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Register a new userPerspective: Surface Freezing in Water: A Nexus of Experiments and Simulations
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Surface freezing is a phenomenon in which crystallization is enhanced at a vapor-liquid interface. In some systems, such as $n$-alkanes, this enhancement is dramatic, and results in the formation of a crystalline layer at the free interface even at temperatures slightly above the equilibrium bulk freezing temperature. There are, however, systems in which the enhancement is purely kinetic, and only involves faster nucleation at or near the interface. The first, thermodynamic, type of surface freezing is easier to confirm in experiments, requiring only the verification of the existence of crystalline order at the interface. The second, kinetic, type of surface freezing is far more difficult to prove experimentally. One material that is suspected of undergoing the second type of surface freezing is liquid water. Despite strong indications that the freezing of liquid water is kinetically enhanced at vapor-liquid interfaces, the findings are far from conclusive, and the topic remains controversial. In this perspective, we present a simple thermodynamic framework to understand conceptually and distinguish these two types of surface freezing. We then briefly survey fifteen years of experimental and computational work aimed at elucidating the surface freezing conundrum in water.
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Experiments in bulk water confirm the existence of two local arrangements of water molecules with different densities, but, because of inevitable freezing at low temperature $T$, can not ascertain whether the two arrangements separate in two phases. To avoid the freezing, new experiments measure the dynamics of water at low $T$ on the surface of proteins, finding a crossover from a non-Arrhenius regime at high $T$ to a regime that is approximately Arrhenius at low $T$. Motivated by these experiments, Kumar et al. [Phys. Rev. Lett. 100, 105701 (2008)] investigated, by Monte Carlo simulations and mean field calculations, the relation of the dynamic crossover with the coexistence of two liquid phases in a cell model for water and predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time $tau_{rm L}$ is approximately independent of pressure $P$; (ii) the Arrhenius activation energy $E_{rm A}(P)$ of the low-$T$ regime decreases upon increasing $P$; (iii) the temperature $T^*(P)$ at which $tau$ reaches a fixed macroscopic time $tau^*geq tau_{rm L}$ decreases upon increasing $P$; in particular, this is true also for the crossover temperature $T_{rm L}(P)$ at which $tau=tau_{rm L}$. Here, we compare these predictions with recent quasi elastic neutron scattering (QENS) experiments performed by X.-Q. Chu {it et al.} on hydrated proteins at different values of $P$. We find that the experiments are consistent with these three predictions.
Water is a ubiquitous liquid with unique physico-chemical properties, whose nature has shaped our planet and life as we know it. Water in restricted geometries has different properties than in bulk. Confinement can prevent low-temperature crystallization into a hexagonal structure, thus creating a state of amorphous water. In this work we introduce a family of synthetic lipids with designed cyclopropyl modification in the hydrophobic chains that exhibit unique liquid-crystalline behaviour at low temperature, enabling maintenance of amorphous water down to 10 K due to nanoconfinement in a bio-mimetic milieu. Small and Wide Angle X-ray Scattering, Elastic and Inelastic Neutron Scattering, Nuclear Magnetic Resonance Spectroscopy and Differential Scanning Calorimetry, complemented by Molecular Dynamics Simulations, unveil a complex lipid/water phase diagram, in which bicontinuous cubic and lamellar liquid crystalline phases containing sub-zero liquid, glassy, or ice water emerge as a competition between the two components, each pushing towards its thermodynamically favoured state.
We investigate the behavior of hydrated sulfonated polysulfones over a range of ion contents through differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and molecular dynamics (MD) simulations. Experimental evidence shows that at comparable ion contents, the spacing between the ionic groups along the polymer backbone can significantly impact the amount of melting water present in the polymer. When we only consider water molecules that can hydrogen bond to four neighboring water molecules as the melting water, the MD simulation results are found to agree with the experimental data. The states of water measured by DSC can therefore be described as aggregated (or bulk-like) for the melting component, and isolated for the nonmelting part. Using this physical picture, a polymer with more aggregated ions has a higher content of melting water, while a polymer at the same ion content but with more dispersed ions has a lower content of melting water. Therefore, ions should be well dispersed to minimize the amount of bulk-like water in ionic polymer membranes.
First systematic spin probe ESR study of water freezing has been conducted using TEMPOL and TEMPO as the probes. The spin probe signature of the water freezing has been described in terms of the collapse of narrow triplet spectrum into a single broad line. This spin probe signature of freezing has been observed at an anomalously low temperature when a milimoler solution of TEMPOL is slowly cooled from room temperature. A systematic observation has revealed a spin probe concentration dependence of these freezing and respective melting points. These results can be explained in terms of localization of spin probe and liquid water, most probably in the interstices of ice grains, in an ice matrix. The lowering of spin probe freezing point, along with the secondary evidences, like spin probe concentration dependence of peak-to-peak width in frozen limit signal, indicates a possible size dependence of these localizations/entrapments with spin probe concentration. A weak concentration dependence of spin probe assisted freezing and melting points, which has been observed for TEMPO in comparison to TEMPOL, indicates different natures of interactions with water of these two probes. This view is also supported by the relaxation behavior of the two probes.
The control of biofilm formation is a challenging goal that has not been reached yet in many aspects. One is the role of van der Waals forces and another the importance of mutual interactions between the adsorbing and the adsorbed biomolecules (critical crowding). Here, a combined exeperimental and theoretical approach is presented that fundamentally probes both aspects. On three model proteins, lysozyme, {alpha}-amylase and bovine serum albumin (BSA), the adsorption kinetics is studied. Composite substrates are used enabling a separation of the short- and the long-range forces. Though usually neglected, experimental evidence is given for the influence of van der Waals forces on the protein adsorption as revealed by in situ ellipsometry. The three proteins were chosen for their different conformational stability in order to investigate the influence of conformational changes on the adsorption kinetics. Monte Carlo simulations are used to develop a model for these experimental results by assuming an internal degree of freedom to represent conformational changes. The simulations also provide data on the distribution of adsorption sites. By in situ atomic force microscopy we can also test this distribution experimentally which opens the possibility to e.g. investigate the interactions between adsorbed proteins.
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