The impact of stoichiometry on the initial steps of crystal formation: Stability and lifetime of charged triple-ion complexes.

Minerals form in natural systems from solutions with varying ratios of their lattice ions, yet non-stoichiometric conditions have generally been overlooked in investigations of new formation (nucleation) of ionic crystals. Here, we investigated the influence of cation:anion ratio in the solution on the initial steps of nucleation by studying positively and negatively charged triple ion complexes and subsequent particle size evolution. Our model systems are carbonates and sulfates of calcium and barium, as it was recently shown that solution stoichiometry affects the timing and rate of their nucleation. Molecular dynamics (MD) simulations and dynamic light scattering (DLS) flow experiments show that nucleation correlates with the stability and lifetime of the initial complexes, which were significantly impacted by the cation:anion stoichiometry and ion type. Specifically, B a S O 4 2 2 - ${{\rm B}{\rm a}{\left({{\rm S}{\rm O}}_{4}\right)}_{2}^{2-}}$ was found to have higher association constants and its lifetime was twofold longer than B a 2 S O 4 2 + ${{{\rm B}{\rm a}}_{2}{{\rm S}{\rm O}}_{4}^{2+}}$ . Similar trends were observed for B a C O 3 ${{{\rm B}{\rm a}{\rm C}{\rm O}}_{3}}$ and C a S O 4 ${{{\rm C}{\rm a}{\rm S}{\rm O}}_{4}}$ . Contrastingly, for C a C O 3 ${{{\rm C}{\rm a}{\rm C}{\rm O}}_{3}}$ , C a C O 3 2 2 - ${{\rm C}{\rm a}{\left({{\rm C}{\rm O}}_{3}\right)}_{2}^{2-}}$ was found to have lower association constants and its lifetime was shorter than C a 2 C O 3 2 + ${{{\rm C}{\rm a}}_{2}{{\rm C}{\rm O}}_{3}^{2+}}$ . These trends in stability and lifetime follow the same asymmetrical behaviour as observed experimentally for particle formation using techniques like DLS. This suggests a causal relationship between the stability and lifetime of the initial charged complexes and the nucleation under non-stoichiometric conditions.

Recalibrating the calcium trap in amino acid carboxyl groups via classical molecular dynamics simulations.

In order to use classical molecular dynamics to complement experiments accurately, it is important to use robust descriptions of the system. The interactions between biomolecules, like aspartic and glutamic acid, and dissolved ions are often studied using standard biomolecular force-fields, where the interactions between biomolecules and cations are often not parameterized explicitly. In this study, we have employed metadynamics simulations to investigate different interactions of Ca with aspartic and glutamic acid and constructed the free energy profiles of Ca2+-carboxylate association. Starting from a generally accepted, AMBER-based force field, the association was substantially over and under-estimated, depending on the choice of water model (TIP3P and SPC/fw, respectively). To rectify this discrepancy, we have replaced the default calcium parameters. Additionally, we modified the σij value in the hetero-atomic Lennard-Jones interaction by 0.5% to further improve the interaction between Ca and carboxylate, based on comparison with the experimentally determined association constant for Ca with the carboxylate group of L-aspartic acid. The corrected description retrieved the structural properties of the ion pair in agreement with the original biomolecule - Ca2+ interaction in AMBER, whilst also producing an association constant comparable to experimental observations. This refined force field was then used to investigate the interactions between amino acids, calcium and carbonate ions during biogenic and biomimetic calcium carbonate mineralisation.

Reconsidering Calcium Dehydration as the Rate-Determining Step in Calcium Mineral Growth.

The dehydration of cations is generally accepted as the rate-limiting step in many processes. Molecular dynamics (MD) can be used to investigate the dynamics of water molecules around cations, and two different methods exist to obtain trajectory-based water dehydration frequencies. Here, these two different post-processing methods (direct method versus survival function) have been implemented to obtain calcium dehydration frequencies from a series of trajectories obtained using a range of accepted force fields. None of the method combinations reproduced the commonly accepted experimental water exchange frequency of 10-8.2 s-1. Instead, our results suggest much faster water dynamics, comparable with more accurate ab initio MD simulations and with experimental values obtained using neutron scattering techniques. We obtained the best agreement using the survival function method to characterize the water dynamics, and we show that different method combinations significantly affect the outcome. Our work strongly suggests that the fast water exchange kinetics around the calcium ions is not rate-limiting for reactions involving dissolved/solvated calcium. Our results further suggest that, for alkali and most of the earth alkali metals, mechanistic rate laws for growth, dissolution, and adsorption, which are based on the principle of rate-limiting cation dehydration, need careful reconsideration.

Simultaneous Real-Time Analysis of Bulk and Bottom Cure of Ultraviolet-Curable Inks Using Fourier Transform Infrared Spectroscopy.

The curing characteristics of an ultraviolet (UV) ink layer are of utmost importance for the development of UV inks. Measuring either bulk or bottom cure in itself is not new and has been the subject of many articles. In this article, two methods are described based on Fourier transform infrared (FT-IR) spectrometry to measure in real time and simultaneously the bulk and bottom cure of a thin UV ink layer. The procedure consists of applying a thin (10-12 µm) layer of UV-curing ink on an attenuated total reflection (ATR) crystal. The bottom cure is measured with ATR. The bulk cure is measured simultaneously with a reflection analysis (method 1) or a transmission analysis (method 2). With both methods, the bulk and bottom cure can be determined. To overcome problems with the interference in the ATR reflection setup, it is recommended to use the ATR transmission setup.