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

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 diimide dye as the probe molecule. The results depict immobile and mobile dye populations that vary with position and RH. The dye molecules are found to be mostly immobile at 16% RH, near the C8 end of the gradient. Their mobility increases toward the CN end and with increasing RH. Single-frame step-size distributions provide evidence that mass transport occurs by both Fickian and desorption-mediated (Levy) mechanisms. Contributions from the latter become more prevalent at the C8 end of the gradient and at high RH. The results of these studies will aid in the development of SAMs for use as molecularly engineered surfaces.