Charged colloids and proteins at an air-water interface: the effect of dielectric substrates on interaction and phase...

We study a two-dimensional (2D) system of macroions, trapped at the interface between air and an aqueous electrolyte solution, in the presence of a dielectric substrate approaching the air-water interface from the water side. Working within the linear Debye-Hückel theory, we investigate how the microion-averaged interaction potential between the macroions is affected by the presence of the dielectric substrate. Using these potentials in a Monte Carlo simulation, we further study the changes in the structural and phase behavior of the 2D colloidal system in response to the approaching substrate. Our scope of investigation covers two classes of colloidal particles, namely, highly charged latex particles of tens of nanometers radius, and protein particles of few nanometers radius carrying relatively small numbers of total charge. Probing the bond-orientational order parameter Phi(6) as a function of the 2D particle surface fraction phi(surf) and the air-water-substrate-water separation distance L, our simulations show that structural formations at the air-water interface are strongly influenced by the presence and the dielectric nature of the supporting substrate. Specifically, our [phi(surf): L] phase diagrams reveal that the transition from the fluid to the crystalline phase is shifted to higher surface fractions, if the approaching substrate is metallic, and to lower surface fractions, if it has a very low dielectric constant. These phase diagrams may be useful for finding materials and substrate interfaces for growing, e.g., 2D crystals of protein particles.