Single-Molecule Perspective on Mass Transport in Condensed Water Layers over Gradient Self-Assembled Monolayers

Abstract

The mass transport of hydrophobic probe molecules on gradient self-assembled monolayers (SAMs) is investigated in the presence of nanometer-thick layers of water condensed from the ambient atmosphere at low, moderate, and high relative humidity (RH). SAM gradients are prepared by vapor-phase deposition of 3-cyanopropyltrichlorosilane (CN) and octyltrichlorosilane (C8) onto silica and silicon substrates. The two-component gradients thus obtained run in opposite directions, with high CN coverage where C8 coverage is low. Gradient formation is confirmed by static water contact angle measurements. Ellipsometry is used to measure the thickness of both the gradient film and the condensed water layer. Atomic force microscopy demonstrates that the water layer comprises micrometer-sized islands of nanometer thickness over hydrophilic regions but is difficult to observe over hydrophobic regions. The diffusion of dye molecules along the gradient is studied by single-molecule tracking using a fluorescent perylene dii...