Unravelling the precipitation pathway of biorelevant minerals: from fundamental studies to biomedical applications

The formation pathway of biominerals archetypes, namely calcium carbonate (CaC) and calcium phosphates (CaPs), was investigated using a multidisciplinary approach. The time-resolved analytical data were collected using a controlled-composition method and interpreted using a computational tool. Some basic-science questions were identified arising from long-lasting debates on the details about the solid formation mechanism. Some of these questions are here addressed by the accurate experimental data collection, the application of new cutting-edge techniques, and the development of a rigorous thermodynamic-kinetic model for the entire solid formation process from solution. The time-resolved data were collected analysing the aqueous species in-situ. In parallel, the amount of chemicals added into the reactor, to maintain iso-pH conditions, was mathematically correlated to the bonded carbonate or phosphate ions, allowing the collection of two independent series of data and the assessment of both the cation and the anion during the solid formation process. This analytical approach went beyond the state-of-the-art. The complex mechanism was partitioned into its sub-processes, and a new strategy was identified to indirectly disclose the nature of the building units involved in the solid growth. Some assumptions embedded in the derivation of the nucleation theory did not compromise the theoretical validity of the framework, and the thermodynamic quantities expressed in the formulation have to be properly related to the nucleating phase and not extended as intensive properties of the solid. As main scientific results, for both CaC and CaPs systems, the experimental data are consistent with a classical nucleation and crystallisation mechanism, where primary, secondary, and diffusion limited growth are taken into account. The associated thermodynamic and kinetic quantities, such as the surface energy for primary and secondary nucleation as well as the nucleation and growth rate are reported and critically discussed in this dissertation. The models allowed detailed investigations in the pre-nucleation zone with the aim of evaluating the presence of clusters at controlled T, pH, ionic strength, and cation/anion ratio against saturation level. An experimental setup was developed to study clusters in-situ: a reactive horizontal micrometric pulsation-free liquid jet was investigated with a focused synchrotron X-ray beam, and the small angle scattering signals were collected for both CaC and CaPs systems. Preliminary results confirm the presence of clusters and attempts to identify their size distribution are ongoing. The agreement between experimental data, standard and cutting-edge analytical investigations, and modelling results allowed the definition of a plausible solid formation pathway, for both the investigated systems, based on the nucleation theory framework, in which the plausible influence of clusters was assessed. Beside the basic-science activities, some applications in the biomedical field were also evaluated. Among them, the controlled growth of CaPs phase on pre-treated Ti implants, e.g., dental implants, was achieved. This application opened exciting perspectives toward the production and the commercialisation of implants characterised by a faster healing time and improved stability. The scientific results were published in peer-reviewed papers and an international patent application; two additional papers are under finalisation.

Characterization of Solid-Liquid Interfaces with High-Resolution Atomic Force Microscop

Maria Ricci

Solid-liquid interfaces are ubiquitous. Despite the unquestioned relevance, many scientific research fields rely on simplified or macroscopic descriptions, discarding the molecular nature of the constituents. However, it is the specific molecular interactions and the complex chemistry in the interfacial region that are fundamental in phenomena such as heterogeneous catalysis, electrochemistry, crystal growth, membrane transport and body-antibody recognition. In particular, the models used to describe the so-called electrical double layer of ionic distributions at charged surfaces in liquid, are based on continuum assumptions. For example, the Gouy-Chapman-Stern model assumes that a dense, homogenously distributed layer with a thickness of a hydrated ion (referred to as the "Stern layer"), is adsorbed onto the charged surface, proceeded by an exponentially decaying diffuse ionic cloud, in which ions are point charges in a continuous dielectric media. In this thesis, I will first illustrate how the assumption of a homogeneous ion lateral density in the Stern Layer fails to describe the actual solid-water interface of several relevant surfaces. Then I will show how the lateral organization of the ions indeed plays a role as important as the vertical modulation of the charge distribution. This is because the molecular nature of the ions, the surface, and the water molecules themselves each contribute to the precise lateral organization of the constituents. As result, the ions can reside on discrete surface sites, defined both laterally and vertically in space. Moreover, the water-mediated interaction can impart a correlation in the reciprocal lateral distribution of the ions. To achieve this level of description, high-resolution atomic force microscopy (AFM) is used. Unlike most of other experimental techniques, AFM is able to characterize interfaces locally at the atomic/molecular level. In fact, in the AFM small-amplitude regime, detection of even single ions adsorbed at the surface of the solid is possible by measuring the perturbation they induce on the local solvation environment. This capability represents an invaluable tool for the exploration of phenomena occurring at the Stern layer of solid-liquid interfaces. Following, I will investigate the properties of several solid-aqueous interfaces in the presence of different ionic concentrations and species, ranging from hard (mica) to soft systems such as organic self-assembled monolayers, lipid bilayers, and the dynamic surface restructuring of calcite when covered with fatty acid molecules. The experimental study, combined with molecular dynamic simulations, reveals a water-induced ion-ion attractive interaction for some ionic species. These results show that water alone is the driving force to induce order within the Stern layer, creating hydration-correlation effects on mica, self-assembled monolayers and possibly lipid bilayers. The local modulation of the hydration properties of the surface is fundamental to highly dynamic systems such as calcite, where steps act as nucleating points for the processes of dissolution and growth. The specific interaction of foreign ions and fatty acids modify these processes as well as the equilibrium between the crystal and the solution. This thesis provides clear experimental evidence of new mechanisms developing at solid-liquid interfaces paving the way for a deeper and more conscious understanding of the colloidal properties of materials.

EPFL2015

In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation

Mohammad Reza Andalibi, Agnese Carino, Andrea Testino

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.

2019

Molecular insights into the electrical double layer of colloidal oxide particles obtained through second harmonic scattering

Marie Bischoff

Photocatalytic applications play an essential role in the search for alternative energy sources and environmental decontamination techniques. It is of fundamental interest to understand on a molecular level the aqueous solid/liquid interface, where photocatalytic reactions occur, to improve the reaction efficiency.Surface charge, ion diffusion and adsorption are all elements that can have a significant impact on reaction rates and, in the case of nanoparticles, colloidal stability. However, not many experimental methods are able to probe the electrical double layer (EDL) and its peculiar properties, particularly when it comes to colloidal nanoparticles directly dispersed in solution. In this thesis we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to study the EDL of colloidal oxide particles in aqueous environments. The insulating oxide silica (SiO2) and the semiconducting metal oxide titanium dioxide (TiO2) are investigated and further compared. We show that from AR-SHS measurements, we can extract the surface potential of the particles with respect to bulk liquid and the surface susceptibility, as a measure of interfacial molecular orientation. This can be achieved without assuming a specific charge distribution in the EDL, and hence without employing a model such as the Gouy-Chapman (GC) or the Gouy-Chapman-Stern (GCS). Through electrophoretic mobility measurements, we additionally obtain the zeta potential. Knowing the surface potential, the surface susceptibility and the zeta potential enables us to establish a molecular-level picture of the oxide particle/liquid interface as a function of ionic strength and pH. First, we provide evidence that a compact layer of hydrated ions is formed at the 300 nm amorphous SiO2/aqueous interface at high pH and high NaCl concentrations (> 1mM). Computation of surface charge density values based on our measured surface potential values and using the GC and GCS models are in good agreement with literature values.Next, we study the EDL of 100 nm amorphous TiO2 particles as a function of NaCl concentration and of basic pH. Three regions, reflecting three different phenomena occurring as a function of ionic strength, can be identified. First, inner-sphere adsorption at the lowest concentrations, then formation of a diffuse layer of counterions and finally, accumulation of hydrated counterions near the interface. Similar regions are observed for 100 nm SiO2 particles as a function of NaCl concentration. The TiO2 surface is found to have a stronger affinity for Na+ ions than SiO2.We further evaluate ion-specific effects at the SiO2 and amorphous TiO2 colloidal aqueous interface. The addition of NaCl, RbCl or CaCl2 to the solution leads to relativedifferences in surface potential and in the evolution of the interfacial H-bonding network as a function of ionic strength, which reveal surface and cation- specific preferences for inner- and outer-sphere adsorption.Finally, we demonstrate that AR-SHS measurements as a function of pH can be used to determine the pKas of 100 nm anatase TiO2 particles. Our data reveals a correlation between the change in orientation of interfacial water molecules and the pKas of surface TiO2 groups. This work shows that AR-SHS is a powerful tool to uncover the EDL structure of colloidal oxide particles, thus contributing to a better understanding of interfacial processes important to a variety of (photo-)catalytic applications.