- The air-liquid interface is a convenient medium to obtain monolayers or multilayers. The molecular structure of such crystalline films can be established by grazing incidence X-ray diffraction (GIXD), complemented by spectroscopic techniques and lattice energy calculations. Formation of two-dimensional clusters using GIXD may provide insight into the first stages of crystal nucleation. Langmuir monolayer can serve as a template for induced nucleation of bulk crystals or to be a model system of natural membranes. A host of architectures composed of water-insoluble molecules can be formed on liquid surfaces. The number of layers and the polymorphism can be controlled. The concept of supramolecular design was applied in two-dimensions with the use of water-insoluble and water-soluble components.