Impact of bulk and surface properties of some biocompatible hydrophobic polymers on the stability of methylene chloride-in-water mini-emulsions used to prepare nanoparticles by emulsification–solvent evaporation

Abstract

The emulsifying and stabilizing ability of several hydrophobic (insoluble in water and soluble in volatile organic solvents) polymers, such as Eudragit RL, Eudragit RS, PLGA, PCL, and their mixtures, with regard to the methylene chloride (MC)-in-water mini-emulsions, has been compared to the viscosity of MC solutions and to the properties of adsorption and spread monolayers of these polymers. Eudragits RS and RL contain ∼2.5 and ∼5 mol% of pendent cationic trimethylammonium (TMA) groups per ∼164 g/mol segments, whereas PLGA and PCL contain 1 and 2 polar carbonyl groups per 130 and 114 g/mol, respectively. The electrostatic attraction between the dipoles, formed by TMA groups and the condensed counter ions in the MC solutions, leads to the contraction of macromolecular coils of Eudragits, whereas the PLGA and PCL macromolecules, interacting by low polar carbonyl groups (with dipole moment μ = 2.7 D) retain more extended conformation in MC. This explains why the characteristic viscosities [ η] of MC solutions are much lower for the former polymers (∼0.1 dL/g) with regard to PLGA and PCL solutions whose [ η] is equal to 0.3 and 0.6 dL/g, respectively. The ionization of TMA groups in contact with the water phase leads to the irreversible adsorption of Eudragits at the MC/water interface and to high decrease of the interfacial tension γ (down to 4 mN/m for the 5% MC solutions). Whereas PLGA and PCL possessing low polar carbonyl groups adsorb poorly at the MC/water interface exhibiting γ ≅ 28 mN/m. Higher stability of spread monolayers of Eudragits ( π* ∼ 40 mN/m) with regard to PLGA and PCL ( π* < 20 mN/m) correlates well with higher interfacial activity of the former with regard to the later. The higher surface potential Δ V of Eudragits (0.9 V) with regard to PLGA (0.3 V) and PCL (0.4 V) is explained by the formation of electric double layer (DL) by the former, whereas the later contribute to the Δ V only by cumulative dipole moments of carbonyl groups. The experimental values of surface potentials correlate well with the Gouy–Chapman model of the DL and the Helmholtz model of the monolayer. The ensemble of experimental results leads to the conclusion that higher emulsifying and stabilizing ability of Eudragits with regard to PLGA and PCL is due to higher adsorption activity of the former which form the corona of polymeric chains with ionized TMA groups around the droplets. It can be postulated that Eudragit polymers have good surface active properties which may allow manufacturing of biocompatible nanoparticles by emulsification–solvent evaporation method without surfactants.