Unique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdf
Trang 1jo u r n al h om ep a g e :w w w e l s e v i e r c o m / l o c a t e / s a a
Department of Chemistry, Faculty of Arts and Sciences, American University of Beirut, P.O Box: 11-0236, Riad El Solh, Beirut, 1107-2020, Lebanon
a r t i c l e i n f o
Article history:
Received 30 September 2010
Received in revised form 17 May 2011
Accepted 24 May 2011
Keywords:
Curcumin
Hydrophilic ionic liquid
Micelle
Surfactant
Spectroscopy
a b s t r a c t Hydrophilicionicliquid,1-butyl-3-methylimidazoliumtetrafluoroburate,modified thepropertiesof aqueoussurfactantsolutionsassociatedwithcurcumin.Becauseofpotentialpharmaceuticalapplications
asanantioxidant,anti-inflammatoryandanti-carcinogenicagent,curcuminhasreceivedampleattention
aspotentialdrug.Theinteractionofcurcuminwithvariouschargedaqueoussurfactantsolutionsshowed
itexistsindeprotonatedenolforminsurfactantsolutions.Thenitroandhydroxylgroupsofo-nitrophenol interactwiththecarbonylandhydroxylgroupsoftheenolformofcurcuminbyforminggroundstate complexthroughhydrogenbondsandofferedinterestinginformationaboutthenatureofthe interac-tionsbetweentheaqueoussurfactantsolutionsandcurcumindependingonchargeofheadgroupofthe surfactant.IL[bmin][BF4]encouragedearlyformationofmicelleincaseofcationicandanionicaqueous surfactantsolutions,butslightlyprolongedmicelleformationinthecaseofneutralaqueoussurfactant solution.However,forcurcuminIL[bmin][BF4]favoredstrongassociation(7-foldincrease)withneutral surfactantsolution,marginallysupportedassociationwithanionicsurfactantsolutionanddiscouraged (∼2-folddecrease)associationwithcationicsurfactantsolution
© 2011 Elsevier B.V All rights reserved
1 Introduction
Micellarsystemsofaqueousoriginhaveimmensetechnological
applicationsas flow field regulators, solubilizing and
emulsify-ingagents,membranemimeticmedia,nanoreactorsforenzymatic
reaction and drug deliverysystem [1–8] It is anticipated that
curcumin,
1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, may find applications as a novel drug in the near
future to control various diseases, including inflammatory
dis-orders,carcinogenesisandoxidativestress-inducedpathogenesis
[9–12].Curcuminhasdrawnintenseinterestrecentlydue toits
potentialpharmaceuticalimportance[13–24].However,curcumin
isverypoorlysolubleinwaterbyreducingitseffectivenessasa
drug.Therefore,variousmethodsarebeingdevelopedtomake
cur-cuminbettersolubleandenhanceeffectivenessofthedrugduring
itsdelivery[16]
Physiochemicalproperties of an aqueoussurfactantsolution
dependontheidentityofthesurfactant.Theaqueoussolutionof
a surfactantat a givenconcentrationposses moreor less fixed
physiochemicalproperties that are difficultto modulate.Other
thanchangingtemperatureandpressure,theusualwayto
mod-ifythephysiochemicalproperties ofa givensurfactantsolution
is to use external additives, such as cosolvents, cosurfactants,
∗ Corresponding author Tel.: +961 1350 000x3985; fax: +961 1365217.
E-mail address: dp03@aub.edu.lb (D Patra).
electrolytes,non-polarorganics,polarorganics,etc.Ionicliquids (ILs) are solvents composed entirely of ions and composed of poorly coordinating ionsand can therefore behighly polaryet non-coordinating[25–27] Theseareimmiscible witha number
of organicsolvents and providenon-aqueous polaralternatives fortwophasesystems.Theyareofparticularinterestbecauseof theirenvironmentallyfriendlynature,theirexcitingfeaturesand theireconomicalconvenience[28–35].Theunusualpropertiesof ILsdemonstrateauniqueroleinalteringthepropertiesofaqueous surfactantsolutionssuchasaggregationnumber[3,4].The effec-tivenessofthismodificationofaqueoussurfactantsolutionsbyIL maylargelydependonthekindandextentofinteraction/sbetween cation/anionoftheILandtheheadgroupofthesurfactant[4] How-ever,hydrophobiceffectofILwithsurfactantmoleculemightplay
arole.InadditionwehypothesizethatILmaydrivethe associa-tionofthedrugmoleculetowardsbettersolubilizationinmicellar system(whichisveryimportantduringdrugdelivery)asperthe headgroupofthesurfactantchargeandphysiochemicalproperties
ofthedrugmolecule
Inordertounderstandthebetterinsightoftherole ofthese interactions of IL during solubilization of poorly water soluble drugsuchascurcumininmicellarsystemsandmicellization,we extendthestudyofinteractionofILandsurfactantsolutions[4] furthertosystemscomposedof various(positiveand negative) charged and uncharged surfactant solutions, curcumin and an
IL(1-butyl-3-methylimidazoliumtetrafluoroburate,[bmin][BF4]) The association of curcumin with various charged surfactant
1386-1425/$ – see front matter © 2011 Elsevier B.V All rights reserved.
Trang 2solutions and fluorescence quenching of curcumin by
o-nitrophenolindifferentsurfactantsolutionsmayexplorethekind
ofinteractionbetweencurcuminandvariouscharged/uncharged
surfactantsolutionswithoutIL.Duetocation/anionoftheIL,itmay
remarkablyaltertheinteractionofcurcuminandsurfactant
solu-tionsbasedonthechargeoftheheadgroupofthesurfactantand
deprotonatedformofcurcumin,thereforeimpactdrug–surfactant
association.Comparativestudyofvariouscharged/uncharged
sur-factantmoleculesmayconcludeimportanceofhydrophobiceffect
ofILduringmicellization
2 Materials and methods
2.1 Materials
Thesurfactants cetyltrimethyl ammoniumbromide (CTAB),
sodium dodecyl sulfate (SDS) and Triton X-100 (TX100) were
obtained from Acros Organics and were dissolved in different
volumesofdoubledistilledwaterforthepreparationofseveral
con-centrationsofsurfactantsolutions.Thestocksolutionsconsistedof
10mMCTAB,100mMSDSand10mMTX100.Curcuminwasalso
obtainedfromAcrosOrganicsandwasusedwithoutfurther
purifi-cation.Topreparethestocksolution,curcuminwasdissolvedin
spectroscopicgradeacetonitrile(AcrosOrganics)sothatthefinal
concentrationofacetonitrileinthesurfactantsolutionsremained
lessthan1%(v/v).1-Butyl-3-methylimidazoliumtetrafluoroburate,
[bmin][BF4]was obtainedfrom Flukaand o-nitrophenolwas a
MerckSchuchardtproduct.Thesolventswereusedwithoutfurther
purification
2.2 Spectroscopicmeasurements
TheabsorptionspectrainvarioussolventsandincationicCTAB,
anionicSDS,andneutralTX100wererecordedatroomtemperature
using a JASCO V-570 UV–VIS–NIR Spectrophotometer
Fluores-cencemeasurementsweredoneonaJOBINYVONHoribaFluorolog
3spectrofluorometer.Theexcitationsourcewasa100WXenon
lamp.ThedetectorusedwasR-928operatingatavoltageof950V
Theexcitationand emissionslitswidthwere5nm.Thespectral
datawerecollectedusingFluorescencesoftwareanddataanalysis
wasmadeusingOrginPro6.0software
3 Results and discussion
3.1 Curcumin–surfactantinteractioninabsenceofIL
Generally,curcuminshowedastrongandintenseabsorption
bandin the 350–480nm wavelength region in all the
investi-gatedsurfactantsolutions Representativeabsorptionspectraof
curcumininvariousconcentrationsofTX100solutionsaredepicted
inFig.1
The interaction between curcumin and micelles can be
describedas:
C+SCSKb
whereC iscurcumin;Sis thesurfactant(CTAB,SDSorTX100);
CSisthecurcumin–surfactantcomplex;andKbistheassociation
constant
Theconcentrationofthemicellizedsurfactantisgivenby:
Sm=Ss−cmc
whereS isthesurfactantconcentration
700 600
500 400
300 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1.0
7-9 6 5
4 3 2 1
Wavelength (nm )
Fig 1.Absorption spectra of curcumin in various aqueous TX100 concentrations.
Table 1
Association rate constants of curcumin with various aqueous surfactant solutions
in the absence and presence of ionic liquid.
Sample cmc used for calculation (mM) K b
TX100 0.2 11,555 M −1
SDS + IL (1%, v/v) 0.95 6315 M −1
CTAB + IL (1%, v/v) 0.1 10,227 M −1
TX100 + IL (1%, v/v) 0.4 82,737 M−1
CTSml
A =
Sm
εs−ε0+K 1
gb(εs−ε0) wherelistheopticalpathlength,εmisthemolarexcitation coeffi-cientofcurcuminfullyboundtomicelles,ε0isthemolarexcitation coefficientofcurcumininthesolvent,CTisthetotalcurcumin con-centrationandA=A− A0whereAistheabsorbanceofcurcumin
inthepresenceofsurfactantsolutionandA0istheabsorbanceof curcuminintheabsenceofmicelle/surfactant
UsingScott’splots[6,36–39],theassociationconstantsofCTAB, SDSandTX100weredeterminedtobe20,467M−1,6193M−1and 11,555M−1(Table1 respectively.Itshouldbenotedthatthecrtical micellarconcentration(cmc)forthecalculationofassociation con-stantsforvariousmicellewasestimatedbyfluorescencemethod
asexplainedlateron.Itisobservedthat,KbCTAB>KbTX100>KbSDS These resultsimplied that thedifferent micelleshave different affinitiesforcurcumin.CationicCTABisboundtocurcuminwith thehighestaffinity,followedbyneutralTX100andthenanionic SDS.Thiscouldbeduetotheelectrostaticinteractionsbetween cur-cuminandthepositivechargeontheheadgroupofCTABpresentin theSternlayerofthemicelle,thusindicatingthatcurcuminatthe givenconditionsismainlyfoundinitsdeprotonatedanionicforms [40](seeSupplement1).InthecaseofSDS,therepulsionbetween deprotonatedenol(anionic)formsofcurcuminandthenegative chargeontheheadgroupofSDSpresentintheSternlayerofthe micellemakeaweakerinteraction,hencedecreasingthe associa-tionrateconstant.However,giventhattheheadgroupofTX100is nonionic,thevalueoftheassociationrateconstantforTX100was
inbetweenthatofCTABandSDS
3.2 Criticalmicellarconcentrationdetermination Fluorescenceexcitationandemissionspectraofcurcuminwith variousconcentrationsofsurfactantnotedthatthefluorescence
Trang 3300 350 400 450 500 550 600 650 700
0.0
4.0x106
8.0x106
1.2x107
1.6x107
2.0x107
10 8-9
Curcumin with [TX100] (1) No TX10 0
(5) 0.1 mM (6) 0.2 mM (7) 0.6 mM (8) 0.8 mM (9) 1.0 mM
0.0 4.0x106 8.0x106 1.2x107 1.6x107 2.0x107 2.4x107
2.8x107
11-12
11-12 10 8-9
6
6 1-5
1-5
Fig 2. Fluorescence excitation and emission spectra of curcumin in various aqueous
TX100 concentrations.
intensityoftheemission and excitationspectraof curcuminin
TX100(shownin Fig.2)and SDS(not shown)increasedasthe
concentrationofthesurfactantwasincreased.However,the
flu-orescence spectra of CTAB exhibited a different behavior (not
shown)
Thefluorescenceintensityinitiallydecreased untilitreached
0.5mMofCTABandoncethecmcwasreached,theintensitystarted
increasingwithconcentration.Aredshiftwasalsoobservedafter
thecmcforCTAB.TheStokes’shiftofcurcumininvarious
concen-trationsofCTAB,SDSandTX100wasdeterminedasthedifference
betweenabsorptionandemissionmaximaobtainedfromthe
cor-rectedspectraonthewavenumberscale[41,42].TheplotofStokes’
shiftversussurfactantconcentrationofferedthreedifferentkinds
ofchange,respectively,forcationic(CTAB),anionic(SDS)and
neu-tral(TX100)surfactantsolutions.InthecaseofCTAB,thevalueof
Stokes’shiftrarelychangedbeforethecmc.Abigjumpof5000cm−1
wasobservedaroundthecmcandafterthecmcitremainedmore
orlessunaltered.ThecmcofCTABwasestimatedbyfindingthe
midpointofthetangentjoiningthetwolines,asshowninFig.3A
ForSDS,Stokes’shiftofcurcuminfordifferentsurfactant
con-centrationsvarieddifferently,itinitiallydecreasedtillthecmcwas
reached.Abovethecmc,itmarginallyincreased.Byextrapolating
thesetwolinearequations,beforeandafterthecmc,with
respec-tivenegativeandpositiveslopes,aminimumintersectingpointwas
obtainedtocalculatethecmc(Fig.3B).Stokes’shiftofcurucmin
increasedwithTX100concentrationuntilcmcwasattainedand
thenitdecreaseddramatically.Inthiscasethemaximumvalueof
Stokes’sshiftwasusedtoestimatecmcasmarkedinFig.3C.Thecmc
valuesestimatedusingStokes’shiftofcurcuminissummarizedin
Table2,thevaluesobtainedwithoutILaresimilartothereported
values[4,5,43]establishingthereliabilityofthemethod.The
differ-Table 2
cmc values of aqueous CTAB, SDS and TX100 solutions in the presence and absence
of ionic liquid.
Sample cmc
Curcumin
(cm −1 )
Pyrene I I /I IIIa Reported b
SDS 7.3 mM 7.0 mM 6.0–8.0 mM
TX100 0.2 mM 0.25–0.5 mM 0.9 mM
SDS + IL (1%, v/v) 0.95 mM 1 mM (2%, v/v) –
CTAB + IL (1%, v/v) 0.1 mM – –
TX100 + IL (1%, v/v) 0.4 mM 0.5–1.0 mM (2%, v/v) –
a From Refs [3,4]
b From Ref [43]
5000 6000 7000 8000 9000
A
-1 )
[CTAB]
CTAB
0.0000 0.0005 0.0010 0.0015 0.0020
35000 40000 45000 50000 55000 60000 65000 70000
cmc of CTAB + IL cmc of CT AB
4000 4200 4400 4600 4800 5000 5200
cmc of SDS + IL
-1 )
[SDS]
SDS
0.00 0 0.005 0.010 0.015 0.020
18000 18200 18400 18600 18800 19000 19200 19400 19600 19800
cmc of S DS
SDS + IL
0.0000 0.0003 0.0006 0.0009 0.0012 0.0015 0.0018
3000 3500 4000 4500 5000 5500
6000
C
-1 )
TX100
0.00 00 0.000 3 0.00 06 0.000 9 0.0012 0.001 5 0.0018
0 1000 2000 3000 4000 5000
cmc of Tx10 0 + I L
cmc of TX100
TX100 + IL
Fig 3. Variation of Stokes’ shift of curcumin in different concentrations of aqueous CTAB (A), SDS (B) and TX100 (C) in the absence and presence of IL.
enttrendsofStokes’sshiftforvarioussurfactantscouldbedueto thevariouskindsofinteractionsbetweenthecharged/uncharged headgroupsofthesurfactantsandthedeprotonatedformsof cur-cumin
3.3 Quenchingstudybyo-nitrophenol o-Nitrophenol can strongly quench the fluorescence of cur-cumin by forming a ground state complex through hydrogen bonding[24]asgiveninScheme1
However,theextenttowhichitquenchesmayhighlydependon theconditionsofthemediuminwhichcurcuminando-nitrophenol
Trang 4OH
Formation cyclic ground state complex of curcumin with o-nitrophenol
Scheme 1. Ground state complex formation of curcumin with o-nitrophenol causing fluorescence quenching of curcumin by o-nitrophenol.
can interact and hence, on the nature of the surfactants The
positionofthefunctionalgroupsino-nitrophenolandthe
geom-etryofthemoleculepredictthelocationofo-nitrophenolinthe
micelle[44].Thebenzeneringofthephenolispushedtowardsthe
hydrocarboncoreandthepolarfunctionalgroupsremaininthe
hydrophiliclayerofthemicelle[44].Giventhatthestoichiometric
ratioofo-nitrophenoltocurcuminis1:1,thenitroandhydroxyl
groupsofthequencherinteractwiththecarbonylandhydroxyl
groupsoftheenolformofcurcuminbymeansofstronghydrogen
bonds[24].Thisassociatedcomplex,whichisformedintheground
state,greatlyquenchesthefluorescenceofcurcuminthroughthe
followingprocess:
h νa
h νa
hνfl
h νfl
Using the Stern Volmer equation [45] the quenching rate
constantKsv ofcurcuminand thequencher,o-nitrophenol, was
determinedas
I0
f
If =1+Ks[oNP]
I0
f
If =1+kq0[oNP]
whereKsvistheSternVolmerrateconstant,I0
f isthefluorescence intensitywithoutthequencher,Ifisthefluorescenceintensitywith
thequencher,kqisthequencherratecoefficient,0isthe
fluores-cencelifetimeofcurcuminwithoutthepresenceofthequencher
and[oNP]istheconcentrationofo-nitrophenol.Fig.4illustrates
thefluorescencespectraofcurcumininthepresenceofSDS
with-outandwithvariousconcentrationsofo-nitrophenol.Theinsertin
Fig.4presentstheSternVolmerplot[45]forcurcumininpresence
ofvariousconcentrationofo-nitrophenol
Thefluorescencespectraofcurcumininwater,CTABandTX100
withoutandwithvariousconcentrationsofo-nitrophenolalong
withtheirrespectiveSternVolmerplotsshowedsimilartrends(not
shown).TheestimatedvaluesofKsvandkqforfluorescence
quench-ingofcurcuminbyo-nitrophenolinwater andvariousmicellar
mediaisdeterminedaspertheSternVolmerequation[45]and giveninTable3.Thequenchingrateconstantofcurcuminby o-nitrophenolinwaterwasdeterminedtobe449M−1incomparison
to3973M−1incationicCTAB.ThehighquenchingrateofCTABis duetothestabilizingelectrostaticinteractionsbetweenthe pos-itively chargedhead groups of themicelles and the negatively chargedenoliccurcumin(seeSupplement1).Thisattractive inter-actionfacilitatesthepenetrationofcurcuminintheSternlayerof themicelleandhencetheformationofthecomplex[CUR–NP].In thecaseofanionicSDS,adecreaseinthequenchingrateconstant wasfoundrelativetothatofwater.Thischangecanbelinkedto
Fig 4.Fluorescence emission spectra of curcumin in SDS in the presence of various concentration of o-nitrophenol The fluorescence intensity decreases with increase
in o-nitrophenol concentration Insert shows Stern Volmer plot for the determina-tion of the quenching rate constant K
Trang 5Table 3
Quenching rate constants of curcumin by o-nitrophenol in water, CTAB, SDS and
TX100 surfactant solutions.
Sample K sv (M−1) k q (( 0av = 2.366 ns)
Water 449 1.9 × 10 11 M −1 s −1
CTAB 3973 1.7 × 10 12 M −1 s −1
SDS 367 1.6 × 10 11 M −1 s −1
TX100 550 2.3 × 10 11 M −1 s −1
association
Fig.5)andfluorescenceexcitationandemission(seeFig.5)
spec-traofcurcumininvarioussurfactantconcentrationsinpresence
ofILshowedtheabsorbanceorfluorescenceintensityofcurcumin
700 600
500 400
300
0.0
0.5
1.0
1.5
2.0
2.5
3.0
7-8 6
5
4 3 2
Wavelength (nm)
(1 ) NO TX100 (2 ) 0.02 mM (3 ) 0.06 mM (4 ) 0.2 mM (5 ) 0.4 mM (6 ) 0.6 mM (7 ) 0.8 mM (8 ) 1.0 mM
Curcumin plus IL with [TX100]
1
700 650 600 550 500 450 400 350
300
0.0
8
8 7
6 4-5
Wave leng th (nm )
0
Curcumin plus IL with TX100
9
9
4-5
1-3
1-3
(1) No TX100 (2) 0.02 mM (3) 0.04 mM (4) 0.06 mM (5) 0.1 mM (6) 0.2 mM (7) 0.4 mM (8) 0.8 mM (9) 1.0 mM
Fig 5. Absorption and fluorescence (excitation and emission) spectra of curcumin
in various aqueous TX100 concentrations in the presence of IL.
inCTAB,SDSandTX100,increasedwithsurfactantconcentration Theassociationconstantsforthethreesurfactantswithcurcumin
in thepresenceofIL weredeterminedasexplained earlierand giveninTable1.TheassociationconstantofCTABinthepresence
ofILdecreasedsignificantlyrelativetoCTABwithoutIL.Thoughthe shorthydrophobiceffectofthetailmayencouragetheILtolocate aroundtheSternlayerofthemicelle,thepositivechargedhead groupwouldrepulsewiththesimilarchargedheadgroupsofCTAB FinallybothCTABandILwillcompetetobindwithdeprotonated formofcurcumin
Thiscompetitioncouldaccountforthedecreaseinthe associa-tionconstantofcurcuminwithCTAB.However,inthecaseofSDSin thepresenceofIL,anincreaseoftheassociationrateconstantwas observedcomparedtoSDSwithoutIL.IntheabsenceofIL,there
isrepulsionbetweenthenegativechargeoftheheadgroup (sul-fateion)ofSDSandthenegativechargeofthedeprotonatedform
ofcurcumin.WhenILisadded,itspositivechargeheadgroupwill actasastabilizerbetweennegativelychargedSDSandnegatively chargedcurcumin(deprotonatedform),thusfacilitatingthe asso-ciationofcurcuminwithSDS.Ontheotherhand,theassociation rateconstantofcurcuminwithTX100increasedsignificantlyinthe presenceofIL.Apossibleexplanationwouldbetheinductionof hydrogenbondinganddipole–dipoleforcesbythepositivecharge
oftheheadgroupoftheILwithTX100[4],assistinginteractionor strongassociationofcurcuminwithneutralsurfactantsolution 3.5 Effectofionicliquid[bmin][BF4]onmicellization
Asdiscussedearlier,thecmcofvariousaqueoussurfactant solu-tionswasevaluatedbasedonthechangeinStokes’shift(seeFig.3)
ofcurcumininthepresenceof1%(v/v)IL.VariationofStokes’shift withsurfactantconcentrationforCTABwithandwithoutILshowed similartrends.Itcouldthereforebeimpliedthatthereisnonew kindoffavorableinteractionbetweentheILandCTAB.However, similarplotsforSDSwithandwithoutILgavetwodifferenttrends indicatingthattheinteractionofcurcuminwithSDSinthe pres-enceandabsenceofILarenotsimilar.Asshownearlier,in the absenceofIL,theStokes’shiftofcurcuminincreasedwithincrease
inSDSconcentrationsuntilcmcwasreached.However,whenIL waspresent,Stokes’shiftcontinuedtodecrease,butata much smallerrate,withincreasingSDSconcentration.Thistrendcould implythatinthecaseofSDS,therecouldbeafavorable interac-tionthatstabilizesthemicellesinthepresenceofIL.ForTX100, variationofStokes’shiftwithsurfactantconcentrationshowed dif-ferenttrendsinthepresenceandabsenceofIL.WithoutIL,there wasabigincreaseinStokes’shiftofcurcuminafterthecmcwas reachedwhereasinthepresenceofIL,therewasanotabledecrease
ofStokes’shiftafterthecmc.Thisimpliesthattheinteractionsof TX100solutionsinthepresenceandabsenceofILareofdifferent nature.ItwasfoundthatcmcofCTABdecreasedwhen 1%(v/v)
ILwasadded(Table2).Thisdecreaseindicatesthatinthe pres-enceofthehydrophilicIL,theformationofmicellesisfavoredat relativelylowerconcentrations.Apossiblereasonforthis observa-tionwouldbethefavorablehydrophobicinteractionofthecarbon chainsofboth CTABand[bmin][BF4]as wellasthecumulative electrostaticinteractionamongCTAB,curcuminand[bmin][BF4] Thus, boththeelectrostaticinteractionand thetendencyof the hydrophobicchainstocometogetherfurtherencouragethe for-mationofmicellesandhencelowersthecmc.Similarly,thecmc
ofSDSdecreasedsignificantlyinthepresenceofIL(Table3).The loweringofthecmcofSDSinthepresenceofILwasalsoreported earlier[3]andthis couldbeattributedtoboththehydrophobic effectandtheattractionbetweentheanionicSDSandthepositively chargedIL.ThecmcofTX100inILincreasesfrom0.2mMto0.4mM
byStokes’shiftmeasurement.Alongwithanarylandaneight car-bonhydrophobicchain(C H ),TX100has100monomoricunits
Trang 6con-tainsa–OHgroupthatinteractsdirectlywiththeheadgroupof
ILviahydrogenbondinganddipole–dipoleinteractions[4].Ifthe
micellarformationofTX100hadtobefavorableinthepresence
ofIL,thentheimmediatelyavailableethericmonomericgroupof
TX100(afterthe–OHgroup)mustinteractwiththeimmediately
availablehydrophobictailofIL(afterthepolarheadgroup)
How-ever,theshorthydrophobictailofILandthepolarmonomericchain
ofTX100makethisinteractionunfavorableatlowconcentrations
Thus,toformmicelles,theethericchainsofTX100mustovercome
thehydrophobiceffectinducedbythetailoftheIL.Thiscausesthe
cmcofTX100toincreaseinthepresenceofIL
4 Conclusion
Theassociationofdye/drugmoleculewithsurfactantsolutions
dependsonthechargeof theheadgroupof thesurfactantand
physiochemicalpropertiesofthedye[36–39].Thepresentbinding
studyofcurcuminwithvarioussurfactantsolutionsandquenching
ofcurcuminbyo-nitrophenolclearly predictelectrostatic
inter-action of head group of surfactant molecule and deprotonated
form of curcumin, while curcumin having greatest affinity for
cationic than non-ionic and finally anionic surfactant solution
Theobservationthatthechangesofassociationofdruglike
cur-cuminwithsurfactantsolutionsaredramaticinthepresenceofIL
[bmin][BF4]comparedtowithoutIL[bmin][BF4]presentsclear
evi-dencetheimportanceofIL[bmin][BF4]inmodulatingassociation
ofcurcuminwithsurfactantsolutions.Theinteractioninvolving
non-ionicTX100surfactantappeartohavemoredramaticeffect
ontheassociationofcurcumin-surfactantsolutionscomparedto
thatinvolvingcationicCTABandthenanionicSDSsurfactantdue
tointeractionsofIL[bmin][BF4],curcuminandheadgroupofthe
surfactant.Thoughthemajorreasonforalternationofaggregation
numberbyIL[bmin][BF4][3,4]isduetoelectrostaticinteractions
betweenheadgroupofthesurfactantandanion[46]orcation[47]
oftheIL[bmin][BF4],ourresultsshowingearlyformationofmicelle
irrespectiveofcationicoranionicaqueoussurfactantsolutionsand
delayinmicelleformationinthecaseofneutralaqueoussurfactant
solutionsuggesthydrophobicinteractionofIL[bmin][BF4]doplay
acrucialrole.Thesefindingswillfurtherenhancepotential
appli-cationofILasamodulatorinsolubilizationinthemicellarsystem,
associationofdrug–surfactantduringdrugdelivery,micellization
andchemistry
Acknowledgements
Financialsupportprovided byLebaneseNationalCouncil for
Scientific Research (LNCSR) and American University of Beirut,
LebanonthroughtheUniversityResearchBoard(URB)and
Long-termFacultyDevelopmentgranttocarryoutthisworkisgreatly
acknowledged
Appendix A Supplementary data
Supplementarydataassociatedwiththisarticlecanbefound,in
theonlineversion,atdoi:10.1016/j.saa.2011.05.064
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