Water droplets & icy particles
at atmospheric conditions
Water droplets at atmospheric conditions |
Motivation/background: Global spatial and temporal distributions of atmospheric water have a clear impact on both our local weather and continental climate patterns. Atmospheric water exhibits a truly rich repertoire of forms and phases in addition to gas, liquid or solid. There is a large variety of intermediates between the three bulk phases and individual molecules, such as clathrates, small clusters, droplets, icy particles and dendritic snow flakes. On microscopic scale water is involved in both homogeneous gas phase reactions and in a rich heterogeneous atmospheric chemistry involving aerosols. Both the bulk content and the surface composition of water clusters are important in condensation and as platform for atmospheric chemical reactions and transformations. Surfaces of ion-containing water-ice clusters provide a platform for numerous heterogeneous reactions. For example, they play a crucial role in heterogeneous chlorine chemistry with well-known consequences in depleting the stratospheric ozone layer in Polar Regions. The sizes of atmospheric water clusters and aerosol particles cover the range from small aggregates of a few molecules to droplets forming clouds. Already the water dimers are assumed to play an important role in the atmospheric photochemistry with consequences to our climate. Charged water clusters are ubiquitous in atmosphere formed by natural air ions. Unlike in the usual picture of electrolyte solutions where the dissolved ions are surrounded by water forming stable hydration shells many ions are found to move to the water surface interfaced to air. The, over a century debated, sign preference in nucleation of water on ions and cloud condensation is an important problem in from environmental point of view as well as in industrial applications . Our work: We have studied local structural, dynamical, electrical and thermodynamical properties of pure and ion-containing water clusters (with N = 20 – 1000) at atmospheric conditions. The local density approaches the bulk value in the center of the cluster already for the smallest clusters. The cluster properties depend on the distance from the center. The cluster size is given by the “radius of equimolecular dividing surface”. The inner part of this surface is slightly negatively charged while the outer part becomes weakly posititively charged, making the whole surface to an electrical double layer generating a surface potential. The average thickness of the surface layer is between one and two molecular diameters. rO(r) dominates over rH(r) on the inner side while it is vice verse on the outer side. Water molecules are aligned in the surface layer with their dipole moments effectively parallel to the surface. This behaviour is observed also in the simulations of the flat water surface. The vapor pressure surrounding a micro-droplet could be, in principal, calculated in computer modeling. However in the case of water the vapor pressure is extremely low and it seems not possible to find its value with the necessary accuracy. Not even in very long simulations. Water condensing around anions gives more compact clusters than with cations. As a consequence, the water–water interactions around an anion become more perturbed. The loss of water-water interaction energy is compensated by stronger water–anion interactions in the first hydration shell. In turn, the first hydration shell of an anion screens the ionic field much more effectively than that for a cation. Ions disrupt partially the hydrogen bond (HB) network, increasing diffusion of water molecules. Lifetimes of HBs do not change due to ions compared to pure water. In simulations of water clusters containing H3O+ the hydronium ion stays close to center independently on temperature and cluster size. Ca 3 waters are found in the first shell at 150K and 300K. The ion is close to the center at all temperatures for all cluster sizes. At 150K three branches of water structures develop rather than a clathrate type of structure (also suggested). At 150 the cluster is still liquid. At 300K the structure is somewhat randomized. Coordination number 3 agrees with previous quantum calculations. In simulations of water clusters containing OH- the hydroxyl ion does not prefer the center at 150K. Both ion and water positions randomized at 300K. Ca 6 waters are found in the first shell at both temperatures. Clusters around OH- are less stable than around H3O+. H3O+ prefers the center but not OH-.
References 1. Egorov, Brodskaya & Laaksonen, J. Chem. Phys., 118, 6380 (2003) |
