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Physical Pharmaceutics-I Unit-III Surface and Interfacial tension (Part-1)(Liquid interface, surface and interfacial tension )

This document discusses liquid interfaces and surface and interfacial tension. It defines a liquid interface as the boundary between phases in contact, with surface referring specifically to the boundary between a liquid and gas. Surface tension is the force per unit length acting at right angles to the liquid surface and arises from cohesive intermolecular forces being imbalanced at the surface. Molecules in the bulk liquid experience equal attractive forces from all sides, while surface molecules only experience inward attraction. This imbalance causes the surface to contract and results in surface tension. Interfacial tension similarly describes the imbalance of forces at the boundary between immiscible liquids. Some examples of liquid surface tensions are provided.

Education◦
IIIrd Semesester B. pharmacy Physical Pharmaceutics-I Unit-III Surface and Interfacial tension (Part-1) (Liquid interface, surface and interfacial tension ) Miss. Pooja D. Bhandare (Assistant professor) Kandhar college of pharmacy
 LIQUID INTERFACE • When phases are in contact with each other, the boundary between them is know as interface. • If one of two phases is a gas or vapour, the term ‘surface’ is generally used instead of interface. • No interface can be observed between two gas phases because they are mutually miscible in all proportions. • The pharmacist encounter several types of interface, while handling dosage form.
• Solid- solid interface are important in pharmacy in the area of adhesion of granules to form a tablet, flow of granules through hopper during tableting. • In the liquid state, the cohesive forces between adjacent molecule are well developed. • Molecules in the bulk liquid are surrounded in all directions by other molecule for which they have an equal attraction. • But molecules at the surface (i.e., at the liquid-air interface) can only develop attractive cohesive forces with other liquid molecules that are situated below and adjacent to them.
• Table 1: Classification of Interfaces Phases Types of Interface Example of interface Gas - Gas No interface possible Gas - Liquid Liquid surface Body of water exposed to atmosphere. Gas - Solid Solid surface Table top Liquid - Liquid Liquid-Liquid interface Emulsion Liquid - Solid Liquid – Solid interface Suspension Solid - Solid Solid - Solid Powder particle
 SURFACE AND INTERFACIAL TENSION • Surface tension is defined as the force, in newtons, acting on the surface of the liquid at right angles to any line of the length surface, 1 metre. • Unit surface tension and interfacial tension:  CGS system : dyne/cm SI system : N/m • Surface tension of liquid can be explained by considering the intermolecular forces between the molecules in the neighbourhood
• In liquid, molecules experience greater attraction from the neighbouring molecules. • Such intermolecular attraction between like molecules are called cohesive forces while attraction between unlike molecules are called adhesive forces.
The phenomenon of surface tension is responsible for the following process:  Formation of spherical globules in emulsion.  Formation of nearly spherical shape of falling water droplet.  Formation of spherical shape of mercury particles on flat surface.  Rise of liquid in capillary tube
Molecule at the surface: • Cohesive (attraction) forces are not properly balanced. • Experienced an inward forces of attraction towards the bulk of liquid. • Surface of liquid gets contracted and molecules at the interface are pulled together. • Force exactly counterbalance the inward pull is surface tension. Molecules in bulk: • Surrounded by other molecules which have equal attraction in all directions. • Net attraction is zero
Interfacial tension is between two immiscible liquids arises as a result of imbalance forces. It is the per unit length existing at the interface between two immiscible liquid phases. Although, in the general sense, all tension may be referred to as interfacial tension, this term is most often used for the attractive forces between immiscible liquids. Generally interfacial tension are less than surface tension because the adhesive forces between two liquid phases forming an interface are greater than when a liquid and a gas phase exist together
• Surface Tension of Some Liquids Liquids Surface tension.mN/m (=dy/cm) Water 71.60 Glycerine 66.00 Ethanol 21.62 Paraffin 30.0 Mercury 480
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