Structure of Liquid H₂SO₄ — H₂O Clusters Around Ions: Thermodynamic Theory

Abstract

The example of H₂SO₄–H₂O clusters was taken because of its importance for air pollution problems. H₂SO₄ is formed by photo-oxidation from the primary pollutant SO₂ by gas-to-particle conversion in the atmosphere. Ionized H₂SO₄ – H₂O clusters are expected to be observed in polluted city air (Cox, 1974). Furthermore H₂SO₄ – H₂O droplets were identified in background atmosphere near Antarctica (Mészdros, 1974), in dependence of the available NH₃-concentration.

In this paper it was calculated by how many H₂O-molecules and H₂SO₄-molecules a single-charged ion is surrounded at 25°C for various relative humidities (RH ≤ 100%) and various activities of sulfuric acid vapor (5 × 10^–3 to 1 × 10^–5) using the Becker-Flood-Doyle theory of the heteromolecular homogeneous nucleation for stable clusters.

The size and composition is given by the minimum of the droplet formation free energy, dependent on the concentrations of gaseous H₂O and H₂SO₄ vapors in the atmosphere. The cluster composition (400–700 pm in size) is shifted to a larger number of H₂O-molecules with increasing RH at nearly constant H₂S0₄-concentration.

The calculated cluster concentration for pure ion-water molecules is compared with the experiment and showed good agreement. The aim of this paper is to predict results of future experiments on the structure of binary mixture droplets around ions. This comparison would test how good the classical thermodynamic theory describes aerosol particles in the range of hundreds of picometers despite using macroscopic concepts (i.e. surface tension).