surfaceandinterfacialphenomena-200417163249.pdf

It covers all the topics of surface tension and interfacial tension from physical Pharmaceutics-I 3rd Semester.

1. Prepared by:
RohitKamboj
Assistant Professor
GGS College of Pharmacy

2. Introduction
Interfaceistheboundarybetweentwoormorephasesexist
together
Severaltypesofinterfacecanexistdependingonwhetherthetwo
adjacentphasesareinsolid,liquidorgaseousstate.
ImportanceofInterfacialphenomenainpharmacy:
Adsorptionofdrugsintosolidadjunctsindosageforms
Penetrationofmoleculesthroughbiologicalmembranes
Emulsionformationandstability
Thedispersionofinsolubleparticlesinliquidmediaforsuspensions.

3. SurfaceandInterfacialTensions
•Intheliquidstate,thecohesiveforcesbetweenadjacent
moleculesarewelldeveloped.
Forthemoleculesinthebulkofaliquid
•Theyaresurroundedinalldirectionsbyothermolecules
forwhichtheyhaveanequalattraction.
Forthemoleculesatthesurface(attheliquid/airinterface)
Onlyattractivecohesiveforceswithotherliquidmolecules
whicharesituatedbelowandadjacenttothem.
Theycandevelopadhesiveforcesofattractionwiththe
moleculesoftheotherphaseintheinterface.
Liquid Interface

4. Surface Tension
ThusSURFACE TENSION [γ]
is the force per unit length that
must be applied parallel to the
surface so as to counterbalance
the net inward pull and has the
units of dyne/cm.
INTERFACIAL TENSIONis the force
per unit length existing at the interface
between two immiscible liquid phases and
has the units of dyne/cm.
If two liquids are completely miscible, no
interfacial tension exists between them.

5. •InternalFactors:Thesurfacetensionisobservedinliquids
duetoIntermolecularattractiveforces.
•ExternalFactors:Whensubstancesareaddedtoliquids,these
alterthesurfaceandinterfacetension.ExampleareSurface
activeagentsdecreasesSurfacetension
•EffectofTemperature:AstempincreasestheSurfacetension
ofliquidsdecreasesduetoenhancekineticenergythatweakens
cohesiveforce.AtcriticaltempSurfacetensionis0.
Factors influencing Surface
and Interfacial tension

6. Methods for measuring surface and interfacial tension
1-Capillary rise method
2-Ring (Du Nouy) tensiometer
3-Drop weight method (Stalagmometer)
4-Drop Count Method
The choice of the method for measuring surface and
interfacial tension depend on:
Whethersurface or interfacial tension is to be determined.
The accuracy desired
The size of sample.

7. 7
Capillary Rise Method
Whenacapillarytubeisplacedinaliquid,itrisesupthe
tubeacertaindistance.Bymeasuringthisrise,itispossible
todeterminethesurfacetensionoftheliquid.Itisnot
possible,toobtaininterfacialtensionsusingthecapillary
risemethod.
Cohesiveforceistheforceexistingbetweenlikemole-
culesinthesurfaceofaliquid
Adhesiveforceistheforceexistingbetweenunlike
molecules,suchasthatbetweenaliquidandthewallofa
glasscapillarytube
WhentheforceofAdhesionisgreaterthanthecohesion,
theliquidissaidtowetthecapillarywall,spreadingoverit,
andrisinginthetube.
The Principle

8. 8
The upward component of the forceresulting from
the surface tension of the liquid at any point on the
circumference is given by:
Thus the total upward force around the inside
circumference of the tube is
Where
Ө= the contact angle between the surface of the
liquid and the capillary wall
2 πr= the inside circumference of the capillary.
For water the angle Өis insignificant, i.e. the liquid wets
the capillary wall so that cosӨ= unity
Cont. angle
water and glass
a = γ cos Ө
a = 2 πrγcos Ө

9. 9
The downward force of gravity
Downward component b = (mass x acceleration)
b = (mass x acceleration + w)
= Volume xdensity xacceleration
= cross-sectional area xheight x density xacceleration
At Maximum height,the opposing forces are in equilibrium
i.e. a=b
Where:
h =the height of the liquid column to the lowest point of the meniscus
p =density of the liquid
g = the acceleration of gravity
w = the weight of the upper part of the meniscus.
2 πrγ= πr
2
h p g
γ= 1/2rh p g
πr
2
x h x p x g

10. Ring (Du Nouy) Tensiometer
theprincipleoftheinstrumentdependsonthefactthat:
theforcenecessarytodetachaplatinum-iridiumring
immersedatthesurfaceorinterfaceisproportionaltothe
surfaceorinterfacialtension.
Theforceofdetachmentisrecordedindynes
onacalibrateddial
Thesurfacetensionisgivenby:
Where:
F =thedetachmentforce
R
1andR
2=theinnerandouterradiiofthering.
γ = F / 2 π(R
1+ R
2)
For measuring surface and interfacial tensions.
The principle

11. 11
Ifthevolumeorweightofadropasitisdetachedfromatipof
knownradiusisdetermined,thesurfaceandinterfacialtensioncan
becalculatedfrom
Where m = the mass of the drop
V = the volume of the drop
p = the density of the liquid
r = the radius of the tip
g = the acceleration due to gravity
The tip must be wetted by the liquid so as
the drop doesn’t climb the outside of the tube.
γ1= vρ1g
2 π rn¹
Drop Weight and Drop volume method
γ2= vρ2g
2 π rn2
Drop Count
γ1/γ2 = ρ1n2/ρ2n1
Drop Weight
γ1/γ2 = W1/W2

12. 12
When a liquid such as oleic acid is placed on the surface of other liquid
like water, it will spread as a film if the adhesion force is greater than the
cohesive forces.
The term film applies to a duplex film as opposed to monomolecular
film.
Duplex films are sufficiently thick so that surface and interface are
independent of one another.
Spreading coefficient
γ
L
γ
S
γ
Ls

13. 13
As surface or interfacial work is equal to surface tension multiplied by the
area increment.
Theworkofcohesion,whichistheenergyrequiredtoseparatethe
moleculesofthespreadingliquidsoasitcanflowoverthesub-layer=
Where2surfaceseachwithasurfacetension=γ
L
Theworkofadhesion,whichistheenergyrequiredtobreaktheattraction
betweentheunlikemolecules=
Where: γ
L =the surface tension of the spreading liquid
γ
S=thesurfacetensionofthesublayerliquid
γ
LS=theinterfacialtensionbetweenthetwoliquids.
Spreadingoccursiftheworkofadhesionisgreaterthantheworkof
cohesion,i.e.Wa>WcorWa-Wc>0
Wc= 2 γ
L
Wa= γ
L+ γ
S-γ
LS

14. Wc= 2 γ
L
Wc= γLΔA + γLΔA
S= WA-
Wc
Wc= 2 γLΔA
ΔA=1cm
Work of Cohesion
Wc= 2γL-----1
γ
L
γ
L
Wa= γ
LΔA + γ
SΔA -γ
LSΔA
Work of Adhesion
Wa= γ
L+ γ
S-γ
LS ------2
γ
L
γ
S

15. 15
SpreadingCoefficientisThedifferencebetween
theworkofadhesionandtheworkofcohesion
S = Wa -Wc=(γ
L+ γ
S-γ
LS ) -2 γ
L
S =γ
S-γ
L-γ
LS
S =γ
S–(γ
L+ γ
LS )
Spreadingoccurs(Sispositive)whenthesurfacetensionofthesub-layer
liquidisgreaterthanthesumofthesurfacetensionofthespreadingliquid
andtheinterfacialtensionbetweenthesub-layerandthespreadingliquid.
If(γ
L+γ
LS)islargerthanY
S,(Sisnegative)thesubstanceformsglobules
orafloatinglensandfailstospreadoverthesurface.

16. 16
Application of Spreading coefficient in pharmacy
Therequirementoffilmcoatstobespreadedoverthe
tabletsurfaces.
Therequirementoflotionswithmineraloilstospreadon
theskinbytheadditionofsurfactants.

17. 17

18. ADSORPTION:
It is surface effect.
E.g. concentration of alkaloid molecule on the surface
of clay.
ABSORPTION:
Gas or liqpenetrates in to the capillary spaces of
absorbing medium.
The taking up of water by a sponge is absorption.
18

19. Positiveadsorption:
Theconcentrationofdissolvedmaterialontheinterface
ishigherthanthatofsolutioninternal.
Negativeadsorption:
Theconcentrationofdissolvedmaterialontheinterface
islessthanthatofsolutioninternal.
19

20. 20
Moleculesandionsthatareadsorbedatinterfacesaretermed
surfaceactiveagents,surfactantsoramphiphile
Themoleculeorionhasacertainaffinityforbothpolarand
nonpolarsolvents.
Dependingonthenumberandnatureofthepolarandnonpolar
groupspresent,theamphiphilemaybehydrophilic,lipophilicorbe
reasonablywell-balancedbetweenthesetwoextremes.
Itistheamphiphilicnatureofsurfaceactiveagentswhichcauses
themtobeadsorbedatinterfaces,whetherthesebeliquid/gasor
liquid/liquid.

21. Surface active agent

22. 22
Functional Classification
According to their pharmaceutical use, surfactants can be
divided into the following groups:
Wetting agents
Solubilizing agents
Emulsifying agents
Dispersing, Suspending and Defloculating agents
Foaming and antifoaming agents
Detergents

23. 23
Ascaleshowingclassificationof
surfactantfunctiononthebasisof
HLBvaluesofsurfactants.
ThehighertheHLBofasurfactant
themorehydrophilicitis.
Example:SpanswithlowHLBare
lipophilic.TweenswithhighHLBare
hydrophilic.

24. Adsorption at Solid interface
Agasorliquidisadsorbedonsolidsurface.
Thematerialusedtoadsorbgasorliquid(solid)is
knownasadsorbent.
Thesubstancethatisattachedtothesurfaceofthe
solidiscalledadsorbate.
Thedegreeofadsorptionofgasbyasoliddependson
1.Natureofadsorbentanditssurfacearea.
2.Natureofadsorbateandthepartialpressureofgas.
3.Temp.
24

25. Type of adsorption.
Physical adsorption Chemical adsorption
Reversible Irreversible
Weak van derWaals forces Strong chemicalbond
Non-specific More specific
Common atlow temp Occurs at high temp
Heat of adsorption is low (20 to 40
kj/mole)
Heat of adsorption is high (40 to 400
kj/mole)
e.g. adsorption of gases on charcoale.g. adsorption of oxygen on silver or
gold.
25
Combination of both type of adsorption is known as sorption.
Desorption:adsorbed molecules or ions are removed from the
solid surface.

26. Adsorption at solid/gas interface
Adsorptionofgasisimportantinfollowingarea
1.Removalofobjectionableodorsfromtheroom.
2.Preventionofobnoxiousgasesenteringintothe
body(gasmasks)
3.Estimationofsurfaceareaandparticlesizeof
powders.
Theamountofgasadsorbedperunitareaorunit
massofsolidismeasuredatdiffpressureofthegas
atconstanttemp.
Thegraphofamtofgasad./unitAormofsolidVs
pressureisknownasadsorptionisotherm.
26

27. Freundlichisotherm
The relationship between pressure of the gas and amt
adsorbed at constant temp has been expressed by
freundlichisotherm eqn:
Where x = wtof gas adsorbed per unit wtof adsorbent,m
P= equilibrium pressure, k and n = constant.
This eqngives curvilinear graph when (x/m) is plotted
against pressure p.
The constant k and n are evaluated from the expand they
depends on temp and nature of the adsorbent and
adsorbate.
27n
kP
m
x
y
/1


28. Freundlich isotherm
28
x/m
p
Log (x/m)
Log p
Slope =
1/n
Intercept =
k

29. Langmuir Adsorption
Thefollowingassumptionsaremadeforthis:
1.Thesurfaceofsolidpossesfixednumberofactive
sitesfortheadsorptionofgases.
2.Atmaxadsorption,thegaslayerthatisfound
aroundthesolidisofonlyonemoleculethick.
3.Therateofadsorption(condensation)is
proportionaltonumberofsitesunoccupied.
4.Therateofevaporation(desorption)isproportional
tothenumberofoccupiedsites.
29

30. Langmuir Adsorption
Theadsorptionofgasonthesolidsurfacedependson
thepressureofthegasintheexperimentalconditions.
Ataparticularpressure,p,
Fractionofsiteoccupied:Ө
Fractionofsitesunoccupied:(1-Ө)
Rateofadsorption:r1=k1(1-Ө)P,
Rateofdesorption:r2=k2Ө,
Wherek1andk2areproportionalityconstantsforthe
processofadsorptionanddesorption.
30

31. At equilibrium : r1 = r2
If we consider,
y= mass of gas adsorbed/g of adsorbrnt
ym= mass of gas that 1 g of adsorbent can take up when a
monolayer is complete.
31)12(
1
1)12(
211
2)1(1
Pkk
Pk
PkPkk
kPkPk
kPk

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