Surface and Interfacial Phenomenon

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

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