The surface and interfacial phenomena play an important role in the formulation of pharmaceutical products. They affect the adsorption process taking place during the preparation of many solid dosage forms, absorption of drug molecules through biologic membranes, formulation and stability of emulsions, quality of dispersion of poorly soluble drugs in liquid media, coating phenomenon of solid dosage forms and also responsible for changes in physical characteristics of solid particles. The interface existing between matters in nature are basically categorized into four types include liquid/liquid, liquid / vapour (gas), solid/vapour and solid/liquid. These phenomena significantly affect the evaluation parameters of the various relevant dosages forms like; adsorption, disintegration, dissolution and bioavailability. Thus, it is an important preformulation study to develop stable and good quality pharmaceutical products.
SURFACE AND INTERFACE
The 'surface' can be defined as a clear boundary between the solid-gas and liquid-gas phases. The interface is the boundary between the two immiscible phases. The interface can be formed between liquid/liquid like; oil and water and between solid/liquid like; insoluble solid/liquid.
To understand the surface tension, one has to know about the cohesive and adhesive force between the like and unlike molecules.
Cohesive forces: The intermolecular attraction between the similar molecules is called the cohesive force. For example, molecules of water in bulk are surrounded by the similar types of molecules and the intermolecular forces between them are the force of attraction.
Adhesive forces: The force between the unlike molecules is called the adhesive force. For example, molecules of water at the surface behave differently than the molecules in the bulk.
Effect of cohesive and adhesive forces: The cohesive forces between the water molecules in the bulk attract the similar molecules but the water molecules behave differently at the surface. The cohesive force of the surface water molecules is higher. Due to higher cohesive force, there is a tendency of surface water molecules to go towards the bulk of the liquid and hence the liquid surface comes under the tension and tend to contract to the smallest possible area i.e., sphere.
- The cohesive and adhesive forces have significant role in the formulation of dispersions.
- If cohesive force is stronger, it keeps the phases separated while, if adhesive force is stronger, miscibility will take place.
- Thus, cohesive force of the molecules at the boundary exists towards bulk and the force which has to be applied to counter-balance this inward force is . called the surface tension.
- The unit of surface tension in CGS system is dynes/ cm (dyne cm) and in SI system, it is Newton/ M (Newton m-1).
Significance of surface tension
The surface tension determines the behavior of liquids in a number of processes and phenomenon.
Wetting or wettability: The wetting of the solid surface depends on the surface tension between solid surface and the liquid. This affects the washing, cleaning and also in the formulation of dispersions. The wettability is increased by reducing the surface tension.
Dispersibility: The interface between solid particles and the liquid phase is formed. A high surface tension has a negative effect on the dispersibility, as contact between similar particles is preferred.
When two immiscible liquids are mixed, they get separated from each other due to the surface tension of each liquid at its surface and form an interface. The surface tension of both the immiscible liquids at the interface is called the interfacial tension. This phenomenon can be understood by describing the function of cohesive force (between like molecules) and the adhesive force (between unlike molecules) between the two immiscible liquids.