Silica prepared in the presence of alkylphosphonium-based ionic liquids and its performance in electrorheological fluids

Abstract

This work highlights the effect of silica particles prepared by a sol–gel process in the presence of different phosphonium-based ionic liquids on the electrorheological behavior of the corresponding suspension in silicone oil. The silica particles were prepared by hydrolysis/condensation of tetraetoxy silane (TEOS) in basic medium and in the presence of three different commercial ionic liquids: tri-isobutyl(methyl)phosphoniumtosylate (IL106), tri(n-butyl)(tetradecyl)phosphonium-dodecylbenzene-sulfonate (IL201) and trihexyl-(tetradecyl)-phosphonium-bis-2,4,4-(trimethylpentyl)-phosphinate (IL104). The resulting material was characterized by Fourier-transform infrared spectroscopy (FT-IR), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM) and dielectric properties. The confinement of the ionic liquids inside the silica particles was suggested by thermogravimetric analysis. The presence of ILs also exerts a strong influence on the morphology and imparts good polarization ability to the silica. The electro-rheological response of the corresponding ionogel suspensions in silicone oil was investigated. A significant ER effect was observed for the fluid containing silica prepared in the presence of IL106. In fact, a very good response under the action of an electrical field corresponding to 3 kV mm⁻¹ was achieved, with a shear stress value as high as 1215 Pa. This behavior may be attributed to the presence of the IL confined in the silica particles and also to the peculiar morphology which favors the formation of a columnar structure to a high extent.

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Introduction

Ionic liquids (ILs) are salts generally constituting of a bulky organic cation and an organic or inorganic anion.^1,2 This peculiar structure, which can be tuned by changing both cation and anion nature, is responsible for their melting points usually being lower than 100 °C and their unique properties such as low volatility, thermal stability, ionic conductivity, etc. Due to their characteristics, ILs have emerged as novel and versatile materials for several applications, such as a new “green solvent” for several organic syntheses,³ polymerization⁴ and sol–gel processes,⁵ as plasticizers,⁶ electrolytes for electrochemical devices,⁷ catalysis,³ etc. In the field of inorganic chemistry, ILs have attracted increasing interest as a unique medium for the preparation of several metals, metal oxides, silica, etc.⁸ Among several inorganic systems, silica is one of the most popular, thanks to its low cost, unique properties and preparation feasibility through the sol–gel process. This technique involves the hydrolysis and condensation of tetra-alkoxysilane and results in better homogeneous materials when compared with the traditional ceramic method.⁹ The mild conditions used in this process enable the incorporation of organic species into silica particles without degradation. ILs can assume different function in this technique. They can act as solvent substituting the water in the sol–gel process,¹⁰ they can be applied as a template for the preparation of mesoporous silica materials,¹¹ and be immobilized, encapsulated or confined in the silica particles.^12–14 ILs confined in silica particles result in a class of material called ionogel and find increased importance in catalysis, biocatalysis, solid electrolyte development and drug release.¹⁴

The confinement of IL inside the silica particles during the sol–gel process affects the ionic conductivity of silica and thereby produces significant change in the dielectric and polarization properties. Such features are very important for the development of electrorheological fluids. Electrorheological (ER) fluids are colloidal suspensions of polarizable solid organic or inorganic particles in a non-conducting liquid,¹⁵ whose rheological properties quickly and reversibly changes from liquid-like to solid-like under application of an external electric field. Several particles have been employed on the development of ER fluids and nowadays the organic–inorganic hybrid nanomaterials have been attracted much attention due to the possibility of tuning their properties by using specific metallo-organic precursors bearing appropriate organic groups.^9,16 Among them, silica is still one of the most studied particles because of the versatility in preparation and intrinsic polarity. Several strategies have been used to improve the polarity of silica particles in order to enhance the ER effect. Some of them include the treatment or preparation of silica with organo-silane bearing polar group,¹⁷ synthesis of silica in the presence of chloride solution of metallic cation,¹⁸ amine¹⁹ and conducting polymers.^20–24 Recently we reported the ER effect of ionogel-based silica material containing phosphonium-based ILs, with a great ER potential, due to the high dielectric constant of the particles.²⁵ The IL containing (11-carboxyl-undecyl)triphenyl-phosphonium cation was confined inside the silica particles giving rise to better permittivity and better ER behavior than pure silica. However, these fluids presented a limitation since they present high current density, which limits the utilization in the electric field range up to 1 kV mm⁻¹. The interesting results obtained in this pioneer work involving silica/ionic liquids for ER fluid applications, prompted us to continue on this subject by investigating other IL systems.

The present work deals with the preparation of silica in the presence of alkylphosphonium-based ILs with different anions and their utilization on the development of ER fluids in silicon oil.

Experimental

Materials

Tetraethoxy silane (TEOS), ammonium hydroxide and absolute ethanol were purchased from Sigma Aldrich. The ionic liquids, triisobutyl(methyl)phosphonium tosylate (IL106), tri(n-butyl)(tetradecyl)phosphoniumdodecylbenzenesulfonate (IL201) and trihexyl-(tetradecyl)-phosphonium-bis-2,4,4-(trimethylpentyl)-phosphinate (IL104) were obtained from CYTEC (Table 1) and used without purification. Silicone oil was purchased from Epoxy Fiber Brazil (Rhodorsil 47V 500) and presents the following characteristics: kinematic viscosity of 475–525 mm² s⁻¹; density of around 0.97 g cm⁻³; conductivity of 1 × 10⁻¹² S cm⁻¹.

Table 1 Ionic liquids chemical formulae

Name	Structure	Supplier
IL106		CYPHOS®IL106
IL201		CYPHOS®IL 201
IL104		CYPHOS®IL 104

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Synthesis of the silica particles and preparation of the silicon oil suspension

Silica particles were synthesized by hydrolysis/condensation of TEOS in basic medium, according to the well-known Stöber process.²⁶ In a typical procedure 18.8 g (0.09 mol) of TEOS was slowly added to a stirred solution containing 70 ml ethanol, 9 ml de-ionized water and 6 ml of ammonium hydroxide. After the TEOS addition, the medium was kept under stirring at room temperature for 4 h. The white precipitate was obtained by centrifugation, followed by washing with water and ethanol four times. The samples were dried at 80 °C for 24 h, under reduced pressure. The ionogels containing the ILs were prepared similarly, with previous addition of 1 g of IL into the water/ethanol solution.

For the preparation of the electrorheological fluids, silica particles and the corresponding ionogels containing different ionic liquids were suspended into silicone oil in a concentration of 30 wt%, with the help of an ultrasonic bath during 30 min, and stored in a desiccator prior to use.

Characterization

Thermogravimetric analysis was performed in a TA Instruments Q50 V20.10 Build 36 analyzer operating at a heating rate of 20 °C min⁻¹, under nitrogen atmosphere (balance gas: nitrogen 40.0 ml min⁻¹ and sample gas: nitrogen 60.0 ml min⁻¹) The TGA profiles were recorded in the temperature range 25–700 °C. The weight of the sample used was about 9–11 mg in all the cases.

Fourier transformed infrared spectra (FTIR) were recorded from KBr pressed discs in a Nicolet thermo scientific, Model iS-50 Spectrometer in the range of 4000–400 cm⁻¹ with 32 scans. The sample (∼1 mg) and KBr (∼99 mg) were ground together in an agate mortar until the sample is well dispersed.

The morphology of the silica particles was evaluated by field-emission gun scanning electron microscopy (FEG-SEM, JEOL JSM-670F) operating at 10 kV. The samples were previously coated with thin layer of carbon.

Dielectric properties were measured using the Solartron SI 1260 gain phase analyzer, interfaced to a Solartron 1296 dielectric interface, operating at a frequency range of 10 mHz to 1 MHz and 1 V. The silica as a powder was previously compressed into disks with surface area 1.33 × 10⁻⁴ m² and thickness of 1 × 10⁻³ m.

The ER properties of the fluids were measured on an Anton Paar Instruments Physical RMC 302 rheometer equipped with plate–plate geometry (PP50/E gap 1 mm) and electrorheological accessory (HVS/ERD80-DC) with a high voltage generator (10 kV, 1 mA).

Results and discussion

Material characteristics

The morphologies of pure silica (SiO₂) and those prepared in the presence of IL106 (SiO₂/IL106), IL104 (SiO₂/IL104) or IL201 (SiO₂/IL201) are shown in Fig. 1. Pure silica presents spherical morphology with broad size distribution with an average size of 170–600 nm. The use of ILs during the sol–gel synthesis resulted in particles with larger particle size and aggregates, suggesting a strong influence of these ILs on the morphology of the silica particles. Similar behavior has been observed in our previous work²⁵ and also reported by Nordström et al.¹³ and Ueno et al.,²⁷ who have attributed to the ionic concentration and electrostatic force. Additionally, the ionic liquid may catalyze the condensation reaction between particles, leading to the formation of non-spherical particles and aggregates.

Fig. 1 FEG-scanning electron microscopy of silica, SiO₂/IL106, SiO₂/IL201 and SiO₂/IL104 particles.

The FTIR spectra of the ionic liquids, pure silica and those prepared in the presence of ILs are shown in Fig. 2. For the spectra of the silica particles, the two absorption regions (4000–2500 and 1750–400 cm⁻¹) were shown separately, for sake of clarity. The broad band centered at 3470 cm⁻¹ in the FTIR spectra of the silica particles (Fig. 2ii) is related to the stretching vibration of –OH groups from silanol moieties (Si–OH) and stretching vibration of –OH from water. The appearance of a small deformation band at 1650 cm⁻¹ (peak A) in all spectra also confirms the presence of adsorbed water.²⁸ The SiO₂ particles are characterized by the broad and intense absorption bands at around 1100 cm⁻¹ (peak B) ascribed to the stretching vibration of the Si–O–Si bonds, a small band at 942 cm⁻¹ (peak C) associated with the Si–OH vibration, and those at 800 cm⁻¹ (peak D) and 460 cm⁻¹ (peak E), which are assigned to the asymmetric vibration of the siloxane network and to the bending vibration of the Si–O bond deflection, respectively.

Fig. 2 (i) FTIR spectra of ionic liquids, (ii) silica (a), SiO₂/IL106 (b), SiO₂/IL201 (c) and SiO₂/IL104 (d) particles.

The presence of the ILs in the silica particles can be suggested from the presence of small absorption bands at 2931 and 2853 cm⁻¹ observed in the samples containing IL104 or IL201. Such bands are attributed to the –CH₂ and –CH₃ groups present in the IL, by comparing with the FTIR spectra of the corresponding pure ILs (Fig. 2i). Such bands are more evident in these samples because of the higher amount of long alkyl groups (in the cation and anion moieties) in these ionic liquids.

Thermogravimetric analysis was employed to evaluate the thermal stability of the modified silica and also the amount of confined IL inside the silica particles. Fig. 3 compares the TGA analysis of pure silica, SiO₂/ILs samples and the pure ionic liquids. The main decomposition temperatures of the pure ionic liquids and the silica particles are also summarized in Table 2. Pure ionic liquids display well defined one stage degradation phenomenon. Contrarily, the silica-based samples display several degradation steps. The mass loss around 7–8% below 120 °C may be attributed to the volatile products adsorbed in the particles, probably water and ethanol resulted from the condensation step of the sol–gel process. The main degradation step of the SiO₂/ILs samples occurs at temperatures lower than that of the corresponding ionic liquid. Similar results have been reported by Singh et al. by studying several imidazolium-based ionic liquid confined inside porous silica matrices.^14,29,30 They have proposed a model denoted “hinged spring model” to explain this phenomenon. The present work is constituted by alkylphosphonium-based ionic liquids but the behavior of the alkyl chains inside the silica pores may be similar. It is important to observe the presence of multiple degradation stages at temperatures higher than those related to the degradation of the pure ionic liquid. Such behavior has been very recently reported by Singh et al. in different imidazolium-based ionic liquid systems, which attributed to the interaction of the cation ring with the oxygen at the silica pore wall and also to the interactions of the counter anion with the hydroxyl groups of the silica.^31,32 Based on the decomposition profiles of the different silica samples prepared in this work, and taking into consideration the interaction mechanism proposed by Singh et al. using different imidazolium-based ILs, it is possible to suggest the confinement of these ionic liquids inside the silica particles.

Fig. 3 TGA curves of (A) pure ionic liquids (IL104, IL106 and IL201) and (B) the corresponding silica particles (SiO₂/IL104, SiO₂/IL106 and SiO₂/IL201).

Table 2 Maximum decomposition temperatures of ILs and the corresponding ionogel particles

Sample	Maximum degradation temperature (°C)
	1^st	2^nd
IL106	467	—
IL201	464	—
IL104	386	—
SiO2/IL106	369	574
SiO2/IL201	420	565
SiO2/IL104	312	540

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To estimate the proportion of ionic liquid in each sample, the mass loss between 200 and 650 °C of the ionogels (silica containing IL) was compared with that obtained in pure silica sample. Based on this it was possible to estimate the proportion of ionic liquid in SiO₂/IL104, SiO₂/IL106 and SiO₂/IL201 as around 1.8, 1.2 and 2%, respectively. These results reveal a very low amount of ionic liquid confined inside the silica particles.

Dielectrical properties of the silica particles

The dielectric properties of pure silica and those containing ILs were evaluated. The relative permittivity dependence with temperature is illustrated in Fig. 4. All systems present an increase of the permittivity with the decreasing in frequency in the low frequency range. This effect may be ascribed to the Maxwell–Wagner–Sillars (MWS) polarization effect, which usually occurs when there is an accumulation of charge carriers at an interface between two materials.³³ Silica prepared in the presence of ILs, displayed higher relative permittivity, indicating that the ILs are confined inside the silica particles, as also observed by TGA analyses. The higher permittivity values in all frequency range were observed for SiO₂/IL104. This behavior may be attributed to the higher mobility of the corresponding IL104 ions.

Fig. 4 Relative permittivity versus frequency for silica, SiO₂/IL106, SiO₂/IL201 and SiO₂/IL104 particles.

Electrorheological behavior

The ER effect of the fluids containing different silica-based particles at the same concentration was evaluated by applying an electric field while measuring the shear stress at a constant shear rate (0.5 s⁻¹), as a function of time. The shear stress was measured under the influence of the electric field and without any applied electric field for three times (cycle off–on as a function of time) as illustrated in Fig. 5. All fluids presented the electrorheological effect, but in different intensities. The fluid containing pure silica displays moderate ER effect. Also the shear stress at an applied electric field of 2 kV mm⁻¹ increases during the time of exposition, indicating that the alignment of the particle with the electric field is not immediate. This behavior is also observed for the second and third cycles. The presence of confined ionic liquid in the silica particles exerts strong influence on the magnitude of the current density of the corresponding ER fluids and consequently on the ER behavior, as indicated in Table 3.

Fig. 5 Shear stress as a function of time for the silica-based fluids in silicone oil, at a constant shear rate = 0.5 s⁻¹ (voltage turned on after 50 s and then turned off after 100 s, three times).

Table 3 Electrorheological feature of different silica-based ER fluids

Sample	Voltage DC (kV mm^−1)	Current density (A m^−2)	Shear stress at 0.5 s^−1 (Pa)
Silica	2	0.041	373–568
SiO2/IL106	2	0.023	708
SiO2/IL106	3	0.048	1215
SiO2/IL201	2	0.099	72
SiO2/IL104	2	0.117	104–149

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The fluid containing SiO₂/IL104 presents a very large current density with the application of a voltage corresponding to 2 kV. This high current density limits the application of the electric field, becoming impossible to control the intensity of strength of device and also to complete the cycle, resulting in the low ER performance of this fluid. Similar behavior was observed for the fluid containing SiO₂/IL201. Contrarily, the fluid containing SiO₂/IL106 presented a great ER effect with high and continuous shear stress and small current density, as indicated in Table 3. In fact, a shear stress as high as 1215 Pa was achieved by applying an electrical field of 3 kV mm⁻¹. Also similar ER behavior was achieved for the second and third cycle of applied voltage corresponding to 2 kV mm⁻¹ and 3 kV mm⁻¹, evidencing the reproducibility of the ER response. The low current density of this system allows performing experiments in higher electric field.

Comparing the ER response of the Si/IL201 and Si/IL106, the former displayed the worst ER behavior, in spite of the same nature of the anion (sulfonate). The difference between them is the size of the alkyl groups located in both cation and anion moieties. IL201 presents long alkyl groups in both moieties, cation and anion, which imparts some affinity with the silicone oil. During the preparation of the silicone oil suspension some molecules of the IL201 previously confined into the silica particles may be extracted by the oil, increasing the ionic conductivity of the oil. This feature should be responsible for the increase of the current density of the suspension, with the application of the electrical field. This phenomenon may also be responsible for the high current density observed for the fluid containing SiO₂/IL104, since this IL also contain long alkyl group in its structure and may be compatible with the silicone oil as well.

Considering the good ER performance of the fluid containing SiO₂/IL106, the viscoelastic behavior of this fluid was compared with that containing pure silica. The test was performed at different electrical fields using frequency sweeps at 0.1–100 rad s⁻¹ and strain of 0.01%. The storage G′ and loss G′′ moduli as a function of frequency, for the silica- and Si/IL106-based fluids samples, are illustrated in Fig. 6. At low frequency there is a dip of G′ and a maximum of G′′ for both compounds, probably because some structural relaxation time of the columns of particles. The G′ values of the silica and SiO₂/IL106-based fluids remain constant with angular frequency. Table 4 summarizes the effect of the electrical field strength on the storage modulus values for SiO₂- and SiO₂/IL106-based fluids. In both cases, a significant increase of G′ is observed as the field strength increases, confirming the ER effect.^34,35 The best ER response was observed for the fluid containing SiO₂/IL106 particles.

Fig. 6 Storage modulus (G′) and loss modulus (G′′) as a function frequency for Si- and Si/IL106-based fluids measured at 3 kV mm⁻¹.

Table 4 Storage modulus at different DC voltage

Electric field (kV mm^−1)	Silica	SiO2/IL106
	G′ (kPa)
0	0.057	0.078
1	21	649
1.5	114	907
2	158	1200
3	294	1870

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To better understanding the effect of the ionic liquid in the silica-like ER fluids, the ER efficiency was determined as a function of electric field, as shown in the Fig. 7. The ER efficiency was determined by (G^′_E − G^′₀)/G^′₀, where G^′_E is the storge modulus with electric field and G^′₀ is the storage modulus without electric field, measured at 10 rad s⁻¹. As expected, the best ER efficiency was achieved with suspensions containing SiO₂/IL106, probably because of the increase in polarity imparted by the presence of the ionic liquid and also to the morphology of the Si/IL106 particles which favors the formation of a dense columnar structure under the applied electrical field, responsible for the increase in storage modulus and shear stress with the applied.

Fig. 7 ER efficiency versus the electric field strength for SiO₂ and SiO₂/IL106-based fluids.

Conclusions

Silica particles were prepared by the classical sol–gel process, assisted by different alkyl phosphonium-based ionic liquids. In spite of the low amount of the IL confined inside the silica particle, as indicated by the TGA data, the presence of the ionic liquids significantly affected the morphology of the particles, resulting an increase of the particle size and the formation of aggregates.

The presence of these ILs exerts strong influence on the ER behavior of the silica-based fluids prepared with silicone oil. Fluids containing SiO₂/104 and SiO₂/201 did not present good ER effect, because these particles generate high current density during the ER experiments, which limit the different features. This behavior may be due to the structure of these Ils, which imparts some affinity with the silicone oil favoring their extraction from the silica particle, increasing the ionic conductivity of the oil. The best ER response was observed for the fluid containing SiO₂/IL106 particle probably because of the adequate polarity and mobility of this particle, imparted by the presence of IL, and also because of the peculiar morphology which permits the formation of columnar structure under the applied electric field in higher extent. In fact, a very good response under the action of an electrical field corresponding to 3 kV mm⁻¹ was achieved, with shear stress value as high as 1215 Pa. This value is superior to those exhibited by several other conventional ER fluids.

Acknowledgements

This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq, Financiadora de Estudos e Projetos—FINEP and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro—FAPERJ. We are sincerely thankful to Central Electronic Microscopy Laboratory, Santa Catarina Federal University (LCME-UFSC).

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