A STUDY ON FACTORS INFLUENCING THE DROPLET SIZE IN NONIONIC OIL-IN-WATER MICROEMULSIONS

The droplet size in oil-in-water microemulsions formed from isopropyl myristate (IPM), sorbitol (S), water and either polysorbate (P) 80, 60 or 40 was determined by total intensity light scattering measurements. Data were analysed using a hard-sphere model to correct for interparticle interactions. Phase studies on systems prepared using P40 have shown an optimum P/S mass ratio of 1:1.5 for microemulsion formation. The droplet size of all microemulsions increased with increasing volume fraction of IPM. At a given IPM content the droplet size was influenced by the structure of the polysorbate, the P/S ratio and the total P + S content. At a constant P/S ratio, the droplet size decreased with change of surfactant in the series P80 > P60 > P40. For microemulsions prepared with each polysorbate, change of the P/S ratio in the series 1:3, 1:2.5, 1:2, 1:1.5 caused a progressive decrease of droplet size. An increase of total P + S content from 40 to 45% w/w at a P/S ratio of 1:2.5 caused a decrease of the droplet size in microemulsions prepared with P80.