Electrical Properties Of Interfaces
- 1. ELECTRICAL PROPERTIESOF INTERFACES The studyof electrical propertiesof interfacesfinds applicationsin the formulationof dosage formsregarding: 1. Stabilizationof colloidal dispersions 2. Preparationof flocculatedsuspensions 3. Stabilization of emulsions Whensolidsare dispersedinaliquid,alarge solid/liquidinterface isobtained,whichmaybecome charged.The originof charge on such an interface canbe accountedas follows. 1. Electrolytes(ions)presentinthe solutionmaygetadsorbedonthe solidsurface.If ionsare absent, the vehicle,(water),maydissociate intohydronium(H3O)+andhydroxyl,(OH)- ionsandgetadsorbed. Adsorptionof hydroxyl ionsismostcommonbecause of the asymmetricnature of the hydroxyl ions 2. Functional groups(suchas carboxyl group) presentonthe surface of the particlesmaygetdissociated and imparta charge.Forexample, -COOH, -NH2groupsonproteinsgetionised. 3. Differencesinthe dielectricconstantsbetweenthe particlesanddispersionmedium are responsible for the originof charge thougha lesscommonsource. The charge on the solidparticle impartscertainchangestoitsenvironmentregardingthe distributionof ions ELECTRICAL DOUBLE LAYER: The concept of the electrical double layeratthe interface canbe illustratedwiththe helpof .Figure 5-20.The solidparticles are dispersed inanaqueous solution containinganelectrolyte
- 2. The interface :aa' isthe solid/liquidinterface.The cationsare assumedtobe presentinsolution.These are adsorbedonthe solidinterface andimpartapositive charge.These cationsare referredaspotential determiningions. TIGHTLY BOUND LAYER: Immediatelyadjacenttothisinterface(aa')isaregionof tightlyboundlayer.This layerextends uptobb'.Once the adsorptioniscomplete,the cationsattracta tew anionsandrepel the approachingcations.Further,thermal motiontendstoproduce equal distributionof ioninsolution. Thus,at equilibrium,some excess anionsare e presentinthis region.However,theirnumberislessthanthe adsorbedcations.Therefore, bb' still possessescharge. Anionsare normallytermedascounterionsorgegenions.When particlesmove relative tothe liquid,thistightlyboundlayeralsomovesalong.Thus,the particlesurface nowextendsuptobb'ratherthan aa'.The boundarybb'is termedasshearplane DIFFUSE SECONDLAYER: This regionisboundbylinesbb'and cc'. In thislayer,excessnegative ionsare present At andbeyondcc' the distributionof ionsisuniform.Onthe whole the systemiselectricallyneutral, eventhoughthe distributionof ionsinunequal indifferentregions. Thus,the electrical double layerconsistsof (1) Tightlyboundlayer
- 3. (2) Diffuse secondlayer n the above example (Figure 5-20),bb'may possessnegativecharge.Itindicatesthatthe numberof anionsismore comparedto that of cationsadsorbedon the interface.However,cc'still maintains electrical neutralityasmentionedearlier. Whenthe interface isadsorbedby negative ions,thenaa'assumesnegative charge,bb'possesses negative charge andcc' will be neutral.Theargumentscanbe appropriatelysequenced.Onsimilarlines dependingonthe distributionof ions,aa'can be negative,bb'maybe positive andcc' will be neutral. Whenwe move fromthe interface (aa') towardsthe bulkof dispersion(dd'),the potentialchangesare representedinFigure5-21.These changesare characterizedandexpressedbydifferentways. NERNSTPOTENTIAL:It isthe potential of the solidsurface itself,aa'owingtothe presence of potential determiningions. Nernstpotential,E,orelectrothermodynamicpotentialisdefinedasthe differenceinpotential between the actual surface andthe electroneutral regionof the solution. ZETA POTENTIAL:It isthe potential observedatthe shearplane,i.e.,bb'line (Figure 5-20). Zetapotential,orelectrokineticpotential isdefinedasthe difference inthe potential betweenthe surface of the tightlyboundlayer(shearplane) andthe electroneutralregionof the solution Zetapotential canalsobe definedasthe workrequiredtobringa unitcharge frominfinitytothe surface of the particles.
- 4. From Figure 5-21, itcan be inferredthatthe potential energydecreasesrapidlyin the initialstage, followedbya gradual decrease towardsthe boundarycc'. The counterions,whichare presentclose tothe surface bb'and inthe region of bb' tocc', may reduce the particle- particle interactions.Hence,the potential energydecreasesmore graduallyinthisregion.Ingeneral, zetapotential ismore importantinthe fieldof pharmacycomparedto Nernstpotential,becausethe electrical double layeralsomoves,whenthe particle isundermotion. APPLICATIONS:Zeta potential governs the degree of repulsions between the adjacent ions of like charges. Hence,itis usedtopredict Particle-particle- interaction.Suchinformationprovidesinsightsaboutthe stabilityof systems containingdispersedparticles.Anoptimumzetapotential desirable forthe maximumstability. If zeta potential fallsbelowaparticularvalue,the attractive forcesexceedthe repulsive forces. Thisresultsinthe aggregationof particles.
- 5. Thisphenomenaisobservedincolloids.Zetapotential decreasesmore rapidlywhenthe concentration of electrolytesisincreasedorthe valencyof counterionsishigher.