Molecular dynamics simulation of gas liquid nucleation in aqueous systems.

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The research on nucleation from the vapour phase has been developed significantly in the last decades. Reasons are the relevance for technical applications, the further development of theory and simulation of nucleation processes and the development of experimental techniques for the investigation of early states of phase formation.The aim of this project is to investigate the microscopic effects on nucleation in binary systems and to relate them to the usually macroscopic experimental obser¬vations. The binary systems are selected to cover the range from highly symmetric to strongly asymmetric mixtures. In addition, the relevance for technical applications as well as the existence of experimental data is considered. Based on these criteria and its broad general significance binary systems with water as component are chosen here for the investigation of binary nucleation.First mixtures of water and heavy water will be investigated. Besides its different mass heavy water also has slightly different interaction parameters. For this highly symmetric system experimental data for nucleation rates and critical cluster mole fraction are available. The analysis of nucleation in such symmetric systems is necessary to obtain data to evaluate the consistency of nucleation theories in the symmetric limit. The microscopic and macroscopic data that will be obtained in this project will be employed for this purpose among others. The system methane/water is in contrast a very asymmetric system. It is relevant for technical applications such as a drying process of natural gas based on a binary nucleation process. The third system consisting of water and 1-butanol is less asymmetric than the methane/water system due to the polarity of butanol. This system is of special interest as besides the nucleation rates, also experimental data of the structure of forming droplets are available. In addition to the variations of the symmetry of the aqueous systems also the influence of pressure on the nucleation shall be analysed. This is of importance for technical processes involving high pressure nucleation. Investigations will be focused on those systems for which it is known that their cluster structure deviates from what is expected from the bulk phase behaviour. These deviations can originate from the preferred adsorption of one species at the cluster surface or from the formation of a liquid-liquid immiscibility, which does not exist in this form in the bulk phase. For the analysis of binary nucleation a method for the construction of the binary plane of the work of formation as function of the number of molecules A and B shall be developed. With this method the statistics of nucleation, cluster growth, and evaporation will be evaluated.