Noncapillary-wave structure at the water-alkane interface

Synchrotron x-ray reflectivity is used to study the interface between bulk water and bulk n-alkanes with carbon numbers 6 through 10, 12, 16, and 22. For all interfaces, except the water-hexane interface, the interfacial widt h disagrees with the prediction from capillary-wave theory. The variation o f interfacial width with carbon number can be described by combining the ca pillary-wave prediction for the width with a contribution from intrinsic st ructure. This intrinsic structure is determined by the gyration radius for the shorter alkanes and by the bulk correlation length for the longer alkan es.