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Journal of Pharmaceutical and Biomedical Analysis
j o ur na l ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j p b a
Analysis of submicron-sized niflumic acid crystals prepared by electrospray crystallization
Rita Ambrus
a, Norbert Radacsi
b, Tímea Szunyogh
a, Antoine E.D.M. van der Heijden
b,c, Joop H. ter Horst
b, Piroska Szabó-Révész
a,∗aDepartmentofPharmaceuticalTechnology,UniversityofSzeged,Eötvös6,H-6720Szeged,Hungary
bProcess&EnergyLaboratory,DelftUniversityofTechnology,Leeghwaterstraat44,2628CADelft,Netherlands
cTechnicalSciences,TNO,2280AARijswijk,Netherlands
a r t i c l e i n f o
Articlehistory:
Received26June2012
Receivedinrevisedform31October2012 Accepted2December2012
Available online xxx
Keywords:
Nanoparticles
Electrospraycrystallization Anti-solventcrystallization Solventevaporation Niflumicacid
Physico-chemicalanalysis
a b s t r a c t
Interestinsubmicron-sizeddrugparticleshasemergedfrombothlaboratoryandindustrialperspectives inthelastdecade.Productionofcrystalsinthenanosizescaleoffersanovelwaytoparticlesfordrug formulationsolvingformulationproblemsofdrugswithlowsolubilityinclassIIoftheBiopharmaceutical ClassificationSystem.Inthisworkniflumicacidnanoparticleswithasizerangeof200–800nmwerepro- ducedbythenovelcrystallizationmethod,electrospraycrystallization.Theirpropertieswerecompared tothosefromevaporativeandanti-solventcrystallizations,usingthesameorganicsolvent,acetone.
Thereisaremarkabledifferenceintheproductcrystalsizedependingontheappliedmethods.Thesize andmorphologywereanalyzedbyscanningelectronmicroscopyandlaserdiffraction.Thestructureof thesampleswasinvestigatedusingdifferentialscanningcalorimetry,Fourier-transformedinfraredspec- troscopyandX-raypowderdiffraction.Theparticlesproducedusingelectrospraycrystallizationprocess wereprobablychangingfromamorphoustocrystallinestateaftertheprocedure.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Niflumicacid(NIF)isanimportantanti-inflammatorydrugand hasaweakanalgesiceffect.Itisprimarilyusedtotreatdifferent formsofrheumatism,likerheumatoid arthritisorarthrosis,and tocureotherinflammatorydiseases[1].However,itspooraque- oussolubilityanddissolutionratearedisadvantages[2].Toachieve optimalpharmacodynamicpropertiessuchasarapidonsetofthe drugeffect,fastdissolutionisimportantforthistypeofdrug.Inour earlierstudies,theaimwastoimprovethesolubilityanddissolu- tionrate,viathepreparationofternarysystemsofNIF,cyclodextrin (CD)andpolyvinylpirrolidone(PVPK-25)indifferentNIFtoCDto PVPratiosbyphysicalmixing,kneading,microwaveirradiationand micronization[3–6].
The interest for nano/micron particles in pharmaceutical research hasemerged recently,since studies have proven that particlesizereductioncaninfluencethemechanismsofdrugdeliv- eryinmanyways[7–14].Thedifferentmethodsusedforparticle sizereductioncanbedividedintotwomaincategories:top–down method,where theraw material is subsequently broken down byusingmillingmethodsuntilmicro-ornanosizedparticlesare
∗Correspondingauthor.Tel.:+3662545575;fax:+3662545571.
E-mailaddress:revesz@pharm.u-szeged.hu(P.Szabó-Révész).
producedandthebottom–upapproach(crystallizationprocedure), thebasicprincipleofwhichinvolvingdissolutionofthedrugina solvent,followedbyadditionofthesolutiontoanon-solvent,asa consequenceofwhichthedrugprecipitates.Thenanosuspension engineeringprocessescurrentlyusedincludeprecipitationusing high-pressurehomogenizationinwater,inmixturesofwaterand water-miscibleliquids,orinnon-aqueousmedia[15–19].Electro- spraycrystallizationasanovelpossibilityinbottom–upprocedure, hasthepotentialtobecomeanefficient,cost-effectivemethodfor theproductionofsubmicron-sizedorganiccrystals[20],offeringa simpleformulationwayofpharmaceuticalingredientswithbene- ficialproperties[21–23].
Inelectrospraycrystallizationahigh,constantpotentialdiffer- enceisappliedtoanozzle,throughwhichaconductivesolution ispumped(Fig.1).Ifthepotentialdifferenceissufficientlyhigh, electrostaticforcesovercomethesurfacetensionandajetofliq- uidisemittedfromtheso-calledTaylor-coneformedatthenozzle.
Atsomedistancefromthenozzle,thejetbecomesunstableand breaksintodroplets,whichareacceleratedtowardthegrounded platebytheelectricfield.Coalescenceofthedropletsisprevented becauseoftheunipolarcharge[21](whichenablesproductionof nano-sizedcrystals).Uponusingasufficientlyvolatilesolventsuch asacetone,solventevaporationoccurs,whichincreasesthespecific surfacechargedensitybecauseofthedecreaseindropletvolume andsurfacearea.Asthesurfacechargedensityreachesacritical 0731-7085/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jpba.2012.12.001
Fig.1.Schematicsoftheelectrospraycrystallizationprocess.Usingasmallnozzle diameter(d),alowconstantflowrate(ϕ),relativelylowsoluteconcentration(c) ajetofhighlychargedsmalldropletswillbeemittedfromtheconeappearingat thenozzletipwhenapplyingahighenoughpotentialdifference(U)atacertain workingdistance(D).
value(Rayleigh-limit)[24]electrostaticforcesovercomethesur- facetensionandthedropletdisruptsintosmallerdropletstoreduce thesurfacechargedensitybycreatingmoresurfacearea.Thisdis- ruptionprocessiscalledCoulomb-fission.Atsomepointduringthis processofdropletevaporationanddisruption,thedrivingforcefor crystallizationbecomessufficientlylargeforcrystalnucleationand growthtooccur.Itisassumedthatcrystallizationisconfinedtothe volumeofthedroplet[25].Therefore,ifthedropletsaresufficiently small,typicallyonecrystalperdropletisformed.Thesecharged submicroncrystalsaccumulateatthegroundedsurfacewherethey losetheirsurfacecharge.
ItisknownthattheelectrospraycrystallizationresultsinNIF particleswithsignificantlyimproveddissolutionrate[26],andthis paperismeanttocharacterizetheproducedsubmicroncrystals.
Thegoalofthisresearchwastoprepareandanalyzesubmicron- sizedNIFcrystals.Herebyelectrospraycrystallizationisintroduced asapotentialinexpensiveandsimplemethodfortheproduction of such submicron-sized drug crystals and compared to con- ventionalevaporative anti-solvent crystallizations.The effectof three differentcrystallizationmethods (electrospray crystalliza- tion,anti-solventcrystallizationandevaporativecrystallization)on
thecrystalstructure,productandsizemicrometricpropertiesis investigatedandcomparedwiththeconventionalNIF.
2. Materialsandmethods
2.1. Materials
Conventional niflumic acid (NIF) (2-[[3-(trifluoromethyl) phenyl]amino]-3-pyridinecarboxylicacid)withbroadcrystalsize distributionandameansizeofaround80mwaspurchasedfrom G.RichterPharmaceuticalFactory,Budapest,Hungary.Forallthe crystallization processes thesolution was prepared with99.8%
acetone,purchasedfromMerck.
2.2. Methods
2.2.1. Preparationprocedure
2.2.1.1. Electrospray crystallization. An eight-nozzle electrospray crystallizationsetupwithdifferentNIFconcentrations(10,20and 30mgml−1)wasusedtoproducethesubmicron-sizedNIF.The setupconsistedofeightnozzles,anozzleholder,agroundedstain- lesssteelcollectorplate,aMeredosTL-EADperistalticpump,which providedanequaldistributionofthesolutionflowoverthenoz- zles,andaWallis±10kVDCpowersupplythatwasusedinpositive DCmodetoprovidethepotentialdifferencebetweenthetipofthe nozzlesandthegroundedplate.Theusednozzleshadalengthof 25.4mmandtheinnerdiameterof0.33mm.Theywerepurchased fromEFD,USA.TheproductdesignatedinthetextasNIF-NANO referstothesampleafter2weeksstorage.
2.2.1.2. Anti-solventcrystallization(AC). Inthismethod,asolution of500mgofNIFin5mlofacetonewasaddedto25mlofanaqueous mediaat roomtemperature (25◦C)that wasacting asan anti- solvent(Fig.2).Duetotheanti-solventeffect(acetoneismiscible withwater,and wateractsasananti-solventforNIF),thedriv- ingforceforcrystallizationwasreachedrapidly,andasuspension ofprecipitatedcrystalswasproducedinwater.Thisprocesswas carriedoutbyusingaT-25typeofUltra-Turraxdispersersimul- taneously(IKA, Staufen,Germany) at 24,000rpm for 10min,as energyinputforsizedecreasing(thegrowthoftheprecipitated crystalcouldbecontrolled).Thisveryhighstirringrateresultsina slighttemperatureincrease(upto33◦C)andenhancestheevapora- tionrateoftheacetonefromthemixture.Theproductwasfiltered anddriedatroomtemperature,theninvestigatedimmediately.The productislabeledasNIF-ACinthepaper.
2.2.1.3. Preparation ofthe sampleby solventevaporation(SE). An amountof250mgNIFwasdissolvedin5mlacetone,andthenthe solventwasevaporatedwiththehelpofacolddryer(HM3Dryer,
Fig.2.Schematicdrawingoftheanti-solventcrystallizationmethod.
MH9150201,Siemens,Germany)at20◦Cuntilalltheacetoneevap- orated. Thenthedry powderwaspulverized and homogenized manuallyinamortartoensureanarrowcrystalsizedistribution.
Thentheproductwasinvestigatedimmediately.TheNIFproduced byevaporativecrystallizationislabeledinthetextasNIF-SEinthe paper.
2.3. Analysis
2.3.1. Scanningelectronmicroscopy(SEM)
Thecrystalsizeand shapewereexaminedbyscanningelec- tronmicroscopy(HitachiS4700,HitachiScientific Ltd.,Japan).A sputtercoatingapparatus(Bio-RadSC502,VGMicrotech,England) wasappliedtoinduceelectricconductivityonthesurfaceofthe samples.Theairpressurewas1.3–13.0mPa.Sampleswerefixed ontoametallicstubwithdouble-sidedconductivetape(diame- ter12mm,Oxon,OxfordInstruments,UK).Imagesweretakenin secondaryelectronimagemodeat10kVaccelerationvoltage.The particlediameterdistributionswereobtainedbyanalyzingseveral SEMimageswiththeImageJsoftwareenvironment[27].Over150 individualparticlemeasurementsweremadeusingatleastfive differentimagesinordertodeterminetheparticlesizedistribution.
2.3.2. Particlesizeanalysis
TheparticlesizedistributionoftheconventionalNIFwasmea- suredbyalaserdiffractometer(MastersizerS,MalvernInstruments Ltd.,Worcestershire,UK)withthefollowingparameters:300RF lens, small volume dispersion unit rotated at 2000rpm, 1.510 refractiveindexfordispersedparticles,and1.330refractiveindex forthedispersionmedium.
2.3.3. Differentialscanningcalorimetry(DSC)
DSCwasemployedtoinvestigatethecrystallizationbehavior andthemeltingbehavioroftheconventionalandsubmicron-sized NIF. The DSCmeasurements weremade witha Mettler Toledo DSC821ethermalanalysissystemwiththeSTARethermalanalysis programV9.1(MettlerInc.,Schwerzenbach,Switzerland).Approx- imately2–5mgofproductwasexaminedinthetemperaturerange between25◦Cand300◦C.Theheatingratewas5◦Cmin−1.Argon wasusedasacarriergas,ataflowrateof10lh−1duringtheDSC investigation.
ThecrystallinityindexofthedifferentNIFsampleswascalcu- latedfromtheheatsoffusion:theratiobetweenthenormalized enthalpyoftheNIFsamplesandthenormalizedenthalpyofthe conventionalNIFindicatestheproductcrystallinity[28].
2.3.4. X-raypowderdiffraction(XRPD)
XRPDwascarriedoutinordertodeterminethecrystallineform and crystallinityoftheproducedmaterials.Samplesweremea- suredwithaBrukerD8Advancediffractometer(BrukerAXSGmbH, Karlsruhe,Germany).Datacollectionwascarriedoutatroomtem- peratureusingmonochromaticCuK˛1radiation(˛=0.154060nm) inthe2regionbetween3◦ and50◦.Formeasurementsdirectly afterelectrospraycrystallization,about10 milligramsofsample wasdirectlydepositedonazerobackgroundholder(Sisinglecrys- tal<510>wafer)andplacedintotheXRDdirectlyafterproduction.
Therecording of thepatternwas relativelyfast,it lasted180s.
Diffractionpatternswererecordedfrom15mintill20hafterpro- ductionatdifferenttimeintervals.
2.3.5. Fouriertransforminfraredspectroscopy(FT-IR)
FT-IRspectroscopywasusedtoinvestigatethechemical sta- bilityof thedrug.FT-IR spectraweremeasuredonan AVATAR 330 FT-IR apparatus (Thermo Nicolet, USA), in the interval 400–4000cm−1,at4cm−1opticalresolution.StandardKBrpellets
Fig.3.Therelationbetweenworkingdistance(D)andpotentialdifference(U).
werepreparedfrom150mgofKBrpressedwith10tonandsamples containing0.5mgofNIFwereused.
3. Resultsanddiscussion
3.1. Electrospraycrystallizationprocessparameters
Theelectrospraycrystallizationprocessparametersweredeter- minedfortheproductionofthedesiredsubmicronproduct.Three processparameterswereidentifiedtohaveamajoreffectonthe product:initialsoluteconcentration(c),potentialdifference(U) and nozzlediameter(d).Theflowrate(ϕ)couldnot beinvesti- gated,sincetheoperationwindowoftheflowratewastoolow,and changesinflowrateresultedininstablejet.Theworkingdistance (D)stronglydepends ontheappliedpotentialdifference:larger distancesdemandhigherpotentialdifference.Fig.3showstherela- tionshipbetweenworkingdistanceDandpotentialdifferenceU.
Whenhigherinitialconcentrationswereused,theaveragecrys- talsizeincreased:thesamedropletwithahigherconcentration containsmoresolutethatcancrystallize,resultinginlargercrystals (Fig.4).Thecrystalshapealsodependsontheinitialconcentra- tion.Athigher concentrations,theobtainedcrystalsweremore needle-like,whileatlowerconcentrationsthecrystalshadsome- whatsphericalshape(Fig.4).
Alowerpotentialdifferenceresultedinagglomerationofcrys- tals.Theincreaseofthenozzlediameteralsoresultsinagglomerate
Fig.4.Therelationshipbetweenthecrystalsizeandshapeandusedsolutioncon- centrationofNIFcrystals,producedbyelectrospraycrystallization.
Fig.5.SEMimagesof(a)conventionalNIF,(b)NIFproducedbyelectrospraycrystallization(NIF-NANO),(c)NIFproducedbyanti-solventcrystallization(NIF-AC)and(d)NIF producedbysolventevaporation(NIF-SE).
formation[26].Alowerpotentialdifferenceorlargernozzlediam- eter results in a lower charge density at the Taylor-cone and thereforeatthedropletsurface.Atalowersurfacechargedensity Coulomb-fissionisoccurringlaterorisabsentandcrystallization mightoccurinlargerdroplets,resultinginagglomeratedcrystals.
Theoptimalprocessparametersforproducingsubmicron-sized NIFcrystals(NIF-NANO)withacompactshape withoutagglom- erationwerefoundtobe:solutionconcentrationof20mgml−1, apotentialdifferenceof+4.7kV,anozzlediameterof0.33mm,a workingdistanceof17mmandaflowrateof1.8mlh−1.Withthese parameterssomewhatprismaticshapeNIFcrystalswithamean sizeofaround500nmwereproduced.Theproductionrateunder theseconditionswas150mgh−1.
3.2. Productsizeandshape
TheconventionalNIFcrystalshadasmoothsurfacewithpris- maticshapewitharound80mmeansizeandabroadcrystalsize distribution(Fig.5A).ThisshapewasalsoseenfortheNIF-SEcrys- tals(Fig.5D).Thecrystalshape ofNIF-SEwasprismaticaswell, anditssizedistributionwasverybroadwithanestimatemean sizeofaround46m,probablyduetotheappliedhomogenization bymanualpulverization.Byusingtheanti-solventcrystallization method(NIF-AC), needle-likeNIF crystalsin thesize ofaround 10mwereproduced(Fig.5C).Thiscrystallizationmethodcom- bined withhighshear mixing isan effectiveprocedure forthe particlesizedecreasingtowardthemicronrange.Theelectrospray crystallizationprocedurecausedthemostremarkablealterations
from allthe used crystallization methods. Afterthe procedure, somewhatsphericalshapedNIFparticleswerefoundwithamean sizeof500nm(Fig.5B).Thesesubmicronparticlesareclustered intoaggregatesupto20minsize,possiblyduetothestrongcohe- siveinteractionsbetweenthehydrophobiccrystalsurfaceswiththe increasedspecificsurfacearea.
Table1presentsthemeanparticlesizeofNIFdeterminedusing aSEMimagesanalysis.Itcanbeseenthatthesolventevaporation withpulverizationofthecrystals(NIF-SE)decreased thecrystal sizetonearlyhalfthatoftheconventionalNIFsize(80m)deter- mined.Theproductproduced bytheanti-solventcrystallization process(AC)resultedincrystalsaround7m.Electrospraycrystal- lizationresultedinnano-sizedparticles,around500nm.Itseems thattheincreaseinsupersaturationintheanti-solventcrystalliza- tionprocessisnotenoughtoobtainsubmicroncrystals.Alsothe crystallizationvolumeshouldbedecreased,asinelectrospraycrys- tallizationthemainreasonofsizedecreasingisthatmicron-sized dropletsarecreatedintheprocess,andcrystallizationcommences inthesmall,confinedvolumeofferedbythedroplets.
Table1
ThecalculatedmeancrystalsizeoftheconventionalNIFandtheproducedcrystals.
Sample Meansize(m)±SD
NIF 80±22.6
NIF-NANO 0.5±0.2
NIF-AC 7.4±3.9
NIF-SE 46.2±26.0
Fig.6. DSCcurveoftheconventionalNIFcomparedwiththerecrystallizedNIFobtainedfromthesolventevaporation(NIF-SE),therecrystallizedNIFobtainedfromthe antisolventcrystallization(NIF-AC)andthesubmicron-sizedNIFfromelectrospraycrystallization(NIF-NANO).
3.3. Structuralanalysis(DSC,XRPDandFT-IR)
The presence of amorphous fraction could be expected by spraying crystallization techniques,since the driving forces for crystallizationarehigh[29,30].Thepresenceofamorphousfrac- tionhasaneffectonthestabilityofthesamplesaswell.Dueto therapidevaporationofthesolventinelectrospraycrystallization, thecrystalstructure might notbuildupcompletely(kinetically formedmoreslowly)duringthecrystallizationprocess[31].Thus structural characterizations wereperformed to checkthe crys- tallinestateoftheproducedNIFcrystals.
3.3.1. Differentialscanningcalorimetry
DSCwasusedtomeasurethemeltingpoint(T)andtheheatof melting.TheonsetToftheconventionalNIFat202.49◦Creflected itsmeltingpoint(Fig.6).TheNIF-ACandNIF-SEsamplespresented similar DSCcurves with a melting point at around 202◦C. The submicron-sizedNIF (NIF-NANO)had awelldetectablemelting pointatalowertemperature(198.93◦C).Thelowermeltingpoint andbroaderpeakofthesubmicron-sizeddrugcanbeexplainedby therelativelylargeproportionofsurfacemoleculescomparedto thebulk.Thevibrationalandpositionalenthalpyandentropyof surfacemoleculesofsubmicron-sizedmaterialsaredifferentfrom thatofmoleculesinsidethebulkofthecrystals,whichcanalter physicalproperties,includingthermalproperties[32].Onewayto testthecrystallinityofthesamplesistodeterminethenormalized heatofmelting,sincethereisarelationbetweentheheatofmelting andthecrystallinefractionofthesample[28].
ThecrystallinityindexofNIFwascalculatedfromthenormal- izedheatofmelting ofthesamples(Table2).Theconventional NIF sample wasassumed to have 100% crystallinity. The sam- pleproducedbysolventevaporation(NIF-SE)showedhigherheat of meltingthan theconventional NIF,probably due tothepul- verizationimpactduringthepreparationprocedure.Thesample fromtheanti-solventcrystallizationmethod(NIF-AC)showed97%
crystallinity.Theelectrospraycrystallizationproduct(NIF-NANO) showedthelowestcrystallinityfromalltheproducedsamples,88%
ofthematerialwasmeasuredtobecrystalline,whichimpliesthat 12%ofthesampleisamorphous.Thecrystallinityindexwasalso
Table2
Thecrystallinityindex(CI)ofthesamplesaccordingtotheDSCmeasurements.
Sample Normalizedintegral(Jg−1) CI(%)
NIF 129.57±0.23 100
NIF-SE 131.22±0.45 100
NIF-AC 125.99±0.36 97
NIF-NANO(2weeksafter preparation)
114.15±0.82 88
ElectrosprayedNIF(15min aftertheprocedure)
105.32±0.39 81
calculatedforasamplethatwasmeasured15minafterthepro- ductionbytheelectrospraycrystallizationprocess.Itscrystallinity indexwas81%,7%lessthantheelectrosprayedsampleafter2weeks (NIF-NANO).ThisshowsthatapartofNIFchangedfromamorphous phasetocrystallinephaseduringthestorage.
TosummarizetheDSCresults,thereasonofmeltingdepression isapartofamorphousstateandparticlessizedecreaseofNIF.
3.3.2. X-raypowderdiffraction
XRPDanalysiswasusedtoexaminethecrystalstructureandit wasalsousedtoassessthecrystallinityoftheproducts.Thechar- acteristicintensityvaluesoftheNIFcanbefoundat8.2,12.9,16.2, 23.2, 25.72 values.Alltheproducedsamplesshowedpeaksat thesamepositionsastheconventionalNIF,buttheirintensitywas lower(Fig.7).Whenthethreeproducedsamplesarecomparedwith eachother,itcanbeseenthattheNIF-SEsamplehasmoreintense peaks,thantheNIF-ACorNIF-NANOsamples,indicatingthatthe NIF-SEsamplehashighercrystallinitythantheothertwoproducts.
ThepeakintensityoftheNIF-ACsampleisinbetweentheNIF-SE andtheNIF-NANOsamples.Fromthethreeproducedmaterials,the NIF-NANOsamplepresentsthelowestpeakintensities,indicating thelowestcrystallinefractionfromthemeasuredproducts.Sincein generaltheevaporativecrystallizationresultsinhighlycrystalline product,itwasassumedthattheNIF-ACandNIF-NANOsamples arepartiallyamorphous.TheparticlesizedecreaseoftheNIF-AC andNIF-NANOsamplesalsocanbethereasonforthemeasured lowerintensitiesintheXRPDpatterns.
Fig.7.XRPDpatternsoftheNIFsamplesproducedbysolventevaporation(NIF-SE), anti-solventcrystallization(NIF-AC)andelectrospraycrystallization(NIF-NANO), comparedwiththeconventionalNIF(NIF).
Fig.8.XRPDpatternoftheproducedNIF-NANOsampletill20haftertheprocedure.
Toinvestigatea possibletransitionbetweenamorphous and crystallinestateoftheNIF-NANOsample,diffractionpatternsof thesamplewasrecordeddirectlyaftersprayingfor20h.Thecrys- tallinitydidnotchangeinthefirst20h(Fig.8).SincetheXRPD patternoftheNIF-NANOsample(whichmeansthenanoparticles 2weeksafterelectrospraycrystallization)showedhigherintensi- tiesthancanbeseenonthe20hdiffractionpattern,probablythe crystallinityincreasedfurtherinthefollowingtwoweeks.
3.3.3. FT-IRanalysis
IR spectroscopy was used to study the chemical stability of the NIF samples. The IR spectrum of NIF displays strong
Fig.9.FT-IRspectraofconventionalNIF,theelectroprayedNIF-NANO,NIF-ACby anti-solventcrystallizationandNIF-SEbysolventevaporationmethods.
absorptionat1607cm−1thatisduetothecharacteristicas(COO) ands(COO)stretchingmodesofcarboxylgroups.Amiddle-strong peakat3163cm–1canbeascribedtotheN–Hvibrationofimine [33,34],andthestrongabsorptionpeakat1523cm–1isowingto theframeworkvibrationofthephenylandpyridinerings.TheFT-IR spectra(Fig.9)of NIF-SEand NIF-NANOdidnot presentdiffer- encescomparedwithNIF.Newbondswerenotdevelopedandold bondsdidnotdisappear.AccordingtotheFT-IRanalysisnochem- icalchangeinthestructurewasdetected,whichmeansthatthe producedsamplesdidnotshowpolymorphictransitionorchemical decomposition.ThisimpliesthattheproducedNIF-NANOcrystals arechemicallystablebeforeandaftertheprocedure.
4. Conclusions
Thisstudyhasshownthatelectrospraycrystallizationasanon- conventionalmethodcanbeusedtoformulatesubmicron-sized NIFcrystalswithameansizeofaround500nm.Carefulselection ofpreparationparametersiscriticaltoachievethesuitablesizeand shape.Lowerinitialsolutionconcentrationsleadtocompact,some- whatsphericalcrystalshape,whilehigherconcentrationsresult in needle-like crystals.The crystallization procedurewas com- paredtotwoconventionalcrystallizationmethods,evaporativeand anti-solventcrystallization,wheremacro-andmicro-sizedcrystals wereprepared.XRPDmeasurementsoftheimmediatelyproduced NIFcrystalsshowedthatbyevaporativeandanti-solventcrystal- lizationtheproductswerehighlycrystallinerightafterproduction andalsoafter2weeks.Howevertheelectrosprayedproductwas partlyamorphousrightaftertheproduction,andonly81%ofthe samplewascrystalline.Thecrystallinityoftheelectrosprayedsam- plehasincreasedduringstorage,accordingtotheDSCresultsthe two-week-old electrosprayedNIFsampleswere88% crystalline.
AccordingtotheFT-IRmeasurementstheNIFcrystalswerechem- icallystablebeforeandaftertheprocedure.
Acknowledgement
The publication/presentation is supported by the European UnionandcofundedbytheEuropeanSocialFund.Projectnumber:
TÁMOP-4.2.2/B-10/1-2010-2012.
Projecttitle:“Broadeningtheknowledgebaseandsupporting thelongtermprofessionalsustainabilityoftheResearchUniversity CenterofExcellenceattheUniversityofSzegedbyensuringthe risinggenerationofexcellentscientists.”
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