Bilayers and micelles self assembly

This study aimed at developing a novel single site model for an amphiphilic molecule, with which free self-assembly of complex structures can be achieved at moderate computing cost. This model is based on Gay-Berne and Lennard-Jones particles and regards the rod particle as amphiphiles and spheres as solvent molecules. By giving the rod-sphere potential a dipolar symmetry, the hydrophobic effect, believed to be the main driver of amphiphilic self-assembly is incorporated. Results obtained thus far indicate that free self-assembly of micellar and bilayer and inverse micelle arrangements can be readily achieved using the generic models employed here, raising the prospect that this approach may well be able to make a link between molecular interactions and the self-assembling structures to which they lead.

Coarse-graining modelling of amphiphilic systems

Here, each amphiphile modelled is as a single rod shaped particle (single-site model) leading to high computational efficiency. Water molecules are represented by smaller spherical particles. This approach has been very successful in modelling liquid crystalline behaviour [2] and, therefore, should also be appropriate for simulating the structures and phases of bio-membrane and amphiphilic systems in general.

Modelling the hydrophobic effect

The hydrophobic effect is modelled through the energy parameter of the rod-sphere interaction and leads to a model with dipolar symmetry in which the spheres favour only one end of the rod. The mathematical function used is an exponential decay, controlled by two parameters. K controls the hydrophobic strength and H controls the hydrophobic to hydrophilic ratio.

Figure 1: plot of the amphiphile-solvent interaction strength.

Simulation results

NVT simulations at constant temperature T=0.7 for different concentrations are shown for the H=80 system. (i.e. 80 per cent of the rod is hydrophobic)

Phase behaviour

Figure 2: low concentration of amphiphiles: 5 per cent, 10 per cent and 20 per cent (in number)

Figure 3: high amphiphiles concentration: 60 per cent,70 per cent and 80 per cent

The ratio H has a big impact on the phase behaviour of these systems: change in the length of the hydrophobic part of the amphiphile induces changes in the stabilities of the curvature-containing aggregates it can adopt.

Micellar phase

The 5 per cent system has been simulated in a bigger simulation box and displays a true micellar phase. (not phase separation)

There are more out there

Mixed surfactants systems (H=80 and H=50)

pore formation due to the presence of micelle forming surfactant H=50 per cent with bilayer forming surfactant H=80 per cent

Ternary system: oil/water/surfactant

Oil-in-water emulsion	Bi-continuous network

Further work outline

• simulate micellar system of mixed/polydisperse surfactants systems
• study phase behaviour of mixed lipid bilayers (both shape and interaction)
• insertion of various object (various size and shape, different hydrophobicity) trough bilayer in order to mimic a coarse-grained protein-membrane system.
• study the effect of the surfactant strength on ternary systems phase behaviour

Downloads

• Presentation: 'Molecular modelling of bio-inspired self-assembling systems' - format PDF
• Videos: lamellar phase self-assembly 1-2, a micelle formation