Viscoelastic surfactant solutions: model systems for rheological research

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Abstract
The molecular constitution of viscoelastic gels is customarily described in terms of networks, which may have a transient or permanent character. Such supermolecular structures are often observed in biological or macromolecular systems, but can even occur in dilute solutions of some detergents. Surfactant molecules in solution, under suitable conditions, assemble reversibly into large aggregates of rod-like geometry. Depending on the ionic strength of the solution, these particles may be completely stiff or semiflexible. At low concentrations, when the lengths of the rods are much smaller than their mean distance of separation, there exists a sol state which is highly sensitive to shear forces. At higher surfactant concentrations, where the lengths of the rod shaped micelles become comparable to their mean distance, one observes gel formation and the solutions exhibit pronounced elastic properties. In analogy to polymer systems, these elongated, rod shaped micelles are strongly entangled; this leads to complicated flow processes. In comparison to solutions of macromolecules, there is, however, one important difference. The anisometric aggregates of surface active compounds are in thermal equilibrium with single monomers. The permanent exchange of surfactant molecules leads to breaking and reformation processes of the rod shaped aggregates. These kinetic processes cause thermal fluctuations, which result in a finite lifetime of the anisometric micelles. In situations where the elongated micelles are breaking within the time scale of observation, the rheological properties are controlled by kinetic processes. It is just under these circumstances, that we obtain very simple scaling laws and linear relations between all rheological quantities. From a theoretical point of view, this corresponds to an ideal case, which is rarely observed in other types of association colloids. The viscoelastic surfactant solutions can therefore be used as simple model systems for studies of fundamental principles of flow. The basic ideas and concepts have a general, universal character which leads to a deeper insight into dynamic properties and rheological mechanisms.