Electrochemical surface properties of bare- and silane-coated silica nanochannels

We present a combined theoretical and experimental analysis of the solid-liquid interface of fused-silica nanofabricated channels with and without a hydrophilic cyanosilane coating. Our theoretical model consists of three parts: (1) a chemical equilibrium model of the wall, (2) a chemical equilibrium model of the bulk electrolyte, and (3) a self-consistent Gouy--Chapman--Stern triple-layer model of the electrochemical double layer coupling (1) and (2). To validate our model, we used both pH-sensitive dye based capillary filling experiments and electro-osmotic current-monitoring measurements. Our model shows that the important fitting parameters are the inner Stern capacitance $C_1$ and the surface reaction constant p$K_+$. We also find that changing the outer Stern capacitance $C_2$ with surface composition results in more accurate fits of experimentally determined $\zeta$ potentials. This model is of value to predict experimentally observed phenomena in nanofluidic systems.