spreading coeficient.pdf

1. PHYSICAL pharmacy
By
Dr. Ahmed R. Gardouh
Lecturer of Pharmaceutics
And Pharmaceutical Technology
Suez Canal University
13/04/2019physical Pharmacy , A.R.Gardouh, PhD1

2. Lecture 7
13/04/2019physical Pharmacy , A.R.Gardouh, PhD2

3. IV-SpreadingCoefficient
Whenasubstanceisplacedonthesurfaceofwater,itwillshowoneoftwo
behaviors:
1.Spreadasafilmiftheforceofadhesionbetweenthesubstancemoleculesandthe
watermoleculesisgreaterthanthecohesiveforcebetweenthesubstancemolecules
themselvessuchasoleicacidonwater.
2.Donotspreadandformseparatespotsifthecohesiveforcebetweenthesubstance
moleculesisgreaterthantheadhesionforcebetweenthesubstancemoleculesandthe
watermoleculessuchasmineraloilonwater.
13/04/2019physical Pharmacy , A.R.Gardouh, PhD3

4. Theworkofadhesion,whichistheenergyrequiredtobreaktheattractionbetween
theunlikemolecule(e.g.oleicacidandwater)canbegivenbytheequation:
Work=SurfacetensionxUnitareachange
Ifweconsiderahypotheticalcylinderofcross–sectionalareaof1cm
2
ofasub-layer
liquid“S”(water),Overlaidwithasimilarsectionofthespreadingliquid“L”(Oleic
acid).
13/04/2019physical Pharmacy , A.R.Gardouh, PhD4

5. Theworkrequiredtoseparatethetwosectionsofliquideachwithacross–sectionalareaof1cm
2
,is
thereforenumericallyrelatedtothesurfaceorinterfacialtensioninvolved,becausetheareaincrementis
unity.Accordingly,theworkdoneequaltothenewlycreatedsurfacetensions,(
L)and(
S)minusthe
interfacialtension(
LS)thathasbeendestroyedintheprocess.Theworkofadhesionis:
Theworkofcohesion,requiredtoseparatethemoleculesofthespreadingliquidsothatitcanflow
overthesub-layer,isobtainedbythesameway:Nointerfacialtensionexistsbetweenthelikemolecules
oftheliquid,andwhenthehypothetical1cm
2
cylinderisdivided,twonewsurfacesarecreated,each
withasurfacetensionof
L.Theworkofcohesionis
13/04/2019physical Pharmacy , A.R.Gardouh, PhD5

6. Withreferencetothespreadingonwatersurface,spreadingoccursif:theworkof
adhesion(attractionforcebetweensubstanceandwater)isgreaterthantheworkof
cohesion(attractionforcebetweenthesubstancemolecules).
Theterm(Wa–Wc)isknownasthespreadingcoefficient“S”.
Where,

S:isthesurfacetensionofthesub-layerliquid,

L:isthesurfacetensionofthespreadingliquid,

LS:istheinterfacialtensionbetweenthem.
N.B.If“S”ispositive,spreadingwilloccur.Ifnegativenospreadingoccurs.
13/04/2019physical Pharmacy , A.R.Gardouh, PhD6

7. Ifthesurfacetensionofthesub-layerisgreaterthanthesumofthesurfacetensionof
thespreadinglayerandtheinterfacialtensionbetweenthetwolayers,Swillbepositive
andspreadingwilloccur.
If(
L+
LS)isgreaterthan
Sthesubstanceformsglobulesorfloatinglensandfailto
spreadoverthesurface,anexampleismineraloilonwater.
Thepreviousdiscussionwasinvolvingtheinitialspreading.Whenequilibriumis
reachedandthewatersurfacebecomesaturatedwiththespreadingmaterial,the
SpreadingCoefficient(S)maybereducedormayevenbecomenegative.Thismeansthat
althoughinitialspreadingmayoccurontheliquidsubstrate,itcanbefollowedby
coalescenceoftheexcessmaterialintoalensifthespreadingcoefficientbecomes
negative.
13/04/2019physical Pharmacy , A.R.Gardouh, PhD7

8. N.B.Inthiscircumstances,
sbecomelessthan(
l+
ls)inwhich
lsdoesnotchangedsincethe
interfacialtensionisdeterminedunderconditionsofmutualsaturation
Itisimportanttoconsiderthetypeofmolecularstructuresthatleadtohighspreading
coefficient,oilmayspreadsoverwaterifitcontainspolargroupsuchasCOOHor
OH,Butwhenthecarbonchainofanacidincreases,theratioofpolar/nonpolar
decreasesandthespreadingcoefficientonwaterdecreases.
Example:
Ifthesurfacetensionofwater“γ
S”is72.8dynes/cmat20°C,thesurface
tensionofbenzene“γ
L”is28.9dynes/cm,andtheinterfacialtensionbetween
benzeneandwater“γ
LS“is35.0dynes/cm,whatistheinitialspreadingcoefficient
“S”?Followingequilibration,
Sis62.2dynes/cmand
Lis28.8.Whatisthefinal
spreadingcoefficient?
Solution:
S = 72.8 –(28.9 + 35.0) = 8.9 dynes/cm
S`= 62.2 –(28.8 + 35.0) = -1.6 dynes/cm
13/04/2019physical Pharmacy , A.R.Gardouh, PhD8

9. V-AdsorptionatLiquidInterfaces:
Surface–activeagents(SAA):
Certainmoleculesandions,whendispersedintheliquid,movetotheinterface.First,They
concentratedattheinterfacethenexceedstheirconcentrationinthebulkoftheliquid.The
adsorptionofthemoleculesattheinterfacereducesthesurfacefreeenergyandthesurfacetension
ofthesystem.Suchaphenomenon,wheretheaddedmoleculesarepartitionedinfavorofthe
interface,istermedpositiveadsorption.Othermaterialssuchasinorganicelectrolytesare
partitionedinfavortothebulk,leadingtonegativeadsorption.
Adsorptiondifferfromabsorptioninthat,theformerissolelysurfaceeffectwhilethesecondis
penetrationoftheabsorbentintothecapillaryspaceoftheabsorbingmedium.
Moleculesandionsthatareadsorbedatinterfacesaretermedsurface–activeagents(SAA)or
surfactants.Theyarealsoknownasamphiphilesoramphipathic,whichsuggeststhatthe
moleculesorionshavecertainaffinityforbothpolarandnonpolarsolvents.Dependingonthe
numberandnatureofthepolarandnonpolargroupspresentintheirmolecules,theamphiphile
maybe:
13/04/2019physical Pharmacy , A.R.Gardouh, PhD9

10. a.Predominantlyhydrophilic(water–loving)
b.Lipophilic(oil–loving)
c.Wellbalancedbetweenthesetwoextremes
Forexamplethestraight–chainalcohols,aminesandacidsareamphiphileswhichare:
Example:
*Ethylalcohol:ismisciblewithwaterinallproportions.
*Amylalcohol:Theaqueoussolubilityismuchreduced
*Cetylalcoholmaybesaidtobestronglylipophilicandinsolubleinwater.
TheAmphiphileNature:
Theamphiphilenatureofthesurface–activeagentsisthecharacterthatcausethemtobeadsorbedat
theinterfaces(liquid/gasorliquid/liquid).Theirshouldbeasuitablebalancebetweenthehydrophilic
andlipophilicgroupstoensuresurfaceactivity.
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11. InanaqueousdispersionofSAA:
a.Thepolargroupsareabletoassociatewithwatermolecules.
b.Thenon-polarportionisrejectedbecausetheadhesiveforceitcandevelopwithwatermoleculesis
smallerthanthecohesiveforcebetweentheadjacentwatermolecules.
c.Asaresulttheamphiphilemoleculeisadsorbedattheinterface.
*Inorderfortheamphiphiletobeconcentratedattheinterface,itmustbebalancedwiththeproper
amountofwater–solubleandoil–solublegroups(e.g.Glycerylmonostearate).
*Ifthemoleculeistoohydrophilic,itwillremainwithinthebodyoftheaqueousphaseandexertsno
effectattheinterface(e.g.Glycerin).
*Likewise,ifitistoolipophilic,itwilldissolvecompletelyintheoilphaseandlittleappearsatthe
interface(e.g.Glyceryltristearate).
13/04/2019physical Pharmacy , A.R.Gardouh, PhD11

12. A-SystemsofHydrophile–LipophileClassification:
Griffindevisedanarbitraryscaleofvaluestoserveasameasureofthehydrophilic–
Lipophilicbalance(HLB)ofsurface-activeagents.
*ThissystemestablishesanHLB
rangeofoptimumefficiencyfor
eachclassofsurfactant.
*ThehighertheHLBofan Hydrophilic
agent,themorehydrophilicitis.
*Thespans,sorbitanesters,
areLipophilicandhavelow
HLBvaluesof1.8to8.6.
*Thetweens,polyoxyethylene Lipophilic
derivativesofthespansare
hydrophilicandhaveHLB
valuesof9.6to16.7.
13/04/2019
Classification of surface active agent
physical Pharmacy , A.R.Gardouh, PhD12

13. ThereareseveralformulaeforcalculatingHLBvaluesofnon-ionicsurfactants:
*TheHLBforpolysorbatesandsorbitanesters(spans)fromthefollowing
formula:
HLB = (E+P) / 5
Where:E,isthepercentagebyweightofoxyethylenechainandP,,isthepercentagebyweightof
polyhydricalcoholgroups(glycerolorsorbitol)
*Ifthesurfactantscontainonlypolyoxyethyleneasthehydrophilicgroupthenthe
formulabecome
HLB = E / 5
*Alternatively,wecancalculatetheHLBvaluesfromthechemicalformulaasthe
following:
13/04/2019physical Pharmacy , A.R.Gardouh, PhD13

14. HLB=7+Ʃ(hydrophilicgroupnumbers)-Ʃ(lipophilicgroupnumbers)
*TheHLBofanumberofpolyhydricalcoholfattyacidesters,suchasglyceryl
monostearate,maybeestimatedbyusingtheformula:
InwhichSisthesaponificationnumberoftheesterandAistheacidnumberofthe
fattyacid.
Example:CalculatetheHLBofpolyoxyethylenesorbitanmonolurate(tween20),
where,S=45.5andA=276,is:
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15. *Inaddition,ithasbeensuggestedthatcertainemulsifyingagentsofagivenHLB
valueappeartoworkbestwithaparticularoilphase,andthishasgivenrisetothe
conceptofarequiredHLBvalueforanyoilsorcombinationofoils.However,thisdoes
notnecessarilymeanthateverysurfactanthavingtherequiredHLBvaluewillproducea
goodemulsion.Forthisreason,mixtureofsurfactantsgivemorestableemulsionsthan
whenusedsingly.TheHLBofamixtureofsurfactantsconsistingoffractionxofAand
(1-x)ofB,isassumedtobeanalgebraicmeanofthetwoHLBnumbers:
HLB
mixt.= x HLB
A+ (1-x) HLB
B
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16. FormulationbytheHLBmethod
Ithasalreadybeenshownthatphysicallystableemulsionsarebestachievedbythepresenceofa
condensedlayerofemulsifyingagentattheoil/waterinterface,andthatthecomplexinterfacialfilms
formedbyablendofanoil-solubleemulsifyingagentwithawater-solubleoneproducesthemost
satisfactoryemulsions.Ausefulmethodhasbeendevisedforcalculatingtherelativequantitiesofthese
agentsnecessarytoproducethemostphysicallystableemulsionforaparticularoil/watertype.Thisis
calledthehydrophile-lipophilebalance(HLB)method.
CalculatethetotalrequiredHLBforthefollowingemulsion:
Liquidparaffin35%
Woolfat 1%
Cetylalcohol 1%
Emulsifiersystem5%
Waterto 100%
Solution:
Thetotalpercentageofoilphaseis37andtheproportionofeachis:
Liquidparaffin35/37x100=94.6%
Woolfat 1/37x100=2.7%
Ceytlalcohol 1/37x100=2.7%
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17. ThetotalrequiredHLBnumberisobtainedasfollows:
Liquidparaffin(HLB12)94.6/100x12=11.4
Woolfat(HLB10) 2.7/100x10=0.3
Cetylalcohol(HLB15) 2.7/100x15=0.4
TotalrequiredHLB =12.1
Assumingthatablendofsarbitanmono-oleate(HLB4.3)andpolyoxyrthylenesorbitan
mono-oleate(HLB15)istobeusedastheemulsifyingsystem,theproportionsofeachtobe
addedtotheemulsiontoprovideanHLBofI2.1arecalculatedasfollows.Let“A”bethe
percentageconcentrationofthehydrophilicand“B”thepercentageofthehydrophobic
surfactantsrequiredtogiveablendhavinganHLBvalueof“X”.Then:
100(x-HLBofB)
A=
(HLBofA-HLBofB)
and B=100–A
13/04/2019physical Pharmacy , A.R.Gardouh, PhD17

18. Inourexample,therefore:
100(12.1–4.3)
A= =72.9
(15–4.3)
B=100–72.9=27.1
Becausethetotalpercentageofemulgentblendintheformulationis5,thepercentageof
eachemulsifierwillbe:
Sorbitanmono-oleate 5x27.1/100=1.36
Polyoxyethylenesorbitanmono-oleate5-1.36 =3.64
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19. 13/04/2019physical Pharmacy , A.R.Gardouh, PhD19

20. 4/13/2019Ahmed R. Gardouh, PhD.20