Interfacial and emulsion stabilising properties of amphiphilic water-soluble poly(ethylene glycol)–poly(lactic acid) copolymers for the fabrication of biocompatible nanoparticles

Abstract

The steric stabilisation of microscopic decane–water–decane films (with a diameter of about 100 μm) and decane-in-water nanoemulsions (with mean droplet size of about 200 nm) by the adsorption layers of biocompatible and biodegradable water-soluble surface-active poly(ethylene glycol)–poly(lactic acid) (PEG–PLA) diblock copolymers with different HLB was studied as a function of both PEG and PLA molecular weights ( MW), varying from 0.5 to 10 kg/mol. In the MW range studied, it was shown that the stability or the lifetime of the microscopic emulsion films (MEF) and of the nanoemulsions depends mainly on the length ( l PLA) of the hydrophobic PLA blocks (it increases with increase of l PLA), and is characterised by a stepwise dependence on the surfactant concentration. A sharp transition, from very stable to unstable MEF and emulsions, occurred at a well-defined critical surfactant concentration C *. The PLA blocks, insoluble in both decane and water, adopt the most energetically favourable “side-on” orientation at the decane–water interface, whereas the PEG blocks are oriented towards the water phase in a “mushroom” or “brush” conformation. The length of the PLA blocks determines the minimal area per adsorbed surfactant molecule at the interface and consequently the critical concentration C *.