Abstract
Dynamic mechanical analysis (DMA) measurements of water confined in nanoporous silica have been performed as a function of temperature and frequency for different pore sizes (2.5-10 nm) at heating and cooling. Most of the data show three processes, P1, P2 and P3, where P1 and P2 depend on measurement frequency and P3 does not. The characteristic shift of P3 with pore size shows that this process corresponds to freezing/melting of "internal water", i.e. in the core of the pores. Thermal expansion data indicate - in agreement with e.g. [A. Taschin, P. Bartolini and R. Torre, Meas. Sci. Technol., 2017, 28, 014009] - that in all our nanoporous systems about 2 layers of water remain liquid much below the freezing point. Dynamic elastic measurements show clear signatures of glass freezing of this supercooled water in the vicinity of P1. Extrapolating the DMA data to the timescale (103 s) of adiabatic calorimetry unveils a systematic behaviour: P1(T) shows a clear size dependence for a broad range of pore diameters, i.e. 2.5 nm ≤ d ≤ 52 nm, implying (together with the corresponding activation energy 0.5 eV) that P1 corresponds to the glass-liquid transition of a few layers of supercooled water at Tg(d). An extrapolation of Tg(d) to d → ∞ yields Tg(∞) ≈ 136 K, the traditional value for bulk water. The small (liquid like) value of Young's modulus in a temperature region above P1 is most naturally explained assuming that the supercooled water in this range is still liquid, implying that Tg values of 160 K or even 210 K - as suggested by various authors - are unlikely.
Highlights
Water is of fundamental importance for all living organisms as well as for abiotic environments
Dynamic mechanical analysis (DMA) measurements of water confined in nanoporous silica have been performed as a function of temperature and frequency for different pore sizes (2.5–10 nm) at heating and cooling
In vapour-deposited amorphous solid water (ASW)[2] as well as in rapidly quenched water below 100 K3,4 a glass to liquid transition was reported from calorimetric measurements to occur at Tg = 136 K, which was assigned to the bulk glass transition temperature of water
Summary
Water is of fundamental importance for all living organisms as well as for abiotic environments. Equal to the size d of the confinement, which limits the characteristic length at a constant value x(TFS) = d and as a result the high-temperature super-Arrhenius dependence (Vogel–Fulcher, power-law) of ta transforms to a low-temperature Arrhenius behavior tb This scenario would imply that the dynamic correlation length x of water should be in the range of 2 nm around TFS E 180 K. In another approach some authors show[22,23] that a powerlaw B(T À Tx)Àg fits the observed super-Arrhenius behaviour of viscosity Z or relaxation time t better than a Vogel–Fulcher law.
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