Dynamical effects of supramolecular aggregates in water-butoxyethanol mixtures studied by viscosity measurements

We have studied the shear viscosity in a water-butoxyethanol (${\mathrm{C}}_{4}$Et) mixture as a function of temperature and concentration. The critical and off-critical mixtures have been studied over a very large T range, and the results are compared with recent light-scattering data confirming the amphiphilic character of the ethoxylated alcohol molecule that originates micellar structures. On this basis the data are discussed, considering the system as a colloidal dispersion of interacting micellar particles, where the attractive interaction plays the main role. In these terms the data near and below the lower critical solution temperature ${\mathit{T}}_{\mathit{c}}$ are well accounted for by the current mode-coupling theory of critical binary fluids. For low temperatures (T\ensuremath{\le}20 \ifmmode^\circ\else\textdegree\fi{}C) where large clusters are present, the \ensuremath{\eta} data are explained by considering the fundamental role of the attractive interaction effects among the dispersed micelles and using in this respect a two-fluid model, a micelle system plus an inert background.