Synchrotron X-ray reflectivity is used to study the electron density profil
e normal to the bulk water-hexane interface. This first measurement of the
microscopic interfacial width of a neat water-oil interface relied upon the
development of a novel technique to flatten the liquid-liquid interface. T
he measurement is interpreted in terms of an error function electron densit
y profile to yield an interfacial width of 0.33 +/- 0.025 nm, Within the co
ntext of capillary wave theory, it is shown that this microscopic parameter
is in agreement with macroscopic measurements of the interfacial tension.
These measurements are compared to computer simulations of water-alkane int
erfaces.