Calorimetric study of Micelle formation in polyethylene glycol monooctyl ether solutions

The micellar solutions of polyethylene glycol monooctyl ethers C8Ej (j = 3, 4, 5) were investigated by employing the high-precision isothermal titration microcalorimeter at seven temperatures from 10° to 40°C. From the thermodynamic analysis of the experimental results, the differential enthalpies of solution of monomer and micelle of C8Ej were obtained separately; they are negative and increase with increasing temperature and decrease with the number j. It was found that C8Ej molecules interact with water molecules more strongly in the monomeric than in micellar state. The enthalpy of micelle formation of C8Ej was positive and decreased with rising temperature, while they increased slightly with increasing j. By comparing the results of ethylene glycol oligomers (C0Ej) with those of C8Ej, it seems reasonable to suppose that the driving force of micelle formation of C8Ej is mainly the increment of entropy caused by dehydration of hydrophilic parts at higher temperatures, while the increment caused by dehydration of hydrophobic parts becomes dominant at lower temperatures in the micellization process.