Zeta potential of microfluidic substrates: 1. Theory, experimental techniques, and effects on separations

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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‐Hückel 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‐ζ 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‐ζ 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 ζ vs. pH dependence measurements from several experiments are compared by normalizing the ζ based on concentration.