SURFACE ANDINTERFACIAL PHENOMENON

Seletted DellnltlonsInterface: The boundary between two phases which exist together.

Surface tension: It is defined as tensile force acting at the surface of liquid which is in contact with gas or it may bebetween two immiscible liquids.

Cohesive forces: The forces acting between the molecules of same phase.

Adhesive force: The forces acting between the molecules of different phase.

Interfacial tension: It is the force per unit length that exist at the interface between two immiscible liquid phases.

Positive adsorption: When some molecules and ions, when dispersed in the liquid, are partitioned in favor of theinterface.

Negative adsorption: The materials (such as inorganic electrolytes) when dispersed in the liquid are partitioned infavor of the bulk.

Surfactants: These are the agents which are used to lower surface tension of liquids and also reduces interfacialtension between two liquids.

AmpbipbiIes: The surfactants which have both polar and non polar groups.

HLB system: HLB is Hydrophilic - Lipophilic Balance. HLB system consist of arbiratory scale in which values areassigned to different surfactants according to their nature.

Solubilization: The process of. increasing the solubility of organic compounds in aqueous system due to presence ofsurface active agents.

Detergency: It is the phenomenon in which dirt or foreign material is removed from the surfaces with the help ofsurfactants.

Adsorption: The accumulation of gas or liquid to the surface of solid .

•

106 W Physical Pharmaceutics-I

6_1 INTRODUCTIONThe boundary between two phases which exist together is known as an interface. It may beliquid-liquid interface or solid-solid interface or liquid-gas interface. In case of gas-gas, nointerface is possible.

6.2 SURFACE & INTERFACIAL TENSIONSSurface tension is defined as tensile force acting at the surface of liquid which is in contact withgas or it may be between two immiscible liquids. Its unit is N/m.

In the liquid state, cohesive forces and adhesive force exist. The forces acting between themolecules of same phase are called cohesive forces. The forces acting between the molecules ofdifferent phase are called adhesive force.

The molecules in the bulk liquid are surrounded in all directions by other molecules. The netforce acting on this molecule is zero. While the molecules at the surface of the liquid experiencean inward force toward the bulk. This force pulls the molecules of the interface together andstretch the surface. It means some tensile force is acting on the surface which is called surfacetension.

Netattraction

Water

Figure 6.1 : Representaion oC surface tension

Interfacial tension: It is the force per unit length that exist at the interface between twoimmiscible liquid phases. If two liquids are completely miscible, no interfacial tension existsbetween them.

Interfacial tensions are less than surface tensions because the adhesive forces between two liquidphases which form an interface are greater than when a liquid and a gas phase exist together.

Suppose a soap film is stretched by applying a force (f) to the movable bar, length (L). Thesurface tension, -y, of the solution forming the film is a function of the force that must be appliedto break the film over the length of the movable bar in contact with the film. Thus

fy=_b_2L

(eq 6.1)

Surface and Interfacial Phenomenon 107

Where

fb is the force required to break the film and

L is the length of the movable bar.

6.3 SURFACE FREE ENERGYThe molecule near the surface of liquid have more potential energy as compared to the moleculesin the bulk of the liquid. This means that as surface of the liquid increases, the more moleculeshave this excessive potential energy. This energy is proportional to the size of the free surface, itis called a surface free energy.For example, a liquid droplets assume a spherical shape because a sphere has the smallest surfacearea per unit volume. To increase the surface of the liquid, work must be done against the surfacetension.When the bar is at a position and mass is added to extend the surface by a distance ds, the workdW (force multiplied by distance) is

dW=jxds

As we know from (equation 6.1) 'Y= ft/2L

So dW = yx2Lxds (eq 6.3)As 2L x ds is equal to the increase in surface area, dA, produced by extending the soap film,Therefore dW = 'Y dA (eq 6.4)For finite changes

(eq 6.2)

W =yflA (eq 6.5)whereW is the work done, or surface free energy increase, expressed in ergs,'Yis the surface tension in dynes/cm, andflA is the increase in area in c.m2.Surface tension can be defined as the surface free energy change per unit area.

Solved problemExercise 6.1 A soap solution having surface tension of 35.07 dy/cm is applied to bar of 6 cm.Calculate work needed to pull wire down by 3 cm.Solution

W=yx2Lxd

W = 35.07 x 2 x 6 x 3 = 1262 ergsAns : 1262 ergs

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6.4 MEASUREMENT OF SURFACE & INTERFACIAL TENSIONSThere are various methods of determination

1. Capillary Rise Method

2. Drop Weight and Drop count methods

3. Wilhelmy Plate Method

4. Ring Detachment Method (Du Novy tensiometer)

6.4.1 Capillary Rise MethodIt measures surface tension only.

Principle: When a capillary is placed in container having liquid, the liquid rises up in the capillaryupto a certain height. The rising of liquid is due to more of adhesive force between liquid andcapillary wall. The rise of liquid in tube will continue until upward movement is balanced bydownward force of gravity due to the weight of liquid .

..-Capillary tube

Liquid

Figure 6.2 : Capillary rise method

Derivation

During upward movement, the surface tension at any point on the circumference of the capillarytube is expressed as = 'Yeose (eq 6.6)

Where

'Yis surface tension of liquid

e is contact angle between surface of liquid and capillary wall

Surface and Interfacial Phenomenon 109

The total upward force along the inside circumference of capillary is given by = 'Y.2lli. Cosf

(eq 6.7)

Where

r is inside diameter of capillary tube

For water, S is considered as 1. So upward force = 2lli 'Y

The countering force relates to the weight of the liquid in capillary rise and given as= n? hpg

(eq 6.8)

At equilibrium

Or

2lli 'Y = lli2 hpg

'Y = ljH hpg (eq 6.9)

This method is very accurate and also used for many liquids. The height (h) can be measured withhelp of travelling microscope. Density is determined using density bottle.

Solved problemExercise 6.2: A given liquid rises 1.19 cm in capillary tube, the radius of which is 0.038cm anddensity of given liquid at 30°C is 0.778 g/crrr'. Calculate surface tension of the liquid

Solution

'Y = ljH hpg

'Y= lj2x 0.038 x 1.19 x 0.778 x 981

'Y= 17.25 dy/cm

Ans .17.25 dy/cm

6.4.2 Drop weight and Drop count methods(1) Drop weight method

Stalagmometer or drop pipette is used in this method. Stalagmometer consist of glass tubewith one marking A above the bulb and other marking B below the bulb. At the tip capillaryis attached.

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A

Bulb

Capillary

B

Figure 6.3: Stalagmometer

The liquid whose surface tension is to be measured is put into beaker. The capillary sucks theliquid upto mark A and then allowed the liquid to fall slowly through capillary tube. Thedrops (approximately 20 to 30) is collected into clean vessel. By this weight of one drop ofliquid can be measured. The surface tension of the liquid is calculated by putting values infollowing equation.

or

w=2JIry

y = w 12JIr

(eq 6.10)

(eq 6.11)

(2) Drop count method

In this method, instead of weight numbers of drops of liquid formed are counted.

As we know

or

y = w 12JIr

y =mg 12JIr

where

m is the mass of one drop

g is gravitational force

n is number of drops

(eq6.12)

(eq 6.13)

y = vdg 12JIr (because m= volume(v) x densityrdj) (eq 6.14)

Surface and Interfacial Phenomenon 111

6.4.3 Wilhebny Plate MethodIn this method, the liquid whose surface tension is to be measured 'is put into container. Arectangular plate which is made of glass, platinum or thin mica is suspended vertically. Theplate is attached to torsion balance. The container is then gradually lowered so that platedetache from the surface of liquid. The reading on the balance is recorded.

Beamt------+ To torsion balance

V Rectangular plate

Figure 6.4: Wilhelmy Plate Method

If liquid completely wets the plate then the detachment force is equal to surface tensionmultiplied by perimeter of surface detached.

WL - W = 2 (L+T) y (eq 6.15)

Where

W L is reading on the balance prior to detachment

W is weight of the plate in air

L is length of the plate

T is thickness of plate

The above equation can also be written as

(eq 6.16)

112 W Physical Pharmaceutics-I

6.4.4 Ring Detachment method or Du Nouy tensiometerIt is used to measure both surface and interfacial tension

Figure 6.S: Du Nouy tensiometer

The liquid whose surface tension is to be measured is put into a container. A platinum orIridium ring (about 4 cm) is suspended in the liquid. The ring is attached to a scale throughtorsion wire. The ring should just touch the surface of liquid. The force required to detach thering is noted from the scale. This detachment force is given by

p = w = 21t (rl + Xi) Y

pY = --,-------:-

21t (r, + r2)

(eq 6.17)

(eq 6.18)

where

P is pull exerted through torsion wire on the ring

w is the force in term of weight

r, and r2 are inner and outer radii of the disc respectively

The error of 25% can occur if correction factor is not applied. So Correction factor is .involved in the equation

(eq 6.19)

Surface and Interfacial Phenomenon 113

Dial reading in dynes . fir = x correction actor2x ring circumference

(eq 6.20)

or

P'Y=-x~4m (eq 6.21)

6.5 SPREADING COEFFICIENTWhen a drop of oleic acid is placed on the surface of water, it will spread as a film. The ability ofone liquid to spread over another is calculated as spreading coefficient. If spreading isspontaneous, the surface free energy of system is reduced. The value of spreading coefficientshould be positive or zero.

The principle of Spreading is important in pharmacy such as for external applications, forabsorption of drug and also for stabilization of emulsion.

The energy involved in separating the molecules of liquid is known as work of cohesion which isexpressed as

(eq 6.22)

The energy involved to separate column of two immiscible liquid at the interface into twosections are called Work of adhesion.

Wa= YL+YS-YLS (eq 6.23)

Where

Ys is interfacial tension of sublayer

YLSis interfacial tension of solid/liquid interface

The spreading coefficient (S) is the difference between Work of adhesion and Work of cohesion.

S = Wa - Wc = YL+ Ys - YLS - 2 YLS = Ys - YL - YLSS = Ys - (YL + YLS)

(1) If Ys> YL + YLS, then S is positive and indicate spreading

(2) If Ys< YL + YLS, then S is negative and indicate no spreading

(eq 6.24)

(eq 6.25)

(eq 6.26)

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Table 6.1 Initial Spreading Coefficient, S, at 20°C

Liquid Spreading Coefficient (S) in dy/cm

Benzene 8.8

Ethyl alcohol 50.4

Oleic acid 24.6

Chloroform 13

Acetic acid 45.2

Liquid paraffin -13.4

6.6 ADSORPTION AT LIQUID INTERFACESWhen some molecules and ions, when dispersed in the liquid, are partitioned in favor of theinterface. This phenomenon is called is positive adsorption. The surface free energy and thesurface tension get decreased in the case of positive adsorption.

Other materials (such as inorganic electrolytes) when dispersed in the liquid are partitioned infavor of the bulk. This phenomenon is called negative adsorption. The surface free energy andthe surface tension get increased in the case of negative adsorption.

6.7 SURFACTANTSThese are the agents which are used to lower surface tension of liquids and also reducesinterfacial tension between two liquids. Surfactants contain hydrophilic (water loving) andhydrophobic (water hating) group. When surfactants are placed in the water, the hydrophilicgroup dissolves in water while hydrophobic group form micelles. The surfactants which haveboth polar and non polar groups are called Amphiphiles.

Surfactants

Anionic Cationic Ampbolytic Non-ionic

1. Anionic surfactants: These are the most common type of surfactants. It contain organictail with negative charge head and small positive molecule like ammonium. This consistof soaps of alkali, arnines, metals, Alkyl sulphates or phosphates and alkyl sulphonates.They have unpleasant taste. They are not suitable for internal use due to irritant action onintestinal mucosa.

Surface and Interfacial Phenomenon 115

2. Cationic surfactants: It contain organic tail with positive charge head and small negativemolecule like chloride. Example include Quaternary ammonium compound such ascetrimide, benzalkonium chloride and benzethonium chloride.

3. Non ionic surfactants: They does not ionize in water because their hydrophilic partconsist of non dissociable molecules. They are pharmaceutic ally important. They arecompatible with both anionic and cationic surfactants. They are resistant to pH change.Example include glycerol and glycol esters (glyceryl monosteartaO, macrogol esters suchas poloxyl stearates, spans and tweens.

4. Ampbolytic surfactants: They depends on the pH of the system. Below a certain pHthey are cationic while above a certain pH they are considered anionic and at intermediatepH they act as zwitterions. Example: lecithin, N-dodecyl alanine.

The surfactants are used in cosmetic products, detergents, floor cleaners, toothpaste,shampoos etc.

6.8 HLB SCALEHLB is Hydrophilic -Lipophilic Balance. HLB system consist of arbiratory scale in whichvalues are assigned to different surfactants according to their nature. This system wasintroduced by Griffin in 1949. The value of 1 to 20 on HLB scale represent lipophillic andhydrophilic part. Higher number indicates that agent is hydrophilic while low HLB valueindicates agent is lipophillic.

t 18 } Solubilizing agents

~:c 15

)} DetergentsQ..

e......12>.:c orw emulsifying agents

+ 9 } Wetting and

) spreading agents~ 6 W/O emulsifying:cQ.. agentseQ.. 3~ } Most antifoaming

+ 0agents

HLB scale

Figure 6.6: HLB scale

HLB value of 1 indicates- surfactants is soluble in oil

HLB value of 20 indicates- surfactants is soluble in water

HLB value (1.8 to 8.6) indicates- surfactants are lipophillic (example Spans)

(eq 6.28)

W Physical Pharmaceutics·'116

HLB value (9.6 to 16.7) indicates- surfactants are hydrophillic (example tweens)

The HLB value of surfactants contain glyceryl monostearate is calculated by

HLB = 20(1- ~)The HLB value of bees wax and lanolin derivatives are calculated by

HLB=E+P5

(eq 6.27)

Where

E is the percentage by weight of ethylene oxide.

P is the percentage by weight of polyhydric alcohol group

In another method the components groups of surfactant are assigned group numbers. Byadding these numbers HLB value is calculated.

HLB = (hydrophilic group number) - (lipophilic group number) + 7 (eq 6.29)

Table 6.2 Hydrophilic-Lipophilic Balance (HLB) Values of Some Ampbiphilic Agents

Substance HLB

Oleic acid 1

Sorbitan tristearate 2.1

Glyceryl monostearate 3.8

Sorbitan monooleate (Span 80) 4.3

Diethylene glycol monolaurate 6.1

Gelatin (Pharmagel B) 9.8

Methyl cellulose (Methocel 15 cps) 10.5

Required HLB

It is also known as critical HLB. It is hydrophilic lipophilic value that is used to prepare o/wand w/o type emulsion.

urface and Interfacial Phenomenon 117

d problem

ercise 6.3 Calculate HLB value of glyceryl monostearate whose saponification number is5 and acid number is 190.

HLB = 20 (I-SIA)HLB = 20 ( 1- 45/190)

HLB = 15.28

Ans.15.28

6.9 SOLUBILIZATION.The process of increasing the solubility of organic compounds in aqueous system due to presenceof surface active agents is called solubilization. Non polar molecules are dissolved in non polarcore of micelle while polar molecules get adsorbed at micellar surface. Amphiphilic groups arearranged in such a way so that their polar groups is toward aqueous phase while lipophillic groupis inside the micelles.

6.10 DETERGENCYIt is the phenomenon in which dirt or foreign material is removed from the surfaces with the helpof surfactants. They have good wetting properties. They act by reducing adhesion forces betweendirt and the surface and dirt get removed. After this surfactant get adsorbed to the particle surfaceand develop charge there which prevent the deposition of dirt on the solid surface.

6.11 ADSORPTION AT SOLID INTERFACEAdsorption is a surface phenomenon. The accumulation of gas or liquid to the surface of solid iscalled adsorption. Adsorption of material at solid interfaces can take place from either an adjacentliquid or gas phase. The material used to adsorb gas is called adsorbent. Adsorbate is thesubstance being adsorbed.

The principles of solid-liquid adsorption are used in decolorizing solutions, adsorptionchromatography, detergency, and wetting.

6.11.1 The Solid-Gas Interface

The degree of adsorption of a gas by a solid depends on the chemical nature of the adsorbent andthe adsorbate, the surface area of the adsorbent, the temperature, and the partial pressure of theadsorbed gas.

The adsorption are generally recognized as

(a) Physical.adsCfPtion, associated with van der Waals forces. It is reversible.

118 ;pJ Physical Pharmaceuti

(b) Chemisorption, in which the adsorbate is attached to the adsorbent by primary chemibonds. It is irreversible unless the bonds are broken.

The removal of the adsorbate from the adsorbent is known as desorption.

The relationship between the amount of gas physically adsorbed on a solid and the equilibriumpressure or concentration at constant temperature yields an adsorption isotherm. Many differenttype of isotherms are observed

Amount adsorbed

v

I

Relative pressure

Figure 6.7: Type of isotherm

Type I is characteristic of microporous solids with a relatively small proportion of the outersurface.

Type 11refers to polymolecular adsorption in nonporous or macroporous adsorbents.

Type III is characteristic of non-porous sorbents with low energy of adsorbent-adsorbateinteraction.

Types IV and V are similar to types 11and Ill, but refer to porous adsorbents.

Type VI isotherms are characteristic of non-porous adsorbents with homogeneous surface.

";~

Surface and Interfacial Phenomenon 119

There are different equations which describe the relation between amount of adsorption andpressure of gas.

Freundlich Adsorption Isotherm

Freundlich Isotherm is given by

Xy=_=kpl/nm

(eq 6.30)

where

y is the mass of gas adsorbed per unit weight of adsorbent,

x is mass of gas adsorbed.

m is mass of adsorbent, and

k and n are constants

P is equilibrium gas pressure

On applying log

x 1log-=logk+-Iog p

m n(eq 6.31)

The plot of log y or log (x/m) against log p give straight line. The constant, log k, is the intercepton the ordinate, and lIn is the slope of the line.

Log (xlm)

-- Log Pressure ---+

Figure 6.8: Adsorption isotherm

Langmuir Adsorption Isotherm

Langmuir developed an equation in which molecules or atoms of gas are adsorbed on active sitesof the solid to form a layer one molecule thick (monolayer). At pressure p, the fraction occupiedby gas molecules is denoted bye, and the fraction of sites not occupied is 1 - e.

120 fV Physical Pharmaceutics-l

The rate, rh of adsorption or condensation of gas molecules on the surface is proportional to theunoccupied spots, 1 - e, and to the pressure, p,

(eq 6.32)

The rate, r2, of evaporation of molecules bound on the surface is proportional to the fraction ofsurface

occupied, e,(eq 6.33)

At equilibrium r1 = r2,

(eq 6.34)

By rearrangement, we obtain

8= kiP = (kl/k2)Pk, +klP 1+(kl/k2)p

(eq 6.35)

On replacing kl/k2 by b and e by ylym, where y is the mass of gas adsorbed per gram ofadsorbent at pressure p and at constant temperature and ym is the mass of gas that 1 g of theadsorbent can adsorb when the monolayer is complete. Inserting these terms into equation

Ymbpy=--I+bp

(eq 6.36)

This is the Langmuir adsorption isotherm.

By inverting equation and multiplying through by p, the equation will be

pIp-=--+-Y bYm s;

(eq 6.37)

A plot of ply against p should yield a straight line, and ym and b can be obtained from the slopeand intercept.

Interfacial Phenomenon 121

PressureFigure 6.9: Langmuir adsorption Isotherm

~~rm:ir and Freundlich equation describe only Type I adsorption isotherm.

- therms are best described by an expression derived by Brunauer, Emmett, and Tellerion).

(eq 6.38)

ressure of the adsorbate

of vapor per gram of adsorbent IS adsorbed,apor pressure when the adsorbent is saturated with adsorbate vapor,

quantity of vapor adsorbed per unit mass of adsorbent when the surface is covered with a=ccomolecular layer, and

nstant proportional to the difference between the heat of adsorption of the gas in the firstthe latent heat of condensation of successive layers.

lid-Liquid Interface

uch as dyes, alkaloids, fatty acids, and even inorganic acids and bases can be adsorbedlution onto solids such as charcoal and alumina. For adsorption of solute molecules from

c::e solution, the pressure in solidi gas system is replaced by concentration

c 1 c-=--+-y bYm Ym

(eq 6.39)

122 ~ Physical Pharmaceutics-l

Where

c is the equilibrium concentration

y is the ratio of mass of the solute per gram of adsorbent at equilibrium.

Ym is adsorptive capacity of solid

b is adsorption coefficient

REVIEW QUESTIONS

VERY SHORT ANSWER QUESTIONS1. What are surfactants?

Answer- These are the agents which are used to lower surface tension of liquids and also reduces interfacialtension between two liquids.

2. Describe HLB system.

Answer- HLB is Hydrophilic -Lipophilic Balance. HLB system consist of arbiratory scale in which values areassigned to different surfactants according to their nature.

3. Define Surface tension

Answer- It is defmed as tensile force acting at the surface of liquid which is in contact with gas or it may bebetween two immiscible liquids.

4. Define Interfacial tension

Answer- It is the force per unit length that exist at the interface between two immiscible liquid phases.

5. What do you mean by Positive adsorption?

Answer- When some molecules and ions, when dispersed in the liquid, are partitioned in favor of the interface.

6. What do you mean by Negative adsorption?

Answer- The materials (such as inorganic electrolytes) when dispersed in the liquid are partitioned in favor ofthe bulk.

SHORT ANSWER QUESTIONS

1. Why Interfacial tensions are less than surface tensions?

Answer- Interfacial tensions are less than surface tensions because the adhesive forces between two liquidphases which form an interface are greater than when a liquid and a gas phase exist together.

2. Differentiate Cohesive forces and Adhesive forces.

Answer- The forces acting between the molecules of same phase are called Cohesive forces and Adhesive forceare the forces acting between the molecules of different phase.

Surface and Interfacial Phenomenon 123

3. Definesurface free energy.Answer- The molecule near the surface of liquid have more potential energy as compared to the molecules in thebulk of the liquid. This means that as surface of the liquid increases, the more molecules have this excessivepotential energy. This energy is proportional to the size of the free surface, it is called a surface free energy.

4. Write classification of surfactaots.Answer- Refer article 6.7

s. Explain solid liquid interface with equationAnswer- Refer article 6.11.2

LONG ANSWER QUESTIONSI. Describe in detail Capillary rise method to determine surface tension of liquid. (refer article 6.4.1)2. Describe in detail Drop weight and Drop count methods to determine surface tension of liquid. (refer article

6.4.2)

3. Describe in detail Du Nouy tensiometer to determine surface tension of liquid. (refer article 6.4.4)

4. Write detailed note on

a. Spreading Coefficient (refer article 6.5)

b. Different methods to determine HLB value (refer article 6.8)

5. Write detailed note on (refer article 6.11.l)

a. Freundlich Adsorption Isotherm

b. Langmuir Adsorption Isotherm

c. BET equation

MULTIPLE CHOICE QUESTIONS1. Apparatus used to determine surface tension of liquid is

a. Capillary tube vi cometer

b. Du Nouy tensiometer

c. Rotometer

d. Rheometer

2. HLB Scale was introduced by

a. Griffin

b. Brunauer

c. Emmett

d. Teller