This thesis describes a detailed investigation of the nanoscale interactions between organic molecules and calcite surfaces in aqueous media. A better understanding of the interactions at such interfaces is of interest for many geochemical and biological systems, as well as for several industrial applications. The starting point for my study was to understand the effect that organic molecules have on calcite surfaces. Calcite is the most stable polymorph of calcium carbonate (CaCO3) and is one of the main components of limestone which is widely used in producing cementitious materials. The effect of organic additives on the properties and behaviour of cements has been widely studied in macroscopic systems but there is still much to learn by exploring the fundamental relationships at the organic-water-mineral interface, at a scale where individual processes can be differentiated, characterized and understood.
In this work, surface interaction between calcite and organic molecules in a variety of solutions was studied using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), which allow us to examine interface processes from a nanoscale perspective. AFM measurements were made in chemical force mapping (CFM) mode, where the changes on the adhesion force between molecularly modified surfaces were measured. The functionalisation was made by the formation of a self assembled monolayer (SAM) using thiol molecules. These spontaneously bind to gold covered tip and surfaces, exposing a desired organic group over the surface. The effect of different cations in solution on the adhesion forces was also evaluated. This was motivated by the presence of cations in aquatic environments and their known influence on the interaction between organic compounds and minerals.
This study is presented as three parts. In the first part, the use of SAM as model organic surfaces facilitates the observation of the effects from Na+, Mg2+, Ca2+, Sr2+ and Ba2+ solutions on the adhesion force. In each system, tip and surface were functionalised to have hydrophobic (-CH3) or hydrophilic (-COO-) groups exposed at the surface. Results show that in the presence of a charged -COO- surface, adhesion forces changed consistently depending on the cation present in the solution. Adhesion forces recorded by CFM were higher than the theoretical adhesion force calculated by a rough approximation of the contributing forces: van der Waals (FvdW), electric double layer (FEDL) and hydration (FHI). Ion bridging between the -COO- groups and the cations appropriately explains ion specific effects observed on the studied systems.
In a second project, a new carboxylate surface -DiCOO-, with two carboxyl terminations on the same molecule, was synthesized to observe the impact of carboxylate structure on the adhesion force. Adhesion between combined surface pairs of -CH3, -COO- and -DiCOO- was measured during exposure to solutions of Na+, Mg2+, Ca2+, Sr2+ and Ba2+. The presence of a -DiCOO- surface resulted in lower adhesion forces and a different sequence in ion response, compared with systems where -COO- surfaces were present. Differences were correlated with the differences in chemical structure, coverage and intermolecular interactions between the carboxylate monolayers. Specific ion effects could be still explained by ion bridging, where differences in complexation of the exposed carboxylates on the interacting monolayers led to preferred interaction between -DiCOO- and Sr2+ or Ca2+, depending on the composition of the systems.
In a final study, calcite surfaces were examined with CFM. -CH3, -COO-, -DiCOO- SAM functionalised tips and calcite tips, modified with glued nanoparticles on it, were used to probe a freshly cleaved calcite surface. The measurements were made with the same solutions that were used in the previous studies, to make it possible to compare the effect of ions on the interactions. Specific ion effects were observed, even when calcite surfaces interacted with nonpolar groups (-CH3), supporting previous results where the presence of a single charged surface, such as calcite, on the interacting system promotes ion bridging. Depending on the functionalisation of the tip, differences in adhesion forces were observed: adhesion between calcite and the -CH3 tip increased the most in the presence of the Sr2+ solution whereas when a polar group, the -COO- or the -DiCOO- tips probed the calcite surface, the Ca2+ solution caused the strongest increase in the adhesion. Certainly there is some equilibration between the solution and the calcite surfaces that would affect the morphology of the surface and possibly also the adhesion force. At pH ~8.3 we expected to have some dissolved Ca2+ ions in the reference solution (~1 mM) but in a much lower concentration than the added divalent cation concentration (12 mM), thus the evaluated cation effect was not significantly affected by equilibration of the calcite surface to the solution. Because calcite surfaces can vary locally in composition and structure, several experiments were needed to gather enough data to make statistically relevant conclusions on calcite-calcite CFM experiments. The variable shape of the calcite tips has probably also an effect resulting in lower adhesion forces when calcite tips interact. In Sr2+ and Ca2+ solutions, calcite-calcite interaction induced an increase in the adhesion force whereas in Mg2+ and Ba2+ solutions, adhesion forces were lower than in the reference solution (0.5 M NaCl). This could be explained by the loss of available surface charge on calcite because of the screening effect of water and cations.
Adhesion forces between tw o calcite surfaces were also measured during the exposure to carboxylate solutions at pH ~8.3. Low molecular weight dicarboxylates were used to allow complete dissolution in water. A general increase in the adhesion force was observed when calcite-calcite systems were exposed to carboxylate solutions compared with the reference solution of 0.5 M NaCl. Thus if there was any effect from calcite dissolution, if will be negligible compared to the effect the added divalent cations, which were at concentration of 12 mM or the reference solution at 0.5 M.
The length of the aliphatic chain of the carboxylate did not have a monotonic effect on the adhesion force but a change in calcite morphology was observed, depending on which carboxylate solution was used. SEM images illustrated the strong effect of carboxylate solutions on calcite surface morphology, such as after exposure to an ethanol solution of -COO- or -DiCOO- thiol, compared with calcite before and after exposure to an ethanol solution of -CH3 thiol.
The overall results of my research demonstrate the key role of cations on the interaction of organic and mineral surfaces, as well as the important effect of the chemical nature of the organic compounds involved in the interaction. Real calcite surfaces are far more complex than any modelled surface. The interplay of concentration, pH, ionic potential and cation hydration makes it difficult to differentiate processes that take place in the water-organic-mineral interface. Ions play an important role on the interaction between calcite and organic molecules just as they play a role in the interaction of surfaces where organic molecules dominate. Ion specific effects depend on the characteristics of the ion (ionic strength, hydration) and on the nature (i.e., polar, non-polar) as well as the chemical structure (monocarboxylate, dicarboxylate, packing density, intermolecular interactions) of the interacting organic species.