Brownian dynamics simulation of a bonded network of reversibly adsorbed particles: Towards a model of protein adsorbed layers

We present computer simulations of adsorbed protein monolayers modelled as networks of spherical particles. This network is generated through the formation of flexible bonds between the particles. The particles are attracted to a flat (fluid/fluid) interface by a potential well, which is truncated in one direction (the solution). As a consequence, unbonded particles establish an equilibrium between the bulk solution and the interface, where they become adsorbed. In an adsorbed monolayer consisting of bonded particles, the individual particles still have the freedom to adsorb and desorb. However, in this case, the monolayer as a whole does not desorb spontaneously. If such a monolayer is strongly compressed, it forms a far thicker interfacial structure with particles extending several particle diameters into the solution. As the interface is compressed, the interfacial pressure increases initially, but then reaches a maximum value, decreases slightly and levels off. Desorption of particles and their movement away from the (crowded) interface reduces the local repulsive interparticle interactions. Upon re-expansion, the particle network completely returns to its original monolayer structure. However, formation of new bonds between particles during the compression-expansion cycle (reflecting the chemical reactivity of the adsorbed protein) introduces a strong element of irreversibility. In this case, a patchy structure is formed at the interface after the expansion. Finally, the effect of strongly adsorbing displacer particles is explored, which can have a similar influence on the adsorbed monolayer as a compression of the interface.