Fluoroalkylated Silicon-Containing Surfaces – Estimation of Solid Surface Energy

By Shreerang S. Chhatre, Jesus O. Guardado, Brian M. Moore, Timothy S. Haddad, Joseph M. Mabry, Gareth H. McKinley, and Robert E. Cohen

The design of robust omniphobic surfaces, which are not wetted by low surface tension liquids such as octane (γ_lv = 21.6 mN/m) and methanol (γ_lv = 22.7 mN/m), requires an appropriately chosen surface

micro/nano-texture in addition to a low solid surface energy (γ_sv). 1H,1H,2H,2H-Heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) offers one of the lowest solid surface energy

values ever reported (γ_sv ≈ 10 mN/m) and has become the molecule of choice for coating textured surfaces. In this work, we synthesize and evaluate a series of related molecules that either retain the POSS cage and differ in fluoroalkyl chain length or that retain the fluorodecyl chains surrounding a linear or cyclic molecular structure. The solid surface energy (γ_sv) of these molecules was estimated using contact angle measurements on flat spin-coated silicon wafer surfaces. Zisman analysis was performed using a homologous series of n-alkanes (15.5 γ_lv 27.5 mN/m), while Girifalco-Good analysis was performed using a set of polar and non-polar liquids with a wider range of liquid surface tension (15.5 γ_lv 72.1 mN/m). The hydrogen bond donating, hydrogen bond accepting, polar and nonpolar (dispersion) contributions to the solid surface energy of each compound were determined by probing the surfaces using a set of three liquid droplets of either acetone, chloroform and dodecane or diiodomethane, dimethyl sulfoxide and water.