in der Helmholtz-Gemeinschaft

Institute of Solid State Research (IFF)

Self-assembling amphiphilic systems

Mixtures of oil, water and amphiphile show a large variety of different mesophases. There are a number of ordered phases of lamellar, hexagonal and cubic symmetry, but also fluid phases with an interesting internal structure. This applies in particular to the microemulsion phase, which consists of two intertwined networks of oil and water channels, which are separated by an amphiphilic monolayer. Several theoretical approaches have been used to describe the thermal behavior of these systems [46]:

• Microscopic lattice models [46,49,78]
• Ginzburg-Landau models [46,63,78]
• Membrane models [46]

Ginzburg-Landau Models

The simplest Ginzburg-Landau model is one with with a single, scalar order parameter - which can be identified with the local concentration difference of oil and water. Such a model has been used to study the behavior of ternary amphiphilic mixtures in considerable detail [46,63,78]. It has been shown to describe several properties of this complex fluid correctly. In particular, the model has been used to calculate

• the elastic bending moduli of surfactant monolayers at the oil/water interface [25,35],
• the phase diagram, which includes lamellar, hexagonal and cubic phases [35],
• the wetting behavior of the oil/water interface by the microemulsion [20,22,55],
• the spectrum of capillary waves of the oil/water interface and of the lamellar phase between walls [20,40],
• the scattering intensity in film contrast [51,60],
• the variation of the internal area, and of the mean and Gaussian curvatures of the amphiphile film in the microemulsion phase [41,51,57],
• the sound attenuation and velocity of balanced microemulsions [43,66], and
• the dynamic scattering intensity in thermal equilibrium [48,50,66].

Ginzburg-Landau models with several order parameters (the local amphiphile concentration and the local amphiphile orientation, in addition to the local concentration difference of oil and water) have been used to calculate

• the phase behavior of aqueous amphiphile solutions [33,58]
• the scattering intensity in film contrast [33,44,58]