Micellization of amphiphilic block copolymers in binary and ternary solvent mixtures

Amphiphilic block copolymers of the poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO) family (commercially available as Pluronics or Poloxamers) are well-known for self-assembling in water (selective solvent for PEO) into micelles with a PPO-rich core and a hydrated PEO corona. The micellization of two PEO–PPO–PEO block copolymers (Pluronic P105: EO37PO56EO37 and Pluronic F127: EO100PO65EO100) has been studied in binary mixed solvents consisting of water and one of the following organic solvents: ethanol, glycerol, d(+)-glucose monohydrate, propylene carbonate, or triacetin, and also in ternary mixtures of water with 50/50 wt% ethanol + glycerol or 50/50 wt% ethanol + propylene carbonate. Glycerol, glucose, propylene carbonate and triacetin were found to promote micellization when added to water. Glycerol and glucose interact favorably with water, and reduce the block copolymer critical micelle concentration (cmc) by dehydrating the PEO–PPO interface as well as changing the bulk solvent properties. Propylene carbonate and triacetin act by locating at the PEO–PPO interface and increasing its hydrophobicity. The addition of ethanol to water provides better solvent conditions for the block copolymers compared to plain water, and disfavors the formation of micelles. In the case of ternary solvents consisting of water, ethanol (that prevents micelle formation), and glycerol or propylene carbonate (that favor micelle formation), the observed changes in the cmc are subtle. For Pluronic P105, the cmc increase is greater for ethanol + propylene carbonate (50/50 wt%) than for ethanol + glycerol (50/50 wt%). For Pluronic F127, the cmcs remain the same as in plain water, i.e., the effects of the two organic solvents compensate each other. The difference between the free energy of micellization in plain water and that in solvent mixtures varies linearly with the cosolvent concentration, and collapses into a single line for each solvent mixture type when normalized with the number of the block copolymer PO units (NPO), indicating that the micelle core is mainly affected by varying solvent condition for different PEO/PPO ratios.

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► Subtle influence of ternary solvents of hydrophilic or hydrophobic solvent + ethanol + water on cmc.
► Both hydrophilic and hydrophobic solvents promote micellization through different mechanisms.
► The importance of solvent cohesive forces on the cmc.
► Good correlation between the free energy of micellization and solubility parameters.