An X-ray Reflectivity Study of the Water-Docosane Interface

Synchrotron X-ray reflectivity is used to study the electron density profile normal to the interface between bulk water and bulk n -docosane (C 22 H 46 ). These measurements are interpreted in terms of an error function electron density profile to yield an interfacial width of 5.7 ± 0.2 Å. In contrast with an earlier measurement on the water−hexane interface, this interfacial width disagrees sharply with the prediction from capillary wave theory, σ cap = 3.5 Å. This width can be accounted for by combining the capillary wave prediction with a contribution from intrinsic structure due to the bulk correlation length of docosane. We also discuss the absence of interfacial freezing at this interface, a phenomenon observed for n -alkanes of a similar chain length at the alkane−vapor and alkane−silicon oxide interfaces.