Interaction forces between colloidal particles in alcohol-water mixtures evaluated by simple model simulations

The interaction force between hydrophilic macroparticles in an alcoholic (or amphiphilic) solution is investigated using a classical molecular dynamics simulation with coarse-grained models. When the fluid employed is a pure amphiphilic liquid, (i) amphiphiles are adsorbed vertically on the surface to form an amphiphile bilayer, and (ii) the surface force becomes strongly repulsive at the separation less than about the thickness of the bilayer. When the fluid is anamphiphile-water mixture, (iii) a bilayer of hydrated amphiphiles is formed near the surface, (iv) the surface force becomes attractive suddenly at the separation less than the length of the amphiphile molecule, and (v) the adhesion force has a maximum at a higher concentration of amphiphiles and the magnitude of this maximum is larger as an amphiphile has a longer hydrophobic tail. It is worth noting that result iv originates from a liquid-to-liquid-phase separation of the amphiphile-water mixture between the surfaces, in which amphiphiles are pushed out into the bulk and water molecules enter to bridge the gap between the surfaces instead. These simulation results are in fair agreement with recent experimental results by an atomic force microscope.