Peculiar properties of water as solute

Water has been investigated for a long time as the most important solvent; the peculiar behavior of water as solute has been studied in binary mixtures with organic solvents, mainly exploring the whole phase diagram. In this Article, we studied the behavior of water in binary mixtures with propylene carbonate in the phase diagram region where water acts as a solute as a function of the water molar fraction X(water). Surface tension measurements, differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the state of water molecules and hydrogen bonds when water is to be considered a solute instead of a solvent, and peculiar and interesting properties were discovered. The interaction of water molecules among themselves and between water and propylene carbonate has been shown to be dependent on the water concentration in the mixtures. All of the measured properties showed a break at X(water) approximately 0.15-0.20 similar to the break due to the critical micellar concentration in surfactant solutions. In particular, from the FTIR spectra, it was possible to deduce that at this concentration water has a transition from pure solute ("multimers" solvated by PC) to cosolvent ("intermediate" and "network" water).