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Journal of Food Engineering 79 (2007) 1104–1109

Research note

Enthalpies of mixing and heat capacities of mixtures containingacetates and ketones with corn oil at 25 �C

C. Gonzalez *, J.M. Resa, R.G. Concha, J.M. Goenaga

Dpto. de Ingenierıa Quımica, Universidad del Paıs Vasco, Apto. 450, Vitoria, Spain

Received 27 February 2004; received in revised form 7 September 2005; accepted 17 January 2006Available online 3 April 2006

Abstract

This study concerns the determination of molar enthalpies of mixing for mixtures composed by acetates (ethyl acetate, propyl acetate,isopropyl acetate, butyl acetate and isobutyl acetate) and ketones (2-butanone, 3-pentanone and 4-methyl-2-pentanone) + corn oil at25 �C and atmospheric conditions. In the design of heat transfer and process equipment employed in the oils and seed processing indus-try, the knowledge of enthalpy of the organic solvent–oil mixtures is a determinant factor. All the binaries show the same trend, adecrease of temperature is observed when the mixture is carried out. An effect endothermic takes place as consequence of the new reor-ganization of molecules. The aim of this study was therefore to calculate the heat capacities at constant pressure for the studied systems.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Enthalpy of mixing; Heat capacity; Mixtures; Corn oil; Ester; Ketone

1. Introduction

Fats and oils as they exist in nature must be processedbefore they are suitable for use as edible fats and oils. Thereare two current methods of recovering oil, squeezing itfrom the seed using a press (hydraulic, screw or a combina-tion) or it is extracted using a solvent, although frequentlythe two methods are combined, depending on the nature ofthe seed and the cost of the operation. Mechanical extrac-tion is simpler and safer but less efficient than solventextraction, in which the flakes (or press meal) are soakedin the solvent, normally hexane.

Oilseeds as natural and biologically active materials, con-tain many colour and flavor precursors as degradation andbreakdown products, becoming refining steps completelynecessary. Corn oils contain relatively large amount ofwaxes that must be removed so that the oil does not cloudwhen refrigerated (winterization or dewaxing). Desired frac-tions from a mixture of triglycerides dissolved in a suitable

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.01.088

* Corresponding author. Fax: +34 945013014.E-mail address: iqpgoorc@vc.ehu.es (C. Gonzalez).

solvent are selectively crystallized at different temperaturesafter which the fractions are separated and the solventremoved. Because of the hazardous nature of hexane andthe lack of knowledge about the behavior with other kindof solvents, this paper continue our research programinvolving the study of bulk and excess properties of binarymixtures containing vegetable oils and organic solvents(Gonzalez, Iglesias, Lanz, & Resa, 1999; Gonzalez, Resa,& Lanz, 2000; Gonzalez, Resa, Ruiz, & Gutierrez, 1996,1997; Resa, Gonzalez, Fanega, Ortiz de Landaluce, & Lanz,2002). We report in this document experimental measure-ments of molar enthalpy of mixing (DHm) as function of sol-vent composition for mixtures involving esters (ethylacetate, propyl acetate, isopropyl acetate, butyl acetateand isobutyl acetate) and ketones (2-butanone, 3-pentanoneand 4-methyl-2-pentanone) at 25 �C and atmospheric pres-sure. Indeed, calculated heat capacities (Cpm) for the studiedmixtures are reported. Corn oil was characterized accordingto spanish standard procedures and its fatty acids composi-tion determined. Physical properties of the corn oil as den-sity (q), refractive index (nD) and speed of sound (u) weremeasured as function of temperature in the range 25–50 �C.

C. Gonzalez et al. / Journal of Food Engineering 79 (2007) 1104–1109 1105

Measurements of bulk properties as the previously cited,and of molar enthalpies provide insight into the moleculararrangements in liquids and help one to understand thethermodynamic properties of liquid mixtures. Much datahas been reported on heat capacities of oils and fats(Coupland & McClements, 1997; Kowalski, 1988; Morad,Mustafa Kamal, Panau, & Yew, 2000), but none refersspecifically to mixtures. Several studies present informationabout physical properties of vegetable oils (Coupland &McClements, 1997; De Dios Alvarado, 1995), or mixturesof fatty acids (Flores Luque, Galan Vallejo, CanteroMoreno, & Quiroga Alonso, 1979; Flores Luque, GomezHerrera, & Galan Vallejo, 1977; Mantell Serrano, MunozCueto, Galan Vallejo, & Rodrıguez Rodrıguez, 1995) butnothing has been found for corn oil with organic solvents.

2. Material and methods

2.1. Reactives

Acetates were supplied by Fluka except of propyl ace-tate supplied by Aldrich, while ketones purchaser was Pro-labo, all the chemicals with a purity better than0.985 mol%. Ultrasonic treatment was used for degassingand molecular sieves (type 3a, 1/16 inch, Fluka) were intro-duced into the bottles to reduce possible water contents insolvents. No more treatment was applied owed to theirhigh purity grade, from the purchaser. The purities of thesolvents were checked by comparing the measured densityand refractive index data with those reported in theliterature.

Corn oil was supplied by Koipe and analyzed to calcu-late its composition in fatty acids. With this aim, we useda SHIMADZU model GC-14B gas chromatographequipped with a flame detector. The chromatographic tech-nique and the chemical procedure for the preparation offatty acids were described in a previous work (Gonzalezet al., 1996). The obtained composition (% mol) is the fol-lowing: palmitic acid (11.97), stearic acid (2.34), araquidicacid (0.77), oleic acid (29.09), linoleic acid (54.97), linolenicacid (0.86). The uncertainty in the % mol data is±0.1% mol. From the experimental composition, the aver-age molar mass (Mw) of corn oil has been calculated,Mw = 872.08 ± 0.05 g mol�1. Other characteristics of the

Table 1Experimental physical properties of corn oil as function of temperature and li

T (�C) Density (kg m�3) Refractive ind

Experimental Literaturea Experimental

25 915.52 917.98 1.4720330 912.02 914.73 1.4702835 908.75 911.48 1.4684240 905.41 908.23 1.4665145 902.00 904.98 1.4648050 899.12 901.73 1.46270

a Coupland and McClements (1997).b Madrid et al. (1997).

oil were measured too: acid value (0.14 mg KOH), saponi-fication value (198 mg KOH/g oil), iodine value (123.7),peroxide value (9.2 meq O2/kg oil), and wetness and vola-tiles (0.04 water % in the oil). These were analyzed follow-ing standard spanish procedures, UNE rules (AMV Ed.,1997) and the obtained values are in agreement with thosereported in bibliography. Other properties of the oil as den-sity, refractive index and speed of sound were measured asfunction of temperature in the range 25–50 �C. Density wasobtained using a vibration tube density meter Anton PaarDMA 58 with a resolution of 1 · 10�5 g cm�3. The oscilla-tor period, s, in the vibrating tube was converted into den-sity (q) by using the equation,

q ¼ A � s2 � B ð1Þwhere A and B are the apparatus constant, determined withthe literature data of pure water and dry air. The refractiveindices, nD, at the sodium line were measured using a Met-tler Toledo RE50 refractometer with a precision of±1 · 10�5. An Anton Paar DSA 48 analyser carried outspeed of sound measurements, with a precision of±1 m s�1 and was also frequently calibrated. These proper-ties are gathered in Table 1, in the studied temperaturesand compared with literature values.

2.2. Preparation of samples

To determinate the mixing enthalpy of the studied mix-tures, all experiments were carried out using a Dewar calo-rimeter and its equipment, the whole set supplied by PhyweSysteme GMBH. Experimental set up and procedure issimilar to that described in Zijlema, Witkamp, and vanRosmalen (1999). To weigh the individual components ofthese mixtures, we use an AND electronic Balance modelHF-20006, with an accuracy of ±2 · 10�3 g, taking careno evaporation of solvent occurs.

2.3. Procedure

The organic solvent contained in an erlenmeyer flask, istemperature equilibrated using a temperature controlledbath. The Dewar vessel with the vegetable oil is heatedby a heating coil. A magnetic heating stirrer and a stirbar provide agitation. To start the measures two liquids

terature values

ex (nD) Speed of sound (m s�1)

Literatureb Experimental Literaturea

1.470–1.474 1449.59 1451.651436.35 1435.501419.70 1419.351403.29 1403.201386.77 1387.051371.00 1370.90

1106 C. Gonzalez et al. / Journal of Food Engineering 79 (2007) 1104–1109

must have reached the same temperature, which is moni-torized connecting the two immersion-type Pt-100 probesto a ‘‘Cobra’’ Interphase (a measure and intelligent controlsystem) connected to a PC. Both components are heateduntil thermal equilibrium is attained at 25 �C and it ismaintained 20 min. Temperature difference between theliquid in the erlenmeyer flask of the temperature-controlledbath and the oil in the calorimeter must not exceed 0.02 �C.After this, the organic solvent is mixed with the oil in theDewar vessel. The temperature change (DTexp) accompany-ing the mixing process is sensed by the thermocouple andregistered in the PC, in all cases a decrease as consequenceof an endothermic effect. The accuracy in measuring thetemperature was ±0.01 �C. All experiments were carriedout in a thermostated room which was kept at the sametemperature of the experiment (25 �C) for a better accuracyof the measures. The enthalpies of mixing and the heatcapacities were determined by heating the mixture withan electrical device, and measuring the electrical power(Wel) needed to heat the mixture the same increase of tem-perature produced when two components were mixed. Theelectrical heat input was controlled by the power supply,connected with a work and power meter that allows adirect lecture of the electrical supplied work. The reliabilityof the apparatus and the method was established by mea-suring enthalpies of standard systems, that is, ben-zene + carbon tetrachloride and chlorobenzene + tolueneat 25 �C. Results were in good agreement with the reporteddata in the literature (Nicolaides & Eckert, 1978; Tanaka &Benson, 1976). The uncertainty in the measured enthalpiesis ±1%.

2.4. Determination of DHm and Cpm

When mixing n1 moles organic solvent with n2 moles ofcorn oil, a temperature change is observed (DTexp) becauseof the interactions between both liquids, in this case adecrease. We communicate heat as electrical power (Wel)to the mixture, to produce a temperature increase (DTind)most nearer as possible to the experimental one. Experi-mental enthalpies of mixing (DHm) are calculated asfollows:

Dhm ¼ Qexp ¼ W el �DT exp

DT ind

� �ð2Þ

Molar enthalpy of mixing, DHm is calculated dividing theenthalpy of mixing by the total number of moles of bothcomponents

DH m ¼ Dhm

n1 þ n2

ð3Þ

Having account enthalpy definition, experimental dataof DHm allows to calculate heat capacities at constant pres-sure, Cpm as follows:

dH ¼ oHoT

� �P

dT þ oHoP

� �T

dP ð4Þ

if P = cte, the second term of equation is equal to zero, so

dH ¼ dqp ¼oHoT

� �P

dT or dH ¼ dqp ¼ Cp � dT ð5Þ

Integrating Eq. (5) between two specific temperature values,

DH m ¼Z T 2

T 1

Cpm � dT ð6Þ

As Cpm varies no significatively in the studied range, heatcapacities of mixtures can de calculated by means ofEq. (7):

DH m ¼ Cpm � ðT 2 � T 1Þ ¼ Cpm � DT exp or Cpm ¼ DH m

DT exp

ð7Þ

3. Results and discussion

3.1. Molar enthalpies of mixing

The mixing enthalpy, DHm is influenced by all theinteractions of the molecules involved, which in turn area function of the mixing ratio, and it is zero for ideal mix-tures (no interactions among molecules take place). In thisstudy enthalpies of mixing, of alkyl acetates (ethyl acetate,propyl acetate, isopropyl acetate, butyl acetate and iso-butyl acetate) and ketones (2-butanone, 3-pentanone and4-methyl-2-pentanone) with corn oil were measured at25 �C. The interactions between two liquids can be thecause of endothermic effects (decreasing supramolecularassemblies) or exothermic ones (formation of strongersupramolecular assemblies of different molecules). Tables2 and 3 report experimental molar enthalpies of mixing,DHm and calculated heat capacities of mixing for the wholeset of binary system. As illustrated in Figs. 1 and 2, valuesof enthalpies of mixing of all the systems are positive overthe entire composition range, pointing out an endothermiceffect (positive values of DHm) when the mixture betweensolvent and corn oil is carried out. To homogenize the mix-ture it is necessary to break intermolecular associationsamong solvent molecules by an expenditure of extra energyand establish new interactions between this one and triglyc-eride molecules. For mixtures concerning acetates, thehighest experimental values were obtained for ethyl acetatesystem, and lower ones for esters increasing its hydrocar-bonated chain. Acetates are weakly polar molecules, thatpresent resonance. Negative charge is distributed amonghydrocarbonated chain carbon atoms, by inductive andhyperconjugative effects. These effects are higher as thelength of the chain increase or ramifications are presentedin the molecule. So, intermolecular forces among acetatesare bigger for ethyl acetate > propyl acetate > isopropylacetate > butyl acetate > isobutyl acetate. Mixtures of cornoil with ramificated esters, present higher values of theproperty than their correspondent linear isomers as

Table 2Experimental enthalpies of mixing and calculated heat capacities of thestudied mixtures (acetates + corn oil) at 25 �C, as function of solventmolar fraction (x1)

x1 DHm (J/mol) Cpm (J/mol �C)

Ethylacetate + corn oil

0.101 213 16370.204 370 14810.298 551 13450.400 696 11230.504 893 10260.547 1030 9710.601 1079 8630.645 1075 7850.701 1071 6950.801 1046 5260.900 771 3690.950 571 278

Propylacetate + corn oil

0.098 149 16620.204 307 14620.304 421 13150.399 566 12300.452 606 11210.496 668 10440.555 696 9530.601 728 8880.700 761 7120.800 718 5530.899 576 3920.950 402 307

Isopropylacetate + corn oil

0.199 348 15120.297 464 13270.396 571 11650.496 716 10530.550 780 9510.600 807 8670.651 835 7800.699 836 6960.800 792 4800.900 557 3840.950 456 304

Butylacetate + corn oil

0.203 200 14320.299 324 13520.401 390 12570.446 476 11340.498 487 10810.552 521 10030.582 531 9480.651 547 8050.701 539 7390.799 515 5850.900 386 4200.950 304 334

Isobutylacetate + corn oil

0.099 84 16800.202 215 14320.302 351 12520.396 425 11480.446 492 10700.498 517 1013

Table 3Experimental enthalpies of mixing and calculated heat capacities of thestudied mixtures (ketones + corn oil) at 25 �C, as function of solventmolar fraction (x1)

x1 DHm (J/mol) Cpm (J/mol �C)

2-Butanone + corn oil

0.098 279 17450.152 369 16060.206 465 14540.304 680 13600.401 881 11910.452 994 11050.500 1038 10170.550 1085 9770.601 1212 8600.701 1270 7020.800 1090 5240.890 825 3600.950 576 267

3-Pentanone + corn oil

0.101 161 17930.150 208 15960.201 334 14520.282 408 12750.400 592 11610.449 659 11180.500 778 10370.547 855 9500.598 876 8940.694 897 6590.800 827 5480.899 503 3750.950 388 261

4-Methyl-2-pentanone + corn oil

0.108 204 17020.197 329 15640.307 439 13290.395 596 11470.501 684 10060.552 742 9520.596 778 8940.701 796 7170.803 695 5600.900 532 3940.950 401 324

Table 2 (continued)

x1 DHm (J/mol) Cpm (J/mol �C)

0.548 554 9230.596 574 8830.650 603 7830.700 616 7090.800 585 5740.900 410 3980.950 288 324

C. Gonzalez et al. / Journal of Food Engineering 79 (2007) 1104–1109 1107

expected. However, it is observed that isopropyl acetateand butyl acetate present higher DHm than their linearisomers, perhaps because of the steric hindrance. Molarfraction of 0.7 gives the maxima for all the studied binaries,as shown in the curves of enthalpies of mixing along

0

200

400

600

800

1000

1200

0.0 0.2 0.4 0.6 0.8 1.0

x1

ΔHm

(J·

mol

-1)

Ethyl acetate

Propyl acetate

Isopropyl acetate

Butyl acetate

Isobutyl acetate

Fig. 1. Enthalpies of mixing of the acetate–corn oil mixtures versus themole fraction of acetate at 25 �C.

0

200

400

600

800

1000

1200

1400

0.0 0.2 0.4 0.6 0.8 1.0

x1

HΔ

m (

J·m

ol-1

)

2-Butanone

3-Pentanone

4-Methyl-2-Pentanone

Fig. 2. Enthalpies of mixing of the ketone–corn oil mixtures versus themole fraction of ketone at 25 �C.

0

200

400

600

800

1000

1200

1400

1600

1800

0.0 0.2 0.4 0.6 0.8 1.0

x1

Cpm

(J/K

·mol

)

Ethyl acetate

Propyl acetate

Isopropyl acetate

Butyl acetate

Isobutyl acetate

Fig. 3. Heat capacities of mixing at constant pressure (Cpm) for the binarysystems acetate + corn oil as function of solvent mole fraction at 25 �C.

Table 4Fitting parameters Ak of Eq. (8) and standard deviations (r) for thestudied mixtures, organic solvents + corn oil at 25 �C

A0 A1 A2 A3 A4 r (J mol�1)

Ethylacetate 3701 3130 140 1405 4826 32Propylacetate 2657 1586 321 2100 3153 13Isopropylacetate 2909 2210 131 277 4615 26Butylacetate 1987 1119 �256 1552 2622 17Isobutylacetate 2082 1349 1195 1654 112-Butanone 4174 3520 2593 453-Pentanone 3037 2749 1339 314-Methyl-2-

pentanone2832 1798 �331 955 3992 23

1108 C. Gonzalez et al. / Journal of Food Engineering 79 (2007) 1104–1109

mole fraction of acetate. For binaries involving ketones +corn oil, the highest values are obtained for 2-butanone,and the lowest ones for the ramificated ketone. As in theprevious case, dipole–dipole interactions among ketonemolecules are broken to establish new weaker ones,between the ester group of triglyceride and the oxygen ofthe ketone.

Experimental data of enthalpies of mixing has been cor-related as a function of composition using the Redlich–Kister polynomial (Redlich & Kister, 1948):

DH m ¼ x1x2

XkP0

Akðx1 � x2Þk ð8Þ

r ¼PðDH calc � DH expÞ2

n

!1=2

ð9Þ

where DHm is the enthalpy of mixing, x1 and x2 are themole fractions of organic solvent and oil, and Ak are the fit-ting parameters. The values of Ak have been obtained by aleast-squares method with all points weighted equally. Theparameters Ak and the standard deviation r calculated withEq. (8), are listed in Table 4.

3.2. Molar heat capacities of mixing

From experimental data, the heat capacities at constant,Cpm pressure of the binary mixtures were calculated too bymeans of Eq. (7), and are represented versus solvent com-position in Figs. 3 and 4. Values of Cpm were decreasingas the mole fraction of solvent increases, and were adjustedto exponential polynomials showing slight variationsamong all the studied systems

Cpm ¼ a � xþ b ð10ÞThe values of the constants a and b for the studied mixturesare given in Table 5.

0

400

800

1200

1600

2000

0.0 0.2 0.4 0.6 0.8 1.0

x1

Cpm

(J/K

·mol

)

2-Butanone

3-Pentanone

4-Methyl-2-Pentanone

Fig. 4. Heat capacities of mixing at constant pressure (Cpm) for the binarysystems ketone + corn oil as function of solvent mole fraction at 25 �C.

Table 5Fitting parameters (a, b) of heat capacities-mole fraction of solvent, forsystems acetates or ketones + corn oil (Eq. (10)), their regressioncoefficients (r) and standard deviations (r)

a b r r

Ethylacetate �1593 1807 0.9988 22Propylacetate �1574 1817 0.9989 21Isopropylacetate �1613 1824 0.9983 24Butylacetate �1535 1817 0.9956 35Isobutylacetate �1511 1761 0.9974 302-Butanone �1677 1865 0.9985 273-Pentanone �1652 1844 0.9936 554-Methyl-2-pentanone �1616 1845 0.9981 29

C. Gonzalez et al. / Journal of Food Engineering 79 (2007) 1104–1109 1109

3.3. Physical properties of the corn oil

In Table 1 experimental values of physical properties(density, refractive index and speed of sound) for the cornoil are reported, in the temperature range 25–50 �C. If rep-resented, a linear decrease of the studied property isobserved when increasing the temperature. Bibliographydata is also given, and good agreement with our values isobserved.

Acknowledgement

The authors are grateful to the University of the BasqueCountry (UPV 00069.125-E-13813/2001) for financial sup-port of this work.

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