Anomalous Diffusion of Electrically Neutral Molecules in Charged Nanochannels

Abstract

Diffusion is the principal means of passive transport whereby ions or molecules driven by thermal motion move along their concentration gradients. This spontaneous process is widespread in nature. For example, as the main form of transport for vital materials through cell membranes, diffusion plays a fundamental role in living cells. Recently, self-supporting membranes that contain single or arrays of nanochannels are attracting increasing attention in nanotechnology, chemistry, physics, and biology. Inspired by biological membrane channel systems, most notably the a-hemolysin channel, artificial analogues of such nanoscopically sized pores have been developed for potential application in controlled growth of nanostructures, nanofiltration devices, bioseparation, and biosensing. Compared to biologically based nanochannels, artificial inorganic nanochannels have potential advantages of being less fragile, more stable, easily tailored, and flexible with regard to surface modification. Anodization of aluminum under appropriate electrochemical conditions yields extended membranes containing self-assembled, uniform, parallel pores with diameters of a few tens to a few hundreds of nanometers, high pore density, and well-defined morphology. Such porous anodic alumina (PAA) membranes could find applications in chemical and biological separations as well as for analytical purposes. Motion of molecules across these charged membranes is the basis of numerous systems of technological and biological interest. Detailed knowledge of such mass-transport behavior is therefore crucial for understanding and optimizing prospective device structures. Although many application models have been successfully established on the basis of the widely studied migration mechanism of ions or charged biomolecules in nanochannels, little is known about the motion of electrically neutral molecules on this size scale. Herein we show a distinct diffusion phenomenon of electrically neutral molecules in charged alumina nanochannels. Mass transfer across a porous membrane is governed by the Nernst–Planck equation. For a one-dimensional system, the mass transfer J along the x axis can be written as Equation (1) where D, C, and z are the diffusion coefficient,