Influence of nanoscale roughness on Derjaguin-Landau-Verwey-Overbeek forces between alkanethiol self-assembled monolayer surfaces

This study addresses the knowledge gap regarding how nanoscale surface roughness and chemical functionalization impact the Derjaguin-Landau-Verwey-Overbeek (DLVO) forces. To this end, force measurements in a spherical-plate geometry were carried out using an atomic force microscope (AFM) on 1-hexadecanethiol self-assembled monolayers (SAMs) on nano-roughened gold (Au) surfaces prepared by the template-stripping method. The measurements were performed in unsalted (DI water) and salted solutions ($[\mathrm{NaCl}]=0.1$ mM) between surfaces of varying roughness. The experimental force curves were compared with the predictions of DLVO theory using a maximum likelihood method to extract the essential parameters describing the electrical double layer (i.e., the Debye length ${\ensuremath{\lambda}}_{D}$ and the surface potentials $\phantom{\rule{4pt}{0ex}}{\ensuremath{\psi}}_{1},\phantom{\rule{4pt}{0ex}}{\ensuremath{\psi}}_{2}$) under different extrapolation assumptions for the van der Waals--Casimir contribution, namely the Drude and Plasma models. For nonfunctionalized Au surfaces both in DI water and in the salted solution, the repulsive electrostatic double-layer force increased with increasing roughness. Moreover, the SAM in DI water modified the surface characteristics reduced the Debye length, where the effect was more pronounced on the rougher surface. In contrast, the SAM in the salted solution displayed a more complicated trend. Finally, no statistically significant differences were observed between the predictions using the two extrapolation models for the van der Waals--Casimir force. However, the Drude model systematically yielded higher likelihood values suggesting that it may better represent the experimental data for liquid media.