The width of the water/2-heptanone liquid-liquid interface

Synchrotron X-ray reflectivity is used to study the electron density as a function of depth through the bulk water/2-heptanone interface. The measured interfacial width of 7.0 ± 0.2 Å is comparable to the value calculated from capillary wave theory (7.3 Å) using the measured interfacial tension of 12.6 mN/m. This result is consistent with capillary wave theory and molecular dynamics simulations that describe a molecularly sharp interface roughened by thermal fluctuations.