Micellization of Polyoxyethylene Monohexadecyl Ethers in Water

Abstract

The micellar behaviour of non-ionic surfactant systems of the polyoxyethylene n-alkanol type has been studied in terms of hydrodynamic, volumetric, light scattering and thermodynamic properties. The intrinsic viscosity of the micelles increased as the ethylene oxide chain length increased. The increment in relative viscosity, and in intrinsic viscosity of the micelles appeared to be due to the hydration of micelles, micellar size and shape. The observed critical micelle concentrations (cmc) indicated that the critical micelle concentration decreased as the ethylene oxide chain length increased. This unusual behaviour has been discussed by considering the decrement in solubility of non-ionics in aqueous solution when the ethylene oxide chain length is increased, and the increment in degree of coiling as the hydrophilic chain length increases. It is likely that the hydrophobic chain coils tightly at the end of the hydrocarbon chain. This is entropically favourable allowing hydrogen bonded water molecules to be released. The volumetric properties of the micelles showed that a volume change occurred during micelle formation. The partial molal volume change decreased with increasing hydrophilic chain length. The relationship between intrinsic viscosity and molecular configuration below cmc has been examined in terms of established hydrodynamic theories. The results show that polyoxyethylene surfactants are randomly coiled in aqueous solution. The addition of small concentrations of aromatic alcohols to the system caused the critical micelle concentration and micellar molecular weight to increase but at higher concentrations the critical micelle concentration decreased. The thermodynamic investigation of the micellization process showed that micelle formation was an entropy directed process. The free energy of micellization decreased as the ethylene oxide chain length increased. This decrement has been attributed to the reduction of structural order of water molecules which arises from the minimization of the hydrocarbon-water interface. This causes the transference of monomers to nonpolar environments.