OINTMENTS, CREAMS AND PASTES; How they work on skin?

Most of the drugs discussed in this section are applied on the skin. The skin has various functions:

Functions of skin

1. protects internal body structure from hostile external environmental hazards (pollution, temperature, humidity  and radiation)
2. limits passage of chemicals into and out of the body
3. acts as a microbilogical barrier
4. stabilizes blood pressure and temperature
5. mediates sensation of heat, cold, touch and pain
6. Expresses emotions, e.g. embarrassment, anger, and anxiety (by sweating)
7. Identifies the individual (color, hair, odor, and texture)

PREPARATION OF EMULSIONS

PREPARATION OF EMULSIONS

Several factors are considered.:

            We need to Specify

·         Type of emulsion, that is whether it will be o/w or o/w.

·         The use of the product i.e. whether it will be for external use, internal use or injectable.

Consider

·         The ingredients taking into accounting the chemical structure, physical properties (m.p., solubility and stability)

·         Dose of the preparation (which has an impact on the overall volume)

·         Incompatibilities of the ingredients

·         Choice of emulgents

PRESERVATION OF EMULSIONS

Some ingredients of emulsions promote growth of micro-organisms by providing nutrients, e.g. bacteria feed on non-ionic and ionic surfactants, glycerin, and Emusifying agents (natural polysaccharides) cause deterioration of emulsions.  .  Oil e.g. arachis oil promote growth of aspergilus, Rhizopus, while liquid paraffin promote growth of some spp. Brucitisses. For these reasons, emulsions should be formulated with a preservative.

FORMATION OF EMULSIONS USING SURFACE ACTIVE AGENT (SAA).

SOLUBILITY THEORY

Consider Sodium Stearate. (C₁₇H₃₅ COONa).  The non-polar hydrocarbon chain is lipophilic while carboxylic (COONa) part is hydrophilic.  The balance of hydrophilic-lipophilic properties determines whether o/w or w/o emulsion results.  Generally, HLB 9 – 12 form o/w and HLB 3 – 6 form w/o emulsion.  A blend of tween 20 and span 20 form o/w.  Span 60 alone forms w/o emulsion.

Bancroft’s Rule:  The type of emulsion is a function of the relative solubility of the SAA, the phase in which it is more soluble, being the continuous phase.  Emulsifiers with high HLB are soluble in water and form oil/water emulsion.  The contrary is true for low HLB.

EMULSIFYING AGENTS

Desirable Properties of Emulsifying Agents.

·         Must be surface active, to reduce surface tension to below 10 dynes/cm

·         Is adsorbed on droplets as non-adhering condensed film

·         Impart enough electrical potential for repulsion of droplets

·         Increase viscosity

·         Effective in low concentration

·         Should be soluble in both phases but not too soluble in either of the phases

EMULSIONS INTRODUCTION

Emulsion is a thermodynamically unstable system consisting of at least two immiscible phase, one of which is dispersed in the other liquid phase, the system being stabilized by an emulsifying agent. One phase is the disperse phase which is distributed throughout the other continuous phase or dispersion medium. Emulsions can be liquids or semi-solids. The latter are referred to as creams. The particle or globule size is 0.1-10μ. When the globule size is < 5μ they are called fine emulsions but can have emulsions <10nm which are called microemulsions.