Time-Dependent Behavior of Cation Transport through Cellulose Acetate-Cationic Polyelectrolyte Membranes

Abstract

Cation transport through a cellulose acetate-poly(N,N-dimethylaminoethyl methacrylate) membrane (CA:PDMAEMA) was studied with scanning electrochemical microscope (SECM) and the thickness increase of the membrane was monitored with ellipsometry. Upon addition of the polyelectrolyte PDMAEMA, the permeability of the probe cation (ferrocenium methanol, FcMeOH) was increased as much as 40-fold. Soaking membranes in an electrolyte solution doubled the permeability in plain CA membranes, whereas for PDMAEMA containing membranes the opposite was observed and the permeability was reduced by 20-40%. This time-dependent behavior is shown to be a result of the presence of PDMAEMA within the membrane matrix, thus providing an interesting platform for controllable membrane permeability. In addition to the biological realm, membranes are used in numerous technical applications in everyday life: air filters, water purification, dialysis instrumentation, chemical synthesis etc. Moreover, the chemical composition of membranes can vary substantially, as shown by the existence of a wide range of different types of membranes such as polymer blends, 1 polymeric ion exchange, 2 electrospun fiber, 3 perovskite type ceramic 4 and liquid 5 membranes. Nevertheless, much of the research challenges in membrane technology are connected to the selective barrier properties of these materials, and in this communication, we study the permeability of cations through polymeric blend membranes.The driving force of ionic transport is the gradient of the electrochemical potential 5 that can formally be divided into concentration (c i ) and Galvani potential (φ) contributions:In the above equation, j i is the flux density (mol cm Equation 1 represents the Nernst-Planck approximation that is valid in moderately concentrated solutions (ionic strength < 1 M). Traditionally, transport through a membrane is considered to take place via the membrane pores. For porous membranes, in addition to the membrane thickness, pore size and pore density are the key factors that control the permeability. In contrast, for transport through nonporous (polymeric) membranes or membranes with a pore size < 1 nm, a solution-diffusion mechanism is suggested. 3,[6][7][8] In this model, solutes are first dissolved into the membrane matrix and then subsequently transferred through it. Partitioning of the solute between the solution and the membrane depends on properties, like their hydrophobicity, as well as the concentration (or pressure) of the solute. Transport through such membranes can also be enhanced by the carrier-facilitated mechanism 9 that commonly occurs in biological membranes where specific proteins can act as carriers.Recently, we introduced a new membrane material made from a blend of cellulose acetate (CA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) 10,11 and this membrane proved to be * Electrochemical Society Member. z E-mail: kirsi.yliniemi@aalto.fi particularly practical for the dissolution control of magnesium. The purpose of this paper is to st...