Experimental Measurements and Correlation of the Solubility of Three

Article pubs.acs.org/jced

Experimental Measurements and Correlation of the Solubility of Three Primary Amides in Supercritical CO2: Acetanilide, Propanamide, and Butanamide José P. Coelho,*,† Greta P. Naydenov,‡ Dragomir S. Yankov,‡ and Roumiana P. Stateva‡ †

Chemical Engineering and Biotechnology Research Center, Instituto Superior de Engenharia de Lisboa (ISEL), 1959-007 Lisboa, Portugal ‡ Institute of Chemical Engineering, Bulgarian Academy of Sciences, Sofia 1113, Bulgária ABSTRACT: Solubilities of three primary amides, namely, acetanilide, propanamide, and butanamide, in supercritical carbon dioxide were measured at T = (308.2, 313.2, and 323.2) K over the pressure range (9.0 to 40.0) MPa by a flow type apparatus. The solubility behavior of the three solids shows an analogous trend with a crossover region of the respective isotherms between (12 to 14) MPa. The solubility of each amide, at the same temperature and pressure, decreases from propanamide to acetanilide. Pure compound properties required for the modeling were estimated, and the solubilities of the amides were correlated by using the Soave−Redlich−Kwong cubic equation of state with an absolute average relative deviation (AARD) from (1.3 to 6.1) %.

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INTRODUCTION Knowledge of the solubility of pure solid substances in a supercritical solvent (e.g., supercritical CO2) is essential for evaluating the viability of supercritical separation processes and for their subsequent design. Hence, both solubility measurements and thermodynamic modeling are of interest to researchers and process engineers and are receiving considerable attention in the literature. A recent review paper1 outlines the wide variety of models that have been applied to correlate the solubility of a solid in a supercritical solvent. Additional details about the models, their advantages and range of applicability can be found in refs 2 to 7, to mention just a few. The most common choice of researchers, however, are equations of state (both cubic and noncubic) because of their simplicity, easy implementation in a software environment, steady convergence performance, comparatively small number of pure component parameters required, and robustness in correlating experimental solubility data.1 Available solid solubility data in supercritical fluids (SCFs) has been summarized and discussed by numerous authors. A recent extensive review on the solubility of over 380 different pure solid compounds, without or with cosolvents, and of 29 binary, two ternary, and one multicomponent solid systems in sub- and supercritical fluids that were published in the literature between 2005 and 2010, should be mentioned particularly.8 The general conclusion of all review papers examined is that new reliable experimental data and proper correlation models for specialty chemicals are still very much needed. The aim of this work is to present new extensive data on the solubility of three primary amides, namely, N-phenylacetamide © 2013 American Chemical Society

(N-phenylethanamide, acetanilide), propanamide (n-propylamide), and butanamide (n-butylamide, butyramide), in supercritical CO2 (SC CO2). Amides have versatile applications including making agrochemicals, dyes, pigments, pharmaceuticals including vitamins, and stabilizers for PVC and polyester. For example, butanamide, an important chemical commodity, is used primarily as a pharmaceutical intermediate; it can be used also for the preparation of β-amodoorganotin compounds.9 Furthermore, butanamide was chosen and tested as an example of a neutral compound that might exhibit some of the growth inhibitory effects on DNA synthesis in hepatoma cells and on cellular proliferation.10 Acetanilide’s applications are numerous. It is used in: the intermediation in rubber accelerator synthesis; dyes and dye intermediate synthesis, and camphor synthesis;11 the production of 4-acetamidobenzenesulfonyl chloride, a key intermediate for the manufacture of sulfa drugs. Acetanilide can also act as a precursor in the synthesis of penicillin and other pharmaceuticals11 and also as an inhibitor in hydrogen peroxide and can stabilize cellulose ester varnishes. Most importantly, acetanilide was the first aniline derivative serendipitously found to possess analgesic as well as antipyretic properties and was quickly introduced into medical practice under the name of Antifebrin in 1886.11 Nowadays, however, acetanilide, because Received: April 15, 2013 Accepted: June 5, 2013 Published: June 18, 2013 2110

dx.doi.org/10.1021/je400357t | J. Chem. Eng. Data 2013, 58, 2110−2115

Journal of Chemical & Engineering Data

Article

solubility ranking is as follows: acetanilide < butanamide < propanamide; namely, propanamide, the component with the lowest molar mass of the three, exhibits the highest solubility in supercritical CO2. There is a crossover of the respective isotherms for each of the three amides which occurs in the interval (12 to 14) MPa approximately. The solubility behavior of acetanilide and of butanamide follows the normal pattern exhibited by typical solids. The same holds for propanamide at T = (308.2 and 313.2) K. However, at T = 323.2 K, and at pressures higher than about p = 30 MPa its solubility deviates from that pattern and increases steeply. The change in propanamide solubility could be a result of a change in the phase behavior of the system. In other words, propanamide might exhibit considerable melting point depression in the presence of CO2, and hence a temperature of 323.2 K could already be above the upper critical end point (UCEP) for the binary. Furthermore, pressures higher than about 30.0 MPa might be above the solid−liquid−gas line and consequently place the system in a vapor−liquid rather than in the solid+SC fluid region. There are cases described in the literature for solids with very pronounced melting point depression. For example, a melting point depression of up to 70 °C for 2-naphthol in the presence of methanol as a solvent16 and a significant melting point depression for naphthalene (normal melting point of naphthalene is 353.65 K) in the presence of CO2, yielding UCRP of 333.25 K, have been reported.17 Therefore, the above hypothesis might be feasible. Still, as pointed out, the solubility of propanamide in SC CO2 was not studied previously, and hence there are no data available to compare our results with. Moreover, there was not an optical view cell to observe the phase behavior of the sample in our experiments. The solubility of butanamide was also not studied previously, while that of acetanilide, in a similar to ours range of temperatures but at pressures up to 22 MPa only, was measured and reported.13 In general, the agreement between our data and those of ref 13 is good; further discussion will be presented in the following sections.

of its unacceptable toxic effects, is no longer used as a drug in its own right. In a previous study we measured and correlated the solubilities of ethanamide (acetamide) and 2-propenamide (acrylamide) in SC CO2.12 In this contribution we extend our research to acetanilide, propanamide, and butanamide and report their solubility in SC CO2 at T = (308.2, 313.2, and 323.2) K and at pressures ranging from (9 to 40) MPa. To the best of our knowledge, the solubilities neither of propanamide nor of butanamide in SC CO2 had been investigated previously, and no such data are available in the open literature. The solubility of acetanilide in SC CO2, as far as we are aware, was measured in a similar range of temperatures and at pressures up to 22 MPa.13 To correlate the solubility of the three primary amides, we applied the Soave−Redlich−Kwong cubic equation of state (SRK CEoS) with the two-binary interaction-parameter-perpair (2PWDW) version of the van der Waals one fluid mixing rule. Furthermore, those of the properties of the three amides, which are not available but are required for the modeling, were estimated and reported.

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EXPERIMENTAL SECTION Chemicals. The chemical name, supplier, and purity of the substances studied are presented in Table 1. Table 1. Name, Source, and Sample Purity of Chemicals chemical name

source

initial mole fraction purity

purification method

carbon dioxide Nphenylacetamide propanamide butanamide

Air Liquid Merck

0.99995 0.99

none none

Merck Fluka

0.98 0.98

none none

Equipment and Procedure. The supercritical fluid extraction experiments were performed in a flow apparatus. This equipment allows carrying out studies at a temperature up to 393.2 K and pressures up to 60.0 MPa (Applied Separations, Spe-ed SFE). A detailed description of the equipment is given elsewhere.12,14,15 The uncertainties of the temperature and pressure measurements were ± 1 K and ± 1.0·105 Pa, respectively. The gravimetric measurements were performed on an analytical balance with an uncertainty of ± 0.1 mg with a coverage factor of 2. The total volume of CO2 was determined with a mass flow meter and totalizer (MV+Tot) from Alicat Scientific, model M-5 SLPM-D. The uncertainty associated with volume measurements was ± 0.5 %. Solubility is expressed as a ratio of the mass of the solid obtained versus the amount of CO2 (volume measured at normal pressure and temperature). Three replicates were performed at each experimental condition, and the solubility was obtained as an average of these results. Relative standard uncertainties ur(y) = 0.05 (5 %) can be used for all of the data.

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THERMODYNAMIC MODELING

Solid-Supercritical Fluid Equilibria. To correlate the solubility of the three solid amides in SC CO2, within the terms of an EoS framework, the following standard formulation, based on the equifugacity condition for a solute is considered: f S (T , P ) = f G (T , P , y , v )

(1)

where f S is the fugacity of the solute in the pure solid phase, f G is fugacity of the solute in the fluid-phase solution, y = (y1, y2, ..., yNc)T is the vector of fluid-phase mole fractions, and v is the molar volume of the fluid. Additional relationships that must be satisfied are the summation to one of the fluid-phase mole fractions. In our case of binary SCF (1) + solid (2) equilibria, the fugacity of the solute in the supercritical phase is calculated according to:

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EXPERIMENTAL RESULTS The solubilities of acetanilide, propanamide, and butanamide in SC CO2 were measured at T = (308.2, 313.2, and 323.2) K and over the pressure range (9 to 40) MPa, and the experimental data obtained are presented in Table 2. The solubility of each of the three primary amides studied increases with pressure at a constant temperature, and the

f 2G = y2 Pφ2G

(2)

where φG2 is the fugacity coefficient, and y2 is the solubility (mole fraction) of the solute in the SCF. 2111



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Table 2. Solubility for Acetanilide (2), Propanamide (2), and Butanamide (2) in SC CO2 (1), Expressed as Mole Fractions, y2, from T = (308.2 to 323.2) Ka

a

Standard uncertainties u: u(T) = 0.1 K; u(P) = 0.1 MPa. Relative standard uncertainty ur(y) = 0.05.

In most cases of practical interest, φs2 is nearly equal to unity (that is a safe assumption taking into consideration that the saturated vapor of the solid solute-SCF (pure) system behaves as an ideal gas); however, φG2 is always far removed from unity and can produce very large enhancement factors (represented by the second multiplier in eq 4). Therefore, the solute solubility is primarily a function of the solid solute pure compound physical properties, the system temperature and pressure, and the fugacity coefficient of the solid solute in the SCF. The calculation of the latter requires a thermodynamic model, and we have chosen the Soave− Redlich−Kwong equationa widely used and reliable representative of the cubic equations of state family. The SRK-EoS Model. The SRK EoS18 was applied with the 2PWDW version of the van der Waals one fluid mixing rule. The expressions for the cross-energy and for the crosscovolume parameters are, respectively:

If the solid vapor pressure is used as the reference fugacity of the solid, then f 2S is: f 2S = P2sφ2s exp

∫P

P

s 2

v2SdP RT

(3)

where Ps2 (T) is the sublimation (vapor) pressure of the pure solid, φs2 is the fugacity coefficient at sublimation pressure, and vs2 is the molar volume of the solid, all at temperature T. Since the solid phase is assumed to be pure and applying the assumption that the molar volume of the solid solute can be treated as a constant with respect to pressure, the following holds for the fugacity of the solute in the solid state: ⎛ v s(P − P2s) ⎞ f 2S = P2sφ2s exp⎜ 2 ⎟ ⎝ ⎠ RT

(3a)

Then, taking into consideration the thermodynamic equilibrium condition (eq 1), the solubility of the solute in the SC CO2, expressed in mole fractions, is calculated according to: y2 =

s s s P2s φ2 exp[v2(P − P2)/RT ] P φ2G

(4) 2112

aij = (aiiajj)0.5 (1 − kij)

(5)

⎛ bii + bjj ⎞ bij = ⎜ ⎟(1 − lij) ⎝ 2 ⎠

(6)



Article

In the above, kij and lij are the energy and size binary interaction parameters, respectively. The unlike-pair interaction parameters, kij and lij, the only adjustable parameters in our case, were estimated by minimizing the absolute average deviation between the experimental and calculated solubility (in mole fractions) of the three solid primary amides in SC CO2 applying a standard optimization procedure. The kij and lij temperature-dependent values obtained are shown in Table 3.

respectively. The solid molar volumes were estimated applying the method of Bondi23 and are also given in Table 3. The availability of reliable experimental data for the critical properties of primary amides is especially a problem as those data are very limited. Although the three amides of interest to our study are of a small to medium molar mass, there are no experimental data for their critical temperatures and pressures available. Thus, those parameters have to be estimated. For acetanilide we used the values reported in Huang et al.,13 while for propanamide and butanamide the values were estimated by us applying the methods of Wakeham et al.24 and Brauner et al.25 The pure components’ critical parameters are also shown in Table 3. The critical parameter values obtained for propanamide and butanamide can be compared neither against other experimental data nor against data estimated by other authors, so we apply as an assessment tool of the reliability of the properties estimated the generalized semitheoretical expression advocated by Zbogar et al.:26

Table 3. Acetanilide, Propanamide, and Butanamide Pure Compound Parameters

Tc/pc = 9.0673 + 0.43309(Q w1.3 + Q w1.95)

(7)

where Tc is in Kelvin and pc is in bar. The dimensionless parameter Qw is a measure of the van der Waals molecular surface area and is calculated as the sum of the group area parameters, Qk Qw =

∑ νkQ k k

a

Reference 19. bEstimated by us. cReference 20. Reference 22 . fReference 13

e

(8)

where vk is the number of times group k appears in the molecule. The group area parameters Qk are available in the UNIFAC tables. The ratios for the critical properties estimated by us for propanamide and butanamide are 13.827 and 15.013, respectively. The above values are in an excellent agreement with those calculated according to the Zbogar et al.26 semiempirical expression, namely, 13.461 and 15.136, respectively. A very good agreement is obtained also for the critical properties of acetanilide, advocated in ref 13the calculated ratio is 18.346 vs 18.174 of Zbogar et al.26 Robustness of Solution. As pointed out by Fornari et al.,27 there are two computational pitfalls that can be encountered in the calculation of solid−fluid equilibrium: The first is a result of the fact that at certain values of temperature, pressure, and binary interaction coefficients, multiple solutions to the equi-fugacity condition (eq 1) can exist. If a successive substitution or some similar approach for solving the equi-fugacity equation is applied then, in principle, only the smallest solubility root will be located. Hence any larger values, if present, that satisfy the equi-fugacity equation might be missed. Second, because the equi-fugacity condition is just a necessary but not sufficient condition for stability, a wrong conclusion can be made about the actual phase equilibria of the system if solutions to eq 1 are not tested for global thermodynamic phase stability. The solution process we implement is organized in such a way that when the EoS has three real roots only the one corresponding with the lowest Gibbs energy is accepted; the other two are rejected. This is extremely important particularly in regions of the thermodynamic phase space where the EoS has multiple roots (e.g., near a three phase locus, critical end points, etc). Furthermore, a stability analysis routine is

d

Reference 21.

Pure-Component Properties. To calculate the solubility of the three solid primary amide data about their fusion and critical properties is required, namely: enthalpy of fusion (ΔHfus), fusion temperature/melting temperature (Tm), solid molar volume, sublimation pressure and critical parameters (Tc and pc). The experimentally measured fusion properties for the three amides are the triple-point temperatures,19 heats of fusion19,20 and enthalpies of sublimation.19,21,22 The data are reported in Table 3. Taking into consideration that the difference between the triple-point temperature and the normal melting temperature of an organic substance is negligible in Table 3, we have shown only the former temperature values for the three amides. The sublimation pressures at the three temperatures of interest to the experiment are calculated integrating the Clayperon equation from the triple-point temperature TTP and pressure pTP and assuming a negligible dependence of the sublimation enthalpy with respect to temperature. The triple-point pressures are calculated from the EoS for each of the solid amides, and the values obtained are given in Table 3, 2113



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interwoven in the algorithm’s environment which assures that the phase configuration with the minimum Gibbs energy is determined. The robustness and efficiency of the algorithm and numerical routines employed as well as their practical implementation are outlined in detail elsewhere (see for example refs 27 and 28) and will not be presented here.

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RESULTS AND DISCUSSION Figures 1 to 3 display the agreement between the measured and correlated solubility for the three amides, namely, acetanilide,

Figure 3. Solubility of butanamide in SC CO2. Symbols represent experimental solubility; lines represent the solubility, correlated by the SRK EoS. ◆, measured, and , correlated solubility at T = 308.2 K; ◆, measured, and ---, correlated solubility at T = 313.2 K; ■, measured, and   , correlated solubility at T = 323.2 K.

Furthermore, the crossover of the respective isotherms of both experiments occurs at approximately 12.5 MPa. Going back to the unexpected solubility behavior exhibited by propanamide at T = 323.2 K and at pressures higher than 30 MPa, we believe that it is not a result of a change in the type of the binary phase behaviorfrom solid-SCF to vapor−liquid. The grounds for this conclusion are that the modeling results (which, as emphasized in the previous section, correspond with the thermodynamically stable solutions) are consonant with the experimental data. Further discussion at this stage is not possible as very few data on the solubility of primary amides in SC CO2 in general are available to draw any substantiated conclusions. The absolute average relative deviations (AARD) between the solubility values measured experimentally, yiexp, and those correlated by the model, yicalc, for each of the three components at the temperatures of interest to the experiment are also presented in Table 4. Obviously, the SRK cubic EoS demonstrates very good correlative abilities with AARD around and lower than 6 % to represent the solubility of the primary amides examined.

Figure 1. Solubility of acetanilide in SC CO2. Symbols represent experimental solubility; lines represent the solubility, correlated by the SRK EoS. This work: ◆, measured, and , correlated solubility at T = 308.2 K; ▲, measured, and ---, correlated solubility at T = 313.2 K; ■, measured, and   , correlated solubility at T = 323.2 K. Experimental solubility results of Huang et al.:13 ∗, T = 308.2 K; ×, T = 318.2 K; +, T = 328.2 K.

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CONCLUSIONS This study presents novel solubility data for three primary amidesacetanilide, propanamide, and butanamideat T = (308.2, 313.3, and 323.2) K and over the pressure range (9.0 to 40.0) MPa. The solubility values, in mole fractions, obtained are in the range (0.137 to 56.85)·10−4, with propanamide exhibiting the highest solubility in SC CO2. In linear amides the introduction of one CH3 group decreases the solubility values from propanamide to butanamide more than two times in the overall region of measurements. The solubility isotherms of the three compounds exhibit a crossover behavior at about 12.5 MPa. Further, the overall solubility of acetanilide measured in this study is in good agreement with the data previously reported.13 The SRK cubic EoS with two binary adjustable parameters was applied to correlate the solubility of the amides and demonstrated good correlative power, particularly after the crossover region.

Figure 2. Solubility of propanamide in SC CO2. Symbols represent experimental solubility; lines represent the solubility, correlated by the SRK EoS. ◆, measured, and , correlated solubility at T = 308.2 K; ▲, measured, and ---, correlated solubility at T = 313.2 K; ■, measured, and   , correlated solubility at T = 323.2 K.

propanamide, and butanamide, respectively, at the temperatures of interest to the experiment. Figure 1 shows as well the experimental data obtained at T = (308.2; 318.2 K; 328.2 K), p = (10.44 to 22.50) MPa and reported in ref 13. Though the temperature and pressure ranges are different, still the general trend observed in both series of experiments is analogous, and the qualitative results obtained are consistent with each other (Figure 1). The agreement, particularly at T = 308.2 K, the temperature at which solubility of acetanilide was measured in both experiments, is very good. 2114



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Table 4. Binary Interaction Parameters (kij, lij) and AARD for the Systems CO2 + Acetanilide, CO2 + Propanamide, and CO2 + Butanamide at the Temperatures of the Experiment CO2 + acetanilide

■

CO2 + propanamide

kij

lij

AARD/%

kij

lij

AARD/%

kij

lij

AARD/%

308.2 313.2 323.2

0.116 0.125 0.137

0.033 0.035 0.041

5.4 4.6 3.9

0.105 0.112 0.129

0.013 0.023 0.047

1.3 3.0 3.2

0.108 0.124 0.133

0.021 0.028 0.044

4.3 6.1 2.2

(15) Coelho, J. P.; Stateva, R. P. Solubility of Red 153 and Blue 1 in Supercritical Carbon Dioxide. J. Chem. Eng. Data 2011, 56, 4686− 4690. (16) Lemert, R. M.; Johnston, K. M. Solid-Liquid-Gas Equilibria in Multicomponent Supercritical Fluid System. Fluid Phase Equilib. 1989, 45, 265−286. (17) Lamb, D. M.; Barbara, T. M.; Jonas, J. NMR Study of Solid Naphthalene Solubilities in Supercritical Carbon Dioxide Near the Upper Critical End Point. J. Phys. Chem. 1986, 90, 4210−4215. (18) Soave, G. Equilibrium constants from a modified RedlichKwong equation of state. Chem. Eng. Sci. 1972, 27, 1107−1203. (19) Thermophysical Properties of Fluid System, National Institute of Standards and Technology; http://webbook.nist.gov/chemistry/ name-ser.html (accessed Jan 2013). (20) Abate, L.; Badea, L.; Blanco, I.; Gatta, G. D. Heat Capacities and Enthalpies of Solid-Solid Transitions and Fusion of a Series of Eleven Primary Alkylamides by Differential Scanning Calorimetry. J. Chem. Eng. Data 2008, 53, 959−965. (21) Aihara, A. Estimation of the Energy of Hydrogen Bonds Formed in Crystals. III. Amides. Bull. Chem. Soc. Jpn. 1960, 33, 1188−94. (22) Perlovitch, G. L.; Volkova, T. V.; Bauer-Brandl, A. Towards an Understanding of the Molecular Mechanism of Solvation of Drug Molecules: A Thermodynamic Approach by Crystal Lattice Energy, Sublimation, and Solubility Exemplified by Paracetamol, Acetanilide, and Phenacetin. J. Pharm. Sci. 2006, 95, 2158−69. (23) Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441−451. (24) Wakeham, W. A.; Cholakov, G. St.; Stateva, R. P. Liquid Density and Critical Properties of Hydrocarbons Estimated from Molecular Structure. J. Chem. Eng. Data 2002, 47, 559−570. (25) Brauner, N.; Stateva, R. P.; Cholakov, G. St.; Shacham, M. Structurally “Targeted” Quantitative Structure-Property Relationship Method for Property Prediction. Ind. Eng. Chem. Res. 2006, 45, 8430− 8437. (26) Zbogar, A.; Da Silva Lopes, F. V.; Kontogeorgis, G. M. Approach suitable for screening estimation methods for critical properties of heavy compounds. Ind. Eng. Chem. Res. 2006, 45, 476−480. (27) Fornari, T.; Luna, P.; Stateva, R. P. The vdW EoS Hundred Years Later, Yet Younger than Before. Application to the Phase Equilibria Modeling of Food-Type Systems for a Green Technology. J. Supercrit. Fluids 2010, 55, 579−593. (28) Sovova, H.; Stateva, R. P.; Galushko, A. A. High-pressure Equilibrium of Menthol + CO2. J. Supercrit. Fluids 2007, 41, 1−9.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+351) 218 317 149. Fax: (+351) 218 317 151. Funding

G.P.N., D.S.Y., and R.P.St. are thankful for the financial support from the Bulgarian Science Fund, Ministry of Education and Science (Contract Grant No.: B01/23). Notes

The authors declare no competing financial interest.

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CO2 + butanamide

T/K

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Experimental Measurements and Correlation of the Solubility of Three

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