Spatially resolved product formation in the reaction of formic acid with calcium carbonate (1014): the role of step density and adsorbed water-assisted ion mobility

Investigation on the influence of additives on the oriented dissolution of calcite

Similar to the crystal growth process, additives have a strong influence on the dissolution process of crystals. Studies on the dissolution process may shed light on understanding the biomineralization and bioinspired crystallization process. The influence of different kinds of additives including surfactants and polymers on the dissolution process of calcite {104} planes was investigated in detail in this work. The additives can be classified into three kinds according to their influence on the dissolution process of calcite under different concentration windows. The additives show three different kinds of dissolution behaviors with the increase of additive concentrations according to the tomographic variation of the calcite surface after the dissolution process. There are four dissolution modes of calcite while changing the additive concentrations in the solution. Rhombohedral etch pits with [4[combining macron]41] and [481[combining macron]] step edges are formed on the calcite {104} planes after the dissolution process at low additive concentrations (mode I). Calcite micropyramids begin to appear on the calcite surface and the densities of micropyramids increase with the increase of the additive concentrations until they cover the entire calcite surface after the dissolution process at medium additive concentrations (mode II). Instead of micropyramids, large pyramids with [481[combining macron]] and [4[combining macron]41] step edges and a size of about 50 μm form after the dissolution process at high additive concentrations (modes III and IV). We propose that the different anisotropic dissolution behaviors of calcite are strongly related to the concentrations and the adsorption features of the additives on the calcite surface. The additives may act as inhibitors of calcite dissolution, possibly through adsorption on calcite surfaces without preferred adsorption, or adsorption at specific kink sites or step edges. The influence of additives on the oriented dissolution of calcite is generally related to the adsorption density and homogeneity of additives on the calcite substrates.

Heterogeneous Reaction of HCOOH on NaCl Particles at Different Relative Humidities

The contribution of volatile organic acids to chloride depletion still remains unclear under ambient conditions in the coast and inland. In this work, the heterogeneous reaction of HCOOH on the NaCl surface at a series of relative humidities (RHs) was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formate was found to be formed on NaCl surface under dry and wet conditions, accompanied by the corresponding chloride depletion. The adsorbed HCOOH and the formation of formate on NaCl surface decreased with increasing RH below 30% RH. The adsorbed HCOOH decreased, while the formation of formate increased with enhanced RH at 45-70% RH. The variation in the formation of formate with RH suggests that chloride depletion may undergo similar changes. Additionally, the mechanism and kinetics for uptake of HCOOH on NaCl surface at various RHs were discussed and analyzed. Our results highlight the role of heterogeneous chemistry of volatile organic acid in the chloride depletion of NaCl in the coast and inland.

Bonding, structural and thermodynamic analysis of dissociative adsorption of H3O+ ion onto calcite (101⁻4) surface: CPMD and DFT calculations

We used density functional theory (DFT) and Car-Parrinello molecular dynamics (CPMD) simulation to investigate the adsorption and bond formation of hydronium ion (H₃O⁺) onto a [Formula: see text] calcite surface. For surface coverage of 25% to 100%, the nature of H₃O⁺ interaction was explored through electron density and energetics in the context of bond critical points. The adsorbate-adsorbent structure was studied by simulation of pair correlation function. The results revealed that dissociation into water molecule(s) and proton(s) complements H₃O⁺ ion(s) adsorbtion. The H₂O molecule adsorbs onto the surface via its O atom, and interacts with surface calcium in a closed-shell mode; the H⁺ ion makes a covalent bond to the surface oxygen while maintaining H-bonding with water. Adsorption energies were diminished by 70-90 kJ mol^-1 when O_bridge-bonded H⁺ ions transferred to the O_terminal manually. While dissociative adsorption of H₃O⁺ ions is spontaneous at all surface coverages tested, the free energy was lowest at 75% coverage. Also, protonation of a completely pre-hydrated calcite surface leads to stronger interaction of water molecules with the surface. This unique outlook on hydrating calcite provides specific insights into biomineralization of this mineral, and helps depict further pH consequences in the field of biomaterial adsorption. Graphical abstract Dissociative adsorption of hydronium ion onto the surface of calcite.

Stereoisomers of hydroxymethanes: Probing structural and spectroscopic features upon substitution

Ab initio studies on CH_x(OH)_4-x (x = 0-3) polyols are carried out to derive their structural and spectroscopic features. Several stereoisomers (both equilibrium structures and transition states) are found. Some are predicted here for the first time. We determined hence their geometrical parameters, vibrational frequencies, electronic excitation energies for the singlet manifold, and IR spectra. While the IR spectra for all polyols present similar shapes, their UV spectra exhibit however distinct band origin that are specific to each polyol and more interestingly to each diasteroisomer. Stereoelectronic effects are also noticed and discussed. It is suggested that UV spectroscopy is an efficient probe to experimentally identify polyols in mixtures involving polyols.

Opposing effects of humidity on rhodochrosite surface oxidation

Rhodochrosite (MnCO3) is a model mineral representing carbonate aerosol particles containing redox-active elements that can influence particle surface reconstruction in humid air, thereby affecting the heterogeneous transformation of important atmospheric constituents such as nitric oxides, sulfur dioxides, and organic acids. Using in situ atomic force microscopy, we show that the surface reconstruction of rhodochrosite in humid oxygen leads to the formation and growth of oxide nanostructures. The oxidative reconstruction consists of two consecutive processes with distinctive time scales, including a long waiting period corresponding to slow nucleation and a rapid expansion phase corresponding to fast growth. By varying the relative humidity from 55 to 78%, we further show that increasing humidity has opposing effects on the two processes, accelerating nucleation from 2.8(±0.2) × 10(-3) to 3.0(±0.2) × 10(-2) h(-1) but decelerating growth from 7.5(±0.3) × 10(-3) to 3.1(±0.1) × 10(-3) μm(2) h(-1). Through quantitative analysis, we propose that nanostructure nucleation is controlled by rhodochrosite surface dissolution, similar to the dissolution-precipitation mechanism proposed for carbonate mineral surface reconstruction in aqueous solution. To explain nanostructure growth in humid oxygen, a new Cabrera-Mott mechanism involving electron tunneling and solid-state diffusion is proposed.

Heterogeneous uptake and adsorption of gas-phase formic acid on oxide and clay particle surfaces: the roles of surface hydroxyl groups and adsorbed water in formic acid adsorption and the impact of formic acid adsorption on water uptake

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth's crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface -OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed.

Salt-mediated self-assembly of thioctic acid on gold nanoparticles

Self-assembled monolayer (SAM) modification is a widely used method to improve the functionality and stability of bulk and nanoscale materials. For instance, the chemical compatibility and utility of solution-phase nanoparticles are often improved using covalently bound SAMs. Herein, solution-phase gold nanoparticles are modified with thioctic acid SAMs in the presence and absence of salt. Molecular packing density on the nanoparticle surfaces is estimated using X-ray photoelectron spectroscopy and increases by ∼20% when molecular self-assembly occurs in the presence versus the absence of salt. We hypothesize that as the ionic strength of the solution increases, pinhole and collapsed-site defects in the SAM are more easily accessible as the electrostatic interaction energy between adjacent molecules decreases, thereby facilitating the subsequent assembly of additional thioctic acid molecules. Significantly, increased SAM packing densities increase the stability of functionalized gold nanoparticles by a factor of 2 relative to nanoparticles functionalized in the absence of salt. These results are expected to improve the reproducible functionalization of solution-phase nanomaterials for various applications.

Growth of manganese oxide nanostructures alters the layout of adhesion on a carbonate substrate

Nanostructures grown under natural conditions can modify the layout of adhesion on mineral surfaces. Using force-volume microscopy and a silicon-nitride probe, we measure changes in adhesion when a patchy overgrowth of manganese oxide nanostructures forms on the surface of rhodochrosite. For the most part, the observations show that the adhesive force to the nanostructures is dominated by van-der-Waals attraction. Measurements made across an area of the surface provide a frequency distribution of adhesive forces, and the mode of this distribution is 166 pN at pH 5.0, increasing to a maximum of 692 pN at pH 7.1, followed by a decrease to 275 pN at pH 9.7. At a few sampling locations over some nanostructures, electrostatic repulsion overtakes van-der-Waals attraction and thus results in negative adhesive forces (i.e., repulsion). Local roughness causes this effect. In comparison to the oxide nanostructures, the exposed rhodochrosite substrate has negligible adhesive force with the probe over the same pH range, suggesting both weak van-der-Waals attraction and weak electrostatic repulsion over this pH range. The quantitative mapping of adhesive force applied more generally to the study of other nanostructures can lead to an improved mechanistic understanding of how nanostructure growth influences contaminant immobilization and bacterial attachment.