Phospholipid monolayers phase diagrams

The results of the thermodynamic and structural experiments on both the fatty acids and the phospholipids can be combined to provide a picture of the phase diagram and the nature of the monolayer phases. 

An extensive analysis of the polymorphism in phospholipid monolayers has been reported by Albrecht et al. (1978) who established a phase diagram of dipal-mitoyl-lecithin monolayers involving four phases. Two of these have the liquid-type of disordered chains (liquid-expanded films). It was proposed that one has disordered chains oriented perpendicular to the water surface and the other has disordered chains in a tilted arrangement. Furthermore two crystalline phases were described, one with tilted chains and the other with perpendicular chains. 

In order to qualitatively understand the dependence of the particle-membrane interaction on particle size and surface treatment type, several simulations were carried out with various particle radii (Rp = 1.6, 2.0, 2.4, 2.8 and 3.2 nm) and parti-cle-hydrophile interaction parameters (%ph = 1.0,0.0, -1.0, —2.0, -3.0). The results of these simulations are summarized in the phase diagram shown in Figure 10.4. It can be seen that, as the charge density of the particle surface is increased, the particle is attrachng more and more phospholipid molecules away from the membrane so that they could adsorb on the particle surface. In particular, while the neutral particles surround themselves with a monolayer of phospholipids, the strongly charged parhcles must surround themselves with a bilayer, thus taking away twice as much phosphohpids as the neutral particles. This qualitative picture is also supported by the analysis of density profiles across the particle and the... 

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