Anomalous Proton Transport across Silica Nanochannel Membranes Investigated by Ion Conductance Measurements

Proton transport plays an important role in many biological and technological processes. Numerous experiments and molecular dynamics simulations have proved the increase of proton mobility in confined nanostructures. In this work, we studied the proton transport across flow-through silica nanochannel membranes (SNMs) with vertically aligned channels, uniform diameter (similar to 2.3 nm), high porosity (16.7%), and ultrasmall thickness (88 nm). Taking into account both the mutual interaction between nanochannels and the contribution of surface conductance, a new theoretical model of ion conductance for SNMs was derived by modifying the conductance model reported previously with a correction factor. The correction factor was estimated by closely matching the experimental conductance of SNMs in KCl and NaCl solutions with the theoretical one calculated by the model. Then the measured conductance of SNMs in HCl solutions was found to be at least four times higher than the calculated value by the model. Given the total conductance across SNMs is dominated by the access conductance instead of channel conductance, the difference between experimental and theoretical conductance values implies either that the theoretical model does not capture the real physics of access conductance or that the two-dimensional nanoconfinement effect exists at the nanochannel entrances. The latter effect likely arises from mutual interaction of neighboring nanochannel entrances.