Light-scattering studies on water–nonionic-amphiphile solutions

The structure of water in aqueous solutions of the polyoxyethylene nonionic amphiphile C₁₀{H}₂₁$(${OCH}₂CH₂{)}₅$OH is studied by depolarized Rayleigh-wing and Brillouin light scattering along an isothermal path crossing the isotropic one-phase region from 0 to 1 amphiphile volume fraction {φ}. Depolarized Rayleigh-wing light scattering shows a well defined slowing down in the water rotational dynamics just where the structure of the system changes from the micellar structure to a block copolymeric melt. The interpretation of the obtained data gives the following structural picture for water: for {φ} lower than 0.75, water is partially bound to the oxyethylene groups of the amphiphile; above {φ}=0.75 all the water present in the system is bound. Brillouin scattering, probing thermodynamical quantities as the bulk modulus, gives information about the collective properties of the system. Two distinct behaviors in the bulk compressibility of the system are observed in two different concentration ranges. In the micellar region, measurements of the dispersion of the sound velocity as a function of the droplet volume fraction show an increased rigidity of the system at high frequencies. This increase in the bulk modulus is consistent with a percolation phenomenon. Micelles build up transient networks due to the attractive interaction. Above this region Brillouin scattering probes the evolution of the system towards the pure amphiphile phase for which a decrease of sound dispersion with {φ} is observed. Strong correlations between amphiphile molecules are present at large amphiphile concentrations.