Surface and interfacial phenomenon

SURFACE AND INTERFACIAL PHENOMENON

Seletted Dellnltlons Interface: 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 be between 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: interface.

When some molecules and ions, when dispersed in the liquid, are partitioned in favor of the

Negative adsorption: favor of the bulk.

The materials (such as inorganic electrolytes) when dispersed in the liquid are partitioned in

Surfactants: These are the agents which are used to lower surface tension of liquids and also reduces interfacial tension 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 are assigned to different surfactants according to their nature.

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

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

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

•

W

106

Physical Pharmaceutics-I

6_1 INTRODUCTION The boundary between two phases which exist together is known as an interface. It may be liquid-liquid interface or solid-solid interface or liquid-gas interface. In case of gas-gas, no interface is possible.

6.2 SURFACE & INTERFACIAL TENSIONS Surface tension is defined as tensile force acting at the surface of liquid which is in contact with gas 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 the molecules of same phase are called cohesive forces. The forces acting between the molecules of different phase are called adhesive force.

The molecules in the bulk liquid are surrounded in all directions by other molecules. The net force acting on this molecule is zero. While the molecules at the surface of the liquid experience an inward force toward the bulk. This force pulls the molecules of the interface together and stretch the surface. It means some tensile force is acting on the surface which is called surface tension.

Net attraction Water Figure 6.1 : Representaion oC surface tension

Interfacial

tension: It is the force per unit length that exist at the interface

immiscible liquid phases. If two liquids are completely between them.

miscible,

no interfacial

between

two

tension exists

Interfacial tensions are less than surface tensions because the adhesive forces between two liquid phases 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). The surface tension, -y, of the solution forming the film is a function of the force that must be applied to break the film over the length of the movable bar in contact with the film. Thus f y=_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 ENERGY The molecule near the surface of liquid have more potential energy as compared to the molecules in the bulk of the liquid. This means that as surface of the liquid increases, the more molecules have this excessive potential energy. This energy is proportional to the size of the free surface, it is called a surface free energy.

For example, a liquid droplets assume a spherical shape because a sphere has the smallest surface area per unit volume. To increase the surface of the liquid, work must be done against the surface tension.

When the bar is at a position and mass is added to extend the surface by a distance ds, the work dW (force multiplied by distance) is

dW=jxds As we know from (equation

(eq 6.2)

6.1) 'Y= ft/2L

dW

So

= 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

W =yflA

(eq 6.5)

where W is the work done, or surface free energy increase, expressed in ergs, 'Yis the surface tension in dynes/cm, and flA is the increase in area in c.m2.

Surface tension can be defined as the surface free energy change per unit area.

Solved problem Exercise 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

Ans : 1262 ergs

= 35.07

x2x6x3

= 1262

ergs

;Pv

108

Physical Pharmaceutics-I

6.4 MEASUREMENT OF SURFACE & INTERFACIAL TENSIONS There 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 Method It measures surface tension only.

Principle: When a capillary is placed in container having liquid, the liquid rises up in the capillary upto a certain height. The rising of liquid is due to more of adhesive force between liquid and capillary wall. The rise of liquid in tube will continue until upward movement is balanced by downward 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 capillary tube 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 2lli 'Y = lli2 hpg 'Y = ljH hpg

Or

(eq 6.9)

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

Solved problem Exercise 6.2: A given liquid rises 1.19 cm in capillary tube, the radius of which is 0.038cm and density of given liquid at 30°C is 0.778 g/crrr'. Calculate surface tension of the liquid Solution 'Y = ljH hpg 'Y=

lj2 x

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 tube with one marking A above the bulb and other marking B below the bulb. At the tip capillary is attached.

fV

110

Physical Pharmaceutics-I

A Bulb

B

Capillary

Figure 6.3: Stalagmometer The liquid whose surface liquid upto mark A and drops (approximately 20 liquid can be measured. following equation.

tension is to be measured is put into beaker. The capillary sucks the then allowed the liquid to fall slowly through capillary tube. The to 30) is collected into clean vessel. By this weight of one drop of The surface tension of the liquid is calculated by putting values in

w=2JIry

or

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

y = w 12JIr y = mg 12JIr

or

(eq6.12) (eq 6.13)

where m is the mass of one drop g is gravitational

force

n is number of drops

y

= vdg 12JIr

(because m= volume(v) x densityrdj)

(eq 6.14)

Surface and Interfacial Phenomenon

111

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

Beam

t------+

V

To torsion balance

Rectangular plate

Figure 6.4: Wilhelmy Plate Method If liquid completely wets the plate then the detachment multiplied by perimeter of surface detached.

WL

-

W = 2 (L+T) y

force is equal

to surface

tension

(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)

W

112

Physical Pharmaceutics-I

6.4.4 Ring Detachment method or Du Nouy tensiometer It 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 or Iridium ring (about 4 cm) is suspended in the liquid. The ring is attached to a scale through torsion wire. The ring should just touch the surface of liquid. The force required to detach the ring is noted from the scale. This detachment force is given by p= w

=

21t

(rl + Xi) Y

(eq 6.17)

p

Y = --,-------:21t

(eq 6.18)

(r, + r2)

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 involved in the equation

factor is not applied.

So Correction

factor is .

(eq 6.19)

Surface and Interfacial Phenomenon

reading in dynes r = 2Dial x ring circumference

113

. fi x correction actor

(eq 6.20)

or P 'Y=-x~

4m

(eq 6.21)

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

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

The energy involved in separating the molecules of liquid is known as work of cohesion which is expressed as (eq 6.22) The energy involved to separate column of two immiscible liquid at the interface into two sections 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 YL

(eq 6.24)

S = Ys - YL - YLS

(eq 6.25)

S = Ys - (YL + YLS)

(eq 6.26)

(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

W

114

Physical Pharmaceutics-I

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 INTERFACES When some molecules and ions, when dispersed in the liquid, are partitioned in favor of the interface. This phenomenon is called is positive adsorption. The surface free energy and the surface tension get decreased in the case of positive adsorption.

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

6.7 SURFACTANTS These are the agents which are used to lower surface tension of liquids and also reduces interfacial tension between two liquids. Surfactants contain hydrophilic (water loving) and hydrophobic (water hating) group. When surfactants are placed in the water, the hydrophilic group dissolves in water while hydrophobic group form micelles. The surfactants which have both 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 organic tail with negative charge head and small positive molecule like ammonium. This consist of 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 on intestinal mucosa.

Surface and Interfacial Phenomenon

115

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

3. Non ionic surfactants: They does not ionize in water because their hydrophilic part consist of non dissociable molecules. They are pharmaceutic ally important. They are compatible with both anionic and cationic surfactants. They are resistant to pH change.

Example include glycerol and glycol esters (glyceryl monosteartaO, macrogol esters such as poloxyl stearates, spans and tweens.

4. Ampbolytic surfactants: They depends on the pH of the system. Below a certain pH they are cationic while above a certain pH they are considered anionic and at intermediate pH 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 SCALE HLB is Hydrophilic -Lipophilic Balance. HLB system consist of arbiratory scale in which values are assigned to different surfactants according to their nature. This system was introduced by Griffin in 1949. The value of 1 to 20 on HLB scale represent lipophillic and hydrophilic part. Higher number indicates that agent is hydrophilic while low HLB value indicates agent is lipophillic.

t

18

~ :c

15

...e...>. :c

12

+

9

~ :c

6

Q..

Q..

e Q..

~

3

+

0

}

Solubilizing agents } Detergents

) )

} Wetting and spreading agents

}

Most antifoaming agents

orw

emulsifying agents

W/O emulsifying agents

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)

W

116

Physical Pharmaceutics¡'

HLB value (9.6 to 16.7) indicates- surfactants are hydrophillic (example tweens) The HLB value of surfactants contain glyceryl mono stearate is calculated by HLB

=

(eq 6.27)

20(1- ~)

The HLB value of bees wax and lanolin derivatives are calculated by (eq 6.28)

HLB=E+P

5

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. By adding these numbers HLB value is calculated.

HLB =

(hydrophilic group number) -

Table 6.2 Hydrophilic-Lipophilic

(lipophilic group number) + 7

(eq 6.29)

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/w and w/o type emulsion.

urface and Interfacial Phenomenon

117

d problem ercise 6.3 Calculate HLB value of glyceryl monostearate whose saponification number is 5 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 presence of surface active agents is called solubilization. Non polar molecules are dissolved in non polar core of micelle while polar molecules get adsorbed at micellar surface. Amphiphilic groups are arranged in such a way so that their polar groups is toward aqueous phase while lipophillic group is inside the micelles.

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

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

The principles of solid-liquid adsorption are used in decolorizing solutions, adsorption chromatography, 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 and the adsorbate, the surface area of the adsorbent, the temperature, and the partial pressure of the adsorbed gas.

The adsorption are generally recognized as (a) Physical.adsCfPtion, associated with van der Waals forces. It is reversible.

;pJ

118

Physical Pharmaceuti

(b) Chemisorption, in which the adsorbate is attached to the adsorbent by primary chemi bonds. 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 equilibrium pressure or concentration at constant temperature yields an adsorption isotherm. Many different type of isotherms are observed I

Amount adsorbed

v

Relative pressure Figure 6.7: Type of isotherm

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

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

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

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 and pressure of gas.

Freundlich Adsorption Isotherm Freundlich Isotherm is given by X

y=_=kpl/n

(eq 6.30)

m 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 1 log-=logk+-Iog m

n

(eq 6.31)

p

The plot of log y or log (x/m) against log p give straight line. The constant, log k, is the intercept on 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 sites of the solid to form a layer one molecule thick (monolayer). At pressure p, the fraction occupied by gas molecules is denoted bye, and the fraction of sites not occupied is 1 - e.

fV

120

Physical Pharmaceutics-l

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

to the

e,

(eq 6.32) The rate, r2, of evaporation surface occupied,

of molecules

bound on the surface is proportional

to the fraction of

e, (eq 6.33)

At equilibrium

r 1 = r2, (eq 6.34)

By rearrangement,

we obtain

8=

= (kl/k2)P

kiP k, +klP

(eq 6.35)

1+(kl/k2)p

e

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

Interfacial Phenomenon

121

Pressure

Figure 6.9: Langmuir adsorption Isotherm

~~rm:ir

and Freundlich equation describe only Type I adsorption isotherm.

- therms are best described by an expression ion).

derived by Brunauer, Emmett, and Teller

(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 first the latent heat of condensation of successive layers.

lid-Liquid Interface uch as dyes, alkaloids, fatty acids, and even inorganic acids and bases can be adsorbed lution 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

y

bYm

c

-=--+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 QUESTIONS 1.

What are surfactants?

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

2.

Describe HLB system.

Answer- HLB is Hydrophilic -Lipophilic Balance. HLB system consist of arbiratory scale in which values are assigned 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 be between 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-

6.

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

What do you mean by Negative adsorption?

Answer- The materials (such as inorganic electrolytes) when dispersed in the liquid are partitioned in favor of the 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 liquid phases 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 force are 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 the bulk of the liquid. This means that as surface of the liquid increases, the more molecules have this excessive potential 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 equation Answer- Refer article 6.11.2

LONG ANSWER QUESTIONS

in

I. 2.

Describe detail Capillary rise method to determine surface tension of liquid. (refer article 6.4.1) 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 QUESTIONS 1.

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