The Double-Layer Interaction in Asymmetric Electrolytes

The force as a function of separation has been measured between two molecularly smooth mica surfaces immersed in dilute solution of cytochrome c . At p H 5.7 this protein, which has the shape of a flattened sphere, carries twelve net positive charges per unit and adsorbs on the negatively charged mica surfaces. The decay length for the asymptotic form of the double-layer forces as a function of the protein concentration is markedly less than the classical Debye screening length. The measured decay lengths are in close agreement with the asymptotic form derived by Mitchell and Ninham for highly asymmetric (here 12:1) electrolyte solutions. Some consequences for colloid stability are discussed.