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1. The effects of methanol clustering on methanol–water nucleation

   https://doi.org/10.1063/5.0120876  

   Sun, Tong; Wilemski, Gerald; Hale, Barbara N.; Wyslouzil, Barbara E. (November 2022, The Journal of Chemical Physics)

   The formation of subcritical methanol clusters in the vapor phase is known to complicate the analysis of nucleation measurements. Here, we investigate how this process affects the onset of binary nucleation as dilute water–methanol mixtures in nitrogen carrier gas expand in a supersonic nozzle. These are the first reported data for water–methanol nucleation in an expansion device. We start by extending an older monomer–dimer–tetramer equilibrium model to include larger clusters, relying on Helmholtz free energy differences derived from Monte Carlo simulations. The model is validated against the pressure/temperature measurements of Laksmono et al. [Phys. Chem. Chem. Phys. 13, 5855 (2011)] for dilute methanol–nitrogen mixtures expanding in a supersonic flow prior to the appearance of liquid droplets. These data are well fit when the maximum cluster size imax is 6–12. The extended equilibrium model is then used to analyze the current data. On the addition of small amounts of water, heat release prior to particle formation is essentially unchanged from that for pure methanol, but liquid formation proceeds at much higher temperatures. Once water comprises more than ∼24 mol % of the condensable vapor, droplet formation begins at temperatures too high for heat release from subcritical cluster formation to perturb the flow. Comparing the experimental results to binary nucleation theory is challenged by the need to extrapolate data to the subcooled region and by the inapplicability of explicit cluster models that require a minimum of 12 molecules in the critical cluster. 

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2. Water Dynamics in a Peptide-appended Pillar[5]arene Artificial Channel in Lipid and Biomimetic Membranes

   https://doi.org/10.3389/fchem.2021.753635  

   Barden, Daniel Ryan; Vashisth, Harish (October 2021, Frontiers in Chemistry)

   Peptide-appended Pillar[5]arene (PAP) is an artificial water channel that can be incorporated into lipid and polymeric membranes to achieve high permeability and enhanced selectivity for angstrom-scale separations [Shen et al. Nat. Commun. 9 :2294 (2018)]. In comparison to commonly studied rigid carbon nanotubes, PAP channels are conformationally flexible, yet these channels allow a high water permeability [Y. Liu and H. Vashisth Phys. Chem. Chem. Phys. 21 :22711 (2019)]. Using molecular dynamics (MD) simulations, we study water dynamics in PAP channels embedded in biological (lipid) and biomimetic (block-copolymer) membranes to probe the effect of the membrane environment on water transport characteristics of PAP channels. We have resolved the free energy surface and local minima for water diffusion within the channel in each type of membrane. We find that water follows single file transport with low free-energy barriers in regions surroundings the central ring of the PAP channel and the single file diffusivity of water correlates with the number of hydrogen bonding sites within the channel, as is known for other sub-nm pore-size synthetic and biological water channels [Horner et al. Sci. Adv. 1 :e1400083 (2015)]. 

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3. Correction: Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond

   https://doi.org/10.1039/D1CP90253A  

   Chapleski, Robert C.; Chowdhury, Azhad U.; Mason, Kyle R.; Sacci, Robert L.; Doughty, Benjamin; Roy, Sharani (January 2022, Physical Chemistry Chemical Physics)

   Correction for ‘Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond’ by Robert C. Chapleski, Jr. et al. , Phys. Chem. Chem. Phys. , 2021, 23 , 23478–23485, DOI: 10.1039/D1CP03587H. 

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4. Fluid–fluid phase transitions in a chiral molecular model

   https://doi.org/10.1063/5.0105851  

   Wang, Yiming; Stillinger, Frank H.; Debenedetti, Pablo G. (August 2022, The Journal of Chemical Physics)

   Molecular chirality is a fundamental phenomenon, underlying both life as we know it and industrial pharmaceutical syntheses. Understanding the symmetry breaking phase transitions exhibited by many chiral molecular substances provides basic insights for topics ranging from the origin of life to the rational design of drug manufacturing processes. In this work, we have performed molecular dynamics simulations to investigate the fluid–fluid phase transitions of a flexible three-dimensional four-site chiral molecular model developed by Latinwo et al. [J. Chem. Phys. 145, 154503 (2016)] and Petsev et al. [J. Chem. Phys. 155, 084105 (2021)]. By introducing a bias favoring local homochiral vs heterochiral interactions, the system exhibits a phase transition from a single achiral phase to a single chiral phase that undergoes infrequent interconversion between the two thermodynamically identical chiral states: the L-rich and D-rich phases. According to the phase rule, this reactive binary system has two independent degrees of freedom and exhibits a density-dependent critical locus. Below the liquid–liquid critical locus, there exists a first-order vapor–liquid coexistence region with a single independent degree of freedom. Our results provide basic thermodynamic and kinetic insights for understanding many-body chiral symmetry breaking phenomena. 

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5. Liquid–liquid criticality in the WAIL water model

   https://doi.org/10.1063/5.0099520  

   Weis, Jack; Sciortino, Francesco; Panagiotopoulos, Athanassios Z.; Debenedetti, Pablo G. (July 2022, The Journal of Chemical Physics)

   The hypothesis that the anomalous behavior of liquid water is related to the existence of a second critical point in deeply supercooled states has long been the subject of intense debate. Recent, sophisticated experiments designed to observe the transformation between the two subcritical liquids on nano- and microsecond time scales, along with demanding numerical simulations based on classical (rigid) models parameterized to reproduce thermodynamic properties of water, have provided support to this hypothesis. A stronger numerical proof requires demonstrating that the critical point, which occurs at temperatures and pressures far from those at which the models were optimized, is robust with respect to model parameterization, specifically with respect to incorporating additional physical effects. Here, we show that a liquid–liquid critical point can be rigorously located also in the WAIL model of water [Pinnick et al., J. Chem. Phys. 137, 014510 (2012)], a model parameterized using ab initio calculations only. The model incorporates two features not present in many previously studied water models: It is both flexible and polarizable, properties which can significantly influence the phase behavior of water. The observation of the critical point in a model in which the water–water interaction is estimated using only quantum ab initio calculations provides strong support to the viewpoint according to which the existence of two distinct liquids is a robust feature in the free energy landscape of supercooled water. 

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