X-Ray Measurements of Noncapillary Spatial Fluctuations from a Liquid Surface

Off-specular diffuse x-ray scattering measurements on both pure water and a homogeneous Langmuir monolayer of poly- $\gamma$-benzyl- $L$-glutamate (PBLG) on water establish the validity of a proposed sum rule for scattering from capillary fluctuations on liquid surfaces. Excess scattering above the predicted capillary contribution is observed when the PBLG monolayer is compressed beyond its elastic limit. This is interpreted in terms of a second-layer inhomogeneity with a surface correlation length of $\sim 1000Å$. Excess off-specular scattering can be used to probe interface correlation lengths from 100 Å to $1\mu \mathrm{m}$.