Counterion

Abstract: This paper summarizes theory, experimental techniques, and the reported data pertaining to the zeta potential of silica and silicon with attention to use as microfluidic substrate materials, particularly for microchip chemical separations. Dependence on cation concentration, buffer and cation type, pH, cation valency, and temperature are discussed. The Debye-Huckel limit, which is often correctly treated as a good approximation for describing the ion concentration in the double layer, can lead to serious errors if it is extended to predict the dependence of zeta potential on the counterion concentration. For indifferent univalent electrolytes (e.g., sodium and potassium), two simple scalings for the dependence of zeta potential on counterion concentration can be derived in high- and low-z limits of the nonlinear Poisson-Boltzman equation solution in the double layer. It is shown that for most situations relevant to microchip separations, the high-z limit is most applicable, leading to the conclusion that the zeta potential on silica substrates is approximately proportional to the logarithm of the molar counterion concentration. The z vs. pH dependence measurements from several experiments are compared by normalizing the z based on concentration.

Abstract: The dependence of polyelectrolyte multilayer thickness on salt concentration, salt type, solvent quality, deposition time, and polymer concentration is evaluated. Polymers are deposited on spinning silicon wafers. For the strong polycation/polyanion pair studied, film thickness is approximately proportional to the number of layers and the salt concentration. The irreversibility of overall molecule adsorption is indicated by the lack of exchange of surface (radiolabeled) for solution polymer. The hydrophobic nature of the driving force for polymer sorption is illustrated by the choice of salt counterion or solvent. Analyzed within the framework of ion exchange, the net energy of ion pair formation is not high, at most a few kT. Salt, competing with polymer segments for the surface, permits localized rearrangements. In the mechanism proposed, excess polymer is accommodated within several layers, rather than in one layer of loops and tails. Steric barriers coupled with slow conformational changes are respons...

Abstract: The structure of a surface with a fairly dense array of polyelectrolytes end-grafted to it is considered. A simple theory for the scaling properties of the corona thickness, counterion distribution, and disjoining pressure between opposing layers is presented which makes contact with previous numerical studies. It is shown that by distributing the charges on a macromolecules the interlayer force is considerably less sensitive to Debye screening by added electrolytes. The flat layer studies are extended to the case of small (spherical) colloidal particles