Condensed matter - solids or liquids, but not gases - might be deeply, homogeneously stretched to absolute negative pressures (metastable regimes of isotropic tension). For instance, we were able to stretch macroscopically-sized liquid solutions down to p ~ - 35 MPa,^[1] a value which still constitutes a world record for large volume liquid samples. Up to now, the absolute record for neat liquids is hold for liquid water in microscopic porous crevices of natural rocks, p ~ -140 MPa.

There are several strategies to force a liquid entering into negative pressure regimes. The most common one is the Berthelot method.^[2] Negative pressures can be either directly measured - via a highly elaborated Bourdon-Berthelot combination methodology^[3,4] or safely, though approximately, estimated through the liquid’s isochoric thermal-pressure coefficient, g_v = a_p/k_T.

In this work, a broad pallete of macroscopically sized samples of ionic liquids was used, in which tensions of about -100 MPa were obtained before cavitation would occur. This value constitutes a new record for large samples. The underlying success for achieving such great absolute negative pressures in ionic liquids as compared to other liquids seems to be the best combination of (i) their negligible vapour pressure and (ii) their generally high viscosity. Good adhesion (wettability) to the container’s walls (in this case Pyrex glass) is a paramount, minimum requirement.

For the first time, NMR studies have been performed on ionic liquids under homogeneous tension. Due to potential hazard issues related to the protection/safety of the NMR instrumentation, negative pressures beyond the value of – 20 MPa were avoided. Upon entering into these metastable regimes of negative pressure, it was possible to observe a sharp increase in the ions self-diffusion coefficients as compared to the equilibrium saturated liquid conditions at the same temperatures.

MD simulations have been performed to gain insights into the ILs structure and interactions under these deep metastable conditions.

References:

1. P. Visak, L.P.N. Rebelo, J. Szydlowski, J. Phys. Chem. B 107 (2003) 9837-9846.
2. Imre, K. Martinás, L.P.N. Rebelo, J. Non-Equil.Thermodyn. 23 (1998) 351-375.
3. I.M. Veiga et al., Int. J. Thermophys. 22 (2001) 1159-1174.
4. P.N. Rebelo et al., in “Liquids Under Negative Pressure”, Eds.: A.R. Imre, H.J. Maris, and P.R. Williams, NATO Science Series, Vol. 84, Kluwer Academic Publishers, Dordrecht, pp. 95 – 108 (2002).