- The packing arrangements of Langmuir films on aqueous solution of simple amphiphiles, such as fatty acids, alcohols, amides, and amino acids, are now established to near atomic resolution by the method of grazing incidence X-ray diffraction (GIXD), complemented by various spectroscopic and lattice energy computational techniques. For simple aliphatic chainlike amphiphilic molecules, it is possible to correlate the extent of two-dimensional (2-D) crystallinity of the Langmuir film with molecular interactions, in terms of the nature and length of the hydrophobic chain, the type of hydrophilic headgroup, and the binding properties thereto of solute ions and molecules from the aqueous subphase. The monolayer packing arrangements of amphiphilic molecules can be engineered for the performance of photoinduced topochemical reactions, and characterized by GIXD. Racemic mixtures of amphiphiles can also be engineered, by taking advantage of intermolecular hydrogen bonding, to undergo a spontaneous separation of the left- and right-handed molecules into 2-D chiral crystals at the air−solution interface. The geometry of binding of molecules or ions from the aqueous subphase to the hydrophilic headgroups can be pinpointed by GIXD, in favorable systems. The ordered binding of solutes to the amphiphile monolayer can lead to induced nucleation of oriented organic and inorganic crystals at the solution interface. GIXD has shown that such an induction can occur via even a partial lattice match, or by structural complementarity, sometimes involving a molecular rearrangement of the amphiphiles. It is possible from monolayer-induced crystallization to glean information on the process of nucleation and on the critical size of the nuclei. A variety of different types of crystalline multilayers, composed of water-insoluble molecules such as bolaform amphiphiles, alkanes, heterosubstituted aromatics, can be formed at the air−solution interface. The number of layers formed and their polymorphic behavior can be controlled, albeit within limits, with the use of tailor-made additives. Their structures can be determined by GIXD, thus providing data on the initial stages of 3-D crystalization. Multilayers, comprising water-insoluble and water-soluble components, assembled in situ at the solution surface leading to thin film supramolecular architectures, have been engineered. These crystals have been found to be oriented vis-à-vis the solution surface and thus amenable to characterization by GIXD and other methods.