Influence of pore geometry on the state of bulk pore water in the pressure-temperature phase space

Abstract

          In recent years, the existence of a second critical point of the liquid-liquid transition of water has been proven. In the pressure-temperature phase space, this point is located in the temperature range –50 °С... –100 °C and at pressure ~ 100 MPa. The exact position of this point is not yet known due to experimental difficulties in achieving the deep supercooling of bulk water. The Widom line, the locus of increased fluctuations in entropy and density, is associated with the second critical point. When approaching the Widom line, a sharp increase in a number of physical quantities was established: heat capacity at constant pressure, isothermal compressibility, volume expansion coefficient. However, the practical significance of these features is not clear, since for pressures close to atmospheric, the temperature on it is –45 °C. At the same time, it
is known that at temperatures below – 41 °C (homogeneous nucleation temperature), chemically pure supercooled bulk water is unstable due to the very rapid formation of ice crystal nuclei. Nevertheless, supercooling of bulk water to –70 °C in nanometre-sized pores is known.
          In the present study, we investigated the possibility of reaching the state on the Widom line at negative pressures, for which, theoretically, the temperature of such a state becomes higher than –45 °C and can reach it positive values at a pressure of –100 MPa. Such a state, in this study, is assumed in the cylindrical hydrophilic pores with a diameter of several nanometres. For the investigation of this possibility and the achievable values of negative pressure (and high temperatures on the Widom line), we measured the low-frequency impedance of a cooled capacitive cell filled with a moistened MCM-41 nanoporous material. In addition, the thermal characteristics were measured in the form of a temperature response of the
medium from a pulsed spot heater at a certain distance from it. The position of the Widom line, associated with the second critical point of water, was determined based on the anomalies of the measured physical values in the temperature range –50 °С…+10 °C. For MCM-41 with an average pore diameter of 3.5 nm, dielectric and thermal extrema were found near –18 °C, which corresponds to a pressure of about –65 MPa.
          Thus, the performed experiments have shown the possibility of reaching the state on the Widom line at temperatures characteristic of ordinary conditions. Consequently, a significant change in the physicochemical characteristics of dispersed moistened media in various natural and artificial objects is possible. The study of other sorbents with cylindrical pores in order to achieve positive temperatures on the Widom line is of interest.