FOR BACHELOR OF PHARMACY

1. SURFACE
&
INTERFACIAL PHENOMENA
PRESENTEDBY ~ADITYA SINGH
~SHABAKHATOON
~ANKITA ROY
PHYSICAL
PHARMACEUTICS

2. CONTENTS
1. LIQUID INTERFACE
2. SURFACEANDINTERFACIALTENSION
3. SURFACEFREEENERGY
4. MEASUREMENTOF SURFACEANDINTERFACIALTENSION
5. SPREADINGCOEFFICIENT
6. ADSORPTION AT LIQUIDINTERFACES
7. SURFACEACTIVEAGENTS
8. HLBCLASSIFICATION
9. SOLUBILIZATION
10. DETERGENCY
11. ADSORPTION AT SOLIDINTERFACES
12. SOLID-GAS & SOLID-LIQUIDINTERFACES

3. INTRODUCTION
 Interface-Whentwo phasescomesin contact with eachOTHER& existtogether, it is called interface.
 SURFACE-THEOUTERMOSTLAYEROF ANYSUBSTANCEis knownas surface.
Buthere,Surfaceis the liquidgas interface or any interface in which gas is on opposite side.
TYPESOF INTERFACES:-
1. SOLIDInterface
2. Liquidinterface

4. LIQUID INTERFACE
 Liquid interface forms whenliquidcomesin associationwithotherstatesof matter(solidor gas)or liquiditself.
eg :- Oil in water(liquid-liquidinterface)
✧Applications:-
 Emulsionformation.
 Adsorptionof drugontosolidin dosage form.
 Infiltration process.
 Biopharmaceuticalstudy.

5. SURFACE AND INTERFACIAL TENSION
 The moleculesinside the liquidinteract equallywith othermoleculesdue to cohesive force fromall side.
But,the moleculesat the surfaceis onlyaffectedby themoleculesbelowit.
 Thereis a force which pullsdown the moleculesof the surface is known as surface tension.
 It is theforce per unitlength that mustbe applied paralleLto the surface.
γ = force/length
Where,
γ- surface tension
 unitof surfacetensionis n/m.

6. SURFACE FREE ENERGY
 Surface free energy is definedas theworkthat is necessaryto increase the surface areaby a unit.
 The moleculesnear the surface of liquidhave morepotential energyas comparedto the moleculesin the bulkof the liquid. It meansthat as
the surface areaof liquid increases,the moremoleculeshave thisexcessive potentialenergy.
 As we know,
W=F×ΔD
W=γ×L×ΔD [F=γ × L] where ,
W= γ×ΔA W=surface free energy
γ = surface tension
ΔA= increase in area

7. MEASUREMENT OF SURFACE AND
INTERFACIAL TENSION
Measurement of surface and interfacial tension are :-
1. Capillaryrise method
2. Dropcount method
3. Dropweightmethod
4. Wilhelmy plateMethod
5. Ring detachment method

8. CAPILLARY RISE METHOD
 Thismethodis usedto measurethesurface tension.
PRINCIPLE:-
1. Whena Capillarytubeis dippedin liquidcontaining beaker,liquid risesup in thecapillarytube UPTO a certainheight.
2. Thisis because The forceof adhesionb/wcapillaryand liquidis greaterthanthecohesiveforce betweentheliquid.
3. DUE TO SurfacetensionLIQUIDRISESBUTGRAVITYALSOACTS ON IT WHICHPULLSIT DOWNWARD.
4. WHENBOTHFORCESAREEQUAL,LIQUIDIS IN ANEQUILIBRIUMANDSTABLE.
DERIVATION:-
Upward force
As we know,
F=γ × L where,
F =2πr.γcosθ γcosθ=S.T & Angle of contact
F=2πr.γ -------(1) 2πr= circumference of capillary

9. DOWNWARDFORCE
F = mgh+w --------(2)
where,
mgh = potential energy w.r.t to gravity
w = weight of liquid
We know that, m=ρv
i.e. m= ρπr²
Substituting the value in eqn (2),we get
F = ρπr²gh + w
Now,liquid is in equilibrium,so both force is
equal.
2πrγcosθ=ρπr²gh + w
For water cosθ = 0
γ = ½ [ ρrgh + w/πr ]
where,
γ- Surface tension of liquid.
ρ- density of liquid
r- radius of capillary
g- gravity acting on liquid
h- height of rised liquid
w- weight

10. DROP COUNT METHOD
 THISMETHODIS ALSOUSEDTO MEASURETHE SURFACETENSIONOF LIQUID.
 IN THISMETHOD, WE USE ANAPPARATUSCALLEDSTALAGMOMETER.
Principle:-
 Firstlytakea liquidwhose s.tis known.
 FilltheSTALAGMOMETERwiththatfluidto pointA. Then close theSTALAGMOMETERfrombottomwiththehelp of finger.
 Now,release liquidslowly dropwiseuntilit reachesto pointb andsimultaneouslycountthe numberof dropandnoteit.
 Now,do somewiththefluidwhose s.t is to be determined.
 So on comparingbothby usingformula,we canfindoutsurfacetension.
Formula:-

11. DROP WEIGHT METHOD
 It is same as dropcountmethod. We use same capillaryor STALAGMOMETER.
 The maindifference is that,we weightthe drop(1drop)Firstlyof thatliquidwhoseSurface tension is known.
 Afterthat,we weighttheother’sfluiddropwhose surface tensionis to be determined.
Case 1:-
m1 = 2πrγ1
CASE 2 :-
m2 = 2πrγ2
On comparing,we get where,
m1 – mass of known liquid
γ1 – surface tension of known liquid
m2 – mass of unknown liquid
γ2 – surface tension of unknown liquid

12. WILHELMY PLATE METHOD
 IT IS USEDTO MEASURETHESURFACETENSIONOF A LIQUID.
PROCEDURE:-
 FIRSTLY,WE PUT THERECTANGULARPLATEIN THATLIQUIDWHOSESURFACETENSIONIS TO BE DETERMINED.
 Now,surfacetensionis appliedon theplatewhichpullit downwardin thatliquid
 We pulltherectangularplate upwardeithsomeforceand surface tensionalsoopposesthis.
 Whenwe detachedouttheplatefromtheliquid,at thattimetheforceappliedis as sameas thesurfacetensionof theliquid.
 Force is displayedon thetorsionbalance.
γ = F/l.Cosθ
where,
γ – surface tension of liquid
F – force applied
l.Cosθ –Perimeter of rectangular plate and angle of contact

13. RING DETACHMENT METHOD
 THISMETHODIS USEDTO MEASUREBOTHSUFACEANDINTERFACIALTENSION.
 It is alsoknownas du nuoy method.
Procedure:-
 In thismethod, a ringmadeup of platinumis attachedto thesurfaceof theliquidwhosesurface tensionis to be determined.
 Ringis liftedslowly,the force f requiredto raise theringfromtheliquid’ssurfaceis measuredand relatedto liquid’ssurface tension.
γ = F/2π(r1 +r2)
where,
γ – surface tension
F – force applied
r1 & r2 – radius of outer and inner surface of ring respectively

14. SPREADING
COEFFICIENT
• SpreadingCOEFFICIENTis definedas theabilityof one
liquidto spreadoveranotherliquid(immiscible).
• It is thedifferencebetweenWorkof adhesionand workof
cohesion.
• Eg :- emulsion,oilin water,etc.
• It occurswhen force of adhesionis morethanforceof
cohesion.
• S = Wa– wc ----------(1)
1. If γs < (γl + γls) thenspreadingdoesn’toccurs.
2. If γs >(γL+ γls) thenspreading occurs.

15. ADSORPTION AT LIQUID
INTERFACE
 Adsorptionis theprocessin whichatoms, ionsor moleculesfroma substance(it couldbe gas, liquidor dissolvedsolid)adhereto a surface of theadsorbent.
 Whenthesubstanceis depositedon thesurface of liquid,itis called positive adsorption.Inthisthesurface free energyand surface tensiondecreases.
 Whenthesubstancemix withintheliquid,itis called negative adsorption(Absorption). In thissurface freeenergy& surfacetensionincreases.

16. SURFACE ACTIVE AGENTS
 Surfactants is definedas thesubstancewhichreducesthesurfaceandinterfacialtensionof A liquidby increasing thierspreadingandwettingproperties.
Eg:-Detergents,soaps,emulsifier,etc
 It helpsin mixingoil in water.
 Foreg :– if we addoil & waterin a container,itis immiscible. So we use surfactantsto decreasetheinterfacialtensionto make it miscible.

17.  TYPESOFSURFACTANTS:-
 ANIONICSURFACTANTS- ITCONTAINSORGANICTAILWITHNEGATIVECHARGEHEAD.
Eg:-alkalimetal& ammoniumsoaps.
 CATIONICSURFACTANTS-ITCONTAINSORGANICTAILWITHPOSITIVECHARGEDHEAD.
Eg:-benzalkoniumchloride.
 AMPHOTERICSURFACTANTS-ITISREFERREDTOASZWITTERIONMOLECULESWHICHCONTAINSBOTHCATIONIC& ANIONICMOLECULES.ITDEPENDSONTHEPHOFTHE
SYSTEMS.
Eg:-lecithin,Aminoaceticacid
 NON-IONICSURFACTANTS-THEYARENON-IONIC& DOESN’TIONIZEDIN WATER. MOSTLYUSEDIN PHARMACEUTICALINDUSTRY.RESISTIVETOPHCHANGE.
eg:-glycerol.

18. HLB CLASSIFICATION
 Introducedby griffinin 1947.
 Hlbscalecontainsdifferentvalue whichindicatesdifferentsurfactants
accordingto theirnature.
 Hlbvalue1 indicates-surfactantsis lipophilic(oilsoluble).
 Hlbvalue20 indicates-surfactantsis hydrophilic(watersoluble).
 Morethe hlbvalue,more hydrophilicit is.

19. SOLUBILIZATION & DETERGENCY
SOLUBILIZATION
 It is definedas thephenomenonin whichthesolubilityof any organic
compoundis increasedby theadditionof surfactants.
 Uses:-
1. Usedto maketwoimmiscibleliquidmiscible.
2. Usedin thepreparationof drug.
DETERGENCY
 Detergentsare basicallymadeup withsurfactants.
 Detergencyis definedas thephenomenonin whichthedirtis removedby
usingdetergent.
 Mechanism:-
It actsby reducingthe adhesiveforce b/wthedirtparticlesand
clothes.

20. ADSORPTION AT SOLID INTERFACE
 Whenthesubstanceget depositedon solid’ssurface,itis saidas adsorptionat solidinterface.
 Adsorbate – substancewhichdepositson thesurface .
 Adsorbent– Surfaceof substanceon which adsorbatedepositsis knownas adsorbent.
 Adsorbate& adsorbentget attachedwitheachotherby someforce of attraction.
 Basedon attractiveforce,itis categorisedintotwotypes
1. Physisorption(physicaladsorption)
2. Chemisorption(chemicaladsorption)

21. PHYSIOSORPTION
 It occursdue to weak attractiveforceslikevanderwaal’s force,etc
 Reversiblein nature.
 Lessactivationenergy.
 Multimolecularlayer.
 Lowtemperatureis favorable.
CHEMISORPTION
 It occursdue to strongforce of attractionlike ionic bond,etc.
 Irreversible in nature.
 Highactivationenergy.
 Unimolecularlayer.
 Hightemperatureis favorable.

22. SOLID-GAS INTERFACE
 Whena gas or vaporis broughtintocontactwitha cleansolidsurface,
is saidto be solidgasinterface.
 Someof it willget attachedto thesurface.
 Adsorbent-Solid
 Adsorbate-Gas
EG :- Charcoal
SOLID-LIQUID INTERFACE
 Whena liquidis broughtin contactwitha cleansolidsurface ,itis said
to be solid-liquidinterface.

23. ➣ANKITA ROY
➣SHABA KHATOON
➣ADITYA SINGH