On the phase behavior of the quaternary system acrylic acid-water-ethanol-CO2 by in situ infrared spectrometry

J. of Supercritical Fluids 116 (2016) 117–125

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu

On the phase behavior of the quaternary system acrylic acid-water-ethanol-CO2 by in situ infrared spectrometry Hongyu Chen-Jolly a , Pierre Guillot a , Emmanuel Mignard a,∗ , Thierry Tassaing b,∗ a b

CNRS, Univ. Bordeaux, Solvay, LOF, UMR 5258, F-33600 Pessac, France CNRS, Univ. Bordeaux, ISM, UMR 5255, F-33400 Talence, France

a r t i c l e

i n f o

Article history: Received 23 March 2016 Received in revised form 20 May 2016 Accepted 20 May 2016 Available online 24 May 2016 Keywords: Phase equilibria Acrylic acid Water Ethanol Supercritical CO2 Infrared spectrometry Polymerization

a b s t r a c t The phase equilibrium of acrylic acid (AA)/carbon dioxide (CO2 ) as well as mixtures containing water, AA, CO2 and/or ethanol (EtOH) has been investigated. An in-situ infrared analysis method combining FTIR and ATR-IR spectrometry techniques has been used to obtain concentrations of each component in both phases, i.e. CO2 -rich and water-rich ones, as a function of pressure and temperature. Measurements have been performed at 65 and 75 ◦ C for pressures ranging between 0.1 and 20 MPa. The solubilities of AA, water and EtOH in the CO2 -rich phase and the concentrations of AA, EtOH and CO2 in the water-rich phase have been obtained. From these measurements, a complete phase diagram of the binary AA-CO2 system as well as the partition coefficient of each component in the quaternary system have been established. It has been demonstrated that AA is in equilibrium between the water- and the CO2 -rich phases and that EtOH limits this transfer. Nevertheless, aqueous droplets of AA in CO2 at high pressure can be used as monomer reservoirs releasing the monomer in the CO2 -rich phase. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Carbon dioxide (CO2 ) is a naturally occurring gas in our environment under standard ambient temperature and pressure, i.e. 25 ◦ C and 0.1 MPa. However, it becomes supercritical above its critical coordinates, i.e. Tc = 31.1 ◦ C and pc = 7.38 MPa. In this state CO2 becomes interesting to the chemist since supercritical CO2 (scCO2 ) can be used as a powerful solvent [1]. In 2005 Ryoji Noyori noted that scCO2 is one of the three main solutions for the development of “green chemistry”, [2] thanks to the specific properties of supercritical fluids and because scCO2 as a solvent has the advantage of being thermodynamically stable, quite chemically inert, non-toxic, non-flammable, inexpensive, easily available, removable after a chemical transformation and recyclable. Hence, the use of scCO2 as a substitute to harmful organic solvents in different chemical processes has been the subject of a particular attention in the last years. [3] To understand the important solvent power of scCO2 , one has to have in mind that scCO2 is characterized by: (i) a low dielectric constant (ε about 2), (ii) no dipole moment due to its symmetrical molecular structure, (iii) a significant quadrupole moment and iv)

∗ Corresponding authors. E-mail address: [email protected] (E. Mignard). http://dx.doi.org/10.1016/j.supflu.2016.05.014 0896-8446/© 2016 Elsevier B.V. All rights reserved.

other possible weak interactions as Lewis acid or base [4]. It has been proven that scCO2 can solubilize light non-polar molecules with low molecular weight as well as some polar ones such as EtOH, acetone, tetrahydrofuran, although water and ionic substances are not soluble [5]. Moreover, supercritical fluids exhibit a liquid-like density and a viscosity close to those of gas. The diffusivities of light molecules in supercritical fluids are much higher than those measured in their respective liquid states; approximately two orders of magnitude higher. Furthermore, it is well-known that all these supercritical fluid properties, as well as dielectric constant and others, can be finely tuned according to the working temperature and/or pressure. In fact, these properties are continuously adjustable by slight variations in pressure and/or temperature from the liquid state to the gas state by passing the critical point. This behaviour is specific to supercritical fluids, and gives them another advantage compared to conventional liquid solvents. In addition to the aforementioned advantages, scCO2 with its easily accessible critical coordinates and specific thermophysical properties, is a versatile “green solvent” with a selective solvation power. It has been widely popularized, even in the chemical industry [6], and is used in particular in extraction processes [7], as well as in other applications, including enhanced oil recovery [8], supercritical chromatography [9] or chemical synthesis [10]. In this latter application, it has been proven that scCO2 could be successfully used in the synthesis of polymeric materials [11]. How-

118

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

ever, if most organic monomers are soluble in scCO2 , almost none of the corresponding polymers are soluble in scCO2 . Therefore, the synthesis of organic polymers in scCO2 is carried out according to heterogeneous polymerization processes. For instance, hydrophilic polymers can be obtained by suspension, e.g. poly(acrylic acid sodium salt) [12], emulsion, e.g. polyacrylamide [13,14], dispersion or precipitation polymerization, e.g. poly(1-vinyl-2-pyrrolidone) [15] or poly(acrylic acid) (PAA) [16]. Among these, linear or cross-linked PAA and its copolymers are important water-soluble systems used as dispersing, thickening or gelling agents in cosmetics and personal care applications. PAA is usually prepared by solution polymerization in aqueous media [17] or heterogeneous polymerization in organic systems like toluene [18], resulting in different polymer sizes and molecular weights. The use of scCO2 as a greener reaction medium is an attractive alternative for the synthesis of hydrophilic polymers like PAA. Romack et al. have reported the successful precipitation polymerization of AA in scCO2 in a batch reactor [16]. The author obtained PAA with a molecular weight of about 150 kg mol−1 after 4 h of reaction. More recently, continuous precipitation polymerization of AA in scCO2 was carried out in Continuous Stirred Tank Reactors (CSTR) [19], following the work done by Charpentier et al. for synthesis of poly(vinylidene fluoride) [20]. Reactions are carried out at 20.7 MPa and at temperatures between 50 and 90 ◦ C for residence times between 12 and 40 min. A wide range of PAA molecular weights was thus obtained; from 5 to 200 kg mol−1 . However, despite these recent works, there is not much quantitative information available in the literature on the phase diagram of AA/CO2 . To the best of our knowledge only the publication of Byun et al. gives the phase diagram of AA in scCO2 at several temperatures [21]. Critical points were obtained by observing the formation of the critical opalescence. The same conclusion can be made for more complex chemical systems including co-solvents such as EtOH or water. The use of EtOH as a co-solvent of scCO2 is quite common and for instance Xu et al. reported on its use in the precipitation polymerization of AA in scCO2 [22]. They obtained PAA of 215–270 kg mol−1 according to the concentration of EtOH used. Finally, there is no published data at all about the quaternary system AA/water/EtOH/CO2 . In order to provide useful information to polymer chemists, the main aim of this article is to give complementary and more accurate data about the phase diagram of AA and CO2 mixtures than previously published, as well as the solubility of ternary or quaternary systems based on mixture of AA, EtOH, water and CO2 . Finally, the partition coefficients in the case of biphasic reaction medium, i.e. liquid water/scCO2 will be given. 2. Experimental It should be noted that experiments involving pressurized fluids can be dangerous and should be performed with the appropriate equipment according to safety conditions recommended by the suppliers. 2.1. Method In order to investigate the phase behaviour of these systems, we have used in-situ IR transmission spectrometry for a quantitative determination of the concentration of all the chemicals solubilized in the CO2 -rich phase. The method, described in detail below, has been successfully applied previously by us and others in phase equilibrium studies for the determination of the CO2 sorption and swelling of liquids [23,24] and polymers [25,26]. One of the main advantages of the method is that molar attenuation coefficients (ε) of C H stretching vibrational modes and combination bands are expected to exhibit little sensitivity to temperature and pressure

conditions [25,27,28]. Moreover, Buback et al. have shown that ε of combination modes of CO2 were almost independent of the CO2 density [29]. Therefore, IR spectrometry allows the determination of the concentration of a compound in a mixture with a statistical error lower than 10 %. Besides, the use of in-situ IR spectrometry allows one to detect any impurities that may be in the mixture, inducing a possible non-negligible effect on the phase behaviour of these systems. However, measuring the IR spectra of the water-rich phase using in-situ IR transmission spectroscopy would require the use of very shorter path length of about 10 ␮m. This might lead to longer time to obtain the equilibrium of the mixtures, due to capillary effects between the windows. For this reason, we have used a high pressure diamond ATR (Attenuated Total Reflection) set-up coupled with a Fourier Transform InfraRed spectrometer (FTIR) in order to analyse the water-rich phase. Indeed, ATR-IR spectrometry is a method well adapted to the analysis of fluids which strongly absorb infrared because of the small penetration depth of the evanescent wave in the sample (less than a few micrometers). Thus, this method has already been applied by other authors [30–33] for studying polymer films, molecular liquids and ionic liquids at highpressure. We have investigated the binary system AA/CO2 , the ternary mixture AA/water/CO2 (10 wt% of AA in water) and three quaternary mixtures AA/water/EtOH/CO2 (10 wt% of AA in water) with initial concentrations of 10, 20 and 30 wt% of EtOH in CO2 using ATR-IR and transmission FTIR spectrometries, and obtained the composition of the liquid AA- or water-rich phases and the CO2 rich phase respectively. We present in this paper the change, as a function of pressure and temperature, of AA, water, EtOH and CO2 concentrations in the two phases of the investigated ternary and quaternary mixtures at 65 and 75 ◦ C for a range of pressures between 0.1 and 20 MPa. We also plot the phase diagram of the AA/CO2 binary system from our measurements and compare our results with the only data available from the literature [21]. 2.2. Chemicals AA (99 %, inhibited with 200 ppm hydroquinone monomethyl ether) was purchased from Aldrich. EtOH absolute was purchased from Merck. CO2 (purity: 99.95 %) was purchased from Air Liquide. All chemicals were used as received without further purification. Ultrapure water of resistivity 18.2 M cm at 25 ◦ C (purified water type 1) was obtained from a filtration device (Synergy, Merck Millipore) coupled to the deionized water system of the laboratory. 2.3. Infrared set-up We have used an infrared spectrometer coupled with two different high pressure cells. The CO2 -rich Phase (CP) was analyzed using a transmission cell (HP-FTIR, High Pressure-Fourier Transform InfraRed), while the Liquid Phase (LP) was analyzed with a reflection cell (HP-ATR, High Pressure-Attenuated Total Reflectance). Fig. 1a shows the complete scheme of the system used. The CO2 tank was connected to a high pressure hand pump to dispense an accurate amount of CO2 into the measurement cell. A rupture disc with a compressive strength up to 50 MPa was installed between the pump and the cell to avoid overpressure. The system was equipped with downstream and upstream pressure-gauges and valves to isolate each part at will. The cell was first filled with the necessary amount of liquid compounds, i.e. AA, water, and/or EtOH, and then CO2 was injected into the cell. The design of the cell determined the choice of the spectrometric technique used, i.e. HP-FTIR or HP-ATR. As shown in Fig. 1b, The infrared transmission experiments were performed using a cubic, home-made, stainless steel cell of

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

119

Fig. 1. Scheme of the high pressure experimental device (a) coupled to the IR spectrometer and the schematic diagrams of the HP cells: (b) Transmission FTIR, (c) ATR-IR.

5.1 mL for high-pressure measurements (up to 50 MPa) [34]. The cell was equipped with two cylindrical silicon windows and the optical path between these windows was 0.49 cm. The sealing was obtained using the unsupported area principle. The windows were positioned on the surface of a stainless steel plug, with a 100 ␮m Kapton® foil placed between the window and the plug to compensate for any imperfections between the two surfaces. Teflon® O-rings were used to ensure the sealing between the plug and the cell body. The cell was heated using cartridge heaters (Watlow Firerod) placed in the periphery of the cell body. A thermocouple located close to a cartridge heater was used to regulate the temperature up to 473 K with a standard uncertainty u(T) = 1 K. The ATR-IR experiments were performed using a diamond ATR accessory suitable for high-pressure measurements (up to 35 MPa) commercially available from Specac Ltd., which has been modified for our measurements (see Fig. 1c). This device has been modified by the addition of a stainless steel cell above the diamond crystal that allows measurements on liquids under high pressure in a closed vessel (volume of 3.2 mL) that can be introduced through the upper port. The cell and the ‘Golden Gate’ accessory are sealed with a gasket in graphite. A magnetically driven stirrer in this cell constantly homogenizes the medium during the experiment. The cell could be heated by a suitable temperature regulator up to 300 ◦ C. Both cells were connected via a stainless steel capillary to a pressurizing system, which permits the regulation of pressure with a standard uncertainty u(P) = 0.1 MPa. The stabilization of the operating conditions was controlled by recording several consecutive spectra. The IR absorption measurements were performed on a ThermoOptek interferometer (type 6700) equipped with a globar source, a KBr/Ge beamsplitter and a DTGS (Deuterated TriGlycine Sulphate) detector in order to investigate the spectral range (400–7500 cm−1 ). Single beam spectra recorded with a 2 cm−1

resolution were obtained after the Fourier transformation of 32 accumulated interferograms.

2.4. Experimental procedure For the determination of the concentration of each component in the CO2 -rich phase, the lower part of the FTIR cell was first filled with a desired volume of liquid in order to get its level well below the incoming infrared beam (see Fig. 1a). In order to determine the concentration in the water-rich phase, the lower part of the high pressure ATR cell was filled with a desirable volume of liquid to be analyzed by the evanescent IR wave coming from the diamond crystal disposed at the bottom of the cell (see Fig. 1b). For AA/CO2 binary system, 1 mL of AA was charged into the FTIR cell and 200 ␮L of AA was added into the ATR cell. Concerning the ternary (AA/water/CO2 ) system, both FTIR and ATR cells were filled with the same initial volume fraction of liquid, 20 vol% (i.e. 1 mL and 630 ␮L of 10 wt% AA in water for the FTIR and ATR cells respectively) as well as regarding the subsequent amount of CO2 injected. Concerning the quaternary (AA/water/EtOH/CO2 ) system, the FTIR and ATR cells were filled as previously described plus the required volume of EtOH. The volume of EtOH added was calculated by considering a single-phase mixture of 10, 20 and 30 wt% of EtOH in CO2 : 0.44, 0.8 and 1.36 mL as well as 0.23, 0.51 and 0.86 mL in the FTIR and the ATR cells respectively. The cells were then heated up to the required temperature. The spectra were recorded for the neat liquid, and then CO2 was added up to the set pressure. The system was kept under isobaric and isothermal conditions for a few minutes. During the stabilization of the operating conditions (weak decrease of pressure between 0.1 and 1 MPa was compensated with the manual pump), consecutive spectra were recorded every 5 min. The equilibrium was considered to be achieved when no change of the spectral bands was noticed,

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

which also shows that no polymerization of AA occurs. The time necessary to reach this equilibrium could be as long as 165 min (10 wt% of AA in water with the addition of 10 wt% of EtOH at 75 ◦ C and 19.5 MPa) and is less and less important as the initial concentration of EtOH increases. The mixture was constantly homogenized during the experiment using a magnetically driven stirrer placed at the bottom of each cell. Some measurements have been performed twice to check for the reproducibility of experiments. In addition, we have performed for the AA/CO2 binary system a third series of measurements that was in agreement with our results and that is reported in the litterature [21].

0.60

2.0 MPa 6.0 MPa 11.0 MPa 12.0 MPa 15.0 MPa

0.45

Absorbance

120

0.30

CO2

AA

0.15

3. Results and discussion 0.00

3.1. Infrared absorption spectra

1000

2.25

1.6 MPa 6.0 MPa 10.0 MPa 11.0 MPa 16.0 MPa

2.00

CO2

Absorbance

1.75 1.50 1.25

AA AA

1.00 0.75 0.50 2500

3000

4500

5000

5500

6000

6500

-1

Wavenumber (cm ) Fig. 2. IR absorption spectra of the AA/CO2 binary mixture in the CO2 -rich phase at 65 ◦ C and at different pressures.

1500

1750 2250

2500

-1

Wavenumber (cm ) Fig. 3. IR absorption spectra of the AA/CO2 binary mixture in the AA-rich phase at 65 ◦ C and at different pressures.

2.0

10 wt% EtOH 20 wt% EtOH 30 wt% EtOH

1.5

Absorbance

3.1.1. AA/CO2 binary mixture The infrared spectra of the AA/CO2 mixture were recorded at two different temperatures (65 and 75 ◦ C) for various pressures ranging between 1.5 and 17 MPa for both the CO2 -rich and the AArich phases. Fig. 2 displays the spectral changes of the IR spectra recorded in the CO2 -rich phase which occur with an increase of CO2 pressure. A number of significant peaks associated with combination bands of CO2 and combination or fundamental modes of AA can be observed. For CO2 , three peaks at 4800, 4950 and 5100 cm−1 can be detected which are assigned to the combination modes 42 + 3 , 1 + 22 + 3 , and 21 + 3 of the CO2 molecule respectively [35]. We recall that the fundamental vibrational modes 1, 2 and 3 are associated to the symetric stretch, the bending mode and the antisymetric stretch of CO2, respectively. For AA, the broad band and the overlapping sharp peaks detected in the range between 2500 and 3200 cm−1 are related to the stretching modes of the O H and C H groups respectively [36]. Moreover, the band at 4500 cm−1 corresponds to combination modes of the COOH group [37] and that one at 6185 cm−1 corresponds to the overtone of the C H stretching vibration of the (R CH ) group of the AA molecule. The intensities of all these contributions increase with CO2 pressure. Fig. 3 illustrates the spectral changes in the wave-number range 950–2500 cm−1 of the AA-rich phase which occur with an increase of CO2 pressure. The characteristic band of dissolved CO2 observed at 2339 cm−1 is assigned to the 3 antisymmetric stretching mode [30]. The other bands observed in the spectrum are associated with AA, for instance, the peak at 984 cm−1 corresponding to the “wagging” vibration of the CH2 group is coupled with the out of plane

1250

AA

0.030 0.025

1.0

0.020 0.015

EtOH

CO2

0.010 0.005 0.000 6120

0.5

6160

6200

6240

6280

H2O 0.0 4250

4500

4750

5000

5250

5500

5750

6000

6250

Wavenumber (cm-1) Fig. 4. IR absorption spectra of the quaternary mixture in the CO2 -rich phase at 75 ◦ C and 19.5 MPa.

deformation of the CH bond [36]. The increase in the CO2 pressure leads to an increase of the CO2 peaks whereas the intensity of the bands of AA decreases. 3.1.2. AA/water/EtOH/CO2 mixtures The infrared spectra of the AA/water/EtOH/CO2 mixtures were recorded at 75 ◦ C and 19.5 MPa for both the CO2 -rich phase and the water-rich phase. Fig. 4 displays the spectral changes of the IR spectra recorded in the CO2 -rich phase with different quantities of EtOH added. The peak of EtOH at 4342 cm−1 is attributed to the combination between the CH2 symmetric stretching mode and a CH3 antisymmetric deformation vibration [38]. The peak at about 5300 cm−1 is assigned to the 2 + 3 combination mode of water [28]. At a first glance, increasing the EtOH quantity leads to an increase of the EtOH peaks and a decrease of the AA peaks at 6185 cm−1 . Fig. 5 displays the spectral changes of the IR spectra recorded in the water-rich phase with different quantity of EtOH added. The peak of EtOH at 2980 cm−1 assigned to CH3 antisymmetric stretch vibration and the peak of H2 O at 1644 cm−1 assigned to the bending mode of water (2 ) can be both observed. As shown in Fig. 5, the intensity of the peaks associated with AA at 984 cm−1 (inset)

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

0.5

AA

0.008

0.4

10 wt% EtOH 20 wt% EtOH 30 wt% EtOH

0.006

Absorbance

0.004 0.002

0.3

0.000 960

970

980

0.2

H2O

0.1

Table 1 Molar attenuation coefficients of different absorption bands associated with AA, CO2 , water and EtOH at 75 ◦ C and 19.5 MPa. CO2 Phase

Wavenumber (cm−1 )

ε × L (L mol−1 )

Peak Area/Height

AA

2665 6185 5090 5300 4342

8.082 0.411 1.839 74.570 0.153

H H A A H

CO2 H2 O EtOH

CO2

990 1000

EtOH

121

AA Phase

Wavenumber (cm−1 )

ε × L (L mol−1 )

Peak Area/Height

AA CO2 H2 O EtOH

984 2339 1644 2980

0.303 1.308 0.002 0.005

A A H H

0.0 1500

2000

2500

0.25

3000

-1

Wavenumber(cm ) Fig. 5. IR absorption spectra of the quaternary mixture in the water-rich phase at 75 ◦ C and 19.5 MPa.

and water at 1644 cm−1 decreases when the quantity of EtOH is increased. 3.2. Data processing for the determination of the concentration of each component in the water- and the CO2 -rich phases

Concentraon of AA (mol.L-1)

1000

65 °C

0.2

75 °C 0.15 0.1 0.05 0 0

The basic relationship between the absorption of electromagnetic waves and the quantity of absorbing material is expressed by the Beer-Lambert law: A = ε × L × c, where A is the sample absorbance; L the optical path length (cm) and c the concentration of the sample (mol L−1 ). Prior to determine the concentration of each component, ε has been determined (see Supporting information for more information). For all IR spectra of the CO2 -rich phase the following peaks were selected for data treatment: for AA the peak at 6185 cm−1 was used when the peak at 2665 cm−1 was saturated. The molar attenuation coefficients for these two bands were determined from spectra measured at 65 ◦ C and 15 MPa from AA solutions of known concentration. Moreover, the molar attenuation coefficient of EtOH was obtained at 75 ◦ C and 15 MPa at 4342 cm−1 and the molar attenuation coefficient of CO2 was obtained at 75 ◦ C and in a pressure range between 2.2 and 12.6 MPa at 5090 cm−1 . We used the molar attenuation coefficient of water at 5300 cm−1 which is known from the literature [28]. In order to estimate the concentration of each component in the water-rich phase, the following calibrations were performed: measurements have been done at 20 ◦ C and at atmospheric pressure with standard mixtures of known concentrations of AA, water and EtOH, while the molar attenuation coefficient of CO2 was obtained at 65 ◦ C and in a pressure range between 5 and 20 MPa. The selected peaks for the AA, CO2 , water and EtOH in all IR spectra of the AA-rich phase were 984, 2339, 1644 and 2980 cm−1 respectively. Although our calibration measurements were determined using neat or two component mixtures, the presence of four components in the case of quaternary mixtures is not expected to have a significant influence on the molar attenuation coefficients. Indeed, we have chosen vibrational and combination modes that are expected to be weakly sensitive to the change of pressure, temperature and solvent effects. Thus, in order to take into account of possible variations of the molar extinction coefficients as well as the error in the absorbance determination and the stability of the spectrometer, we have evaluated a relative standard uncertainty ur (S) = 0.05 on the solubility data using our set-up and our data processing. Table 1 lists all molar attenuation coefficients used in this works.

0.05

0.1

0.15

0.2

0.25

0.3

Density (g/mL) Fig. 6. Concentration of AA in the CO2 -rich phase as a function of the density of CO2 at 65 and 75 ◦ C (binary system).

From the concentration obtained for the AA-CO2 binary mixture, we have determined the phase diagram as a function of pressure at 65 and 75 ◦ C (see Ref. [23] for the details of this method). 3.3. Mutual solubility of AA and CO2 3.3.1. Concentration of AA in the CO2 -rich phase The concentration (or solubility) of a solute in scCO2 is usually reported as a function of the density of neat CO2 as it has been rationalised by the semiempirical-based model, i.e. Chrastil equation [39]: SA = k exp

a T



+b

(1)

where SA is the solubility of the solute A in the CO2 -rich phase (g mL−1 ),  is the density of the of the pure CO2 (g mL−1 ) calculated from tabulated PVT data [40], T is the temperature (K), k is an adjustable factor which represents the average number of CO2 molecules in the solvated complex. The constant a is dependent on the heats of solvation and vaporization of the solute according to: a=

Hsolvation + Hvaporization R

(2)

where R is the universal gas constant. And the constant b in Eq. (1) is dependent on the molecular weights MA and MCO2 of the solute A and the CO2 respectively:





b = ln MA + k × MCO2 + q − k × ln(MCO2 )

(3)

where q is another constant. The evolution of the concentration of AA in the CO2 -rich phase as a function of the density of neat CO2 at two different temperatures is reported in Fig. 6. As expected, at constant temperature the solubility of AA increases with the density of the supercritical medium. These solubilities are reported in

122

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125 16

T (K)

k

a

b

338.15 348.15

2.9001 2.6897

255.9795 309.8110

1.2356 0.9873

16.0 65°C

Concentraon of AA (mol.L-1)

14.0 75°C

12.0 10.0

Concentraon of CO2 (mol.L-1)

Table 2 Fitting parameters obtained for the Charstil model.

6.0

10 8 6

65°C

4

75°C

2 0

2

4

6

8

10

12

14

Pressure (MPa)

y = -1.1697x + 16.771 R² = 0.9807

4.0

12

0

y = -1.0164x + 16.571 R² = 0.9899

8.0

14

Fig. 8. Concentration of CO2 incorporated into the AA-rich phase as a function of the CO2 pressure at 65 and 75 ◦ C (binary system).

2.0

16

0.0 0

2

4

6

8

10

12

14

Pressure (MPa)

a limited range of density because the binary system is no longer in biphasic conditions above a maximum in density. For example above 0.3 g mL−1 at 65 ◦ C, the system becomes a homogeneous phase. Regardless of the temperature, the concentration of AA in the CO2 -rich phase increases as the density of the CO2 increases and does not exceed a value of 0.4 mol L−1 at the highest density investigated. The fits obtained from the Chrastil equation are in rather good agreement with the experimental data, in particular for density higher than 0.15 g mL−1 . The Chrastil parameters under the temperature and pressure range investigated are reported in Table 2. Furthermore, the solubility of AA in CO2 is a bit higher at 75 ◦ C than 65 ◦ C at a constant density of CO2 . This can be explained as increasing the temperature results in the increase of the vapor pressure of the solute, which favours its solubility in the CO2 -rich phase. Finally, as the solubility of AA in the CO2 -rich phase is low below the density maximum, we can assume that the concentration of CO2 in the CO2 -rich phase is equal to that of neat CO2 . 3.3.2. Concentration of AA and CO2 into the AA-rich phase Fig. 7 displays the concentration of the AA in the AA-rich phase as a function of the CO2 pressure at two different temperatures. In both thermal conditions, as the CO2 pressure increases the AA concentration decreases. As expected from the previous result, it may result from the mass transfer of AA from the AA-rich phase to the CO2 -rich one. Though this decrease seems much more important than the increase observed in the CO2 -rich phase, one must consider the evolution of the mass of the solute instead of its molar concentration to conclude. Indeed, the sorption of CO2 into the AArich phase must also be considered since it produces the swelling of the liquid phase and thus the dilution of AA. However, the exact volume of each phase is not known, and thus the variation of the mass of the AA cannot be determined. Furthermore, for a given pressure, the decrease in the concentration of AA is more important at 65 ◦ C than at 75 ◦ C. This might be due to the decrease in density of the CO2 rich-phase upon increasing the temperature at a fixed pressure and therefore a reduction of the solubility of CO2 in the liquid phase (see below). Finally, the concentration of CO2 sorbed into the AA-rich phase is reported in Fig. 8 as a function of the CO2 pressure at 65 and 75 ◦ C. In each condition, the concentration of sorbed CO2 increases with the pressure up to a limit since above a critical pressure the

65 °C

12

Pressure (MPa)

Fig. 7. Concentration of AA in the AA-rich phase as a function of the density of CO2 at 65 and 75 ◦ C (binary system).

45 °C Literature

14

65 °C Literature

10

75 °C 85 °C Literature

8 6 4 2 0 0

20

40

60

80

100

wAA (wt. %) Fig. 9. Phase diagram of the AA/CO2 mixures at 65 and 75 ◦ C. Fits were obtained by using the Shah equatrion and are for illustration purpose only.

binary system becomes homogeneous, as mentioned previously. At a constant pressure within the investigated range, increasing the temperature leads to a decrease of the CO2 quantity incorporated into the AA-rich phase. This behaviour is consistent with the temperature dependence of the concentration of AA in the AA-rich phase mentioned above. Thus, it seems that the CO2 sorption into the AA-rich phase is mainly responsible for the decrease in the concentration of AA in this phase. 3.3.3. Phase diagram of the AA/CO2 mixture From the concentration data reported above, we have determined the phase diagram of the AA/CO2 mixture at 65 and 75 ◦ C within a range of pressure between 1.5 and 13 MPa (see Fig. 9 and Table 3). Whatever the temperature, at low molar fraction of AA a rapid increase of the pressure can be observed, which emphasises the limited solubility of AA in the CO2 -rich phase at low pressure. In contrast, regarding the AA-rich phase, we observe that a significant quantity of CO2 can be inserted into the AA-rich phase at relatively moderate pressures. Indeed, at 75 ◦ C, a weight fraction of CO2 about 0.5 can be reached at pressures around 8 MPa. Finally, a homogeneous phase can be obtained when the pressure is greater than 12.5 or 13.5 MPa at 65 and 75 ◦ C respectively. Hence, to perform a precipitation polymerization of the AA in scCO2 , one must work above these (p,T) coordinates. Our results are compared with previously reported data at 45, 65 and 80 ◦ C by Herbert et al. [21]. The observed tendency as a function of pressure and temperature is in overall good qualitative agreement, and our data obtained at 65 ◦ C by our methodology enables us to obtain the phase diagram in the full molar fraction range. The small observed discrepancies might be related to uncertainties that are associated with the different experimental methods.

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125 0.25

p (MPa)

Weight fraction (%)

65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75

1.58 3.72 6.02 7.46 8.42 9.62 10.39 11.05 10.00 9.00 7.97 7.04 6.00 5.00 4.03 3.05 2.00 2.54 5.06 7.56 8.52 8.95 9.50 10.00 10.10 11.88 13.10 12.00 11.00 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00

1.88 1.26 1.43 1.79 2.27 3.35 4.83 20.68 30.81 39.72 46.78 52.90 60.16 66.71 73.03 78.38 86.02 1.96 1.63 2.08 2.53 3.3 3.79 4.31 5.78 8.18 19.35 26.71 34.16 40.6 47.03 52.75 58.06 63.17 69.49 74.68 80.26 85.29

3.4. Mutual solubility of AA, water, EtOH and CO2 Prior to work on the quaternary system, a ternary mixture of AA/water/CO2 was studied. It consisted of 1 mL of 10 wt% AA in water under various pressures of CO2 from 2.5 to 27 MPa at 75 ◦ C. The infrared spectra of the CO2 -rich phase were recorded using the FTIR cell of 5.1 mL total volume. The solubility of AA and water in the CO2 -rich phase were estimated from the evolution of the intensity of the characteristic peaks of each compound (see Section 3.2 and Supporting information). Fig. 10(A) shows the concentration of AA or water in mol L−1 as a function of pressure, while Fig. 10(B) shows the fraction of AA or water in% which was transferred from the liquid to the CO2 -rich phase according to the initial quantity of AA or water respectively, and assuming a constant volume of 4.1 mL of this latter phase. When the pressure of the medium is less than 11 MPa, the solubilities of AA and water are low, i.e. few percent. At higher pressures, the solubility of AA in the CO2 -rich phase increases as expected according to the pressure. However, at equivalent pressure and temperature, the AA content in the CO2 rich phase is lower than that obtained for the binary system AA/CO2 . Thus, in the presence of water, there is a partitioning of AA between the two phases (i.e. water-rich and CO2 -rich phases) in favor of the water-rich phase. A maximum of 27% of the initial content of AA is thus soluble in the CO2 -rich phase. Regarding the solubility of water in the CO2 -rich phase, it remains low even at the higher pressure investigated here, i.e. <2.5 wt%. Several accurate quantities of EtOH were then added to the 10 wt% of AA aqueous solution. Thus, the initial concentration of AA in the water-EtOH mixture decreases as more EtOH is added to

(A)

0.2

0.15

0.1

0.05

30

0

(B) AA Water

20

10

Solubility (wt%)

T ( C)

0 0

5

10

15

20

25

30

Pressure (MPa) Fig. 10. Solubility at the equilibrium of AA and water in the CO2 -rich phase (ternary system) at 75 ◦ C vs. pressure in mol/L (A) and wt% (mass transferred vs. initial mass) (B).

it. Fig. 11 shows the solubility of AA in CO2 -rich phase obtained at the equilibrium as a function of the initial concentration of AA in the water-rich phase for solutions of 0, 10, 20 and 30 wt% of EtOH. From this figure, one can see that if EtOH is added to the 10 wt% of AA/water mixture, then there is less solubilized AA in the CO2 -rich phase. And if more EtOH is added to the initial AA/water mixture, then the quantity of AA transferred to the CO2 -rich phase decreases. Though AA is diluted in these initial liquid mixtures, EtOH does not facilitate the transfer of the AA to the CO2 rich-phase at 75 ◦ C and 19.5 MPa. Depending on the spectra showed in Figs. 4 and 5, the concentration of each component can be easily obtained in each case (see Table 4), and the partition coefficient (K) of each component

30.00

0 wt % EtOH 10 wt % EtOH

Solubility of AA (wt %)

◦

Solubility (mol.L-1)

Table 3 Experimental data for the AA/CO2 binary mixture system.

123

10.00; 21.84

20 wt % EtOH 20.00

30 wt % EtOH

10.00

7.42; 7.75 6.13; 5.65 4.82; 2.94

0.00 2.00

4.00

6.00

8.00

10.00

12.00

Inial concentraon of AA (wt %) Fig. 11. Influence of the amount of EtOH on the solubility of the AA in the CO2 -rich phase at the equilibrium (mass of transferred AA vs. initial mass in wt%) as a function of its initial concentration in the water-rich one. The (x, y) coordinates are written next to their respective point.

124

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

Table 4 Concentration of each compound in the CO2 - and AA-rich phases at 65 ◦ C and 19.5 MPa according to the amount of EtOH in the quaternary system. Concentration in the CO2 -rich phase (mol L−1 )

EtOH (%)

10 20 30

Concentration in the liquid phase (mol L−1 )

AA

H2 O

EtOH

CO2

AA

H2 O

EtOH

CO2

0.029 0.024 0.015

0.28 0.34 0.37

0.88 1.33 1.70

10.97 10.55 10.07

0.49 0.39 0.27

50.83 38.01 27.78

3.07 4.86 5.65

2.84 4.58 6.55

Table 5 Partition coefficients as a function of the initial weight percentage of EtOH at 75 ◦ C and 19.5 MPa in the quaternary system. EtOH (%)

K (AA)

K (H2 O)

K (EtOH)

K (CO2 )

10 20 30

0.060 0.062 0.056

0.006 0.009 0.013

0.288 0.275 0.301

3.860 2.305 1.538

4.0

0.35 0.30

3.0

0.25

K (H2O)

0.15

K (EtOH)

K

0.20

2.0

K (CO2)

K (AA)

K (CO2)

0.10

1.0

0.05 0.00

0.0 0

5

10

15

20

25

30

35

EtOH (wt %) Fig. 12. Evolution of the partition coefficient K of each component in three quaternary systems at 75 ◦ C and 19.5 MPa as a function of the initial concentration of EtOH.

between CO2 -rich phase and water-rich phase can be calculated according to: K (Solute) =

[Solute]CO2 [Solute]H2 O

(4)

The results are reported in Table 5 and Fig. 12. These results show that upon an increase of the quantity of EtOH, the partition coefficient of water, K (H2 O), slightly increases whereas that of CO2 decreases. This behaviour might be explained by specific interactions occurring between EtOH with CO2 as well as water, promoting the mass transfer of these compounds between the two phases. Indeed, due to the increase of the initial concentration of EtOH in the water phase, EtOH molecules can break up to some extent the strong hydrogen bond network existing in water. And as a result, CO2 can be dissolved more easily in the water-rich phase. On the other hand, due to the increase of the initial concentration of EtOH in the CO2 -rich phase, EtOH acts as a co-solvent which promotes the solubility of water in the CO2 phase, while it limits the solubility of AA in this one. Regarding the case of AA and EtOH, their respective partition coefficients are almost constant upon an increase of the initial quantity of EtOH: about 30% for EtOH and 6% for AA at 75 ◦ C and 19.5 MPa. However, the concentration of EtOH increases in both phases whereas that of AA decreases in both phases. Therefore, the main result of this study is that the addition of EtOH into a biphasic ternary mixture AA-water-CO2 promotes the mutual solubility of water and CO2 and leads to a decrease of the concentration of AA, in particular in the aqueous phase.

4. Conclusion The combination of in situ transmission FTIR and ATR-IR methods allowed us to study the thermodynamic equilibrium of AA, water, EtOH and CO2 mixtures as a function of temperature for a wide range of pressure. For the binary system, the solubility of AA in scCO2 has been quantified. The solubility of AA increases with the pressure at constant temperature, or with the temperature at constant density. From these data, we have established the phase diagram of the binary system AA/CO2 in the full weight fraction range at 65 and 75 ◦ C. The AA/CO2 mixture at 75 ◦ C becomes homogeneous at moderate pressure, i.e. 13.5 MPa. The study on the ternary system AA/water/CO2 allowed us to determine the solubility of an aqueous solution of AA (10 wt%) in the CO2 -rich phase as a function of pressure at 75 ◦ C. The water added to the binary mixture is almost not soluble in CO2 , but it acts as a hinderer which limits the transfer of AA from the water-rich phase to the CO2 -rich one. Furthermore, CO2 swells the water-rich phase and then dilutes the AA in the liquid phase. Despite these observations, the main conclusion of the study of this ternary mixture is that droplets of aqueous solution of AA surrounding by a CO2 -rich continuous phase can be used as a monomer reservoir in a heterogeneous polymerization system. In other words, if one wants to control the transfer of AA over time and thus the monomer concentration in a reactive CO2 -rich phase, the use of aqueous droplets could be an effective synthetic pathway. Then, the quaternary system AA/water/EtOH/CO2 was studied since EtOH is a co-solvent, commonly used with CO2 . For instance, it could be used in addition to the ternary system in order to allow the solubilisation and the injection of the initiator and/or an organic comonomer in the CO2 -rich continuous phase. In this work, the partition coefficient of each compound in three quaternary mixtures with an initial concentration of EtOH of 10, 20 and 30 wt% have been determined. It has been found that the addition of EtOH to the biphasic ternary mixture AA-water-CO2 favours the mutual solubility of water and CO2 and leads to the dilution of AA in both phases. But it does not fully hamper the transfer of AA from the possible droplet reservoir to the CO2 -rich continuous phase; EtOH moderates this transfer. Hence, for a biphasic reaction system where the mutual solubility of the water and CO2 should be as low as possible and the transfer of the AA from the aqueous phase to the CO2 -rich one must be as important as possible, the quantity of EtOH in the mixture should be minimized. This possible droplet-based heterogeneous polymerization process will be studied in more detail.

Acknowledgment The authors acknowledge the “Region Aquitaine” for the PhD fellowship of H. Chen-Jolly and Solvay for their strong support.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.supflu.2016.05. 014.

H. Chen-Jolly et al. / J. of Supercritical Fluids 116 (2016) 117–125

References [1] J.A. Hyatt, Liquid and supercritical carbon dioxide as organic solvents, J. Org. Chem. 49 (1984) 5097–5101. [2] R. Noyori, Pursuing practical elegance in chemical synthesis, Chem. Commun. (2005) 1807–1811. [3] M. Besnard, T. Tassaing, Y. Danten, J.M. Andanson, J.C. Soetens, F. Cansell, et al., Bringing together fundamental and applied science: the supercritical fluids route, J. Mol. Liq. 125 (2006) 88–99. [4] P. Raveendran, Y. Ikushima, S.L. Wallen, Polar attributes of supercritical carbon dioxide, Acc. Chem. Res. 38 (2005) 478–485. [5] R.B. Gupta, J.J. Shim, Solubility in Supercritical Carbon Dioxide, CRC Press, Boca Raton, 2007. [6] Z. Knez, E. Markocic, M. Leitgeb, M. Primozic, M. Knez Hrncic, M. Skerget, Industrial applications of supercritical fluids: a review, Energy 77 (2014) 235–243. [7] M.M.R. De Melo, A.J.D. Silvestre, C.M. Silva, Supercritical fluid extraction of vegetable matrices: applications, trends and future perspectives of a convincing green technology, J. Supercrit. Fluids 92 (2014) 115–176. [8] A.-C. Aycaguer, M. Lev-On, A.M. Winer, Reducing carbon dioxide emissions with enhanced oil recovery Projects: a life cycle assessment approach, Energy Fuels 15 (2001) 303–308. [9] G. Guiochon, A. Tarafder, Fundamental challenges and opportunities for preparative supercritical fluid chromatography, J. Chromatogr. A. 1218 (2011) 1037–1114. [10] E.J. Beckman, Supercritical and near-critical CO2 in green chemical synthesis and processing, J. Supercrit. Fluids 28 (2004) 121–191. [11] C. Boyère, C. Jérôme, A. Debuigne, Input of supercritical carbon dioxide to polymer synthesis: an overview, Eur. Polym. J. 61 (2014) 45–63. [12] Y.A. Hussain, T. Liu, G.W. Roberts, Synthesis of cross-linked, partially neutralized Poly(Acrylic acid) by suspension polymerization in supercritical carbon dioxide, Ind. Eng. Chem. Res. 51 (2012) 11401–11408. [13] F. Adamsky, E. Beckman, Inverse emulsion polymerization of acrylamide in supercritical carbon dioxide, Macromolecules 27 (1994) 312–314. [14] S. Villarroya, K.J. Thurecht, S.M. Howdle, HRP-mediated inverse emulsion polymerisation of acrylamide in supercritical carbon dioxide, Green Chem. 10 (2008) 863–867. [15] T. Carson, J. Lizotte, J.M. Desimone, Dispersion polymeriza- tion of 1-vinyl-2-pyrrolidone in supercritical carbon dioxide, Biotechnol. Bioeng. 33 (2000) 1917–1920. [16] T.J. Romack, E.E. Maury, J.M. DeSimone, Precipitation polymerization of acrylic acid in supercritical carbon dioxide, Macromolecules 28 (1995) 912–915. [17] R.A. Scott, N.A. Peppas, Kinetic study of acrylic acid solution polymerization, AIChE J. 43 (1997) 135–144. [18] C. Bunyakan, D. Hunkeler, Precipitation polymerization of acrylic acid in toluene. I: synthesis, characterization and kinetics, Polymer 40 (1999) 6213–6224. [19] T. Liu, J.M. DeSimone, G.W. Roberts, Kinetics of the precipitation polymerization of acrylic acid in supercritical carbon dioxide: the locus of polymerization, Chem. Eng. Sci. 61 (2006) 3129–3139. [20] P.A. Charpentier, Continuous polymerization in supercritical carbon dioxide: chian-growth precipitation polymerizations, Macromolecules 32 (1999) 5973–5975. [21] H. Byun, B.M. Hasch, M.A. McHugh, Phase behavior and modeling of the systems CO2-acetonitrile and CO2-acrylic acid, Fluid Phase Equilib. 115 (1996) 179–192.

125

[22] Q. Xu, B. Han, H. Yan, Effect of cosolvents on the precipitation polymerization of acrylic acid in supercritical carbon dioxide, Polymer 42 (2001) 1369–1373. [23] S. Foltran, E. Cloutet, H. Cramail, T. Tassaing, In situ FTIR investigation of the solubility and swelling of model epoxides in supercritical CO2 , J. Supercrit. Fluids 63 (2012) 52–58. [24] E. Girard, T. Tassaing, C. Ladavie`re, Distinctive features of solubility of RAFT/MADIX-Derived partially trifluoromethylated poly (vinyl acetate) in supercritical CO2 , Macromolecules 45 (2012) 9674–9681. [25] T. Guadagno, S.G. Kazarian, High-pressure CO2 -expanded solvents: simultaneous measurement of CO2 sorption and swelling of liquid polymers with in-Situ near-IR spectroscopy, J. Phys. Chem. B 108 (2004) 13995–13999. [26] P. Vitoux, T. Tassaing, F. Cansell, S. Marre, C. Aymonier, In situ IR spectroscopy and ab initio calculations to study polymer swelling by supercritical CO2 , J. Phys. Chem. B. 113 (2009) 897–905. [27] F. Palombo, T. Tassaing, Y. Danten, M. Besnard, Hydrogen bonding in liquid and supercritical 1-octanol and 2-octanol assessed by near and midinfrared spectroscopy, J. Chem. Phys. 125 (2006) 094503. [28] R. Oparin, T. Tassaing, Y. Danten, M. Besnard, Structural evolution of aqueous NaCl solutions dissolved in supercritical carbon dioxide under isobaric heating by mid and near infrared spectroscopy, J. Chem. Phys. 122 (2005) 094505. [29] M. Buback, Kinetics and selectivity of chemical processes in fluid solutions, Pure Appl. Chem. 68 (1996) 1501–1507. [30] N. Flichy, S. Kazarian, An ATR-IR study of poly (dimethylsiloxane) under high-pressure carbon dioxide: simultaneous measurement of sorption and swelling, J. Phys. Chem. B 106 (2002) 754–759. [31] Irene Pasquali, J.-M. Andanson, Sergei G. Kazarian, R. Bettini, Measurement of CO2 sorption and PEG 1500 swelling by ATR-IR spectroscopy, J. Supercrit. Fluids 45 (2008) 384–390. [32] J.-M. Andanson, F. Jutz, A. Baiker, Supercritical CO2/ionic liquid systems: what can we extract from infrared and Raman spectra? J. Phys. Chem. B 113 (2009) 10249–10254. [33] M.W.G. Alexander, A. Novitskiy, Jie Ke, Gurbuz Comak, Martyn Poliakoff, A modified golden gate attenuated total reflection (ATR) cell for monitoring phase transitions in multicomponent fluids at high temperatures, Appl. Spectrosc. 65 (2011) 885–891. [34] P. Chambon, E. Cloutet, H. Cramail, T. Tassaing, M. Besnard, Synthesis of core-shell polyurethane-polydimethylsiloxane particles in cyclohexane and in supercritical carbon dioxide used as dispersant media: a comparative investigation, Polymer 46 (2005) 1057–1066. [35] L. Ma, L. Zhang, J.-C. Yang, X.-M. Xie, Improvement in the water-absorbing properties of superabsorbent polymers (acrylic acid-co-acrylamide) in supercritical CO2 , J. Appl. Polym. Sci. 86 (2002) 2272–2278. [36] J. Umemura, S. Hayashi, Infrared spectra and molecular configurations of liquid and crystalline acrylic acids, Bull. Inst. Chem. Res. 52 (1974) 585–595. [37] J. Dong, Y. Ozaki, K. Nakashima, Infrared, Raman, and near-infrared spectroscopic evidence for the coexistence of various hydrogen-bond forms in poly (acrylic acid), Macromolecules 30 (1997) 1111–1117. [38] J.-P. Perchard, M.-L. Josien, Vibrational spectra of 12 isotopic species of monomeric ethanols, J. Chim. Phys. Phys. - Chim. Biol. 65 (1968) 1834–1855. [39] J. Chrastil, Solubility of solids and liquids in supercritical gases, J. Phys. Chem. 86 (1982) 3016–3021. [40] NIST, http://webbook.nist.gov/chemistry/ (accessed 09/2014).