Surface-dependent chemical equilibrium constants and capacitances for bare and 3-cyanopropyldimethylchlorosilane 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 3-cyanopropyldimethylchlorosilane (cyanosilane) coating. We develop a model that relaxes the assumption that the surface parameters C1, C2, and pK+ are constant and independent of surface composition. Our theoretical model consists of three parts: (i) a chemical equilibrium model of the bare or coated wall, (ii) a chemical equilibrium model of the buffered bulk electrolyte, and (iii) a self-consistent Gouy–Chapman–Stern triple-layer model of the electrochemical double layer coupling these two equilibrium models. To validate our model, we used both pH-sensitive dye-based capillary filling experiments as well as electro-osmotic current-monitoring measurements. Using our model we predict the dependence of ζ potential, surface charge density, and capillary filling length ratio on ionic strength for different surface compositions, which can be difficult to achieve otherwise.

Graphical abstractElectrochemical properties of the solid–liquid interface in bare and cyanosilane-coated silica nanochannels are investigated using capillary filling and EO-flow. Modeling uses Gouy–Chapman–Stern triple-layer theory coupled with bulk-electrolyte chemistry. Figure optionsDownload full-size imageDownload high-quality image (141 K)Download as PowerPoint slideResearch highlights► Fabrication and modeling of bare and cyanosilane-coated fused silica nanochannels. ► Experiment: EO flow and capillary filling with pH-sensitive dye in nanochannels. ► Modeling: extended 3-layer model needed to capture observed surface dependence. ► Modeling/experiment show surface dependence of surface capacitances and pK constants. ► Coated versus bare: pK, surface capacitance, and cation adsorption significantly higher.