Experimental Measurement of Extreme Phase Transition Temperatures for Water Confined inside Carbon Nanotubes

Fluid phase transitions inside single, isolated carbon nanotubes (CNT) are predicted to deviate substantially from classical thermodynamics and also allow the study of ice nanotube (ice-NT) properties.  Herein, we measure, using two different techniques, the diameter dependent phase boundaries of ice-NTs within isolated CNTs 1.05, 1.06, 1.15, 1.24, and 1.52nm in diameter using Raman spectroscopy.  The results reveal both an exquisite sensitivity to diameter and substantially larger temperature elevations of the melting transition than theoretically predicted by as much as 100°C.  Dynamic water filling and reversible freezing transitions were marked by 2 to 5cm^-1 shifts in the radial breathing mode (RBM) frequency, revealing reversible melting at 138°C and 102°C for 1.05 and 1.06nm single and double-walled CNTs, respectively.  A near-ambient phase change at 15°C was observed for 1.52nm CNT, whereas freezing inside 1.24nm tube was suppressed at -30°C.  We find that the interior aqueous phase also decreases the axial thermal conductivity of the CNT reversibly by as much as 500%, allowing digital control of the heat flux.  These extreme phase transitions enable the study of ice-NT at high temperatures and their potential utilization as novel phase change materials.