Crystallization of (012) oriented calcite single crystals underneath monolayers of tetra(carboxymethoxy)calix[4]arenes

Abstract

The amphiphilic 5,11,17,23-tetrakis(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(carboxymethoxy)calix[4]arene, 1, forms stable monolayers at the air–water interface which can be employed as supramolecular templates for induced calcite crystallization. Uniform, (012) oriented CaCO₃ (calcite) single crystals grow underneath monolayers of 1 at low compression (π = 0.1–0.5 mN m⁻¹), while more randomly oriented single crystals are obtained at higher surface pressure (π = 5–20 mN m⁻¹). The use of easy-to-synthesize calixarenes for this particular area of crystal engineering is reported here for the first time. A range of structural analysis methods is employed in order to obtain a refined picture of the structural factors that govern the growth of uniformly oriented calcite crystals underneath monolayers of 1. Thus, the crystal structures of 5,11,17,23-tetrakis(1,1,3,3-tetramethylbutyl)-25,26,27,28-tetra(carboxymethoxy)calix[4]arene, 1, as well as of its Ca salt, 2, were solved and analysed in terms of typical supramolecular packing arrangements and coordination motifs. The Langmuir isotherms point to a liquid-condensed state of the monolayers of 1 throughout the investigated compression range. Brewster angle microscopic observation of the monolayer morphology at low surface pressure reveals a highly viscous consistency, which does not change upon further compression. Grazing incidence X-ray diffraction (GIXD) investigations on the monolayer structure provide no indications for the occurrence of a two-dimensional lattice of the alkyl chains. Considering the non-crystalline, highly dynamic state of the monolayer during crystal maturation, an epitaxial correlation based on geometric matching of lattice positions between the monolayer and the overgrowing calcite crystals is ruled out. We, therefore, suggest that non-specific monolayer properties such as average charge density or mean dipole moment are the main determinants for templated calcite growth in the present and related cases.