Unexpected Effects of Activator Molecules' Polarity on the

Jun 14, 2017 - Titanium dioxide nanoparticles, obtained using the sol–gel method and modified with organic solvents, such as acetone, acetonitrile, ...
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Unexpected Effects of Activator Molecules’ Polarity on the Electroreological Activity of Titanium Dioxide Nanopowders A.V. Agafonov,*,†,§ O.I. Davydova,† A.S. Krayev,† O.S. Ivanova,‡ O.L. Evdokimova,† T.V. Gerasimova,† A.E. Baranchikov,‡ V.V. Kozik,§ and V.K. Ivanov‡,∥ †

Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow 119991, Russia § National Research Tomsk State University, Tomsk 634050, Russia ∥ Institute of Fine Chemical Technologies, Moscow Technological University, Moscow 119454, Russia ‡

S Supporting Information *

ABSTRACT: Titanium dioxide nanoparticles, obtained using the sol−gel method and modified with organic solvents, such as acetone, acetonitrile, benzene, diethyl ether, dimethyl sulfoxide, toluene, and chloroform, were used as the filler of polydimethylsiloxane-based electrorheological fluids. The effect of electric field strength on the shear stress and yield stress of electrorheological fluids was investigated, as well as the spectra of their dielectric relaxation in the frequency range from 25 to 106 Hz. Modification of titanium dioxide by polar molecules was found to enhance the electrorheological effect, as compared with unmodified TiO2, in accordance with the widely accepted concept of polar molecule dominated electrorheological effect (PM-ER). The most unexpected result of this study was an increase in the electrorheological effect during the application of nonpolar solvents with zero or near-zero dipole moments as the modifiers. It is suggested that nonpolar solvents, besides providing additional polarization effects at the filler particles interface, alter the internal pressure in the gaps between the particles. As a result, the filler particles are attracted to one another, leading to an increase in their aggregation and the formation of a network of bonds between the particles through liquid bridge contacts. Such changes in the electrorheological fluid structure result in a significant increase in the mechanical strength of the structures that arise when an electric field is applied, and an increase in the observed electrorheological effect in comparison with the unmodified titanium dioxide.

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INTRODUCTION The electrorheological effect (ERE) is a rapid and reversible change of viscoplastic properties of suspensions of polarizable materials in dielectric media under applied electric field.1−4 The trigger for ERE is the redistribution of particles in the interelectrode space, and formation of chain structures.5,6 The changes in suspensions’ structure have been confirmed, e.g., by direct microscopic observations. This effect is of particular theoretical and practical interest and, nowadays, is considered to be a result of electric polarization forces’ action.1−4 Therefore, the polarization of dispersed phase particles in the electric field is of prime importance. In this context, induced polarization in the interface layer of two substances with different dielectric constants is of particular interest. Relatively recently, a so-called “giant” electrorheological effect has been discovered in the ER fluid prepared from barium titanyl oxalate nanoparticles covered with urea.7 This effect has been characterized by the electrically controlled transition from a liquid to a solid state reaching a yield stress in the electric field of about 130 kPa. This figure represents a more than 10-fold © 2017 American Chemical Society

increase in the electrorheological effect value predicted by a simple polarization model. This giant effect has been confirmed by other researchers, who have used nanomaterials as filling agents for the ER fluids, which have consisted of a dielectric core and a shell formed by adsorbed polar compounds.8−10 The yield stress of newly developed ER fluids amounts to hundreds of kPa, and exceed that of ER fluids with native particles by several orders of magnitude.11 According to the proposed interpretation,12 in the absence of an external electric field, polar molecules position themselves on the surface of filling agent nanoparticles in compliance with adsorption interactions with the corresponding active sites (physical or chemical). When the external electric field is applied, polar molecules migrate into the gaps between filler particles, with maximum electric field strength. The appearance of a highly polar molecule in the gap between nanoparticles results in a Received: May 2, 2017 Revised: June 13, 2017 Published: June 14, 2017 6732

DOI: 10.1021/acs.jpcb.7b04131 J. Phys. Chem. B 2017, 121, 6732−6738

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The Journal of Physical Chemistry B significant increase in local electric field strength and strength of interparticle forces.13,14 The impact of polar molecules in the ER effect is dictated by two types of interaction: dipole−dipole interaction between polar molecules (Fd−d), and dipole−charge interaction on the surface of polarized particles (Fd−ch).12 It is noteworthy that the dipole−dipole interactions are insignificant in comparison to dipole−charge interactions, and the resulting interaction can be described by the following equation: Fd − ch = A ·3φρeμ2 E /πrε0εf d 2

particles in the aqueous solution was measured using a Malvern Zeta Sizer Nano analyzer. An X-ray phase analysis of the powders was performed with a Bruker D8 ADVANCE X-ray diffractometer. The texture parameters of the samples were analyzed using nitrogen adsorption at 77 K (Quantachrome Instruments Nova 1200). Elemental analysis of the modified powders was carried out on a FlashEA 1112 CHNS analyzer. All the solvents and precursors used in this study were obtained from Sigma-Aldrich. As a dispersion medium for electrorheological fluids, we used polydimethylsiloxane PMS-20 (PENTA Silicones) with the following characteristics: dynamic viscosity 20 cSt, surface tension 19 mN/m, liquid state range −60 ÷ +200 °C, dielectric strength 14 kV/mm, dielectric constant 2.4, dielectric loss angle tangent 0.0001. Preparation of Nanodispersed Filling Agents of ER Fluids. We used a nanopowder of titanium dioxide, obtained by the sol−gel method in ethanol, as an ER fluid filler.16 Titanium isopropoxide (SIGMA) was added dropwise, with intensive stirring, to a mixture of ethanol, water (4%), and diethyl amine (0.5%) to provide complete hydrolysis of titanium isopropoxide.17 After 24 h, the loose, white precipitate was separated using centrifugation and dried in a vacuum at 50 °C. The resulting X-ray amorphous powder consisted of particles of 20−150 nm size, according to scanning electron microscopy (Figure 1). The specific surface area of the powder,

(1)

where parameter A refers to the properties of the dielectric fluid and particles (shape of particles and adsorption energy); φ, volume fraction of particles in the suspension; ρ, surface density of adsorbed polar molecules; e, unit charge; μ, dipole moment; d, polar molecule size; r, particle radius; ε0 and ε, dielectric constants of the vacuum and dispersion medium. As follows from eq 1, with all else equal, small molecules with large dipole moment (μ2/d2) values are preferred as ER effect promoters. Consequently, a study of the relative influence, on the ER effect, of organic solvents of different polarity adsorbed on the filler particles appears to be promising, for the design of efficient ER fluids. Among the promising oxide-based filling agents for ER fluids is nanodisperse titanium dioxide powder, for which there is a huge body of data characterizing it as a filler for ER fluids. In the present study, we investigated several organic solvents as TiO2 surface modifying agents. These solvents belong to different classes and have different dipole moments, including nonpolar solvents with zero dipole moment. The dipole moment values and calculated polarization density of molecules for the chosen solvents are listed in Table 1. Table 1. Values of Molar Volume (V), Surface Tension (σ), and Dynamic Viscosity (η) of Organic Solvents, Dipole Moments (μ) of Solvent Molecules, and Calculated Values of the Polarization Density (μ/V) of These Molecules15 solvent acetone acetonitrile benzene diethyl ether dimethyl sulfoxide toluene chloroform

μ (D)

V (cm3)

μ/V×100 (D/cm3)

2.84 3.97 0.00 1.15 3.96

73.4 52.2 89.1 103.9 71.0

3.8 7.6 0.0 1.1 5.6

25.2 28.7 28.9 16.7 42.9

0.31 0.37 0.60 0.22 1.99

0.37 1.15

106.3 80.1

0.3 1.4

28.4 27.5

0.56 0.54

σ (mN/m) η (cSt)

Figure 1. SEM image of titanium dioxide powder.

determined using the BET model, amounted to 170 m2/g. The ζ-potential of titanium dioxide nanoparticles, in an aqueous dispersion of pH 7, was −28 mV, with an average hydrodynamic diameter of 300 nm. According to IR spectroscopy data, the powder was hydrated titanium dioxide containing a number of alkoxy groups (Figure 2). According to thermal analysis data, the content of hydrated titanium dioxide in the powder was about 90% by weight, and the content of the organic phase was about 10% by weight.16 For the surface modification of TiO2 particles, a number of organic solvents of different nature were applied (Table 1). The following method of surface modification of TiO2 particles was used: a sample of titanium dioxide powder was placed in a weighting cup and distributed as a thin layer on the bottom of the cup, followed by the addition of an organic solvent (10% by weight of the powder sample). The weighting cup was kept for 4 h at the boiling temperature of the corresponding solvent, then for 10 days at room temperature,

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EXPERIMENTAL SECTION The investigation of the physicochemical properties of the materials obtained was performed using high-resolution scanning electron microscopy (SEM), IR spectroscopy, powder X-ray diffraction, thermal analysis, low-temperature nitrogen adsorption, dynamic light scattering, and measurement of the dielectric properties of ER fluids and the electrorheological effect. SEM images of titanium dioxide powders were obtained with a Carl Zeiss NVision 40 scanning electron microscope. The IR spectra of the powders in the KBr matrix were recorded with a VERTEX 80v infrared Fourier spectrometer. A complex thermal analysis of the powders was carried out using a NETZSCH DSC 204 F1 Phoenix analyzer. The ζ-potential of 6733

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present study, suspensions in polydimethylsiloxane PMS-20 were used. Weight fractions of the solid phase were 10 and 45% (5.8 and 26.0% by volume, respectively). Measurements of the shear stress of the ER fluids in an electric field were carried out on a modified RN-211 viscometer with an error of 5%. The testing of ER fluid tension in an electric field was carried out using a specially designed setup, consisting of a screw press with an electric field strength applied of up to 8 kV/mm between the electrodes. The movement rate of the upper electrode of the press was controlled by means of a high-precision screw and a computerized stepper motor. In this work, the stretching rate of the ER fluid was 0.0003 mm/sec. The profile of the electrorheological fluid in the space between the electrodes was kept constant under tension and compression. The sensitivity of the measuring system was ±5 Pa. The details of the measuring methods applied for the rheological characteristics of suspensions in the electric field have been reported in previous work.20,21 A specially designed cylindrical cell of capacitor type, with polished, stainless steel electrodes, was applied to analyze the dielectric spectra of electrorheological suspensions based on titanium dioxide, modified with various solvents. The measurements of the frequency dependence of the dielectric constant and the dielectric loss tangent of ER fluids were carried out using an impedance and amplitude-phase characteristics Solartron SI 1260 Impedance/Gain-Phase analyzer. The method of measuring the dielectric characteristics of a suspension, and the design of a measuring cell, have been described in detail in a previous paper.20

Figure 2. IR-spectra of samples: titanium dioxide (1); titanium dioxide with solvents adsorbed: acetone (2); acetonitrile (3); dimethyl sulfoxide (4); chloroform (5).

with periodic shaking. The presence of solvent molecules on the surface of titanium dioxide particles was confirmed by elemental analysis and IR spectroscopy. The IR spectra for the powders obtained are presented in Figure 2. For the interpretation of IR spectra, we used previously published data.18,19 The spectra of modified powders contained characteristic absorption bands of solvents, highlighted by ovals in the figure. The IR spectrum of an acetone modified titanium dioxide sample contained an absorption band of enolized acetone (ν = 1640 cm−1), in addition to the characteristic band of stretching vibrations of the carbonyl bond CO of the acetone molecule (ν = 1720 cm−1). The bands which correspond to deformation and stretching vibrations of the Ti−O−C bond (ν = 1045 cm−1, ν = 1167 cm−1) in the spectrum of acetone modified titania (Figure 2, spectrum 2) were shifted to lower frequencies in comparison with the native titanium dioxide. These shifts were evidence of the weakening of the corresponding connections through the formation of new bonds, including hydrogen bonds. The characteristic absorption band of Ti−O− Ti bond stretching vibrations (ν = 626 cm−1) also shifted to the region of lower frequencies. Analogous shifts to the lowerfrequency region, although less pronounced than in the case of acetone, were observed for Ti−O−C bond vibrations in the IR spectra of TiO2-chloroform systems (Figure 2, spectrum 5), TiO2-dimethyl sulfoxide (Figure 2, spectrum 4), and TiO2acetonitrile (Figure 2, spectrum 3). Thus, the IR spectra of the systems studied suggested the chemisorption of polar solvent molecules on the surface of titanium dioxide nanoparticles. According to CHNS analysis, the content of polar solvents in samples of modified titanium dioxide amounted to some 6−7% (with an accuracy of 0.1%). Nonpolar solvents, such as toluene and benzene, wetted the powder surface well, but did not change the IR spectra. Electrorheological and Dielectric Measurements. Electrorheological fluids were prepared by thoroughly grinding the powders obtained with polydimethylsiloxane in an agate mortar until a stable suspension was formed. Powders for the preparation of electrorheological fluids were used immediately after opening the weighting cups. Electrorheological measurements were carried out on freshly prepared liquids. In the

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RESULTS AND DISCUSSION An ER fluid containing 45 wt % of native titanium dioxide nanopowder exhibited a significant electrorheological effect (Figure 3), due to the polarization of the hydrate forms present on the surface of the TiO2 nanoparticles.21,22 These forms play the role of highly polarizable small molecules, according to the generally accepted point of view.12−14 The experimental data obtained, characterizing the effect of organic solvent molecules adsorbed on the titanium dioxide

Figure 3. Relationship between the shear stress of 45 wt % suspensions based on TiO2, modified with organic solvents, and the strength of the electric field applied, at a shear rate of 42 s−1. (1) TiO2; (2) TiO2-acetone; (3) TiO2-acetonitrile; (4) TiO2-DMSO; (5) TiO2chlorophorm; (6) TiO2-diethyl ether; (7) TiO2-toluene. 6734

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The Journal of Physical Chemistry B powder the shear stress at a shear rate of 42 s−1, as a function of electric field strength, are shown in Figure 3. The increase in shear stress values for the suspensions upon the application of the electric field is due to the particle arrangement and the formation of chain-like structures (Figure S1), which has been repeatedly observed and discussed earlier.5,6 As shown in Figure 3, the modifying solvents can be divided into two groups, according to their influence on the shear stress value. The members of the first group caused an increase in electrorheological activity in comparison to unmodified TiO2. The electrorheological effect increases in the following sequence of filling agents: TiO2-chloroform ≥ TiO2-acetone> TiO2-diethyl ether. What is noteworthy is that, up to field strengths of 4 kV/mm, these solvents had almost the same influence on the electrorheological effect. Modification of titanium dioxide by DMSO molecules virtually did not change the magnitude of the shear stress, up to the maximum field strength. Once this maximum was reached, the TiO2-DMSO filling agent became less efficient than the unmodified TiO2. The second group included solvents that lowered the value of the yield strength in an electric field. This group included acetonitrile and toluene. An analogous sequence, in terms of the change in yield strengths for systems based on titanium dioxide, modified by solvent molecules, was obtained by stretching suspensions in an electric field (Figure 4). For the systems under investigation,

For suspensions of solvent-modified powders, the maximum values of the dielectric loss angles tangents were substantially lower than the corresponding values for a suspension of native titanium dioxide. This clearly showed a decrease in the conductivity of titanium dioxide suspensions after the introduction of modifying additives. These relationships suggest that the dielectric relaxation spectra of modified suspensions in the low-frequency region resulted from the superposition of two contributions, the Debye contribution and the contribution of conductivity, characterizing the interphase polarization, which distinguished them from the spectra of the unmodified titanium dioxide. This may indicate the formation of a bonding network between the dispersed phase particles in the ER fluids based on solventmodified TiO2 powders. At the same time, both native titanium dioxide and titanium dioxide, modified with polar molecules, demonstrated the Maxwell−Wagner type of polarization. As Figure 5 shows, the modification of the titanium dioxide powder with organic solvents led to a shift of the maxima for the frequency dependences of the dielectric loss angle tangents to higher frequencies (shorter relaxation times). The effect of the modifying solvent on dielectric relaxation time, according to the data shown in Figure 5, varied in the series: TiO2 > TiO2DMSO > TiO2-acetone > TiO2-chloroform > TiO2-acetonitrile. In terms of the magnitude of the effect of the electric field on the yield stress, the ER fluid filling agents were arranged in a row: TiO2-chloroform > TiO2-acetone > TiO2-DMSO > TiO2acetonitrile ≈ TiO2. Comparing these dependences, it can be noted that a decrease in dielectric relaxation time promotes the growth of the ER effect. The only exception is the TiO2acetonitrile system, which does not conform to this general dependence, instead having a relaxation maximum above 105 Hz, which, as a rule, leads to a decrease in the electrorheological effect.23 It should be noted that the polarity of the molecules given in Table 1 does not correlate with the value of the electrorheological effect observed in the presence of the corresponding activators. The most unexpected result of the study was an increase in the electrorheological effect during the application of nonpolar solvents as modifiers with zero or near-zero dipole moments (see Figure 6). This contradicts the generally accepted concept of the electrorheological effect activated by polar molecules.12−14 As can be seen from Figure 6, the molecules of nonpolar solvents caused a substantial increase in the electrorheological effect, and, according to the data presented in Figure 7, the effect of the modifying solvent on dielectric relaxation time varied in the series: TiO2-toluene > TiO2benzene ≥ TiO2-diethyl ether. The decrease in the electrorheological effect for suspensions containing 45 wt % of solvent-modified TiO2 occurred in the same sequence (Figure 6). It is of particular interest that modification with toluene and benzene almost did not shift the position of the relaxation maximum in comparison with native titanium dioxide, while modification with diethyl ether shifted the maximum to lower frequencies. Consequently, the effect of adsorbed molecules on the magnitude of the electrorheological effect, in addition to the influence of the electric field on dipole−dipole and charge− dipole interactions, enhancing electrostatic attraction, was associated with other types of interaction in electrorheological systems.

Figure 4. Relationship between the yield stress of 45 wt % suspensions of TiO2, modified by polar solvents, and the strength of the electric field applied, at a tensile strain of 0.003 mm/s. (1) TiO2; (2) TiO2acetone; (3) TiO2-acetonitrile; (4) TiO2-DMSO; (5) TiO2-chloroform.

the effect of the modifying solvent on the magnitude of the electrorheological effect decreased in the series: TiO2-chloroform > TiO2-acetone > TiO2-DMSO > TiO2-acetonitrile = unmodified TiO2. It is worth mentioning that chloroform is considered to be a low-polar solvent (μ = 1.15 D). The strength of the ER fluids under stretching significantly exceeded their shear strength. The yield stress/shear stress ratio for the corresponding filling agents amounted to 30−35. Overall, rheological data correlated well with the data of the dielectric relaxation spectra. The extreme nature of the dependences of tgδ for 10 wt % suspensions of the materials studied in the frequency range 25÷103 Hz (Figure 5) indicated that polarization in these systems caused relaxation-type losses. 6735

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Figure 5. Frequency distributions of permittivity (left) and dielectric loss tangents (right) for 10 wt % titanium dioxide suspensions modified by molecules of organic solvents. (1) TiO2; (2) TiO2-acetone; (3) TiO2-acetonitrile; (4) TiO2-DMSO; (5) TiO2-chloroform.

where R is the particle radius, r1, r2, meniscus radii. Radii r1 and r2 depend on the amount of liquid phase covering the particles of the ER fluid filler, the surface tension, the wetting angle, and the particle radius. When the state of equilibrium is reached (when the forces of mutual attraction of the particles are completely balanced out by the repulsive forces), the rapprochement distance of particles depends on viscosity (η), the surface tension of the particle covering the particle (σ), and particle size (R): Δλ /λ = 3σt /2ηR

(3)

The parameter t has a physical meaning of time, and its dimension is chosen in such a way as to bring the right-hand side of eq 3 to a dimensionless form. Expressions 2 and 3 are interdependent, as the radii of curvature r1 and r2 change as the particles approach each other, thereby changing the force of attraction. Thus, the presence of solvents with lower viscosity and greater (in comparison to the dispersion medium) surface tension on the surface of nanoparticles leads to two effects: the wetting of particles of the dispersed phase increases, and surface tension forces make the particles coalesce into larger associates consisting of many particles. The structures formed in this way include liquid phase junctions and possess high mechanical strength.25,26 The improvement in the wettability of the filler particles in the ER fluids promotes an increase in the electrorheological effect, as shown earlier by Shen et al.27 Within the framework of the model considered, the influence of polar, nonpolar, and weakly polar solvents in increasing the electrorheological effect can be explained by the formation of aggregates and a network of bonds due to the surface tension forces between the modified particles in the electrorheological fluid. Thus, as follows from Table 1, the value of the contribution described by eq 2, estimated from the ratio of surface tension to viscosity for the solvents under study, changes in the following sequence: acetone −81.3; acetonitrile −77.5; diethyl ether −75.9; chloroform −50.9; toluene −50.7;

Figure 6. Relationship between yield stresses for 45 wt % TiO2 suspensions, activated with nonpolar solvents, and the strength of the electric field applied. (1) TiO2; (2) TiO2-diethyl ether; (3) TiO2benzene; (4) TiO2-toluene.

A layer of liquid (polar or nonpolar) formed on the surface of particles; this layer had different properties to the properties of the dispersion medium, (in the case of the current research, the silicone oil). When the filler particles contacted one another, these layers merged to form regions with the shape of a concave lens (Figure 8). An excessive pressure arose due to surface tension, which was directed toward the center of the curvature of the lens, and extruded the fluid from the contact zone of the particles toward the dispersion medium. As a result, there was a convergence of particles initially located at a distance of λ by a magnitude of Δλ. The force F, drawing particles together, may be considered on the basis of the theory proposed by Frenkel:24−26 F = σ[πR2 sin 2 φ(1/r1 + 1/r2) + 2πR sin φsin(φ + θ )] (2) 6736

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Figure 7. Frequency distributions of permittivity (left) and dielectric loss tangents (right) for 45 wt % titanium dioxide suspensions modified with organic solvents. (1) TiO2-diethyl ether; (2) TiO2-benzene; (3) TiO2-toluene.

benzene −48.1; DMSO − 21.5. It is obvious that the formation of such structures contributes to increasing the electrorheological effect in electric fields, in comparison with ER fluids with intact filling agents. The effect of particle aggregation on the electrorheological effect, due to the change of dispersion medium properties, was demonstrated earlier.28 It is interesting to note that the model describing the formation of interactions between particles due to viscous flow caused by surface tension forces predicts that the use of a mixture of oils with different viscosities in the composition of electrorheological fluids should lead to an increase in the electrorheological effect. Indeed, such an effect has been demonstrated previously.21 In two-phase disperse systems, such as solid phase/liquid medium, the properties of the dispersion medium have a significant effect on the processes of aggregation of particles, and, therefore, on the magnitude of the electrorheological effect.25−29 Thus, when the powder is mixed with oil, microgranules of the dispersed phase are formed and the resulting suspensions are structured in an electric field, and the formation of cohesive structures is achieved as a result of the formation of contacts between microgranules. The contacts can occur through thin interlayers of liquid on the surface of microgranules, but the greatest strength of the structures formed is provided by the direct contacts between the particles of the dispersed phase, without a liquid interlayer. Dispersion media with lower viscosity are characterized by higher mobility, and their use contributes to an increase in the proportion of direct contacts between particles. Thus, the structures which form are more resistant to external dynamic loads−tension, shear, and compression.

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CONCLUSIONS In the present study, we have demonstrated that nonpolar solvents are efficient modifying additives to titanium dioxide used as a filling agent of ER fluids. The adsorption of nonpolar solvent molecules on the surface of TiO2 particles not only leads to additional polarization effects in the interface, but also

Figure 8. Interaction between filler nanoparticles due to surface tension forces and electrorheological fluid solidification in an applied electric field.

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(13) Lu, K.; Shen, R.; Wang, X.; Sun, G.; Wen, W.; Liu, J. Polar molecule type electrorheological fluids. Int. J. Mod. Phys. B 2007, 21, 4798−4805. (14) Tan, P.; Tian, W. J.; Wu, X. F.; Huang, J. Y.; Zhou, L. W.; Huang, J. P. Saturated orientational polarization of polar molecules in giant electrorheological fluids. J. Phys. Chem. B 2009, 113, 9092−9097. (15) Dean Lange’s Handbook of Chemistry; Dean, J. A., Ed.; Mc GrawHill: New York, 1998. (16) Davydova, O. I.; Agafonov, A. V.; Kraev, A. S.; Trusova, T. A. The effect exerted by the type of the solvent and precursor in sol-gel preparation of titanium dioxide on its electrorheological activity. Russ. J. Appl. Chem. 2010, 83, 14−17. (17) Hanaor, D. A. H.; Chironi, I.; Karatchevtseva, I.; Triani, G.; Sorrell, C. C. Single and mixed phase TiO2 powders prepared by excess hydrolysis of titanium alkoxide. Adv. Appl. Ceram. 2012, 111, 149−158. (18) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric identification of organic compounds; John Wiley & Sons: New York, USA, 2005. (19) Urlaub, R.; Posset, U.; Thull, R. FT-IR spectroscopic investigations on sol-gel-derived coatings from acid-modified titanium alkoxides. J. Non-Cryst. Solids 2000, 265, 276−281. (20) Agafonov, A. V.; Krayev, A. S.; Davydova, O. I.; Ivanov, K. V.; Shekunova, T. O.; Baranchikov, A. E.; Ivanova, O. S.; Borilo, L. P.; Garshev, A. V.; Kozik, V. V.; et al. Nanocrystalline ceria: a novel material for electrorheological fluids. RSC Adv. 2016, 6, 88851−88858. (21) Davydova, O. I.; Kraev, A. S.; Redozubov, A. A.; Trusova, T. A.; Agafonov, A. V. Effect of polydimethylsiloxane viscosity on the electrorheological activity of dispersions based on it. Russ. J. Phys. Chem. A 2016, 90, 1269−1273. (22) Liu, X.; Guo, J.; Cheng, Y.; Xu, G.; Li, Y.; Cui, P. Synthesis and electrorheological properties of polar molecule-dominated TiO2 particles with high yield stress. Rheol. Acta 2010, 49, 837−843. (23) Sun, Y.; Thomas, M.; Masounave, J. An experimental investigation of the dielectric properties of electrorheological fluids. Smart Mater. Struct. 2009, 18, 024004−024013. (24) Frenkel, J. Viscous flow of crystalline bodies under the action of surface tension. J. Phys. (Paris) 1945, 9, 385−391. (25) Lu, N.; Wu, B.; Tan, C. P. Tensile strength characteristics of unsaturated sands. J. Geotech. Geoenviron. Eng. 2007, 133, 144−154. (26) Radjai, F.; Richefeu, V. Bond anisotropy and cohesion of wet granular materials. Philos. Trans. R. Soc., A 2009, 367, 5123−5138. (27) Shen, C.; Wen, W.; Yang, S.; Sheng, P. Wetting-induced electrorheological effect. J. Appl. Phys. 2006, 99, 106104−106107. (28) Gong, X.; Wu, J.; Huang, X.; Wen, W.; Sheng, P. Influence of liquid phase on nanoparticle-based giant electrorheological fluid. Nanotechnology 2008, 19, 165602−165609. (29) Uriev, N. B. Physicochemical dynamics of disperse systems. Russ. Chem. Rev. 2004, 73, 37−58.

changes the internal pressure of the liquid at the points of contact of the particles. As a result, an increase in the number of interparticle contacts occurs, which leads to a significant increase in the mechanical strength of the structures that arise when an electric field is applied, and an increase in the observed electrorheological effect in comparison with the unmodified titanium dioxide.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b04131. Optical microscopy images illustrating the changes in electrorheological fluid structure upon application of an electric field (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A.E. Baranchikov: 0000-0002-2378-7446 V.K. Ivanov: 0000-0003-2343-2140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to express their gratitude to the Russian Science Foundation (project No. 16-13-10399) for the financial support of the study. The research was performed using the equipment of the JRC PMR IGIC RAS.

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REFERENCES

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DOI: 10.1021/acs.jpcb.7b04131 J. Phys. Chem. B 2017, 121, 6732−6738