(The) effects of solvent quality on adsorption and depletion of molecules at Interfaces

Abstract

There are two main phenomena, either adsorption or depletion, when a molecule approaches to a surface. An attractive force builds an adsorption layer while a repulsive force creates a repulsive layer. The formation of an adsorption or depletion layer at an interface depends on the properties of a solvent and the specific interaction energy between the molecule and surface. First, the issue of depletion phenomenon of rod molecules at surface is addressed. An extensive study of surface depletion based on binary mixtures of n-octane and n-tetracosane was presented, when the relatively longer chain molecules solidify below the theta temperature. In low concentrations of n-tetracosane, unique phenomena on the surface tension profile induced by surface depletion was founded. It depends on the concentration of n-tetracosane within a limited volume fraction range : 0.08~0.21. The results are satisfactorily described by the depletion dynamics with the values of the change of the surface tension immediate after the crystallization occurs within the volume fraction range 0.08~0.11 Second, a adsorbed sulfonated polystyrene on a silicon surface was studied in various solvents: pH4 buffered aqueous, organic and aqueous/organic mixed solutions by using neutron reflectivity. The unique interfacial behavior of the polymer brush prepared by the polyelectrolyte random copolymer having 12% and 27% sulfonate level was described. The specially designed liquid cells were used for neutron reflectivity, atomic force microscope and ellipsometry to investigate the behavior in different solvents,. The change of thickness, roughness and width of interfacial region were observed.