Surface and Interfacial Phenomenon

Surface and Interfacial Phenomenon


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 γ

Or, S= γsL+ 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  


-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) 


  • 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).HLB

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  


-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)  


  • 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  

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