Solvent effects on the gelation performance of melamine and 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester in water-organic mixtures

A two-component gel of melamine (M) and 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester (P) in binary solvent mixtures has been investigated. PM formed opaque gels at a certain water volume fraction in methanol-water mixtures. The minimum gelation concentration (MGC) decreased with increasing the water content, whereas the gel-to-sol transition temperature (T-gel) increased firstly and had a decline at 60% water content. Scanning electron microscopy (SEM) images revealed that the gel fibers formed macroscopic aggregates when an appropriate amount of water was added. When the water content was increased from 25% to 50%, the network in the mixed-solvents became more uniform and denser and when the water content was increased to 60%, fibers packed more closely together and formed a sheet-like structure. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy patterns (FTIR) of xerogels showed that the gelator had a similar packing mode in the gel state, which was different from the precipitation state. The solvent viscosity had a good correlation with SEM images and gel-to-sol temperature. Hansen solubility parameters and Flory-Huggins parameters were calculated to estimate the gelator-solvent interaction. The results indicated that gelation occurred when the gelator-solvent interaction was neither too strong nor too weak, otherwise solution or precipitate was obtained.