From nucleation to engineering of crystalline architectures at air-liquid interfaces

The packing arrangements of Langmuir films on aqueous solution of simple am phiphiles, 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 e nergy 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 he adgroup, and the binding properties thereto of solute ions and molecules fr om the aqueous subphase. The monolayer packing arrangements of amphiphilic molecules can be engineered for the performance of photoinduced topochemica l 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 molecul es 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 h eadgroups can be pinpointed by GIXD, in favorable systems. The ordered bind ing of solutes to the amphiphile monolayer can lead to induced nucleation o f oriented organic and inorganic crystals at the solution interface. GIXD h as shown that such an induction can occur via even a partial lattice match, or by structural complementarity, sometimes involving a molecular rearrang ement of the amphiphiles. It is possible from monolayer-induced crystalliza tion 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, alkan es, heterosubstituted aromatics, can be formed at the air-solution interfac e. The number of layers formed and their polymorphic behavior can be contro lled, albeit within limits, with the use of tailor-made additives. Their st ructures can be determined by GIXD, thus providing data on the initial stag es of 3-D crystalization. Multilayers, comprising water-insoluble and water -soluble components, assembled in situ at the solution surface leading to t hin film supramolecular architectures, have been engineered. These crystals have been found to be oriented vis-a-vis the solution surface and thus ame nable to characterization by GIXD and other methods.