study of 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.