The puzzling first-order phase transition in water–glycerol mixtures

Abstract

Over the last decade, discussions on a possible liquid-liquid transition (LLT) have strongly intensified. The LLT proposed by several authors focused mostly on explaining the anomalous properties of water in a deeply supercooled state. However, there have been no direct experimental observations yet of LLT in bulk water in the so-called 'no man's land', where water exists only in the crystalline states. Recently, a novel experimental strategy to detect LLT in water has been employed using water-glycerol (W-G) mixtures, because glycerol can generate a strong hindrance for water crystallization. As a result, the observed first-order phase transition at a concentration of glycerol around cg≈ 20 mol% was ascribed to the LLT. Here we show unambiguously that the first order phase transition in W-G mixtures is caused by the ice formation. We provide additional dielectric measurements, applying specific annealing temperature protocols in order to reinforce this conclusion. We also provide an explanation, why such a phase transition occurs only in the narrow glycerol concentration range. These results clearly demonstrate the danger of analysis of phase-separating liquids to gain better insights into water dynamics. These liquids have complex phase behavior that is affected by temperature, phase stability and segregation, viscosity and nucleation, and finally by crystallization, that might lead to significant misinterpretations.

2H NMR Studies on the Dynamics of Pure and Mixed Hydrogen-Bonded Liquids in Confinement

“…In Figure 4a, we observe that 〈T 1 〉 is highly comparable in all studied samples, including bulk glycerol, implying that glycerol reorientation is basically ( ) F t for bulk and confined glycerol at 204 K together with Kohlrausch fits and (b) correlation times τ p of bulk and confined glycerols from SLR and STE studies together with DS data for the bulk liquid from the literature [13]. In the SLR analyses, CD and HN spectral densities were used for bulk and confined glycerol, respectively.…”

“…[42]. In addition, we study the dependence of protein dynamics on the composition and, thus, on the viscosity of the solvent, by applying 13 C NMR to the above described mixtures of elastin, glycerol, and water [33]. ( ) F t with Kohlrausch fits.…”

“…For the most part, we utilize 2 H NMR to investigate rotational motion of deuterated compounds. In doing so, we combine spin-lattice relaxation (SLR) with stimulated-echo (STE) measurements to cover a broad dynamic range, ~10 −10 −10 0 s. In addition, 13 C NMR line-shape analysis (LSA) is performed to study protein dynamics. Throughout this contribution, we strive for a broad overview by comparing previous findings with new results.…”

“…Such HB mixtures often do not obey simple solution behavior. In particular, in alcohol-water solutions, coexistence of polar and apolar groups results in complex structural and dynamical properties as well as delicate phase behaviors, including micro-phase segregation [11][12][13]. For example, concentration-dependent maxima of structural relaxation times τ and glass transition temperatures T g were observed [14].…”

“…Then, the intensity and width of the observed resonances depend on molecular dynamics. Specifically, 13 C CP MAS NMR spectra probe molecular reorientation on a time scale 10 −5 s via a broadening of the lines and a reduction of their intensities. …”