PhD Defence Naveen Kumar

Naveen Kumar is PhD student in the MESA+ research group Physics of Complex Fluids. His promoter is Frieder Mugele.

surface charge characterization of solid-liquid interfaces using atomic force microscopy

In this thesis, we investigate the charging behavior and ion adsorption at solid-liquid interfaces. These phenomena play a crucial role in a wide range of fields, such as colloid science, self-assembly, wetting, electrochemistry, and molecular biology. The aim of this work is to understand the scientific challenges in enhanced oil recovery (EOR) processes. Surface charge properties of rock/clay surface plays an important role in understanding various fluid-rock/clay interactions in an oil reservoir. In this regard, we characterize the surface charge properties of rock/clay-water interface in various electrolyte solutions. In chapter 2, we provide an overview of the rock/clay materials, experiemental techniques and theoretical models used in the research. Firstly, clays in terms of their morphology and charge characteristics are discussed. We introduce kaolinite and mica as representative of 1:1 and 2:1 layer type clay mineral, respectively. Secondly, the techniques used to characterize the rock/clay-water interface i.e. AFM, contact angle goniometry and Langmuir trough are explained. We use dynamic force spectroscopy to record the tip-sample interaction forces in different aqueous conditions. We briefly discuss the force inversion formulae for amplitude modulation (AM) and frequency modulation (FM) modes. Further, a detailed description is presented of the theoretical model used to calculate the surface charge by analyzing the tip-sample interaction forces using DLVO theory with charge regulation boundary conditions. Lastly, we discuss the numerical approach to solve the Poisson-Boltzmann equation using a point and shoot method. As kaolinite is the most abundant clay material, we start our research by exploring its charging behavior. In Chapter 3, we determine the surface charge distribution on the two facets of kaolinite by selectively orient them on mica or sapphire substrates. The surface charge density is extracted from the measured force distance curves using DLVO theory. It is found that the surface charge is heterogeneously distributed on the basal plane of the kaolinite particles. The surface charge values vary from + 0.015 to 0 e/nm² for gibbsite facet and 0 to -0.045 e/nm² for silica facet. Using sharp AFM tips (tip radius ~ 3 nm) in non-contact AM-AFM, we resolved the atomic structure of the two facets of kaolinite and extract their lattice parameters. The combination of AFM spectroscopy and high resolution imaging offers a unique experimental capability to investigate charge characteristics of complex clay systems. In Chapter 3 we discuss only a single case: one concentration and a fixed pH, although oil reservoirs are much more complicated. Therefore, in Chapter 4, using the same methods and techniques, we investigate the effect of salt concentration and pH on the surface charge of kaolinite particles. The surface charge of the silica facet is always negative and increases in magnitude with increasing pH. While for the gibbsite facet, the surface charge is positive for 4 < pH < 6, and becomes negative for higher pH ~ 7. Further, it is found that the surface charge of the gibbsite facet at pH 6, increases up to a concentration of 10 mM CaCl₂ and starts to decrease upon further increasing the salt concentration to 50 mM. With the help of atomic scale imaging and the DFT calculations, we demonstrate that the surface charge is increased due to Ca²⁺ ion adsorption, while it is reduced due to Cl^- ion adsorption at higher CaCl₂ concentrations. After surface charge characterization of kaolinite particles, we focus in Chapter 5 our attention on the charging behavior of mica, which is a 2:1 type clay material. Specifically, we study the effect of pH, salt type and salt concentration on the surface charge of mica. It is observed that the surface charge of mica decreases with increasing monovalent salt concentration. As we go to a bigger monovalent cation radius (Li⁺ < Na⁺ < Cs⁺), the decrease in surface charge is more pronounced. The surface charge of mica in presence of divalent ions is reversed and becomes positive above a concentration of ~ 20 mM. In atomic scale images, we observe that the divalent ions (Ca²⁺) adsorb strongly to the mica surface and alter the hexagonal surface pattern to a rectangular pattern. In presence of Na⁺ ions, we observe a hexagonal surface pattern, which is due to the mica crystal lattice. While in case of Cs⁺ ions, some domain formation is observed, partially covering the mica lattice. The surface charge of mica is also pH dependent. It is observed that the effect of pH on surface charge of mica is significant in the range of 4 to 6, while charge changes slightly with further increasing the pH from 6 to 9. In Chapter 6, we investigate the adsorption/desorption of model oil to the model rock surface. Langmuir-Blodgett (LB) films of stearic acid (representative of polar organic components of oil) are deposited on silica in the presence of Ca²⁺ and/or Na⁺ ions. Large differences in macroscopic wettability (contact angles) are observed between the monolayer prepared in Ca²⁺ and Na⁺ ions sub-phases. Both contact angle and AFM imaging experiments reveal that the LB films prepared in presence of divalent ions (Ca²⁺) are more stable compared to monovalent ions (Na⁺). The observations on varying the composition of the droplets corroborated the stabilizing effect of Ca²⁺. We attribute these findings to the cation-bridging ability of Ca²⁺ ions, which can bind the negatively charged stearate groups to the negatively charged substrates.