Pharmaceutical Engineering: Surfactants I [PHENt 203]

Study Notes

Surface active agents (SAA) are molecules with a hydrophilic head and a hydrophobic tail.
Due to their structure, SAAs have an affinity for boundaries or interfaces between phases.
SAA molecules are amphipathic, having both hydrophilic and hydrophobic portions.

Surfactant molecules align themselves at the interface between aqueous and oily phases.
The bonds formed between the polar heads of the surfactant molecules and water are weaker than water cohesive forces, leading to a reduction in interfacial tension.
When the water surface is saturated, and more surfactant molecules are added, they form micelles.
Micelles are aggregates of molecules in a colloidal solution with a hydrophobic core and a hydrophilic surface.
The critical micelle concentration (CMC) is the concentration above which micelles form.

Micelles have a hydrophobic core and a hydrophilic surface.
The relationship between surface/interfacial tension and surfactant concentration is that at low surfactant concentration, the surfactant molecules arrange on the surface, and the surface tension of the solution starts to rapidly decrease.
When the surface becomes saturated, the addition of surfactant molecules leads to the formation of micelles.
The critical micelle concentration (CMC) is the point at which micelles form.

CMC is not a constant value and depends on temperature, solute concentrations, and the structure of the SAA.
Traube's Rule states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples.

Solubilization of poorly water-soluble drugs: Micelles can solubilize poorly soluble drugs, allowing for increased solubility.
Formulation of emulsions: Surfactants are used to prepare emulsions, which are fine dispersions of minute droplets of one liquid in another.

Micelles can solubilize poorly soluble drugs, allowing for increased solubility.
Not all surfactants have the same solubilization capacity.
Solubilization capacity is measured by the ability of a surfactant to solubilize a solute.

Emulsions are fine dispersions of minute droplets of one liquid in another.
To prepare emulsions, you need to mix and add an appropriate amount of surfactant.
Mixing alone will result in a temporary mix that will eventually separate, while surfactant addition alone will not result in particle size reduction and dispersion.

Anionic surfactants: Have a negatively charged head in solution, e.g., soaps, sulfate, and sulfonate.
Cationic surfactants: Have a positively charged head in solution, e.g., cetyl trimethylammonium bromide (CTAB).
Non-ionic surfactants: Do not have an electrical charge in solution, e.g., polyoxyethylene oxide derivatives, sorbitan fatty acid esters.

Identify the type of surfactant based on its head group and determine its application.
Consider the properties of the surfactant, such as toxicity, solubility, and antiseptic effect.
Determine whether the surfactant is suitable for oral, parenteral, or topical use.