Critical Packing Density of Water-Mediated Nonstick Self-Assembled Monolayer Coatings

Nanoparticle-mineral surface interactions are relevant in many biological and geological applications. We have previously studied nanoparticle coatings based on closely packed bicomponent polyol-fluoroalkane self-assembled monolayers (SAMs) that can have tunable stickiness on calcite surfaces by changing the compositions of the fluoroalkanes in the SAMs, where the coatings show nonstick properties if the fluoroalkanes can effectively perturb the hydration layers on the calcite surfaces. However, when applying coatings on nanoparticles, it can be challenging to predict the maximum achievable coating density. Here, we study how would the water-mediated SAM-calcite interactions change with different SAM coating densities. Molecular dynamics simulations show that compositionally repulsive closely packed polyol-fluoroalkane SAMs become adhesive to calcite surfaces with decreasing coating densities. Our modeling shows that this results from the collapsing of the fluoroalkanes into the voids of the SAMs, where the fluoroalkanes can no longer perturb the hydration layers on calcite surfaces. Interestingly, we find that the nonstick-stick transition occurs when the volume fractions of the voids on the SAMs are greater than the volume fractions of the hydrophilic coating molecules.