Nanocrystalline Oligo(ethylene sulfide)- b-poly(ethylene glycol) Micelles: Structure and Stability

Micelle formation generally relies on hydrophobic or electrostatic interactions between distinct regions of amphiphilic molecules. In this work, a different mechanism is considered in which nanocrystalline domains are formed from short ethylene sulfide oligomers at the core of the micelles, leading to exceptionally stable, uniform micellar structures. The structure and thermodynamic properties of the resulting micelles are examined through a combination of experiments, theory, and simulations. It is found that in oligo(ethylene sulfide)-b-poly(ethylene glycol) amphiphiles as few as three ethylene sulfide monomers are sufficient to form a highly crystalline core, surrounded by a water-soluble ethylene glycol corona of arbitrary size. Sulfur-sulfur interactions induce formation of rhombohedral lattice crystalline regions, which exhibit well-defined intramolecular and intermolecular order. An atomistic model is used to determine the free energy of the micelles; the critical micelle concentration (CMC) is found to be extremely small, on the order of 10^-8 mol/L. The size distribution of these micelles is nearly monodisperse. The crystalline core also includes amorphous regions that could serve as hosts for other molecules. Taken together, these properties serve to underscore that controlled crystallization provides a useful and underexploited mechanism for assembly of ultrastable micelles for applications in a variety of settings, including drug delivery and immunology.