liquid–liquid equilibria of linalool + ethanol + water, water + ethanol + limonene, and...

Journal of Solution Chemistry, Vol. 33, No. 5, May 2004 ( C© 2004)

Liquid–Liquid Equilibria of Linalool + Ethanol +Water, Water + Ethanol + Limonene, andLimonene + Linalool + Water Systems

Alberto Arce,1,∗ Alicia Marchiaro,2 and Ana Soto1

Received March 12, 2004; revised April 15, 2004

The present work was undertaken to determine liquid–liquid equilibria for ternarysystems involved in the citrus essential oil terpeneless using dilute alcohol. Tie-line datahave been determined for the linalool + ethanol + water, water + ethanol + limonene,and limonene + linalool + water ternary systems at 298.15 K. The experimentaldata were satisfactorily correlated using the UNIQUAC and NRTL equations, and theobtained binary interaction parameters are reported. The UNIFAC group-contributionmethod did not allow adequate predictions of liquid–liquid equilibria involved in thisstudy.

KEY WORDS: LLE; limonene; linalool; ethanol; essential oil.

1. INTRODUCTION

Essential oils are plant products, usually somewhat volatile, giving the odorsand tastes characteristic of the particular plant, thus possessing the essence.They are basically a mixture of terpenic hydrocarbons and oxygenated deriva-tives. Among their components, oxygenated compounds are generally consideredpreferable due to odor, and their content has become a definitive parameter inestablishing the price of the oil and representing its quality.

Citrus oil is an essential oil, the two main components are the terpene hy-drocarbon limonene and the oxygenated terpenoid linalool. The present work wasundertaken to evaluate the aqueous solutions of ethanol as agents for their sep-aration. With this in mind, we have determined liquid–liquid equilibria (LLE)for linalool + ethanol + water, water + ethanol + limonene, and limonene +1Department of Chemical Engineering, University of Santiago de Compostela, E-15782 Santiago,Spain; e-mail: [email protected].

2Present address: Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia, Provincia delChubut Argentina.

561

0095-9782/04/0500-0561/0 C© 2004 Springer Science+Business Media, Inc.

562 Arce, Marchiaro, and Soto

Fig. 1. Experimental tie-lines for linalool + ethanol + water, water + ethanol+ limonene, and limonene + linalool + water ternary systems at 298.15 K.

linalool + water ternary systems at 298.15 K. In the literature(1) we have onlyfound LLE data for the water + ethanol + limonene system at 293 K.

The experimental data have also been correlated using the UNIQUAC andNRTL equations, and have been compared with the predictions of the UNIFACgroup contribution method.

2. EXPERIMENTAL

All chemicals used were chromatography-grade products, supplied by Flukawith nominal purities of 98 mass% for limonene (4-isopropenyl-1-methyl-cyclohexene) and 97 mass% for linalool (3,7-dimethyl-1,6-octenedien-3-ol), andby Merk with nominal purity of 99.8 mass% for ethanol. Water was purified us-ing a Milli-Q plus system. These purities were verified by gas chromatographyand the chemicals were used without further purification. Their densities and re-fractive indices were measured at 298.15 K and atmospheric pressure, and arecompared with published values(2,3) in Table I. Densities were measured with anAnton Paar DMA 60/602 densimeter precise to within ±10−5 g-cm−3. Refractiveindices were measured with an Atago RX-5000 refractometer with an accuracyof ±4 × 10−5.

First, solubility curves at 298.15 K were determined by the cloud-pointmethod.(4) These curves were used to calibrate the gas chromatograph by aninternal standard method. The chromatograph used was an Hewlett-Packard 5890Series II apparatus equipped with a thermal conductivity detector and an HP-FFAP:

Liquid–Liquid Equilibria for Ternary Solvents 563

Table I. Densities (ρ) and Refractive Indices (nD) of the Pure Components at298.15 K and Atmospheric Pressure

ρ (g-cm−3) nD

Component Experimental Literature Experimental Literature

Limonene 0.83717 0.8383(2) 1.47027 1.4701(2)

Linalool 0.85774 0.85760(3) 1.45970 Not foundEthanol 0.78522 0.78493(2) 1.35920 1.35941(2)

Water 0.99704 0.99704(2) 1.33250 1.33250(2)

25 m × 0.2 mm × 0.3 µm capillary column. Helium was used as the mobile phase.The injection volume was 0.5 µL, and the split ratio was 1:100. Separation wasperformed to 403.15 K under isothermal conditions. The greatest errors in thedetermination of the mole fraction compositions using the calibration curves were±0.003.

The tie-line data were determined by analysis of the two layers of heteroge-neous mixtures. In each case, a mixture with a bulk composition in the immiscibil-ity region was placed inside an equilibrium cell, where it was stirred for 1 h in orderto ensure intimate contact between the phases and was then left standing for 4 h toachieve thermodynamic equilibrium (the time necessary to attain equilibrium, novariation of compositions with time, was established in preliminary experiments).The whole procedure was carried out at constant temperature using water from athermostat (Selecta Ultraterm 6000383) and the water temperature was measuredwith a thermometer Heraeus Quat 100 precise to within ±0.01 K. When ther-modynamic equilibrium had been attained, samples of both liquid phases werecollected and analyzed by gas chromatography.

3. RESULTS

The experimental tie-line data for linalool + ethanol + water, water +ethanol + limonene, and limonene + linalool + water systems at 298.15 K arelisted in Tables II, III, and IV, respectively. Figure 1 shows experimental tie-linesfor these equilibria.

4. DATA TREATMENT

4.1. Correlation

The experimental data were correlated using the widely employed NRTL(5)

and UNIQUAC(6) equations. The value of the NRTL nonrandomness parameterwas set successively to 0.1, 0.2, and 0.3, and the results given below are in eachcase the best of these three sets. The UNIQUAC structural parameters r and q


Table II. Experimental Tie-Lines of the System Linalool (1) + Ethanol (2) + Water (3)

[T = 298.15 K]

Aqueous phase Organic phase

x1 x2 x3 x1 x2 x3

0.0002 0.0000 0.9998 0.8099 0.0000 0.19010.0003 0.0194 0.9803 0.7228 0.0681 0.20920.0003 0.0441 0.9556 0.6230 0.1496 0.22740.0004 0.0623 0.9373 0.5491 0.2061 0.24480.0005 0.0847 0.9148 0.4360 0.2784 0.28560.0006 0.1107 0.8887 0.3614 0.3324 0.30620.0009 0.1260 0.8731 0.3041 0.3587 0.33720.0017 0.1487 0.8496 0.2287 0.3804 0.39090.0035 0.1712 0.8253 0.1704 0.3787 0.45090.0083 0.2032 0.7885 0.1205 0.3601 0.5194

Note. Compositions are given as mole fractions.

were taken from literature.(7−9) The computer program used to fit these equationsminimizes the objective function

FO =∑

k

∑i

∑j

(xi jk − x̂i jk)2 (1)

Table III. Experimental Tie-Lines of the System Water (1) + Ethanol (2) + Limonene (3)

[T = 298.15 K]


x1 x2 x3 x1 x2 x3

0.0000 0.0065 0.9935 0.9924 0.0053 0.00220.0000 0.0962 0.9038 0.9826 0.0157 0.00170.0001 0.1468 0.8532 0.9696 0.0256 0.00480.0003 0.2158 0.7839 0.9798 0.0172 0.00300.0008 0.2604 0.7389 0.9659 0.0317 0.00240.0016 0.3350 0.6634 0.9520 0.0450 0.00300.0043 0.4108 0.5848 0.9342 0.0602 0.00560.0066 0.4509 0.5425 0.9326 0.0611 0.00640.0121 0.5117 0.4763 0.9199 0.0737 0.00640.0204 0.5613 0.4182 0.9197 0.0742 0.00610.0498 0.6748 0.2754 0.8097 0.1698 0.02040.0726 0.6959 0.2315 0.7669 0.2098 0.02330.0996 0.7005 0.2000 0.7164 0.2558 0.02780.2624 0.6134 0.1242 0.5263 0.4141 0.0595



Table IV. Experimental Tie-Lines of the System Limonene (1) + Linalool (2) +Water (3)

[T = 298.15 K]


x1 x2 x3 x1 x2 x3

0.0000 0.0000 1.0000 0.9978 0.0000 0.00220.0000 0.0000 1.0000 0.9141 0.0824 0.00350.0000 0.0001 0.9999 0.8022 0.1848 0.01300.0000 0.0001 0.9999 0.7371 0.2431 0.01980.0000 0.0001 0.9999 0.6465 0.3263 0.02720.0000 0.0001 0.9999 0.5653 0.3912 0.04350.0000 0.0001 0.9999 0.4386 0.4989 0.06250.0000 0.0001 0.9999 0.3107 0.5914 0.09790.0000 0.0001 0.9999 0.2395 0.6491 0.11150.0000 0.0001 0.9999 0.1808 0.6849 0.13430.0000 0.0001 0.9999 0.0889 0.7477 0.16340.0000 0.0002 0.9998 0.0101 0.7935 0.1964


where xi jk is the experimental mole fraction component i in phase j on tie-line k,x̂i jk is the value calculated using the parameters being optimized.

Table V lists the binary interaction parameters obtained for the ternary sys-tems, and for both UNIQUAC and NRTL equations. The quality of the correlationis measured by the residual function F

F = 100

∑

k

∑i

∑j

(xi jk − x̂i jk)2

6M

0.5

(2)

where M is the number of experimental tie-lines.Figure 2 compares experimental tie-lines for linalool + ethanol + water,

water + ethanol + limonene, and limonene + linalool + water systems with thosecalculated using the NRTL (α = 0.1) equation.

4.2. Prediction

Figure 3 compares the experimental data with those predicted by the UNIFACmethod(10) using interaction and structural parameters taken from literature.(11) Thecorresponding F values, Eq. (2), at 298.15 K are 5.74, 2.79, and 3.24, for linalool +ethanol + water, water + ethanol + limonene, and limonene + linalool + watersystems, respectively.


Table V. Binary Interaction Parameters and Residual Functions, Eq. (2), for UNIQUAC and NRTLModels Fitted to LLE Data for Linalool + Ethanol + Water, Water + Ethanol + Limonene, and

Limonene + Linalool + Water Systems at 298.15 K

Model Pair i– j Parameters (J-mol−1) F

Linalool (1) + ethanol (2) + water (3)NRTL (α = 0.1) 1–2 −124.24 2492.50

1–3 −4715.43 21325.952–3 −5358.37 11483.59 0.5192

UNIQUAC 1–2 2394.36 −583.441–3 4070.65 44.8902–3 −605.51 926.57 0.6373

Water (1) + ethanol (2) + limonene (3)NRTL (α = 0.1) 1–2 25212.54 −13092.41

1–3 26840.47 4268.602–3 5677.56 −716.43 0.8517

UNIQUAC 1–2 953.74 −831.331–3 3781.88 10441.522–3 −615.71 3691.14 2.0005

Limonene (1) + linalool (2) + water (3)NRTL (α = 0.1) 1–2 −7312.14 146.41

1–3 7772.34 8645.722–3 −4888.77 20891.34 0.1599

UNIQUAC 1–2 1460.48 −1515.001–3 9258.17 3831.242–3 3246.14 569.75 0.1143

Structural parameters for the UNIQUAC equation

Limonene(6) Linalool(6) Ethanol(7) Water(8)

r 6.2783 7.0356 2.1055 0.92q 5.2080 6.0600 1.972 1.40

5. CONCLUSIONS

With the aim to evaluate the possibility of using aqueous solutions of ethanolas agents for limonene–linalool mixtures separation, we have determined liquid–liquid equilibrium (LLE) for linalool + ethanol + water, water + ethanol +limonene, and limonene + linalool + water ternary systems at 298.15 K.

Limonene and linalool are completely soluble in ethanol but not quite com-pletely soluble in water. Ternary equilibria established between ethanol andwater with limonene and linalool, have opposite tie-lines slopes. The next stepof our work is to determine quaternary liquid–liquid equilibria. Thiswill allow us to evaluate the influence of limonene–linalool ratio in essentialoil, and the percentage of water the solvent, on the effectiveness ofextraction.


Fig. 2. Experimental tie-lines for linalool + ethanol + water, water + ethanol + limonene, andlimonene + linalool + water systems at 298.15 K (——o), and those obtained optimizing a NRTL(α = 0.1) model (- - -∇).

These ternary liquid–liquid equilibria were satisfactorily fitted by both theUNIQUAC and the NRTL equations in most of the cases, only in the UNIQUACcorrelation of water + ethanol + limonene did we find high deviations between theexperimental and correlated data. The corresponding optimized binary interactionparameters are reported in Table V.

The experimental data were not satisfactorily predicted by the UNIFACmethod. This method was able to predict tie-lines slopes but deviations foundbetween experimental and predicted compositions were very high. Predictionmethods frequently provide rough approximations, therefore, it will be alwaysnecessary to obtain at least a few reliable experimental results.


Fig. 3. Experimental tie-lines for linalool + ethanol + water, water + ethanol + limonene, andlimonene + linalool + water systems at 298.15 K (——o), and those predicted using the UNIFACmethod (- - - �).

ACKNOWLEDGMENTS

The authors are grateful to the Ministerio of Ciencia y Tecnologı́a (Spain) forfinancial support under project PPQ2003-01236. A.M. is grateful to the EuropeanUnion for financial support (Project ALFA-PROQUIFAR).

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