Surface charge characterization of gibbsite nanoparticles - An atomic force microscope study
Due to the COVID-19 crisis measures the PhD defence of Aram Klaassen will take place (partly) online.
The PhD defence can be followed by a live stream.
Aram Klaassen is a PhD student in the research group Physics of Complex Fluids (PCF). His supervisor is prof.dr. F.G. Mugele from the Faculty of Science and Technology (TNW).
The interaction of water with mineral surfaces, silica and alumina, is one of the most important chemical reactions occurring in nature. Interfacial water, surface charge and hydration properties play a major role in dissolution, CO2 sequestration, precipitation and sorption processes affecting the composition and quality of natural waters. Mapping the mineral nanoparticles effective surface charge and hydration structure is therefore essential for (geo)chemists and environmental scientists to understand the molecular processes.
Currently, the chemical modeling of relatively simple surfaces fails when incorporating for a wide range of salt concentrations and pH values. This is further hampered by the lack of systematic studies that probe a wide range of electrolyte solutions. But also questions like how water molecules form self organized layers at the interface, how these layers depend on the surface crystallinity, whether they are surface charge dependent and the origin of the monotonic hydration are still largely unanswered. In this thesis, I will address some of those questions, in the following order.
In chapter 2 we will try to further understand the implications of photothermal excitation on the measured forces in Atomic Force Microscopy. At this point often the same force inversion routine is used for acoustic and photothermal excitation, which does not necessarily give equal results. The force inversion routine also depends on the tip-sample geometry, which will be given for commonly used tip-sample geometries. Furthermore, the procedure to extract diffuse layer charge values from the forces will be described, using charge regulation boundary conditions. Since the charge regulation boundary conditions require a chemical model, which is not always available, we develop an exponential surface charge model. This model enables the separation of diffuse layer charge/potential calculations and surface chemistry. Finally, we explore the added value of principal component analysis, which in other scientific fields can improve the data quality significantly.
A colloidal probe has a large interaction area and can be used when the primary interest is to measure the interaction force precisely, at costs of the lateral resolution. In chapter 3 we will characterize the diffuse layer charge using colloidal probe force spectroscopy of a typical rock surface, in this case silica. To do so, we accurately determine the diffuse layer charge over a wide range of pH and sodium chloride concentration, without any specific chemical knowledge of the surface. The diffuse layer charge values serve as an input for the chemical modeling, with the aim to verify whether the surface can be described with chemical equilibrium constants. Usually these constants are determined for smaller ranges of pH and salt concentration, where possible fitting issues or deviations are less likely to occur.
As discussed in the introduction, clays can have a significant effect on the amount of released oil. Often, in studies where the influence of these clays is analyzed, the crystallographic planes of these minerals are modeled as smooth surfaces. The surfaces do not have any broken bonds, lattice imperfections or other defects. In other words, the clay surfaces have a homogeneous distributed diffuse layer charge density. In chapter 4 we measure the diffuse layer charge of gibbsite. We do this using sharp tips, with the aim to capture the homogeneity, or heterogeneity of the diffuse layer charge distribution of gibbsite. We show that even the surface of synthesized gibbsite contains many defects, and that these defects are more pH responsive than the basal plane of gibbsite. Surface defects are often expected to have an impact on clay mineral surfaces, however, its impact on the diffuse layer charge is shown here for the first time.
The affinity of oil towards clay minerals is in a large part determined by interaction forces such as electrostatics and hydration forces. In most studies only one part of these interaction forces is studied, like in chapter 3 and 4. In chapter 5 we measure the lateral distribution of hydration forces, while simultaneously measuring the electrostatic force on silica and gibbsite. Usually, these are two separate measurements, because of the different length scales involved. Combining both is unprecedented and combining both allows us to bridge the gap between colloidal scale continuum DLVO forces and molecular scale hydration forces.
In previous chapters we have used monovalent salt for our ambient electrolyte solution. However, as mentioned previously, it is the divalent ions that play an important role in retaining oil in the reservoir. In previous work, it was shown that in pH 6 solutions the calcium ions adsorb to the basal plane of gibbsite. DFT calculations were able to reproduce this and to ensure charge neutrality OH- ions were introduced. One of the observations was that these ions were involved in stabilizing the calcium ions on the gibbsite surface. To study the validity of these findings, the diffuse layer charge of gibbsite was measured in various pH solutions in chapter 6. At high concentrations (> 50 mM) the anion adsorbs on top of the calcium ion. To investigate whether this is chemically specific, we also use solutions of calcium with different anions.
