The effect of surface charge regulation on conductivity in fluidic nanochannels

• Buffering from channel walls results in constant conductivities over some range.
• In the vicinity of the pI, conductivities can be reduced relative to bulk.
• In the vicinity of pH = 7, conductivity can be reduced even when adding electrolyte.

The precise electrostatic potential distribution is very important for the electrokinetic transport in fluidic channels. This is especially valid for small nanochannels where the electric double layers formed at the walls are comparable to the channel width. It can be expected that due to the large surface to volume ratio in such systems, they will exhibit properties that are not detectable in larger channels, capillaries and pores. We present a detailed numerical analysis of the current transport in fluidic nanochannels. It is based on solving the Poisson–Boltzmann equation with charge regulation boundary conditions that account for the surface-aqueous solution chemical equilibria. The focus is on studying the effect of the pH on the current transport. The pH is varied by adding either HCl or KOH. The analysis predicts non-monotonous and sometimes counterintuitive dependence of the conductivity on the pH. The channel conductivity exhibits practically no change over a range of pH values due to a buffering exerted by the chemical groups at the walls. An unexpected drop of the conductivity is observed around the wall isoelectric point and also in the vicinity of pH = 7 even though the concentration of ions in the channel increases. These observations are explained in the framework of charge regulation theory.

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