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Fluid Phase Equilibria 340 (2013) 7–10 Contents lists available at SciVerse ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Measurement and modeling of epicatechin solubility in supercritical carbon dio
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  FluidPhaseEquilibria 340 (2013) 7–10 ContentslistsavailableatSciVerseScienceDirect Fluid   Phase   Equilibria  journalhome   page:www.elsevier.com/locate/fluid Measurement   and   modeling   of    epicatechin   solubility   in   supercritical   carbondioxide   fluid Felycia   Edi-Soetaredjo a ,   Suryadi   Ismadji b ,   Yi-Hsu    Ju a , ∗ a DepartmentofChemicalEngineering,NationalTaiwanUniversityofScienceandTechnology,43,Sec.4.KeelungRd.,Taipei,Taiwan b DepartmentofChemicalEngineering,WidyaMandalaSurabayaCatholicUniversity,Kalijudan37,Surabaya60114,Indonesia a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received4August2012Receivedinrevisedform20October2012Accepted6December2012 Available online 20 December 2012 Keywords: DensitybasedmodelEpicatechinSolubilitySupercriticalcarbondioxide a   b   s   t   r   a   c   t The   experimental   equilibrium   solubility   of    epicatechin   in   supercritical   carbon   dioxide   was   measuredat   several   temperatures   (313.15,   323.15,   333.15   and   343.15   K)   and   pressure   inthe   range   of    12–26   MPa.The   solubilities   of    epicatechin   in   supercritical   CO 2  increased   with   pressure   and   temperature.   The   effectof    increasing   temperature   on   the   solubility   is   more   significant   at   high   pressure   thanlow   pressure.   Theexperimental   data   were   fitted   very   well   by   three   density-based   models   of    Chrastil,   del   Valle   and   Aguileraandthe   model   of    Méndez-Santiago   and   Teja.   Reasonable   fitting   parameters   of    the   models   were   obtained. © 2012 Elsevier B.V. All rights reserved. 1.Introduction Epicatechin(( − )- cis -3,3  ,4  ,5,7-pentahydroxyflavane(2 R ,3 R )-2-(3,4-dihydroxyphenyl)-3,4-dihydro-1(2 H  )-benzopyran-3,5,7-tri-ol)isaflavonolbelongingtothegroupofflavonoidsandwidelypresentinfruitsandvegetables.Highquantitiesofepicatechincanbefoundincocoa[1],   tea[2–4]andgrapes[5–7].Epicatechin isalsofoundinstapleplantsuchassago( Metroxylonsago )[8].Epicatechinexertsbeneficialhealth-relatedeffectssinceitactsasfreeradicalscavengersandinhibitorsofeicosainoidbiosyn-thesis[9,10].   Epicatechinalsoreduceslow-densitylipoproteininoxidation,riskofstroke,heartfailure,canceranddiabetes[1,9,11,12].Theinitialstepintherecoveryandpurificationofflavonoidsfromplantmaterialsisextraction.Soxhletextractionsusingorganic–aqueoussolventsarecommonlyused;howeverthismethodistimeconsumingandusesalotoforganicsolvents.Recently,microwave-assistedextraction[13],acceleratedsolvent extraction[14,15]andsupercriticalfluidextractionwereemployed toobtainbetterrecoveriesandtoreduceextractiontime[16,17].Amongtheseextractionmethods,supercriticalfluidextraction(SFE)offersanenvironmentalfriendlychoiceintermsofusinglessamountoforganicsolvent.SFEworksintheabsenceoflightandairthatcausedegradationofflavonoids.Amongmanysuper-criticalfluids,carbondioxideisextremelyattractivebecauseofit ∗ Correspondingauthor.Tel.:+886227376611;fax:+886227376644. E-mailaddress: yhju@mail.ntust.edu.tw (Y.-H.Ju). hasrelativelylowcriticaltemperature(304.15K),islesspossibletocausedegradationofthermallylabilecompoundsandiseasilyseparablefromtheextractedsolutes.Otherbenefitsinusingsuper-criticalcarbondioxide(SCCO 2 )areitisnontoxic,nonflammablesandinexpensive.ProcessdesignbasedonSFEandthedeterminationofopti-mumoperatingconditionsrequireknowledgeofphaseequilibriaandcompoundsolubilityinasupercriticalfluid.Inthelasttwodecades,manyattemptshavebeenreportedonthesolubilityof flavonoidcompounds.Songetal.[18]determinedthesolubil-ityofepigallocatechingallateinSCCO 2  withethanolcosolvent(0.044and0.084molefraction)atatemperaturerangingfrom313to333Kandpressurefrom15to35MPa.Theirsolubilitydatawerecorrelatedusingathermodynamicmodel,amodifiedChrastilmodelandtheMéndez-SantiagoandTeja[19]model.Cháferetal.[20]reportedexperimentalmeasurementsandthermodynamicmodelingofthesolubilityofquercetinwithethanolasthecosol-vent(5–30%)at313.15Kandpressurerangingfrom8to12MPa.ThesameoperatingconditionswereusedbyBernaetal.[21]forcatechinandCháferetal.[22]f orepicatechinandthedata weremodeledusingequationsofstate(EOS):Peng–RobinsonandSoave–Redlich–Kwongmodels.Othersolubilitydataofhydroxy-cinnamicacidsinSCCO 2  werereportedfor4-hydroxycinnamicacid(  p -coumaricacid),3,4-dihydroxycinnamicacid(caffeicacid),4-hydroxy-3-methoxycinnamicacid(ferulicacid)[23–25].TheexistingsolubilitydataforepicatechininSCCO 2 +ethanol[22]was   limitedatonefixedtemperature(313K)andanarrowpressurerangeof8–12MPa.Widerrangesoftemperatureandpressureareessentialinordertodetermineoptimumoperating 0378-3812/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2012.12.005  8  F.Edi-Soetaredjoetal./FluidPhaseEquilibria  340 (2013) 7–10 conditionsforseparationofepicatechinfromplantmaterialsusingSCCO 2 .EpicatechinisalmostinsolubleinSCCO 2  duetoitsstrongpolarity;thereforeethanolisthebestchoiceascosolventespeciallyforfoodandpharmaceuticalapplications.Intheirreport,Cháferetal.[22]usedethanolascosolventintherangeof5–30%.Thepres- enceofhighamountofethanolintheirsystemmay   havechangedthesupercriticalconditionofCO 2  intosubcritical[26–29].    Joungetal.[29]f oundthatthemaximumconcentrationofethanolin thesystemshouldbe6.49mol%at8.15MPa   and313.15Kinordertoobtainsupercriticalcondition.Undersubcriticalcondition,mostepicatechindissolvedinethanolratherthaninSCCO 2 .Furthersep-arationofepicatechinfromethanolisneededinordertoobtainpureepicatechin.Consideringtheimportanceofsolubilitydataofsolidsinsuper-criticalfluidsforthedevelopingoftheextractionprocessandthedependencyofsolubilityondensity,temperatureandpressureof supercriticalfluids;extensiveamountsofdataareneeded.More-over,thepresentofcosolventinSCCO 2 extractionproducesimpureextractwiththecosolventasanimpurity.Therefore,inordertoobtainpureextract,separationofextractandcosolventisneeded.Theobjectiveofthispaperistomeasurethesolubilitiesofepi-catechininSCCO 2  withoutcosolventatdifferenttemperatures(313.15,323.15,333.15,and343.15K)andpressures(12–26MPa).Theexperimentaldatawerethencorrelatedusingdensity-basedmodels(Chrastil[30],delValleandAguilera[31]andMéndez- SantiagoandTeja[19]models).Tothebestofknowledge,thereisnoliteratureavailableonthesolubilityofepicatechininSCCO 2 intherangeofpressuresandtemperaturesstudiedinthiswork. 2.Experimental  2.1.Materials HPLCgrade( − )-epicatechinwithapurityof98wt.%wasobtainedfromSigma–Aldrich(Singapore)andwasusedwithoutanyfurtherpurification.AnalyticalgradeethanolwassuppliedbyMerck(Darmstadt,Germany)andwasusedasasolventtocollectextractforfurtheranalysis.Foodgradecarbondioxide(99%purity)wasusedasthesupercriticalsolventandsuppliedasliquidCO 2  byAnekaGasPtyLtd(Indonesia)withapurityof99.9%.  2.2.Experimentalprocedure TheexperimentsofepicatechinsolubilityinsupercriticalCO 2 werecarriedoutinastaticsystemconsistsofa50ml   longequil-ibrationcolumn(Swagelok,USA),ahighpressurepump(EldexAA-100-S-2-CE,USA)andapressuretransducer(DruckPTX611,USA)withadigitalprocessindicator(DruckDPI280,USA)whichgivesapressuremeasurementuncertaintyof  ± 0.01MPa.Thesys-temtemperaturewascontrolledbyaheatingchamber(OvenMemmert,Germany).Theuncertaintyoftemperaturemeasure-mentintheheatingchamberis ± 1K.Allfittingandtubingusedinthissystemweremadeofstainlesssteel316(Swagelok,USA).Themaximumworkingpressureandtemperatureofthesupercriticalextractionsystemwere40MPa   and373.15K,respectively.Epicatechinsolubilitydatawereobtainedbyadding100mg   of ( − )-epicatechininasampleholderintheequilibrationcolumn.Thesystemwasheatedtoadesiredtemperature(313.15,323.15,333.15,or343.15K).Subsequently,liquidCO 2  waspumpedintothesystemusingthehigh-pressurepumpuntiladesiredpressurewasreached(12–26MPa).Afterequilibriumconditionwas   achievedin4h(insignificantincreaseofepicatechinconcentrationinCO 2  wasobservedafter3h),theoutputvalvewasreleased,andthesampleflowedintoacollectorcontainingaknownamountofethanoltoseparateepicatechinandCO 2 .Atleastthreereplicationsoftrialswerecarriedoutforeverysetoftemperatureandpressure,andtheequilibriumcompositionwasrepresentedbyaverageofthethreereplications.Theuncertaintyofeachmeasurementwaswithin ± 2%.ThedeterminationofepicatechinconcentrationinethanolwasbasedonthemethodofZuoetal.[32]byusingaHPLC(JascoHPLC PU-2089plus)withaUV–visdetector(UV-2077plus). 3.Resultsanddiscussion TheexperimentalequilibriumsolubilitydataofepicatechinandtheresultofcalculateddensitiesofsupercriticalSCCO 2  atvari-ouspressuresandtemperaturesusingtheStryjekandVera[33]modificationofthePeng–RobinsonequationofstatearereportedinTable1.ThesolubilityofepicatechininSCCO 2  increaseswithincreasingpressureandtemperature.Atconstanttemperature,increasingpressureraisesSCCO 2  densityandincreasesitsabilitytodissolvesolute.Atconstantpressure,increasingtemperatureraisessolutevaporpressureaswellasthediffusivitiesofbothsolventandsolute.Thecorrelationofexperimentalsolubilitydatawas   investigatedusingseveralsemi-empiricalmodels.Chrastil[30]proposedthe firstmodelfordensity-basedcorrelation.Itisbasedonthehypoth-esisthatonemoleculeofasoluteAassociateswith k moleculesof asolventBtoformonemoleculeofsolvato-complex  AB k  inequi-libriumwiththesystem.Thedefinitionoftheequilibriumconstantthroughthermodynamicconsiderationresultedinthefollowingmodelforthesolubility: c  1  =  k exp  aT   + b   (1)where c  1  istheconcentrationofthesoluteinthegas(gl − 1 ),  isthedensityofthegas(gl − 1 ), k isanassociationnumber, a isafunctionoftheenthalpyofsolvationandenthalpyofvaporization(K − 1 ),and b isafunctionofassociationnumberandmolecularweightsofthesoluteandsupercriticalfluids.VariousmodificationsonChrastilmodelwereproposedsuchasmodifiedChrastilbyGarlapatiandMadras[34,35],   modifiedChrastilbyWang[36],andmodifiedChrastilbydelValleandAguil- era[31].AmongthesemodifiedChrastilmodels,delValleand Aguilera[31]claimedthattheirmodelfittedwellfortempera-turesfrom293to353Kandpressurebetween15and88MPa   andadequatelypredictedsolubilityunder100gl − 1 withintheregionsuggestedforcommercialsupercriticalfluidextractionoffoodcomponents(Eq.(2)). c  1  =  k  exp  b  + a  T   + dr  2   (2)Thephysicalmeaningoftheparameters k  , b  ,and a  aresimilartoChrastilmodel.Theparameter d  isintroducedtocompensatethevariationofenthalpyofvaporization(  H  vap )withtemperature.Multivariablenon-linearregressionanalysisofallexperimentaldatawas   performedtoestimatetheconstantsinChrastilmodelandmodifiedChristilmodelbydelValleandAguilera.Thequalityofalldatacorrelationsisquantifiedbythesumofsquarederrors(SSE),definedasfollows:SSE =  c  1(exp) − c  1(cal)  2 N   1 / 2 (3)where c  1(exp)  istheactualsolubilityofepicatechininSCCO 2 , c  1(cal) isthecalculatedsolubility,and N  isthenumberofexperimen-taldata.Multivariablenon-linearregressiontechniqueemployedaniterativecurvefittingprocedure.Aninitialestimationforeachparameterwasprovided,andthencalculationofapoint-by-pointsumofsquares(Eq.(3))   foreachiterationwas   conducteduntilconvergencecriteriawerefulfilled.  10  F.Edi-Soetaredjoetal./FluidPhaseEquilibria  340 (2013) 7–10 Fig.3. Méndez-SantiagoandTejamodelfittings(wiremesh)andexperimentalsolubilitydata(  ). andenthalpyofvaporizationinChrastilmodelispresentedintheparameter a ( a =  H  / R ).However,delValleandAguilera[31]modifiedthisparametertocompensatethevariationofenthalpyofvaporizationwithtemperature(  H  ( T  )= R ( a  +2 d / T  )).Theheatofsolution(  H  )forepicatechinandCO 2  usingChrastilmodelis89.7kJ/mol,whileusingdelValleandAguileratheheatof solutionfallsinarangeof87.3–89.9kJ/mol.Fromtheanalysesof allparametersforbothmodels,weconcludethatthesolubilityof epicatechininSCCO 2  canbewellfittedbybothmodels.Anothersemi-empiricaldensitybasedmodelwas   proposedbyMéndez-SantiagoandTeja[19]basedonthetheoryofdilutesolu- tions.ThismodeltookintoaccountHenry’slaw,infinitedilutionpartitioncoefficientsnearthecriticalpointofthesolventandClausius–Clapeyron-typeexpressionforthesublimationpressure.Theequationhasthreeadjustableparameters(  A  , B  and C   ),definedasfollow: T  ln( c  1 P  ) =  A  + B   + C   T  (4)Fig.3showsthattheMéndez-SantiagoandTejamodelcanrepre-senttheexperimentalsolubilitydataofepicatechininSCCO 2  well.Valuesof   A  , B  and C   are − 15,230,3.008and4.108,respectively,andtheSSEis1.18%.Thethreeconstants(  A  , B  and C   )aretheresultofmultivariablenon-linearregressionanalysis,whichisindepen-dentoftemperatureandpressurewithoutanysignificantphysicalmeaning,therefore,canbeusedtoextrapolatesolubilitydatatoothertemperature[19]. 4.Conclusion SolubilitydataofepicatechininSCCO 2 ,inwiderrangesof temperatureandpressure,wereobtainedinthisstudy.Theexper-imentaldatawas   fittedbyusingdensity-basedmodels(Chrastil,delValleandAguileraandMéndez-SantiagoandTeja).Themodelsfittedverywelltotheexperimentaldataandresultedinreasonablevaluesofthefittingparameters. 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