Proton assisted dissolution of the dental hard tissue enamel as a non-bacterial process

The overall aim of this thesis was to examine the kinetics of proton-promoted dissolution of the dental hard tissue enamel as a non-bacterial process and the evaluation of inhibitors with the intent of minimising the dissolution process and effectively protecting the surface. A novel approach was taken, utilising scanning electrochemical microscopy (SECM) to galvanostatically generate controllable and well defined proton fluxes in defined areas of the surface. The resulting etch pits formed on the surface were characterised by optical microscopy and white light interferometry (WLI), which quantitatively determined etch pit dimensions. A theoretical finite element model (FEM) was used to elucidate the kinetics of dissolution based upon the analysis of the shape and dimensions of etch pits produced. A heterogeneous rate constant of dissolution of 0.08 ± 0.04 cm s-1 was attributed to untreated enamel, whereas 2 min treatment with 1000 ppm sodium fluoride (NaF) decreased this rate constant slightly to 0.05 ± 0.03 cm s-1. The impact of fluoride on the rate of proton attack was evident from the formation of shallower broader etch pits. In relation to both acid erosion and caries, the two most relevant acids pertinent to enamel dissolution are citric acid and lactic acid. These acids were investigated by protonating their respective sodium salts in-situ to produce localised weak acid directly under the probe tip. This permitted the surrounding enamel sample to remain largely unaltered giving a true surface for comparison, whilst allowing evaluation of the kinetics in the presence of each weak acid. Etching in the presence of lactic acid, showed a surface controlled process with a rate constant of 0.1 ± 0.03 cm s-1. Etching in the presence of the triprotic citric acid, also yielded a surface controlled process with a rate constant of 0.35 ± 2.6 cm s-1. Calcite was also investigated using SECM, WLI and FEM to validate the use of these techniques. The kinetic data extrapolated was comparable to rate constants found in literature, confirming the validity of these methods. In this case, a novel approach was the use of experimental data to parameterise the finite element model directly. Confocal laser scanning microscopy (CLSM) coupled with SECM was used to visualise proton fluxes from the tip of the UME. This allowed, not only, correlation of the current applied to the probe tip with the pH, but also quantitative data on the spread of protons across a particular surface. Rate constants found for untreated and fluoride-treated enamel were comparable to those found in SECM etching, however, zinc ion treatment proved to result in much greater inhibition of dissolution than fluoride

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

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

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

Small and medium-sized companies' view of sustainability reporting and perception of its core concepts : A qualitative study before the introduction of CSRD

Den Europeiska Unionen vill med Corporate Sustainability Reporting Directive (CSRD) standardisera hållbarhetsredovisning för företag som är verksamma inom unionen. Återigen kommer fler och fler företag omfattas av reglerna att behöva upprätta en obligatorisk hållbarhetsredovisning – vilket kan utgöra en utmaning för mindre erfarna små och medelstora företag. Denna studie undersökte små och medelstora företags syn på hållbarhetsredovisning samt deras uppfattning för dess kärnbegrepp; närmare bestämt ESG och materialitet. Studien utfördes genom semistrukturerade intervjuer med sex företag och med hjälp av sju respondenter. Studien visade att det finns en överväldigande positiv inställning till hållbarhetsredovisning bland de studerade företagen. Hursomhelst framgick det tecken på att företagen var bristfällig i vissa aspekter. Till exempel upplevde samtliga företag svårigheter med att koppla delar av sin verksamhet till hållbarhet på en bolagsstyrningsnivå (eng. governance). Liknande resultat upptäcktes för materialitet. Alla företag upplevdes ha svårigheter att begripa innebörden av begreppet ’materialitet’. Studien visade, trots svårigheten att begripa innebörden av materialitet, att de studerade företagen visade tecken på att vara effektiva i identifieringen av materiella hållbarhetsaspekter.The European Union wants to standardize sustainability reporting with the implementation of the Corporate Sustainability Reporting Directive (CSRD). Once again, more and more enterprises will be subject to the mandatory requirements to disclose sustainability information – which may pose a challenge for less experienced small and medium sized enterprises. This study examines the perception of small and medium sized enterprises on sustainability reporting and its foundational concepts; namely environmental, social and governance (ESG) and materiality. The study was conducted on six individual companies and seven interviewees in total. The study concludes that there exists an overwhelmingly positive perception of sustainability reporting amongst the studied enterprises. However, there were signs that the studied companies were deficient in certain aspects. For example, all enterprises experienced difficulties in identifying aspects of their business that could be linked with governance sustainability. Similar results were discovered on the materiality topic. All enterprises had difficulties in understanding the term ‘materiality’. The study concludes that, despite the difficulty understanding materiality, the studied enterprises showed signs of being efficient in identifying material sustainability topics

Small and medium-sized companies' view of sustainability reporting and perception of its core concepts : A qualitative study before the introduction of CSRD

Den Europeiska Unionen vill med Corporate Sustainability Reporting Directive (CSRD) standardisera hållbarhetsredovisning för företag som är verksamma inom unionen. Återigen kommer fler och fler företag omfattas av reglerna att behöva upprätta en obligatorisk hållbarhetsredovisning – vilket kan utgöra en utmaning för mindre erfarna små och medelstora företag. Denna studie undersökte små och medelstora företags syn på hållbarhetsredovisning samt deras uppfattning för dess kärnbegrepp; närmare bestämt ESG och materialitet. Studien utfördes genom semistrukturerade intervjuer med sex företag och med hjälp av sju respondenter. Studien visade att det finns en överväldigande positiv inställning till hållbarhetsredovisning bland de studerade företagen. Hursomhelst framgick det tecken på att företagen var bristfällig i vissa aspekter. Till exempel upplevde samtliga företag svårigheter med att koppla delar av sin verksamhet till hållbarhet på en bolagsstyrningsnivå (eng. governance). Liknande resultat upptäcktes för materialitet. Alla företag upplevdes ha svårigheter att begripa innebörden av begreppet ’materialitet’. Studien visade, trots svårigheten att begripa innebörden av materialitet, att de studerade företagen visade tecken på att vara effektiva i identifieringen av materiella hållbarhetsaspekter.The European Union wants to standardize sustainability reporting with the implementation of the Corporate Sustainability Reporting Directive (CSRD). Once again, more and more enterprises will be subject to the mandatory requirements to disclose sustainability information – which may pose a challenge for less experienced small and medium sized enterprises. This study examines the perception of small and medium sized enterprises on sustainability reporting and its foundational concepts; namely environmental, social and governance (ESG) and materiality. The study was conducted on six individual companies and seven interviewees in total. The study concludes that there exists an overwhelmingly positive perception of sustainability reporting amongst the studied enterprises. However, there were signs that the studied companies were deficient in certain aspects. For example, all enterprises experienced difficulties in identifying aspects of their business that could be linked with governance sustainability. Similar results were discovered on the materiality topic. All enterprises had difficulties in understanding the term ‘materiality’. The study concludes that, despite the difficulty understanding materiality, the studied enterprises showed signs of being efficient in identifying material sustainability topics

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