Synchrotron X-ray Scattering and Monte Carlo Simulations of Structure and Forces in Silicate Nanoplatelet Dispersions

Abstract

Clays are the world’s most widely used natural material, however, little is known regarding the microstructure as well as the forces involved in clay-water interactions, and their influence on the swelling properties. The utilization of clay platelets is nowadays a key in a number of biological and industrial applications e.g. nuclear waste management. Bentonites from different natural sources and pure Na/Ca montmorillonite platelets have been studied experimentally and theoretically. Small angle X-ray scattering (SAXS), dynamic light scattering (DLS), nuclear magnetic resonance (NMR), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and Cryogenic-TEM were applied to provide direct information about the structure of dry clays as well as clay platelets in equilibrium with a bulk solution of given ionic composition, temperature and pH. Monte Carlo (MC) simulations have been used to predict the osmotic pressure of montmorillonite dispersions. The swelling behavior is mainly regulated by counterion valency and surface charge density. Divalent counterions and high surface charge density lead to a limited swelling, while monovalent counterions favor a large swelling. This thesis has also investigated the aggregation of nanoplatelets in clay-water systems, in order to understand the effect of platelet size on the structure and swelling behavior. A new twist on aggregation phenomenon is that, really small platelets (~ 20 nm) do not form a tactoid, whereas larger platelets give rise to larger tactoids. The platelet size controls the aggregation and microstructure of silicate platelets into tactoids following an empirical relation as: N ≈ δ + 𝛼 D_eff, where N is the number of platelets per tactoid, D_eff is the effective diameter of platelets, δ and 𝛼 are constants.

Full text

M. S. MEEHDI Synchrotron X-ray Scattering and Monte Carlo Simulations of Structure and Forces in Silicate Nanoplatelet Dispersions 2014
ISBN 978-91-7422-353-8
Theoretical Chemistry, Lund University
Printed by M edia-Tryck, Lund U niversity, Swe den
M. SEGAD MEEHDI | FACULTY OF SCIENCE | LUND UNIVERSITY
DOCTORAL THESIS
Synchrotron X-ray Scattering
and Monte Carlo Simulations of
Structure and Forces in Silicate
Nanoplatelet Dispersions
Photographer: Paul K Robbins, Monde Photo
Photo of M. Segad (middle) with the deputy chair (Tracey Hanley, right)
and co-chair (Jill Trewhella, left) was taken during the ceremony of
best poster prize at the international small-angle scattering conference
(SAS2012) hosted by the Australian nuclear science and technology
organization (ANSTO) and held in Sydney, Australia.
M.S. studied natural sciences and obtained a B.Sc. in chemistry and
a M.Sc. in physical chemistry. He began his graduate studies in 2009
with a PhD project entitled “Structure, Forces and Swelling Properties:
Experimental and Theoretical Studies of Nanoplatelet Dispersions” at
the Center for Chemistry and Chemical Engineering, Lund University.
The general idea of this thesis is to combine several experimental
methods with Monte Carlo simulations to investigate the swelling
properties of natural materials and to gain an understanding of the
forces in colloidal dispersions. Besides his research interests in the
application of X-ray and neutron scattering techniques to characterize
nanostructured materials, he is also involved in X-ray characterisation of
intrinsically disordered proteins.