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Surface and Interfacial Phenomenon

Learning Objectives of Surface and Interfacial Phenomenon

At the end of this lecture, student will be able to

– Explain the terms surface and interfacial tension and Surface and Interfacial Phenomenon

– Describe the difference between surface and interface

– Explain the relationship between the cohesive and adhesive forces with surface and interfacial tension

– Explain the application of surface and interfacial tension

– Describe the method of determination of surface tension by capillary rise method

– Describe the method of determination of surface and interfacial tension by tensiometer

– Explain the concept of surface free energy and its importance in pharmaceutical preparations

– Describe work of cohesion, adhesion and spreading coefficient for different types of interfaces

– Explain the classification of surface active agents and their applications in pharmacy

– Describe the adsorption at solid interfaces

– Explain the differences between physical and chemical adsorption

– Describe the Freundlich adsorption isotherm and its Applications

– Describe the mechanism of adsorption on solid/gas and solid/liquid interfaces

– Explain Langmuir adsorption isotherms and its applications

– Describe multimolecular layer adsorption

– Explain different types of adsorption isotherms

– Explain the concept of HLB and its applications in pharmacy

– Describe the concept of required HLB in the preparation of pharmaceutical formulations

– Explain soluble monomolecular film and its applications

– Describe the adsorption at solid-liquid interface

– Explain contact angle and wetting phenomenon

– Explain the electrical properties of interfaces and the effect of electrolytes

Introduction to Surface and Interfacial Phenomenon

Interface is the boundary that forms between two phases like solid and liquid
The term surface is normally used to denote interface when one of the phases is gas
Solid- liquid interfaces are important in pharmacy in the area of adhesion of granules to form a tablet, flow of granules through hopper during tableting

Surface and Interfacial Tension-General Principle

Surface Tension is defined as the force, in dynes, acting on the surface of the liquid at right angles to any line of length of surface 1 centimeter
Units of surface tension are Dyne/cm in CGS system and Newton’s/ meter in MKS system.
Surface tension is responsible for the following processes:

– Spherical globules in emulsions

– Nearly spherical shape of falling water droplets

– Spherical shape of mercury particles on a flat surface

– Rise of liquid in capillary tube

– Lower meniscus of water in glass tubes

In a liquid, molecules experience greater attraction from the neighbouring molecules and such intermolecular force of attraction between like molecules are called cohesive forces of attraction
Surface tension denotes cohesive forces of interaction in a liquid
An example of water in a beaker and the intermolecular forces of attraction can explain about the surface tension

Interfacial tension is defined as the force per unit length existing at the interface between two immiscible liquids
Units are dyne/cm (CGS system) and Newton/meter (MKS system)
Interfacial tensions are less than the surface tensions
Interfacial tension indicates the strength of the adhesive forces between immiscible liquids
Internal factors – intermolecular forces of attraction is a measure of the magnitude of the surface tension
External factors – presence of electrolyte causes a slight increase in the surface tension

– Surface active agent’s decreases surface tension

– Increase in temperature causes decrease in surface tension

Determination of Surface and Interfacial Tension

Different methods to determine surface and interfacial tensions are:

– Capillary rise method

– The Du Nouy tensiometer

– Bubble pressure

– Drop weight (drop count)

Capillary Rise method

When a capillary tube is placed in the liquid contained in a beaker, the liquid rises up the in the tube to a certain distance
The rise of the liquid is because of the adhesive forces between the liquid molecules and glass
Rise of the liquid will continue until the upward movement is just balanced by the downward force of gravity

Upward component

Surface tension of the liquid (γ) at any point on the circumference is given by:

Upward component,

a = γ.Cosθ…………………(1)

Where, γ=surface tension of the liquid

θ=contact angle between the surface of the liquid and capillary wall

The total upward force around the inside circumference (2πr) of the tube is:

Upward component,

a= γ. 2 πr Cosθ…………….(2)

Where r is the inside diameter of the capillary tube

For water θ is zero, so, cos θ = 1 and equation (1) changes to upward component,

a= 2 πr. γ………….(3) Downward component

Counteracting force is gravity and depends upon the weight of liquid in the capillary rise Downward component,

b=mass x acceleration

=volume x density x acceleration

=cross sectional area x height x density x acceleration

= πr2 x h x ρ x g……………(4)

At equilibrium the opposing forces are equal i.e., a=b

2 πr. γ = πr2 x h x ρ x g…………..(5)

γ = 1/2rhρg…………….(6)

Equation (6) is used to measure the surface tension by capillary rise method

The DuNouy Ring Method

The method is widely used to measure surface and interfacial tension
The force required to detach the platinum iridium ring immersed at the interface or surface is measured

Upward pull

Upward pull= dial reading in dynes………….(7)

Downward pull

The weight of the liquid that is adhered to the ring, acts as the downward force

Downward pull= mg= γ .2 πr.2………….(8)

At equilibrium:

Upward pull=downward pull

dial reading= γ .2πr.2

An error of 25% is possible, so a correction factor is applied

Surface Free Energy

Surface tension maintains the surface area of a liquid to a minimum value
Surface free energy is defined as the work required to increase the area of a liquid by 1 cm2
Surface free energy is equal to the surface tension

Derivation of surface free energy

A rectangular wire ABCD, the side of AD=L which is movable
A drop of soap solution is placed to form a film within the frame
The side AD remains stable initially on account of surface tension
When force is applied (a hanging mass) downward, the film gets stretched as the movable bar AD goes down until the film breaks
If the applied force is less than that is required for breaking, the film would retract on account of surface tension
If force ‘f’ applied on AD (downward component), it shifts the movable wire to a distance ‘d’ to A’D’
The work done ‘W’ is given by

W=f x d…………….(1)

The above force acts against the surface tension (upward component), γ, of the liquid
The force acting on the surface is:

f= γ x 2L……..(2)

Substituting eq. (2) in equation (1) gives

W= γ x 2L x d…………(3)

Since, 2L x d = ΔA, produced by extending the soap film, eq. (3) changes t

W= γ x ΔA…………(4)

or, ΔG= γ x ΔA

where, W= work done or surface free energy increase (ΔG), expressed in ergs (mJm-2)

Spreading Coefficient

Spreading coefficient can be analysed by considering the cohesive and adhesive forces operating between two molecules

Work of cohesion

It is the energy required to separate the molecules of the spreading liquid
When a hypothetical cylinder is divided, two new surfaces are created

Work of cohesion= Wc=2γL………….(5)

Where, γL = surface tension of the liquid (L)

Work of adhesion

It is the energy required to bring out the adhesion between the unlike molecules
The work done is equal to:

Work of adhesion= Wa= γL + γs + γLs………(6)

where, γs= interfacial tension of sublayer

γLs=interfacial tension of liquid/solid surface

Spreading of liquid occurs when adhesive forces (Wa) are stronger than the cohesive forces (Wc)
The spreading coefficient is obtained by the equation:

S= Wa-Wc =(γL + γs – γLs)-2 γL

Or, S= γs(γL+ γLs)

If, γs> (γL+ γLs), S is positive, indicating spreading
If γs< (γL+ γLs), S is negative, indicating no spreading
Spreading coefficient of substance can be increased by:

– The prescence of polar functional groups such as –COOH, -OH etc., in the structure

– Reducing the nonpolar chain length

Surface Active Agents

Surface active agents are the substances which preferentially get adsorbed at the interface and exhibit self-association in the bulk of the liquid at a specific concentration
These are polymer like substances which have both polar and non-polar groups so that they remain at the interface and reduce the interfacial tension
They are also termed as amphiphiles
Depending on the number and the nature of the groups they may be classified as:

-Predominantly lipophilic

-predominantly hydrophilic

-Well balanced

Surface Active Agents-Applications

Pharmaceutical adjuvants like

-solubilizing agents

-wetting agents

-detergents

-suspending agents

-emulsifying agents

-foaming agents

Influence on drug action
Antibacterial activity

Adsorption at Solid Interfaces

Adsorption of a gas or a liquid onto a solid surface is important in pharmacy
Material used to adsorb gases or liquids is termed as adsorbent
The substance that is attached to the surface of the solid is called adsorbate
Depending on the nature of interactions, adsorption is classified into physical adsorption (physisorption) and chemical adsorption (chemisorption)

Physical adsorption	Chemical adsorption
Reversible	Irreversible
Weak van der Waals forces	Strong chemical bonds
Nonspecific	More specific
Common at low temperature	Occurs at high temperature
Heat of adsorption is low	Heat of adsorption is high
Example –adsorption of gases on charcoal	Example-adsorption of oxygen on silver or gold

The combination of both types of adsorption is termed as ‘Sorption’
Phenomenon opposite to adsorption is desorption
In thermodynamic terms adsorption is a surface phenomenon
Greater the surface area greater is the adsorption
The relationship between the surface free energy and surface tension is given by:

W = ΔG =γΔA……………(4)

Where, W= work done to obtain division of particles

ΔA =increase in the surface area

ΔG= increase in the surface free energy

Adsorption at Solid/Gas Interface

Adsorption of a gas onto a solid surface is important in pharmacy due to :

-removal of objectionable odours from the rooms

-prevention of obnoxious gases entering into the body

-estimation of surface area and particle size of powders

In the study of adsorption, the amount of gas adsorbed per unit area or unit mass of solid is measured at different pressures of the gas
Study is usally conducted at constant temperature and graphs are plotted, which are known as adsorption isotherms

Adsorption at Solid/Gas Interface-Freundlich Isotherm

Freundlich isotherm gives the relationship between pressure of the gas and amount adsorbed at constant temperature
The equation is:

where, x= weight of gas adsorbed per unit weight of adsorbent,

P= equilibrium pressure,

K and n = constants

Converting equation (5) into logarithmic form

Adsorption at Solid/Gas Interface-Langmuir Adsorption Isotherm

Langmuir adsorption isotherm can be represented by:

Where, y= mass of gas adsorbed per gram of adsorbent

ym= mass of gas that 1g of adsorbent can take up when a monolayer is complete

b= k1/k2 (constant)

p= pressure

Inverting equation (1) and multiplying by ‘p’ gives:

Adsorption at Solid/Gas Interface- Multi-molecular Adsorption (BET equation)

• Sometimes gases adsorb as multi-molecular layers on solids

Where, P= pressure of the adsorbate, in mm Hg

y= mass of the vapour per gram

P0= vapour pressure at saturation

ym= amount of vapour adsorbed per unit mass of adsorbent (surface is covered with monomolecular layer) b= constant, proportional to heat of adsorption and latent heat of condensation of subsequent layer

Adsorption at Solid/Gas Interface- Adsorption Isotherms

Adsorption isotherms are the plots between the amount of gas adsorbed on a solid against the equilibrium pressure or concentration at constant temperature

Type I

This isotherm represents an increase in the adsorption with increasing pressure followed by levelling off • Levelling off is due to saturation of entire surface by formation of monomolecular layer
It represent Freundlich or Langmuir adsorption isotherm

Type II

Occurs when gases undergo physical adsorption onto nonporous solids
First inflection point represents, formation of monolayer, when pressure in increased multilayer formation occurs
Isotherm is described by BET equation

Type III

The heat of adsorption of gas in the first layer is less than the latent heat of condensation of subsequent layers
In BET equation the constant ‘b’ is smaller than 2

Type IV

Plot represents the adsorption of gases on porous solids
First point of inflection extrapolated to zero represents the monomolecular layer adsorption
Condensation within the pores of the solid and the multi-molecular layer is represented by further adsorption

Type V

Seen rarely and indicates capillary condensation
The adsorption reaches a limiting value before P0 is attained

Hydrophilic-Lipophilic Balance (HLB)

HLB is an arbitrary scale that indicates the extent of hydrophilic lipophilic balance (HLB)

Hydrophilic-Lipophilic-Balance

The higher the HLB of an agent, the more the Hydrophilicity
Spans (sorbitan ester) are lipophilic and have low HLB values (1.8-8.6)
Tweens (polyoxyetylene derivative of span) are hydrophilic and have high HLB values (9.6-16.7)
HLB scale is used to identify the optimum efficiency of a variety of surfactants
Method 1

HLB= Σ(hydrophilic group number)-Σ(lipophilic group number) +7

Method 2

Where, E= percent by weight of ethylene oxide chain

P=percent by weight of polyhydric alcohol groups

Method 3

where, S= saponification number of the ester

A=acid number of the fatty acid

Required HLB (RHLB)

Required HLB (critical HLB) is the hydrophilic-lipophilic value that is desired in order to prepare a stable emulsion of o/w or w/o type
A blend of surface active agents are used in the preparation of emulsions and the blend is estimated based on the nature of the oil phase
HLB of a mixture of two surfactants containing the fraction f, of A and (1-f) of B is an algebraic mean of the two HLB values

HLBmixture = f.HLBA + (1-f).HLBB

Soluble Monomolecular Films

When a small drop of polar-short chain alcohol is added to water with an increasing concentration it completely covers the surface with a monomolecular film

Applications

-Stabilization of emulsions

– Wetting and detergency

– Membrane models

The following parameters are evaluated:

-Surface tension

-Surface excess

-Concentration of amphiphiles in the bulk

The number of molecules per unit area of the surface can be estimated using Gibbs equation:

Where, ᴦ= moles of solute adsorbed/unit area or surface excess

R= ideal gas constant

T=absolute temperature

γ =change in the surface tension

da2=change in the solute activity at a

Surface excess is the amount of the amphiphiles per unit area of surface in excess of that in the bulk liquid
For dilute solutions activity term can be replaced by solute concentration, c:

Since the term integral dc/c is equal to d (ln c), the Gibbs equation can be written as

Equation (2) and (3) is applicable to the absorption of surfactants
From equation (3), surface tension is plotted ag

Adsorption at Solid/Liquid interface

Solute present in a solution may often adsorb on the solid- liquid interface
Adsorption phenomenon find applications in many ways:

-Reduced absorption

-Antidote in poisoning

-Purification and reduced toxicity

-reduced drug content

-Separation of substances in a mixture

Adsorption at Solid/Liquid interface-Wetting Phenomenon

Wetting is an adsorption process in which an intimate contact of the solids with liquid phase is achieved • The importance of wetting phenomenon are:

– In the preparation of suspensions and emulsions

-Mixing of powders with binding agents in granulation process

-Film coating of tablets

-Dissolution of tablets or capsules

Surfactants are used to aid in the wetting of powders
Contact angle can be defined as an angle between the liquid droplets and surface over which it spreads
Contact angle can take any value between 0 digree to 180 digree

Contact angle can be estimated by placing a drop of liquid on a solid surface
The forces acting at equilibrium:

γS= γLS + γL. Cos θ……………(4)

Whereθ is the contact angle

Equation (4) can be written as:

− γSL + γS

Cos θ = ———- ………….(5)

γL

Ideal wetting is Cos θ=1 or θ=0

Adsorption at Solid/Liquid Interface- Critical Surface Tension

The surface tension obtained to Cos θ=1 is known as critical surface tension

Electrical Properties of Interfaces

The electrical properties of interfaces finds applications in:

-Stabilization of colloidal dispersions

-Preparation of flocculated suspensions

-Stabilization of emulsions

The origin of charge on interface can be accounted as:

– Electrolytes present on the surface may get adsorbed on the solid surface

– Functional groups present on the surface of the particles dissociated and impart charge

– Differences between the dielectric constants between the particles and dispersion medium

Electrical Properties of Interfaces-Electrical Double Layer

The electrical double layer can be illustrated by:

The electrical double layer is consisting of

– Tightly bound layer

– Diffuse second layer

The cations at the interface are potential determining ions
The anions are termed as counter ions or gegenions
Nernst potential, E (Electrothermodynamic potential) is defined as the difference in potential between the actual surface and the electroneutral region of the solution
Zeta potential,ζ (electrokinetic potential) is defined as the difference in potential between the surface of the tightly bound layer and the electroneutral region of the solution
Zeta potential is work required to bring a unit charge from infinity to the surface of the particles
Zeta potential governs the degree of repulsions between the adjacent ions of like charges
It is used to predict particle –particle interaction and an optimum zeta potential is desirable for the maximum stability

Surface and Interfacial Phenomenon Summary

Surface tension – Force acting on the surface of the liquid at right angle
Surface tension is the measurement of cohesive forces of attraction
Interfacial tension is the measurement of adhesive forces of attraction
Interfacial tension – Force acting at the interface of two liquids
Applications of surface and interfacial tension – To know the properties of various liquids used the the pharmaceutical preparations
Surface tension – Force acting on the surface of the liquid at right angle
Interfacial tension – Force acting at the interface of two liquids
Methods of determination – By capillary rise method and DuNouy tensiomete
Surface free energy – The free energy associated with the surface of a compound
Importance of surface free energy – Deals with the stability of different pharmaceutical formulations • Spreading coefficient – Differences between work of cohesion and work of adhesion
A higher spreading coefficient signifies a lesser surface free energy
Surfactants – Substances with both hydrophilic and lipophilic property
Applications of surfactants – They are used as different adjuvants in pharmaceutical preparation • Material used to adsorb gases or liquids is termed as adsorbent
The substance that is attached to the surface of the solid is called adsorbate
The relationship between the surface free energy and surface tension is given by:

W = ΔG =γΔA

Freundlich adsorption isotherm – Relationship between the pressure of the gas and amount adsorbed at constant temperature
Langmuir adsorption isotherm – this isotherm explains about monomolecular layer adsorption
Bet equation – Explains about multimolecular layer adsorption
Adsorption isotherm – These are the plots of amount of gas or liquid adsorbed onto an unit mass of solid at an equilibrium pressure
HLB scale – An arbitrary scaled notes Hydrophilicity and lipophilicity of surfactants and different pharmaceutical substances
Application of HLB – Used to identify the optimum efficiency of a variety of surfactant
Required HLB- It is the hydrophilic – lipophilic value that is desired to prepare a stable emulsion
When a small drop of polar – short chain alcohol is added to water with an increasing concentration it completely covers the surface with a monomolecular film
Wetting – An adsorption process in which an intimate contact of the solids with liquid phase is achieved
Contact angle – It is defined as an angle between the liquid droplet and surface over which it spreads
Nernst potential – Defined as the difference in potential between the actual surface and the electroneutral region of the solution
Zeta potential – Defined as the difference in potential between the surface of the tightly bound layer and the electroneutral region of the solution