Contact Angles of Surface Nanobubbles on Mixed Self-Assembled Monolayers with Systematically Varied Macroscopic Wettability by Atomic Force Microscopy

The dependence of the properties of so-called surface nanobubbles at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (theta(macro)) of the SAMs spanned the range between 107 degrees +/- 1 degrees and 15 degrees +/- 3 degrees. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles theta(nano) < 86 degrees were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of theta(nano) calculated from these data were found to change from 167 degrees +/- 3 degrees to 33 degrees +/- 58 degrees, when theta(macro) decreased from 107 degrees +/- 1 degrees to 37 degrees +/- 3 degrees. While the values for theta(nano) significantly exceeded those of theta(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.