Direct measurement of interactions in supermolecular fluids and liquid crystals

Abstract

The understanding of the nature of the interactions between amphiphilic aggregates or within the assemblies of the molecular architecture is of fundamental importance. Direct measurement of forces between two unsupported self-assembled aggregates in solution is a formidable task. A new application of the surface force apparatus demonstrates that the interactions operating in micellar solutions or in a stack of membranes, such as a multilamellar mesophase, are not so intractable. For these two illustrating examples, the following features are emerging: — In aqueous micellar systems, where the electrostatic interactions dominate, the competition between a double-layer repulsion and a depletion attraction has been identified. At large volume micellar fractions, the depletion and oscillatory structural effects are enhanced. — In lyotropic lamellar mesophases, the compressibility of the stack normal to the layers is related directly to the free energy density of interlayer interactions, so that measurements of layer compressibility allow the nature of the forces to be inferred. The method presented here suffers from none of the difficulties usually encountered in the measurement of elastic properties of smectic phases (combination of several elastic moduli, model-dependence, etc). Moreover structural information about the confined mesophase, such as reticular spacing and the nature and dynamics of packing defects, is obtained. © 1992, IUPAC.