Colloid Adsorption onto Responsive Membranes.

The adsorption of colloids with varying sizes and charges onto a surface carrying both negative and positive charges representing a membrane has been investigated by using a simple model employing Monte Carlo simulations. The membrane is made of positive and negative charges (headgroups) that are allowed to move along the membrane, simulating the translational diffusion of the lipids, and are also allow to protrude into the solution, giving rise to a fluid and soft membrane. When an uncharged colloid is placed in the vicinity of the membrane, a short-range repulsion between the colloid and the membrane is observed and the membrane will deflect to avoid the contact with the colloid. When the colloid is charged, the membrane response is two-fold: the headgroups of the membrane move towards the colloid as to partly embrace it, and the positive headgroups of the membrane approach the oppositely charged colloid, inducing the demixing of the lipids (polarization) of the membrane. The presence of protrusions enhances the polarization of the membrane. Potential of mean force calculations show that protrusions give rise to a more long-ranged attractive colloid-membrane potential which however has a smaller magnitude at short separations.