The document discusses the electrical properties of interfaces and the electrical double layer that forms at solid-liquid interfaces. When solid particles are dispersed in a liquid, ions from the solution or functional groups on the particle surface can impart a charge to the interface. This gives rise to an electrical double layer consisting of a tightly bound layer and a diffuse second layer. The zeta potential, which is the potential at the boundary between these layers, governs particle interactions and can be used to predict stability. An optimal zeta potential is needed to provide sufficient repulsion between particles and prevent aggregation.

1. ELECTRICAL PROPERTIES OF INTERFACES 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 thedielectricconstantsbetween theparticlesand
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.The cationsare 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

5. (2) Diffusesecond layer
n theabove 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.

6. ZETA POTENTIAL: Itisthe potentialobservedat 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.

7. From Figure 5-21,it can be inferredthatthe potentialenergydecreases
rapidly in theinitialstage,followedby a gradual decrease

8. 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.