Confinement of matter on the nanometre scale can induce phase transitions not seen in bulk systems. In the case of water, so-called drying transitions occur on this scale as a result of strong hydrogen-bonding between water molecules, which can cause the liquid to recede from nonpolar surfaces to form a vapour layer separating the bulk phase from the surface. Here we report molecular dynamics simulations showing spontaneous and continuous filling of a nonpolar carbon nanotube with a one-dimensionally ordered chain of water molecules. Although the molecules forming the chain are in chemical and thermal equilibrium with the surrounding bath, we observe pulse-like transmission of water through the nanotube. These transmission bursts result from the tight hydrogen-bonding network inside the tube, which ensures that density fluctuations in the surrounding bath lead to concerted and rapid motion along the tube axis. We also find that a minute reduction in the attraction between the tube wall and water dramatically affects pore hydration, leading to sharp, two-state transitions between empty and filled states on a nanosecond timescale. These observations suggest that carbon nanotubes, with their rigid nonpolar structures, might be exploited as unique molecular channels for water and protons, with the channel occupancy and conductivity tunable by changes in the local channel polarity and solvent conditions.

Analytic formulas, within the mean spherical approximation, are given through fifth order for the inverse temperature expansion of the energy and the pair correlation function at contact for a fluid whose molecules interact via a Yukawa potential with a hard core. This series converges rapidly if the Yukawa potential has a long range. Reliable results are obtained from this series even when the potential has a short range.

We discuss the structure of alkali metal ions, halide ions, and uncharged solutes at infinite dilution in supercritical water solutions, at solvent densities of 0.35, 0.20, and 0.997 g cm-3 at a temperature of 683 K using the SPC/E model for water. This model has critical constants (T c = 640 K, ρ c = 0.29 g cm-3) which compare well with the corresponding values (T c = 647 K, ρ c = 0.322 g cm-3) for real water. The solute−water pair correlation functions are qualitatively different for the charged and uncharged solutes at 683 K at both 0.35 and 0.20 g cm-3 solvent densities, with water expelled from the immediate vicinity of the uncharged solute but retained and compressed in the neighborhood of a small ion. Increasing the solvent density to 0.997 g cm-3 at 683 K leads to dramatic changes in the solvent structure around an uncharged solute, with the formation of hydrogen-bonded cages analogous to those observed at room temperature (298 K) at the same solvent density. The primary hydration numbers of the ions at 683 K and solvent density of 0.35 g cm-3 are nearly the same as the corresponding values at room temperature at a solvent density of 0.997 g cm-3. The partial molar volumes of the ions and uncharged species at the supercritical temperature are different in sign and are explained in terms of a simple model. The dynamics of ions and uncharged solutes under the same supercritical conditions are discussed in the companion to this paper.

We discuss the results of a molecular dynamics (MD) study of the residence times of water in the primary hydration shells of ions and uncharged solutes at infinite dilution in supercritical water at 683 K, as well as the solvation dynamics and diffusion coefficients of these species. The SPC/E model is used to represent water in this paper and its companion, where structural aspects of the same systems are discussed. The residence times at 683 K are found to be lower than those under ambient conditions and only weakly dependent on the size and sign of the charge on the ions. In contrast to earlier studies at room temperature (298 K at a solvent density of 0.997 g cm -3 ), the solvation dynamics of sodium and chloride ions at 683 K and a solvent density of 0.35 g cm -3 are very similar to each other and indicate a solvent relaxation time of 0.8 ps. The ion diffusion coefficients are larger in magnitude at 683 K and less differentiated by size and charge sign at low solvent density (e0.35 g cm -3 ) than they are at room temperature. In the low-density range, an uncharged solute of the same size as an ion diffuses 2-9 times faster than the corresponding charged species, with the smaller solutes moving much faster than the larger ones. Increasing the solvent density to 0.997 g cm -3 at 683 K decreases the diffusion coefficients of ions and uncharged solutes. At this density, the diffusion coefficients pass through distinct maxima as a function of size at both 298 and 683 K. An explanation of the diffusion coefficients of charged and uncharged species at low solvent density is sought in terms of a semicontinuum theory. The importance of the time scale of the solvent density fluctuations in controlling diffusion as distinct from the spatial extent of these fluctuations (determined by the correlation length) is emphasized. These observations suggest that the mechanisms of solute diffusion at supercritical conditions and at room temperature may be fundamentally different.

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