On the development of SAFT-γ force fields for ionic surfactants and theories of micellization
File(s)
Author(s)
Kiesel, Matthias
Type
Thesis or dissertation
Abstract
Surfactants have exceptional influence on the behaviour of systems they are part of.
They lower the interfacial tension at phase boundaries and cause micellization inside
bulk phases. Molecular modelling and simulation is used in this thesis to gain insight
into the behaviour of anionic and non-ionic surfactants in aqueous solution and make a
step towards the modelling of such systems.
For use in molecular simulation, the statistical association fluid theory for Mie potentials
(SAFT-VR Mie) and its extension to charged systems is shown to be applicable
for the parameterisation of coarse-grained force fields for strong electrolytes for use in
direct molecular simulation. The resulting strong electrolyte models are found to reproduce
the osmotic coefficients and densities of various strong electrolyte in aqueous
solution well.
Furthermore, the transferability of these strong electrolyte models to the molecular
simulation of sodium dodecyl sulfate (SDS), an anionic surfactant, is thoroughly
investigated. The micellization behaviour, such as the free surfactant concentration,
equilibration times and micelle shape is found to be qualitatively well reproduced with
good agreement for micelle size. At interfaces between water and air as well as water
and n-dodecane, the SDS model correctly predicts the lowering of the interfacial
tension and onset of micellization at higher surfactant concentrations. Artefacts such
5
as the partitioning of charged species into apolar phases remain challenges for further
improvements.
In addition to molecular simulation, a SAFT-γ Mie enhanced theory of micellization
for the description of non-ionic surfactants is developed in this work. In this enhanced
version, the interfacial tensions are calculated via the square-gradient theory and experimental
correlations for the transfer of surfactant tails from the aqueous phase into the
micelle are replaced with SAFT-γ Mie calculations. The reduced parameterisation effort
compared to other micellization theories allows for the extension towards more complex
surfactant compositions while the quality of description of the micellization theory is
improved simultaneously.
Altogether, the work presented in this thesis is a step towards a more predictive
description of surfactants in aqueous solution utilising SAFT.
They lower the interfacial tension at phase boundaries and cause micellization inside
bulk phases. Molecular modelling and simulation is used in this thesis to gain insight
into the behaviour of anionic and non-ionic surfactants in aqueous solution and make a
step towards the modelling of such systems.
For use in molecular simulation, the statistical association fluid theory for Mie potentials
(SAFT-VR Mie) and its extension to charged systems is shown to be applicable
for the parameterisation of coarse-grained force fields for strong electrolytes for use in
direct molecular simulation. The resulting strong electrolyte models are found to reproduce
the osmotic coefficients and densities of various strong electrolyte in aqueous
solution well.
Furthermore, the transferability of these strong electrolyte models to the molecular
simulation of sodium dodecyl sulfate (SDS), an anionic surfactant, is thoroughly
investigated. The micellization behaviour, such as the free surfactant concentration,
equilibration times and micelle shape is found to be qualitatively well reproduced with
good agreement for micelle size. At interfaces between water and air as well as water
and n-dodecane, the SDS model correctly predicts the lowering of the interfacial
tension and onset of micellization at higher surfactant concentrations. Artefacts such
5
as the partitioning of charged species into apolar phases remain challenges for further
improvements.
In addition to molecular simulation, a SAFT-γ Mie enhanced theory of micellization
for the description of non-ionic surfactants is developed in this work. In this enhanced
version, the interfacial tensions are calculated via the square-gradient theory and experimental
correlations for the transfer of surfactant tails from the aqueous phase into the
micelle are replaced with SAFT-γ Mie calculations. The reduced parameterisation effort
compared to other micellization theories allows for the extension towards more complex
surfactant compositions while the quality of description of the micellization theory is
improved simultaneously.
Altogether, the work presented in this thesis is a step towards a more predictive
description of surfactants in aqueous solution utilising SAFT.
Version
Open Access
Date Issued
2023-02
Date Awarded
2023-07
Copyright Statement
Creative Commons Attribution Licence
Advisor
Galindo, Amparo
Jackson, George
Sponsor
Procter & Gamble Company
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)