1 Impact of tunable oligo-phosphonates on barium sulfate crystallization Mark I. Ogden ^ , Colin L. Raston # , Tomoko Radomirovic ^ , Franca Jones* ^ ^Chemistry Department, Curtin University, GPO Box U1987, Perth WA 6845. # Centre for NanoScale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park, SA 5042, Australia Abstract Calixarenes can be used as well-defined scaffolds for investigating structure-activity relationships of additives and their impact on crystallization. In this work we present the crystal growth modification of barium sulfate by p-phosphonic acid calix[n]arene, which vary in size, n = 4, 5, 6 and 8, and thus vary in the size of the internal cavity for the same functionality in the upper rim. The tetrameric, hexameric and octameric macrocycles induce nanoparticle formation with clear superstructure. In the case of the hexameric calix[6]arene, the initial mesocrystalline superstructure fuses over time forming almost hollow spheres, while the mesocrystals formed in the presence of the tetramer and octamer are stable over an extended period. The pentameric calix[5]arene forms more disordered aggregates of single crystals. Thermogravimetric data show that a significant proportion of the weight of the barium sulfate containing solid is the macrocycle, regardless of the choice of macrocycle. Keywords: crystallization, crystal growth modification, additives, calixarene, barium sulfate Corresponding author contact details: Chemistry Department, Curtin University, GPO Box U1987, Perth WA 6845. Email: F.Jones@curtin.edu.au Phone: (618) 9266 7677 Fax: (618) 9266 4699