al.,2014 [216] who prepared poly-Ɛ- caprolactone nanoparticles by solvent displacement method and investigated the effect of polymer concentration on particle size. It was found that increasing polymer concentration from (0.5 to 0.8% w/v) at surfactant concentration (0.5% w/v) resulted in increasing particle size while, at the same surfactant concentration and increasing polymer concentration to (1% w/v) particle size decreased. Also, increasing polymer concentration from (0.5 to 0.8% w/v) at surfactant concentration (1% w/v) resulted in decreasing particle size while, at the same surfactant concentration and polymer concentration was increased to (1% w/v) particle size increased. These results were attributed to that at low polymer concentration and high surfactant concentration, the solubility of polymer in acetone/water mixture might have increased due to the solubilizing effect of the surfactant leading to slower rate of polymer precipitation and formation of larger particles. While at higher polymer concentration the effect of surfactant on solubility was less marked leading to higher precipitation rate and the formation of smaller particles [216]. The higher polymer concentration might also results in increasing viscosity of the organic phase, which might decrease the diffusion rate and might lower the rate of Ostwald ripening for the more viscous solutions so smaller particles were produced [217]. Eudragit S100 nanoparticles were smaller than those of HPMC phthalate HP55 while maintaining the same formulation conditions; this may be due to that polymer molecular weight that influences nanoparticle size as the higher polymer molecular weight, the smaller the nanoparticles [218].According to this, molecular weight of Eudragit S100 (150000 g/mole) [128] is greater than molecular weight of HPMC phthalate HP55 (78000 g/mole) [133], Eudragit S100 nanoparticles of were smalle>