Interfacial electrostatics guiding the crystallization of CaCO3 underneath monolayers of calixarenes and resorcarenes

Abstract

The amphiphilic octaacids rccc-4,6,10,12,16,18,22,24-octakis-O-(carboxymethyl)-2,8,14,20-tetra(n-undecyl)resorc[4]arene (1) and 5,11,17,23,29,35,41,47-octakis(1,1,3,3-tetramethylbutyl)-49,50,51,52,53,54,55,56-octa(carboxymethoxy)calix[8]arene (2) form stable monolayers at the air–water interface which induce growth of CaCO₃ crystals at the monolayer–solution interface. Uniformly (012) oriented rhombohedral calcite single crystals grow underneath a monolayer of 2, whereas crystallization under a monolayer of 1 preferentially leads to formation of acicular aggregates of aragonite crystals. While polymorph selection and orientations of the CaCO₃ crystals critically depend on the average charge density of the monolayer, the molecular shape and the particular Ca coordination behaviour of the amphiphiles that form the monolayer are of minor importance. CaCO₃ crystal growth underneath monolayers of macrocyclic amphiphiles is briefly reviewed and the present experimental observations are compared to previous related investigations on “template-induced” biomimetic mineralization.