Selective Crystallization of BaF2 under a Compressed Langmuir Monolayer of Behenic Acid

Selective crystallization of BaF 2 crystals under a compressed Langmuir monolayer of behenic acid [CH 3 (CH 2 ) 20 COOH] has been studied by using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analysis. It was found that, in the absence of a monolayer, three kinds of crystals (Ba 2 ClF 3, BaClF, and BaF 2 ) can be obtained by mixing BaCl 2 with a NH 4 F solution. However, in the presence of the monolayer of behenic acid, only BaF 2 crystals appear at the monolayer−subphase interface and crystals have a special crystal face (100). During this process of crystallization, the monolayer plays a very important role and acts as a template that can preferentially select a special crystal and a special crystal face. The above results can be explained in terms of a specific molecular interaction between ions and the headgroups of the monolayer and specific electrostatic, geometric, and stereochemical interactions at the organic−inorganic interface.