Ureasil-Poly(ethylene oxide) Hybrid Matrix for Selective Adsorption

Mar 13, 2014 - This result suggests that, in defiance of possible differences in the diffusion rates, the kinetics do not control the adsorption pheno...
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Ureasil-Poly(ethylene oxide) Hybrid Matrix for Selective Adsorption and Separation of Dyes from Water Eduardo F. Molina,*,† Renato L. T. Parreira,† Emerson H. De Faria,† Hudson W. Pereira de Carvalho,‡ Giovanni F. Caramori,§ Daniel F. Coimbra,§ Eduardo J. Nassar,† and Katia J. Ciuffi† †

Universidade de Franca, Av. Dr. Armando Salles Oliveira 201, 14404-600 Franca-SP, Brazil Karlsruhe Institute of Technology (KIT), Engesserstraβe 20, 76131 Karlsruhe, Germany § Universidade Federal de Santa Catarina, Departamento de Química, 88040-900 Florianópolis, SC, Brazil ‡

S Supporting Information *

ABSTRACT: Herein, we present a cross-linked ureasil-polyether-siloxane hybrid (labeled PEO500) that can function as a stimuli-sensitive material; it swells or shrinks in response to changes in the environmental conditions and it can also, effectively and selectively, remove dyes from water solution. We also developed a methodology to separate a mixture of cationic and anionic dyes present in water. Addition of PEO500 to an aqueous solution of the anionic orange II (OII) or the ponceau S (PS) dye rendered the solution colorless, but an aqueous solution of cationic methylene blue (MB) remained unchanged after 2 h of contact with the insoluble matrix. In situ small-angle X-ray scattering (SAXS) showed that the distance of siloxane nanodomains are strongly affected by the swelling or shriking. By in situ UV−vis adsorption experiments, we found that the kinetics of OII and PS removal followed a pseudo-first-order rate equation. We accomplished B3LYP calculations, to establish which sites on the matrix interacted with the dyes and to investigate the nature of the matrix-dye chemical bonds. On the basis of the experimental and theoretical investigations, we proposed some mechanisms to explain how PEO500 adsorbs anionic dyes efficiently. This “smart” matrix is potentially applicable as an efficient, fast, selective, and convenient device in water treatment and stimuli-sensitive response materials.

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eletrochromic devices,14 full-color display components,15 ionic conductors,16 and drug delivery systems.17,18 Moreover, the existence of polar chemical groups and the presence of poly(oxyalkylenes) allow ureasil to dissolve organic substances. In the case of the ureasil hybrid, the covalent bonds between the polymer and the siloxane backbone hinder phase separation and preserve the mechanical and thermal properties of the material, making cross-linked ureasil-polyether materials versatile matrixes that are also rubbery, flexible, transparent, and insoluble.19,20 The use of polymer−nanoparticles for cation binding has been studied and is covered appropriately in the literature.21−24 However, the nanoparticles of clays such as the Laponites can flocculate and form gels in water, making the separation of dyebound clay particles from the rest of the sample difficult.25,26 Recently, Jiang and Yin et al.27 investigated how hydrophilic dyes and a series of core cross-linked hybrid nanoparticles based on hyperbranched poly(ether amine) nanoparticles (hPEA NPs) interacted in aqueous solution; these authors demonstrated the multiresponses of the materials to temper-

INTRODUCTION To protect the environment as well as water sources on a global scale, the effective removal of organic contaminants from water is crucial. Indeed, discharging pollutants, such as organics, anions, heavy metal ions, dyes, and microbes, into the aqueous environment dramatically threatens human health.1 The effluents originating from the dyeing, paper and pulp, tannery, and paint industries contain dyes with high chemical oxygen demand;2−4 therefore, treating industrial emissions before releasing them into the environment is essential. The use of highly efficient adsorbents has led to significant progress in the removal of organic solvents, oils, and dyes from water. Such advanced adsorbents should specifically bind to contaminants and offer the possibility of being enriched with the target pollutants; additionally, they should easily separate the contaminants and be inexpensive.5−11 In this context, organic−inorganic hybrid materials based on siloxane− polyether, the so-called “ureasils”, represent an interesting alternative. Organic−inorganic hybrid materials consisting of crosslinked ureasil-poly(ethylene oxide) (PEO) efficiently adsorb dyes and metal ions.12,13 Because they bear different interaction sites, such as silanol, urea, and ether-type oxygen atoms (Scheme 1), these materials also find applications in solid-state © 2014 American Chemical Society

Received: December 17, 2013 Revised: February 25, 2014 Published: March 13, 2014 3857

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Scheme 1. Chemical Structure of the PEO500 Hybrid with the Possible Interaction Sites: Silanol (Green), Urea (Red), and Ether-Type Oxygen Atoms (Blue)

swelling−shrinking behavior of the matrix and in situ UV−vis to monitor the adsorption experiments. To prepare PEO500, we conducted one-pot sol−gel chemistry at room temperature using commercially available poly(ether amines), which furnished a rubbery material with a cylindrical shape (∼20 mm diameter × 3 mm thickness).

ature, pH, and ionic strength. Moreover, they were able to disperse hPEA NPs directly in water, to observe that the nanoparticles self-assembled into micelles comprising a hydrophobic core (PPO and trimethoxysilane groups) and a hydrophilic shell (PEO). These researchers also verified that the host and guest interacted more strongly when hPEA NPs displayed increasingly hydrophobic chains.28 However, hPEA NPs presented poor mechanical stability (fragility). To circumvent this problem, the authors introduced poly(vinyl alcohol) (PVA) into the hPEA NPs, to enhance mechanical strength and achieve reinforced materials that can adsorb dyes selectively.29 Ureasil hybrids represent an efficient and selective strategy to separate dyes. The ureasil xerogel is insoluble in aqueous medium, so pH and ionic strength do not affect the stability and dispersity of these matrixes. When Bekiari and Lianos12 used a ureasil gel to adsorb dyes from aqueous solutions, they found that the main factor determining the dye-adsorption ability of these materials was the hydrophobic/hydrophilic balance. These authors also verified that the poly(oxyalkylene) chains played an important role in dyes adsorption: these longchain components adsorbed hydrophobic dyes more effectively.12 Inspired by the considerations above and by the relevance of host−guest interactions between polymeric nanoparticles and guest molecules, in this paper we used a ureasil gel as a selective and efficient material to bind anions and separate dyes from a mixture containing cationic along with anionic dyes. More specifically, we used ureasil-PEO500, where PEO and 500 stand for ureasil-poly(oxyethylene) and the molecular weight of the polymer (Mw), respectively. We will demonstrate that the xerogel has stimuli-sensitive character: it swells or shrinks in response to changes in the environmental conditions. In contrast to Bekiari and Lianos,11 who showed that ureasil adsorbs several types of dye molecules but questioned the existence of hybrid−dye interactions, in this study we aimed to (i) evaluate how the contact time between PEO500 and the dye affected adsorption until the dye concentration in the surrounding solution no longer changed, (ii) develop a method for the dynamic separation of a mixture of anionic and cationic dyes controlled by the contact time between the hybrid matrix and the dyes solution, (iii) qualitatively investigate the nature of the interactions taking place between the anionic dye and the polymeric fragment by computational calculations, (iv) examine the nanostructure of the hybrids and assess how the swelling− shrinking effect affects this structure before and after their contact with the dyes solution, and (v) evaluate the pseudofirst-order/pseudo-second-order kinetic parameters. We used in situ small-angle X-ray scattering (SAXS) to investigate the

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EXPERIMENTAL SECTION

Materials. O,O′-bis(2-aminopropyl)-poly(ethylene oxide) with a molecular weight of 500 g mol −1 (Jeffamine ED-600), 3isocyanatopropyltriethoxysilane (ICPTES), ethanol (CH3CH2OH), tetrahydrofuran (THF), methylene blue (MB), orange II (OII), and ponseau S (PS) were purchased from Sigma-Aldrich. All these chemicals were used as received. Synthesis of the Hybrid Material. The ureasil cross-link agent was covalently bound to both ends of the macromer polyether18,30 by reacting the terminal aminopropyl groups of the functionalized PEO [O,O′-bis(2-aminopropyl)-poly(ethylene oxide)] with 3-(isocyanatopropyl)-triethoxysilane at a 1:2 molar ratio. These commercially available reagents (Fluka, Aldrich) were stirred together in THF under heating at 60 °C for 15 h. The THF solvent was eliminated by evaporation at 60 °C, to give the hybrid precursor (EtO)3Si(CH2)3NH(OC)NHCHCH3CH2−(polyether)-CH2CH3CHNH(OC)NH(CH2)3Si(OEt)3. This synthesis was adopted for PEO with average molecular weight of 500 g mol−1, labeled PEO500 hybrid hereafter. In a second step, silanol moieties were generated. Condensation reactions followed to afford cross-linking ureasil nodes. Hydrolysis of −(SiOCH2CH3)3 was initiated by adding 3 mL of a water/ethanol mixture (0.03 V/V) containing 110 mg kg−1 HCl catalyst (oral toxicity limit = 900 mg kg−1) to 1.5 g of the precursor. Finally, cylindrical monolithic xerogels of approximately 20 mm diameter and 3 mm height were obtained after drying under vacuum at 70 °C for 24 h. In situ UV−vis Adsorption Experiments. For all the adsorption experiments, the concentration of the dye in the aqueous solution was determined by UV−vis spectroscopy, conducted on an Agilent Technologies Cary 60 dual beam spectrophotometer fitted with a fiber optic coupler equipped with a solarization-resistant immersion probe. This arrangement made the technique sensitive to the dye molecules present in the aqueous solution. Figure SI of the Supporting Information depicts the setup employed for the in situ UV−vis experiments. UV−vis absorption data were recorded within the wavelength range 200−800 nm; the acquisition scan rate was 600 nm min−1, which allowed recording of a full spectrum within 60 s. Standard stock aqueous solutions with different concentrations of the dyes were measured by UV−vis (same conditions as the adsorption experiments) and used to construct the calibration curve employed in the quantitative determination of the dye remaining in the solution after the adsorption assay. For the experiments, approximately 0.5 g of the adsorbent PEO500 was immersed in 30 mL of an aqueous dye solution containing a 50 mg L−1 dye; the system was maintained under controlled stirring. The monolithic xerogel PEO500 was left in contact with the dye solution until equilibrium was reached, as verified by achievement of a constant dye concentration in 3858

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Table 1. Molecular Structures and Abbreviations of the Investigated Dyesa

a

λmax: maximum absorption wavelength. According to the Su-Li EDA approach,31 the interaction energy ΔEHF int can be decomposed into different terms: electrostatic, exchange, repulsion, and polarization (eq 3). The electrostatic, repulsion, and exchange terms are isolated according to the HS method.39 The latter method is based on spin−orbitals, so it can be used to treat both closed and open-shell systems described by RHF, ROHF, or UHF wave functions.

the surrounding solution. Table 1 summarizes the molecular structures of the three model dyes used here. Separation of the Dyes Mixture. The separation experiments were conducted using the following mixtures of cationic and anionic dyes: MB-OII and MB-PS. The PEO500 xerogel mass used in the assays was approximately 0.5 g; the concentration of each dye in the solution was 50 mg L−1. The dye concentration in the solution and the removal of the target dyes were traced and determined by in situ UV− vis spectra as cited above. Computational Details. The geometries of the complexes were optimized without constraints by using Gaussian 0931 and employing the B3LYP/6-31+G(2d,p) level of theory. Wang et al.32 reported that the 6-31+G(2d,p) basis set provides a good accuracy/efficiency relationship for larger water clusters or other complicated systems. The nature of the stationary points was determined by performing the harmonic vibration analyses. The energy decomposition analysis developed by Su and Li,33 the so-called Su-Li EDA approach, was carried out for complexes consisting of hybrid−dyes. The R-B3LYP/6-31+G(2d,p) model was used, and the dye (q = −1) and the polymer (q = 0) were considered as interacting fragments. The Su-Li EDA calculations were conducted as implemented in the quantum chemistry package GAMESS-US.34,35 This approach35 is based on the EDA methods of Kitaura and Morokuma,36,37 Ziegler and Rauk,38 and Hayes and Stone39 and serves to study not only covalent bonds but also internal rotation barriers, interaction energies in molecular clusters, nonbonding interactions, metal−ligand interactions, and ionic bonds. Su-Li EDA focuses on instantaneous interactions between fragments, A, of the molecule, X, to give ΔEHF int , which is the energy difference between the molecule and its fragments in the frozen geometry of the compound. For a set of orthonormal molecular Hartree−Fock (HF) spin orbitals, EHF can be written in terms of orbital energy integrals (see eq 1), in which i and j run over occupied spin orbitals and the one-electron and two-electron Coulomb exchange integrals are given by hI,⟨ii|jj⟩, and ⟨ij|ij⟩, respectively; Enuc represents the nuclear repulsion energy. For a molecule X consisting of A fragments, the total HF interaction energy, ΔEHF int , can be written as eq 2, in which |ΦX⟩ and |ΦA⟩ represent the single-determinant wave functions of the molecule and fragment, respectively. i

EHF =

∑ hi + α ,β

−

1 2

i

1 2

i

j

∑ ∑ ⟨ii|jj⟩ − α ,β α ,β

1 2

i

HF ΔE int = ⟨ΦX |HX |ΦX ⟩ −

j

A

(5)

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α

RESULTS AND DISCUSSION Removal of Dyes from Water. We studied how the PEO500 hybrid matrix behaved with respect to the adsorption of three hydrophilic dyes (see Table 1 for the structures) in water. We conducted the in situ UV−vis equilibrium experiments using the setup depicted in Figure S1 of the

(1)

∑ ⟨ΦA|HA|ΦA⟩

(4)

Small-Angle X-ray Scattering (SAXS) Experiments. The materials nanostructure and the effect of swelling−shrinkage on the structure of the hybrid were monitored by small-angle X-ray scattering (SAXS) experiments, performed at the D01A-SAXS2 beamline of the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil). A vertical position-sensitive X-ray detector and a multichannel analyzer were used to record the SAXS intensity, I(q), as a function of the modulus of the scattering vector q = (4π/λ) sin(θ/2), where θ corresponds to the scattering angle and λ is the wavelength of the Xray (λ = 0.148 nm). The distance between the sample and the detector was 700 mm; 30 s was necessary for each data collection. The beam center was calibrated using silver behenate with the primary reflection peak at 1.076 nm−1. The in situ scattering experiments (under water or dye solution) were accomplished via a controlled-temperature sweep (25 °C) with the same time for each data collection (30 s). The shrinking effect was evaluated after the swelling experiments, when the sample was placed under constant N2 flow.

j β

ΔE disp = ΔE CCSD(T ) − ΔEHF

KS ΔE int = ΔE ele + ΔE ex + ΔErep + ΔEpol + ΔE disp

∑ ∑ ⟨ij|ij⟩ + Enuc β

(3)

The polarization term is defined as the relaxation energy on going from monomers to the supermolecular orbitals. The dispersion energy (eq 4) is derived through correlation methods such as MP2 or CCSD(T). A DFT version of Su-Li EDA was also implemented in GAMESS-US; in this case, the total interaction energy, ΔEKS, is decomposed into electrostatic (ΔEele), exchange (ΔEex), repulsion (ΔErep), polarization (ΔEpol), and dispersion (ΔEdisp) terms (eq 5), which depend on the employed exchange and correlation functionals. Further details can be found in Su-Li’s article.33 The Boys and Bernardi40 counterpoise method was also implemented in the Su-Li EDA approach, to correct the basis set superposition error (BSSE).

∑ ∑ ⟨ij|ij⟩ α

HF ΔE int = ΔE ele + ΔE ex + ΔErep + ΔEpol

(2) 3859

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immersion into the OII dye solution, this material acquired a deep orange color. In addition, the diameter of the xerogel (right) augmented, because the free volume increased as a result of the swollen network.41 We plotted the concentration of OII in solution as a function of the contact time with PEO500 (Figure 1b). The equilibrium assays revealed that the xerogel removed 50% of the OII dye in the solution after 20 min of immersion; the final amount of OII in the supernatant was 0.7 mg L−1 at the end of the experiments (120 min). Whereas the initial OII solution had a deep orange color, it became clear after 120 min in contact with PEO500 (inset of Figure 1b). Therefore, the PEO500 hybrid efficiently removed the anionic dye from water, confirming the strong affinity of this xerogel for OII anions. Next, we immersed the PEO500 xerogel in a solution containing the cationic dye methylene blue (MB). Figure 2a contains the UV−vis data of the MB dye solution collected during the adsorption experiments as a function of the contact time with PEO500. The inset of Figure 2a illustrates photographs of the xerogel: the volume of the matrix increased because the network swells. However, PEO500 remained

Supporting Information. The initial concentration of the dyes was fixed at 50 mg L−1; the mass of the PEO500 xerogel immersed in the dye solution was approximately 0.5 g in all experiments. Because the PEO500 hybrid matrix is insoluble in water and presents swelling properties, water and small solute molecules can freely diffuse into the xerogel. We initiated our studies using a solution of the anionic dye orange II (OII). The UV− vis setup allowed recording of a full spectrum within 60 s, and the number of spectra as a function of contact time was higher. Figure 1a presents the UV−vis data of the OII dye solution

Figure 1. (a) Time evolution of the UV−vis spectra collected during adsorption experiments using a solution of the orange II dye at 50 mg L−1 and the PEO500 xerogel. Inset: photographs of the PEO xerogel before and after 120 min of contact time with the OII dye solution; (b) plot of the dye concentration vs time. Inset: photographs of the orange II solution in water before and after contact with the PEO500 xerogel.

collected during the adsorption experiments as a function of the contact time with the PEO500 xerogel. Immersion of the xerogel into the OII solution diminished the bands at 485, 410, 308, 260, and 230 nm, typical of the OII dye molecules, indicating that the dye concentration in the supernatant significantly decreased after 100 min (Figure S2 of the Supporting Information display absorbance at λmax = 485 nm vs time). In accordance with the inset in Figure 1a, the PEO500 xerogel was initially transparent (left); after 120 min of

Figure 2. (a) Time evolution of the UV−vis spectra collected during adsorption experiments using a solution of the methylene blue dye at 50 mg L−1 and the PEO500 xerogel. Inset: photographs of the PEO xerogel before and after 120 min of contact time with the dye solution. (b) Plot of the dye concentration versus time. Inset: photographs of the methylene blue solution in water before and after contact with the PEO500 xerogel. 3860

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Figure 3. (a) One-pot separation of a mixture of the MB and OII dyes: plot of the dyes concentration versus time. (b) Photograph of the solution containing a mixture of the MB and OII dyes. (c) PEO500 xerogel before and after adsorption experiments.

transparent after contact with the MB solution, showing that the xerogel did not adsorb MB. Indeed, the intensity of the bands typical of MB at 665 and 610 nm remained unaltered during contact with PEO500 (Figure S3 of the Supporting Information). Moreover, the photographs of the MB solution in Figure 2b did not reveal any differences in the color of the solution before and after the adsorption experiments. Separation of Mixed Dyes. The fact that PEO500 selectively adsorbed OII motivated us to use this xerogel to separate the anionic OII dye from a solution containing OII along with the cationic dye MB. During the experiment, we employed a solution containing 50 mg L−1 of each dye in deionized water; again, we used approximately 0.5 g of the PEO500 xerogel throughout the assay. Figure 3a shows the OII and MB concentrations in solution as a function of the contact time with PEO500. The xerogel efficiently removed the anionic OII dye from the starting solution within a contact time of 120 min; in contrast, the MB concentration in this same solution remained constant throughout this period. Figure 3b corresponds to the photographs of the solution containing both OII and MB before and after the adsorption experiment. At the start of the assay, the solution had a dark-brown color, reflecting the simultaneous presence of MB and OII. After contact with PEO500, this solution became blue. Figure 3c contains pictures of the xerogel recorded before and after the adsorption experiments. PEO500 was initially transparent. Along the experiment, it acquired a deep orange color, because OII dye molecules adsorbed into the matrix (Figure 3c). To confirm that PEO500 has strong affinity for anionic dyes, we assayed another mixture containing the anionic Ponceau S (PS) along with the cationic dye MB (see structures in Table 1 and the UV−vis data collected during the adsorption

experiments in Figure 4a). The intensity of the characteristic band of the PS dye molecules at 535 nm decreased; the bands relative to the MB dye at 665 and 610 nm did not vary. Figure 4b displays the plot of the dye concentration in the solution containing a mixture of PS and MB as a function of the contact time with PEO500. The xerogel exhibited strong affinity for the anionic PS molecules: the PS concentration decreased from 50 to 16 mg L−1 after 120 min, which corresponded to 68% removal efficiency. The MB concentration became only slightly lower during this same time period. Figure 4c displays photographs of the solution containing a mixture of MB and PS before and after the adsorption experiment. The initially deep-purple solution became light blue as a function of the contact time with the PEO500 matrix. Figure 4d depicts the PEO500 xerogel before and after the adsorption assays. The transparent xerogel acquired a red color along the assay, attesting to the presence of PS molecules inside the hybrid network. The molecular structures of the PEO500 xerogel and of the dyes probably accounted for the selective adsorption of anionic dyes by this matrix. PEO500 possibly displayed charge-based specificity for negatively charged molecules.27,42,43 Upon immersion in water, the amine groups on PEO500 must have acquired a positive charge, which promoted their interaction with the negatively charged OII and PS molecules. Moreover, the sulfonate groups (SO3−) present in OII and PS may have interacted with the urea groups (−NH−CO−NH−) in PEO500, to establish strong hydrogen bonds via the sulfonate oxygens of the dyes.44,45 Such attraction between the sulfonate anion and the urea group agrees with the theoretical investigation by Saito et al.:46 these authors correlated the remarkable effect of the LiPF6 salt on the hydrogen-bonding 3861

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Figure 4. (a) Time evolution of the UV−vis spectra collected during the adsorption experiments using a mixture of the MB and PS dyes (50 mg L−1) and PEO500. (b) One-pot separation of the mixed MB and PS dyes: plot of the dye concentration versus time. (c) Photograph of the solution containing a mixture of the MB and PS dyes. (d) PEO500 xerogel before and after the adsorption experiments.

interactions involving the PF6− anions and the hydrogen atoms of the urea −NH group. To confirm this type of interaction, we conducted computational calculations using the dye molecule and a polymeric fragment. We aimed to qualitatively demonstrate the nature of the interactions governing the adsorption of anionic dyes by the polymeric PEO500 matrix. We tested various forms of interaction and selected four complexes that interacted via hydrogen bonds (Figure 5, panels a−d). Su-Li EDA results (Table 2) clearly showed that all the complexes (Figure 5, panels a−d) exhibited stabilizing interaction energies because all the ΔEKS int values were negative. EDA revealed that the dye−polymer interactions were slightly more electrostatic than covalent. Indeed, the electrostatic component (ΔEele) contributed the most, from a 50.07 to

57.69% contribution versus a 42.31 to 49.13% contribution from the covalent component (ΔEorb) (Table 2). The polarization term values (ΔEpol) which ranged from −5.16 to −16.66 kcal mol−1, modulated the considerable contribution from the covalent character. The dispersion component values (ΔEdisp) lay between −5.23 and −10.39 kcal mol−1. The values achieved for the positive repulsion component (ΔErep) corroborated such situation. EDA also evidenced that the most stabilizing dye−polymer configuration was C: it presented the largest electrostatic and orbital contributions as a straightforward consequence of the number of stabilizing hydrogen bonds it contained. On the other hand, configuration B was the least stabilizing because it exhibited the smallest electrostatic and covalent contributions. EDA revealed that SO3− participated as a hydrogen-bond acceptor in cases C and 3862

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Figure 5. Equilibrium geometries for the anionic dye−polymeric fragment complexes [B3LYP/6-31+G(2d,p)]. Distances are given in angstroms.

Table 2. Su-Li EDA Results (kcal mol−1) for the Different Conformations of the Supramolecular Complex (Dye− Polymeric Fragment) at the R-B3LYP/6-31+G(2d,p) Level of Theorya Su-Li EDA ΔE

ele

ΔEex ΔErep ΔEpol ΔEdisp ΔEorb b ΔEKS int

A

B

C

D

−18.76 (50.87%)c −9.17 33.75 −11.83 −6.29 −18.12 (49.13%)c −12.29

−10.42 (50.07%)c −2.49 15.03 −5.16 −5.23 −10.39 (49.93%)c −8.26

−34.24 (55.93%)c −12.87 49.07 −16.66 −10.32 −26.98 (44.07%)c −25.02

−26.17 (57.69%)c −9.87 33.35 −13.69 −5.50 −19.19 (42.31%)c −21.88

and anionic dyes due to greater removal of the anionic molecule by the PEO500 hybrid. Nanostructural Behavior of the Ureasil−PEO Hybrid. We analyzed the nanostructural homogeneity of PEO500 by SAXS and in situ SAXS to assess how swelling−shrinkage affected these materials. We conducted experiments in aqueous dye solution at 25 ± 0.5 °C before and after adsorption of the dye molecules (OII or PS) into the matrix; we also accomplished control assays in pure water. The SAXS curves recorded for PEO500 displayed a single broad peak with maximum located at qmax values of ∼0.22 Å−1 both before and after adsorption of the dye (OII or PS, Figure 6). This peak evidenced strong spatial correlation between the regularly spaced cross-linking ureasil nodes bound at the extremities of

The dye (q = −1) and polymer (q = 0) were considered fragments. ΔEorb = ΔEpol + ΔEdisp cValues in parentheses give the percentage of attractive interactions ΔEorb + ΔEele. a b

D, so the interactions were more stabilizing. On the other hand, no significant hydrogen bonds existed in configuration B, especially those involving the SO3− group, so the interaction energy was very small. Additionally, the distance of this interaction in B (2.508 Å) was the largest among the four conformations. As for configuration A, we verified an intermediate situation: it bore just one significant OH···O C hydrogen bond, which provided a total interaction energy of −12.29 kcal mol−1. Because the PEO500 hybrid was stable in water, it retained its cylindrical shape during the whole separation experiment. As a result of the hydrophilic character of PEO, the xerogel free volume increased after contact with the dye aqueous solution. The network swelled and was able to accommodate the anionic dye. Therefore, it was possible to smartly separate the cationic

Figure 6. SAXS patterns of the dry PEO500 matrixes after the adsorption of the OII or PS dye. 3863

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Figure 7. Time dependence of in situ SAXS curves obtained for the PEO500 hybrid immersed in (a) water and (b) OII dye solution at 25 °C and (c) nanoscopic expansion-shrinking ratio (Δξ/ξd) obtained for the PEO500 hybrid in water and in OII dye solution at 25 °C.

we replaced the aqueous medium with a constant N2 flow and evaluated the shrinking behavior of the matrix. In this case, the average correlation distance between the cross-link nodes decreased, displacing the peak position to higher q. In other words, the structure of the PEO hybrid returned to the initial nanoscopic state. The SAXS curves collected during the swelling−shrinking experiments carried out in OII dye solution at 25 °C resembled the SAXS curves obtained for the experiments accomplished in water (Figure 7b). The elongation-shrinking ratio (Δξ/ξd) is an important structural parameter to assess cross-linked network hydration. Considering the SAXS results, it was possible to calculate the relative elongation-shrinking ratio, (ξs − ξd)/ξd, from the average distance between the nodes measured in the dry state (ξd) and after different swelling periods (ξs). Figure 7c represents the Δξ/ξd evolution as a function of the time that the hydrophilic PEO500 matrix remained immersed in water or in the OII dye solution at room temperature. In both cases, the xerogel reached an expansion factor of ∼0.2 and returned to the initial state after contact with N2 flow. For a contact time shorter than 10 min, the expansion behavior was very close in

the PEO chain.30 The presence of OII or PS dye molecules in the xerogel (dry samples) did not alter the average correlation distance between two adjacent nodes (ξd = 28 Å, as determined by the equation ξd = 2π/qmax). The less intense peak [I(q)] in the SAXS curves of PEO500 containing PS or OII revealed a smaller electronic density contrast between the siloxane nodes and the polymeric phase. This phenomenon resulted from solvation of the dye molecules occupying the void space between the PEO500 polymeric chains. A previous study reported that platinum species solvated by ether-type oxygen atoms in a ureasil matrix behaved in the same way: the typical peak of diureasil PPO400 decrease upon platinum incorporation into the matrix network.47 Figure 7 illustrates the time evolution of selected SAXS curves of the sample PEO500 recorded during the swelling− shrinking experiments conducted in the water and dye solution at 25 °C. The hydrophilic nature of the PEO chains promoted hydration of the hybrid matrix during the swelling experiment in water (first 60 min), shifting the peak position to low q values, while the distance between the siloxane nodes (ξd) increased from 28 to 34 Å (Figure 7a). After 60 min of swelling, 3864

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both media, indicating similar hydration of the PEO matrix regardless of the presence of the dye. Nevertheless, when the xerogel adsorbed the OII dye (after the first 10 min of the experiment), the diffusion of water molecules inside the matrix became slightly hindered. Indeed, the system involving OII took longer to reach the equilibrium (expansion): 53 min versus 40 min in the case of water. In the same way, OII also affected the PEO500 shrinking behavior: in water and in OII dye solution, the PEO500 hybrid took 20 and 30 min to return to its initial nanoscopic structure under contact with N2 flow, respectively (Figure 7c). This suggests that the dye molecules also hindered the flow of water molecules out of the xerogel during the shrinking experiment. These results clearly attested that the PEO500 hybrid matrix functioned as a stimuli-sensitive material that swells or shrinks in response to changes in the environmental conditions. Most interestingly, the process was reversible, being potentially applicable in the controlled delivery of active species like drugs and in the regulation of soil humidity. Effects of Contact Time between PEO500 and the OII or PS Dye. We studied the kinetics of OII and PS dye adsorption onto PEO500. Figure 8 compares the quantity of

OII and PS dyes adsorbed by the matrix, Qe, as a function of time. To calculate Qe at different times (1 min intervals), we used eq 6: (C0 − Ce)V m

(7)

t 1 t = + 2 Qt Qe k 2Q e

(8)

where Qt and Qe are the adsorption capacity at time t and at equilibrium, respectively (mg g−1); k1 is the rate constant of the pseudo-first-order adsorption (min−1); and k2 is the rate constant of the pseudo-second-order adsorption (g mg−1 min−1). We obtained the values of Qe, k1, and the correlation coefficient r2 from the linear plot of ln(Qe − Qt) versus t (Figure 9, panels a−b) for the pseudo-first-order model. In the same way, we determined Qe, k2, and r2 from the slope and intercept of the plot of t/Qt versus t (Figure 9, panels c−d) for the pseudo-second-order model. Table 3 compares the pseudo-first- and pseudo-second-order adsorption constants. Both the pseudo-first-order and the pseudo-second-order kinetic model fitted the whole range of contact time satisfactorily (Figure 9); the correlation coefficient (R2) values were higher than 0.98 in both situations. However, Qe,calc, obtained using the pseudo-first-order and pseudosecond-order equations were slightly different: the pseudofirst-order equation furnished values much closer to the experimental Qe,exp, as compared with the pseudo-secondorder equation. Therefore, we concluded that the adsorption of PS and OII into the PEO500 matrix followed the pseudo-firstorder equation. From the pseudo-second order adsorption constants provided in Table 3 and the kinetic model curve shown in Figure 9, we can observe that the adsorption rates are virtually the same for OII and PS dye. Nevertheless, the structure of the hydration shell around PS is expected to be larger than OII (since the PS present charge is −4, whereas OII present charge is −1). This result suggests that, in defiance of possible differences in the diffusion rates, the kinetics do not control the adsorption phenomena presented in this work. The influence of dye concentration on the adsorption capacity and the adsorption isotherms of OII and PS dye by PEO500 hybrid matrix was also verified (Figure S4−S5 and Table S1 of the Supporting Information). The adsorption constants have shown that interaction between OII with the PEO500 hybrid is favored, which the adsorption capacity for OII is higher than for PS. These findings agree with the theoretical calculations. In accordance with the Figure 5c and Table 2, the most stable adsorption conformation takes place by the sulfonic and hydroxyl sites, which are only found in the OII dye molecule. The pH may play an important role on the adsorption, since both dye molecules and the siloxane-polyether hybrid might be ionized or neutral according to the pH. In the present study, the pH was adjusted to 7, in which the siloxane−polyether hybrid matrix and MB dye is expected to be positively charged since the urea and amine groups will be protonated. On the other hand, the sulfonic groups of the azo dyes are dissociated, and the overall charge presented by these molecules is negative. These statements are corroborated by the high uptake of anionic dye (OII and PS), whereas only small adsorption was verified for MB. Further studies have been conducted to investigate in deep the role of both pH and temperature on the adsorption process of these molecules on the siloxane− polyether hybrids. These encouraging results confirmed the advantages of removing anionic dyes from water using the ureasil-PEO500

Figure 8. Effect of the contact time on the adsorption of OII and PS onto the PEO500 hybrid.

Qe =

k 1 1 = l + Qt Q et Qe

(6)

where Qe is the amount of dye adsorbed onto the adsorbent at equilibrium (mg g−1), C0 is the initial concentration (mg L−1) of the dye in solution, Ce is the concentration (mg L−1) of the dye in solution at equilibrium, m is the mass (g) of adsorbent used in the assay, and V is the volume (L) of the dye solution. The adsorption capacity of the PEO500 xerogel augmented as the contact time with the anionic dye increased. The final Qe values were high: 2.65 and 2.58 for OII and PS, respectively. In both cases, the system reached the adsorption equilibrium after 115 min. To better investigate the adsorption mechanism, we employed pseudo-first-order and pseudo-second-order models to assess the adsorption kinetics of the dyes onto the PEO500 hybrid matrix. Equations 7 and 8 give the linear forms of the pseudo-first-order and pseudo-second-order models, respectively:48−50 3865

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Figure 9. Plot of the pseudo-first-order kinetic model for the (a) OII and (b) PS and pseudo-second-order kinetic model for the (c) OII and (db) PS dyes using the PEO500 xerogel as adsorbent.

Table 3. Comparison of the Pseudo-First and Pseudo-Second Order Adsorption Constants at Initial Dye Concentration of 50 mg L−1 pseudo first order −1

−1

pseudo second order −1

2

dye

Qe,exp (mg g )

k1 (min )

Qe,calc (mg g )

R

OII PS

2.65 2.58

25.13 42.15

3.00 3.50

0.9985 0.9969

−1

k2 (g mg

−1

min )

0.0092 0.0076

Qe,calc (mg g−1)

R2

3.45 3.53

0.9983 0.9946

efficiency of 10% and 12%, respectively. By successively replacing the alkaline solution of NaOH (pH = 12) by a fresh one (20 mL × 3 changes × 60 min each immersion), around 50% of the OII dye desorption was achieved. Therefore, this result points out the possibility of reuse of the PEO500 matrix. We anticipate that PEO systems containing adsorbed dyes can help monitor gaseous carbon dioxide and oxygen. For example, exposure of the xerogel containing dye molecules to a gas (e.g., oxygen) shifts the UV−vis band characteristic of the dye molecule present in the hybrid matrix. We shall focus future research on the development of a new approach to optically monitor gas using ureasil−polyether materials containing dye molecules.

hybrid matrix. We will subject these materials to more intense studies, to elucidate the role played by the molecular weight and the nature of the polyether chains (hydrophobic or hydrophilic) in the interactions between the dyes and the matrix. Above all, this xerogel is extremely interesting because it offers easy and fast separation of anionic dyes from a mixture containing anionic along with cationic dyes. Finally, preliminary desorption studies of dyes were also carried out. For this purpose, we collected the PEO500 with adsorbed dye and immersed it in deionized water at 25 °C. After 24 h of immersion in water, the xerogel still retained the color of the adsorbed dye (OII or PS). Thus we repeated the aforementioned procedure using acidic solution of HCl(aq) (pH = 2.0), alkaline solution of NaOH(aq) (pH = 10), and methanol to find the ideal eluent for desorption. The desorption of OII dye by acidic solution was low (nearly 5%). The alkaline solution (pH = 10) and methanol presented desorption

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CONCLUSION We were able to easily and rapidly separate anionic dyes from a mixture containing anionic along with cationic dyes using a 3866

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(6) Walcarius, A.; Mercier, L. Mesoporous organosilica adsorbents: Nanoengineered materials for removal of organic and inorganic pollutants. J. Mater. Chem. 2010, 20, 4478−4511. (7) Yu, L.; Zou, R.; Zhang, Z.; Song, G.; Chen, Z.; Yang, J.; Hu, J. A Zn2GeO4−ethylenediamine hybrid nanoribbon membrane as a recyclable adsorbent for the highly efficient removal of heavy metals from contaminated water. Chem. Commun. 2011, 47, 10719−10721. (8) Klein, T. Y.; Wehling, J.; Treccani, L.; Rezwan, K. Design options for a more sustainable urban water environment. Environ. Sci. Technol. 2013, 47, 1065−1072. (9) Lei, W.; Portehault, D.; Liu, D.; Qin, S.; Chen, Y. Porous boron nitride nanosheets for effective water cleaning. Nat. Commun. 2013, 4, 1−4. (10) Him, J. H.; Fang, B.; Song, M. Y.; Yu, J. Topological transformation of thioether-bridged organosilicas into nanostructured functional materials. Chem. Mater. 2012, 24, 2256−2264. (11) Yuan, W.; Lu, Z.; Li, C. M. Self-assembling microsized materials to fabricate multifunctional hierarchical nanostructures on macroscale substrates. J. Mater. Chem. A 2013, 1, 6416−6424. (12) Bekiari, V.; Lianos, P. Ureasil gels as a highly efficient adsorbent for water purification. Chem. Mater. 2006, 18, 4142−4146. (13) Bekiari, V.; Lianos, P. Use of Ureasil gels to extract ions from aqueous solutions. J. Hazard. Mater. 2007, 147, 184−187. (14) Barbosa, P. C.; Silva, M. M.; Smith, M. J.; Gonçalves, A.; Fortunato, E.; Nunes, S. C.; Bermudez, V. Z. Di-ureasil xerogels containing lithium bis(trifluoromethanesulfonyl)imide for application in solid-state electrochromic devices. Electrochim. Acta 2009, 54, 1002−1009. (15) Sun, Z.; Bai, F.; Wu, H.; Boye, D. M.; Fan, H. Monodisperse fluorescent organic/inorganic composite nanoparticles: Tuning full color spectrum. Chem. Mater. 2012, 24, 3415−3419. (16) Chaker, J. A.; Santilli, C. V.; Pulcinelli, S. H.; Dahmouche, K.; Briois, V.; Judeinstein, P. Multi-scale structural description of siloxane−PPO hybrid ionic conductors doped by sodium salts. J. Mater. Chem. 2007, 17, 744−757. (17) Molina, E. F.; Marçal, L.; Carvalho, H. W. P.; Nassar, E. J.; Ciuffi, K. J. Tri-ureasil gel as a multifunctional organic−inorganic hybrid matrix. Polym. Chem. 2013, 4, 1575−1582. (18) Lopes, L.; Molina, E. F.; Chiavacci, L. A.; Santilli, C. V.; Briois, V.; Pulcinelli, S. H. Drug−matrix interaction of sodium diclofenac incorporated into ureasil−poly(ethylene oxide) hybrid materials. RSC Adv. 2012, 2, 5629−5636. (19) Molina, E. F.; Pulcinelli, S. H.; Santilli, C. V.; Blanchandin, S.; Briois, V. Controlled Cisplatin Delivery from Ureasil-PEO1900 Hybrid Matrix. J. Phys. Chem. B 2010, 114, 3461−3466. (20) Molina, E. F.; Santilli, C. V.; Pulcinelli, S. H.; Blanchandin, S.; Baudelet, F.; Briois, V. Multi-spectroscopic monitoring of cisplatinderived species delivery from ureasil polyether hybrid matrix. Phase Transitions 2008, 84, 687−699. (21) Wheeler, P. A.; Wang, J.; Mathias, L. J. Poly(methyl methacrylate)/Laponite Nanocomposites: Exploring Covalent and Ionic Clay Modifications. Chem. Mater. 2006, 18, 3937−3945. (22) Appel, E. A.; Barrio, J.; Loh, X. J.; Scherman, O. A. Supramolecular polymeric hydrogels. Chem. Soc. Rev. 2012, 41, 6195−6214. (23) Ruiz-Hitzky, E.; Aranda, P.; Dardera, M.; Rytwo, G. J. Hybrid materials based on clays for environmental and biomedical applications. Mater. Chem. 2010, 20, 9306−9321. (24) Yuan, W.; Dong, H.; Li, C. H.; Cui, X.; Yu, L.; Lu, Z.; Zhou, Q. pH-Controlled Construction of Chitosan/Alginate Multilayer Film: Characterization and Application for Antibody Immobilization. Langmuir 2007, 23, 13046−13052. (25) Thomas, P. C.; Cipriano, B. H.; Raghavan, S. R. Nanoparticlecrosslinked hydrogels as a class of efficient materials for separation and ion exchange. Soft Matter 2011, 7, 8192−8197. (26) Murchid, A.; Delville, A.; Lambard, J.; lecolier, E.; Levitz, P. Phase diagram of colloidal dispersions of anisotropic charged particles: equilibrium properties, structure, and rheology of laponite suspensions. Langmuir 1995, 11, 1942−1950.

stimuli-sensitive ureasil-PEO hybrid matrix. The active site of the OII dye and the urea groups present in the PEO hybrid matrix played an important role in the capacity of the xerogel to adsorb anionic species. By means of theoretical investigations, we obtained evidence that the dye and the matrix established hydrogen bonds. Small-angle X-ray scattering studies revealed that the nanostructure of the PEO material remained unchanged after adsorption of the OII or PS dye. Moreover, we verified that the hybrid matrix functioned as a stimuli-sensitive material that swells or shrinks in response to alterations in the environmental conditions. This process was reversible and is potentially applicable to control the delivery of active species like drugs and to regulate soil humidity. The adsorption kinetics results showed that the system fitted a pseudo-first-order model. Moreover, this hybrid matrix is an ideal candidate for environmental remediation. In conclusion, the ureasil−PEO matrix is a great promise for the smart separation of dyes in aqueous medium with a view to water treatment.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental setup of the in situ UV−vis measurements, additional spectra, and adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +55 16 37116989. Fax: +55 16 37116989. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support received from the Brazilian agencies CAPES, CNPq, and FAPESP (Project 2012/ 05514-0). The authors want to thank Brazilian Synchrotron Light Laboratory (LNLS) for providing beamtime at the D01ASAXS2 beamline and for assistance with the X-ray scattering experiments. We would also like to thank Celso V. Santilli and Sandra H. Pulcinelli for allowing us to use their laboratory premises (UNESP/Araraquara) during the UV−vis adsorption experiments.

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