Hydrophobicity of Perfluoroalkyl Isocyanate Monolayers on Oxidized Aluminum Surfaces

• Published in: Langmuir 2009-06, Vol.25 (12), p.6834-6840
• Main Authors: Hozumi, Atsushi, Kim, Bokyung, McCarthy, Thomas J
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Interfaces: Adsorption, Reactions, Films, Forces; Surface physical chemistry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la804127z

Perfluoroalkyl isocyanate-derived monolayers were prepared on oxidized aluminum surfaces by chemical vapor deposition (CVD) of 1H,1H,2H,2H-perfluorodecyl isocyanate (CF3[CF2]7CH2CH2 NCO, i.e., RF-NCO). Two types of oxidized aluminum substrates were studied: electrochemically polished aluminum plates (AlAl2 O3 ) and AlAl2 O3 that was treated with boiling water (BWAlAl2 O3 ). X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) confirmed that this CVD method produces a monolayer with a thickness of ∼1.2 nm on AlAl2 O3 through a carbamate linkage without discernible change in surface morphology. After monolayer deposition, hydrophilic AlAl2 O3 surfaces become hydrophobic. Advancing and receding water contact angles are θA/θR = 125°/103°. AFM indicates that BWAlAl2 O3 surfaces contain 10−15-nm-size pebble-like features. This is due to both chemical and physical erosion of the aluminum surface caused by the boiling water treatment. Due to the presence of these topographical features, the CVD of RF-NCO on BWAlAl2 O3 induces superhydrophobicity (θA/θR = 167°/165°). AlAl2 O3 and BWAlAl2 O3 based “control surfaces” were prepared with the conventional mono- and trifunctional organosilanes, 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (CF3[CF2]7CH2CH2Si[CH3]2Cl, RF-SiMe2Cl) and 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (CF3[CF2]7CH2CH2Si[OCH3]3, RF-Si[OMe]3) for comparison. In the case of RF-Si(OMe)3, the θA/θR values for AlAl2 O3 and BWAlAl2 O3 are 125°/110° and 170°/167°, respectively. RF-SiMe2Cl-treated aluminum surfaces are much less hydrophobic. The θA/θR values for AlAl2 O3 and BWAlAl2 O3 are 107°/69° and 137°/87°, respectively. XPS analysis of surface coverage is consistent with the contact angle behavior and the water sensitivity. Stability of these three monolayers in water is in the order: RF-Si(OMe)3 > RF-NCO ≫ RF-SiMe2Cl.