L. Magnus Bergström

By L. Magnus Bergström

A comprehensive, yet simple, theoretical model for droplet microemulsions is presented. The model combines thermodynamics of self-assembly with bending elasticity theory and relates microemulsion properties, such as average droplet size, polydispersity, interfacial tension and solubilisation capacity with the three bending elasticity constants, spontaneous curvature (H 0), bending rigidity (kc) and saddle-splay constant (k¯c). In addition, the self-association entropy constant (ks) explicitly determines various microemulsion properties. The average droplet size is shown to increase with increasing effective bending constant, defined as keff=2kc+k¯c+ks, as well as with decreasing magnitudes of H0. The polydispersity decreases with increasing values of keff, but does not at all depend on H0. The model predicts ultra-low interfacial tensions, the values of which decrease considerably with increasing droplet radius, in agreement with experiments. The solubilisation capacity increases as the number of droplets is increased with increasing surfactant concentration. In addition, an enhanced solubilisation effect is obtained as the size of the droplets increases with increasing surfactant concentration, as a result of self-association entropy effects. It is demonstrated that self-association entropy effects favour smaller droplet size as well as larger droplet polydispersity.

Part of the book: Properties and Uses of Microemulsions

By Lars Magnus Bergström

By means of combining bending elasticity theory with solution thermodynamics of small systems, we demonstrate that unilamellar vesicular liposomes can be thermodynamically stable with a wide range of average sizes depending on the various bending elasticity constants. The average vesicle size increases with increasing bending rigidity (k_c) and saddle-splay constant (k¯c), and with decreasing spontaneous curvature (H₀). Bilayer aggregates predominate over micelles at lower values of the spontaneous curvature, in the regime of which H₀ favours large vesicles. However, small unilamellar vesicles may be favoured by low values of k_c, rather than high H₀. Mixing two amphiphilic components with different spontaneous curvatures gives rise to an explicit contribution that always brings down k_c. As a result, the amphiphilic component with high positive spontaneous curvature prefers to be located in the outer, positively curved, monolayer, whereas the other compound prefers to be located in the inner, negatively curved, layer. In contrast to unilamellar vesicles, geometrically open disks can only be thermodynamically stable close to the micelle-to-bilayer transition in a dilute solution of non-interacting bilayer aggregates. However, in more concentrated solutions, above the overpacking limit of vesicles, disks may be more favourable than vesicles due to more favourable packing conditions.

Part of the book: Liposomes - A Modern Approach in Research [Working title]