Nguyen, Thai TH and Khan, Azeem and Bruce, Layla M. and Forbes, Clarissa and O'Leary ...

25 Pages·2017·3.82 MB·English

crystals
Article
The Effect of Ultrasound on the Crystallisation of
Paracetamol in the Presence of Structurally
Similar Impurities
ThaiT.H.Nguyen1 ID,AzeemKhan1,LaylaM.Bruce1 ID,ClarissaForbes1,2,3,
RichardL.O’Leary2andChrisJ.Price1,3,* ID
1 DepartmentofChemical&ProcessEngineering,UniversityofStrathclyde,GlasgowG11XQ,UK;
[email protected](T.T.H.N.);[email protected](A.K.);[email protected](L.M.B.);
[email protected](C.F.)
2 TheCentreforUltrasonicEngineering,DepartmentofElectronic&ElectricalEngineering,
UniversityofStrathclyde,GlasgowG11XW,UK;[email protected]
3 EPSRCCentreforContinuousManufacturingandCrystallisation,UniversityofStrathclyde,
GlasgowG11RD,UK
* Correspondence:[email protected]
AcademicEditor:JudyLee
Received:17August2017;Accepted:26September2017;Published:30September2017
Abstract: Sono-crystallisationhasbeenusedtoenhancecrystallineproductqualityparticularlyin
termsofpurity,particlesizeandsizedistribution.Inthiswork,theeffectofimpuritiesandultrasound
on crystallisation processes (nucleation temperature, yield) and crystal properties (crystal size
distributiondeterminedbyFocusedBeamReflectanceMeasurement(FBRM),crystalhabit,filtration
rate and impurity content in the crystal product by Liquid Chromatography-Mass Spectroscopy
(LC-MS))wereinvestigatedinbulksuspensioncrystallisationexperimentswithandwithoutthe
useofultrasound. Theresultsdemonstratethatultrasonicinterventionhasasignificanteffecton
both crystallisation and product crystal properties. It increases the nucleation rate resulting in
smallerparticlesandanarrowerParticleSizeDistribution(PSD),theyieldhasbeenshowntobe
increaseashastheproductpurity. Theeffectofultrasoundistoreducethelevelacetanilideimpurity
incorporated during growth from a 2 mol% solution of the selected impurity from 0.85 mol% to
0.35mol%andlikewiseultrasoundreducestheuptakeofmetacetamolfrom1.88mol%to1.52mol%.
Keywords: suspension crystallisation; impurities; crystal size distribution; filtration rate; optical
microscopy;imageanalysis;crystalmorphology;purification;HPLC;ultrasoundintensity
1. Introduction
Crystallisationistheprincipalpurificationtechniqueinthepharmaceutical,finechemical,paint,
pigmentsandagrochemicalsector. Itallowsthedesiredproducttobeobtainedinapurestateleaving
unwantedorhazardousimpuritiesinsolutionwheretheycanberemovedeffectivelybyfiltration
and washing. During crystallisation, molecules of the product assemble together to form crystals
witharegular3Dpackingarrangementknownasacrystallattice. Purificationoccursbymolecular
recognitionatthesolution-latticeinterface. Themismatchbetweensomeimpuritymoleculesandthe
crystallatticeissosignificantthattheimpuritymoleculecannotbeincorporatedinthelatticeandis
rejected. However,ifthemolecularmismatchissmallenough,theimpuritymoleculecanbecome
incorporatedintothecrystallattice. Iftheportionoftheimpuritymoleculeprojectingoutfromthe
crystalsurfaceisdifferentfromtheadjacentmoleculesinthesurfacelayerofthecrystalthisislikely
todisruptandretardsubsequentgrowthatthatgrowthsiteandadjacentsitesonthatcrystalface.
Crystals2017,7,294;doi:10.3390/cryst7100294 www.mdpi.com/journal/crystals
Crystals2017,7,294 2of24
Increasingthecrystallisationdrivingforce,i.e.,increasingsupersaturationlevel,reducesselectivity
allowingeasierincorporationoftheimpurityintothecrystallatticeandsubsequentovergrowth.Inone
oftheauthor’sindustrialexperience,typicalfeedstreamstoindustrialcrystallisationcontainseveral
mole%ofimpuritiessointeractionsbetweenimpuritymoleculesandthegrowingcrystalsurfaceare
veryfrequentandcanhaveundesirableconsequences. Impuritypoisoningofgrowingcrystalfaces
increases processing time and slows the approach to equilibrium. In some instances, a significant
amountofproducthastobeleftinsolutionandlostinthewastestreaminorderfortheprocesstobe
runwithaplausiblebatchduration. Sometimestheproductissoimpurethatithastobere-crystallised
tomeettherequiredspecification.
Mostimpuritiesthatarereadilyincorporatedintothegrowingcrystallatticeandactascrystal
growth inhibitors are close analogues of the active pharmaceutical ingredient and are produced
inadvertentlyasby-productsordegradantsduringthesyntheticprocess. Aswellasretardingcrystal
growththeyalsomodifycrystalhabit. Asaconsequence,otherattributesoftheisolatedcrystalssuch
asbulkdensityandpowderflowaremodifiedandcaninfluenceboththesubsequentformulation
processanddrugproductperformance. Severalrouteshavebeenidentified[1]bywhichimpurities
becomeentrappedinfinalcrystallineproduct: (i)Depositiononcrystalsurfacesduetotheincomplete
removalofimpuremotherliquorduringfiltrationandwashing;(ii)incorporationwithinthecrystal
latticebymolecularrecognition/substitutionor(iii)entrapmentofmultipleimpuritymoleculeswithin
inclusionsorbetweenindividualcrystallitesinagglomerates. Theimpactofspecificimpuritieson
crystallisationprocessesdependson;thestructureoftheimpurity,itsconcentration,thecrystallisation
solvent, the supersaturation and the temperature of the crystallising solution. The presence of
impurities during the crystallisation process can significantly modify the kinetics of nucleation,
the growth rate of specific crystal faces, crystal morphology (habit) and the polymorphic phase
crystallized. Theseundesiredeffectscancausecostlydelaysduringtheresearchanddevelopment
processwithinthepharmaceuticalindustry.Therearemanycasestudiesofimpurityeffectsreportedin
thecrystallisationliterature[2–5].Theeffectofincreasingamountsoftheimpurityp-acetoxyacetanilide
on paracetamol nucleation is to increase the metastable zone width and lengthen the induction
time[3,6,7]. Ithasbeenreportedthatthisduetoitsabilitytodisrupttheformationandgrowthofthe
criticalnucleus[3,8]. Impuritieshavebeenreportedbothtosuppressandoccasionallytoaccelerate
crystalgrowthprocesses[9]. N-phosphonomethylphosphoricacid(PMG)crystallisationisreported
tobeaffectedbytheimpurities,iminobismethylenephosphonicacidandaminomethylphosphoric
acidbyselectiveabsorptionontothe(100)faceinhibitingthegrowthofthisface;whiletheimpurity
N-phosphonomethyliminodiaceticacid(PIDA)doesnotchangecrystalhabitnoticeably[10]. Also,
animpurityhasbeenreportedtoaffectthesolution-mediatedphasetransformationofthemetastable
AformtothestableBformofanactivepharmaceuticalingredient[11]. Thepresenceofimpurities
iswidelyreportedtoinfluencecrystalgrowthandcrystalmorphology[9,12–15]. Thisisthecasefor
aminoacidsonthehabitofglycinecrystals[16],orureacrystallisationinthepresenceofbiuret[15].
Thechangeofcrystalhabitisduetoadifferenceintheextentofadsorptionandmagnitudeofbinding
energiesondifferentcrystalfaces[16]. Forthisreason,identification,quantificationandreductionof
impuritiesintheearlystagesofsyntheticprocessdevelopmentcanavoidproblemsincrystallisation
development,enhanceproductperformanceandreducedevelopmenttime[17].
Ultrasound(US)hasbeenusedinthechemicalandfoodindustriestoenhancecrystallineproduct
quality in terms of particle size, size distribution and impurity and is termed sono-crystallisation.
Ultrasound has been widely reported to influence the primary nucleation process accelerating
nucleation kinetics, this is typically expressed in terms of reducing the induction time and
MSZW [18–33]. Ultrasound can also increase the rate of secondary nucleation, this is manifested
as a reduction on the product crystal size distribution [20,22,24,31,32,34–43]. Ultrasound can
also influence crystal growth [20,25,26,36,40,44–46] although the effect on crystal growth is not as
dramatic as on nucleation and arises largely from enhanced mass transfer [46] and can influence
crystal morphology. Other reported outcomes of ultrasonic intervention include influencing;
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which polymorph nucleates [23,47,48], crystal structure [30], crystal properties [49] and reaction
crystallisation[45]. Severalproposedmechanismsoftheeffectofultrasoundoncrystallisationhave
beenpublished.Theseincludetheformationoflocalhotspotswhicharisefromthelargeenergyrelease
onthecollapseofcavitationbubblescreatinghighlylocalisedregionsofextremelyhightemperature
and pressure [50,51], or due to rapid cooling which follows; shockwaves released from cavitation
bubbles[52,53],promotingmasstransferandcollisionsbetweencrystalsandadjacentsurfaces[54].
These effects of ultrasound on crystallisation have been demonstrated on pharmaceuticals and
commoditychemicalsincluding;lactose[35,39,42,55],alpha-dextrosemonohydrate[20],glycine[48],
p-aminobenzoic acid [47], adipic acid [28], benzoic acid [31], acetylsalicylic acid [27], protein [19]
variousfoodproducts[30,56,57]andinthecrystallisationofinorganicmaterialssuchaspotassium
sulphate[26],potassiumdihydrogenphosphate[32]andcalcite[34]. Ultrasoundhasalsobeenutilized
incontinuouscrystallisation[28,58–60]. Whilesomeauthorsmentionthatultrasoundcanimprove
crystal purity [54]; there is very limited data on the effect of ultrasound on the purity of the final
crystallineproduct. Thisworkfocusesontheuseofultrasoundinsuspensioncrystallisationprocesses
to improve product purity, yield and crystal size distribution. The objective is to develop a bulk
suspension sonocrystallisation process at lab scale which exploits ultrasound to improve product
purity, yield and crystal habit. The research hypothesis is that through active intervention in the
molecularprocessesoccurringatgrowingsurfacesitmaybepossibletoremoveimpuritymolecules
fromthesurfacesandreducetheentrapmentofimpuritiesinsideagglomeratesformedduringgrowth.
Thesemeasurementsaimtoincreaseunderstandingofhowultrasonicinterventionsaffectproduct
puritybothdirectlythroughinfluencingimpurityincorporationduringgrowthandindirectlythrough
influencingnucleationkinetics,crystalsizedistributionandagglomeration,whichallinfluencethe
purityoffinalproductsbymodifyingtheextenttowhichimpuritiesareremovedduringfiltration
andwashing.
Paracetamol(Acetaminophen)hasbeenusedasamodelcompoundforthisstudybecauseits
crystallisationbehaviouriswellcharacterised[2–5]andmultiplestructurallyrelatedimpuritiesare
readilyavailable. Threepolymorphsofparacetamolhavebeenreported[61–63]butsofaronlyforms
IandIIhavebeenobtainedassinglecrystalswhichhaveallowedthecrystalstructuretobesolved.
PolymorphI(monoclinic,P21/n)isthethermodynamicallystableformatroomtemperature,thiscan
bepreparedaslargesinglecrystalsfromsolutionsinvarioussolvents. PolymorphII(orthorhombic,
Pcab) is metastable at ambient conditions. Crystals of paracetamol form I exhibit three dominant
crystal habit faces; {100}, {001} and {110}. Individual paracetamol crystals exhibit different crystal
habits,forexamplesignificantchangesofthegrowthrateandresultantmorphologyofparacetamol
singlecrystalsareobservedinpure[64]andimpuresystems[4].
2. MaterialsandMethodology
2.1. Materials
Paracetamol (BioXtra grade, purity ≥ 99.0%, Lot number 637515L263) was obtained from
Sigma-Aldrich(St. Louis,MO,USA).TheimpuritiesinvestigatedwereAcetanilide99.0%Lotnumber
STBD0193VandMetacetamol97.0%LotnumberMKBX4643VbothalsoobtainedfromSigma-Aldrich,
all three samples were analysed on receipt to confirm the reported assay. The basis for selecting
acetanilideisthatitisstructurallysimilartoparacetamolbutlacksthehydroxylgroupandsowhile
it may be incorporated into the crystal lattice it is deficient in a hydrogen bonding opportunity.
Metacetamol was selected as a positional isomer which is also likely to be incorporated into the
crystallattice;however,themisplacedhydroxylgroupislikelytodisruptthelocalhydrogenbonding
arrangement. Importantlybotharemoresolublethanparacetamolin3-methyl-1-butanol,theselected
solventensuringlittleriskofthemcrystallizingindependentlyduringtheexperiments.
The crystallisation solvent 3-methyl-1-butanol (isoamyl alcohol) was selected because the
solubility relationship with temperature for paracetamol is appropriately steep over the chosen
Crystals2017,7,294 4of24
temperature range and the product crystals tend not to form agglomerates. 3-methyl-1-butanol
wasobtainedfromSigma-Aldrich98%,n-heptane99%fromAlfaAesar(WardHill,MA,USA)usedfor
filtercakewashingfollowingfiltration. HPLCgrademethanolandwaterusedfortheanalysiswere
purchasedfromSigma-Aldrich.
2.2. EquipmentApparatus
2.2.1. SolubilityMeasurements
GravimetricMethod
In order to evaluate the effect of impurities on the crystallisation of paracetamol it was first
necessarytomeasuretheextenttowhichtheimpuritiesaffectthesolubilityofparacetamolinthe
crystallisationsolvent. SolubilitymeasurementswereperformedbyequilibrationusingaStuartSI60D
Incubator(Stuart,Staffordshire,UK).AgitationoftheequilibratingsampleswasprovidedbyaThermo
scientific submersible telesystem 15.20 magnetic stirrer plate (Thermo Fisher Scientific, Waltham,
MA,USA).ThetemperaturewasrecordedusinganOmegaHH800SWtemperaturesensor(Omega,
Manchester, UK).SamplesweretakenusinganEppendorfpipetteweighedonanA&DGRseries
balance(A&DInstrumentsLimited,Manchester,UK)andevaporatedtodrynessinaMemmertGmbH
vacuumovenV0200(MemmertGmbH+Co. KG,Schwabach,Germany)at55◦C.
DetectionoftheDissolutionTemperatureMethodUsingCrystalline-TechnobisCrystallisationSystems
Solubility measurements of acetanilide and metacetamol were also conducted in a Technobis
Crystalline system, a multi-reactor crystallisation platform which has eight independent Peltier
heater-coolers able to maintain temperatures from −15 ◦C to 150 ◦C. Each 8 mL (34 × 10 mm)
crystallizerisequippedwithathree-blademarineimpellerwithoverheadstirring.Eachvesselposition
isequippedwithaturbiditysensorandacameraallowingturbidityandimagestoberecordedand
synchronisedwiththetemperaturetimeprofile.
2.2.2. Sono-CrystallisationStudies
Aschematicdiagramoftheexperimentalequipmentusedtoevaluatetheeffectofultrasoundon
crystalpurityisshowninFigure1. Thesystemcomprisedtwocellcultureflasks,doublesidearm,
Celstir® 125mLplacedontoasubmersibleTelesystem15.20stirrerfromThermoScientific(Waltham,
MA,USA)immersedinaXUB25ultrasonicbathfromGrantInstruments(Cambridge,UK)operatingat
35±3kHz. ThetemperatureofthewaterintheultrasonicbathandhencethesolutionsintheCelstir®
flaskswascontrolledusinganImmersionCompactThermostat(ICC)fromIKA(StaufenimBreisgau,
Germany)whichwasconnectedtoaHaileaHC-150Achiller(GuangDong,China). Additionalcooling
capacity was provided by a stainless-steel dip coil running around the base of the ultrasonic bath
andconnectedtoaLauderLCK1913,ECORE420Gthermocirculator(LaudaBrinkmann,NJ,USA).
ThetemperatureofsolutionswasmonitoredusingaprecisePPL-GMH-3750thermometerandPt100
probe(±0.03◦C)(GHMMesstechnikGmbHStandortGreisinger,Regenstauf,Germany). Thepoint
ofnucleationwasdeterminedbyvisualobservation,initiallyafewparticleswereobservedandthe
solution gradually became cloudy. On completion of the crystallisation (~3–3.5 h after nucleation
processoccurs)theproductcrystalsizedistributionwasmeasuredinsitubyFocusedBeamReflectance
(FBRM)(MettlerToledo,Greifensee,Switzerland). TheproductsuspensionwasfilteredusingaVac
Master10system(BiotageLtd.,Uppsala,Sweden)whichhadbeenmodifiedtoaccommodate50mL
graduatedcylinderswhichallowedthefiltrationratetobemeasured,filtrationtobehaltedatdryland,
andthemotherliquorandindividualwashliquorstobesegregatedforsubsequentassay.Thefiltration
drivingforcewasmonitoredandcontrolledusingaBüchiV800vacuumcontroller(BüchiUKLtd.,
Oldham,UK).
Crystals2017,7,294 5of24
Crystals 2017, 7, 294 5 of 24

Figure 1. (a) Schematic diagram and (b) Annotated photograph showing the experiment setup for
Figure1. (a)Schematicdiagramand(b)Annotatedphotographshowingtheexperimentsetupfor
suspension cooling crystallisation of paracetamol in the presence of impurities using an ultrasonic
suspensioncoboatlhin. gcrystallisationofparacetamolinthepresenceofimpuritiesusinganultrasonicbath.
2.3. Experimental Method
Chemicalanalysisofproductcrystals,motherliquorsandwashliquorswasperformedusingan
AgilentHigh2P.3e.1r. fSoorlumbialintyc eLiquidChromatography6130dualsourcemodel(Agilent,SantaClara,CA,
USA)providedwithathermostatedautosampler,athermostatedcolumncompartmentandasingle
Gravimetric Method
wavelengthultraviolet(UV)detectoranda2.7µmPoroshell120EC-C18Column: 4.6×75mmto
Solubility measurements were performed by equilibration using a Stuart SI60D Incubator
determinetheopcerraytsedta olvlienr ethpe rtoemdpuecrattuprue rriatnyg.e 25 to 50 °C. The sample size employed to prepare saturated
solutions to determine solubility was 15 mL. The effect of impurities on solubility was determined by
2.3. ExperimenditsaslolMvinegt hao dknown target amount of metacetamol and/or acetanilide in the solvent prior to the
addition of paracetamol. Individual vials were prepared with specific impurity loadings and then
equilibrated with an excess of paracetamol. The vials were placed onto a Thermo scientific
2.3.1. Solubility
submersible telesystem 15.20 magnetic stirrer plate and agitated at 190 rpm. The temperature was
kept within ±0.2 °C of the desired temperature which was recorded with an Omega precise
GravimetricMethod
thermocouple. After equilibration for 24 h, the excess solid paracetamol was allowed to settle for 1 h
without agitation. A 2 mL sample of the clear saturated solution was withdrawn using an Eppendorf
SolubilitymeasurementswereperformedbyequilibrationusingaStuartSI60DIncubatoroperated
pipette taking great care not to disturb the settled solids. The samples were then transferred into a
overthetemplaeberlaletdu raendr awnegiegh2ed5 tvoia5l. 0T◦hCe .sTamhpeles avmialp cloenstaiizneinge mthpe losaytuerdatetdo psorleuptioanr ewsaas tuwreaigtheedd solutionsto
determinesoilmumbeildiitaytewly. aTshi1s 5sammpLl.inTg hperoecefsfse cwtaso freipmeapteud rfiotri eeascho neqsuoililburbatieldit vyiawl aansd dtheet searmmpilense wderbey dissolving
placed in a fume hood and allowed to evaporate to dryness for 24 h, after evaporation the samples,
a known target amount of metacetamol and/or acetanilide in the solvent prior to the addition of
were then transferred to a vacuum oven and allowed to dry for a further 48 h at 55 °C. The dry residue
paracetamol. Individualvialswerepreparedwithspecificimpurityloadingsandthenequilibrated

withanexcessofparacetamol. ThevialswereplacedontoaThermoscientificsubmersibletelesystem
15.20magneticstirrerplateandagitatedat190rpm. Thetemperaturewaskeptwithin±0.2◦Cofthe
desiredtemperaturewhichwasrecordedwithanOmegaprecisethermocouple. Afterequilibrationfor
24h,theexcesssolidparacetamolwasallowedtosettlefor1hwithoutagitation. A2mLsampleofthe
clearsaturatedsolutionwaswithdrawnusinganEppendorfpipettetakinggreatcarenottodisturb
thesettledsolids. Thesampleswerethentransferredintoalabelledandweighedvial. Thesample
vialcontainingthesaturatedsolutionwasweighedimmediately. Thissamplingprocesswasrepeated
foreachequilibratedvialandthesampleswereplacedinafumehoodandallowedtoevaporateto
drynessfor24h,afterevaporationthesamples,werethentransferredtoavacuumovenandallowed
todryforafurther48hat55◦C.Thedryresiduemass,wasdeterminedandthesolubility,expressed
inmgofsolute/gofsolvent,wascalculatedwithsubtractionoftheknownamountofimpurityinthe
Crystals2017,7,294 6of24
drysamplesbasedonthepresumptionthattheimpurityremainedevenlydistributedthroughoutthe
Crystals 2017, 7, 294 6 of 24
liquidphase.
mass, was determined and the solubility, expressed in mg of solute/g of solvent, was calculated with
DissolutionTemperatureDetectionMethod
subtraction of the known amount of impurity in the dry samples based on the presumption that the
Aimspeurriietsy orefmsuaisnpeedn esvioennlsy odfisktrniobuwtendc tohnrocuenghtroautti othneo lifqmuiedt apcheatsaem. ol(50–130mg/g)andacetanilide
(80–200mg/g)in3-methyl-1-butanolwerepreparedin8mLvials. Thevialswereheatedfrom5◦Cto
70◦CDaitsshoeluattiinong Traemtep0e.1ra◦tuCr/e mDiente.cTtihoen tMemetphoedra tureofdissolutionwasdeterminedatthepointatwhich
alltheparAti cselersiews oefr esucsopmenpslieotnesl yofd kisnsoowlvne cdo.nBceynmtraatkioinn gofm meeatsauceretammeonl t(s50a–c1r3o0s smagr/ga)n agnedo afcceotamnpiliodsei tions
theso(8lu0–b2i0li0t ymcgu/gr)v ienw 3-amsedtheytel-r1m-biunteadnoal nwderteh eprseoplaurbedil iitny 8a mtsLp veicailfis.c Ttheem vpiaelrsa wtuerree shewaittehdi fnrotmhi s5 r°aCn tgoe can
70 °C at heating rate 0.1 °C/min. The temperature of dissolution was determined at the point at which
beinterpolated.
all the particles were completely dissolved. By making measurements across a range of compositions
2.3.2.thUel tsroalusobniliitcyI cnutrevnes iwtyasM deetaesrumrienmede nantsd the solubility at specific temperatures within this range can
be interpolated.
ThesonomechanicalactivityfromtheXUB25ultrasonicbathwascharacterisedbymeasurement
of aco2.u3.s2t.i cUlitnrtaesnonsiicty Inutesninsigtya MNeaHsu4r0e0m0ePnVts DF needle hydrophone (Precision Acoustics Ltd., Dorset,
UK).TheexperimentalarrangementwasasshowninFigure1exceptthelidoftheCelstirvesselwas
The sonomechanical activity from the XUB25 ultrasonic bath was characterised by measurement
replaocfe dacwouitshtica ipntleungsimtya unsuinfagc atu NreHd40fr0o0m PVPDVFC nteheadtlea lhloydwreodphtohneen (ePerdecliesihoynd Arocopuhsotincse Ltotdb.,e Dpoorsseitti,o ned
25 mUmKf)r. oTmhe tehxpeebriamseenotafl tahreraCngeelmsteirntv wesasse als, sahliogwnne din wFiigtuhret h1e excceenpttr tahlea lixdi sofo tfheth Ceelssatimr vee.ssIenl worads er to
makerempleaacseud rwemithe na tpolufga mcoaunsutfiaccitnutreedn sfirtoymw PiVthCi nthtahte alCloewlsetdir tvhee snseeeld,lteh heytdimroephdoonme atoin bew paovseitfioornmed from
theN2H5 m40m00 frhoymd trhoep bhaosne eofw thaes Creelcsotirrd veedsswel,i tahligannedA wgiiltehn tthTe eccehntnroall oagxiise osfI tnhfien snamiVei.s Iino norXde2r0 t2o4 m-Aakdei gital
measurement of acoustic intensity within the Celstir vessel, the time domain waveform from the
oscilloscope(AgilentTechnologies,SouthQueensferry,UK).Atimingtriggerforthemeasurement
NH4000 hydrophone was recorded with an Agilent Technologies InfinniVision X2024-A digital
wasprovidedviaasubmersiblepiezoelectrictransducer,sensitiveattheoperatingfrequencyofthe
oscilloscope (Agilent Technologies, South Queensferry, UK). A timing trigger for the measurement
XUB25bath,andpositionedonthetopsurfaceofthestirrerplate.
was provided via a submersible piezoelectric transducer, sensitive at the operating frequency of the
Priortostartingthemeasurements,theNH4000hydrophonewaswettedandsoakedfor1hour
XUB25 bath, and positioned on the top surface of the stirrer plate.
indeionisedwater. AswasdescribedinSection2.3.3,thecrystallisationwasundertakenusingorganic
Prior to starting the measurements, the NH4000 hydrophone was wetted and soaked for 1 hour
solveinnt dwehioicnhiseisd inwcaotemr.p Aatsi bwleasw ditehsctrhibeePdV iDn FSehcytidorno p2.h3.o3n, ethteip c.rIynstoarlldiseartitoon pwroatse cutntdheerthakyednr oupshinogn etip
durinogrgtahneicm seoalsvuenret mwehnictho fist hiencaocmoupasttiibclien wteinthsi ttyhei nPtVhDeFc rhyystdarlolpishaotinoen tmip.e dIniu omrd,ethr etoh ypdrorotepcth othnee was
placehdyidnrsoipdheonane t8ipm dmuridngia tmhee mteeralsautreexmreunbt boef rthseh aecaotuhsfiticll eindtewnsiitthy dine tihoen icsreydstawlliastaetrio.nT mheedCiuelmst, itrhve essel
usedhfoyrdrsoupshpoennes iwonasc prylascteadll iisnastiidoen aenx p8 emrimm edniatmswetears lfialtleexd ruwbibthera snheaalitqhu foiltleodf wthiethc dryesiotanliisseadt iwonatseor.l vent;
The Celstir vessel used for suspension crystallisation experiments was filled with an aliquot of the
thehydrophone,completewithprotectivesheath,wasimmersedintothecrystallisationmediumas
crystalisation solvent; the hydrophone, complete with protective sheath, was immersed into the
showninFigure2. Measurementsofacousticintensitywerethenundertakenwiththecrystallisation
crystallisation medium as shown in Figure 2. Measurements of acoustic intensity were then
vesselplacedinsidetheXUB25bath,itwaslocatedaroundthedefinedpositionsonthestirrerplateat
undertaken with the crystallisation vessel placed inside the XUB25 bath, it was located around the
thebottomoftheultrasonicbath—allmeasurementswererecordedat30◦C.Thepeakinstantaneous
defined positions on the stirrer plate at the bottom of the ultrasonic bath—all measurements were
acousticintensityoftherecordedwaveformwasdeterminedatboth50%and100%powersettingon
recorded at 30 °C. The peak instantaneous acoustic intensity of the recorded waveform was
theXdUeBte2r5mbinaetdh .at both 50% and 100% power setting on the XUB25 bath.

Figure 2. Photograph of the hydrophone arrangement within the Celstir vessel set-up for ultrasonic
Figure2.PhotographofthehydrophonearrangementwithintheCelstirvesselset-upforultrasonic
intensity measurement.
intensitymeasurement.
Crystals2017,7,294 7of24
2.3.3. Sono-Crystallisation
A 150 mL suspension containing 12.65 g of paracetamol and 1% or 2% mol of metacetamol
and/oracetanilidein115g3-methyl-1-butanolwaspreparedincellcultureflasks,Celstir®125mL.
Theparacetamolsuspensionin3-methyl-1-butanolwiththepresenceofimpurities,wasimmersedand
sonicatedintheultrasonicbath,XUB25,operatingat35±3kHzandwascontinuouslystirredata
rateof~150rpmusingasubmersibleTelesystem15.20. Thesolutionwasheatedto65◦Candheldat
65◦Cfor15mintoensurecompletedissolutionwhichwasverifiedbyvisualinspectionbeforeslow
coolingto15◦Cover2h. Duringthecoolingcrystallisationprocess,thenucleationtemperatureofthe
solutionswasrecorded. Attheendofthecrystallisation(3hafternucleation)andimmediatelyprior
toisolation,thecrystalsizedistributionwasmeasuredinsituusinganFBRMtodeterminethemean
chordlengthdistribution. Thentheproductcrystalswereseparatedfromthemotherliquorsusing
themodifiedVacMaster10systematpressuredrivingforceof200mbarusinga70mLIsolutefilter
tubewithapolyethylenefilterwithanominalporesizeof10µm(BiotageLtd.,Uppsala,Sweden).
Thetimerequiredtocollectfiltratevolumesof5mL,10mL,15mLetc. wererecorded. Thefiltration
was halted at “dry land” the point where the filter cake surface is first exposed above the mother
liquors. Themassofthesolventwetcakewasmeasured. Fromthefiltrationdata,thefiltrationrate
andfiltercakeresistancewereobtainedusingDarcy’sequation[65].
dt µαC µR
= V+ m (1)
dV ∆PA2 A∆P
where: tistime(s); V isvolume(m3); αiscakeresistance(m/kg); µisthemotherliquorviscosity
(kg/s.m);∆Pispressuredifference(Pa);Cisconcentrationofsolids(kg/m3);Aisareaofcake(m2);
R isresistanceoffiltermedium(m−1).
m
Inthiswork,filtrationwascarriedoutatconstantpressuredifference,∆P=200mbar. Integration
Darcy’sequationgives:
t µαC µR
= V+ m (2)
V 2∆PA2 A∆P
Plottingt/VversusVwillallowthecakeresistanceαtobedetermined.
Theratiooftheconcentrationofimpurityintheproductcrystalstotheconcentrationofimpurity
relativetoparacetamolinthemotherliquorswasalsodeterminedbydissolutionofproductcrystals
andHPLCAnalysis.
2.3.4. ImpurityAnalysisUsingHPLC
TheseparationofanalyteswasachievedusinganAgilentPoroshellcolumnoperatedat40◦C
with an isocratic elution of 1 mL/min, the mobile phase composition was 80% water and 20%
methanol. Thedetectionwavelengthwas243nm,whichrepresentsthemaximumabsorbanceforboth
paracetamolandmetacetamol[66]whilestillremainingclosetothelambdamaxforacetanilideat
242nm[67]. Thematerialcorrespondingtoeachpeakinthechromatogramwasidentifiedusingmass
spectroscopy. Theanalyticalsampleusedtodeterminethepurityoftheproductcrystalswasprepared
byweighing14mgofdryproductcrystalsintoa100mLvolumetricflaskanddissolvingthemina
5%methanol/95%watermixturetoprepareasolutionwithaconcentrationwhichwaswithinthe
concentrationrangeusedtocalibratetheinstrument. Motherliquorsweresimilarlydilutedbymassto
ensureappropriatecompositionforassay.
3. Results
3.1. Solubility
Thesolubilitycurvesofparacetmol,metacetamolandacetanilidein3-methyl-1-butanolprepared
usingtheisothermalequilibrationandgravimetricanalysisandpolythermaldissolutiontemperature
detection method are shown in Figure 3. These data are summarised in Table 1. Full details of
Crystals2017,7,294 8of24
Crystals 2017, 7, 294 8 of 24
the gravimetric studies and dissolution temperature detection are given in Section S1 within the
ESI†dmetaaitlesr oiaf ltsh.e gravimetric studies and dissolution temperature detection are given in Section S1 within
tTheh eESsIo†l umbaitleitryiaolsf. paracetamol,metacetamolandacetanilideincreasewithincreasingtemperature.
ParacetamThoel issotlhuebilleiatys tsoof lupbalreacinet3am-moel,t hmyle-t1a-cbeutatamnool laanndd aacecetatanniilliiddee isinthcreeamseo stwsiothlu binlecraenadsinegx hibits
the stetrmopnegreastutrde.e pPearnadceetnacmyolo nis ttehme pleearsat tusorleub(Flei ginu r3e-m3)e.thEyxl-a1-mbuintainnogl tahned daacteatapnirleidsee niste tdhei nmFoisgt ure 3,
soluble and exhibits the strongest dependency on temperature (Figure 3). Examining the data
therewasadiscrepancyinsolubilityofmetacetamolobtainedbyisothermalequilibrationfollowedby
presented in Figure 3, there was a discrepancy in solubility of metacetamol obtained by isothermal
gravimetricanalysisandbyobservingthedissolutiontemperatureduringheatingusingtheTechnobis
equilibration followed by gravimetric analysis and by observing the dissolution temperature during
Crystalline. Determining the dissolution temperature using the Crystalline involves identifying a
heating using the Technobis Crystalline. Determining the dissolution temperature using the
clearpointatwhichdissolutionispresumedtohaveoccurred. Theclearpointwasdeterminedusing
Crystalline involves identifying a clear point at which dissolution is presumed to have occurred. The
bothtransmissivitymeasurementandexaminingimagesfromthecrystallinecamera. Thedissolution
clear point was determined using both transmissivity measurement and examining images from the
kinetcircysstdaellpineen dcaomnertah.e Tshize edaisnsdolushtiaopn ekoinfetthices cdreypsetanlds odnis tshoel vsiinzeg aanndd sthhaepier aobf itlhitey ctroysitmalps eddisesothlveinpga ssage
ofligahntd( tthraeinr sambiilsitsyiv tiot yim)opredtoe tbhee cplaesasralgyer oefs olilgvhetd (tirnanasnmiimssiavgitey()c oarm toe rbae bclaesaerdly mreesaoslvuerde mine annt )i.mAaglteh ough
them(ceaamsuerrae mbaesnedtw maesapsuerrefmoremnte)d. Aalttahomuogdh etshteh meaetaisnugrermateenot fw0a.1s ◦pCer/fomrmine,dth aet pao mssoidbeilsitt yheoaftoinvge rrsahteo oting
thedoifs 0s.o1l °uCti/monint,e tmhep peorsastiubirleityre omf oavineresdh.oMotientga ctheet mdiosslohluatsioanv teemryptehraintunree eredmlea-ilnikede. cMryetsatcaeltmmoolr hpahso logy;
a very thin needle-like crystal morphology; the crystals appear to persist some while after they could
thecrystalsappeartopersistsomewhileaftertheycouldhavedissolvedinanisothermalexperiment.
have dissolved in an isothermal experiment. As a result complete dissolution is detected at slightly
Asaresultcompletedissolutionisdetectedatslightlyhighertemperaturethanseeninthegravimetric
higher temperature than seen in the gravimetric method. Interestingly, there is no similar discrepancy
method. Interestingly,thereisnosimilardiscrepancyinthedatacollectedusingthetwomethods
in the data collected using the two methods for acetanilide, this is believed to be due to the higher
foracetanilide,thisisbelievedtobeduetothehighersolubilityofacetanilideandthemorerapid
solubility of acetanilide and the more rapid dissolution which may be linked to the thin plate-like
dissolutionwhichmaybelinkedtothethinplate-likecrystalhabit.
crystal habit.
300
250
Paracetamol (Gravimetric)
)
g
g/
m200 Metacetamol
n ( (Gravimetric)
o
ti150 Metacetamol (Crystalline)
a
r
t
n
e Acetanilide (Gravimetric)
c100
n
o
C
Acetanilide (Crystalline)
50
0
0 10 20 30 40 50 60
Temperature (°C)

FiguFriegu3.reS o3.l uSobliulibtiylitoyf opfa praarcaecteatmamool,l,m meettaacceettaammool laanndd acaecteatnainlidileid ine i3n-m3e-mthyelt-h1y-blu-1ta-bnuolt aonbotalinoebdt abiyn edby
isothiseormthearlmeqaul ileiqburialtibiorantifoonll ofwolelodwbeydg rbayv imgraevtriimceatnriacl yasnisalaynsdis baynodb sbeyrv ionbgsetrhveindgi sstohleu tdioisnsotelumtiponer ature
duritnegmhpeearattiunrge udsuirninggt hheeaTtiencgh unsoibnigs tCher yTsetcahlnlionbei.s Crystalline.
Table 1. Molecular weights, melting points, enthalpy of fusion and measured solubilities of
Table 1. Molecular weights, melting points, enthalpy of fusion and measured solubilities of
paracetamol, metacetamol and acetanilide determined using the gravimetric method.
paracetamol,metacetamolandacetanilidedeterminedusingthegravimetricmethod.
Measured Solubility in
Compounds Molecular Weight (g/mol) Melting Point (K) ΔHfusion (kJ mol-1) 3-meMtheyals-1u-rbeudtSanoolul b(miligty/gi)n
ComPaproaucentdasmol MolecularW1e5i1g.1h6t (g/mol) M44e3l.t6in [g68P];o 4in42t.(9K [)69] ∆2H7.f1u s[i6o8n](; k3J0.m72o l[-619)] 2255543 °.-◦8CmC ethy4l70-41 4°.-3Cb0 u ◦tCano51l52(4°m.C35g 5/g◦)C
Metacetamol 151.16 420 [70,71] 26.0 [71]; 28.8 [70] 81.4 116.7 162.6
ParaAcceetatamnoillide 15113.156.17 443.6[6388]7;.454 [26.99][ 69] 27.1[6280].;3300 [.6792][ 69] 16524..88 241.784 .3 - 124.3
Metacetamol 151.16 420[70,71] 26.0[71];28.8[70] 81.4 116.7 162.6
Acetanilide 135.17 387.5[69] 20.30[69] 162.8 241.8 -
The solubility of a crystalline organic material typically depends on the lattice structure
(polymorph), melting point, molecular weight and solvent choice. The measured solubility of
pTahraecestoamluobli alintdy moeftaacectarmysotla allrien reelaotrigvealnyi scimmilaatre irni acolmtypparicisaolnly wdithe paecentdansiloidne, tthhies ilsa ctotincseistsetnrut cture
(poly morph), melting point, molecular weight and solvent choice. The measured solubility of
paracetamolandmetacetamolarerelativelysimilarincomparisonwithacetanilide,thisisconsistent
Crystals2017,7,294 9of24
withtheirrelationshipaspositionalisomerscontainingthesamefunctionalgroups. Theparacetamol
crystalstructureisdominatedbyhydrogen-bondedmolecularchainspackedina‘herringbone’pattern.
Similarly,Cmryesttalas c20e1t7a, 7m, 29o4l crystalsareboundtogetherbyanetworkofhydrogenbondin9g ofi 2n4t eractions.
Acetanilidehoweverhasnohydroxylgroupinthepara-positionsolacksaprotondonor. Inaddition,
with their relationship as positional isomers containing the same functional groups. The paracetamol
the melting point and the enthalpy of fusion decrease in the order Paracetamol > Metacetamol >
crystal structure is dominated by hydrogen-bonded molecular chains packed in a ‘herring bone’
Acetanilidpeat(tseerne. TSiambillearl1y), .mFeotarceatcaemtoaln cirlyidstealst haere cbroyusntda ltosgtreuthcetru brye ais nseutwcohrkt hoaf thmydorolegecnu lbeosndairneg boundby
weakerforicnetesratchtaionnsi.n Apceatraanicleidtea mhoowledveure hatso nloa hckydorofxtyhl egrhoyupd rino txhyel pgarrao-upopsirteiodnu scoi lnacgkst ha eprooptopno drotnuonr.i tytoform
In addition, the melting point and the enthalpy of fusion decrease in the order Paracetamol >
ahydrogenbondingnetwork. Acetanilidehasasubstantiallylowerheatoffusionandalowermelting
Metacetamol > Acetanilide (see Table 1). For acetanilide the crystal structure is such that molecules
pointthanparacetamolandmetacetamol. Therelativesolubilityispresumedtoaffecttheimpurity
are bound by weaker forces than in paracetamol due to lack of the hydroxyl group reducing the
segregatioonppoofrtthuneitfiyn tao lfcorrmys ata hllyidnreogperno bdouncdti.ng network. Acetanilide has a substantially lower heat of
The dfuastiaonp arneds ean tloewderi nmFelitginugr epo4init ntdhainc aptearatcheatatmtohle anpdr emseetnacceetaomfo1l. –T4h%e roeflattihvee ssoelulebciltietyd isi mpurities
doesnotspigrensiufimceadn ttol yaffaefcfte tchte tihmepusroitlyu sbeiglrietygaotifonp oafr tahcee ftinaaml coryl.stAaltlin2e5 p◦rCoduacntd. 40◦C,itwasshownthatthe
The data presented in Figure 4 indicate that the presence of 1–4% of the selected impurities does
solubilityissimilarorincreasesslightlywiththepresenceoftheselectedimpurities. At55◦C,thedata
not significantly affect the solubility of paracetamol. At 25 °C and 40 °C, it was shown that the
showsincrsoelausbeilditys cisa stitmerilaarn odr itnhceretarseesn dsligishtlleys wsipthr othneo purnesceendce. oIft tchaen sebleectceodn imclpuudreitdiest. hAatt 5t5h °eCe, fthfeec tof1–4%
metacetamdoatla/ sahcoewtas ninilcirdeaeseodn sctahteters oanludb thilei ttyrenodf pisa lersasc pertoanmouonlciesd.m Ito cdane sbte. concluded that the effect of
1–4% metacetamol/acetanilide on the solubility of paracetamol is modest.

Figure 4. Solubility of paracetamol in 3-methyl-1-butanol with the presence of impurities
Figure4.Solubilityofparacetamolin3-methyl-1-butanolwiththepresenceofimpurities(metacetamol
(metacetamol and acetanilide) obtained by equilibration and gravimetric assay.
andacetanilide)obtainedbyequilibrationandgravimetricassay.
3.2. Ultrasonic Intensity Measurements
3.2. UltrasonicTIhnet eunltsriatsyouMndea isnuternemsiteyn mtseasurements reveal that the local ultrasonic field is highly chaotic
and intensities range from 0.12 to 0.25 mW/cm2 at 50% ultrasonic power and from 0.9 to 1.68 mW/cm2
Theultrasoundintensitymeasurementsrevealthatthelocalultrasonicfieldishighlychaoticand
at 100% power (see Table 2). It is clear from this data that the % ultrasound power is not a linear
intensitiesdreasncrgipetofrr.o Cmry0st.a1l2listaotio0n.2 e5xmpeWrim/ecnmts 2waetre5 0co%nduulcttreads oatn picospitoiownse r1 aanndd 2fr oanmd 0th.9e tuoltr1a.s6o8unmd W/cm2at
100%poweinrte(nsesietyT aapbplelie2d) .dIutriisngc lethaersefr oexmpetrhimisendtas tcaatnh baet tchhear%actuerlitsreads oasu; nzderpo o(0w merWis/cnmo2)t laowlin, e50a%r descriptor.
ultrasound power (0.19 ± 0.06 mW/cm2) and high, 100% ultrasound power (1.45 ± 0.21 mW/cm2).
Crystallisationexperimentswereconductedatpositions1and2andtheultrasoundintensityapplied
during these experiments can be characterised as; zero (0 mW/cm2) low, 50% ultrasound power
Table 2. The ultrasonic intensity was measured at four positions within the ultrasonic bath.
(0.19±0.06mW/cm2)andhigh,100%ultrasoundpower(1.45±0.21mW/cm2).
Ultrasonic Intensity (mW/cm2)
Positions on the Stirrer plate at the Bottom of the Ultrasonic Bath
50% Ultrasonic Power 100% Ultrasonic Power
Table2.Theultrasonicinte1 nsitywasmeasuredatfourpo0s.i1t3i ±o 0n.0s2 withintheu1l.2t5r a± s0o.1n3 icbath.
2 0.25 ± 0.03 1.68 ± 0.25
3 0.24 ± 0.02 1.32 ± 0.09
4 0.12 ±U 0l.t0r4a sonicIntensity0.9(m ± W0.0/7c m2)
PositionsontheStirrerplateattAhveeBraogteto vmaluoefs theUltrasonicBath 50%U0l.t1r9a ±so 0n.0i7c Power 110.209% ± 0U.3l2t rasonicPower
1 0.13±0.02 1.25±0.13

2 0.25±0.03 1.68±0.25
3 0.24±0.02 1.32±0.09
4 0.12±0.04 0.9±0.07
Averagevalues 0.19±0.07 1.29±0.32
Crystals2017,7,294 10of24
3.3. Sonocrystallisation
The data comprise 11 suspension crystallisation experiments designed using the Design of
Experiments software MODDE. (Umetrics Modde, Version 11, MKS Instruments, Umea, Sweden)
to facilitate multivariate analysis. A full factorial (2 level) method was used to create a data set
that was then analyzed using multiple linear regression (MLR) methods to measure effects of the
three independent variables on the experimental responses. The parameters investigated were;
theconcentrationsofthetwoimpuritieswheretheparameterrangewasselectedas0–2mol%and
ultrasonicpowerwheretheparameterrangewas0%to100%powersettingsontheultrasonicbath.
Whentheultrasonicpowerwasmeasuredthecenterpoint(50%ultrasoundpower)theintensitywas
foundtobe0.19±0.06mW/cm2andat100%ultrasoundpowertheintensitywas1.45±0.21mW/cm2.
Theresponsesanalyzedwerenucleationtemperature,particlesize,filtrationrate,productyieldand
thepercentageofimpuritiesintheproductcrystals.
Theeffectofthepercentageofimpuritiespresentinsolutionatthestartofcrystallisationand
ultrasonicpoweron;nucleationtemperature,crystalsizedistribution,filtrationrateandprocessyield
atadefinedcrystallisationendpointaresummarisedinTable3.
Table3. Summaryofcrystallisationtemperature,crystalsizedistribution,filtrationtime(for50mL
suspensionsolution),filtrationrate,cakeresistanceandprocessyieldasafunctionofthepercentageof
impuritiesandultrasonicpower.
Experiments Tcrys(◦C) PSLDenMgethan(µCmho)rd FTiilmtraeti(os)n RFaitletr(amtio3/ns) Cake(mRe/ksigs)tance Yield
Pureparacetamol 41.3 47–55 209 1.90×10−7 7.03×1011 57.7%
(100%powerUS)
Pureparacetamol 36.0 85–100 95 4.29×10−7 2.62×109 54.3%
(noUS)
2mol%Metacetamol 36.4 47 361 1.07×10−7 1.52×1010 45.5%
(100%powerUS)
Aftermaintaining
2mol%Metacetamol at15◦Cfor 100–109 45 1.06×10−7 9.02×108 39.1%
(noUS)
approx.1.5h
2mol%Acetanilide 43 40 561 7.32×10−7 1.88×1010 56.2%
(US100%P)
2mol%Acetanilide 15.5◦C 105 54 7.01×10−7 1.02×109 55.3%
(noUS)
2mol%Metacetamol&
Acetanilide 34.4 37.3 346 1.19×10−7 1.51×1010 42.2%
(US100%power)
Aftermaintaining
2mol%Metacetamol& at15◦Cfor 87–90 48 9.61×10−7 1.14×109 32.0%
Acetanilide(noUS)
approx.2.5h
1mol%Metacetamol&
1mol%Acetanilide 44 87 61 7.19×10−7 1.38×109 40.6%
(50%USpower)
3.3.1. CrystallisationTemperature
Thenucleationtemperaturemeasuredwithandwithouttheuseofultrasoundforcrystallisation
ofparacetamolinthepresenceofmetacetamoland/oracetanilideisshowninTable3. Eventhough
nucleation is a stochastic process and some scatter may be anticipated among the measured data
the experimental results clearly show that ultrasound has a significant effect on the nucleation
process. Forexample,inthecaseofparacetamolcrystallisationinthepresenceof2%metacetamol,
the ultrasound treated sample started to nucleate at around 36 ◦C, and was in the final stages of
crystallisationbythetimethesuspensionhadbeencooledto15◦C.Bycontrastthesamplenottreated
withultrasoundonlystartedtonucleateafterholdingat15◦Cforapproximately1.5h. Thisisbelieved
tobeduetotheslownucleationandcrystalgrowthkineticsinthepresenceofimpurities. Ultrasound
canpromotenucleationandgeneratesubstantialnumbersofsecondarynucleifromexistingparent