Underwater Superoleophobicity of Pseudozwitterionic SAMs: Effects of Chain Length and Ionic Strength

Surfaces with controlled oil wettability in water have great potential for numerous underwater applications. In this work, we proposed two schemes, alkyl chain length dependent and ionic strength dependent, to achieve controllable oelophobic surfaces. The underwater oil-resistant property of the obtained self-assembled monolayers (SAMs) was evaluated by using an oil droplet (1,2-dichloroethane) as a detecting probe. The oleophobicity of SAM surfaces could be modulated from superoleophilic (contact angle of ca. 0 degrees) to superoleophobic (contact angle over 170 degrees) by controlling the chain length difference between negatively charged HS(CH2)(n)COO--SAM (n = 17, 16, 14, 12, 10, 8, 6, 4) and positively charged HS(CH2)(5)N(CH3)(3)(+)-SAM. The observed phenomena could be explained by interchain interactions between charged -N(CH3)(3)(+) and -COO-, in addition with the bending effect of the long chain in mixed-charged (pseudozwitterionic) SAMs. Furthermore, the effect of ionic strength on mixed-charged SAMs (negatively charged HS(CH2)(m)COO--SAM and positively charged HS(CH2)(8)N(CH3)(3)(+)-SAM, m = 8, 7, 6, 5, 4, 3) is also studied. Higher ionic strength could promote underwater superoleophobicity to an ideal oil contact angle of 180 degrees. The additional ions markedly neutralized the effect of interchain interaction among charged head groups, which contributed to the formation of a more robust hydration network. This work provides two stratagies for preparation of hydrophilic mixed-charged surfaces with tunable underwater oleophobicity, which could not only help the fabrication of tunable underwater oil wetting surfaces, but also be potentially useful in numerous important applications, such as microfluidic devices, bioadhesion, chemical microreactors, and antifouling materials.