The two most prominent and ubiquitous features of glasses at low temperatures, namely the presence of tunneling two-level systems and the so-called boson peak in the reduced vibrational density of states, are shown to persist essentially unchanged in highly stabilized glasses, contrary to what was usually envisaged. Specifically, we have measured the specific heat of 110 million-year-old amber samples from El Soplao (Spain), both at very low temperatures and around the glass transition Tg. In particular, the amount of two-level systems, assessed at the lowest temperatures, was surprisingly found to be exactly the same for the pristine hyperaged amber as for the, subsequently, partially and fully rejuvenated samples.

Deep eutectic solvents (DESs) are lately expanding their use to more demanding applications upon aqueous dilution thanks to the preservation of the most appealing properties of the original DESs while overcoming some of their most important drawbacks limiting their performance, like viscosity. Both experimental and theoretical works have studied this dilution regime, the so-called “water-in-DES” system, at near-to stoichiometric amounts to the original DES. Herein, we rather studied the high-dilution range of the “water-in-DES” system looking for enhanced performance because of the interesting properties (a further drop of viscosity) and cost (water is cheap) that it offers. In particular, we found that, in the “water-in-DES” system of a ternary DES composed of resorcinol, urea and choline chloride (e.g., RUChClnW, where n represents mol of water per mole of ternary DES), the tetrahedral structure of water was distorted as a consequence of its incorporation, as an additional hydrogen bond donor or hydrogen bond acceptor, into the hydrogen bond complexes formed among the original DES components . DSC confirmed the formation of a new eutectic, with a melting point below that of its respective components, the original ternary DES and water. This depression in the melting point was also observed in the same regime of reline and malicine aqueous dilutions, thus suggesting the universality of this simple procedure (i.e., water addition to reach the high-dilution range of the “water-in-DES” system) to obtain deeper eutectics eventually providing enhanced performances and lower cost.

In this work, we review, extend and discuss low-temperature specific-heat experiments, that we have conducted on different molecular (hydrogen-bonded) solids both in crystalline and glassy (either amorphous or orientationally disordered) phases. In particular, we have measured the low-temperature specific heat Cp for a set of simple, well known alcohols: glycerol, propanol and ethanol. For glycerol, we have prepared and measured Cp of both glass and crystal phases down to 0.5 K. The same has been done for propanol, in this case comparing the strikingly different values observed for the two chemical isomers, 1-propanol and 2-propanol. Moreover, ethanol exhibits a very interesting polymorphism presenting three different solid phases at low temperature: a fully ordered (monoclinic) crystal, an orientationally disordered (cubic) crystal or ‘orientational glass’ and the ordinary structural (amorphous) glass. By measuring and comparing the low-temperature specific heat of the three phases, in the boson peak range 2–10 K as well as in the tunnelling-state range below 1 K, we provide a quantitative confirmation that ‘glassy behaviour’, either concerning low-temperature properties or the glass-transition phenomenon itself, is not directly related to the lack of long-range crystalline order occurring in amorphous solids.