Pore conductivity control at the hundred-nanometer scale: an experimental and theoretical study.
We report on the observation of an unexpected mechanism that controls conductivity at the 100-nm scale on track-etched polycarbonate membranes. Transport measurements of positively charged methyl viologen performed by absorption spectroscopy under various pH conditions demonstrate that for 100-nm-diameter pores at pH 2 conductivity is blocked, while at pH 5 the ions move through the membrane according to diffusion laws. An oppositely charged molecular ion, naphthalene disulfonate, in the same membrane, shows the opposite trend: diffusion of the negative ion at pH 2 and very low conductivity at pH 5. The influence of parameters such as ionic strength and membrane surface coating are also investigated. A theoretical study of the system shows that at the 100-nm scale the magnitude of the electric field in the vicinity of the pores is too small to account for the experimental observations; rather, it is the surface trapping of the mobile ion (Cl- or Na+) that gives rise to the observed control of the conductivity. This surprising effect has potential applications for high-throughput separation of large molecules and bio-organisms.
Létant
Sonia E
SE
Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA. letant1@llnl.gov
Schaldach
Charlene M
CM
Johnson
Mackenzie R
MR
Sawvel
April
A
Bourcier
William L
WL
Wilson
William D
WD
D002712:Chlorides; D004058:Diffusion; D004553:Electric Conductivity; D006863:Hydrogen-Ion Concentration; D008567:Membranes, Artificial; D008956:Models, Chemical; D010269:Paraquat / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
D002712:Chlorides; D008567:Membranes, Artificial; D010269:Paraquat