Surfactants

Introduction

The term "surfactant" is a contraction of "surface active agent".

• Surfactants reduce the surface tension between two ingredients (or phrases) in a preparation to make them more miscible (easier to mix together).

Their amphiphilic nature - possessing both hydrophilic (water-loving) and hydrophobic (water-fearing, lipid loving) components - allows them to interact with diverse substances.

• In a mixture of oil and water, the hydrophobic end of the surfactant interacts with the oil, while the hydrophilic end interacts with the water, facilitating their combination.

Surfactants are known by various names, depending on their specific function and application, as described below.

Wetting Agents

Wetting agents, also called levigating agents, reduce the surface tension between a liquid and a solid.

• Insoluble powders can be made into a paste with the addition of a small amount of wetting agent, facilitating grinding (particle size reduction) and mixing.

Suspending Agents

A suspension is a mixture in which solid particles are dispersed throughout a liquid.

• Suspending agents (or dispersing agents) help maintain this dispersion, preventing the solid particles from settling.
• This ensures a consistent dose in products like medications, by keeping the active ingredients evenly distributed.
• While suspending agents improve dispersion, they do not prevent settling indefinitely; suspensions should be shaken before use to re-disperse the solid.

Foaming Agents

Surfactants are crucial in the production of detergents and soaps.

• They from micelles, which encapsulate and lift lipophilic (oil-loving) dirt, allowing it to be washed away.

NOTE: In non-sterile compounding, however, anti-foaming agents (e.g. simethicone) are more commonly used.

Emulsifiers

An emulsion is a mixture of two or more liquids which are immiscible (not able to be blended together), such as oil and water.

• Emulsifiers (emulgents) stabilize these mixtures by preventing the dispersed droplets of one liquid from coalescing (combining) and separating from the other (i.e. keeping the liquid droplets dispersed throughout the liquid vehicle).

A key consideration when choosing the surfactant is whether the desired emulsion is water-in-oil (w/o) or oil-in-water (o/w).

• Water-in-oil emulsions are often used topically due to their less palatable taste.
• Oral products are typically oil-in-water emulsions.

The hydrophilic-lipophilic balance (HLB) number, ranging from 0 to 20, helps determine the appropriate surfactant for a given emulsion.

• Surfactants with low HLB values (<10) are more lipophilic and are suitable for water-in-oil (w/o) emulsions.
• Conversely, surfactants with high HLB values (>10) are more hydrophilic and are used for oil-in-water (o/w) emulsions.