Surface Forces Apparatus Measurements of Interactions between Rough and Reactive Calcite Surfaces

Wettability of rock minerals and the underlying surface forces: A review of the implications for oil recovery and geological storage of CO2

The wettability of subsurface minerals is a critical factor influencing the pore-scale displacement of fluids in underground reservoirs. As such, it plays a key role in hydrocarbon production and greenhouse gas geo-sequestration. We present a comprehensive and critical review of the current state of knowledge on the intermolecular forces governing wettability of rock minerals most relevant to subsurface fluid storage and recovery. In this review we first provide a detailed summary of the available data, both experimental and theoretical, from the perspective of the fundamental intermolecular and surface forces, specifically considering the roles played by the surface chemistry, fluid properties, as well as other significant factors. We subsequently offer an analysis of the effects of chemical additives such as surfactants and nanoparticles that have emerged as viable means for manipulating wettability. In each example, we highlight the practical implications for hydrocarbon production and CO2 geo-storage as two of the most important current applications. As the physico-chemical mechanisms governing the wetting phenomena are the main focus, special emphasis is placed on nano-scale experimental approaches along with atomic-scale modeling that specifically probe the underlying intermolecular and surface forces. Lastly, we discuss the gaps in the current state of knowledge and outline future research directions to further our fundamental understanding of the interactions and their impact on the wetting characteristics of Earth's minerals.

Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals

HYPOTHESIS

This study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties.

EXPERIMENTS

This work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3-10 % RH) by AFM.

FINDINGS

Three logarithmic regimes for the velocity-dependence of friction are identified. BelowTc ∼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W1) to velocity-strengthening friction (S1). AboveTc ∼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S0) precedes velocity-weakening friction (W1). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above Tc -likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S0). These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.

The surface force balance: direct measurement of interactions in fluids and soft matter

Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is thesurface force balance(SFB), orsurface forces apparatus, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1 nm resolution, over separations from about 1 µm down to contact. The measured interaction forcevs.distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structurein situ. Here, we provide an overview the operating principles and capabilities of the SFB with particular focus on the recent developments and future possibilities of this remarkable technique.

The dynamic nature of natural and fatty acid modified calcite surfaces

Calcium carbonate, particularly in the form of calcite, is an abundant mineral widely used in both human-made products and biological systems. The calcite surface possesses a high surface energy, making it susceptible to the adsorption of organic contaminants. Moreover, the surface is also reactive towards a range of chemicals, including water. Consequently, studying and maintaining a clean and stable calcite surface is only possible under ultrahigh vacuum conditions and for limited amounts of time. When exposed to air or solution, the calcite surface undergoes rapid transformations, demanding a comprehensive understanding of the properties of calcite surfaces in different environments. Similarly, attention must also be directed towards the kinetics of changes, whether induced by fluctuating environments or at constant condition. All these aspects are encompassed in the expression "dynamic nature", and are of crucial importance in the context of the diverse applications of calcite. In many instances, the calcite surface is modified by adsorption of fatty acids to impart a desired nonpolar character. Although the binding between carboxylic acid groups and calcite surfaces is strong, the fatty acid layer used for surface modification undergoes significant alterations when exposed to water vapour and liquid water droplets. Therefore, it is also crucial to understand the dynamic nature of the adsorbed layer. This review article provides a comprehensive overview of the current understanding of both the dynamics of the calcite surface as well as when modified by fatty acid surface treatments.

A comprehensive review on the behavior and evolution of oil droplets during oil/water separation by membranes

Membrane separation technology has significant advantages for treating oil-in-water emulsions. Understanding the evolution of oil droplets could reveal the interfacial and colloidal interactions, facilitate the design of advanced membranes, and improve the separation performances. This review on the characteristic behavior and evolution of oil droplets focuses on the advanced analytical techniques, and the subsequent fouling as well as demulsification effects during membrane separation. A detailed introduction is provided on microscopic observations and numerical simulations of the dynamic evolution of oil droplets, featuring real-time in-situ visualization and accurate reconstruction, respectively. Characteristic behaviors of these oil droplets include attachment, pinning, wetting, spreading, blockage, intrusion, coalescence, and detachment, which have been quantified by specific proposed parameters and criteria. The fouling process can be evaluated using Hermia and resistance models. The related adhesion force and intrusion pressure as well as droplet-droplet/membrane interfacial interactions can be accurately quantified using various force analysis methods and advanced force measurement techniques. It is encouraging to note that oil coalescence has been achieved through various effects such as electrostatic interactions, mechanical actions, Laplace pressure/surface free energy gradients, and synergistic effects on functional membranes. When oil droplets become destabilized and coalesce into larger ones, the functional membranes can overcome the limitations of size-sieving effect to attain higher separation efficiency. This not only bypasses the trade-off between permeability and rejection, but also significantly reduces membrane fouling. Finally, the challenges and potential research directions in membrane separation are proposed. We hope this review will support the engineering of advanced materials for oil/water separation and research on interface science in general.

Interaction between Hydrated Smectite Clay Particles as a Function of Salinity (0-1 M) and Counterion Type (Na, K, Ca)

Swelling clay minerals control the hydrologic and mechanical properties of many soils, sediments, and sedimentary rocks. This important and well-known phenomenon remains challenging to predict because it emerges from complex multiscale couplings between aqueous chemistry and colloidal interaction mechanics in nanoporous clay assemblages, for which predictive models remain elusive. In particular, the predominant theory of colloidal interactions across fluid films, the widely used Derjaguin-Landau-Verwey-Overbeek model, fails to predict the ubiquitous existence of stable swelling states at interparticle distances below 3 nm that are stabilized by specific inter-atomic interactions in overlapping electrical double layers between the charged clay surfaces. Atomistic simulations have the potential to generate detailed insights into the mechanisms of these interactions. Recently, we developed a metadynamics-based molecular dynamics simulation methodology that can predict the free energy of interaction between parallel smectite clay particles in a wide range of interparticle distances (from 0.3 to 3 nm) and salinities (from 0.0 to 1.0 M NaCl). Here, we extend this work by characterizing the sensitivity of interparticle interactions to counterion type (Na, K, Ca). We establish a detailed picture of the free energy of interaction of parallel clay particles across water films as the sum of five interaction mechanisms with different sensitivities to salinity, counterion type, and interparticle distance.

Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C₃S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C₃S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C₃S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C₃S paste. We further prepared thin films of C₃S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C₃S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C₃S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C₃S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

Limits to Crystallization Pressure

Crystallization pressure drives deformation and damage in monuments, buildings, and the Earth's crust. Even though the phenomenon has been known for 170 years, there is no agreement between theoretical calculations of the maximum attainable pressure and experimentally measured pressures. We have therefore developed a novel experimental technique to image the nanoconfined crystallization process while controlling the pressure and applied it to calcite. The results show that displacement by crystallization pressure is arrested at pressures well below the thermodynamic limit. We use existing molecular dynamics simulations and atomic force microscopy data to construct a robust model of the disjoining pressure in this system and thereby calculate the absolute distance between the surfaces. On the basis of the high-resolution experiments and modeling, we formulate a novel mechanism for the transition between damage and adhesion by crystallization that may find application in Earth and materials sciences and in conservation of cultural heritage.

Cohesion Gain Induced by Nanosilica Consolidants for Monumental Stone Restoration

Mineral nanoparticle suspensions with consolidating properties have been successfully applied in the restoration of weathered architectural surfaces. However, the design of these consolidants is usually stone-specific and based on trial and error, which prevents their robust operation for a wide range of highly heterogeneous monumental stone materials. In this work, we develop a facile and versatile method to systematically study the consolidating mechanisms in action using a surface forces apparatus (SFA) with real-time force sensing and an X-ray surface forces apparatus (X-SFA). We directly assess the mechanical tensile strength of nanosilica-treated single mineral contacts and show a sharp increase in their cohesion. The smallest used nanoparticles provide an order of magnitude stronger contacts. We further resolve the microstructures and forces acting during evaporation-driven, capillary-force-induced nanoparticle aggregation processes, highlighting the importance of the interactions between the nanoparticles and the confining mineral walls. Our novel SFA-based approach offers insight into nano- and microscale mechanisms of consolidating silica treatments, and it can aid the design of nanomaterials used in stone consolidation.

Influence of Thin Film Deposition on AFM Cantilever Tips in Adhesion and Young’s Modulus of MEMS Surfaces

Adhesion is a critical factor in microelectromechanical systems (MEMSs) and is influenced by many parameters. In important fields, such as microassembly, an improved understanding of adhesion can result in higher precision. This study examines the influence of deposition of gold and titanium onto the atomic force microscope (AFM) tips in adhesion forces and Young’s modulus, between a few MEMS substrates (silicon, gold, and silver) and the AFM tips. It was found that, except for gold substrate, an AFM tip coated with gold has the highest adhesion force of 42.67 nN for silicon substrates, whereas the titanium-coated AFM tip decreases the force for all the samples. This study suggests that such changes must be taken into account while studying the adhesion force. The final results indicate that utilizing gold substrate with titanium AFM tip led to the lowest adhesion force, which could be useful in adhesion force measurement during microassembly.

Ca2+ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy

Solution composition-sensitive disjoining pressure acting between the mineral surfaces in fluid-filled granular rocks and materials controls their cohesion, facilitates the transport of dissolved species, and may sustain volume-expanding reactions leading to fracturing or pore sealing. Although calcite is one of the most abundant minerals in the Earth's crust, there is still no complete understanding of how the most common inorganic ions affect the disjoining pressure (and thus the attractive or repulsive forces) operating between calcite surfaces. In this atomic force microscopy study, we measured adhesion acting between two cleaved (104) calcite surfaces in solutions containing low and high concentrations of Ca²⁺ ions. We detected only low adhesion between calcite surfaces, which was weakly modulated by the varying Ca²⁺ concentration. Our results show that the more hydrated calcium ions decrease the adhesion between calcite surfaces with respect to monovalent Na⁺ at a given ionic strength, and thus Ca²⁺ can sustain relatively thick water films between contacting calcite grains even at high overburden pressures. These findings suggest a possible loss of cohesion and continued progress of reaction-induced fracturing for weakly charged minerals in the presence of strongly hydrated ionic species.

Nanoscale Wear and Mechanical Properties of Calcite: Effects of Stearic Acid Modification and Water Vapor

Understanding the wear of mineral fillers is crucial for controlling industrial processes, and in the present work, we examine the wear resistance and nanomechanical properties of bare calcite and stearic acid-modified calcite surfaces under dry and humid conditions at the nanoscale. Measurements under different loads allow us to probe the situation in the absence and presence of abrasive wear. The sliding motion is in general characterized by irregular stick-slip events that at higher loads lead to abrasion of the brittle calcite surface. Bare calcite is hydrophilic, and under humid conditions, a thin water layer is present on the surface. This water layer does not affect the friction force. However, it slightly decreases the wear depth and strongly influences the distribution of wear particles. In contrast, stearic acid-modified surfaces are hydrophobic. Nevertheless, humidity affects the wear characteristics by decreasing the binding strength of stearic acid at higher humidity. A complete monolayer coverage of calcite by stearic acid results in a significant reduction in wear but only a moderate reduction in friction forces at low humidity and no reduction at 75% relative humidity (RH). Thus, our data suggest that the wear reduction does not result from a lowering of the friction force but rather from an increased ductility of the surface region as offered by the stearic acid layer. An incomplete monolayer of stearic acid on the calcite surface provides no reduction in wear regardless of the RH investigated. Clearly, the wear properties of modified calcite surfaces depend crucially on the packing density of the surface modifier and also on the air humidity.

Nanoscale insight into the relation between pressure solution of calcite and interfacial friction

Pressure solution of carbonate-based rocks participates in many geophysical and geochemical processes, but fundamental knowledge of the interfacial processes is still lacking. By concurrently pressing and sliding two single calcite crystals past each other, the pressure solution rate and the friction force between the crystals were concurrently measured in calcium-carbonate saturated water with an extended surface forces apparatus. These studies reveal that both a decrease and an increase in frictional strength can originate from the pressure-solution of calcite single crystals. By conducting nanoscale force measurements with an atomic force microscope, ion specific effects were unveiled at the level of a single asperity. Pressure solution is promoted when the interfacial water layers of calcite remain undisturbed under stress (e.g. with Ca(II)) and the dissolved ions and water lubricate the interface - a phenomenon called pressure-solution facilitated slip. The mechanically induced disruption of the hydration layers of the calcite surface (e.g. with Mg(II) and low Ni(II) concentration) correlates with the more fluid-like and lubricious behavior of the confined fluid in the absence of pressure solution. Charge neutralization of the calcite surface leads to an abrupt change of calcite's hydration layers, which promotes pressure-solution facilitated slip. This work advances the fundamental understanding of physicochemical interactions occurring at confined surfaces of stressed calcite.

Nanoscale Forces between Basal Mica Surfaces in Dicarboxylic Acid Solutions: Implications for Clay Aggregation in the Presence of Soluble Organic Acids

The stability of organomineral aggregates in soils has a key influence on nutrient cycling, erosion, and soil productivity. Both clay minerals with distinct basal and edge surfaces and organic molecules with reactive functional groups offer rich bonding environments. While clay edges often promote strong inner-sphere bonding of -COOH-laden organics, we explore typically weaker, outer-sphere bonding of such molecules onto basal planes and its significance in organomineral interactions. In this surface force apparatus study, we probed face-specific interactions of negatively charged mica basal surfaces in solutions containing carboxyl-bearing, low-molecular-weight dicarboxylic acids (DAs). Our experiments provide distance-resolved, nanometer-range measurements of forces acting between two (001) mica surfaces and simultaneously probe DA adsorption. We show that background inorganic ions display crucial importance in nanoscale forces acting between basal mica surfaces and in DA adsorption: Na⁺ contributes to strong repulsion and little binding of dicarboxylic anions, while small amounts of Ca²⁺ are sufficient to screen the basal surface charge of mica, facilitate strong adhesion, and enhance dicarboxylic anion adsorption by acting as cationic bridges. Despite reversible and weak adsorption of DAs, we resolve their multilayer binding via assembly of hydrophobic chains in the presence of Ca²⁺, pointing the importance of abundant, less reactive basal clay surfaces in organomineral interactions.

Molecular dynamics simulations of the colloidal interaction between smectite clay nanoparticles in liquid water

Colloidal interactions between clay nanoparticles have been studied extensively because of their strong influence on the hydrology and mechanics of many soils and sedimentary media. The predominant theory used to describe these interactions is the Derjaguin-Landau-Verwey-Overbeek (DLVO) model, a framework widely applied in colloidal and interfacial science that accurately predicts the interactions between charged surfaces across water films at distances greater than ~ 3 nm (i.e., ten water monolayers). Unfortunately, the DLVO model is inaccurate at the shorter interparticle distances that predominate in most subsurface environments. For example, it inherently cannot predict the existence of equilibrium states wherein clay particles adopt interparticle distances equal to the thickness of one, two, or three water monolayers. Molecular dynamics (MD) simulations have the potential to provide detailed information on the free energy of interaction between clay nanoparticles; however, they have only been used to examine clay swelling and aggregation at interparticle distances below 1 nm. We present the first MD simulation predictions of the free energy of interaction of smectite clay nanoparticles in the entire range of interparticle distances from the large interparticle distances where the DLVO model is accurate (>3 nm) to the short-range swelling states where non-DLVO interactions predominate (<1 nm). Our simulations examine a range of salinities (0.0 to 1.0 M NaCl) and counterion types (Na, K, Ca) and establish a detailed picture of the breakdown of the DLVO model. In particular, they confirm previous theoretical suggestions of the existence of a strong non-DLVO attraction with a range of ~ 3 nm arising from specific ion-clay Coulomb interactions in the electrical double layer.

Mechanisms of Phase Transformation and Creating Mechanical Strength in a Sustainable Calcium Carbonate Cement

Calcium carbonate cements have been synthesized by mixing amorphous calcium carbonate and vaterite powders with water to form a cement paste and study how mechanical strength is created during the setting reaction. In-situ X-ray diffraction (XRD) was used to monitor the transformation of amorphous calcium carbonate (ACC) and vaterite phases into calcite and a rotational rheometer was used to monitor the strength evolution. There are two characteristic timescales of the strengthening of the cement paste. The short timescale of the order 1 h is controlled by smoothening of the vaterite grains, allowing closer and therefore adhesive contacts between the grains. The long timescale of the order 10-50 h is controlled by the phase transformation of vaterite into calcite. This transformation is, unlike in previous studies using stirred reactors, found to be mainly controlled by diffusion in the liquid phase. The evolution of shear strength with solid volume fraction is best explained by a fractal model of the paste structure.

Crystallization in Confinement

Many crystallization processes of great importance, including frost heave, biomineralization, the synthesis of nanomaterials, and scale formation, occur in small volumes rather than bulk solution. Here, the influence of confinement on crystallization processes is described, drawing together information from fields as diverse as bioinspired mineralization, templating, pharmaceuticals, colloidal crystallization, and geochemistry. Experiments are principally conducted within confining systems that offer well-defined environments, varying from droplets in microfluidic devices, to cylindrical pores in filtration membranes, to nanoporous glasses and carbon nanotubes. Dramatic effects are observed, including a stabilization of metastable polymorphs, a depression of freezing points, and the formation of crystals with preferred orientations, modified morphologies, and even structures not seen in bulk. Confinement is also shown to influence crystallization processes over length scales ranging from the atomic to hundreds of micrometers, and to originate from a wide range of mechanisms. The development of an enhanced understanding of the influence of confinement on crystal nucleation and growth will not only provide superior insight into crystallization processes in many real-world environments, but will also enable this phenomenon to be used to control crystallization in applications including nanomaterial synthesis, heavy metal remediation, and the prevention of weathering.

The influence of surface roughness on the adhesive interactions and phase behavior of suspensions of calcite nanoparticles

We investigate the impact of nanoparticle roughness on the phase behaviour of suspensions in models of calcium carbonate nanoparticles. We use a Derjaguin approach that incorporates roughness effects and interactions between the nanoparticles modelled with a combination of DLVO forces and hydration forces, derived using experimental data and atomistic molecular dynamics simulations, respectively. Roughness effects, such as atomic steps or terraces appearing in mineral surfaces result in very different effective inter-nanoparticle potentials. Using stochastic Langevin Dynamics computer simulations and the effective interparticle interactions we demonstrate that relatively small changes in the roughness of the particles modify significantly the stability of the suspensions. We propose that the sensitivity of the phase behavior to the roughness is connected to the short length scale of the adhesive attraction arising from the ordering of water layers confined between calcite surfaces. Particles with smooth surfaces feature strong adhesive forces, and form gel fractal structures, while small surface roughness, of the order of atomic steps in mineral faces, stabilize the suspension. We believe that our work helps to rationalize the contrasting experimental results that have been obtained recently using nanoparticles or extended surfaces, which provide support for the existence of adhesive or repulsive interactions, respectively. We further use our model to analyze the synergistic effects of roughness, pH and ion concentration on the phase behavior of suspensions, connecting with recent experiments using calcium carbonate nanoparticles.

Effect of Divalent Cations on the Interaction of Carboxylate Self-Assembled Monolayers

Interactions between organic molecules in aqueous environments, whether in the fluid phase or adsorbed on solids, are often affected by the cations present in the solution. We investigated, at nanometer scale, how surface carboxylate interactions are influenced by dissolved divalent cations: Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. Self-assembled monolayer (SAM) surfaces with exposed terminations of alkyl, -CH₃, carboxylate, -COO^- , or dicarboxylate, -DiCOO^-, were deposited on gold-coated tips and substrates. We used atomic force microscopy (AFM), in chemical force mapping (CFM) mode, to measure adhesion forces between various combinations of SAMs on the tip and substrate, in solutions of 0.5 M NaCl, that contained 0.012 M of one of the divalent cations. The type of cation, the number of carboxyl groups that interact, and their structure on the SAM influenced adhesion between the surfaces. The effect of the reference solution, which only contains Na⁺ cations, on adhesion force was mainly attributed to van der Waals and hydrophobic forces, explaining the lower force in systems that are more hydrophilic, i.e., -COO^--COO^-, and higher force for more hydrophobic systems. For charged surfaces, i.e., -COO^- and -DiCOO^-, in divalent cation solutions results were consistent with ion bridging. The inclusion of a hydrophobic surface, i.e., the -CH₃-COO^- or -CH₃-DiCOO^- system, decreased the possibility for strong cation bridging with the charged surface, resulting in lower adhesion. For systems including -COO^-, the adhesion force series followed the inverse cation hydrated radius trend (Na⁺ ≈ Mg²⁺ < Sr²⁺ < Ca²⁺ < Ba²⁺) whereas -DiCOO^- was responsible for lower adhesion force and modified trends, depending on the corresponding surface in the system. Differences in force magnitude between the monolayers were correlated with lower charge availability on the -DiCOO^- surface as a result of fewer active sites, probably because of the tendency of exposed malonate surface groups to interact between them, as well as high rigidity, resulting from the molecule structure. The characteristic response of the -DiCOO^- surface in solutions of Sr²⁺ and Ca²⁺ was correlated with possible malonate complexation modes. Comparison with previous studies suggested that the strong response of a -DiCOO^- surface to Sr²⁺ resulted from bidentate chelation, whereas Ca²⁺ response was attributed to alpha-mode association to malonate.

Nucleation in confinement generates long-range repulsion between rough calcite surfaces

Fluid-induced alteration of rocks and mineral-based materials often starts at confined mineral interfaces where nm-thick water films can persist even at high overburden pressures and at low vapor pressures. These films enable transport of reactants and affect forces acting between mineral surfaces. However, the feedback between the surface forces and reactivity of confined solids is not fully understood. We used the surface forces apparatus (SFA) to follow surface reactivity in confinement and measure nm-range forces between two rough calcite surfaces in NaCl, CaCl₂, or MgCl₂ solutions with ionic strength of 0.01, 0.1 or 1 M. We observed long-range repulsion that could not be explained by changes in calcite surface roughness, surface damage, or by electrostatic or hydration repulsion, but was correlated with precipitation events which started at µm-thick separations. We observed a submicron-sized precipitate that formed in the confined solution. This liquid-like viscous precipitate did not undergo any spontaneous ripening into larger crystals, which suggested that confinement prevented its dehydration. Nucleation was significantly postponed in the presence of Mg²⁺. The long-range repulsion generated by nucleation between confined mineral surfaces can have a crucial influence on evolution of the microstructure and therefore the macroscopic strength of rocks and materials.

Simple ions control the elasticity of calcite gels via interparticle forces

Suspensions of calcite in water are employed in many industrial fields such as paper filling, pharmaceutics or heritage conservation. Whereas organics are generally used to tune the rheological properties of the paste, we also expect simple ions to be able to control the suspension rheology via the interparticle forces. We have thus investigated the impact of calcium, sodium and hydroxide ions on the elasticity of a colloidal gel of nanocalcite. We confront our macroscopic measurements to DLVO interaction potentials, based on chemical speciation and measurements of the zeta potential. Upon addition of calcium hydroxide, we observe a minimum in shear modulus, correlated to a maximum in the DLVO energy barrier, due to two competing effects: Calcium adsorption onto calcite surface rises the zeta potential, while increasing salt concentration induces stronger electrostatic screening. We also demonstrate that the addition of sodium hydroxide completely screens the surface charge and leads to a more rigid paste. A second important result is that carbonation of the calcite suspensions by the atmospheric CO₂ leads to a convergent high elasticity of the colloidal gels, whatever their initial value, also well rationalized by DLVO theory and resulting from a decrease in zeta potential.

Iceland spar calcite: Humidity and time effects on surface properties and their reversibility

Understanding the complex and dynamic nature of calcite surfaces under ambient conditions is important for optimizing industrial applications. It is essential to identify processes, their reversibility, and the relevant properties of CaCO₃ solid-liquid and solid-gas interfaces under different environmental conditions, such as at increased relative humidity (RH). This work elucidates changes in surface properties on freshly cleaved calcite (topography, wettability and surface forces) as a function of time (≤28 h) at controlled humidity (≤3-95 %RH) and temperature (25.5 °C), evaluated with atomic force microscopy (AFM) and contact angle techniques. In the presence of humidity, the wettability decreased, liquid water capillary forces dominated over van der Waals forces, and surface domains, such as hillocks, height about 7.0 Å, and trenches, depth about -3.5 Å, appeared and grew primarily in lateral dimensions. Hillocks demonstrated lower adhesion and higher deformation in AFM experiments. We propose that the growing surface domains were formed by ion dissolution and diffusion followed by formation of hydrated salt of CaCO₃. Upon drying, the height of the hillocks decreased by about 50% suggesting their alteration into dehydrated or less hydrated CaCO₃. However, the process was not entirely reversible and crystallization of new domains continued at a reduced rate.