Observation and Thermodynamic Analysis of Reversible Fluid Isobars in Isolated Single Digit Nanopore Carbon Nanotubes
Authors: Kuehne Matthias , Faucher Samuel , Misra Rahul Prasanna , Oliaei Hananeh , Li Hao-Kun , Yang Jingfan , Penn Aubrey , Tu Yu-Ming , Xu Xintong , Yuan Zhe , Li Sylvia Xin , He Guangwei , Zhang Ge , Koman Volodymyr B. , Majumdar Arun , Aluru Narayana R. , Blankschtein Daniel , Strano Michael S. 
Published: September 2023

Recent interest in one-dimensionally confined fluids, where confinement approaches molecular dimensions, has demonstrated exceptionally high fluxes from slip flow and large distortions of fluid phase boundaries [1,2]. The Center for Enhanced Nanofluidic Transport (CENT) was recently formed as an intellectual hub for studying extreme fluid confinement in what we label Single Digit Nanopores (SDNs). To this end, in this work, we note that predicting such phenomena for a given conduit dimension has been confounded by a dearth of fundamental thermodynamic measurements and analysis as a function of confinement diameter and wall composition. Herein, we utilize Raman spectroscopy and carbon nanotubes with diameters less than 3 nm to identify and study newly observed, thermally driven, fluid adsorbed phases under conditions at constant ambient pressure, tracing what we label a fluid isobar as a function of temperature. We show that the Raman radial breathing(-like) vibrational modes (RBLMs) can serve as fluid sensors to trace the dependence of the fluid isobar and allow study of basic thermodynamics of fluid phase transition as a function of conduit curvature and confinement volume. We study fluids on the exterior and interior of double- and single- walled carbon nanotubes spanning 0.9 to 3.3 nm in diameter, showing that the fluid location can be disambiguated on double walled tubes by using an adaptation of elastic shell theory that allows deconvolution of inner and outer shell frequencies with fluid location. We find that the RBLM FWHM systematically changes as a unique signature of the location of the fluid phase near the CNT wall. A Buckingham potential is used along with electron diffraction assigned DWNT to provide a high accuracy estimate of the carbon-carbon coupling constant for two elastic shells, an important structural parameter for DWNT. The thermodynamic analysis of isobars measured for more than 25 CNTs under various environmental conditions is conducted by comparison to a new fluid equation of state that provides a continuum description of the fluid isobar under confinement. Our work sets forth a new and reliable approach for studying phase change of different fluids in large numbers of isolated single digit nanopores that may inform the refinement of fluid force fields and motivate theoretical refinements to the calculation of thermodynamic properties under confinement.

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