Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.