Surface and Interfacial Phenomenon
Contents of
This Chapter
•General principles of surface and interfacial tension
•Applications of surface and interfacial tension
•Different factors affecting surface and interfacial tension
•Applications of surface and interfacial tension
•Different methods of determination of surface and interfacial
tension
•Energetics of liquid surfaces – surface free energy and spreading
coefficient
•Surface active agents and its effect in interfacial tension
• Differences between physical and chemical adsorption
•Adsorption at solid interfaces
•Thermodynamic stability at solid interfaces
•Freundlich adsorption isotherm
•Adsorption at solid gas interface continued……..
•Adsorption isotherms
•Concept of HLB scale and method of estimation of HLB
•Applications of HLB
•Concept of Required HLB
•Soluble monomolecular films
•Adsorption at solid- liquid interface
•Electrical properties of interfaces
•Electrical double layer
Learning
Objectives
• At the end of this lecture, student will be able to
– Explain the terms surface and interfacial tension
– 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
Summery
• 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
