Dynamic Contact Angle Studies of Self-Assembled Thin Films from Fluorinated Alkyltrichlorosilanes

Dynamic contact angle studies on self-assembled thin films from C 7 F 15 CH 2 OCH 2 CH 2 CH 2 SiCl 3 reveal a range of interesting behavior. Solution-based processing conditions have been identified that allow preparation of essentially monolayer films on quartz exhibiting water adv/rec contact angles of 119/104 ± 2° and extremely low contact angle hysteresis (hexadecane adv/rec = 74/70°, heptane adv/rec = 56/55°) with hydrocarbon liquids. This compound provides an example of a fluorinated trichlorosilane that is able to deliver low-hysteresis films by deposition at room temperature. Adsorption of silane oligomers, formed by hydrolysis and condensation reactions in solution, was also found to occur, slower than but competitive with monolayer formation. This process became more significant as dip coating times increased. Ellipsometric data on silicon wafers confirmed that film thicknesses increased with dip time, while AFM imaging showed that the oligomeric material was deposited in the form of particulates. The effects of this process on water dynamic contact angles are discussed. We also compare contact angles on these films with those on films prepared from C n F 2 n +1 CH 2 CH 2 SiCl 3 ( n = 6, 8, 10) and draw some conclusions regarding structure−property effects in these systems. Finally, we propose a mechanism that can account qualitatively for the bulk of the results observed here. Its central feature is reaction of the fluorinated alkyltrichlorosilane with surface-adsorbed water to yield a self-assembled monolayer consisting of silanetriol molecules hydrogen-bonded to the substrate. Effects of high humidity aging on dynamic contact angles of these films suggest that they are at most only lightly cross-linked when prepared under conditions utilized here.