Concise overview of surface tension and interfacial phenomena.

1. George Wild Science of Medicines
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Surface Tension
The extentof inter-surface/interfacial contactbetweentwophaseswill be adeterminantinthe
interactionbetweenthe twophases;the bulkpropertiesof the twophasesinisolate haslittle todo
withthis.Atthe boundarybetween liquidandair,the watermoleculesatthe surface experience an
imbalance of attractive forcesthatcausesa netdownwardforce that wouldcause the surface to
contract to formspherical structures,where are geometricallystable.Thisdoesnottendtohappen
and insteadthe surface existsundertension;the surface tensionof the liquid(γ).Thisarisesdue to
longdistance interactionssuchasdipole,induceddipoleanddispersion(vanderWaals) forces.
Surface tensionmaybe measuredusingthe rise of aliquidina capillarytube,butismore commonly
measuredusingdetachmentmethods.These methodsuse eitherathinplate (Wilhelmymethod)or
ring(Du Nuoymethod) placedatthe surface of a liquid(andattachedtoa microbalance arm) to
measure the force requiredtopull free fromthe surface of thatliquid.The surface tensionmaybe
calculatedbydividingthe force requiredbythe perimeterof the plate.
 The surface tensionof waterisunusuallyhighdue tothe extentof interactionswithinthe
bulk of the liquid.The presence of organicimpuritieslike methanolreduce the surface
tensionbecause theyhave alowaffinityforthe solventandsoare pushedto the surface to
preserve water-waterinteractions.Thismeansthatthe methanol isquite concentratedat
the surface (althoughthere are still watermoleculesthere) andsothe surface hasa
methanol qualitytoit,causingthe surface tensiontodrop.
 The methanol issaidto be surface active here,asit alignsat the liquid/airboundary.
 The presence of inorganicmaterialsmayincrease the surface tensionbystrengthening
bondingwithinwaterandaccumulatinginthe bulkratherthanat the surface.
Solid Wettability
The wettabilityof solidsisameasure of the affinityof the solidsurface forcertainliquid
environments.Itisimportanttoconsider,asproblemsinvolvingwettabilitycouldmeanthat
potentiallysoluble drugsdonotadequatelybecome available inthe body.The difference between
solidandliquidsurface tensionsisthatthe solidsurface tensionisnotuniformthroughoutthe entire
surface;crystallisationandproduction/millingtechniquescause differenttexturesandmolecular
orientationsthatcarrytheirownsurface energies.Thismeansthatthere maybe some hydrophobic
and hydrophilicregions.
The surface tensionof solidscanbe measuredusingthe contactangle (),whichisthe angle formed
by the tangentto the waterdropletat the three phase interface. The three interfacial tensionsare
relatedbyYoung’sequation;γLV cos() = γSV - γSL. Contactangleslowerthan90 indicate good
wetting,whilstthose closerto180 indicate poorwetting.
 γSV will act to aidspreadingof the liquidacrossthe surface
 γSL will actto preventspreadingof the liquidacrossthe surface
 γLV will actalong the tangentto the drop

2. George Wild Science of Medicines
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The contact angle canbe measureddirectlyusingavideoimage whenadroplethasbeenplacedon
a flatsurface,but inrealitythe ideaof a completelyhomogenous,perfectlyflatandsmoothsurface
uponwhicha pure liquidcanbe placedisnotachievable.A raindropfallingonaplane of glassthat
sitshorizontallywill remainmotionlesswithequalcontactanglesall arounditscircumference;thisis
the equilibriumcontactangle (E).If the glassplane isthenmovedvertically,the dropletfallsinthe
shape of a leadingedge andarecedingedge.Atthe leadingedgethe contactangle isgreatestand
thisisthe advancingcontactangle (A),whereasatthe trailingedge,the contactangle issmallest
and thisisthe recedingcontactangle (R).
 The difference betweenthese twoanglesistermedthe contactangle hysteresis.This
feature of an interface occursbecause of surface roughnessandvariationinsurface
heterogeneity.
It’salso possible todeterminethe contactangle throughusingthe Wilhelmyplate method,given
that the surface tensionof the liquidisknown;F= p γLV cos(),where pisthe perimeterof the plate.
Althoughit’spossible toformsmoothplatesfrompolymericmaterials,mostdrugsubstancesare dry
powdersthatare notso easilymanipulated.Impurities,differingfacesandproductionmethods
meanthat contact anglesobtainedfordrugsolidsare usuallyaveragesof the contactanglesfor
manyof the faces.Thisis complicatedfurtherbythe fact that manysolids,despite beingcrystalswill
have slightlyamorphousregionswhichare subjecttonatural structural change.
 The most widely-usedmethodforobtainingacontactangle isthroughcompactionto
produce a flat,smoothsurface,butthe veryprocessof compactioncreatesanentirelynew
surface and doesnotnecessarilyrepresentthe contactanglesthatwouldformfromthe
variousotherfacesfromwhichit forms.