Liquid−Solid Phase Transitions in Nanoscale Mixtures of Water and Organic Substances by the Data of NMR Spectroscopy

The process of water clustering in the interparticle gaps of hydrophilic (A-300) and hydrophobic (AM1) silicas in different media was studied using ¹H NMR spectroscopy. It has been established that when equal amounts (100 mg/g) of water and oil are introduced into the interparticle gaps of compacted hydrophilic or hydrophobic silica by grinding under the influence of mechanical load, the water transforms into a nanosized state with cluster radii in the range of 1-50 nm. In air, the main part of water is in a strongly associated state with a network of hydrogen bonds similar to liquid water. Replacing air with a chloroform medium leads to the stabilization of weakly associated forms of water, which are observed in the NMR spectra in the form of one or several signals with chemical shifts δ_H = 1-2 ppm. A comparison of the intensities of the NMR signals of water and oil allows us to conclude that the oil is partially frozen not only in air, but also in chloroform, which has unlimited solubility in relation to oil. In the medium of acetone, which is capable of dissolving both water and oil, in the interparticle gaps of hydrophobic silica, the formation of several types of clusters of strongly and weakly associated water is observed, existing as spatially separated nanodroplets, slowly (on the NMR time scale) exchanging protons or molecules with each other. It has been shown that the hydrophobic walls of silica particles have such an ordering effect on clusters of water and acetone, oil or TMS located in the interparticle gaps that a significant part of it turns into a solid state at temperatures (up to 287 K), which is several tens of degrees higher than the bulk freezing temperature.