Electrical properties of interfaces

Electrical properties of interfaces

• 1.
  ELECTRICAL PROPERTIES OFINTERFACES The study of electricalproperties of interfacesfindsapplications inthe formulation of dosage forms regarding: 1. Stabilizationofcolloidal dispersions 2. Preparationof flocculatedsuspensions 3. Stabilizationofemulsions When solidsare dispersed in a liquid, a largesolid/liquid interfaceis obtained,which may become charged. The origin of charge on such an interfacecan be accountedasfollows. 1. Electrolytes(ions) present in the solutionmay get adsorbed on the solidsurface.If ionsare absent,the vehicle,(water), may dissociate into hydronium (H3O)+ andhydroxyl, (OH)- ionsandget adsorbed. Adsorption of hydroxyl ionsismost common because of the asymmetric natureof thehydroxyl ions 2. Functionalgroups (such ascarboxyl group) present onthe surface of theparticlesmay getdissociated and impart a charge.Forexample, - COOH, -NH2 groups on proteinsget ionised.
• 2.
  3. Differencesin thedielectricconstantsbetweentheparticlesand dispersion medium are responsible forthe origin of charge though a lesscommon source. The charge on the solidparticle imparts certainchanges to its environmentregarding the distribution ofions ELECTRICAL DOUBLE LAYER: The concept of theelectricaldouble layerattheinterface can be illustratedwiththe helpof.Figure 5-20.The solid particlesare dispersed in anaqueoussolutioncontainingan electrolyte
• 3.
  The interface:aa' isthesolid/liquidinterface.Thecationsare assumed to be present in solution.These are adsorbed on thesolidinterface and impart a positivecharge.These cationsare referredaspotential determining ions.
• 4.
  TIGHTLY BOUND LAYER:Immediatelyadjacent to this interface(aa')isa region of tightlybound layer.This layerextends upto bb'. Once theadsorption iscomplete, the cationsattract a tewanions andrepel the approaching cations. Further,thermal motion tendsto produce equal distribution of ion in solution.Thus, atequilibrium, some excess anionsaree present in this region. However,theirnumber islessthan theadsorbed cations.Therefore, bb' stillpossesses charge. Anions are normallytermed ascounterionsorgegenions. When particles move relativetotheliquid, thistightlybound layeralso movesalong. Thus,theparticle surface now extendsupto bb' ratherthanaa'.The boundary bb' is termedasshearplane DIFFUSE SECOND LAYER: Thisregion isbound by linesbb' andcc'. In thislayer,excess negativeionsare present At and beyond cc' thedistribution ofionsisuniform. On the wholethe system iselectricallyneutral,eventhough thedistribution of ionsin unequal indifferent regions. Thus, the electricaldouble layerconsists of (1) Tightlybound layer
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  (2) Diffusesecond layer ntheabove example (Figure 5-20), bb' may possess negativecharge.It indicatesthatthenumber of anionsismore compared to thatof cations adsorbed on the interface.However,cc' stillmaintainselectrical neutralityasmentionedearlier. When theinterface isadsorbed by negativeions, thenaa'assumes negativecharge,bb' possesses negativecharge and cc' willbe neutral.Theargumentscan be appropriately sequenced. On similar linesdepending on thedistribution of ions, aa'can be negative,bb' may be positive andcc' will be neutral. When we move from theinterface(aa')towards thebulkof dispersion (dd'), thepotentialchangesare representedin Figure 5-21.These changesare characterizedand expressed by differentways. NERNST POTENTIAL: Itisthe potentialofthesolid surface itself,aa' owingto thepresence of potentialdeterminingions. Nernst potential,E,orelectrothermodynamic potentialisdefined as thedifference in potentialbetweentheactual surface andthe electroneutralregionof thesolution.
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  ZETA POTENTIAL: Itisthepotentialobservedat theshearplane,i.e., bb' line(Figure 5-20). Zeta potential,orelectrokineticpotentialisdefinedasthedifference in thepotentialbetweenthe surface of thetightlybound layer(shear plane)and theelectroneutralregionof thesolution Zeta potentialcan also be definedasthework required tobring a unit charge from infinitytothe surface ofthe particles.
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  From Figure 5-21,itcan be inferredthatthe potentialenergydecreases rapidly in theinitialstage,followedby a gradual decrease
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  towardsthe boundary cc'.The counterions, which are present close to thesurface bb' and in theregion of bb' tocc', may reduce theparticle- particle interactions.Hence, the potentialenergydecreases more gradually inthisregion.In general,zetapotentialismore important in thefieldof pharmacy compared toNernst potential,because theelectricaldouble layeralso moves,whenthe particle isundermotion. APPLICATIONS:Zeta potentialgovernsthe degree of repulsions betweenthe adjacentionsof like charges. Hence, it isused topredict Particle-particle-interaction.Such informationprovides insights about thestabilityof systems containingdispersed particles. An optimum zeta potentialdesirable forthe maximum stability. If zetapotentialfallsbelow aparticular value,theattractiveforces exceed therepulsive forces. Thisresultsin theaggregation of particles. Thisphenomena isobserved in colloids. Zeta potentialdecreasesmore rapidly when theconcentration of electrolytesisincreasedor the valencyof counterionsishigher.