Crystalline architectures at the air-liquid interface: From nucleation to engineering

Ordered molecular clusters with a length scale down to few nanometers are currently attracting wide attention in the physical and biological sciences. The design and preparation of functional materials such as thin-layered microstructures, reagent films for biosensors, and devices for optoelectronics
requires knowledge and control of nanoarchitectures from the very early stages of self-organization. This requirement touches upon the control of nucleation, growth, morphology and structure of crystals, particularly at interfaces. The recent development of various methods for the elucidation of molecular ordering at interfaces provides a means to probe the early stages of molecular assembly. One route involves the spontaneous formation, at the air-water interface, of
crystalline films ranging from one to several layers thick, composed of water-insoluble molecules in pure form or complexed with water-soluble ions or
molecules from the aqueous subphase. In this review much use is made of grazing incidence X-ray diffraction (GIXD) using synchrotron radiation that provides structural information at the subnanometer scale of crystalline films at the air-liquid interface [1,2]. The topics described here shall encompass the spontaneous separation of racemates of amphiphilic molecules into enantiomorphous two-dimensional (2D) domains, the formation of alkane multilayers, the assembly of trilayers containing interdigitated molecules, the self-organization of supramolecular thin film architectures, the early stages of crystalline aggregation of cholesterol, and the organization of channel-forming molecules.