Micellization of nonionic amphiphiles in solvent mixtures
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Amphiphilic block copolymers of the poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) family (commercially available as Pluronics) 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 block copolymers (Pluronic P105: EO 37 PO 56 EO 37 and Pluronic F127: EO 100 PO 65 EO 100 ) has been studied in mixed solvents consisting of water and one of the following cosolvents: ethanol, glycerol, D(+)-glucose monohydrate, propylene carbonate, or triacetin, and also in ternary mixtures of water with ethanol+glycerol (50/50%wt) or ethanol+propylene carbonate (50/50%wt). The conformation of PEO-PPO block copolymers in dilute solution of the mixed solvent of water and ethanol or glycerol has been determined using viscosity measurements. The polymer coil is found to expand in the presence of water-ethanol mixed solvent and contract in the presence of water-glycerol mixed solvent. Addition of glycerol, glucose, propylene carbonate or triacetin to water promotes micellization when compared with water, while addition of ethanol to water disfavors the formation of micelles, i.e., the water-ethanol mixture provides better solvent conditions for the block copolymers than water. Glycerol and glucose provide favorable interactions with water and reduce the cmc by dehydrating both PEO and PPO blocks. In the case of propylene carbonate and triacetin, the reduction in cmc is due to the decrease in water-solvent interactions and increase in the hydrophobicity of the block copolymer. A correlation has been observed between the free energy of cosolvent and the number of PPO blocks present in the block copolymer, which can be used for the determination of the cmc of different PEO-PPO block copolymers in mixed solvents. This thesis also summarizes the recent literature on association properties of nonionic C m E n type (m: number of carbon atoms in alkyl chain, n: number of ethylene oxide segments) surfactants and nonionic block copolymers in aqueous solution, non-aqueous solvent, and in mixtures of water and non-aqueous solvents. The effect of the solvent on critical micelle concentration (cmc), the thermodynamic parameters of micellization, and the micelle structural parameters are discussed.