Measuring Hydrophobic Interactions with Three-Dimensional Nanometer Resolution

We investigate forces between two nanoscopic hydrophobic surfaces under pure water. One of the surfaces is a multiwall carbon nanotube AFM tip with a radius of curvature of 8.5 nm, the other a hydrophobic domain in a mixed self-assembled monolayer. The monolayer has domains consisting of hydrophobic dodecanethiol, surrounded by domains of shorter alkanethiols with hydrophilic end groups. On samples with larger hydrophobic domains (30−80 nm), nanobubbles cover the surface when immersed in water. On samples with smaller domains (10−50 nm), nanobubble formation is greatly diminished, and it is possible to do force measurements that are unaffected by nanobubbles. By using dynamic AFM at a frequency of 1.16 Mhz, which is at least 20 times higher than commonly used in liquid, force−distance profiles are measured with high spatial resolution. On hydrophobic domains we find an attractive hydration force from distances of 5 nm and closer that reaches a maximum of 0.1 nN at a separation of 1.5 nm. Surprisingly, we see a smooth transition to repulsive forces at closer separations. Attractive forces are well-fitted by an exponential decay with 0.6 nm decay length, and we find no evidence for a long-range (>5 nm) attractive force on these samples.