Glass transition of LiCl aqueous solutions confined in mesoporous silica

Abstract

The thermal transitions of confined LiCl aqueous solutions were studied by differential scanning calorimetry for solutions with salt concentrations with eutectic (R = 7) and subeutectic (R > 7) compositions (R = moles of water/moles of LiCl). The confinement media consist of mesoporous silica with pore diameters between 2 nm and 58 nm, with a small negative surface charge density. The vitrification of confined LiCl aqueous solutions was observed in all samples, expanding the vitrification region up to R = 15, and probably beyond for cooling rates of ≈1000 K/min. Ice crystallization was observed in some samples, except for those confined in the narrower pores. The onset and endpoint glass transition temperatures for the confined eutectic samples increase by 2 K and 5 K, respectively, for the smallest pore diameters (2 nm), which is equivalent to the effect of applying a pressure of up to 100 MPa to the bulk sample. This behavior is opposite of that reported for aqueous subeutectic NaCl solutions confined in silica glasses of similar sizes. We speculate that this is due to the fact that the mechanism of double confinement of the NaCl solution, between the pore wall and the precipitated ice, is not operative for LiCl solutions. Instead, the Li + ions might force the hydration water in to a high-density state.

Anion type-dependent confinement effect on glass transitions of solutions of LiTFSI and LiFSI

“…As a reference, LiCl solutions vitrify at a deeper supercooled state and experience almost negligible changes in T g when confined in nanopores (Fig. 2(f)), as reported by Corti et al [19] However, for LiFSI solutions in zones I and II, confinement significantly decreases T g at LiFSI•4.5H 2 O. This effect gradually weakens as water content increases and ultimately disappears at LiFSI•13H 2 O (see Fig.…”

“…One objective is to understand the structural and dynamic inhomogeneity in deeply supercooled glass-forming liquids. [13][14][15][16][17][18][19][20][21][22][23][24][25] It is reasonable to propose that the size of the spatial constraint should be the upper limit of the heterogeneous domains of glass forming liquids. [23,26] Hence, when it comes to confinement, it is expected that nanometer-sized pores or channels, especially with diameters less than 2 nm, will reduce the glass transition temperature T g of confined liquids, as compared to the T g of bulk liquids.…”

“…These two factors induced vitrification of eutectic NaCl solution [16] as well as relatively diluted LiCl solutions. [19] These solutions only undergo total vitrification under high pressures when in their bulk states.…”

NMR studies on the influence of silica confinements on local and diffusive dynamics in LiCl aqueous solutions approaching their glass transitions

“…Consistent with these studies at room temperature, it was found in calorimetric measurements at cryogenic temperatures that the glass transition temperature Tg is only slightly higher in such narrow silica confinements. 20 Electrolyte solutions are also interesting from a different perspective, namely, for improving our understanding of the various anomalies of water. Most researchers agree that the water anomalies originate in the supercooled liquid regime.…”

“…26,27 Against this backdrop, it is intriguing to ascertain confinement effects for water-mimetic liquids, in particular, for LiCl solutions. While the aforementioned studies addressed this problem at high or low temperatures, [17][18][19][20] it is the goal of this contribution to investigate the dynamics of glass-forming LiCl solutions in silica pores of various diameters d throughout the whole supercooled temperature range, especially near the proposed second critical point of bulk water at ∼225 K.…”