10−2 , (2) 0. The concentration of TiO2 gradually decreases from 0.03 g/L in a neutral area, to 0.02 g/L in sour and alkaline areas.

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where H is the shortest distance between particles surfaces, λ is the descriptive wave length equal to 0.1 µm, A131 is the constant of dispersion interaction of solid particles 1 through dispersion medium 3, r is the particle radius,   2  kT ezϕ0 2 exp(−χH ) VR (H ) = 16εε0 (10) th2 r , ez kT 4 2r + H where ε and ε0 are the relative dielectric permittivity of the medium and dielectric constant, correspondingly, k is the Boltzmann constant, T is the temperature, e is the charge of electron, z is the valency of counterion, ϕ0 and χ are the surface potential and converse thickness of electrical double layer, correspondingly. Particles radius was taken, for calculation, equal to radius of primary particles of titanium dioxide r = 0.115 µm, and a constant of dispersion interaction as A131 = 10kT [32]. The quantity conversed to thickness of electrical double layer was calculated according to the formula [23]:   2  2 2 χ = e (11) (zi ci ) (εε0 kT ), i=1

where ci is the concentration of additives of monobasic acid or monoacid alkali at pH medium change (i = 1) and electrolyte (i = 2). Such calculations were made for one concentration of surfactant ATLAS G-3300 (1.57 × 10−3 mol/L) and for one concentration of surfactant TRITON X-100 (5.5 × 10−5 mol/L), and the value of zeta-potential was used instead of surface one. The dependence of height of energetic barrier upon medium pH is shown in Fig. 7. In relation to suspensions in the presence of TRITON X100, correlation between the height of power barrier and aggregative stability of suspensions is absent. As in a isoelectric point at pH ∼ 4, where, according to calculations by the theory of DLVO there is no power barrier, the diameter of particles is considerably smaller than diameter at pH of suspen-

sions, where this barrier is maximal (pH ∼ 10). It confirms a conclusion from concentration dependence of aggregative stability of suspensions, that aggregation of particles of titanium dioxide is controlled by hydrophobic co-operation. Obviously, that largeness of particles of titanium dioxide for concentration 5.5 × 10−5 mol/L in comparison with concentration 4.5 × 10−4 mol/L can be explained by the high degree of hydrophobization (Fig. 2b). For suspensions with ATLAS G-3300 the power barrier decreases with the increase of medium pH, passing through a small minimum at pH ∼ 8–10, and the diameter of particles practically remains unchanged. This effect can be explained by move of ψδ -planes to depth of solution on the thickness of adsorption layer. Such moving is supporting strengthening of the ionic-electrostatic repulsion, as in Eq. (10) it is necessary to replace H by H − 2δ, where δ is the thickness of adsorptions layers. By such move of ψδ -planes to depth of solution can be explained the fact of considerably higher aggregative stability of suspensions with ATLAS G-3300 in sour and in alkaline areas as compared to aquatic suspensions, though zeta-potential (modulus of value) is almost equal in both suspensions. 4. Conclusions 1. Complex researches of zeta-potential, the dimension of particles, conductivity and pH of titanium dioxide in water suspensions was shown, that in the processes of stabilizing of these suspensions the secondary processes are important and having place after interaction of surface groups with low molecular components of water mediums, particularly with H+ and OH− ions. The character of dependence of zeta-potential of particles from the concentration of surfactant is determined by correlation of intensity of three processes: (a) equilibrium processes of co-operation of amphoteric hydroxylic groups with a proton; (b) binding of molecules of surfactant with surface hydroxylic groups with formation of surface complexes; (c) lateral co-operations of alkylphenyl hydrocarbonic chains with formation of supramolecular structures of molecules of surfactant on a solid surface. 2. It was confirmed that anionic surfactant ATLAS G-3300 stabilizes the water TiO2 suspensions much better than nonionic surfactant TRITON X-100 in the wide range of suspensions’ pH changing and was shown that in the processes of stabilization of the nonionic surfactants hydrophobic interaction is important, but in the processes of stabilization of ionic surfactants moving of ψδ -planes’ potential to depth of solution has important role. References

Fig. 7. Dependence of height of energetic barrier Vmax /kT versus pH of suspensions, containing ATLAS G-3300 (Q; 1.57 × 10−3 mol/L) and TRITON X-100 (a; 5.5 × 10−5 mol/L).

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