Microemulsions Definition &Composition: Self emulsifying drug delivery systems (SEDDS) described as homogeneous mixtures of natural or synthetic solid or liquid surfactants and cosurfactants. Also “Microemulsions” might defined as are liquid dispersions of water and oil that are made homogenous, transparent (or translucent) and thermodynamically stable by the addition of relatively large amounts of a surfactant and a co-surfactant and having diameter of the droplets in the range of 100 – 1000 A (10 – 100 nm). Compare: ME, Emulsions and Micellar systems. (see figure) Physicochemical properties of ME: Thermodynamic stability, They form transparent or transluscent dispersions, Self emulsifying properties, They have excellent solubilisation capacity. Droplet size (mostly)<100 nm, They form O/W or O/W or bi-continuous systems. (see figure)

Figure. Structures of Micellar solution, Microemulsion and Emulsion

Energy Change of ME formation: For o/w dispersion of liquid paraffin, γo/w=55 mJ.m-2, ∆G= γ ∆ A* calculated at different droplet sizes, d=1 cm = 0.01 m ∆G = 55 mJ.m-2 * (3.1416*[0.01]2 m2) = 0.017 mJ, d= 10 um >> (1cm)3/(10 um)3 =V1/V2 1 cm globule >>109 globules, ∆G = 109{55 mJ.m-2 * 3.1416 *[0.05]2 m2) = 17.28 mJ d= 1um (1000 nm) ∆G = 172.8 mJ Sphere area = pi*sq. diameter = 4*pi*sq. radius

Role of Cosurfactant: ME droplet (about 100nm) >> huge interfacial area, But ME are thermodynamically stable. HOW? γo/w cyclohexane/water)=42 mJ. m-2 Add 10-4g/ml SDS>> γo/w = 2 mJ. m-2 Add 20% pentanol>> γo/w = 10 mJ. m-2 Add of both >> very low γo/w value >> spontaneous ME formation.

Ternary phase diagrams in ME formulation