Effects of ion size, ion valence and pH of electrolyte solutions on EOF velocity in single nanochannels

Abstract

Electroosmotic flow (EOF) in nanochannels as small as 85 nm and in small microchannels is systematically investigated by using the current-slope method. The effects of ion size, ion valence and pH of electrolyte solutions on the velocity of electroosmotic flow are experimentally studied. The results show that the large size of hydrated ions leads to higher EOF velocity in large nanochannels and microchannels, and, however, lower EOF velocity in small nanochannels with an overlapped electric double layer (EDL). Furthermore, the EOF velocity is proportional to the pH of the electrolyte solution and contrariwise proportional to the valence of counter-ions. It is also observed that the EOF velocity depends on the channel size in nanochannels, even without overlapped EDL. Several models of electric double layer (EDL) are proposed to explain the experimental results and provide an improved understanding of the effects of ion size, ion valence and pH of electrolyte solutions on the EOF velocity in nanochannels in terms of EDL structure.

Keywords: Electric double layer; Electroosmotic flow; Ion size; Ion valence; Nanochannel; pH.