Molecular Mobility under Nanometer Scale Confinement

Abstract

The mobility of organic molecules under nanoscale confinement differs greatly from that in the bulk. In this study we show that the conventional free volume dependent mobility relationship explained by the free volume theory of diffusion breaks down for diffusion of linear alkane molecules in organosilicate films with connected nanoporosity. Alkane mobility under such nanoscale confinement was observed to decrease with chain length and was lower than that reported in the bulk. While the activation energy for diffusion was similar to that in the bulk, it was found to decrease with chain length exactly opposite to the trend observed in the bulk. This suggests an increasing molecular free volume with chain length. The effects of molecular polarity and pore size on diffusion were also demonstrated. Molecular mobility was found to be suppressed with increasing molecular polarity and decreasing pore size.