Double Layer Forces: the Role of Molecular Solvents

Abstract

The dielectric continuum model has rightfully been and continues to be a major

workhorse for theory and modelling in Surface and Colloid Chemistry. Due to

the implicit description of water, entering only as a scaling constant for

charge-charge interactions, one would not expect it to work for short

distances. However, considerable evidence has been accumulated over the years

which shows that this is not the case, and the dielectric continuum model

gives a reasonable description sometimes on lengthscales which approach the

size of a water molecule!



A first part of this thesis concerns theoretical modeling of the salt

dependent water uptake of a complex between DNA and CTA

(hexadecyltrimethylammonium). The electrostatic component of the free energy

was treated at the Poisson-Boltzmann level. Despite the demanding conditions,

with high electric fields, low water content and complex geometry, the

dielectric description of water works. The agreement between theory and

experiment is quantitative.



To try to understand why the primitive model is working under such conditions,

double layer forces with an explicit description of the solvent have been

studied through Monte Carlo simulations. A simple and previously

well-characterized system of two infinite, like-charged plates with only

neutralizing counterions was chosen. The solvent was treated as

Lennard-Jones dipoles (Stockmayer fluid) and, for comparison, also at the

primitive model level. The forces between like-charged plates in the primitive

model agree qualitatively with the molecular solvent ones and the implicit

solvent gives a reasonable description of the dielectric screening. As

expected, the molecular solvent introduces extra effects, namely packing. The

phenomenon of ion-ion correlation attraction, i.e., the attraction of the two

like-charged plates, is also reproduced in a molecular solvent. One of the

studies includes an analysis of the regime between the dielectric continuum

model and the full explicit molecular solvent. This is done by progressively

reducing the size of the molecular solvent, doubling its number density, and

keeping the dielectric properties constant.



The comparison between the two solvent models requires being able to calculate

the dielectric constant for a general dipolar fluid. A simulation methodology

has also been developed and tested for this purpose.

• author: Pegado, Luis
• supervisor: Håkan Wennerström; Bo Jönsson
• opponent: Professor Delville, Alfred, Université d'Orléans
• organization: Physical Chemistry
• publishing date: 2010
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: Dielectric continuum model, molecular solvent, Stockmayer fluid, electrical double layer, ion-ion correlations, Monte Carlo simulations, thermodynamic modeling, pressure curves, interaction free energy, packing effects
• pages: 145 pages
• defense location: Lecture hall C, Center for Chemistry and Chemical Engineering, Lund, Sweden
• defense date: 2010-10-21 10:30:00
• ISBN: 978-91-7422-252-4
• language: English
• LU publication?: yes
• id: 5df21a32-f929-424e-b38f-a97396136a8f (old id 1681982)
• date added to LUP: 2016-04-04 13:25:28
• date last changed: 2018-11-21 21:13:54