Two immiscible fluids will phase separate when mixed. Under appropriate conditions a dispersed or colloidal state can be maintained, namely when the interface between the two fluids can be covered with stabilising components. Molecules which can serve that purpose by accumulating (adsorbing) at the surface are generally called 'surface active agents', or 'surfactants' for short. Surfactants should be amphiphilic, that is, posses moieties miscible/compatible with both fluids; they form, in a sense, a two-dimensional (2D) phase at the boundary between two three-dimensional (3D) phases.
In the present proposal we consider interfaces carrying a mixture of two different surfactants (say, A and B). Depending on the conditions such a mixture may phase separate at the interface, i.e. in two dimensions, forming regions which differ in composition, an A-rich and a B-rich phase. Now if we have a third surface active component, L, which shares properties with both A and B (e.g., in terms of tail length and/or mixing properties) then this component will tend to accumulate at the 2D boundaries between these domains (which are lines rather than surfaces) and it will stabilise the 2D phase regions (see figure). Similarly to the way surfactants can change and control 3D dispersion of immiscible components, this third component will change/control 2D dispersion of immiscible (surfactant) systems. Analogously to the term 'surfactant' this third surfactant can be called ?lineactant? (line-active agent) since its 2D behavior resembles 3D behavior of surfactants. Of course all lineactants are also surfactants. Examples of lineactants could be tri-block copolymers of the type AnBmCk, where B is a hydrophilic block, and A and C are hydrophobic blocks, chosen such that the corresponding tri-block copolymers AnBmAn and CkBmCk would phase separate at the surface. Another example could be di-glycerides having one long and one short hydrophobic chain, which could act as lineactants stabilizing demixed films of mono-glycerides comprising molecules having only short or only long hydrophobic chains.
Lineactants will be used for two purposes: (i) controlled self-assembly and (ii) making designed surfaces. By cleverly using lineactants one could essentially develop 2D colloid science. This would imply making 2D foams, emulsions and more complex systems such as 2D bi-continuous systems. One might even consider 2D 'Pickering' emulsions stabilized by 2D colloidal particles (see figure 2-4). Eventually, it would be important to understand how self assembly in 2D influences properties of complex colloidal systems in 3D.
The applications of alternative types of lineactants for controlling properties and behavior of phospholipid bi-layers such as cell membranes could find direct application in biology and medicine. |
