Direct observation of reversible liquid–liquid transition in a trehalose aqueous solution

Abstract

Water forms two glassy waters, low-density and high-density amorphs, which undergo a reversible polyamorphic transition with the change in pressure. The two glassy waters transform into the different liquids, low-density liquid (LDL) and high-density liquid (HDL), at high temperatures. It is predicted that the two liquid waters also undergo a liquid–liquid transition (LLT). However, the reversible LLT, particularly the LDL-to-HDL transition, has not been observed directly due to rapid crystallization. Here, I prepared a glassy dilute trehalose aqueous solution (0.020 molar fraction) without segregation and measured the isothermal volume change at 0.01 to 1.00 GPa below 160 K. The polyamorphic transition and the glass-to-liquid transition for the high-density and low-density solutions were examined, and the liquid region where both LDL and HDL existed was determined. The results show that the reversible polyamorphic transition induced by the pressure change above 140 K is the LLT. That is, the transition from LDL to HDL is observed. Moreover, the pressure hysteresis of LLT suggests strongly that the LLT has a first-order nature. The direct observation of the reversible LLT in the trehalose aqueous solution has implications for understanding not only the liquid–liquid critical point hypothesis of pure water but also the relation between aqueous solution and water polyamorphism.

Transformation process of ice crystallized from a glassy dilute trehalose aqueous solution

“…The PXRD patterns of HDS and LDS are similar to those of HDA and LDA of pure water, respectively indicating that the HDS-to-LDS transition of TRaq solution is related to the polyamorphic transition of pure water (HDA-to-LDA transition). 37 The LDS crystallized at B160 K, and the T x is consistent with the previous study on the liquid-liquid transition of TRaq solution. 37 In the previous high-pressure experiment, 37 the sample immediately after crystallization amorphized to HDS around 0.3 GPa when compressed at B160 K. The polyamorphic transition in this amorphized HDS was the same as that in the original HDS made by PLCV.…”

“…37 The LDS crystallized at B160 K, and the T x is consistent with the previous study on the liquid-liquid transition of TRaq solution. 37 In the previous high-pressure experiment, 37 the sample immediately after crystallization amorphized to HDS around 0.3 GPa when compressed at B160 K. The polyamorphic transition in this amorphized HDS was the same as that in the original HDS made by PLCV. This indicates that the trehalose solutes are dispersed homogeneously in the sample immediately after the crystallization because the polyamorphic transition depends on the solute concentration.…”

“…The HDS-to-LDS transition and the crystallization of LDS are consistent with the results in the previous study. 37 As the temperature increased further, the appearance of the crystalline sample became a little cloudy, but the sample was transparent enough to see a copper plate behind the sample. The sample suddenly changes from transparent to opaque around 250 K, suggesting the occurrence of macroscopic segregation.…”

“…31,32 Previously, we prepared a glass of dilute aqueous solution, in which the solute molecules dispersed homogeneously, by cooling at 0.3 GPa (the pressure liquid cooling vitrification: PLCV) and studied the polyamorphic behaviors in glassy aqueous solution. [33][34][35][36][37] The solvent water in glassy dilute aqueous solution made using the PLCV method is similar to a high-density amorph (HDA) of pure water. The glassy aqueous solution undergoes the reversible polyamorphic transition between lowdensity and high-density states with the change of pressure, which is related to the polyamorphic transition between lowdensity amorph (LDA) and HDA of pure water.…”

“…38 Recently, the polyamorphic transition in the trehalose aqueous solution was examined and the direct observation of the reversible liquid-liquid transition has been reported. 37 On the other hand, we have examined the transformation of ice formed in the dilute glycerol aqueous solution by heating using powder X-ray diffraction (PXRD) measurements. 31,32 When high-density glass of the glycerol aqueous solution (0.07 molar fraction), which is prepared using the PLCV method, is heated from 77 K, it transforms into a low-density glassy aqueous solution at B130 K. By heating up to B148 K, the low-density glassy solvent water crystalizes to nano-sized ice Isd, that is with few stacking faults and has highly cubicity.…”