Selective crystallization of BaF2 under a compressed Langmuir monolayer ofbehenic acid

Selective crystallization of BaF2 crystals under a compressed Langmuir mono layer of behenic acid [CH3(CH2)(20)COOH] has been studied by using X-ray di ffraction, 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 (Ba2ClF3, BaClF, and BaF2) can be obta ined by mixing BaCl2 with a NH4F solution. However, in the presence of the monolayer of behenic acid, only BaF2 crystals appear at the monolayer-subph ase interface and crystals have a special crystal face (100). During this p rocess of crystallization, the monolayer plays a very important role and ac ts as a template that can preferentially select a special crystal and a spe cial crystal face. The above results can be explained in terms of a specifi c molecular interaction between ions and the headgroups of the monolayer an d specific electrostatic, geometric, and stereochemical interactions at the organic-inorganic interface.