Mechanism of the pH-induced aggregation reaction between melamine and phosphate

Abstract

The mechanism of the pH-induced aggregation reaction between melamine (Mel) and phosphate (Ps) has been studied using a combination of resonance Rayleigh scattering (RRS), FTIR, UV-visible spectra, and single-crystal X-ray and powder X-ray diffraction for the first time. The results confirm that the aggregation reaction between Mel and Ps is highly pH-dependent. Changes in pH dramatically influence the molar ratio of Mel to melaminium (Hmel⁺) and subsequently affect the synergistic nature of hydrogen bonding, electrostatic and π–π stacking interactions, resulting in the reversibility of the aggregation and dissolution process. The composition of the Mel–Ps aggregates has been characterized as 2[Hmel⁺]·[HPO₄²⁻]·3H₂O. Mechanism analysis demonstrated that there exist two different methods for the transformation of Mel into Hmel⁺ by considering the aggregation reaction process and conditions.