The word surfactant is an abbreviation for the surface-active agent. A surfactant is characterized by its tendency to adsorb at surfaces and interfaces. Examples of interfaces involving a liquid phase include suspension (solid-liquid), emulsion (liquid-liquid) and foam (liquid-vapor). In many formulated products several types of interfaces are present at the same time. Surfactants are interfacially active compounds. They consist of a polar head group and a non-polar hydrocarbon chain. Surfactants can be classified according to the charge of their polar head group:
The Hydrophilic-Lipophilic Balance (HLB) of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule.
The cloud point of a nonionic surfactant or glycol solution is the temperature at which the mixture starts to phase-separate, and two phases appear, thus becoming cloudy. Wetting, cleaning, and foaming characteristics can be different above and below the cloud point. Generally, nonionic surfactants show optimal effectiveness when used near or below their cloud point. Low-foam surfactants should be used at temperatures slightly above their cloud point. Cloud point is typically measured using 1% aqueous surfactant solutions.
The critical micelle concentration (CMC) is the surfactant concentration at and above which micelles are formed. It can be determined for surfactant solutions by measuring the surface tension at different concentrations. Below the CMC, the surface tension decreases with the increasing surfactant concentration, as the number of surfactants at the interface increases. Above the CMC, in contrast, the surface tension of the solution is constant because the interfacial surfactant concentration does not change anymore.
Surfactants are widely used and many topical pharmaceutical formulations, cosmetics, antiseptics, shampoos, detergents, creams, and lotions contain surfactants. Surface active products are for their amphiphilic properties used as emulsifiers, suspending, wetting, solubilizing, and stabilizing agents.
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