An X-ray reflectivity study of the water-docosane interface

Synchrotron X-ray reflectivity is used to study the electron density profil e normal to the interface between bulk water and bulk n-docosane (C22H46), These measurements are interpreted in terms of an error function electron d ensity profile to yield an interfacial width of 5.7 +/- 0.2 Angstrom. In co ntrast with an earlier measurement on the water-hexane interface, this inte rfacial width disagrees sharply with the prediction from capillary wave the ory, sigma(cap) = 3.5 Angstrom. This width can be accounted for by combinin g the capillary wave prediction with a contribution from intrinsic structur e due to the bulk correlation length of docosane, We also discuss the absen ce of interfacial freezing at this interface, a phenomenon observed fur n-a lkanes of a similar chain length at the alkane-vapor and alkane-silicon oxi de interfaces.