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Ion-exchange equilibria of aliphatic amino acid cations on a

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Ion-exchange equilibria of aliphatic amino acid cations on a

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238V.S.Soldatovetal./Reactive&FunctionalPolymers38(1998)237–247

beenextensivelyinvestigated.Thelatestreviewwaspresentedinarecentpaper[3].

Thereareseveralfactorspredeterminingthecomplexityofthesesystems:differentdistribu-tionofionicformsofaminoaciddependingonthesolutionpH;multicomponentnessofthesystemincluding,atleastinrealcases,one

more(exceptH1andNH1

3CHRCOOH)cation;dependenceoftheequilibriumcoef?cientofbinaryion-exchangeequilibriawithaminoonthedegreeofionexchange.Inter-pretative[3–6]describingthesepro-cessestakeintoaccountallthesefactorsassum-ingirregularityofexchangesitesofresins.Liquidionexchangershavecompletelydif-ferentstructuretothepolymericonesandcomparisonofregularitiesofionexchangeinbothsystemscangivenewinformationontheroleofpolymericstructureandmicroenviron-mentoftheexchangesitesinselectivityofsorptionofaminoacids.

Suchanapproachwasused?rstinthework

ofSoldatovandHogfeldt¨onequilibriaof

inorganiccations[7]andgaveabetterunder-standingoftheroleofthepolymericmatrixinionexchange.Themodelofion-exchange

equilibriausedbyHogfeldt¨was?rstreported

forionexchangeonthedinonylnaphthalenesulphonicacidin[8].Thismodelwasfurtherdevelopedrecently[9]andinthatformwillbeusedinthepresentpapertodescribetheaminoacidion-exchangeequilibria.

Itistheaimofthepresentpapertostudyion-exchangeequilibriabetweencationsofthemostwidespreadaliphatica-aminoacids(Gly1,Ala1,Val1,Leu1)andthehydrogenionondinonylnaphthalenesulphonicacidandcomparethesedatawiththoseonsulphostyreneionexchangers.

2.Experimental

Asolutionof0.1Mdinonylnaphthalenesulphonicacid(HD)inn-heptanewastakenasaliquidsulphonictypeionexchanger.Itwasa

productofKingsOrganicChemical(CT,USA).Itconsistsofamixtureofisomerswithgeneralformula

wheretheC9H19-radicalsareofabranchedstructure[8,10].

TheacidnumberoftheHDsamplewas1.86mmol/g.Thetheoreticalvalueis2.0.Analyticalgradeglycine,L-a-alanine,L-valineandL-leucinewereusedintheexperiments.EquilibriaofionexchangeH1–NH13CHRCOOHwerestudiedinthefollowingway.

Athermostatedvessel(T52060.58C)con-taining30.00mlofwater,1.00mloftheinitialaminoacidsolution(15.0000g/l)and20.00mloftheHDsolutionwassuppliedwithamag-neticstirrer.Itwassealedwithacapcontainingopeningsfortheglasselectrode,additionandremovalofaliquots.ThepHadjustmentwasachievedbyaddingsmallamountsof1MHClandwaskeptconstantat1.6060.05.Themix-turewasstirredfor2hwhichguaranteedtheequilibriumestablishment(10minintheslow-estcase).Aftercompleteseparationofthemixtureintotwolayers(5h)aprobeofaqueoussolution1.00mlwastakenoutforanalysisandthesameamountoftheinitialsolutionoftheaminoacidwasaddedtothevessel.WhennecessarythepHwasadjusted.Theprocedurewasrepeatedtoobtaintheextractionisotherminthedesiredrangeoftheaminoacidequilib-riumconcentration.

Theaminoacidconcentrationinthealiquotwasdeterminedcolorimetricallybytheninhydrinmethod[11,12].Theamountoftheextractedaminoacidwascalculatedfromthedifferencebetweentheaminoacidamountsintheinitialandequilibriumaqueoussolutions.Theamountofthehydrogenionintheex-tractantwasdeterminedasthedifferencebe-

V.S.Soldatovetal./Reactive&FunctionalPolymers38(1998)237–247239

Table1

ExperimentaldataondistributionoftheaminoacidcationsbetweentheHDheptanesolutionandtheaqueoussolutionatpH1.6

C(mM)AXAGly–H5.660.2078.130.26510.70.31913.90.36315.70.42522.90.49248.50.62477.30.67481.30.687176.50.763296.40.809Ala1–H1

0.3370.0801.120.1542.020.2242.920.2953.930.3634.880.4315.390.5066.960.5649.650.60811.90.64868.70.672126.30.706250.60.715Val1–H1

0.1710.0610.2990.1210.3410.1840.6400.2430.8960.3021.150.3621.450.4212.050.4752.390.5323.500.5775.330.6105.630.6678.530.67821.30.705157.90.706Leu1

–H1

0.076a

0.2210.114b0.2650.152b0.3090.305b0.3520.419b0.3940.648c0.4803.090.66115.80.69725.50.70527.40.71236.60.73264.80.75581.20.781111.00.791

pH1.51.b

pH1.52.c

pH1.57.

tweentheexchangecapacityoftheliquidionexchangerandthequantityoftheabsorbedaminoacid,countingallofitasauni-chargedcation.

Forthepointscorrespondingtominorcon-centrationofoneoftheexchangeableions,theexperimentwascarriedoutinseparatetestvesselsusingconventionalbatchtechnique.Thisallowedremovaloflargervolumesofthesolutionforanalysisandtoanalysethephaseoftheliquidionexchangerafterreextractingtheaminoacidwith1.5MNaOH.

Forcomparison,thesametechniquewasusedtomeasuretheequilibriumofH1–Na1ex-change.Theresultsobtainedcompletelyco-incidedwiththosereportedearlierforthissystem[7].Theexperimentaldataaresumma-rizedinTable1.

3.Resultsanddiscussion

InFig.1theaminoacidcationdistributiondiagramsarepresented.TheaminoacidcationconcentrationwascomputedfromthesolutionpHandthetotalaminoacidconcentrationusingtheequilibriumconstantsofthefollowingequilibria:

NH1?NH121

3CHRCOOH3CHRCOO1H(1)NH12213CHRCOO?NH2CHRCOO1H

(2)

accordingtoequation

CCA

A15]]]]]]]11K]

2(3)

1/CH11K1K2/CH1

whereK1andK2areequilibriumconstantsofdissociationequilibria(1)and(2)respectively.TheaminoacidcationisdenotedasA1,theHDsaltsasADorA1D2(ifmeaningrequires).Undertheconditionsofourexperiments(pH1.60)practicallyalltheaminoacidsexistedinthecationicform.Thisminimisedtheotherpossible(apartfromionexchange)sorptionprocesses.Thelargestpercentofthedipolarform(16%)occurredinthecaseofvaline.

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240V.S.Soldatovetal./Reactive&FunctionalPolymers38(1998)237–247

Fig.1.(a)DistributiondiagramoftheA1ionsintheH1–A1ionexchangeontheliquidionexchanger.A15Leu1(1),Val1(2),Ala1(3),]Gly1(4),Na1(5).(b)InitialpartoftheX5f(X)curve.

Therefore,thedominatingprocessresponsible

forextractionoftheaminoacidswasequivalent

ionexchangewhichcanbeformulatedasan

usualion-exchangeequilibrium

(H1D2)org1(A1)aq?(A1D1)org1(H1)aq

(4)

InFig.1aandbthedistributiondiagramsofthe

aminoacidcationsarepresented.

Threestraightforwardobservationsfollow

fromthese?gures:

(a)Atlowloadingoftheliquidionexchanger

withtheA1cationsselectiveextractiontakes

placeinallcases.

(b)Selectivityofextractionissubstantially

increasedwithincreasingsizeofthehydro-

carbonradical.](c)Athighloading(X.0.8)inversionofthe

selectivityoccursinallcasesandH1ion

becomespreferredtotheliquidionexchanger.

Theselectivitycoef?cientshavebeencom-

putedfromtheequilibriumdataas

]XA1XH1k5]](5)]]XA1XH1

]TheirdependenciesofXarepresentedinFig.]Fig.2.Dependencelogk5f(X),seeFig.1.Thepointscorre-]spondingtologkatX50arecomputedfromEq.(7).2.Itisinterestingtocomparetheliquidandresinoussulphonicionexchangers.Thelatterhavebeenstudiedbymanyauthors[5,6,13–16]andtheirdataareillustratedinTable2.Differ-enceinthekvaluesismainlyduetotheirdependenceontheioniccompositionandcross-linkageoftheresins.Insomeworks[13,14,16]itwasassumedthattheselectivitycoef?cientis

V.S.Soldatovetal./Reactive&FunctionalPolymers38(1998)237–247

Table2

Averageselectivitycoef?cientoftheH1–A1exchangeonsulphostyreneionexchangersA1

k

8%DVB[13]

4%DVB[14]5%DVB[15]7%DVB[16]8%DVB[5]241

8%DVB[6]Gly12.01.61.7Ala12.01.62Val1222Leu1

3.2

2

2

atrueconstantanditsvalueisgivenwithout

mentioningtherelative]

X.Intheotherworks,thethermodynamicconstants[15]ormeanselectivitycoef?cients[5,6]aregiven.Unex-plainablylowvaluesofkwerereportedin[16].Forcomparisonoftheselectivitycoef?cientsexperimentaldatafrom[5]weretakenforthe

same]

X50.3.Table3showsthattheeffectofincreasinghydrophobicradicalsisclearlyseeninthecaseoftheliquidionexchangerandthedifferencebetweentheaminoacidselectivityhereismuchgreaterthaninthepolymericionexchanger.Thiscreatesafavourableprecondi-tionfordevelopmentoftechnologyfortheirselectiveextractionandseparationusingtheHD.

ReturningtoFig.2,inallthecasesclosetoconstantlogkvaluesareobservedtractantloadingdegreebelow]

attheex-X50.5followedbyasharpdecreaseofthesevaluesatthehigherloading.Thecompletetransferoftheliquidionexchangerintotheformsofaminoacidcationswaspracticallyimpossible.Themaximumload-ingdegreeswere:forglycine0.80,foralanine0.70,forvaline0.70,forleucine0.85.Similarresultswereobservedforsorptionofbulkyorganicionsbypolymericionexchangers[17].

Table3

Selectivitycoef?cientsoftheH1–A1exchangeresinousionexchangersofsulphonictypeat]

ontheliquidand

X50.3Ion-exchangeequilibrium

kHD

8%DVB[5]Gly1–H11.31.6aAla1–H13.51.5Val1–H117.81.5Leu1–H1

100.0

4.9

a

5%DVB[15].

0.25220.511.01.621.021.2

3.0

2.9

Theyareexplainedbythesieveeffectcausedbystericobstaclestotheirpermeationintodif?cult-to-accesspartsofthepolymernetwork.Inaliquidionexchangersuchaneffectisexcluded.Thisfactmakesitnecessarytoreviseinsomecasesinterpretationofsimilareffectsobservedinionexchangeandlookfortheotherreasonsfortheirappearance.

ItisknownthationicpairsI1D2formedbothinorganicandorganiccationsarestronglyasso-ciated[8,10,18].TheHDmeanassociationnumberinheptaneinequilibriumwithwaterdeterminedbyvapourpressureosmometryis10.6[10]andthatforValDis21.8[19].Thedatafortheotherstudiedaminoacidsareabsent,neverthelessitcanbesuggestedthattheyarenotverymuchdifferentfromthoseforvaline.Ithasalsobeenshown[7,19]thatintheprocessofionexchangetheIDmicellesunder-goanumberofconversionsinsize,andproba-blyinstructure.SometimesacertaingeometricformisprescribedtotheassociatesofHDanditssalts[20].Butourresearchshowsthattheassociatesarepolydisperseandnonglobular[10,21].

TherearenoreliabledataonthemechanismofionexchangeinmicellarIDsolutions.ItisassumedinRef.[22]thatintheexchangeofhydrogenandmetalionsthe?rststageofionexchangeisinvasionofoneforeignionintoeachhydrogenmicelle.Thefurtherconversionofthehydrogenformintoametalionicformisnotconsidered.InRef.[7]itwasshownthatcontinuoussubstitutionofoneionbyanotherwithoutchangingofthemicellestructureispossibleonlyiftheexchangeableionsaresimilarinproperties(e.g.,Na1andK1).Ifthe

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