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Supercritical CO2–ionic liquid mixtures for modification of organoclays
Journal of Colloid and Interface Science 353 (2011) 225–230
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Supercritical CO2–ionic liquid mixtures for modification of organoclays Sébastien Livi, Jannick Duchet-Rumeau ⇑, Jean-François Gérard Université de Lyon, F-69003 Lyon, France INSA Lyon, F-69621 Villeurbanne, France CNRS, UMR 5223, Ingénierie des Matériaux Polymères, France
a r t i c l e
i n f o
Article history: Received 13 July 2010 Accepted 17 September 2010
Keywords: Ionic liquid Supercritical CO2 Organoclay Cationic exchange
a b s t r a c t The use of supercritical CO2 as solvent in the modification of montmorillonite by imidazolium and phosphonium ionic liquids bearing long alkyl chains (C18) known for their excellent thermal stability is described. The objective is to combine the environmentally friendly character of ionic liquids and supercritical carbon dioxide for the organophilic treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites, cationically exchanged under supercritical carbon dioxide with or without co-solvent, have been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and compared to montmorillonites treated by conventional cationic exchange. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since the 1980s, the industrial and academic research have a growing interest in the design of polymer/clay nanocomposites. According to the literature, the insertion of lamellar silicates in a polymer matrix can have beneficial effects on the final properties of the nanocomposite. The most often reported improvements are for thermo-mechanical behavior [1] and gas barrier properties [2,3] due to nanometer-range dimensions and high aspect ratio of layered silicates leading to a large amount of interfacial zones. Natural clays, such as montmorillonite (MMT) are frequently used for that purpose [4]. However, the dispersion of non-modified clay due to the poor interfacial interactions is very limited. Indeed, it is necessary to modify the surface of the pristine MMT, mostly by cationic exchange using ammonium salts [5,6] to improve the compatibility between the polymer and lamellar fillers during the processing step of nanocomposites, i.e. to improve the final dispersion state and to get a full/large development of the interfaces. Ionic liquids are organic salts with melting points below 100 °C, which are considered as green solvents. The most commonly used are imidazolium [7], pyridinium [8] and phosphonium [9] salts. They are used in several applications: chemical reaction medium, i.e. as solvents, electrolytes in batteries, lubricants, plasticizers, and catalysts. Recently, ionic liquids were used as surfactants to replace the conventional alkyl ammonium salts that show a lower
⇑ Corresponding author. Fax: +33 4 72438527. E-mail address: [email protected] (J. Duchet-Rumeau). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.09.049
thermal stability [10–12]. Despite many benefits, very few studies using such cations for layered silicates intercalation have been reported in the literature may be due of the actual higher cost of these surfactants compared to ammonium salts [13,14,7]. Moreover, thermally stable and commercially available ionic liquids which could be considered as surfactants only incorporate short aliphatic chains (up to C14). In this work, ionic liquids bearing long alkyl chains based on imidazolium (two chains in C18) and on phosphonium salts with three benzyl groups and only one long aliphatic chain (one chain in C18) were synthesized. As these ionic liquids are used as clay intercalants, the long alkyl chains and the aromatic groups could cause expansion of the distance between the layers and could contribute to a better intercalation of the clay platelets. To perform an environmentally friendly MMT surface treatment, the properties of ionic liquids and supercritical CO2 were combined. The use of CO2 in the supercritical state should make useless the addition of any solvent for that the cationic exchange succeeds. The main interest of supercritical CO2 is to design a clean surface treatment compared to the ones using conventional solvents. In fact, carbon dioxide has a high diffusivity like a gas, low surface tension (close to zero), viscosity and density like a liquid which gives it a high solvency power tuneable by adjusting pressure [15]. In this work, the combination of supercritical CO2 and ionic liquids should lead to an environmentally benign cationic exchange process to modify the MMT with imidazolium and phosphonium salts with long alkyl chains. The objectives of this work is to design a process without use of solvent and to identify the supercritical CO2 exposure effects on the physico-chemical properties resulting of modified clays.
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2. Experimental 2.1. Materials A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with intercalated sodium ions was chosen as pristine clay. This one was provided by Süd Chemie (Germany). The Nanofil 757 has a cationic exchange capacity of 95 meq/100 g and is described by the following formula Na0,65[Al, Fe]4Si8O20 (OH)4. All chemicals necessary to the synthesis of ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl (95%), and all the solvents (toluene, sodium methanoate, pentane, and acetonitrile) were supplied from Aldrich and used as received. 2.2. Synthesis of phosphonium and imidazolium salts 2.2.1. Synthesis of octadecyltriphenylphosphonium salt In a 100 mL flask was placed under a positive nitrogen pressure, 1 equiv. of triphenylphosphine (5 g) and 1 equiv. octadecyl iodide (7.3 g). The stirred suspensions were allowed to react for 24 h at 120 °C in toluene (20 mL), a yellow precipitate was formed. The reaction mixture was then filtered and washed repeatedly with pentane. Most of the solvent was removed under vacuum. The chemical structure of the salts was confirmed by 13C NMR spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer [21]. The assignment of 13C NMR resonance peaks is reported below. 13 C NMR (CDCl3): d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37– 29.66; 30.24; 31.85 (P-CH2); 118.45; 130.43; 133.70; 135.15 (P-C). 2.2.2. Synthesis of N-octadecyl-N0 -octadecylimidazolium salt A solution of sodium methoxide was prepared from 1 equiv. of sodium (0.465 g) in dry freshly distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottomed, three-necked flask equipped with a condenser, under nitrogen atmosphere, and magnetic stirring. Imidazole (1.37 g) diluted in acetonitrile (10 mL) was then added into the stirred mixture of sodium methoxide previously cooled at room temperature. After 15 min, a white precipitate was formed. The suspension was then concentrated under reduced pressure for 1 h. The dried white powder was dissolved in acetonitrile and a solution of octadecyl iodide (7.70 g) diluted in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen at room temperature. The mixture was stirred for 1 h, then heated under reflux at 85 °C for about 24 h. A solution of octadecyl iodide (7.70 g) diluted in acetonitrile (10 mL) was added to the mixture at room temperature. The stirred suspension was heated under reflux at 100 °C for about 24 h leaving brownish viscous oil in each case. After cooling to room temperature, the solvent was removed by evaporation under vacuum, the beige solid
was filtered, washed repeatedly with pentane and dried. Purification of the resulting imidazolium salts was accomplished by crystallization from ethyl acetate/acetonitrile: 75/25 mixture. The assignment of 13C NMR resonance peaks evidences the success of the ionic liquid synthesis. 13 C NMR (CDCl3): d 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35–29.69; 30.24; 31.91 (CH2); 50.10; (CH2N@); 50.32 (CH2NA); 121.69; 122.48 (@CN); 136.88 (NAC@N). 2.3. Organic modification of montmorillonite The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionised water. The amount of surfactant added was about 2 CEC, based on the cation exchanged capacity (CEC = 95 meq/100 g) of the MMT used [9]. This dispersion was mixed and stirred vigorously at 80 °C for 6 h, followed by filtration and continuous washing at 80 °C with deionised water until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The solvent was removed by evaporation under vacuum. The modified montmorillonite was then dried for 12 h, at a suitable temperature (not greater than 80 °C). The imidazolium, phosphonium and the quaternary ammonium ions used for the exchange reactions are presented in Table 1. The following abbreviations were used to design the different montmorillonites: MMT-Na+ means the pristine montmorillonite. A phosphonium-montmorillonite denoted MMT-P was obtained when octadecyltriphenylphosphonium iodide was used as surfactant. An imidazolium-montmorillonite, denoted MMT-I, was obtained when the N-octadecyl-N0 -octadecylimidazolium iodide was used like intercalation agent. As supercritical carbon dioxide was used as solvent instead of water, the procedure was the following one: 2 g of untreated MMT and an excess of surfactant (2 CEC: imidazolium and phosphonium ionic liquid) were placed into a 300 mL high pressure reactor. Then, an initial loading of the autoclave at a pressure of 50 bars and at a temperature of 20 °C was made. The temperature used was chosen based on the melting temperatures of the imidazolium and phosphonium ionic liquids, i.e. 71 and 85 °C, respectively. Initially, tests were made at these melting temperatures without to give results because of their high viscosity. Higher temperatures i.e. 80 °C for the imidazolium ionic liquid and 90 °C for the phosphonium salt, were chosen to perform synthesis. Once the reactor temperature setpoint, the pressure displayed was 70 bars for the imidazolium-modified montmorillonite and 80 bars for the phosphonium-modified montmorillonite. After 6 h of reaction and depressurization different settings to minimize the losses of modified clays, the autoclave was depressurized at a rate of 3.6 bar per second. A phosphonium-montmorillonite, denoted MMT-PCO2 and an imidazolium-montmorillonite denoted MMT-ICO2 , were obtained in the supercritical carbon dioxide. When
Table 1 Designation of pristine and both synthesized ionic liquid modified montmorillonite (MMT). Trade name
Intercalant
Nanofil 757
None
H37C18
N
N
P
C18H37
C18H37
Cationic exchange process
Designation
Water Supercritical CO2 Supercritical CO2 (+10% water)
MMT-Na+ MMT-I MMT-ICO2 MMT-IðCO2 þwaterÞ
Water Supercritical CO2 Supercritical CO2 (+10% water)
MMT-P MMT-PCO2 MMT-PðCO2 þwaterÞ
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
10 mL of water was added as a co-solvent, the nomenclature is as follows for MMT-PðCO2 þwaterÞ and MMT-IðCO2 þwaterÞ . The structure of synthesized phosphonium and imidazolium ionic liquids are described in Table 1. 2.4. Characterization of the treated clays Thermogravimetric analysis (TGA) of organically modified clays and composites were performed on a Q500 thermogravimetric analyser (TA instruments). The samples were heated from 30 to 800 °C at a rate of 20 K min1 under nitrogen flow. Differential scanning calorimetry (DSC) analyses of nanocomposites were performed on a Q20 (TA instruments). The samples were kept for 1 min at 200 °C to erase the thermal history before being heated or cooled at a rate of 10 K min1 under nitrogen flow. Surface energy of modified clays was determined with the sessile drop method on a GBX goniometer. From contact angle measurements performed with water and diiodomethane as test liquids on modified clays as pressed discs, polar and dispersive components of surface energy were determined by using Owens– Wendt theory. Wide angle X-ray diffraction spectra (WAXD) were collected on a Bruker D8 Advance X-ray diffractometer at the H. Longchambon diffractometry center. A bent quartz monochromator was used to select the Cu Ka1 radiation (k = 0.15406 nm) and run under operating conditions of 45 mA and 33 kV in a Bragg–Brentano geometry. The angle range scanned was 1–10°2h for the modified clays and 1–30°2h for the nanocomposite materials.
3. Results and discussion 3.1. Effect of supercritical carbon dioxide as an exchange solvent on the thermal degradation of the modified MMT 3.1.1. Thermal stability of imidazolium-modified montmorillonite Thermogravimetric analysis (TGA) may help to identify intercalated as well as physisorbed species of modified clay by investigating the degradation mechanisms and the effects of functionalization on the thermal stability. Fig. 1 shows the evolution of the weight loss as a function of temperature and the corresponding derivative curves performed on imidazolium-modified montmorillonites either in water at room temperature and pressure or in supercritical carbon dioxide. The thermal degradation of imidazolium-modified MMT reveal three weight losses whatever the cationic exchange process used.
(a) (b) (a') (b')
(b) (a)
(a') (b')
0.4
Weight (%)
0.3 80 0.2 60 0.1
40
0
200
400
Temperature (°C)
600
Deriv. Weight (%/°C)
100
0.0 800 Universal V4.2E TA Instruments
Fig. 1. Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I (a, a0 ) and MMT-ICO2 (b, b0 ) before washing (heating rate: 20 K min1; nitrogen atmosphere).
227
According to the literature [16–19], as a layered clay is modified with an organic cation, two kinds of interactions between organic cations and inorganic clay can take place: (i) Van der Waals bonds as the organic species are physically adsorbed on the clay surface and. (ii) Ionic bonds as the species display ionic interactions inside galleries in the clays. The first weight loss corresponds to a partial physisorption on the edges (bearing polar SiOH groups) or on the external surface of the platelets since the peak decreases after washing. The fact that it does not completely disappear after intensive washing is associated to a part of ionic liquid is well intercalated but in a peripheral position respect to the clay gallery as reported by Davis et al. [20] or is physisorbed from p-SiOH interactions at the edges of the lamellar silicates. Such part of surfactant can not be washed away easily (since it underwent cationic exchange) but it is no thermally stabilized by the presence of the inorganic silicate platelets in a confined position. As a consequence, it degrades at the same temperature as the physisorbed surfactant. On the other side, the degradation which is evidenced in a temperature range from 400 to 500 °C corresponds to the well intercalated species. Indeed, organics inside clay galleries display higher temperatures of degradation as reported in previous works [20,21]. Indeed, on the MMT-I DTG curves, the first peak at 340 °C corresponds to physical adsorption onto clay surface and the second and third peak at about 420 °C and 480 °C are related to the imidazolium ionic liquid species really intercalated between clay layers. The same signature on TGA curve is clear evidence that the cationic exchange of montmorillonite with imidazolium ionic liquid is possible using only supercritical carbon dioxide as a solvent. Up to now, only phosphonium ionic liquids with much shorter alkyl were considered in ScCO2 to modify MMT but working at very high pressures while using a small amount of co-solvent [22]. The influence of a co-solvent on the modification of lamellar silicates in supercritical carbon dioxide was also considered. However, there is a significant drawback, i.e. the formation of sticky powders due to the presence of ionic liquid excess adsorbed on the surface of montmorillonite having a very high viscosity. As a consequence, the easiest solution is to reduce the amount of ionic liquid introduced into the autoclave from the use of a co-solvent as increasing pressure (75 bars) up to reduce the viscosity of ionic liquid required. According to the literature [23], the solubility of ionic liquid in supercritical CO2 remains extremely low and it is necessary to use organic co-solvents. Wu et al. [24] studied the effects of organic solvents as acetone or ethanol in ScCO2. Large increase of the solubility of the ionic liquid in ScCO2 was reported. This phenomenon is explained by strong interaction of solvent with the ionic liquid due mainly to their high polarity. For this knowledge, we selected the most polar solvent, i.e. water, that has the huge advantage of being a green solvent (such as supercritical CO2 and ionic liquids) to design an environmentally sustainable cationic exchange process. However, compared to organic solvents, it has the disadvantage of being CO2-phobic but water could be used as a real cosolvent due to the fact that the synthesized ionic liquids are soluble in water. With water as co-solvent, the thermal degradation of imidazolium-modified montmorillonite is quite different from one of the modified montmorillonite by conventional cationic exchanges, i.e. in aqueous solution or ScCO2 medium process (Fig. 2). The modified montmorillonite in supercritical carbon dioxide combined with co-solvent shows both a much earlier degradation of physisorbed species (240 °C versus 340 °C) but in the opposite a delayed degradation of intercalated species (540 °C versus 420/490 °C). Washing with methyl alcohol that is the more suitable solvent of ionic liquids removes the physically adsorbed species corresponding to the first degradation peak (see Table 2).
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
0.6
80
0.4
(a')
(b')
60
0.2 (a)
40
20
0.0
(b)
0
200
400
Deriv. Weight (%/°C)
Weight (%)
100
0.8
-0.2 800
600
Universal V4.2E TA Instruments
Temperature (°C)
Fig. 2. Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I (a, a0 ) and MMT-IðCO2 þwaterÞ (b, b0 ) before washing (heating rate: 20 K min1; nitrogen atmosphere).
Table 2 Relative mass loss of physically adsorbed and intercalated species determined by TGA (imidazolium-modified montmorillonites either in water or ScCO2). Sample
Cationic exchange process
Physically adsorbed species (%)
Intercalated species (%)
MMT-I bw MMT-I aw MMT-ICO2 bw MMT-ICO2 aw MMT-IðCO2 þwaterÞ bw
Water
31 – 27 – 47
18 23 22 25 18
9
33
Supercritical CO2 Supercritical CO2 (+10% water)
MMT-IðCO2 þwaterÞ aw
bw: before washing with MeOH, aw: after washing with MeOH.
Table 2 summarizes the relative mass losses of the physically adsorbed species and the intercalated species for the imidazolium-modified montmorillonites either from a water solution or from the supercritical CO2 medium before and after washing with or without co-solvent. The results for imidazolium-modified montmorillonite (MMT-I) from the water solution or under ScCO2 are similar before and after washing. In the case of MMT-IðCO2 þwaterÞ , when comparing with the imidazolium-modified MMT before washing with a standard cationic exchange process, the weight percent of physisorbed species is significantly higher (a difference of 16%). After washing, we found almost the same difference (10% instead of 16%) for the intercalated species. Thus, the use of the supercritical CO2 in presence of co-solvent leads to an increase of the intercalated species ratio up to 33%. This means that the combined effect of supercritical CO2 and the solubility of imidazolium salts in water play an important role on the intercalation process. In order to have a better understanding of the role of the various components, imidazolium ionic liquids were introduced alone in the autoclave, heated at 80 °C under pressure of 70 bars for 6 h. The melting temperature of the imidazolium ionic liquid after synthesis denoted as C18I and after exposure to supercritical CO2, denoted as C18ICO2 are reported in Table 3.
After 6 h in supercritical carbon dioxide, a strong decrease of the melting point is observed. This value remains about the same as the exposure time increases, this effect could be explained by the presence of ScCO2 which remains soluble in the ionic liquid. In conclusion, there is a solubility limit of the ionic liquid in the CO2 phase. According to the literature reported on the influence of supercritical CO2 on ionic liquids [25,26], this phenomenon was also observed for example by Kazarian et al. [27] who reported that the melting point of imidazolium ionic liquids based on a C16 chain and a fluoride anion was reduced from 25 °C after a ScCO2 treatment under a pressure of 70 bars. Later, another study on the phosphonium and ammonium salts [28] showed in the both cases an important decrease of 100 °C but at higher pressure of exposure (150 bars). Recently, Scurto et al. [29] concluded that CO2 interacted with the ionic liquid due to the establishment of weak Lewis acid–Lewis base interactions between basic moieties of the organic salt and the acidic carbon of CO2. In conclusion, the decrease of the melting temperature of the ionic liquid after exposure to the supercritical CO2 explains that in presence of co-solvent in which ionic liquid is soluble that the cationic exchange will be optimized and that a better intercalation of imidazolium salt in the clay layer galleries will take place. 3.1.2. Thermal stability of phosphonium-modified montmorillonite The same approach was considered for the modification of montmorillonite with phosphonium ions. The cationic exchange was performed in water under atmospheric pressure but also in supercritical carbon dioxide. The TGA analysis performed on phosphonium-modified MMT before washing with methyl alcohol are reported in Fig. 3. The same MMT modification realized under supercritical fluid shows the same TGA analysis which evidences the success of the cationic exchange in these conditions. As previously, we led the same experiment with the presence of co-solvent such as water. Fig. 4 displays the TGA and DTG curves of phosphonium-modified montmorillonites MMT-P and MMTPðCO2 þwaterÞ . With the presence of a co-solvent, a shift of intercalated species is observed (570 °C versus 510 °C) whereas the physically adsorbed species show the same degradation behavior (320 °C versus 330 °C). The DSC data realized on phosphonium ionic liquid after synthesis and after treatment with ScCO2 show the same behavior as imidazolium ions in Table 4. The melting temperature of the phosphonium salt is significantly reduced after treatment with supercritical carbon dioxide
120
(a) (b) (a') (b') 1.0
0.003
100
80 (a')
Ionic liquid
Exposure under ScCO2 Time (h)
Melting temperature (°C)
C18I C18ICO2
0 6 24
71 38 33
0.8
0.6
0.4
0.002
0.001
0.2
(b')
60
Table 3 Effect of exposure to supercritical CO2 on the melting temperature of imidazolium ionic liquid at 80 °C.
0.004
0.000 (a)
Deriv. Log[Weight] (1/°C)
(a) (b) (a') (b')
Deriv. Weight (%/°C)
120
Weight (%)
228
0.0
(b)
40
0
200
400
Temperature (°C)
600
-0.001 800 Universal V4.2E TA Instruments
Fig. 3. Effect of ScCO2 on the thermal degradation of phosphonium-modified montmorillonite by TGA; derivative of the TGA curves MMT-P (a, a0 ) versus MMT-PCO2 (b, b0 ) before washing (heating rate: 20 K min1, nitrogen atmosphere).
229
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
(a) (b) (a') (b')
Weight (%)
100
1.2 1.0
Deriv. Weight (%/°C)
120
0.8 0.6
80 0.4
(a)
60
0.2
(a')
(b)
(b')
diffraction peak at 2.3°2h, corresponding to an interlayer distance of 3.7 nm, distance similar to one characteristic of a paraffinic conformation with trans–trans positions of the alkyl chain. The CO2 step leads to a shift of diffraction peak towards lower angles with an interlayer distance of 4.1 nm, corresponding to diffraction peak at 2.1°2h. The use of water as a co-solvent leads to reduce the effects of swelling by ionic liquid which is solubilized in water. The interlayer distance was found to be similar to one of the imidazolium-modified montmorillonite performed by conventional cationic exchange.
0.0 40
0
200
400
-0.2 800
600
Temperature (°C)
Universal V4.2E TA Instruments
Fig. 4. Effect of ScCO2 combined with water as co-solvent on the thermal degradation of phosphonium-modified montmorillonite by TGA: the derivative of the TGA curves MMT-P (a, a0 ) versus MMT-PðCO2 þwaterÞ (b, b0 ) before washing (heating rate: 20 K min1, nitrogen atmosphere).
Table 4 Effect of exposure to supercritical CO2 on the melting temperature of phosphonium ionic liquid at 90 °C. Ionic liquid
Exposure under ScCO2 time (h)
Melting temperature (°C)
C18P C18PCO2
0 6 24
85 57 55
(28 °C), in the same order of magnitude than for imidazoliumbased ionic liquid. We can expect that, according to the melting temperature depression, the solubility of both montmorillonites exchanged with imidazolium and phosphonium ionic liquids in ScCO2 are similar.
3.2. Structural analysis
(a) (b) (c)
2
3
2000 1500 1000
3.7 nm
(c) (a)
500
(c) 3.7 nm 1
(a) (b) (c)
2500
(a) 3.7 nm
0
4.1 nm (b) 3.7 nm
3000
4.1 nm (b)
34000 32000 30000 28000 26000 24000 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0
Intensity (u.a)
Intensity (u.a)
3.2.1. Imidazolium-modified montmorillonite The effect of the cationic exchange process on the MMT intercalation was studied by X-ray diffraction and reported in Fig. 5. Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which correspond to the d-spacing of MMT-Na+ reported in literature [16]. After organophilic treatment by a conventional cationic exchange, the MMT-I displays a (0 0 1)
3.2.2. Phosphonium-modified montmorillonite Fig. 6 shows the X-ray diffraction spectra performed on MMT-P, MMT-PCO2 and MMT-PðCO2 þwaterÞ . After organic treatment in water by the phosphonium salt, the MMT-P displays a (0 0 1) diffraction peak at 2.1°2h, corresponding to an interlayer distance of 4.2 nm. This value could be explained by the swelling of layered silicates due to steric volume of the three ring structure and the alkyl chain. For the MMT-PCO2 , the diffraction peak located at 1.8°2h, i.e. reveal an interlayer distance of 4.9 nm, this slight increase can be explained by the ionic liquid swelling under the effect of supercritical CO2. The diffraction spectrum displays also a small peak at 3.1°2h, corresponding to physically phosphonium salt adsorbed on the surface of montmorillonite. By using water as co-solvent, the diffraction peak is located at 2.0°2h, i.e. indicating a distance of 4.4 nm for MMT-PðCO2 þwaterÞ , close to the one of the phosphonium- treated MMT by conventional cationic exchange. In all the cases, the spectra show intense, thin, and regular diffraction peaks which suggest a modification on a long range order. The resulting structure of the ionic liquid modified montmorillonites, i.e. the d001 distances, as well as the amount of intercalated ionic liquids species can be explained from the solubility parameters of the various components : ionic liquids, water, and supercritical CO2 the spreading coefficient. In fact, as a conventional route is used, i.e. water solution of ionic liquid as exchange process medium, the intercalation of imidazolium or phosphonium alkylmodified species proceeds from the well-known exchange process described for quaternary ammonium intercalants [30,31]. As supercritical CO2 is used as medium for intercalation, the driving force is the spreading of ionic liquid species onto the MMT surface as the surface polarity of montmorillonite matches the surface tension (or solubility parameter) of the ionic liquid better than the ScCO2 medium. In the later case, i.e. involving water as co-solvent,
0 4
5
6
7
8
9
10
2θ Fig. 5. Effect of the cationic exchange process on the interlayers distance measured by X-ray diffraction on imidazolium ionic liquid modified MMT: (a) MMT-I; (b) MMT-ICO2 ; (c) MMT-IðCO2 þwaterÞ .
0
1
2
3
4
5
6
7
8
9
10
2θ Fig. 6. Effect of the cationic exchange process on the interlayers distance measured by X-ray diffraction on phosphonium ionic liquid modified MMT: (a) MMT-P; (b) MMT-PCO2 ; (c) MMT-PðCO2 þwaterÞ .
230
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230 Table 5 Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders). Montmorillonite +
MMT-Na MMT-P MMT-I MMT-PðCO2 þwaterÞ MMT-IðCO2 þwaterÞ
Hwater (°)
HCH2 I2 (°)
c polar (mN m1)
c dispersive (mN m1)
c total (mN m1)
22.9 ± 0.9 88.9 ± 0.1 92.8 ± 0.1 81.5 ± 0.1 125.2 ± 0.5
33.6 ± 0.8 49.4 ± 0.6 55.5 ± 0.6 45.5 ± 0.7 41.9 ± 0.7
30 2 1 3.6 3.6
43 35 31 36.8 38.5
73 37 32 40.4 42.1
as this one is not soluble in ScCO2, the ionic liquids remain in the water phase leading to a highly concentrated ionic liquids–water phase, which spreads onto the polar montmorillonite surface. As a consequence, the process involving water as co-solvent is similar to the conventional water-solution based protocol. The solubility of ionic liquids in supercritical CO2 became of a charge interest as such an understanding requires modelling approaches [32–34] which could be used for practical purposes [35]. 3.3. Surface energies of ionic liquids-modified montmorillonites The contact angles and surface energy determined by the sessile drop method on pressed powder are collected in Table 5. Both ionic liquids based on phosphonium and imidazolium salts make the montmorillonite more hydrophobic with a surface energy close to the surface energy of a polyolefin [36]. The polar components are very low which is an evidence that the hydroxyl groups are well covered by the organic species, especially C18 chains. The steric hindrance of imidazolium and phosphonium ionic liquids causes an efficient screening of the hydrophilic surface of lamellar silicates. For imidazolium and phosphonium-modified montmorillonites under supercritical carbon dioxide without co-solvent, the values (no reported here) are the same as ones obtained by conventional cationic exchange whereas the montmorillonites MMT-IðCO2 þwaterÞ co and MMT-PðCO2 þwaterÞ co, i.e. by using water as a co-solvent, display slightly higher values of surface energy. 4. Conclusions In this work, we demonstrated that it is possible to modify lamellar silicates by phosphonium and imidazolium ionic liquids using solvents as water and supercritical CO2. The resulting properties are improved thermal stability of intercalated species and better intercalation between the layers of montmorillonite. This process can be improved and requires many additional studies on the interactions between different components which are considered. Nevertheless, this study highlights that several solvents, such as water, ionic liquids, and supercritical CO2, which are among the most promising components of green chemistry, could be used as relevant surface treatment of layered minerals. References [1] L. Le Pluart, J. Duchet, H. Sautereau, Polymer 46 (26) (2005) 12267.
[2] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci., Part B: Polym. Phys. 44 (2) (2006) 431. [3] M.A. Osman, V. Mittal, M. Morbidelli, U.W. Suter, Macromolecules 36 (2003) 9851. [4] S.M. Auerbach, K.A. Carrado, P.K. Dutta, Handbook of Layered Materials, Taylor & Francis, New York, 2004. [5] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J. Colloid Interface Sci. 284 (2) (2005) 667. [6] H. He, J. Duchet, J. Galy, J.F. Gerard, J. Colloid Interface Sci. 295 (2006) 202. [7] V. Mittal, Eur. Polym. J. 43 (2007) 3727. [8] G. Chigwada, D. Wang, et al., Polym. Degrad. Stab. 91 (4) (2006) 848–855. [9] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. 14 (11) (2002) 4837. [10] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochim. Acta 409 (2004) 3. [11] J. Zhu, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater. 13 (2001) 3774. [12] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven, Thermochim. Acta 357–358 (2000) 97. [13] F.A. Bottino, E. Fabbri, I.L. Fragala, G. Malandrino, A. Orestano, F. Pilati, Macromol. Rapid Commun. 24 (2003) 1079. [14] J.W. Gilman, W.H. Awad, R.D. Davis, J. Shields, R.H. Harris, C. Davis, Chem. Mater. 14 (2002) 3776. [15] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. 31 (2006) 19– 43. [16] J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, Chem. Mater. 12 (2000) 1866. [17] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. 78 (7) (2002) 645. [18] J.W. Gilman, Appl. Clay Sci. 15 (1999) 31. [19] C. Lotti, C.S. Isaac, M.C. Branciforti, R. Alves, S. Liberman, R. Bretas, Eur. Polym. J. 44 (2008) 1346. [20] R.D. Davis, J.W. Gilman, T.E. Sutto, Clays Clay Miner. 52 (2) (2004) 171. [21] S. Livi, J. Duchet-Rumeau, T.-N. Pham, J.-F. Gérard, J. Colloid, Interface Sci. 349 (2010) 424–433. [22] E. Naveau, C. Calberg, C. Detrembleur, S. Bourbigot, C. Jérôme, M. Alexandre, Polymer 50 (2009) 1438–1446. [23] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. Supercrit. Fluids 43 (2007) 150–180. [24] W. Wu, J. Zhang, B. Han, J. Chen, Z. Liu, T. Jiang, J. He, W. Li, Chem. Commun. (2003) 1412–1413. [25] V. Najdanovic-Visak, A. Serbanovic, J.M.S.S. Esperança, H.J.R. Guedes, L.P.N. Rebelo, M. Nunes da Ponte, Chem. Phys. Chem. 4 (2003) 520. [26] M. Roth, J. Chromatogr. A 1216 (2009) 1861–1880. [27] S.G. Kazarian, N. Sakellarios, C.M. Gordon, Chem. Commun. (2002) 1314. [28] A.M. Scurto, W. Leitner, Chem. Commun. (2006) 3681. [29] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J. Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. 47 (2008) 493. [30] C.B. Hedley, G. Yuan, B.K.G. Theng, Appl. Clay Sci. 35 (2007) 180–188. [31] A. Vasquez, M. Lopez, G. Kortaberria, L. Martin, I. Mondragon, Appl. Clay Sci. 41 (2008) 24–36. [32] X. Ji, H. Adidharma, Fluid Phase Equilib. 293 (2010) 141–150. [33] J. Kumelan, J. Chem. Thermodyn. 37 (6) (2005) 595–602. [34] J.S. Torrecilla, J. Palomar, J. Garcia, E. Rojo, F. Rodriguez, Chemiom. Intell. Lab. Syst. 93 (2) (2008) 149–159. [35] M.G. Freire, C.M.S.S. Neves, S.P.M. Ventura, M.J. Patras, I.M. Marrucho, J. Oliveira, J.A.P. Coutinho, Fluid Phase Equilib. 294 (2010) 234–240. [36] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. 194 (1) (2002) 241.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Supercritical CO2–ionic liquid mixtures for modification of organoclays Sébastien Livi, Jannick Duchet-Rumeau ⇑, Jean-François Gérard Université de Lyon, F-69003 Lyon, France INSA Lyon, F-69621 Villeurbanne, France CNRS, UMR 5223, Ingénierie des Matériaux Polymères, France
a r t i c l e
i n f o
Article history: Received 13 July 2010 Accepted 17 September 2010
Keywords: Ionic liquid Supercritical CO2 Organoclay Cationic exchange
a b s t r a c t The use of supercritical CO2 as solvent in the modification of montmorillonite by imidazolium and phosphonium ionic liquids bearing long alkyl chains (C18) known for their excellent thermal stability is described. The objective is to combine the environmentally friendly character of ionic liquids and supercritical carbon dioxide for the organophilic treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites, cationically exchanged under supercritical carbon dioxide with or without co-solvent, have been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and compared to montmorillonites treated by conventional cationic exchange. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since the 1980s, the industrial and academic research have a growing interest in the design of polymer/clay nanocomposites. According to the literature, the insertion of lamellar silicates in a polymer matrix can have beneficial effects on the final properties of the nanocomposite. The most often reported improvements are for thermo-mechanical behavior [1] and gas barrier properties [2,3] due to nanometer-range dimensions and high aspect ratio of layered silicates leading to a large amount of interfacial zones. Natural clays, such as montmorillonite (MMT) are frequently used for that purpose [4]. However, the dispersion of non-modified clay due to the poor interfacial interactions is very limited. Indeed, it is necessary to modify the surface of the pristine MMT, mostly by cationic exchange using ammonium salts [5,6] to improve the compatibility between the polymer and lamellar fillers during the processing step of nanocomposites, i.e. to improve the final dispersion state and to get a full/large development of the interfaces. Ionic liquids are organic salts with melting points below 100 °C, which are considered as green solvents. The most commonly used are imidazolium [7], pyridinium [8] and phosphonium [9] salts. They are used in several applications: chemical reaction medium, i.e. as solvents, electrolytes in batteries, lubricants, plasticizers, and catalysts. Recently, ionic liquids were used as surfactants to replace the conventional alkyl ammonium salts that show a lower
⇑ Corresponding author. Fax: +33 4 72438527. E-mail address: [email protected] (J. Duchet-Rumeau). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.09.049
thermal stability [10–12]. Despite many benefits, very few studies using such cations for layered silicates intercalation have been reported in the literature may be due of the actual higher cost of these surfactants compared to ammonium salts [13,14,7]. Moreover, thermally stable and commercially available ionic liquids which could be considered as surfactants only incorporate short aliphatic chains (up to C14). In this work, ionic liquids bearing long alkyl chains based on imidazolium (two chains in C18) and on phosphonium salts with three benzyl groups and only one long aliphatic chain (one chain in C18) were synthesized. As these ionic liquids are used as clay intercalants, the long alkyl chains and the aromatic groups could cause expansion of the distance between the layers and could contribute to a better intercalation of the clay platelets. To perform an environmentally friendly MMT surface treatment, the properties of ionic liquids and supercritical CO2 were combined. The use of CO2 in the supercritical state should make useless the addition of any solvent for that the cationic exchange succeeds. The main interest of supercritical CO2 is to design a clean surface treatment compared to the ones using conventional solvents. In fact, carbon dioxide has a high diffusivity like a gas, low surface tension (close to zero), viscosity and density like a liquid which gives it a high solvency power tuneable by adjusting pressure [15]. In this work, the combination of supercritical CO2 and ionic liquids should lead to an environmentally benign cationic exchange process to modify the MMT with imidazolium and phosphonium salts with long alkyl chains. The objectives of this work is to design a process without use of solvent and to identify the supercritical CO2 exposure effects on the physico-chemical properties resulting of modified clays.
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2. Experimental 2.1. Materials A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with intercalated sodium ions was chosen as pristine clay. This one was provided by Süd Chemie (Germany). The Nanofil 757 has a cationic exchange capacity of 95 meq/100 g and is described by the following formula Na0,65[Al, Fe]4Si8O20 (OH)4. All chemicals necessary to the synthesis of ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl (95%), and all the solvents (toluene, sodium methanoate, pentane, and acetonitrile) were supplied from Aldrich and used as received. 2.2. Synthesis of phosphonium and imidazolium salts 2.2.1. Synthesis of octadecyltriphenylphosphonium salt In a 100 mL flask was placed under a positive nitrogen pressure, 1 equiv. of triphenylphosphine (5 g) and 1 equiv. octadecyl iodide (7.3 g). The stirred suspensions were allowed to react for 24 h at 120 °C in toluene (20 mL), a yellow precipitate was formed. The reaction mixture was then filtered and washed repeatedly with pentane. Most of the solvent was removed under vacuum. The chemical structure of the salts was confirmed by 13C NMR spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer [21]. The assignment of 13C NMR resonance peaks is reported below. 13 C NMR (CDCl3): d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37– 29.66; 30.24; 31.85 (P-CH2); 118.45; 130.43; 133.70; 135.15 (P-C). 2.2.2. Synthesis of N-octadecyl-N0 -octadecylimidazolium salt A solution of sodium methoxide was prepared from 1 equiv. of sodium (0.465 g) in dry freshly distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottomed, three-necked flask equipped with a condenser, under nitrogen atmosphere, and magnetic stirring. Imidazole (1.37 g) diluted in acetonitrile (10 mL) was then added into the stirred mixture of sodium methoxide previously cooled at room temperature. After 15 min, a white precipitate was formed. The suspension was then concentrated under reduced pressure for 1 h. The dried white powder was dissolved in acetonitrile and a solution of octadecyl iodide (7.70 g) diluted in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen at room temperature. The mixture was stirred for 1 h, then heated under reflux at 85 °C for about 24 h. A solution of octadecyl iodide (7.70 g) diluted in acetonitrile (10 mL) was added to the mixture at room temperature. The stirred suspension was heated under reflux at 100 °C for about 24 h leaving brownish viscous oil in each case. After cooling to room temperature, the solvent was removed by evaporation under vacuum, the beige solid
was filtered, washed repeatedly with pentane and dried. Purification of the resulting imidazolium salts was accomplished by crystallization from ethyl acetate/acetonitrile: 75/25 mixture. The assignment of 13C NMR resonance peaks evidences the success of the ionic liquid synthesis. 13 C NMR (CDCl3): d 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35–29.69; 30.24; 31.91 (CH2); 50.10; (CH2N@); 50.32 (CH2NA); 121.69; 122.48 (@CN); 136.88 (NAC@N). 2.3. Organic modification of montmorillonite The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionised water. The amount of surfactant added was about 2 CEC, based on the cation exchanged capacity (CEC = 95 meq/100 g) of the MMT used [9]. This dispersion was mixed and stirred vigorously at 80 °C for 6 h, followed by filtration and continuous washing at 80 °C with deionised water until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The solvent was removed by evaporation under vacuum. The modified montmorillonite was then dried for 12 h, at a suitable temperature (not greater than 80 °C). The imidazolium, phosphonium and the quaternary ammonium ions used for the exchange reactions are presented in Table 1. The following abbreviations were used to design the different montmorillonites: MMT-Na+ means the pristine montmorillonite. A phosphonium-montmorillonite denoted MMT-P was obtained when octadecyltriphenylphosphonium iodide was used as surfactant. An imidazolium-montmorillonite, denoted MMT-I, was obtained when the N-octadecyl-N0 -octadecylimidazolium iodide was used like intercalation agent. As supercritical carbon dioxide was used as solvent instead of water, the procedure was the following one: 2 g of untreated MMT and an excess of surfactant (2 CEC: imidazolium and phosphonium ionic liquid) were placed into a 300 mL high pressure reactor. Then, an initial loading of the autoclave at a pressure of 50 bars and at a temperature of 20 °C was made. The temperature used was chosen based on the melting temperatures of the imidazolium and phosphonium ionic liquids, i.e. 71 and 85 °C, respectively. Initially, tests were made at these melting temperatures without to give results because of their high viscosity. Higher temperatures i.e. 80 °C for the imidazolium ionic liquid and 90 °C for the phosphonium salt, were chosen to perform synthesis. Once the reactor temperature setpoint, the pressure displayed was 70 bars for the imidazolium-modified montmorillonite and 80 bars for the phosphonium-modified montmorillonite. After 6 h of reaction and depressurization different settings to minimize the losses of modified clays, the autoclave was depressurized at a rate of 3.6 bar per second. A phosphonium-montmorillonite, denoted MMT-PCO2 and an imidazolium-montmorillonite denoted MMT-ICO2 , were obtained in the supercritical carbon dioxide. When
Table 1 Designation of pristine and both synthesized ionic liquid modified montmorillonite (MMT). Trade name
Intercalant
Nanofil 757
None
H37C18
N
N
P
C18H37
C18H37
Cationic exchange process
Designation
Water Supercritical CO2 Supercritical CO2 (+10% water)
MMT-Na+ MMT-I MMT-ICO2 MMT-IðCO2 þwaterÞ
Water Supercritical CO2 Supercritical CO2 (+10% water)
MMT-P MMT-PCO2 MMT-PðCO2 þwaterÞ
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
10 mL of water was added as a co-solvent, the nomenclature is as follows for MMT-PðCO2 þwaterÞ and MMT-IðCO2 þwaterÞ . The structure of synthesized phosphonium and imidazolium ionic liquids are described in Table 1. 2.4. Characterization of the treated clays Thermogravimetric analysis (TGA) of organically modified clays and composites were performed on a Q500 thermogravimetric analyser (TA instruments). The samples were heated from 30 to 800 °C at a rate of 20 K min1 under nitrogen flow. Differential scanning calorimetry (DSC) analyses of nanocomposites were performed on a Q20 (TA instruments). The samples were kept for 1 min at 200 °C to erase the thermal history before being heated or cooled at a rate of 10 K min1 under nitrogen flow. Surface energy of modified clays was determined with the sessile drop method on a GBX goniometer. From contact angle measurements performed with water and diiodomethane as test liquids on modified clays as pressed discs, polar and dispersive components of surface energy were determined by using Owens– Wendt theory. Wide angle X-ray diffraction spectra (WAXD) were collected on a Bruker D8 Advance X-ray diffractometer at the H. Longchambon diffractometry center. A bent quartz monochromator was used to select the Cu Ka1 radiation (k = 0.15406 nm) and run under operating conditions of 45 mA and 33 kV in a Bragg–Brentano geometry. The angle range scanned was 1–10°2h for the modified clays and 1–30°2h for the nanocomposite materials.
3. Results and discussion 3.1. Effect of supercritical carbon dioxide as an exchange solvent on the thermal degradation of the modified MMT 3.1.1. Thermal stability of imidazolium-modified montmorillonite Thermogravimetric analysis (TGA) may help to identify intercalated as well as physisorbed species of modified clay by investigating the degradation mechanisms and the effects of functionalization on the thermal stability. Fig. 1 shows the evolution of the weight loss as a function of temperature and the corresponding derivative curves performed on imidazolium-modified montmorillonites either in water at room temperature and pressure or in supercritical carbon dioxide. The thermal degradation of imidazolium-modified MMT reveal three weight losses whatever the cationic exchange process used.
(a) (b) (a') (b')
(b) (a)
(a') (b')
0.4
Weight (%)
0.3 80 0.2 60 0.1
40
0
200
400
Temperature (°C)
600
Deriv. Weight (%/°C)
100
0.0 800 Universal V4.2E TA Instruments
Fig. 1. Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I (a, a0 ) and MMT-ICO2 (b, b0 ) before washing (heating rate: 20 K min1; nitrogen atmosphere).
227
According to the literature [16–19], as a layered clay is modified with an organic cation, two kinds of interactions between organic cations and inorganic clay can take place: (i) Van der Waals bonds as the organic species are physically adsorbed on the clay surface and. (ii) Ionic bonds as the species display ionic interactions inside galleries in the clays. The first weight loss corresponds to a partial physisorption on the edges (bearing polar SiOH groups) or on the external surface of the platelets since the peak decreases after washing. The fact that it does not completely disappear after intensive washing is associated to a part of ionic liquid is well intercalated but in a peripheral position respect to the clay gallery as reported by Davis et al. [20] or is physisorbed from p-SiOH interactions at the edges of the lamellar silicates. Such part of surfactant can not be washed away easily (since it underwent cationic exchange) but it is no thermally stabilized by the presence of the inorganic silicate platelets in a confined position. As a consequence, it degrades at the same temperature as the physisorbed surfactant. On the other side, the degradation which is evidenced in a temperature range from 400 to 500 °C corresponds to the well intercalated species. Indeed, organics inside clay galleries display higher temperatures of degradation as reported in previous works [20,21]. Indeed, on the MMT-I DTG curves, the first peak at 340 °C corresponds to physical adsorption onto clay surface and the second and third peak at about 420 °C and 480 °C are related to the imidazolium ionic liquid species really intercalated between clay layers. The same signature on TGA curve is clear evidence that the cationic exchange of montmorillonite with imidazolium ionic liquid is possible using only supercritical carbon dioxide as a solvent. Up to now, only phosphonium ionic liquids with much shorter alkyl were considered in ScCO2 to modify MMT but working at very high pressures while using a small amount of co-solvent [22]. The influence of a co-solvent on the modification of lamellar silicates in supercritical carbon dioxide was also considered. However, there is a significant drawback, i.e. the formation of sticky powders due to the presence of ionic liquid excess adsorbed on the surface of montmorillonite having a very high viscosity. As a consequence, the easiest solution is to reduce the amount of ionic liquid introduced into the autoclave from the use of a co-solvent as increasing pressure (75 bars) up to reduce the viscosity of ionic liquid required. According to the literature [23], the solubility of ionic liquid in supercritical CO2 remains extremely low and it is necessary to use organic co-solvents. Wu et al. [24] studied the effects of organic solvents as acetone or ethanol in ScCO2. Large increase of the solubility of the ionic liquid in ScCO2 was reported. This phenomenon is explained by strong interaction of solvent with the ionic liquid due mainly to their high polarity. For this knowledge, we selected the most polar solvent, i.e. water, that has the huge advantage of being a green solvent (such as supercritical CO2 and ionic liquids) to design an environmentally sustainable cationic exchange process. However, compared to organic solvents, it has the disadvantage of being CO2-phobic but water could be used as a real cosolvent due to the fact that the synthesized ionic liquids are soluble in water. With water as co-solvent, the thermal degradation of imidazolium-modified montmorillonite is quite different from one of the modified montmorillonite by conventional cationic exchanges, i.e. in aqueous solution or ScCO2 medium process (Fig. 2). The modified montmorillonite in supercritical carbon dioxide combined with co-solvent shows both a much earlier degradation of physisorbed species (240 °C versus 340 °C) but in the opposite a delayed degradation of intercalated species (540 °C versus 420/490 °C). Washing with methyl alcohol that is the more suitable solvent of ionic liquids removes the physically adsorbed species corresponding to the first degradation peak (see Table 2).
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
0.6
80
0.4
(a')
(b')
60
0.2 (a)
40
20
0.0
(b)
0
200
400
Deriv. Weight (%/°C)
Weight (%)
100
0.8
-0.2 800
600
Universal V4.2E TA Instruments
Temperature (°C)
Fig. 2. Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I (a, a0 ) and MMT-IðCO2 þwaterÞ (b, b0 ) before washing (heating rate: 20 K min1; nitrogen atmosphere).
Table 2 Relative mass loss of physically adsorbed and intercalated species determined by TGA (imidazolium-modified montmorillonites either in water or ScCO2). Sample
Cationic exchange process
Physically adsorbed species (%)
Intercalated species (%)
MMT-I bw MMT-I aw MMT-ICO2 bw MMT-ICO2 aw MMT-IðCO2 þwaterÞ bw
Water
31 – 27 – 47
18 23 22 25 18
9
33
Supercritical CO2 Supercritical CO2 (+10% water)
MMT-IðCO2 þwaterÞ aw
bw: before washing with MeOH, aw: after washing with MeOH.
Table 2 summarizes the relative mass losses of the physically adsorbed species and the intercalated species for the imidazolium-modified montmorillonites either from a water solution or from the supercritical CO2 medium before and after washing with or without co-solvent. The results for imidazolium-modified montmorillonite (MMT-I) from the water solution or under ScCO2 are similar before and after washing. In the case of MMT-IðCO2 þwaterÞ , when comparing with the imidazolium-modified MMT before washing with a standard cationic exchange process, the weight percent of physisorbed species is significantly higher (a difference of 16%). After washing, we found almost the same difference (10% instead of 16%) for the intercalated species. Thus, the use of the supercritical CO2 in presence of co-solvent leads to an increase of the intercalated species ratio up to 33%. This means that the combined effect of supercritical CO2 and the solubility of imidazolium salts in water play an important role on the intercalation process. In order to have a better understanding of the role of the various components, imidazolium ionic liquids were introduced alone in the autoclave, heated at 80 °C under pressure of 70 bars for 6 h. The melting temperature of the imidazolium ionic liquid after synthesis denoted as C18I and after exposure to supercritical CO2, denoted as C18ICO2 are reported in Table 3.
After 6 h in supercritical carbon dioxide, a strong decrease of the melting point is observed. This value remains about the same as the exposure time increases, this effect could be explained by the presence of ScCO2 which remains soluble in the ionic liquid. In conclusion, there is a solubility limit of the ionic liquid in the CO2 phase. According to the literature reported on the influence of supercritical CO2 on ionic liquids [25,26], this phenomenon was also observed for example by Kazarian et al. [27] who reported that the melting point of imidazolium ionic liquids based on a C16 chain and a fluoride anion was reduced from 25 °C after a ScCO2 treatment under a pressure of 70 bars. Later, another study on the phosphonium and ammonium salts [28] showed in the both cases an important decrease of 100 °C but at higher pressure of exposure (150 bars). Recently, Scurto et al. [29] concluded that CO2 interacted with the ionic liquid due to the establishment of weak Lewis acid–Lewis base interactions between basic moieties of the organic salt and the acidic carbon of CO2. In conclusion, the decrease of the melting temperature of the ionic liquid after exposure to the supercritical CO2 explains that in presence of co-solvent in which ionic liquid is soluble that the cationic exchange will be optimized and that a better intercalation of imidazolium salt in the clay layer galleries will take place. 3.1.2. Thermal stability of phosphonium-modified montmorillonite The same approach was considered for the modification of montmorillonite with phosphonium ions. The cationic exchange was performed in water under atmospheric pressure but also in supercritical carbon dioxide. The TGA analysis performed on phosphonium-modified MMT before washing with methyl alcohol are reported in Fig. 3. The same MMT modification realized under supercritical fluid shows the same TGA analysis which evidences the success of the cationic exchange in these conditions. As previously, we led the same experiment with the presence of co-solvent such as water. Fig. 4 displays the TGA and DTG curves of phosphonium-modified montmorillonites MMT-P and MMTPðCO2 þwaterÞ . With the presence of a co-solvent, a shift of intercalated species is observed (570 °C versus 510 °C) whereas the physically adsorbed species show the same degradation behavior (320 °C versus 330 °C). The DSC data realized on phosphonium ionic liquid after synthesis and after treatment with ScCO2 show the same behavior as imidazolium ions in Table 4. The melting temperature of the phosphonium salt is significantly reduced after treatment with supercritical carbon dioxide
120
(a) (b) (a') (b') 1.0
0.003
100
80 (a')
Ionic liquid
Exposure under ScCO2 Time (h)
Melting temperature (°C)
C18I C18ICO2
0 6 24
71 38 33
0.8
0.6
0.4
0.002
0.001
0.2
(b')
60
Table 3 Effect of exposure to supercritical CO2 on the melting temperature of imidazolium ionic liquid at 80 °C.
0.004
0.000 (a)
Deriv. Log[Weight] (1/°C)
(a) (b) (a') (b')
Deriv. Weight (%/°C)
120
Weight (%)
228
0.0
(b)
40
0
200
400
Temperature (°C)
600
-0.001 800 Universal V4.2E TA Instruments
Fig. 3. Effect of ScCO2 on the thermal degradation of phosphonium-modified montmorillonite by TGA; derivative of the TGA curves MMT-P (a, a0 ) versus MMT-PCO2 (b, b0 ) before washing (heating rate: 20 K min1, nitrogen atmosphere).
229
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230
(a) (b) (a') (b')
Weight (%)
100
1.2 1.0
Deriv. Weight (%/°C)
120
0.8 0.6
80 0.4
(a)
60
0.2
(a')
(b)
(b')
diffraction peak at 2.3°2h, corresponding to an interlayer distance of 3.7 nm, distance similar to one characteristic of a paraffinic conformation with trans–trans positions of the alkyl chain. The CO2 step leads to a shift of diffraction peak towards lower angles with an interlayer distance of 4.1 nm, corresponding to diffraction peak at 2.1°2h. The use of water as a co-solvent leads to reduce the effects of swelling by ionic liquid which is solubilized in water. The interlayer distance was found to be similar to one of the imidazolium-modified montmorillonite performed by conventional cationic exchange.
0.0 40
0
200
400
-0.2 800
600
Temperature (°C)
Universal V4.2E TA Instruments
Fig. 4. Effect of ScCO2 combined with water as co-solvent on the thermal degradation of phosphonium-modified montmorillonite by TGA: the derivative of the TGA curves MMT-P (a, a0 ) versus MMT-PðCO2 þwaterÞ (b, b0 ) before washing (heating rate: 20 K min1, nitrogen atmosphere).
Table 4 Effect of exposure to supercritical CO2 on the melting temperature of phosphonium ionic liquid at 90 °C. Ionic liquid
Exposure under ScCO2 time (h)
Melting temperature (°C)
C18P C18PCO2
0 6 24
85 57 55
(28 °C), in the same order of magnitude than for imidazoliumbased ionic liquid. We can expect that, according to the melting temperature depression, the solubility of both montmorillonites exchanged with imidazolium and phosphonium ionic liquids in ScCO2 are similar.
3.2. Structural analysis
(a) (b) (c)
2
3
2000 1500 1000
3.7 nm
(c) (a)
500
(c) 3.7 nm 1
(a) (b) (c)
2500
(a) 3.7 nm
0
4.1 nm (b) 3.7 nm
3000
4.1 nm (b)
34000 32000 30000 28000 26000 24000 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0
Intensity (u.a)
Intensity (u.a)
3.2.1. Imidazolium-modified montmorillonite The effect of the cationic exchange process on the MMT intercalation was studied by X-ray diffraction and reported in Fig. 5. Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which correspond to the d-spacing of MMT-Na+ reported in literature [16]. After organophilic treatment by a conventional cationic exchange, the MMT-I displays a (0 0 1)
3.2.2. Phosphonium-modified montmorillonite Fig. 6 shows the X-ray diffraction spectra performed on MMT-P, MMT-PCO2 and MMT-PðCO2 þwaterÞ . After organic treatment in water by the phosphonium salt, the MMT-P displays a (0 0 1) diffraction peak at 2.1°2h, corresponding to an interlayer distance of 4.2 nm. This value could be explained by the swelling of layered silicates due to steric volume of the three ring structure and the alkyl chain. For the MMT-PCO2 , the diffraction peak located at 1.8°2h, i.e. reveal an interlayer distance of 4.9 nm, this slight increase can be explained by the ionic liquid swelling under the effect of supercritical CO2. The diffraction spectrum displays also a small peak at 3.1°2h, corresponding to physically phosphonium salt adsorbed on the surface of montmorillonite. By using water as co-solvent, the diffraction peak is located at 2.0°2h, i.e. indicating a distance of 4.4 nm for MMT-PðCO2 þwaterÞ , close to the one of the phosphonium- treated MMT by conventional cationic exchange. In all the cases, the spectra show intense, thin, and regular diffraction peaks which suggest a modification on a long range order. The resulting structure of the ionic liquid modified montmorillonites, i.e. the d001 distances, as well as the amount of intercalated ionic liquids species can be explained from the solubility parameters of the various components : ionic liquids, water, and supercritical CO2 the spreading coefficient. In fact, as a conventional route is used, i.e. water solution of ionic liquid as exchange process medium, the intercalation of imidazolium or phosphonium alkylmodified species proceeds from the well-known exchange process described for quaternary ammonium intercalants [30,31]. As supercritical CO2 is used as medium for intercalation, the driving force is the spreading of ionic liquid species onto the MMT surface as the surface polarity of montmorillonite matches the surface tension (or solubility parameter) of the ionic liquid better than the ScCO2 medium. In the later case, i.e. involving water as co-solvent,
0 4
5
6
7
8
9
10
2θ Fig. 5. Effect of the cationic exchange process on the interlayers distance measured by X-ray diffraction on imidazolium ionic liquid modified MMT: (a) MMT-I; (b) MMT-ICO2 ; (c) MMT-IðCO2 þwaterÞ .
0
1
2
3
4
5
6
7
8
9
10
2θ Fig. 6. Effect of the cationic exchange process on the interlayers distance measured by X-ray diffraction on phosphonium ionic liquid modified MMT: (a) MMT-P; (b) MMT-PCO2 ; (c) MMT-PðCO2 þwaterÞ .
230
S. Livi et al. / Journal of Colloid and Interface Science 353 (2011) 225–230 Table 5 Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders). Montmorillonite +
MMT-Na MMT-P MMT-I MMT-PðCO2 þwaterÞ MMT-IðCO2 þwaterÞ
Hwater (°)
HCH2 I2 (°)
c polar (mN m1)
c dispersive (mN m1)
c total (mN m1)
22.9 ± 0.9 88.9 ± 0.1 92.8 ± 0.1 81.5 ± 0.1 125.2 ± 0.5
33.6 ± 0.8 49.4 ± 0.6 55.5 ± 0.6 45.5 ± 0.7 41.9 ± 0.7
30 2 1 3.6 3.6
43 35 31 36.8 38.5
73 37 32 40.4 42.1
as this one is not soluble in ScCO2, the ionic liquids remain in the water phase leading to a highly concentrated ionic liquids–water phase, which spreads onto the polar montmorillonite surface. As a consequence, the process involving water as co-solvent is similar to the conventional water-solution based protocol. The solubility of ionic liquids in supercritical CO2 became of a charge interest as such an understanding requires modelling approaches [32–34] which could be used for practical purposes [35]. 3.3. Surface energies of ionic liquids-modified montmorillonites The contact angles and surface energy determined by the sessile drop method on pressed powder are collected in Table 5. Both ionic liquids based on phosphonium and imidazolium salts make the montmorillonite more hydrophobic with a surface energy close to the surface energy of a polyolefin [36]. The polar components are very low which is an evidence that the hydroxyl groups are well covered by the organic species, especially C18 chains. The steric hindrance of imidazolium and phosphonium ionic liquids causes an efficient screening of the hydrophilic surface of lamellar silicates. For imidazolium and phosphonium-modified montmorillonites under supercritical carbon dioxide without co-solvent, the values (no reported here) are the same as ones obtained by conventional cationic exchange whereas the montmorillonites MMT-IðCO2 þwaterÞ co and MMT-PðCO2 þwaterÞ co, i.e. by using water as a co-solvent, display slightly higher values of surface energy. 4. Conclusions In this work, we demonstrated that it is possible to modify lamellar silicates by phosphonium and imidazolium ionic liquids using solvents as water and supercritical CO2. The resulting properties are improved thermal stability of intercalated species and better intercalation between the layers of montmorillonite. This process can be improved and requires many additional studies on the interactions between different components which are considered. Nevertheless, this study highlights that several solvents, such as water, ionic liquids, and supercritical CO2, which are among the most promising components of green chemistry, could be used as relevant surface treatment of layered minerals. References [1] L. Le Pluart, J. Duchet, H. Sautereau, Polymer 46 (26) (2005) 12267.
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