Study of the mechanism for controlled crystallization of BaF2 under two kinds of monolayers

Controlled crystallization of BaF2 under two different kinds of monolayers, octadecylamine [CH3(CH2)(17)NH2] and hexadecanol [CH3(CH2)(14)CH2OH], has been studied by using x-ray diffraction (XRD) and scanning electron microsc ope. It was found that the monolayer headgroup, the degree of ionization of the headgroup, etc., had a complicated effect on the selectivity of monola yers for crystal and on the morphology and orientation of crystals grown un der the compressed monolayers. At pH = 7.0, XRD analysis showed that (100)- oriented BaF2 crystals were formed under the octadecylamine monolayer, whil e several kinds of crystals were found under the hexadecanol monolayer. In comparison, at pH = 8.5, both (100)-oriented BaF2 and (111)-oriented Ba(NO3 )(2) crystals were obtained under the monolayer of octadecylamine. However, crystals formed under hexadecanol monolayer consist of BaF2, Ba(NO3)(2), e tc. The detailed mechanism for crystallization was discussed in terms of th e specific interaction and lattice matching between the monolayer headgroup and the nucleating species.