The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its crucial importance in many biological, technological, and atmospheric processes. A long-term controversy pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface. Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported by surface selective nonlinear vibrational spectroscopy, which reveals among other things that the hydronium cations are present at the air/solution interface. The ion specific propensities for the air/solution interface have important implications for a whole range of heterogeneous physical and chemical processes, including atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence.

Aquaporins are efficient, yet strictly selective water channels. Remarkably, proton permeation is fully blocked, in contrast to most other water-filled pores which are known to conduct protons well. Blocking of protons by aquaporins is essential to maintain the electrochemical gradient across cellular and subcellular membranes. We studied the mechanism of proton exclusion in aquaporin-1 by multiple non-equilibrium molecular dynamics simulations that also allow proton transfer reactions. From the simulations, an effective free energy profile for the proton motion along the channel was determined with a maximum-likelihood approach. The results indicate that the main barrier is not, as had previously been speculated, caused by the interruption of the hydrogen-bonded water chain, but rather by an electrostatic field centered around the fingerprint Asn-Pro-Ala (NPA) motif. Hydrogen bond interruption only forms a secondary barrier located at the ar/R constriction region. The calculated main barrier height of 25-30 kJ mol(-1) matches the barrier height for the passage of protons across pure lipid bilayers and, therefore, suffices to prevent major leakage of protons through aquaporins. Conventional molecular dynamics simulations additionally showed that negatively charged hydroxide ions are prevented from being trapped within the NPA region by two adjacent electrostatic barriers of opposite polarity.

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H(2)O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (λ) between 5 and 6 H(2)O/SO(3)(-) as the percolation threshold of Nafion. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the λ value is increased from 5 to 15. The proton diffusion coefficient in Nafion at a λ value of 15 is about 1.1 × 10(-5) cm(2)/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity.