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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Solid-solid interfaces as critical regions in rocks and materials: probing forces, electrochemical reactions, friction and reactivity.

Alternative title: Grenseflater mellom faste stoffer som kritiske områder i stein og materialer: krefter, elektrokjemiske reaksjoner, friksjon og reaktivitet

Awarded: NOK 3.2 mill.

The overall strength of rocks and granular materials is often associated with processes that operate at contacts between individual solid grains. The overarching goal of this project was to recognize which processes make the solid-solid interfaces weak, and how to convert the weak interfaces into strong ones. Although we see the destructive effects of weak interfaces at a macroscopic scale (earthquakes, rock compaction, subsidence, and general material failure), the very mechanisms governing the interfacial strength are frequently operating at much smaller scales (10-9 m). To recognize these mechanisms and be able to modify them, we need analytical methods that enable us to investigate the relevant nano-processes. In this experimental project, the tool to investigate the interfaces was the Surface Forces Apparatus (SFA), which allows studying surface forces (adhesion and repulsion), surface reactivity, and surface corrosion processes. In this project, we focused on several minerals: mica, which is an atomically smooth model surface to resolve nanoscale surface forces; calcite, which is of major importance in geological environments (such as drinking water aquifers, seismic regions, and hydrocarbon reservoirs), and calcium silicate, which is a precursor for extremely cohesive calcium silicate hydrate in Portland cement. Experiments performed with atomically smooth mica surfaces helped us to recognize the influence of ions on the interfacial properties and aggregation of mica. We showed that adhesion between mica surfaces is strongly cation-dependent and related these measurements to the interfacial properties of mica and the hydration properties of the studied cations (publication under preparation). We then explored how simple ions, such as Na+ and Ca2+, influence the adsorption of organic molecules onto mica. We used dicarboxylic acids that comprise a good model organic compound for low-weight organic matter present in the environment. While Ca2+ enhances the adsorption of dicarboxylic anions by acting as a cationic bridge, Na+ does not favor any significant binding of the organic ions (published: https://doi.org/10.1021/acs.langmuir.0c02290 ). We further used an electrochemical version of the SFA (EC-SFA) to show how simple inorganic ions are transported under confinement when the gap between two contacting surfaces is only a few nanometers thick. We could visualize this process by contacting mica against a polarizable gold surface. Ions were added or removed from the gap between the surfaces by changing the surface charge of gold. We could then estimate the speed of ion exchange (publication submitted). We then explored surface forces acting between two calcite surfaces. Here, we showed that Ca2+ ions decrease adhesion between two calcite surfaces, as the low surface charge of calcite does not promote attractive ion correlation forces (published: https://doi.org/10.1021/acsearthspacechem.1c00220 ). In the context of the project, these works underline that forces between mineral surfaces depend on both solution composition and mineral type, with consequences for porosity development, compaction, and cohesive properties of mineral-based materials. Later in the project, we developed a methodology to study the interfacial properties of calcite in the EC-SFA. Our EC-SFA experiments demonstrate variable adhesion between calcite surfaces and electrochemically modulated gold surfaces, which allows us to determine the surface charge properties of calcite in a range of geologically and environmentally relevant solution conditions. These findings have implications for the strength of fluid-saturated carbonates (publication under preparation). We also used EC-SFA to induce the growth of calcite crystals in a confined geometry. We relate these findings to the strength of calcite contacts, where moderately repulsive forces and no cementing properties were associated with calcite nucleation in nanosized pores (publication under preparation). The subproject focused on calcium silicate surfaces that provide cohesion upon their hydration. Such reactions provide early-stage mechanical strength to Portland cement. Our findings confirm that calcium silicate surfaces become highly adhesive upon their exposure to water, irrespective of their high surface roughness. Our methodology is important in the context of developing binding materials alternative to calcium carbonate-derived types of cement (published: https://doi.org/10.1021/acs.langmuir.2c02783 ). In the last subproject, we studied cementing properties of nanoparticle suspensions upon their aggregation in confined spaces. Using SFA, we could quantify the force needed to break the cemented contact between two consolidated surfaces. These findings have important implications for architecture conservation, where nanoparticle mineral suspensions are commonly used as consolidating agents applied to weathered stones (published: https://doi.org/10.1021/acs.langmuir.2c00486 ).

The project outcomes include the new interdisciplinary knowledge and understanding of several major phenomena related to mineral-water-mineral interaction zones and significant methodological developments. In detail, the project generated a significant amount of new experimental results that helped to identify interfacial phenomena relevant for the strength of solid-solid contacts in aqueous solutions. We generated quantitative data on adhesive and repulsive surface forces acting between mineral surfaces in the presence of simple salt ions, organic molecules, and nanoparticles. We also generated data forces acting between mineral surfaces with changing properties, by changing a surface charge of one of the mineral surfaces, or by allowing mineral nucleation and growth on one of the surfaces. We extended surface forces apparatus (SFA) technique to study many different, and also reactive, mineral surfaces, expanding the use of the technique to scientific disciplines, in which it was never or scarcely used. After the completed mobility period, we introduced the newly developed methodologies to the host organization: NJORD centre at the University of Oslo. The main impact of our project is to advance the basic knowledge about the strength of solid-solid interfaces and to provide a method to study these interfaces from a new surface forces-based perspective. Our methodological developments can likely provide more data in the future that cannot be accessed with any other experimental method. The results were obtained in experiments at the nanoscale but the knowledge can advance understanding of the processes observed at the macroscale. As such, in the long-term, the project outputs will likely have impact on many applied science fields including geotechnical engineering, soil science, geophysics, rock compaction, subsurface reservoir characterization, environmental engineering, materials engineering, and cultural heritage conservation.

Solid-solid interfaces are extremely important regions in rocks and materials. The interfaces, which are common boundaries of two alike or dissimilar solids, are the most active regions in a bulk material. A range of paramount phenomena can occur in these contact spaces. To name a few, the interfaces: enable and govern mass transport within the material; allow movement in a specific direction along the interface; confine water and other phases in narrow spaces, endowing these phases with unexpected properties; alter reactivity of contacting surfaces; and initiate surface corrosion processes. The overarching goal of this project is to recognize which processes make the interfaces weak, and how to convert the weak interfaces into the strong ones. Although we see destructive effects of weak interfaces at a macroscopic scale (earthquakes, rock compaction and subsidence, general material failure), the very mechanisms governing the interfacial strength are frequently operating at very small scales (nanometers). To recognize these mechanisms and be able to modify them, we need analytical methods that enable us to investigate the relevant nano-processes. In this experimental project, we will address the interfacial processes from the 4 main perspectives: 1) normal forces acting at the solid-solid contacts (adhesion and repulsion); 2) friction forces between two surfaces that move laterally; 3) surface reactivity in confinement; and 4) electrochemical reactions (corrosion) at interfaces. We believe that this holistic approach is necessary to improve our understanding of the interfacial processes at solid-solid contacts. There is one single instrument that allows looking at these four processes at the same time - the Surface Forces Apparatus (SFA), and we aim to use it in this project. Using the SFA, we will be able to understand how these phenomena are interconnected and affect each other, and which processes may lead to interface weakening or strengthening.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek