Aqueous self-assembly of arginine and K8SiW11O39: fine-tuning the formation of a coacervate intended for sprayable anticorrosive coatings†
Coacervates are commonly thought to be formed from the liquid–liquid phase separation of macromolecules, such as oppositely charged polyelectrolytes, proteins or peptides. Unlike conventional systems, we here show an entirely novel coacervate obtained from the self-assembly of arginine (Arg) and K8[α-SiW11O39] (SiW11) in water. The formation of the coacervate Arg/SiW11 is confirmed by combined techniques, including turbidity, rheology, optical microscopy, and scanning and transmission electron microscopy. Assessment of the rheological response reveals that the complex coacervate exhibits shear thinning behaviour. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, elemental analysis and thermogravimetric analysis are used to characterize the coacervate. The thermodynamic parameters of the coacervation are monitored by isothermal titration calorimetry (ITC), which identifies that the formation of the coacervate by mixing of Arg and SiW11 is driven by a combination of entropic and enthalpic effects. The resultant coacervate shows a typical upper critical solution temperature (UCST) phenomenon, which is strongly dependent on the concentration of the species. Furthermore, we demonstrate that the coacervation could be tuned by stoichiometry and pH. A phase diagram for the complexation of Arg and SiW11 thus has been constructed using turbidity measurements. Such a phase diagram is a very useful tool for the preparation of coacervates from a specific combination of Arg and SiW11. Finally, the acid induced gelation of the coacervate has been explored to fabricate an anticorrosive coating to protect a copper plate from exposure to acid vapour.