Electrokinetic flow in a pH-regulated, cylindrical nanochannel containing multiple ionic species

Abstract

Considering the wide applications of the electrokinetic flow regulated by pH, we model the flow of an electrolyte solution containing multiple ionic species in a charge-regulated cylindrical nanochannel. This extends previous analyses, where only two kinds of ionic species are usually considered, and a charged surface assumed to maintain at either constant potential or constant charge density, to a case closer to reality. Adopting a fused silica channel containing an aqueous NaCl background salt solution with its pH adjusted by HCl and NaOH as an example, we show that if the density of the functional groups on the channel surface increases (decreases), it approaches a constant potential (charge density) surface; if that density is low, the channel behavior is similar to that of a constant charge density channel at high salt concentration and large channel radius. Several interesting results are observed, for example, the volumetric flow rate of a small channel has a local maximum as salt concentration varies, which is not seen in a constant potential or charge density channel.