Spherical Micelles and Other Self-Assembled Structures in Dilute Aqueous Mixtures of Poly(Ethylene Glycol) Lipids

In this work we report on the characteristics of dilute mixtures of poly(ethylene glycol) derivatized lipids (PEG lipids) in aqueous solution. We show that for PEG lipids with PEG of molecular weight 750, 2000, and 5000 covalently coupled to DSPE (distearoyl phosphatidylethanolamine), spherical micelles are formed. The hydrodynamic radii and aggregation numbers of the micelles are determined as a function of temperature. The hydrodynamic thickness of the polymer layer is also determined, and the experimental values are compared with theoretically predicted values using a starlike polymer scaling model. We show a quantitative agreement between the experimental and theoretically predicted values. The phospholipid anchor carries a negative charge, and the surface potential of the micelles is estimated using a fluorescent probe titration technique. The experimental values are compared with values of the surface potential calculated using a program that solves the Poisson−Boltzmann equation numerically for the spherical geometry. A reasonable agreement between the experiments and the calculations is found. Based on the range of the electrostatic interaction, we infer that the steric repulsion caused by the overlap of the polymeric coronas dominates the intermicellar interactions. In addition to spherical micelles, we also observe some interesting aggregates in samples containing PEG(750)-DSPE which are found to be of lamellar character. For the PEG(2000) and PEG(5000) lipids no lamellar aggregates are observed, but it is found that micellar solutions of these lipids are only metastable at temperatures below approximately 60 °C.