Deuterium MAS NMR and Local Molecular Dynamic Model to Study Adsorption–Desorption Kinetics of a Dipeptide at the Inner Surfaces of SBA-15

This work presents a deuterium magic angle spinning (MAS) NMR study of the adsorption–desorption dynamics of glycine-(2,2)-<i>d</i>₂-alanine dipeptide adsorbed at the inner surfaces of mesoporous SBA-15 silica under different hydration levels and temperatures. The experimental and theoretical challenges posed by the strong quadrupolar interaction of the rigid CD₂ group, 3-fold bigger than that of the rotating methyl CD₃, were addressed. Deuterium MAS NMR spectra modulated by exchange were analyzed using theoretically calculated exchange spectra based on the two-site Bloch–McConnel exchange equation represented in Floquet space. To solve this equation, which is composed of a high dimensional Floquet exchange matrix, our former computational approach was modified to reduce the overall computation time by orders of magnitude so as to yield more accurate exchange parameters from the spectral analysis. The adsorption–desorption kinetics of minutely hydrated silica surfaces is understood to originate from the diffusion of water molecules into and out of adsorbate binding sites, thereby gating the dynamic behavior of the adsorbate via increase or reduction of the size of the surrounding water cluster. Molecular dynamic (MD) simulations were employed to model the dynamic behavior of the adsorbate at the two states. Deviations between the MD and experimental observations are attributed to the simplified surface modeling, thereby highlighting the importance of experimental MAS NMR data to improve future modeling of realistic functional surfaces

Exposed and Buried Biomineral Interfaces in the Aragonitic Shell of <i>Perna canaliculus</i> Revealed by Solid-State NMR

A comprehensive molecular description of the inorganic–bioorganic interfaces and internal structure of the aragonitic shells of Perna canaliculus is derived by employing solid-state NMR spectroscopy. The primary component of the shell, the highly ordered aragonite polymorph of CaCO₃, is shown to possess a small fraction of disordered carbonates whose average chemical-structural identity is similar to that of aragonite. These disordered carbonates were found to interact with bioorganics, bicarbonates, and water molecules and are denoted as interfacial. Characterization of the bleached and of the annealed shells enables the distinguishing of two classes of interfacial carbonates: exposed, solvent accessible, which interact primarily with bioorganics, and buried, solvent inaccessible, which interact exclusively with spatially separated water and bicarbonates. Shell annealing shows that the decomposition of the buried bicarbonate defects correlates with removal of lattice distortions, as detected by XRD, a phenomenon often found in biogenic calcium carbonates. The solid-state NMR investigation exposes the molecular bioorganic–inorganic interfaces in a mollusk shell and demonstrates the unique capability of NMR to determine comprehensively the structure of biogenic composite materials

Phosphate–Water Interplay Tunes Amorphous Calcium Carbonate Metastability: Spontaneous Phase Separation and Crystallization vs Stabilization Viewed by Solid State NMR

Organisms tune the metastability of amorphous calcium carbonates (ACC), often by incorporation of additives such as phosphate ions and water molecules, to serve diverse functions, such as modulating the availability of calcium reserves or constructing complex skeletal scaffolds. Although the effect of additive distribution on ACC was noted for several biogenic and synthetic systems, the molecular mechanisms by which additives govern ACC stability are not well understood. By precipitating ACC in the presence of different PO₄^3– concentrations and regulating the initial water content, we identify conditions yielding either kinetically locked or spontaneously transforming coprecipitates. Solid state NMR, supported by FTIR, XRD, and electron microscopy, define the interactions of phosphate and water within the initial amorphous matrix, showing that initially the coprecipitates are homogeneous molecular dispersions of structural water and phosphate in ACC, and a small fraction of P-rich phases. Monitoring the transformations of the homogeneous phase shows that PO₄^3– and waters are extracted first, and they phase separate, leading to solid–solid transformation of ACC to calcite; small part of ACC forms vaterite that subsequently converts to calcite. The simultaneous water–PO₄^3– extraction is the key for the subsequent water-mediated accumulation and crystallization of hydroxyapatite (HAp) and carbonated hydroxyapatite. The thermodynamic driving force for the transformations is calcite crystallization, yet it is gated by specific combinations of water–phosphate levels in the initial amorphous coprecipitates. The molecular details of the spontaneously transforming ACC and of the stabilized ACC modulated by phosphate and water at ambient conditions, provide insight into biogenic and biomimetic pathways

Thermal conductivity-structure-processing relationships for amorphous nano-porous organo-silicate thin films

© 2019, Springer Science+Business Media, LLC, part of Springer Nature. While numerous thermal conductivity investigations of amorphous dielectrics have been reported, relatively few have attempted to correlate to the influence of processing conditions and the resulting atomic structure. In this regard, we have investigated the influence of growth conditions, post deposition curing, elemental composition, atomic structure, and nano-porosity on the thermal conductivity for a series of organo-silicate (SiOCH) thin films. Time-domain thermoreflectance (TDTR) was specifically utilized to measure thermal conductivity while the influence of growth conditions and post deposition curing on composition, mass density, atomic structure, and porosity were examined using nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS), Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), ellipsometric porosimetry (EP), and positronium annihilation lifetime spectroscopy (PALS). Analytical models describing the thermal conductivity dependence on mass density and vol% porosity were found to generally over-predict the measured thermal conductivity, but improved agreement was obtained when considering only the heat carrying network density determined by FTIR. Ashby’s semi-empirical relation, which assumes only 1/3 of the heat carrying bonds are aligned to the heat transport direction, was also found to reasonably describe the observed trends. However, the thermal conductivity results were best described via a model proposed by Sumirat (J Porous Mater 9:439 (2006)) which considers the effect of both vol% porosity and phonon scattering by nanometer sized pores. Post-deposition curing was additionally observed to increase thermal conductivity despite an increase in nano-porosity. This effect was attributed to an increase in the Si–O–Si network bonding produced by the cure

Small but effective: potent light-weight additives modulate prenucleation clusters by specific interactions on the molecular level

Small-molecular-weight (MW) additives can strongly impact amorphous calcium carbonate (ACC), playing an elusive role in biogenic, geologic, and industrial calcification. Here, we present molecular mechanisms by which additives regulate stability and composition of both CaCO3 solutions and solid ACC. Potent antiscalants inhibit ACC precipitation by interacting with prenucleation clusters (PNC); they specifically trigger and integrate into PNCs or feed PNC growth. Only PNC-interacting additives are traceable in ACC, considerably stabilizing it against crystallization. The selective incorporation of potent additives in PNCs is a reliable chemical label that provides conclusive chemical evidence that ACC is a molecular precipitate derived PNCs. Our results reveal additive-cluster interactions beyond established mechanistic conceptions. They reassess the role of small-MW molecules in crystallization and biomineralization, while breaking grounds for new sustainable antiscalants