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

The steric stabilisation of microscopic decane-water-decane films (with a diameter of about 100 mu m) 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*. (C) 1998 Elsevier Science B.V. All rights reserved.