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Glycine in aerosol water droplets: a critical assessment of Köhler theory by predicting surface tension from molecular dynamics simulations
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).ORCID iD: 0000-0001-6508-8355
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2011 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, Vol. 11, no 2, 519-527 p.Article in journal (Refereed) Published
Abstract [en]

Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Kohler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Kohler equation could be improved by incorporating surface tension corrections.

Place, publisher, year, edition, pages
2011. Vol. 11, no 2, 519-527 p.
National Category
Theoretical Chemistry Meteorology and Atmospheric Sciences
URN: urn:nbn:se:kth:diva-31645DOI: 10.5194/acp-11-519-2011ISI: 000286722300008ScopusID: 2-s2.0-78751504505OAI: diva2:405125
Swedish Research Council, 2009-3614Swedish e‐Science Research Center
QC 20110321Available from: 2011-03-21 Created: 2011-03-21 Last updated: 2012-05-23Bibliographically approved
In thesis
1. Applications of Molecular Dynamics in Atmospheric and Solution Chemistry
Open this publication in new window or tab >>Applications of Molecular Dynamics in Atmospheric and Solution Chemistry
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the applications of molecular dynamics simulation techniques in the fields of solution chemistry and atmospheric chemistry. The work behind the thesis takes account of the fast development of computer hardware, which has made computationally intensive simulations become more and more popular in disciplines like pharmacy, biology and materials science. In molecular dynamics simulations using classical force fields, the atoms are represented by mass points with partial charges and the inter-atomic interactions are modeled by approximate potential functions that produce satisfactory results at an economical computational cost. The three-dimensional trajectory of a many-body system is generated by integrating Newton’s equations of motion, and subsequent statistical analysis on the trajectories provides microscopic insight into the physical properties of the system.

The applications in this thesis of molecular dynamics simulations in solution chemistry comprise four aspects: the 113Cd nuclear magnetic resonance shielding constant of aqua Cd(II) ions, paramagnetic 19F nuclear magnetic resonance shift in fluorinated cysteine, solvation free energies and structures of metal ions, and protein adsorption onto TiO2. In the studies of nuclear magnetic resonance parameters, the relativistic effect of the 113Cd nucleus and the paramagnetic shift of 19F induced by triplet O2 are well reproduced by a combined molecular dynamics and density functional theory approach. The simulation of the aqua Cd(II) ion is also extended to several other monovalent, divalent and trivalent metal ions, where careful parameterization of the metal ions ensures the reproduction of experimental solvation structures and free energies. Molecular dynamics simulations also provided insight into the mechanism of protein adsorption onto the TiO2 surface by suggesting that the interfacial water molecules play an important role of mediating the adsorption and that the hydroxylated TiO2 surface has a large affinity to the proteins.

The applications of molecular dynamics simulations in atmospheric chemistry are mainly focused on two types of organic components in aerosol droplets: humic-like compounds and amino acids. The humic-like substances, including cis-pinonic acid, pinic acid and pinonaldehyde, are surface-active organic compounds that are able to depress the surface tension of water droplets, as revealed by both experimental measurements and theoretical computations. These compounds either concentrate on the droplet surface or aggregate inside the droplet. Their effects on the surface tension can be modeled by the Langmuir-Szyszkowski equation. The amino acids are not strong surfactants and their influence on the surface tension is much smaller. Simulations show that the zwitterionic forms of serine, glycine and alanine have hydrophilic characteristics, while those of valine, methionine and phenylalanine are hydrophobic. The curvature dependence of the surface tension is also analyzed, and a slight improvement in the Köhler equation is obtained by introducing surface tension corrections for droplets containing glycine and serine.

Through several examples it is shown that molecular dynamics simulations serve as a promising tool in the study of aqueous systems. Both solute-solvent interactions and interfaces can be treated properly by choosing suitable potential functions and parameters. Specifically, molecular dynamics simulations provide a microscopic picture that evolves with time, making it possible to follow the dynamic processes such as protein adsorption or atmospheric droplet formation. Moreover, molecular dynamics simulations treat a large number of molecules and give a statistical description of the system; therefore it is convenient to compare the simulated results with experimentally measured data. The simulations can provide hints for better design of experiments, while experimental data can be fed into the refinement of the simulation model. As an important complementary to experiments, molecular dynamics simulations will continue to play significant roles in the research fields of physics, chemistry, materials science, biology and medicine.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology (KTH), 2011. viii, 54 p.
Trita-BIO-Report, ISSN 1654-2312 ; 2011:10
National Category
Theoretical Chemistry
urn:nbn:se:kth:diva-33309 (URN)978-91-7415-963-9 (ISBN)
Public defence
2011-05-26, FB52, AlbaNova, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Swedish e‐Science Research Center
QC 20110511Available from: 2011-05-11 Created: 2011-05-03 Last updated: 2012-05-24Bibliographically approved

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