Molecular-Scale Topographic Design of Multicomponent Self-Assembled Monolayers for Environmentally Adaptable, Lyophobic Coating

Hypothesis The water wettability of a hydrophilic surface is dictated by the hydration of the region less than 1nm below the surface plane, yet it hitherto remains a topic rarely touched upon how a subtle change in surface structural features at molecular level affects the surface wettability and its response to the environmental nature. Experiments Binary self-assembled monolayers (SAMs) consisting of plain and functional thiols were constructed, where the surface fraction of the functional thiols was varied from 0.1 to 0.3, 0.6, 0.8 and 1.0 and the difference in CH2 unit number between two types of thiols – defined as height difference between surface polar (OH, NH2, COOH, or H2PO3) and non-polar (CH3) groups – from 0 to 1, 3, and 5. The surface wettability of as-prepared binary SAMs and their surface energy were assessed in both air/water/solid and oil/water/solid triphasic systems. Findings With the relative height of surface H2PO3 groups being deliberately set as 3 in particular, as-prepared binary SAMs gain unique environmentally adaptable surface lyophobicity, namely, they can be both oleophobic in water and hydrophobic in oil. Thanks to this new environmentally adaptable surface lyophobicity, such binary SAM coating enables copper meshes to realize selective and efficient oil/water separation without need of pre-wetting. Both the polar and dispersion interaction components of surface energy of the binary SAM coatings are found crucial to effective oil/water separation of the coated copper meshes; the former defines critical intrusion pressure and the latter must be larger than 22 nN/m.