Thermodynamics of Water Phase Change in Isolated Single Digit Nanopores

With support from the Chemical Structure and Dynamics (CSD) program in the Division of Chemistry, Professor Matthias Kuehne of Brown University is investigating the thermodynamics of water phase change under extreme nanoconfinement in pores with diameters on the order of 1 nm. The behavior of water in such nanometer-sized cavities is known to deviate strongly from bulk properties, yet prior studies of confined water have reported widely divergent phase transition temperatures for a given pore size. Professor Kuehne and his students will use a unique combination of photoluminescence spectroscopy, Raman excitation spectroscopy, electromechanical resonance measurements, and transient electrothermal methods to address this knowledge gap. Their studies will advance the fundamental understanding of the phase behavior, mass density, and heat capacity of water in well-defined cylindrical nanopores relevant to green energy technology and water desalination. The project outcomes are expected to ultimately benefit society by informing the engineering of advanced separation membranes and electrochemical devices, and by training the next generation of scientists and engineers in this important area of research. As part of the project's broader impacts, two educational videos will also be produced to engage the general public on topics related to fluids at the nanoscale. Professor Kuehne's research examines three key aspects of water thermodynamics under cylindrical nanoconfinement between 4-500 K. First, photoluminescence and Raman excitation spectroscopy will be used to precisely determine water phase transition temperatures in isolated carbon nanotubes based on changes in the nanotube's optical and vibrational properties. Second, the total and interfacial mass densities of the confined water will be measured by combining electromechanical resonance techniques with Raman spectroscopy. Third, a transient electrothermal method will be coupled with Raman spectroscopy to determine the volumetric heat capacity. The research leverages the Kuehne lab's expertise in fabricating carbon nanotube nanofluidic devices and combines advanced optical and electrical measurement techniques in an innovative approach. Together, these complementary techniques will enable more reliable mapping of the phase diagram of water in single-digit nanopores and extracting enthalpies of phase change. The results will aid in resolving contradictions in reported phase transition temperatures for a given pore size and in developing accurate molecular models of nanoconfined water. The experimental tools developed in this project will further pave the way for systematic investigations of other confined fluids and electrolytes beyond water in future work. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.