The role of ether-functionalized ionic liquids in the sol–gel process: Effects on the initial alkoxide hydrolysis steps

Journal of Colloid and Interface Science 447 (2015) 77–84

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The role of ether-functionalized ionic liquids in the sol–gel process: Effects on the initial alkoxide hydrolysis steps Ricardo K. Donato a,b,⇑, Marino Lavorgna c, Pellegrino Musto d, Katarzyna Z. Donato a,b,c, Alessandro Jager a, Petr Šteˇpánek a, Henri S. Schrekker b, Libor Mateˇjka a,⇑ a

´ Sq. 2, 162 06 Prague 6, Czech Republic Institute of Macromolecular Chemistry, Heyrovsky Laboratory of Technological Processes and Catalysis, Institute of Chemistry, Universidade Federal do Rio Grande do Sul-UFRGS, Av. Bento Gonçalves, P.O. Box 15003, 9500 Porto Alegre, RS, Brazil c Institute for Composite and Biomedical Materials, P.le E. Fermi 1, Loc. Granatello, 80055 Portici, NA, Italy d Institute of Chemistry and Technology of Polymers, Via Campi Flegrei 34, 80078 Pozzuoli, NA, Italy b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 29 December 2014 Accepted 29 January 2015 Available online 7 February 2015 Keywords: Ether-functionalized ionic liquids Sol–gel silica Multiple hydrogen-bonds Dynamic template Morphology control Time-resolved kinetics Hydrolysis control Imidazolium ring mobility

a b s t r a c t The ether-functionalized imidazolium ionic liquids (IL) applied in the silica sol–gel process demonstrated a defined coordination potential. These IL display the capacity to control the system organization from the reactions’ first moments through a dynamic system-assembling ability, being the sum of ionic and physical interactions, i.e. Coulomb forces, H-bonding and London forces. The initial hydrolysis steps of tetraethyl orthosilicate (TEOS) in the presence of these IL were followed by Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS), both in time-resolved experiments, in an attempt to correlate the structuring and the bonding dynamics of these systems. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The sol–gel process is a powerful source for the preparation of surface and interface controlled materials, allowing the physicochemical properties tuning from the first stages of preparation.

⇑ Corresponding authors at: Laboratory of Technological Processes and Catalysis, Institute of Chemistry, Universidade Federal do Rio Grande do Sul-UFRGS, Av. Bento Gonçalves, P.O. Box 15003, 9500 Porto Alegre, RS, Brazil (R.K. Donato). E-mail address: [email protected] (R.K. Donato). http://dx.doi.org/10.1016/j.jcis.2015.01.079 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

Technology driven materials with unique properties can be prepared, including highly homogenous structures or finely controlled secondary phases [1,2]. Ionic liquids (IL) are salts with ionic– covalent crystalline structures presenting melting temperatures up to 100 °C. Due to the unusual imidazolium derived IL’s characteristics, e.g. structural organization and high thermal and chemical stability, these have shown great potential as templates for the synthesis of inorganic materials. Some IL do not only act as templates, but also stabilize the structures formed, thus working as an efficient template–solvent and in some cases a

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template–solvent–reactant [3,4]. This explains the interest in understanding the physicochemical characteristics of IL-solvent mixtures. The pure imidazolium IL exist as self-assembled polymeric supramolecular structures, driven by hydrogen bonding (H-bond) and p–p stacking, as well as Coulomb coupling and London forces depending on the IL molecular structure, among the cations and anions. Differently, IL-solvents mixtures present highly complex structures (inclusion structures, triple ions, ion pairs, solvent separated ions), which are concentration dependent [3]. The elucidation of these structures, and their correlation with the formation conditions, are of great interest for applications in colloidal solution-based systems. For instance, this structural organization of the original sol determines the continuous polycondensation and subsequent network formation [2,5]. Nowadays, ether-functionalized IL are popular due to successful applications in the fields of deep-eutectic solvents, electrochemistry and materials with biological interfaces [6,7]. In general, the functionalization of imidazolium cations with alkyloxy or alkyloxyalkyl groups leads to reduced viscosity, crystallinity, melting temperature and glass transition temperature; and also to increased polarity, hydrophilicity and H-bond capability, when compared to their aliphatic analogs [6]. Furthermore, this set of physicochemical changes leads to increased water solubility and conductivity [8–10]. These characteristics are a reflex of the high chain flexibility predominating over the polarity of the ether groups, the higher rotational freedom and reduced lattice energy promoted by the ether groups [9,11,12]. Also, the ether chains compete with anions for cation interactions and, as a result, the cation–anion interactions are weakened, resulting in the formation of cation dimers in ether-functionalized IL [13,14]. Interestingly, the alkoxy chain can be designed in different lengths and shapes, allowing a fine tuning of these physicochemical properties for particular applications [6]. Since the first reports on using IL as templates for the sol–gel process and materials science [15], their structural similarities to usual surfactants, summed to their unique properties, make them ideal candidates for replacing traditional surfactants applied in the preparation of periodic porous networks [16,17]. This ability of IL to act as template is due to their interfacial interaction with the growing material. Based on the special molecular structures of some of these imidazolium based IL, a mechanism called hydrogen bond-co-p–p stack was proposed [16,18]. This should be responsible for the IL induced formation of the tridimensional porous systems in the formation of, e.g., silica via the sol–gel method. In this particular case, the H-bond induced IL anion–silanol interaction forces the orientation of this anion. Consequently, the cation also tends to align with the growing silica surface, driven by Coulomb coupling forces with the anion. The fluid state of the sol system favors the necessary molecular relocation, which is further promoted by the additional p-interactions among the imidazolium rings [18]. The use of this strategy, despite of being fairly recent, is already quite consolidated in the literature, including successful attempts of our group to obtain morphologically controlled sol–gel silica by using different classes of IL [5,19]. This approach was extended to the in situ formation of silica-filler polymer nanocomposite systems, where the IL could serve as multifunctional agents, providing materials with significantly improved mechanical properties [20– 22]. Ether-functionalized imidazolium IL also showed media dependent electrical conductivity enhancements [23], forming in the presence of water H3O+ and a new species that inhibits charge transfer processes on an electrode surface [8]. The H3O+ formation in the presence of water allows these IL to act as bifunctional morphology controller-catalysts when applied into the sol–gel process [19]. Importantly, previous works focused only on the IL’s participation in the final stages of sol–gel process, when the condensed

phase was dominant in the growing network [5,16–19,21]. However, information about the initial sol–gel stage involving the hydrolysis step and IL’s interactions with the reaction mixture, in spite of its crucial role in the system structure evolution, is very scarce. Differently, this study is an attempt to elucidate the ether-functionalized IL’s mechanism of action in the phase structure evolution (by DSL) and the kinetics (by FTIR) of TEOS hydrolysis in the sol–gel process, since the first reaction moments. Herein, is also reported about the individual roles of IL’s cation side chain length and anion (Fig. 1), as well as the synergic action with the applied co-solvent. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS), isopropyl alcohol (iPrOH), ethyl alcohol (EtOH) and hydrochloric acid (HCl) were used as received (Sigma–Aldrich). 2.2. IL synthesis A straightforward synthetic pathway was chosen for the preparation of halide-free ether-functionalized imidazolium IL, as presented in the literature [24]. The first reaction step consisted in the preparation of methanesulfonate ester alkylating agents by the treatment of the corresponding alcohols (i.e., 2-methoxyethanol and triethylene glycol monomethyl ether) with methanesulfonyl chloride, in the presence of triethylamine. The obtained alkylating agents were than reacted with 1-methylimidazole to obtain the respective IL: 1-monoethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate (C3OMImMeS), 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate (C7O3MImMeS) and 1-triethylene glycol monomethyl ether-3-methylimidazolium tetrafluoroborate (C7O3MImBF4) (Fig. 1). 1H and 13C NMR confirmed the purity of the synthesized IL. 2.3. Synthesis of sol–gel silica Hydrolysis and condensation of TEOS was performed in either iPrOH or EtOH solutions, using IL (2.5 wt.% relative to the content

Fig. 1. Ether-functionalized IL used in this study.

R.K. Donato et al. / Journal of Colloid and Interface Science 447 (2015) 77–84

of TEOS) and (in some cases) HCl (2 mol.% relative to the content of TEOS) as catalyst. The experiments were performed with a 45:55 volume ratio of TEOS:iPrOH or TEOS:EtOH and 1:2 molar ratio of TEOS:H2O. The system compositions are summarized in the Supplementary Information (Table SI1). Homogenization of the alcohol-IL-water mixtures was followed by the addition of TEOS, which marked the beginning of the hydrolysis reaction. 2.4. Time-resolved Fourier transform infrared spectrometry analysis Real time attenuated total reflection infrared Fourier transform spectrometry (FTIR–ATR) was performed to investigate both the TEOS hydrolysis mechanism in the presence of IL and to understand the dynamic H-bonding network that evolves among hydrolyzed TEOS, alcohol co-solvents (iPrOH and EtOH), water and IL. The FTIR spectra were collected with a Spectrum-GX (PerkinElmer, Norwalk, CT), equipped with a germanium/KBr beam splitter and a wide-band DTGS detector. All the measurements were carried out at 25 °C. The instrumental parameters for data collection were as follows; resolution 4 cm1, spectral range 4000– 400 cm1, 64 acquisition scans for high-resolution spectra and 1 scan for real time spectra. The SPECAC (Mod. Benchmark) ATR accessory was used with temperature control from ambient to 200 °C, outfitted with a metal boat, wherein the co-solvent, IL, water and TEOS, subsequently, were poured with a calibrated syringe. The solution was stirred by pipet plunging during the IR data collection. After TEOS addition, automated data acquisition started, using software package for time-resolved spectroscopy (Timebase from Perkin–Elmer). The signals were acquired directly as absorbance spectra at specific predetermined time intervals, using the empty metallic boat profile as background. The acquisition time intervals were about 3 s during the experiments, which typically lasted about 30 min. The beginning of the reaction was considered from the moment of TEOS addition to the mixture of the other components (i.e. co-solvent, IL and water). 2.5. Time-resolved dynamic light scattering experiments (DLS) The dynamic light scattering (DLS) experiments were performed to probe the average scattering intensity and size of the nano and micro structures, using a Zetasizer Nano ZS instrument (Malvern Instruments, U.K.). The samples were measured at a constant temperature of 25 ± 1 °C. The average scattering intensity (I) was obtained directly from the derived count rate. The hydrodynamic radii (RH) of the structures were determined from the measured intensity correlation functions g2 (t) converted to distribution of relaxation times and, subsequently, to size distributions using the StokesEinstein equation (Eq. (1));

RH ¼

kB Tq2 s 6pg

79

the measurements, the cuvette was quickly shaken 3 times for homogenization. Each measurement was comprised of 5 s duration runs, constituting a 30 min analysis.

3. Results and discussion Previously, IL have shown to induce variable morphologies in silica sol–gel systems, dependent both on the anion and cation side chain [5,19,21]. As a consequence, understanding the role of IL in the structure evolution during the sol–gel process is of fundamental importance. Interestingly, the ether-functionalized IL in combination with iPrOH showed an atypical phenomenon during the sol structure evolution. The early stages of TEOS hydrolysis, in the presence of either IL C3OMImMeS or C7O3MImMeS in combination with iPrOH, displayed a transient behavior, going from transparent to turbid and back to transparent state. The transition period observed and the phase structuring development varied depending on the IL cation side-chain length, where the shorter side-chain of IL C3OMImMeS produced later transition periods. The transient turbidity effects were observed only for the TEOS–iPrOH system, when applied MeS anion based IL. No turbidity occurred in the EtOH systems or by using IL with BF4 anion. The phase structure evolution in the early stages of the sol–gel process was studied by dynamic light scattering (DLS). This revealed a transient formation of clusters responsible for the turbidity period. Two maxima of correlation distance intensities are shown in Fig. 2, which are in agreement with the individual time regimen of the visual turbidity phenomena for each IL. The structure evolution was more premature when applying C7O3MImMeS (after 1 min) compared to C3OMImMeS (after 4 min), which is discussed in details below. Such phenomena were not observed in IL-free systems; or when using EtOH as a co-solvent; or IL C7O3MImBF4, independent of the co-solvent (iPrOH or EtOH). This implies that the IL cation and anion, as well as the applied co-solvent, had crucial roles on TEOS hydrolysis mechanism and organization of these colloidal systems, corroborating with previous applications of these systems by our group [5,19,21,22]. In an attempt to understand how and where this IL interference occurred in the silica formation, the sol–gel process course was followed by FTIR. This investigation provided information about the reactants consumption kinetics and new species’ formation, including also important H-bond interactions taking place in the reaction mixture.

ð1Þ

where kB is the Boltzmann constant, T is the absolute temperature,

g is the viscosity of the solvent, s is the relaxation time related to the diffusion movement of the nano and micro structures, and q is the scattering vector given by Eq. (2);

q¼

  4pn h sin k 2

ð2Þ

where n is the refractive index of the solvent (nwater = 1.33), k is the wavelength of the incident beam (k = 633 nm), and h is the scattering angle (h = 173°). The samples were prepared by filtering the water–co-solvent–IL solutions and TEOS (in a separate filter) with a 3 mL syringe equipped with a polyvinylidene fluoride (PVDF) 0.45 lm filter, and then added to a square glass cell (12 mm). Before starting

Fig. 2. DLS average intensity in function of time for iPrOH–C3OMImMeS (a) and iPrOH–C7O3MImMeS (b) TEOS based sol–gel systems. Shadowed areas mark the observed turbidity phenomenon range for each IL.

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3.1. Physical interactions in the reaction medium The TEOS hydrolysis media, composed of alcoholic co-solvent, water and IL, are characterized by a complex network of physical interactions (especially H-bond) established among the several components. These led to red or blue shifts (i.e. decrease or increase in wavenumber, respectively) of specific FTIR bands of the individual components. Selected FTIR spectra regions of (a) C7O3MImMeS, (b) iPrOH, (c) iPrOH–C7O3MImMeS mixture and (d) iPrOH–C7O3MImMeS–H2O mixtures are shown in the Supplementary Information (Fig. SI1 and Table SI2) [26,31–34]. Several characteristic band assignments for co-solvents (i.e. iPrOH and EtOH), TEOS and IL (C3OMImMeS, C7O3MImMeS and C7O3MImBF4) were used to follow the systems as they evolved or shifted during the components mixing, thus characterizing interactions in the solutions (Table 1, Figs. SI1 and SI2). In Supplementary Information (Fig. SI3) are shown the spectra of the neat C7O3MImMeS, as well as the iPrOH–C7O3MImMeS and iPrOH–C7O3MImMeS–H2O solutions in the region between 1800 and 700 cm1. When considering the systems before applying TEOS, the IL’s MeSO3 anion band at 1038 cm1, ascribed to the symmetric S–O 1 stretching of the SO in the presence 3 group, shifted to 1042 cm of iPrOH and to 1047 cm1 in the presence of both iPrOH and H2O. This wavelength increase (blueshift) was a direct consequence of the acidic behavior of this IL in the presence of water [8,19]. This could lead to the formation of acidic species, such as H3O+, which can compete with the imidazolium cation for interacting with the MeSO3 anion, affecting its vibration modes. Moreover, these acidic species could catalyze the hydrolysis reaction. Similar behavior was also observed for sulfonic ionomeric membranes, where a decrease in the sulfonate-cation binding strength occurred by increasing the size of the cation in the contact ion pair [25]. Thus, the blueshift was mainly ascribed to the stronger interaction between acidic species and anions. However, the solvation effect of water molecules (hydration) cannot be neglected. The band at 770 cm1, ascribed to the stretching of the C–S bond in the MeSO3 anion, shifted to higher frequencies as a consequence of the chemical environment modification of the SO 3 groups [26]. The mixture EtOH–IL–H2O also presents H-bond among IL, EtOH and water molecules. The IL band at 770 cm1 was blueshifted, whereas the band at 1040 cm1 is overlapped by the EtOH bands. Thus, also in this case, using IL with MeSO3 anion, the formation of acidic species during hydrolysis of TEOS took place, as observed for the iPrOH system (Table 1).

a b c d e

Systems

ds(CH3)a

m(C–C–C)b

m(C–S)c

C7O3MimMeS iPrOH–C7O3MimMeS iPrOH–C7O3MimMeS–H2O C3OmimMeS iPrOH–C3OmimMeS iPrOH–C3OMImMeS–H2O iPrOH–C7O3MImBF4 iPrOH–C7O3MImBF4–H2O EtOH–C7O3MimMeS EtOH–C7O3MimMeS–H2O EtOH–C3OmimMeS EtOH–C3OMImMeS–H2O EtOH–C7O3MImBF4 EtOH–C7O3MImBF4–H2O

– 1379 1384 – 1379 1382 1379 1382 1379 1381 1379 1381 1380 1381

– 950 946 – 950 948 950 947 – – – – – –

770 – 778 765 – 780 – – – 778 – 780 – –

(+3)d (+3)d (+2)d (+2)d (+1)d

3.2. TEOS hydrolysis The TEOS hydrolysis kinetics were determined by real-time FTIR spectrometry. The sequence of spectra of the iPrOH–C7O3-

Table 1 Main absorbance band frequencies for the applied systems.

(+5)d

The absorbance band at 1576 cm1 in C7O3MImMeS is ascribed to the in plane C–C and C–N stretching vibrations of the imidazolium ring. This band was still evident in the spectrum of the iPrOH–C7O3MImMeS system, but showed a clear transient effect with the addition of water (Fig. 3). This spectral feature could be ascribed to the p-packing of imidazolium rings and loss of their vibrational freedom, which packing arrangement was favored by both water and hydronium ions. Likely the hydroxyl ion (OH), which competes with the MeSO3 anion in the interaction with the imidazolium cation, allowed a denser IL packing, diminishing the ring mobility with a consequent formation of micelle structures, wherein the iPrOH molecules are tightly constrained. Similar results were also observed for the C3OMImMeS–iPrOH system. The addition of water to the iPrOH–C7O3MImMeS or EtOH– C7O3MImMeS solutions affected the alcohol bands in the regions 1250–1550 cm1 and 2700–3000 cm1. The two bands at 1380 and 1370 cm1 for iPrOH and at 1380 cm1 for EtOH, ascribed to the umbrella mode of CH3 groups, blueshifted as a consequence of the modification in the chemical environment. In analogous systems, using sodium dodecyl sulfate as surfactant, blueshifts for the CH2 groups asymmetric (+1.7 cm1) and symmetric (+0.9 cm1) stretching bands were observed, as a consequence of the increase of alkyl group packing density [27]. Simultaneously, wavenumber reductions (redshifts) at 950 cm1 and 815 or 880 cm1, were observed and ascribed to the CH3 umbrella modes and C–O bond stretching for iPrOH and EtOH, respectively. These spectral evidences confirm that specific H-bonds took place in the different media, which were dependent on the synergistic effect between IL and co-solvent. The typical absorbance band frequency shifts are presented in Table 1. The most significant shifts of the absorbance bands, implying the strongest interactions, occurred in the presence of iPrOH and C7O3MImMeS. The H-bond strengths were weaker in the presence of C3OMImMeS and EtOH. These spectral features, alongside with the imidazolium ring packing, confirmed that the more hydrophobic iPrOH interacted through the OH groups via H-bond with water–IL, whereas the hydrophobic part packs in a confined region. As for the MeSO3 anions, it is worth noting that in the presence of water there was always a frequency increase ascribed to the S–O and C–S bond stretching, regardless the IL or the co-solvent. Thus, suggesting that strongly coordinative groups surrounded the MeS anion.

(+8)e

(+5)e

(+ 8)e (+5)e

CH3 stretching modes for iPrOH and EtOH. CH3 umbrella modes for iPrOH and EtOH. C–S stretching modes for MeSO3 anion. CH3 stretching mode blueshifts for iPrOH and EtOH after water addition. C–S stretching mode blueshifts for MeSO3 anion after iPrOH and water addition.

Fig. 3. Time dependent FTIR spectra during hydrolysis of TEOS in the iPrOH–C7O3MImMeS system.

R.K. Donato et al. / Journal of Colloid and Interface Science 447 (2015) 77–84

MImMeS–H2O–TEOS solution showed the evolution during TEOS hydrolysis, where the main peaks for following water consumption (OH bending at 1650 cm1) and the in plane C–C and C–N stretching vibrations of the imidazolium ring (at 1570 cm1) are indicated (Fig. 3). The reaction described in Eq. (3) was followed by monitoring the water consumption and EtOH formation through the bands at 1650 cm1 and 878 cm1, ascribed respectively to bending mode of water and m(C–O)/(C–C) of EtOH. The consumption of TEOS was not monitored through the band at 780 cm1, assigned to the Si–O asymmetric stretching vibration, due to the overlap with IL, making it unreliable. Also other TEOS related bands, e.g. 1168 cm1 and 812 cm1, were overlapped by the iPrOH CH3 and CH2 rocking modes.

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Importantly, in this investigation the water consumption was a reliable source to follow the system hydrolysis, as the investigation was exclusively about the initial hydrolysis step, where the contribution of water formation from TEOS condensation is insignificant, especially in systems under acidic conditions [2]. However, other reference parameters should be adopted for following the later hydrolysis stages, when condensation is more pronounced. Moreover, the comparison of these systems under the same conditions, but without IL, was not possible, as the TEOS hydrolysis was too slow and the contribution of its condensation was significant.

tion, the maximum observed (at 5.5 min) coincided with the region of the fastest water consumption and EtOH production. When hydrolysis started, the packed organization of IL decreased, due to the modification of the hydrophobic/hydrophilic character of the medium by the production of EtOH and consumption of water, and became less ordered until a maximum of disorder was reached. Only after established this unpacked arrangement, the above mentioned acidic species became available in higher extent as catalysts for TEOS hydrolysis, accelerating the reaction. At this point, however, the medium character changed due to the production of silanol groups and the vibration of imidazolium ring became likely hindered by the formation of siloxane domains. The TEOS hydrolysis kinetic curves of the iPrOH–C3OMImMeS system are reported in Fig. 4(2). In comparison to the C7O3MImMeS system, the hydrolysis proceeded in a slower rate. In fact, after 30 min of reaction the curves related to both water and EtOH did not reach a quasi-equilibrium plateau as determined in the case of C7O3MImMeS. However, the band associated with the imidazolium ring, which was not detectable at the beginning of the hydrolysis, increased rapidly after the addition of TEOS and formed an oscillating state, with a first maximum at about 13 min, which persisted during all the investigated time. Interestingly, both the water consumption and the imidazolium ring mobility maxima took about 3 times longer to establish in this system, compared to the iPrOH–C7O3MImMeS.

3.2.1. TEOS hydrolysis in iPrOH Fig. 4 displays the kinetic curves for the evolution of different phenomena. These include the peaks at; (i) 1650 cm1, related to the consumption of water (OH bending reduction); (ii) 880 cm1, corresponding to the production of EtOH; and (iii) 1570 cm1, showing the evolution of the imidazolium ring mobility and thus the extent of ring packing. For C7O3MImMeS in the presence of iPrOH, the water consumption kinetics in Fig. 4(1) exhibited a sigmoidal course, involving a slower induction period at the beginning of the hydrolysis reaction, followed by a faster reaction, until it reached a semi-plateau. Also the kinetic curve related to the formation of EtOH showed a similar course. The imidazolium ring curve grew during the slow initial hydrolysis, thus implying an increase in ring mobility. This indicated that the initially packed organization of C7O3MImMeS, with a limited vibrational freedom, became gradually disordered, up to the arrangement showed the highest ring mobility. When this ring curve was correlated to the water consumption and EtOH forma-

3.2.2. TEOS Hydrolysis in EtOH For the systems with EtOH, TEOS hydrolysis was evaluated through both the water consumption and TEOS depletion. In this case, it was possible to monitor the direct TEOS depletion from the CH3 rocking band at 1171 cm1, since it did not present overlapping bands like in the iPrOH system. The hydrolysis was much faster than with iPrOH and did not show the sigmoidal character with an induction period. This could be ascribed to the effective catalyst action of the acidic species produced by MeSO3 IL–water interaction, which were more available to TEOS from the beginning of the process. The water–EtOH solution has a higher dielectric constant compared to iPrOH–water, promoting the ionic dissociation in a greater extent [28]. Interestingly, also for the EtOH systems, when C7O3MImMeS was used, the hydrolysis process was about 3 times faster than for C3OMImMeS, suggesting that similar phenomena could be happening but in a much faster pace. In addition, no change in the imidazolium ring mobility (p packing and unpacking) and no induction period could be observed for EtOH system (Supplementary Information, Fig. SI4).

SiðOEtÞ4 þ nH2 O $ SiðOHÞn ðOEtÞ4n þ nEtOH

ð3Þ

Fig. 4. Kinetic curves of water consumption (a), EtOH formation (b) and imidazolium ring freedom (c), for the iPrOH–C7O3MImMeS (1) and iPrOH–C3OMImMeS (2) systems.

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Fig. 6. Water consumption, determined from the FTIR peak at 1790–1590 cm1 during hydrolysis for the systems iPrOH–C3OMImMeS (a), iPrOH–C7O3MImMeS (b) and EtOH–C7O3MImMeS (c). Fig. 5. Kinetic curves for the consumption of water (a), for the formation of EtOH (b) and for the imidazolium ring mobility (c) in iPrOH–HCl–C7O3MImMeS system.

3.2.3. TEOS Hydrolysis in iPrOH and HCl As a manner of correlating the contribution of acid catalyst availability and the solvent polarity, HCl was applied as catalyst in the iPrOH–C7O3MImMeS system. The acidification of this system, as expected, resulted in acceleration of the hydrolysis making the system behave much more similar to the one with EtOH. The initial step of hydrolysis was abruptly shortened, with respect to the system without HCl, but the process dynamics were comparable. The curve corresponding to the imidazolium ring freedom (Fig. 5, curve c) initially grew and started decreasing almost simultaneously with the promotion of the fast initial hydrolysis step. The maximum in the mobility peak was still observable in this curve, this system was characterized by an intermediary behavior between the system with EtOH and that with iPrOH. 3.3. Phase structure evolution during hydrolysis of TEOS The FTIR studies revealed that the MeSO3 anion based IL, in the presence of iPrOH, formed a packed structure within the TEOS free system and turned gradually into an unpacked structure when TEOS hydrolysis proceeded. At higher degrees of hydrolysis, the imidazolium packed structure reestablished, due to IL-silanol interactions. For C7O3MImMeS the process was quite fast, whereas for C3OMImMeS the process was slower and the structure seemed to oscillate over the 30 min investigated. A complex system was observed during the hydrolysis process, wherein the structural modifications depended on both the IL cation side-chain length and the co-solvent nature. Alongside these features, it was necessary to take into account the formation of new acidic species based on H2O-ether-functionalized IL with MeSO3 anion, as previously reported by our group [8], which could also be affected by the co-solvent. Taking the water consumption as a parameter and considering the system with C7O3MImMeS, initially the hydrolysis in iPrOH was slower than in EtOH, but appeared reaching the same speed of consumption after approximately 9 min (Fig. 6). This was ascribed to the steric effect of iPrOH, which was prominent in the early stages of hydrolysis [29]. This effect became irrelevant after silanols and EtOH started to be formed (see Eq. (3)). The system with iPrOH–C3OMImMeS presented slower water consumption, when compared to the iPrOH–C7O3MImMeS system. The kinetic behavior correlated with the number of ethylene glycol (EG) segments in the IL’s cation side-chain, where C7O3MImMeS

(with 3 EG segments) consumed water 3 times faster than C3OMImMeS (with 1 EG segment). This fact seems to be related to the ether-functionalized IL’s ability to form multiple H-bonds with increasing ether functionalities [6,7], creating and stabilizing increasingly complex structures with the newly formed silanols and co-solvents. This was confirmed by the spectral features associated with the imidazolium ring during the process. In the case of the iPrOH–C3OMImMeS system, the imidazolium rings changed from a packed to a free to vibrate arrangement with the beginning of hydrolysis, keeping in an oscillating pattern throughout the whole investigated time (Fig. 4(2), curve c). On the contrary, in the case of iPrOH–C7O3MImMeS system, the IL exhibited a defined transient process from immobilized into more mobile and back to immobilized structure again (Fig. 4(1), curve c). To validate the sol aggregation phenomenon and evaluate the system’s initial organization state, time dependent dynamic light scattering (DLS) experiments of the systems were carried out. The average light scattering intensities and size distributions gave valuable information about the dynamics and the dimensions of aggregates in the solution during the sol–gel hydrolysis. The water depletion kinetics followed by FTIR and structure evolution determined by DLS with both IL are compared in the Figs. 2 and 7. The system iPrOH–C3OMImMeS presented a scattering intensity with two maxima in function of time. At the beginning, the scattering intensities were constant (around 40 Kcps) and no sign of structural organization or large particle sizes could be detected from the correlation function (Supplementary Information, Fig. SI5). At around 4.5 min a sharp increase in the scattering intensity with values around 9000 Kcps was observed (Figs. 2 and 7). The increase in the scattering intensity was related to the formation of aggregates with the hydrodynamic diameter (2Rh = D) in the micrometric scale (Supplementary Information, Fig. SI5). This time range correlated with the first change in the water consumption rate (Fig. 7a). At about 6.5 min, the scattering intensity decreased to values around 2500 Kcps and the hydrodynamic diameter of the aggregates entered in the sub-micron range limit. Later, the scattering intensity increased and reached a maximum intensity value at about 10 min and the size of the aggregates corresponded to a few micrometers (around 2.5 lm). Finally, the scattering intensity droped until it reached the initial values (at 13 min) and no large aggregates could be detected. This time frame was coincident with the maximum of imidazolium ring freedom and a defined increase in the water consumption rate. The phenomenon time window of about 8 min correlated with the turbidity process observed (Fig. 2).

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Fig. 7. Time correlation between water consumption (1790–1590 cm1 FTIR band), imidazolium ring freedom (1570 cm1 FTIR band) and aggregation phenomena (DLS) in the sol–gel systems with iPrOH–C3OMImMeS (a) and iPrOH–C7O3MImMeS (b).

As previously described by the FTIR experiments, the iPrOH–C7O3MImMeS system showed sigmoidal shaped water consumption and the IL structure evolution exhibited a transient process. The scattering intensities and the corresponding cluster sizes involved during the hydrolysis process, correlated to FTIR, are shown in Fig. 7b. In comparison to iPrOH–C3OMImMeS, the initial aggrega-

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tion process for iPrOH–C7O3MImMeS was about 3 times faster. Furthermore, the higher scattering intensities observed from the beginning of the experiments (200 Kcps, Fig. 2) suggested structures with larger sizes (Supplementary Information, Fig. SI5). The initial hydrodynamic diameter observed for iPrOH–C7O3MImMeS was around 200 nm. The Fig. 7b showed an increase in the scattering intensity in the first minute of reaction with a maximum of 5400 Kcps. This corresponded to an increase in the structure sizes from 200 nm to 800 nm (Supplementary Information, Fig SI5). The intensity decreased after a few seconds and remained constant for about 5 min. The structural size observed during this time period was about 400 nm (Supplementary Information, Fig SI5). Around 7 min the average light scattering intensity increased reaching a maximum value of about 8000 Kcps, followed by its wane until stabilization at average intensity 300 Kcps. The structure size observed at the maximum scattering intensity was around 1.6 lm. These larger aggregates disappeared after a few seconds and the remaining scattering intensity was related to sparkly diffusive structures without organization. The hydrodynamic diameter variations along the time and the aggregates size increase gave a strong indication of an organization process taking place during the water consumption event. When studying these systems without IL or using C7O3MImBF4 instead of C7O3MImMeS, none of these events were observed. No structural organization at higher levels was observed by intensity or size in function of time. The scattering intensity observed was very low and only very small structures could be observed, probably related to monomers of TEOS. These results demonstrate the important role of the IL’s anion for the observed phenomenon of structural organization. A probable reason for this exclusive effect of MeSO3 based IL is the higher coordination strength of this anion in comparison to the BF4 anion, producing a higher potential to form intermolecular H-bonds [3]. The MeSO3 anion has a more externalized charge stabilized by oxygen atoms, while BF4 anion has it more internalized within the boron atom (Fig. 1). Synergistically with these IL’s capacity to form multiple H-bonds, this allowed the formation of different metastable aggregates that organized the system throughout the whole hydrolysis process, strongly influencing the structure and morphology of the final products. Summarizing all the effects observed, the ether-functionalized IL, especially C3OMImMeS and C7O3MImMeS, seemed to act as dynamic-mobile template-catalysts, able to stabilize metastable micellar structures. Before TEOS addition, randomized interactions

Fig. 8. Schematic representation of the proposed IL-induced sol–gel organization in the initial hydrolysis phase. Interactions among IL, water and co-solvent before TEOS addition (a); TEOS addition and phase-coalescence into a dynamic core–shell structure (b); IL multiple H-bond interaction with silanols formed from TEOS hydrolysis (c).

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among IL, iPrOH and water started competing and causing shifts in the FTIR profiles of the system components (Fig. 8a and Table 1). When TEOS was added, a coalescence stage took place. At this point, the IL’s multiple interacting capacity started acting and a first system structuring could be observed by the DLS, followed by the increase in both water consumption and IL’s imidazolium ring freedom to vibrate (Fig. 7). This could lead to the formation of core–shell structures where the core consisted in non-hydrolyzed TEOS, the shell involved iPrOH and the mobile phase was composed of water and iPrOH. The IL could be acting at the core–shell interphase, both stabilizing its structure and transporting water from the mobile phase to the TEOS core (Fig. 8b). Once the water coordinated to IL forming acidic species, these were able reaching the TEOS phase. Thus, hydrolysis acceleration took place, explaining the similarities of the system profiles with acid catalyzed ones. As the different ether functionalized IL’s cation side chain lengths produce different coordinative (H-bond) capacities, this could justify the difference observed for the coalescence structures formed by either C3OMImMeS or C7O3MImMeS. Similar transport process of acidic species through a hydrophobic environment was already reported using coordinative organometallic compounds [30]. This dynamic templating and multifunctional behavior of the ether-functionalized IL would cause the controlled and ordered hydrolysis of TEOS, forming silanol species at the core–shell interphase, which could be also stabilized by the IL (Fig. 8c). This water transport phenomenon was dependent on the number of etherfunctionalities in the IL’s cation side-chain, what would justify the difference in the systems kinetics using either C7O3MImMeS or C3OMImMeS. It was evident from Fig. 7 that a similar set of processes developed in both systems, but all of them seemed happening about 3 times faster for C7O3MImMeS compared to C3OMImMeS. These dynamic and synergistic systems led to the sol–gel process taking place into a confined space, forming wellorganized silica with a narrow particle size distribution, as reported in our previous works [5,21,22]. 4. Conclusions The use of ether-functionalized IL in the sol–gel process allowed producing a dynamic templating and catalyzing effect, since the initial sol–gel silica formation steps. Particularly, the MeSO3 anion derived ether-functionalized IL caused these effects and iPrOH, as the co-solvent, enhanced this behavior. Under these conditions, multiple H-bonding environments induced the organization of the complex sol–gel system, where the components coalesced into a core–shell structure that isolated the unhydrolysed TEOS into the core. The time-resolved analyses showed important correlations between water consumption, solution structuration and imidazolium ring packing, where the rates of water consumption changed together with the increasing imidazolium ring freedom and solution structuration. This indicated a water transport phenomenon, in which the content of water transported to the core, allowing TEOS to hydrolyze, was proportional to the number of ether segments into the IL cation side chain. Moreover, MeSO3 derived IL in water formed acidic species which were available to promote the hydrolysis of TEOS only when the hydrophilic characters of the mixture reached peculiar values, catalyzing the hydrolysis. During the system’s evolution, the IL turned from ‘‘packed IL’’ (where the H3O+ was not available for hydrolysis) into ‘‘disordered IL’’ (corresponding to the maximum intensity of the imidazolium ring) to successively ‘‘constrained IL’’ (where the formed silanols could constrain the imidazolium ring vibration). These features suggested the ether-functionalized IL as potential multi-task

agents for colloidal systems, presenting dynamic and multifunctional behavior. Acknowledgments The authors are grateful for the financial support from the Brazilian agencies CAPES, CNPq and FAPERGS, as well as for the cooperation project ‘‘New sustainable approaches in the synthesis of epoxy–silica hybrids with tunable properties (2013–2015)’’ by CNR of Italy and AVCR of the Czech Republic. Financial support of the Grant Agency of the Czech Republic (P108/12/1459) and grant LH – KONTAKT II LH14292 from Ministry of Education, Youth and Sports of the Czech Republic are gratefully appreciated by A. Jager and P. Šteˇpánek. R. K. Donato is thankful to FAPERGS–CAPES for the DOCFIX post-doctoral fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.01.079. References [1] L.L. Hench, D.R. Ulrich, Ultrastructure Processing of Ceramics Glasses and Composites, Wiley-Interscience, 1984. [2] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, 1990. [3] J. Dupont, J. Braz. Chem. Soc. 15 (2004) 341. [4] J.P. Hallett, T. Welton, Chem. Rev. 111 (2011) 3508. [5] R.K. Donato, M.V. Migliorini, M.A. Benvegnú, M.P. Stracke, M.A. Gelesky, F.A. Pavan, C.M.L. Schrekker, E.V. Benvenutti, J. Dupont, H.S. Schrekker, J. Sol–Gel Sci. Technol. 49 (2009) 71. [6] S. Tang, G.A. Baker, H. Zhao, Chem. Soc. Rev. 41 (2012) 4030. [7] Z.J. Chen, T. Xue, J.-M. Lee, RSC Adv. 2 (2012) 10564. [8] M.V. Migliorini, R.K. Donato, M.A. Benvegnú, J. Dupont, R.S. Gonçalves, H.S. Schrekker, Cat. Commun. 9 (2008) 971. [9] Z.-B. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J. 11 (2005) 752. [10] H.S. Schrekker, M.P. Stracke, C.M.L. Schrekker, J. Dupont, Ind. Eng. Chem. Res. 46 (2007) 7389. [11] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwen, Thermochim. Acta 357–358 (2000) 97. [12] Z. Duan, Y. Gu, J. Zhang, L. Zhu, Y. Deng, J. Mol. Catal. A: Chem. 250 (2006) 163. [13] G.D. Smith, O. Borodin, L. Li, H. Kim, Q. Liu, J.E. Bara, D.L. Gin, R. Nobel, Phys. Chem. Chem. Phys. 10 (2008) 6301. [14] Z. Fei, W.H. Ang, D. Zhao, R. Scopelliti, E.E. Zvereva, S.A. Katsyuba, P.J. Dyson, J. Phys. Chem. B 111 (2007) 10095. [15] C.J. Adams, A.E. Bradley, K.R. Seddon, Aus. J. Chem. 54 (2001) 679. [16] Y. Zhou, Curr. Nanosci. 1 (2005) 35. [17] Y. Liu, M. Wang, Z. Li, H. Liu, P. He, J. Li, Langmuir 21 (2005) 1618. [18] Y. Zhou, J.H. Schattka, M. Antonietti, Nano Lett. 4 (2004) 477. [19] M.V. Migliorini, R.K. Donato, M.A. Benvegnú, R.S. Gonçalves, H.S. Schrekker, J. Sol–Gel Sci. Technol. 48 (2008) 272. [20] R.K. Donato, M.A. Benvegnú, L.G. Furlan, R.S. Mauler, H.S. Schrekker, J. Appl. Polym. Sci. 116 (2010) 304. [21] R.K. Donato, L. Mateˇjka, H.S. Schrekker, J. Pleštil, A. Jigounov, J. Brus, M. Šlouf, J. Mater. Chem. 21 (2011) 13801. [22] R.K. Donato, K.Z. Donato, H.S. Schrekker, L. Mateˇjka, J. Mater. Chem. 22 (2012) 9939. [23] R.K. Donato, M.V. Migliorini, M.A. Benvegnú, J. Dupont, R.S. Gonçalves, H.S. Schrekker, J. Solid State Electrochem. 11 (2007) 1481. [24] H.S. Schrekker, D.O. Silva, M.A. Gelesky, M.P. Stracke, C.M.L. Schrekker, R.S. Gonçalves, J. Dupont, J. Braz. Chem. Soc. 19 (2008) 426. [25] W. Kujawski, Q.T. Nguyen, J. Neel, J. Appl. Polym. Sci. 44 (1992) 951. [26] M.I. Tejedor-Tejedor, L. Paredes, M.A. Anderson, Chem. Mater. 10 (1998) 3410. [27] R.B. Viana, A.B.F. da Silva, A.S. Pimentel, Adv. Phys. Chem. doi:10.1155/2012/ 903272. [28] C.-H. Ma, T.L. Yu, H.-L. Lin, Y.-T. Huang, Y.-L. Chen, U.-S. Jeng, Y.-H. Lai, Y.-S. Sun, Polymer 50 (2009) 1764. [29] T.N.M. Bernards, M.J. van Bommel, A.H. Boonstra, J. Non-Cryst. Solids 134 (1991) 1. [30] A. Giannetto, S. Lanza, F. Puntoriero, M. Cordaroa, S. Campagna, Chem. Commun. 49 (2013) 7611. [31] P.F. Rossi, G. Busca, V. Lorenzelli, O. Saur, J.C. Lavalley, Langmuir 3 (1987) 52. [32] M.-J. Tang, M.-Q. Li, T. Zhu, Sci. China Chem. 53 (2010) 2657. [33] F. Shi, Y. Deng, Spectrochim. Acta A Mol. Biomol. Spectrosc. 62 (2005) 239. [34] T. Moumene, E.H. Belarbi, B. Haddad, D. Villemin, O. Abbas, B. Khelifa, S. Bresson, J. Mol. Struct. 1065–1066 (2014) 86.