Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by MC simulations of isotherms of density fluctuation restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P and SPCE model and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest, that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids is discussed.

Coexistence curves of water in cylindrical and slitlike nanopores of different size and water-substrate interaction strength were simulated in the Gibbs ensemble. The two-phase coexistence regions cover a wide range of pore filling level and temperature, including ambient temperature. Five different kinds of two-phase coexistence are observed. A single liquid-vapor coexistence is observed in hydrophobic and moderately hydrophilic pores. Surface transitions split from the main liquid-vapor coexistence region, when the water-substrate interaction becomes comparable or stronger than the water-water pair interaction. In this case prewetting, one and two layering transitions were observed. The critical temperature of the first layering transition decreases with strengthening water-substrate interaction towards the critical temperature expected for two-dimensional systems and is not sensitive to the variation of pore size and shape. Liquid-vapor phase transition in a pore with a wall which is already covered with two water layers is most typical for hydrophilic pores. The critical temperature of this transition is very sensitive to the pore size, in contrast to the liquid-vapor critical temperature in hydrophobic pores. The observed rich phase behavior of water in pores evidences that the knowledge of coexistence curves is of crucial importance for the analysis of experimental results and a prerequiste of meaningful simulations.

Three distinct liquid–liquid coexistence regions were observed for ST2 model water by restricted ensemble Monte Carlo simulations of the isotherms of homogenized systems and by phase equilibria simulations in the Gibbs ensemble. The lowest density liquid–liquid transition meets the liquid–vapor phase transition at a triple point and ends in a metastable critical point. A percolation analysis evidences, that the phase separations at the lowest and highest densities can be attributed to the separation of differently coordinated water molecules. The densities of the obtained four phases of supercooled water correlate with experimentally observed densities of amorphous ice.

The formation of spanning hydrogen-bonded water networks on protein surfaces by a percolation transition is closely connected with the onset of their biological activity. To analyze the structure of the hydration water at this important threshold, we performed the first computer simulation study of the percolation transition of water in a model protein powder and on the surface of a single protein molecule. The formation of an infinite water network in the protein powder occurs as a 2D percolation transition at a critical hydration level, which is close to the values observed experimentally. The formation of a spanning 2D water network on a single rigid protein molecule can be described by adapting the cluster analysis of conventional percolation studies to the characterization of the connectivity of the hydration water on the surface of finite objects. Strong fluctuations of the surface water network are observed close to the percolation threshold. Our simulations also furnish a microscopic picture for understanding the specific values of the experimentally observed hydration levels, where different steps of increasing mobility in the hydrated powder are observed.

The first simulation study of the percolation of hydrogen bonded water clusters in the vicinity of the region of immiscibility of an aqueous solution (of tetrahydrofuran) is reported. Percolation of water is found in a wide concentration range on both sides of the liquid–liquid coexistence curve. An infinite cluster appears with a probability of 50% at a water fraction significantly lower than the one corresponding to the organic-rich branch of the coexistence curve. The fractal dimension df of the infinite clusters at this threshold is found close to the two-dimensional (2D) value, df(2D)≅1.9. Three-dimensional (3D) percolation clusters form at the organic-rich branch of the coexistence curve. At this water concentration the fractal dimension of an infinite cluster reaches the 3D value df(3D)≅2.5 and the cluster size distribution follows a power law with an exponent τ≅2.2. The analysis of the clustering of the organic (tetrahydrofuran) molecules indicates that the immiscibility gap of an aqueous solution corresponds to the concentration interval where both components are above their respective percolation threshold.