In certain surfactant solutions, such as aqueous cetyltrimethylammonium bromide (CTAB) in 0.1 M KBr, the amphiphiles are found to assemble reversibly into long, flexible worm-like micelles. Above a few per cent surfactant, these form an entangled viscoelastic fluid, reminiscent of a polymer solution. We review some recent experimental and theoretical progress concerning the equilibrium statistics and dynamics of these systems, which differ from normal polymer solutions in that the chains can break and reform reversibly.<. This entropy gain is enhanced by the fact that the excluded volume repulsion on scales less than < is also reduced by breaking the chain; this effect is accounted for by the term t 9 in the prefactor of log<.

From seashells to DNA, chirality is expressed at every level of biological structures. In self-assembled structures it may emerge cooperatively from chirality at the molecular scale. Amphiphilic molecules, for example, can form a variety of aggregates and mesophases that express the chirality of their constituent molecules at a supramolecular scale of micrometres. Quantitative prediction of the large-scale chirality based on that at the molecular scale remains a largely unsolved problem. Furthermore, experimental control over the expression of chirality at the supramolecular level is difficult to achieve: mixing of different enantiomers usually results in phase separation. Here we present an experimental and theoretical description of a system in which chirality can be varied continuously and controllably ('tuned') in micrometre-scale structures. We observe the formation of twisted ribbons consisting of bilayers of gemini surfactants (two surfactant molecules covalently linked at their charged head groups). We find that the degree of twist and the pitch of the ribbons can be tuned by the introduction of opposite-handed chiral counterions in various proportions. This degree of control might be of practical value; for example, in the use of the helical structures as templates for helical crystallization of macromolecules.

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Rheological experiments were carried out on aqueous micellar solutions of the "dimeric" surfactant ethanediyl-a,a>-bis(dodecyldimethylammonium bromide) as a function of surfactant volume fraction and temperature. The aim was to investigate the effect of electrostatic interactions on the micellar size through the linear viscoelastic properties of the system. The zero-shear viscosity results indicate a sharp crossover from a dilute regime in which the micelle growth is very weak to a regime of rapid growth, in agreement with the theoretical predictions. However, upon increasing further the volume fraction, a maximum of the zero-shear viscosity is observed which suggests that the micellar length also goes through a maximum. This is confirmed by the ánalysis of the viscoelastic spectra on the basis of the recently developed models taking into account both reptation and Rouse processes. The decrease of micellar length observed in the high volume fraction range has been interpreted as due to the decrease of the effective end-cap energy associated with the theoretically predicted increase of the micelle ionization degree with volume fraction. The effect of the electrostatic interactions also appears in the behavior of the plateau modulus that exhibits a larger volume fraction dependence than in highly screened micelles.

Ionized poly (acrylic acid) gels were studied both at concentrations close to the concentration of preparation and at swelling equilibrium. In the first experimental condition, the introduction of electrostatic interactions decreases the shear modulus. The addition of salt screens these interactions and allows one to recover the shear modulus of unneutralized gels. The correlation of these effects with light scattering results suggests that they are related to a change of the gel microstructure with electrostatic interactions. The swelling equilibrium of these gels if found to scale like the ratio of the ionization degree to the Debye-Hiickel screening parameter with an exponent 6/5. The shear modulus at swelling equilibrium is given by the simple affine deformation law for not too high swelling degrees (<200). For larger swelling ratios, the shear modulus increases with swelling ratio due to deviations from Gaussian elasticity. These results can be partly explained by a recently proposed model. Finally, the cooperative diffusion coefficient can be measured by kinetics of swelling experiments and its behavior does not follow the predictions of the same model, possibly due to the coupling of cooperative diffusion with the establishment of a Donnan equilibrium.