Hydrogen bonding between protic media and organic halides has long remained a point of controversy. 1-6 While it is known that fluoride ions form the strongest hydrogen bonds [F--H--F]-, there is no clear evidence establishing the hydrogen bonding of fluorine atoms covalently bound to carbon. We feel that a resolution of this controversy will likely offer insight into a variety of fields ranging from drug design, where halogens are often substituted for strongly H-bonding OH groups, 4, 5 to coatings applications, where H-bonding interactions can largely govern interfacial properties. 7

To evaluate the degree of hydrogen bonding between fluorocarbons and protic media, we report here the wetting properties of specifically fluorinated self-assembled monolayers (SAMs) on gold. Organic thin films prepared via self-assembly are known to be highly ordered and well-defined. 8 To probe their wetting properties, we employ contact angle measurements, which are remarkably sensitive to the chemical composition and physical structure of the outermost few angstroms of organic thin films. 9 For our systematic study, we chose hexadecane, acetonitrile, N, N-dimethylformamide (DMF), water, and glycerol as contacting liquids. This combination of nonpolar (hexadecane), polar aprotic (acetonitrile and DMF), and polar protic (water and glycerol) solvents permitted us to resolve the dispersive and polar contributions to the overall interfacial properties. 10 As part of the present study, we prepared SAMs from simple alkanethiols (CH3 (CH2) nSH with n) 9-15) and their CF3-terminated analogues (CF3 (CH2) nSH with n) 9-15). 11 Previous studies with atomic force microscopy (AFM) 12 and polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) 13, 14 revealed that both types of films are highly ordered and similarly crystalline with indistinguishably different spacings between the headgroups. In Figure 1, we compare the advancing contact angles (θa) of the test liquids on the CH3-terminated SAMs to those on the CF3-terminated SAMs. For hexadecane, the contact angles are ca. 18 higher on the fluorinated surfaces than on the hydrocarbon surfaces. This result is not surprising and can be