Correlation between specific ion adsorption at the air/water interface and long-range interactions in colloidal systems

Specific ion effects are of high impact in colloid science and dominate processes in aqueous systems from protein folding or precipitation to ordering of particles or macromolecules in bulk solutions. Due to the large internal interface of colloidal systems especially interfacial ion effects are of importance. This paper presents a new insight into the specific ion effects at the air/water interface of monovalent electrolyte solutions and their consequences for long-range interactions in colloidal systems. Solely, in an asymmetric film (i.e. wetting film) one can determine the sign and precise value of the surface potential of the free air/water surface. It is shown that the all over charges of the interfacial region, which are affected by the type of ion, dominate the interfacial forces even over several tens of nm. This is of interest for tailoring the stability of colloidal systems. It is clearly shown that the air/water interface is negatively charged and that both anions and cations affect the surface potential even at very low electrolyte concentrations (10(-4) M).