The interfacial structures of a solution-cast stearic acid film before and after immersion in water were studied by X-ray scattering combined with atomic force microscopy. It is shown that the interfaces of the as-deposited film are self-affine. After the film is immersed in water for a long time, water molecules penetrate into the hydrophilic interfaces, enlarging the distance between the adjacent bilayers. Theoretical fit indicates that the film takes on a liquid crystal character after being swollen by water. The interfaces in both cases are partially correlated and are well represented by the given correlation functions, except for a small discrepancy between the theory and the experiment on length scales shorter than 60 nm. The results help to provide a deeper insight into the interfaces of lipid multilayers during the intercalation of small molecules.