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Coarse-Grained Models of Ionic Solutions

Szparaga, Ryan LU (2015)
Abstract (Swedish)
Popular Abstract in English

Typically, when we speak of ionic solutions we mean a salt (as solute) dissolved in some liquid (the solvent). In this thesis, we predominantly consider ionic solutions for which there is no solvent---room-temperature ionic liquids (RTILs).



It follows that these liquids have a large concentration of charged particles, a property that causes them to be effective electrolytes in capacitors. A capacitor works as a rechargeable battery, storing a net charge (an excess adsorption of positive or negative particles) onto the surface of an electrode when a voltage is applied, and releasing them when connected to a circuit, creating a current to power some electrical components. The... (More)
Popular Abstract in English

Typically, when we speak of ionic solutions we mean a salt (as solute) dissolved in some liquid (the solvent). In this thesis, we predominantly consider ionic solutions for which there is no solvent---room-temperature ionic liquids (RTILs).



It follows that these liquids have a large concentration of charged particles, a property that causes them to be effective electrolytes in capacitors. A capacitor works as a rechargeable battery, storing a net charge (an excess adsorption of positive or negative particles) onto the surface of an electrode when a voltage is applied, and releasing them when connected to a circuit, creating a current to power some electrical components. The nature of this adsorption (or the `electric double layer') is therefore of interest to us. Another appealing feature of RTILs is the ability to control their physical properties by changing their chemical structures. Synthesizing ionic liquids is expensive, and there are vast numbers of possible compounds so there is a necessity to describe these chemicals theoretically, to explain and predict their behaviour.



However, the old theories of electrolyte-electrode behaviour fail under these high concentrations. Computer simulations of RTILs with all-atom forcefields can be very time-consuming, so to save time and gain a broader understanding, we can use less detailed, `coarse-grained' models. These simple models also allow easy implementation into a classical density functional theory framework, an approximate semi-analytical theory capable of delivering results of comparable accuracy to simulation, but at a fraction of the time. (Less)
Abstract
Room-temperature ionic liquids (RTILs) are compounds composed entirely of ions, which are liquid at temperatures below 100 degrees Celsius. Their high ionic strength and strong coupling make them useful for a number of applications, e.g. as electrolytes in supercapacitors. These properties also make them interesting subjects for theoretical research, as the canonical theories of the electrical double layer fail under these conditions. We implement coarse-grained models of RTILs into Monte Carlo (MC) simulations and Classical Density Functional Theory (DFT) to investigate their behaviour at electrode surfaces.

A prewetting transition is found for a dilute RTIL and solvent mixture, with a large differential capacitance spike found... (More)
Room-temperature ionic liquids (RTILs) are compounds composed entirely of ions, which are liquid at temperatures below 100 degrees Celsius. Their high ionic strength and strong coupling make them useful for a number of applications, e.g. as electrolytes in supercapacitors. These properties also make them interesting subjects for theoretical research, as the canonical theories of the electrical double layer fail under these conditions. We implement coarse-grained models of RTILs into Monte Carlo (MC) simulations and Classical Density Functional Theory (DFT) to investigate their behaviour at electrode surfaces.

A prewetting transition is found for a dilute RTIL and solvent mixture, with a large differential capacitance spike found in the sub critical region. Investigation of capillary condensation for a similar mixture in a pore yields another differential capacitance spike, which decreases away from critical conditions. DFT and MC are compared for a new model RTIL with the former containing a new innovation in describing charge-charge correlations, which gives good agreement for fluid structures at a surface. Pressure-distance curves are computed with DFT for a homologous series of aromatic RTILs, showing the range of interactions to increase with alkyl chain length. Increasing surface charge density causes the amplitude of the interaction free energy curves to decrease.

Image charge interactions are examined for a primitive model electrolyte using MC and a new image-corrected Poisson-Boltzmann DFT formulism (iPB), that record good agreement. Increasing salt concentration enhances the desolvation repulsion, and incorporating a basic model for specific adsorption predicts that a colloidal dispersion can be stabilised in this way.

The final paper discusses DFT formulism in detail for ionic systems, including new results. We find that increasing the surface exclusion zone reduces the differential capacitance, raising the temperature can enhance the differential capacitance and specific adsorption of charged components diminishes the characteristic minimum of the 'camel-hump'. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Univ.Prof.Dr. Kahl, Gerhard, Vienna University of Technology
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Monte Carlo, Classical Density Functional Theory, Coarse-Grained Models, Ionic Liquids, Prewetting, Capillary Condensation, Electric Double Layer, Differential Capacitance.
pages
170 pages
publisher
Division of Theoretical Chemistry, Department of Chemistry, Lund University
defense location
Lecture Hall B
defense date
2015-03-13 10:30
ISBN
978-91-7422-390-3
language
English
LU publication?
yes
id
12f3468f-9af7-41a8-8a41-e57037871090 (old id 5049384)
date added to LUP
2015-02-25 11:55:02
date last changed
2016-09-19 08:45:06
@misc{12f3468f-9af7-41a8-8a41-e57037871090,
  abstract     = {Room-temperature ionic liquids (RTILs) are compounds composed entirely of ions, which are liquid at temperatures below 100 degrees Celsius. Their high ionic strength and strong coupling make them useful for a number of applications, e.g. as electrolytes in supercapacitors. These properties also make them interesting subjects for theoretical research, as the canonical theories of the electrical double layer fail under these conditions. We implement coarse-grained models of RTILs into Monte Carlo (MC) simulations and Classical Density Functional Theory (DFT) to investigate their behaviour at electrode surfaces. <br/><br>
 A prewetting transition is found for a dilute RTIL and solvent mixture, with a large differential capacitance spike found in the sub critical region. Investigation of capillary condensation for a similar mixture in a pore yields another differential capacitance spike, which decreases away from critical conditions. DFT and MC are compared for a new model RTIL with the former containing a new innovation in describing charge-charge correlations, which gives good agreement for fluid structures at a surface. Pressure-distance curves are computed with DFT for a homologous series of aromatic RTILs, showing the range of interactions to increase with alkyl chain length. Increasing surface charge density causes the amplitude of the interaction free energy curves to decrease. <br/><br>
 Image charge interactions are examined for a primitive model electrolyte using MC and a new image-corrected Poisson-Boltzmann DFT formulism (iPB), that record good agreement. Increasing salt concentration enhances the desolvation repulsion, and incorporating a basic model for specific adsorption predicts that a colloidal dispersion can be stabilised in this way.<br/><br>
 The final paper discusses DFT formulism in detail for ionic systems, including new results. We find that increasing the surface exclusion zone reduces the differential capacitance, raising the temperature can enhance the differential capacitance and specific adsorption of charged components diminishes the characteristic minimum of the 'camel-hump'.},
  author       = {Szparaga, Ryan},
  isbn         = {978-91-7422-390-3},
  keyword      = {Monte Carlo,Classical Density Functional Theory,Coarse-Grained Models,Ionic Liquids,Prewetting,Capillary Condensation,Electric Double Layer,Differential Capacitance.},
  language     = {eng},
  pages        = {170},
  publisher    = {ARRAY(0x82237c0)},
  title        = {Coarse-Grained Models of Ionic Solutions},
  year         = {2015},
}