Studying the Crystallization of Polyoxometalates from Colloidal Softoxometalates

Abstract

Understanding crystallization of polyoxometalates (POMs) is challenging. The chemistry of the reactive oxometalates renders any physical modeling impossible as it is not clear how the speciation in solution occurs. In this study we circumvent this problem by using a model system of ammonium heptamolybdate tetrahydrate which has a robust molecular structure and shows phase changes from true solution to colloidal softoxometalates (SOMs) to crystals as a function of concentration or volume fraction without any chemical changes (problem of speciation) in the system. Temperature and ionic strength variation studies have been conducted with respect to volume fraction to construct a phase diagram showing the transitions from true solution to colloidal SOMs to crystals. The stabilization of the SOM phase has been shown to take place via the established counterion condensation model in colloidal oxometalates. The transition from colloidal to crystalline phase has been observed at volume fractions of 0.1 and 0.19 at elevated temperature. These phase transitions have been studied using a laser set up of Mueller matrix polarimetry, Raman spectroscopy, and electron spray ionization mass spectrometry. The crystallization of POMs from the colloidal SOM phase has been tested at the corresponding volume fractions by measurement of the osmotic compressibility of the colloidal SOM phase by light scattering using the Baxter type model. The study thus puts forward a clear picture in the matter of crystallization of POMs. When chemical speciation does not occur in the system, oxometalates form colloids or the SOM phase which undergoes a phase transition to the crystalline POM phase. This study thus leads to the immediate causal conclusion that all POM crystallizations are in fact phase transitions from colloidal (SOM) to crystalline phase once the chemistry of speciation is complete. Hence understanding the colloidal SOM phase in detail could lead to crystal engineering of the POMs in a facile and controlled manner.