Crystal growth and dissolution of gypsum and analogous materials : a multi-scale approach

This thesis is concerned with the growth and dissolution of gypsum and analogous crystalline materials, with the aim of understanding the kinetic and mechanistic processes at the mineral-solution interface. The research conducted was a collaborative project sponsored by Saint-Gobain Gypsum. First, an image processing (IP) software package was developed to meet highly specialised IP needs and expedite the extraction of vital surface information from images produced in the growth and dissolution studies carried out in this thesis. A simple but powerful morphological analysis of characteristic etch pit features formed on the basal plane of gypsum was proposed, to aid the determination of intrinsic dissolution kinetics. Limiting the study to short times produced microscopic active features, which exhibited high and quantitative mass transport rates. At early times, the reaction was surface controlled, with the edge planes dominating the process, revealing anisotropic step propagation kinetics. With time, an increased contribution from mass transport was observed, suggesting that at later times, the basal plane dominated reaction kinetics. Common ion effects indicated a greater impact of Ca2+ than SO42- in reducing dissolution rates while inert ions enhanced dissolution in a directionspecific way. With this approach, microscopic phenomena were related to macroscopic measurements thus reconciling experimental length scales. Dissolution of the basal (010) and edge (001) surfaces of gypsum and polycrystalline anhydrite, were probed at the bulk scale by coupling the channel flow cell (CFC) technique which displays high mass transport rates, with off-line spectrometric measurements of dissolved Ca2+. Quantitative modelling of the diffusion-reaction within the CFC yielded a linear rate law for the dissolution process. Rates from the basal plane and anhydrite were found to be consistent with other bulk measurements, while the highly reactive edge plane exhibited high rates indicating a transport-limited process. Sodium trimetaphosphate, a common humid-creep inhibitor was found to significantly retard basal plane dissolution rates. Further CFC studies were carried out on industrially-relevant, chemically modified CaSO4 based materials, using a simple flux approach. It was found that models proposing a dissolution-precipitation pathway as the mode of action of humid-creep inhibitors were less plausible than those proposing a surface binding pathway. Finally, the influence of solution stoichiometry, r = (aCa2+ / aSO42-) on the growth kinetics of microscopic gypsum crystals was determined at a constant supersaturation. Crystal growth was found to be entirely controlled by surface kinetics over the range of r, with the edge planes dominating the process. The highest lateral rates were found at r = 1, diminishing sharply at r ≠ 1, and indicating strong plane-specific dependence on Ca2+ and SO42- availability. Additionally, dramatic changes in the morphology of grown crystals were observed. Propagation of steps on the basal face revealed a complex polynuclear layer-by-layer growth process for this surface. Macroscopic growth rates compared well to previous bulk measurements indicating that the approach used provided a comprehensive multi-scale view of gypsum growth processes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Crystal growth and dissolution of gypsum and analogous materials : a multi-scale approach

This thesis is concerned with the growth and dissolution of gypsum and analogous crystalline materials, with the aim of understanding the kinetic and mechanistic processes at the mineral-solution interface. The research conducted was a collaborative project sponsored by Saint-Gobain Gypsum. First, an image processing (IP) software package was developed to meet highly specialised IP needs and expedite the extraction of vital surface information from images produced in the growth and dissolution studies carried out in this thesis. A simple but powerful morphological analysis of characteristic etch pit features formed on the basal plane of gypsum was proposed, to aid the determination of intrinsic dissolution kinetics. Limiting the study to short times produced microscopic active features, which exhibited high and quantitative mass transport rates. At early times, the reaction was surface controlled, with the edge planes dominating the process, revealing anisotropic step propagation kinetics. With time, an increased contribution from mass transport was observed, suggesting that at later times, the basal plane dominated reaction kinetics. Common ion effects indicated a greater impact of Ca2+ than SO42- in reducing dissolution rates while inert ions enhanced dissolution in a directionspecific way. With this approach, microscopic phenomena were related to macroscopic measurements thus reconciling experimental length scales. Dissolution of the basal (010) and edge (001) surfaces of gypsum and polycrystalline anhydrite, were probed at the bulk scale by coupling the channel flow cell (CFC) technique which displays high mass transport rates, with off-line spectrometric measurements of dissolved Ca2+. Quantitative modelling of the diffusion-reaction within the CFC yielded a linear rate law for the dissolution process. Rates from the basal plane and anhydrite were found to be consistent with other bulk measurements, while the highly reactive edge plane exhibited high rates indicating a transport-limited process. Sodium trimetaphosphate, a common humid-creep inhibitor was found to significantly retard basal plane dissolution rates. Further CFC studies were carried out on industrially-relevant, chemically modified CaSO4 based materials, using a simple flux approach. It was found that models proposing a dissolution-precipitation pathway as the mode of action of humid-creep inhibitors were less plausible than those proposing a surface binding pathway. Finally, the influence of solution stoichiometry, r = (aCa2+ / aSO42-) on the growth kinetics of microscopic gypsum crystals was determined at a constant supersaturation. Crystal growth was found to be entirely controlled by surface kinetics over the range of r, with the edge planes dominating the process. The highest lateral rates were found at r = 1, diminishing sharply at r ≠ 1, and indicating strong plane-specific dependence on Ca2+ and SO42- availability. Additionally, dramatic changes in the morphology of grown crystals were observed. Propagation of steps on the basal face revealed a complex polynuclear layer-by-layer growth process for this surface. Macroscopic growth rates compared well to previous bulk measurements indicating that the approach used provided a comprehensive multi-scale view of gypsum growth processes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Importance of mass transport and spatially heterogeneous flux processes for in situ atomic force microscopy measurements of crystal growth and dissolution kinetics

It is well-established that important information about the dissolution and growth of crystals can be obtained by the investigation of step movement on single-crystal faces via in situ AFM. However, a potential drawback of this approach for kinetic measurements is that the small region of investigation may not be representative of the overall surface. It is shown that the investigation of local processes without accounting for the processes outside the region of interest can lead to significant misinterpretation of the data collected. Taking the case of gypsum dissolution as an example, we critically analyze literature data and develop 3 different finite element method models that treat in detail the coupled mass transport–surface kinetic problem pertaining to dissolution processes in a typical AFM environment. It is shown that mass transport cannot be neglected when performing in situ AFM on macroscopic surfaces even with high-convection fluid cells. Moreover, crystal dissolution kinetics determined by AFM is mainly influenced by processes occurring in areas of the surface outside the region of interest. When this is recognized, and appropriate models are applied, step velocities due to dissolution are consistent with expectations based on macroscopic measurements, and the kinetic gap that is often apparent between nanoscale and macroscopic measurements is closed. This study provides a framework for the detailed analysis of AFM kinetic data that has wide utility and applicability

Holistic approach to dissolution kinetics : linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca2+ has a much greater impact in reducing dissolution rates compared to SO42−. With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements

Dual-barrel conductance micropipet as a new approach to the study of ionic crystal dissolution kinetics

A new approach to the study of ionic crystal dissolution kinetics is described, based on the use of a dual-barrel theta conductance micropipet. The solution in the pipet is undersaturated with respect to the crystal of interest, and when the meniscus at the end of the micropipet makes contact with a selected region of the crystal surface, dissolution occurs causing the solution composition to change. This is observed, with better than 1 ms time resolution, as a change in the ion conductance current, measured across a potential bias between an electrode in each barrel of the pipet. Key attributes of this new technique are: (i) dissolution can be targeted at a single crystal surface; (ii) multiple measurements can be made quickly and easily by moving the pipet to a new location on the surface; (iii) materials with a wide range of kinetics and solubilities are open to study because the duration of dissolution is controlled by the meniscus contact time; (iv) fast kinetics are readily amenable to study because of the intrinsically high mass transport rates within tapered micropipets; (v) the experimental geometry is well-defined, permitting finite element method modeling to allow quantitative analysis of experimental data. Herein, we study the dissolution of NaCl as an example system, with dissolution induced for just a few milliseconds, and estimate a first-order heterogeneous rate constant of 7.5 (±2.5) × 10–5 cm s–1 (equivalent surface dissolution flux ca. 0.5 μmol cm–2 s–1 into a completely undersaturated solution). Ionic crystals form a huge class of materials whose dissolution properties are of considerable interest, and we thus anticipate that this new localized microscale surface approach will have considerable applicability in the future

Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101̅4) cleavage surface (from natural “Iceland Spar”) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved “mirror” surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 μm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) × 10–4 m s–1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites

Crystal growth and dissolution of gypsum and analogous materials : a multi-scale approach

This thesis is concerned with the growth and dissolution of gypsum and analogous crystalline materials, with the aim of understanding the kinetic and mechanistic processes at the mineral-solution interface. The research conducted was a collaborative project sponsored by Saint-Gobain Gypsum. First, an image processing (IP) software package was developed to meet highly specialised IP needs and expedite the extraction of vital surface information from images produced in the growth and dissolution studies carried out in this thesis. A simple but powerful morphological analysis of characteristic etch pit features formed on the basal plane of gypsum was proposed, to aid the determination of intrinsic dissolution kinetics. Limiting the study to short times produced microscopic active features, which exhibited high and quantitative mass transport rates. At early times, the reaction was surface controlled, with the edge planes dominating the process, revealing anisotropic step propagation kinetics. With time, an increased contribution from mass transport was observed, suggesting that at later times, the basal plane dominated reaction kinetics. Common ion effects indicated a greater impact of Ca2+ than SO42- in reducing dissolution rates while inert ions enhanced dissolution in a directionspecific way. With this approach, microscopic phenomena were related to macroscopic measurements thus reconciling experimental length scales. Dissolution of the basal (010) and edge (001) surfaces of gypsum and polycrystalline anhydrite, were probed at the bulk scale by coupling the channel flow cell (CFC) technique which displays high mass transport rates, with off-line spectrometric measurements of dissolved Ca2+. Quantitative modelling of the diffusion-reaction within the CFC yielded a linear rate law for the dissolution process. Rates from the basal plane and anhydrite were found to be consistent with other bulk measurements, while the highly reactive edge plane exhibited high rates indicating a transport-limited process. Sodium trimetaphosphate, a common humid-creep inhibitor was found to significantly retard basal plane dissolution rates. Further CFC studies were carried out on industrially-relevant, chemically modified CaSO4 based materials, using a simple flux approach. It was found that models proposing a dissolution-precipitation pathway as the mode of action of humid-creep inhibitors were less plausible than those proposing a surface binding pathway. Finally, the influence of solution stoichiometry, r = (aCa2+ / aSO42-) on the growth kinetics of microscopic gypsum crystals was determined at a constant supersaturation. Crystal growth was found to be entirely controlled by surface kinetics over the range of r, with the edge planes dominating the process. The highest lateral rates were found at r = 1, diminishing sharply at r ≠ 1, and indicating strong plane-specific dependence on Ca2+ and SO42- availability. Additionally, dramatic changes in the morphology of grown crystals were observed. Propagation of steps on the basal face revealed a complex polynuclear layer-by-layer growth process for this surface. Macroscopic growth rates compared well to previous bulk measurements indicating that the approach used provided a comprehensive multi-scale view of gypsum growth processes

Scanning electrochemical microscopy as a quantitative probe of acid-induced dissolution: theory and application to dental enamel

This Article reports the use of scanning electrochemical microscopy (SECM) for the quantitative study of acid-induced dissolution. An ultramicroelectrode (UME) is used to generate a flux of protons galvanostatically just above a sample surface, creating controlled acid challenges relevant to acid erosion. The electrochemical technique produces etch features in the sample, which are characterized by white light interferometry (WLI). The technique has been applied to bovine enamel where understanding the kinetics of dissolution is important in the context of acid erosion. Dissolution has been observed as a fast process, but the high rates of mass transport in SECM allow the surface kinetics of dissolution to be evaluated. Key attributes of SECM for these studies are the ability to deliver high, controllable, and local acid challenges in a defined way and that multiple dissolution measurements can be performed on one sample, eliminating intersample variability effects. A novel moving boundary finite element model has been designed to describe the etching process, which allows the etch kinetics to be evaluated quantitatively, simply by measuring the size and shape of etch features over time