Journal of Supercritical Fluids 23 (2002) 113–121

Solubility of some phenolic compounds contained in grapeseeds, in supercritical carbon dioxide

Ruth Murga, Marıa Teresa Sanz, Sagrario Beltran *, Jose Luis CabezasDepartamento de Ingenierıa Quımica, Uni�ersidad de Burgos, Pl. Misael Banuelos s/n, 09001 Burgos, Spain

Received 18 May 2001; received in revised form 5 February 2002; accepted 9 February 2002

Abstract

The solubility of some natural, low molecular weight phenolic compounds, 3,4-dihydroxy benzoic acid (protocate-chuic acid), methyl 3,4,5-trihydroxybenzoate (gallic acid methyl ester or methyl gallate), and 3,4-dihydroxy benzalde-hyde (protocatechualdehyde), in supercritical carbon dioxide (SC-CO2) has been determined at pressures from 10 to50 MPa and temperatures from 313 to 333 K. These phenolic compounds are contained in grape seeds and othernatural substrates. The data presented in this work are valuable to know the possibility of separation from theirnatural matrices by supercritical extraction with carbon dioxide. Data modeling has been carried out by using thePeng–Robinson equation of state (PR-EOS) to describe the behavior of the supercritical fluid (SCF) phase. Twosemiempirical density dependent correlations, specifically, the Chrastil model, and a model that assumes a linearcorrelation between the enhancement factor and the density of the solvent, have also been used for data correlation.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Solubility; Supercritical carbon dioxide; Natural phenolic compounds; Grape seeds

www.elsevier.com/locate/supflu

1. Introduction

A large variety of complex phenols is widelyfound in natural products [1]. The interest in thesephenolic compounds is as a result of their knownhealth benefits due to their antioxidant activity asfree radical scavengers [2]. In phenolic acids andtheir esters, this activity depends on the numberof hydroxyl groups. Considering two of the com-pounds studied in this work, methyl gallate, with

three available hydroxyl groups, shows double theantioxidant activity of protocatechuic acid thathas only two hydroxyl groups available [3].

The separation and isolation of these phenoliccompounds from their natural sources is actuallyvery complex, reason for which, many of them arenot produced at a large scale and are not com-mercially available. Such is the case of most of theproanthocyanidins.

Murga et al. [4] reported the supercritical car-bon dioxide (SC-CO2) extraction of some complexphenols from grape seeds. They concluded that byadjusting variables such as pressure and amountof cosolvent, the great variety of phenolic com-

* Corresponding author. Tel.: +34-947-258-809; fax: +34-947-258-831.

E-mail address: beltran@ubu.es (S. Beltran).

0896-8446/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0 896 -8446 (02 )00033 -5

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121114

pounds contained in grape seeds might be selec-tively extracted by supercritical fluid (SCF) extrac-tion. For designing such a process, the solubilityof the phenolic compounds in the SCF to be usedas extractive solvent, needs to be known. Solubil-ity data of natural compounds having antioxidantproperties are very limited although several datasets may be found in the literature [5–8]

The work presented in this paper is part of abroader project to study the possibility of usingSC-CO2 to selectively extract some natural com-plex phenols found in grape seeds. Particularly,solubilities of protocatechuic acid, methyl gallate,and protocatechualdehyde in pure SC-CO2 havebeen experimentally determined using the dynamicanalytical method.

Because of the limited amount of experimentaldata for solid-SCF systems, there is a great inter-est on thermodynamic models able to predict thephase behavior of such systems. In this work twotypes of models have been used: a rigorous ther-modynamic method [9] where the Peng–Robinsonequation of state (PR-EOS) was used to describethe behavior of the SCF-phase, and two semiem-pirical density dependent correlations, specifically,the Chrastil model, and a model that assumes alinear correlation between the enhancement factor

and the density of the solvent. These semiempiri-cal correlations are widely used and, althoughthey are not capable of predicting unknown phaseequilibria, they are useful tools for experimentaldata correlation.

2. Experimental

2.1. Chemicals

The solutes used in this work, protocatechuicacid, methyl gallate, and protocatechualdehydewere supplied by Sigma, (�97% purity). Themolecular weight (MW) and normal melting point(mp) of these solutes are reported in Table 1together with the estimated normal boiling points(Tb), critical temperatures (Tc) and pressures (Pc),and acentric factors (�). Glass beads 30/60 mesh(Phase Separations) were used to distribute thesolute in the equilibrium cell. The carbon dioxideused as supercritical solvent (SCF-TP quality) wassupplied by Air Liquide. HPLC-grade methanoland acetonitrile (Lab-Scan, Dublin, Ireland) andpuriss. p.a. (98%) formic acid (Fluka, Buchs,Switzerland) were used as solvents for sampleanalyses.

Table 1Physical properties of the solutes

* Sigma data base (www.sigma-alddrich.com/saws.nsf/msdselp).** Estimated by the Joback method implemented in PE [16].

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121 115

Fig. 1. Schematic diagram of the dynamic equilibrium apparatus.

2.2. Apparatus and procedure

The solubility of the solids was experimentallydetermined using the dynamic analytical method.The employed apparatus was specially designed tooperate at pressures up to 50 MPa and at temper-atures up to 120 °C. It is schematically shown inFig. 1. Two syringe pumps (ISCO 260 DM), thatwork alternatively, provide an uninterrupted flowof liquid CO2 compressed up to the desired oper-ating pressure. These pumps also allow to fix thesolvent flow that was adjusted to be as low aspossible in order to achieve a good solute–solventcontact. The pressurized solvent was pre-heatedup to the extraction temperature before enteringthe equilibrium cell. The equilibrium cell containsthree compartments (height=29 mm, internal di-ameter=8 mm) placed one above the other andfitted at their bottom with fritted disks that allowa small pressure drop while a good dispersion ofthe SC-CO2 is achieved. Each of the stages wasfilled with the solute (�200 mg) distributed inglass beads in order to improve the solute–solventcontact and avoid channeling when the SC-sol-vent passes through. The cell is placed inside anoven where temperature was controlled (�0.1 K)and measured using a calibrated 4-wires 100 �platinum probe. The saturated SCF that leavesthe cell, goes through a transfer line where acalibrated pressure transducer (Druck, modelPTX 661) and a back pressure regulator (BPR)(Tescom, model 26-1721) were placed. The BPRallowed pressure to be constant within �0.5%.An accurate pressure calibrator (Druck, modelDPI 145) allows calibration of the pressure trans-ducer. A 5-�m in-line filter and a column packed

with stainless steel balls were used for solutetrapping after CO2 depressurization. Filter andtrap were rinsed repeatedly with a solvent(methanol/water, 20/80) in order to collect thesolute quantitatively and prepare the sample foranalysis. The depressurized CO2 was quantifiedwith a totalizer flow-meter.

Analysis of the solutes was carried out off-lineby using an HPLC diode array detector (Hewlett-Packard 1100 series). Quantification was made ata wavelength of 280 nm. Calibration curves, ob-tained in the range where the Lambert–Beer lawwas valid, allowed quantitative analysis of thesamples.

Solids solubility was determined by carryingout at least five experiments in which the totalamount of SC-CO2 that flowed through the cellwas varied, hence varying the total amount ofsolute dissolved. The plot of the amount of soluteversus the amount of CO2 used to dissolve it, is alinear function, whose slope is the solubility at thetemperature and pressure at which the operationwas carried out [10]. The standard error (SE) ofeach estimated solubility, that indicates the qual-ity of the linear fit [11], is given next to everysolubility datum in Table 3, and is plotted withthe solubility data in Figs. 2–4.

2.3. Equipment calibration

Calibration of the equipment was performed bydetermining some known solubility data of a so-lute whose chemical structure includes a phenolgroup as the solutes subject of this work. Thesolute chosen, p-coumaric acid, may also befound in grape seeds and has antioxidant capabil-

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121116

ities. Choi et al. [5] determined the solubility ofthis solute using also the dynamic analyticalmethod. The results that they obtained are listedin Table 2 together with the results obtained inthis work. It may be observed that the relativestandard deviations (RSD) between the solubilitydata determined in this work and those reportedby Choi et al., are lower than 10% in most cases,which is within the experimental error of bothdata sets.

3. Results and discussion

The solubility of protocatechuic acid, methylgallate, and protocatechualdehyde in SC-CO2, un-der different conditions of pressure, P, and tem-perature, T, are listed in Table 3. The CO2 densityat the given pressure and temperature, obtainedfrom the IUPAC international thermodynamictables [12], and the standard errors (S.E.) forevery solubility data, have also been included. Thetemperatures studied range from 313 to 333 Kand the pressures from 10 to 50 MPa.

The results show that solubility increased withpressure, at constant temperature, in all cases.This was expected, since the density of CO2 in-creases with pressure causing a decrease of theintermolecular distances which increases the so-lute–solvent interaction. The effect of tempera-ture is more complex and cross-over phenomena[13] around 15 MPa, are observed in all cases. Atpressures above the cross-over pressure, the solu-bility increases with temperature while at pres-sures below the cross-over pressure, the solubilitydecreases with increasing temperature.

Under the same conditions of temperature andpressure, protocatechualdehyde shows a highersolubility in SC-CO2 than the other two solutesstudied. This was the result expected since proto-catechualdehyde presents the lowest melting pointand size [14]. On the contrary, protocatechuic acidshows a slightly lower solubility than methyl gal-late although they have similar melting points andthe acid is smaller than methyl gallate. This maybe due to the higher polarity of protocatechuicacid that is being dissolved in a non-polar solventas CO2. In solids of low molecular weight, the

influence of their functional groups is very high[13]. The fluid phase in equilibrium with the non-dissolved solid is far from being an ideal solutionand the dipole moment has a large influence [15].

Modeling the solubility of substances in SCFs isimportant for SCF-process design. The most rig-orous methods consist of calculating the fugacityof the components in the SC-phase assuming thatit behaves as a dense gas or as an expandedliquid. It is most common to treat the SCF-phaseas a dense gas. In that case, the solubility of thesolute in the SCF is calculated by means of Eq.(1) that considers a solid phase, formed by thepure solute (2), in equilibrium with a fluid phaseformed by a mixture of solvent (1) and solute (2)[9].

y2=P 2

s

PE (1)

where

E=�2

s

�2

exp��2

s(P−P 2s)

RTn

(2)

P2s is the saturation (vapor) pressure of the pure

solid, �2 the fugacity coefficient at the systempressure P, �2

s the fugacity coefficient at satura-tion pressure P2

s, and �2s the solid molar volume,

all at the system temperature T. The quantity E,nearly always greater than unity, is called theenhancement factor, and is defined as the ratiobetween the observed equilibrium solubility andthat predicted by the ideal gas law at the same

Table 2Experimental solubility data of p-coumaric acid in supercriti-cal CO2 and comparison with data from Choi et al. [5]

y 108 [5]T (K) y 108 (thisP (MPa) RSD (%)work)

8.5313 0.69 0.79 14.4910 2.86 2.65 7.3415 15.45 14.70 4.8520 20.31 20.86 2.7125 26.9827.30 1.17

0.598.5 0.52323 11.8610 0.79 0.63 20.2515 16.63 16.25 2.2920 26.28 27.54 4.7925 36.29 38.54 6.20

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121 117

Table 3Experimental solubility data of protocatechuic acid, methyl gallate and protocatechualdehyde, in SC-CO2: P, T, and y expressed assolute mol fraction

�CO2, (kg/m3) Protocatechuic acid Methyl gallate ProtocatechualdehydeT (K) P (MPa)

y 107 SE 107 y 107 SE 107 y 107 SE 107

10313 615 0.438 0.022 1.200 0.033 4.277 0.452781 1.057 0.102 3.56115 0.137 28.312 0.876841 2.079 0.240 5.568 0.07120 39.137 0.621881 7.64825 0.749911 3.478 0.300 10.228 0.81930 57.078 0.806936 4.163 0.052 12.42735 1.261 74.528 2.689

40 957 4.739 0.755 14.985 1.530 93.997 7.91945 976 5.140 0.466 19.024 1.362 101.007 11.402

992 5.319 0.042 22.13050 1.860 105.786 2.80110323 408 0.247 0.028 0.263 0.026 2.868 0.032

701 1.207 0.120 4.97115 0.173 31.003 0.401785 3.042 0.274 9.297 0.18020 60.604 1.500835 12.06925 1.03987230 7.173 0.337 16.442 1.661 82.207 2.562900 9.752 0.651 20.02735 1.802 98.235 1.121

40 924 11.130 0.216 22.441 0.950 124.585 9.18945 945 11.993 0.601 25.811 1.222 153.230 1.692

963 12.907 0.598 30.49550 1.030 185.443 13.90110333 295 0.023 0.003 0.191 0.007 1.306 0.157

607 0.953 0.101 5.56815 0.562 42.442 2.173725 5.254 0.348 10.435 0.23920 83.291 3.330788 19.16725 1.881

30 831 10.211 1.250 24.090 0.849 165.720 3.722864 15.473 0.756 29.96935 1.621 206.504 9.461891 18.140 0.223 35.203 1.70040 309.705 11.410914 22.271 1.322 39.81645 1.811 367.788 7.482935 25.945 1.430 42.397 1.14050 457.859 37.412

The density of the SC-CO2, �CO2, at each P and T is also given.

temperature and pressure, as inferred from Eq.(1).

The PR-EOS (Eq. (3)) is the equation mostwidely used for calculating the fugacitycoefficients.

P=RT

�−b−

a(T)�(�+b)+b(�−b)

(3)

where � is the molar volume of the componentand parameters a and b have been calculatedaccording to the quadratic mixing rules:

a(T)=y12a11(T)+2y1y2a12(T)+y2

2a22(T) (4)

b=y12b11+2y1y2b12+y2

2b22 (5)

where yi stands for the concentration of the com-ponents in the SC-phase, aii and bii refer to purecomponent values, and aij and bij are binaryparameters that are usually written in the form:

a12(T)=�a11(T)a22(T)(1−k12) (6)

b12=b11+b22

2(1− l12) (7)

The interaction parameter k12 accounts for in-termolecular interactions between the species inthe mixture, and l12, for the different size of thespecies.

The PR-EOS gives good quantitative fits for awide variety of systems, but critical properties (Tc

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121118

Fig. 2. Experimental solubility of protocatechuic acid in SC-CO2. (�) 313 K, (�) 323 K, (�) 333 K. The error barsrepresent the standard error (S.E.) of each solubility data. Thecontinuous lines represent the solubility isotherms calculatedwith the PR-EOS.

Fig. 3. Experimental solubility of methyl gallate in SC-CO2.(�) 313 K, (�) 323 K, (�) 333 K. The error bars representthe standard error (S.E.) of each solubility data. The continu-ous lines represent the solubility isotherms calculated with thePR-EOS.

and Pc) and acentric factors of solute and solventneed to be known for calculating aii and bii. Whilethese properties are available for CO2, this is notthe case for the solids subject of this work. There-fore, they have been estimated by the Jobackgroup contribution method [16] and are listed inTable 1. This group contribution method has alsobeen used for prediction of the normal boilingtemperature of the solute, also listed in Table 1.

The interaction parameter k12, and l12, havebeen obtained by fitting the experimental solubil-

ity data to Eq. (1). The regression procedure wascarried out by minimizing the average absoluterelative deviation (AARD) between experimental(yexp) and calculated (ycal) solubility data, usingthe PE 2000 (Phase Equilibria) program devel-oped by Prof. Brunner’s group [16].

AARD(%)=�100

n� � �yexp−ycal�

yexp

(8)

where n is the number of solubility data used forobtaining the parameters.

Table 4Results of the solubility data correlation through the PR-EOS: number of data points used in the correlation (n), binary interactionsparameters (kij and lij), and average absolute relative deviations (AARD)

Parameters of PR-EOST (K)nSolute

AARD (%)lijkij

10.65−0.6352−0.1054Protocatechuic acid 3138−0.59878 13.80323 −0.1082

333 −0.1170 −0.6033 9.318313 −0.1405 −0.5605 21.25Methyl gallate 9

21.84−0.8515−0.23193239−0.28229 −0.9973 22.88333

8 313Protocatechualdehyde 0.1990 −0.0456 13.7512.57−0.08740.18293238

8 333 0.2902 0.2702 24.61

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121 119

Table 5Results of the solubility data correlation considering a linear correlation between the enhancement factor and the density of thesolvent: number of data points used in the correlation (n), parameters of Eq. (9) (A and B), average absolute relative deviations(AARD), and �-squared (�2)

T (K)Solute Parameters of Eq. (9)n

A B 102 AARD (%) �2

313 10.44Protocatechuic acid 1.1458 12.12 0.9850323 7.25 1.3888 16.40 0.9691

8 333 6.57 1.352 7.73 0.99829Methyl gallate 313 10.60 1.444 11.98 0.9962

323 12.03 1.1349 3.00 0.9995333 11.14 1.089 6.67 0.99749313 0.596 1.2728 5.93Protocatechualdehyde 0.9976

8 323 2.33 1.017 10.38 0.99513338 1.20 1.132 13.57 0.9953

Fig. 4. Experimental solubility of protocatechualdehyde inSC-CO2. (�) 313 K, (�) 323 K, (�) 333 K. The error barsrepresent the standard error (S.E.) of each solubility data. Thecontinuous lines represent the solubility isotherms calculatedwith the PR-EOS.

for data correlation. One of them assumes a linearfunctionality between the enhancement factor, ascalculated from Eq. (1), and the density of thesolvent [17]; that is,

ln E=A+B� (9)

This model has been shown to be one of thebest available routes among the empirical models[18]. The linear relation between the enhancementfactor and the density is a useful equation fordata correlation because of its simplicity and goodfits. The results of data correlation to this modelare reported in Table 5 together with the AARDbetween experimental and calculated solubility.The goodness of the linear correlation is indicatedby �2.

The second empirical model used, was theChrastil model [19] that assumes the formation ofa solvato complex between molecules of the SC-solvent and the solute at equilibrium. This modelleads to the following linear relationship amongsolubility and CO2 density for a giventemperature:

ln C=k* ln �+a/T+b (10)

where the solubility, C, is calculated by means ofEq. (11):

C=�×MWsolute×yMWCO2

× (1−y)(11)

In Eqs. (10) and (11), � is the CO2 density in

The values of the interaction parameters k12,and l12 for the different temperatures are listed inTable 4 together with the corresponding AARD.The PR-EOS was applied to describe the behaviorof the fluid phase in a wide range of pressures, i.e.from 10 to 50 MPa. That may be one of thereasons why the AARD values are not as low asdesired. The continuous lines in Figs. 2–4 repre-sent the solubility curves as predicted by Eq. (1)and the PR-EOS.

Two semiempirical models have also been used

R. Murga et al. / J. of Supercritical Fluids 23 (2002) 113–121120

Table 6Results of the solubility data correlation through the Chrastil model: number of data points used in the correlation (n), parametersof Chrastil equation (a, b and k), average absolute relative deviations (AARD), and �-squared (�2)

Parameters of Chrastil modelSolute n

k a b AARD (%) �2

6.653Protocatechuic acid −859524 −24.64 19.89 0.97346.073 −7139 −24.1327 11.28Methyl gallate 0.99006.018 −8802 −17.02 16.63Protocatechualdehyde 0.985124

g/L, C is the solubility of the solute expressedas (g of solute)/(L of solvent), T is the tempera-ture in K, MW is the molecular weight, y is theequilibrium mole fraction of the solute in theSC-phase, and a, b and k are adjustableparameters of the model. The constant k is anassociation factor that represents the number ofCO2 molecules in the complex, a depends on thevaporization and solvation enthalpies of the so-lute, and b depends on the molecular weights ofthe solute and solvent.

The optimum values of the parameters a, b,and k were obtained by nonlinear regression ofthe experimental data using the Marquardt al-gorithm. These values are reported in Table 6together with the AARD between experimentaland calculated solubility, C. The goodness ofthe correlation is indicated by �2.

Eq. (10) has the advantage of having onlythree parameters to fit all the experimental data,no matter at which temperature they were ob-tained. Besides, the properties of the solid neednot be estimated.

The correlation improves largely when lowsolubility data are not taken into account forcorrelation since solubilities are in the same or-der of magnitude as the experimental error. Forinstance, if the solubility data obtained at 10MPa are excluded from the correlation of proto-catechuic acid, the AARD decreases down to8% and �2 increases up to 0.9907. In that casethe Chrastil parameters found were k=8.393,a= −9894.9 and b= −32.42.

Acknowledgements

Financial support provided by the CICYT(QUI96-0691), and the EU, CICYT and JCyL(1FD97-1471-QUI) is gratefully acknowledged.

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