Nanotechnology has given us a large number of new materials with which to confine fluid phases, altering their thermodynamic and transport properties considerably. Over the past decade, my laboratory at MIT has been centrally focused on quantitatively understanding these alternations, and ultimately using them to solve longstanding research challenges. In this presentation, I will highlight our use of a distinct platform to this end – the interior of carbon nanotubes as nanofluidic conduits. This platform was utilized by our team in the Center for Nanofluidic Transport at MIT to formulate and validate the first confined fluid thermodynamic Equation of State (EOS) as a new tool with which to solve problems in nanofluidics. The thermodynamic fluid EOS is central to the design of chemical engineering unit operations, from distillation to liquid-liquid extraction. Although adsorptive processes within porous media have featured heavily in recent chemical engineering innovation, including carbon capture and energy storage devices, there is no equivalent predictive EOS for fluids subjected to nanoconfined volumes. We validate the first of such EOS using the interior of carbon nanotubes of precise diameters from 0.72 to 1.64 nm subjected to variable laser heating, measuring the interior density using the vibration of the encapsulating graphene wall to map the phase behavior versus temperature from ambient to 600 K. The resulting traces of density versus temperature at constant pressure, or isobars, are well described by a confined fluid equation state that builds on the work of Sandler and co-workers. We find that for water, the enthalpy versus entropy of confined adsorption exhibits a linear compensation, similar to what is observed for activated carbon. Reformulating the EOS for a 2D slit shaped pore is able to predict the well-known maximum in capillary pressure at 1.3 nm spacing for a rectilinear graphene pore. Isobars are measured and described even in the limit of single file water for a CNT of 0.72 nm. The validation of this EOS for nanopore systems is a significant advance in engineering prediction for the narrowest and technologically important materials.
