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 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)  

• 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).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  

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