Structural simulation of molecular clusters and vapor-liquid interface

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Title: Structural simulation of molecular clusters and vapor-liquid interface
Author: Toivola, Martta
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Publisher: Helsingin yliopisto
Date: 2014-06-19
Belongs to series: Helsingin yliopiston verkkojulkaisut
Thesis level: Doctoral dissertation (article-based)
Abstract: The atmosphere is an inhomogeneous system of vapors, and liquid as well as solid particles. The nucleation process is an important source of particles, observed at most sites where particle measurements have been carried out. Sulfuric acid plays an important role in the first steps of particle formation, but a complete picture of process, after several years of research, is still unclear. Computational methods provide valuable information to support experimental observations, and sometimes they are the only way to obtain information about the system. In this thesis, molecular level simulations have been performed for inhomogeneous systems. The description of surface composition has been connected to the unphysical behavior of classical nucleation theory in the case of a surface-active mixture. Density functional theory for classical fluid has been employed to test three well known phenomenological models for surface composition. Simple models presented by Ebenhart, and Kulmala and Laaksonen who modified Eberhart's model, give good agreement with computed composition. The quality of molecular dynamic simulations depend on simulation conditions and the interaction potentials used. Indirect nucleation simulations performed for argon indicate that a vapor phase of tens of atoms is large enough to correct predict the density distribution, surface tension and equilibrium cluster size. The repulsive wall of a spherical simulation box disturb only vapor density and temperature distributions. The use of a Nose-Hoover thermostat simplifies the simulation procedure, and the system properties such as the density profile are indistinguishable from the constant energy simulations. The uncertainty concerning the reliability of molecular dynamic simulation results increases when a chemically active molecular system is simulated. Proton transfer reactions appearing in the sulfuric acid - water system are modelled by simulating bisulfate-oxonium ion pairs instead of neutral molecules. Structural differences between systems including ion pairs and neutral molecules are observed as expected. However, all the observed differences may also be simulation artefacts. Atmospheric sulfuric acid is mainly formed via oxidation of sulfur dioxcide. Byproducts of the oxidation chain are proposed to participate in nucleation, and dimer formation of sulfur-containing compounds with sulfuric acid, as well as the hydration of these dimer by one water molecule, are studied using quantum chemical calculations. A stability study of dimers and trimers shows that only peroxidisulfuric acid-sulfuric acid dimer is more stable than sulfuric acid dimer Peroxidisulfuric acid may also replace a sulfuric acid in a sulfuric acid - water cluster, but the second replacement is unfavorable, and thus peroxidisulfuric acid does not enhance sulfuric acid addition to the cluster.Ilmakehässä kaasumolekyylit muodostavat molekyyliryppäitä, jotka suotuisissa olosuhteissa kasvavat kaasujen tiivistymisen ja rypästörmäysten kautta kiinteiksi tai nestemäisiksi hiukkasiksi. Nykytietämyksen mukaan kaasu-hiukkas-muuntuma on olleellinen hiukkaslähde ilmakehässä. Rikkihapolla on merkittävä rooli hiukkasmuodustuksen alkuvaiheissa, mutta kaikkia hiukkasmuodostukseen vaikuttavia tekijöitä ei vielä tunneta. Mittaustekniikat kehittyvät koko ajan mutta on yhä prosesseja ja hiukkasten ominaisuuksia, joita voidaan tutkia vain laskennallisin menetelmin. Laskennalliset molekyylimallinnusmenetelmät antavat sekä uutta tietoa tutkittavasta kohteesta, että tukevat mittaustuloksia. Tässä väitöskirjatyössä on käytetty molekyylimallinnusmenetelmiä neste-kaasu-rajapintojen sekä molekyyliryppäiden rakenteiden tutkimiseen.
Subject: fysiikka
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