First-Principles Prediction of Surface Wetting.

We have developed a method for predicting the solvation contribution to solid-liquid interfacial tension (IFT) based on density functional theory and the implicit solvent model COSMO-RS. Our method can be used to predict wetting behavior for a solid surface in contact with two liquids. We benchmarked our method against measurements of contact angle from water-in-oil on silica wafers and a range of self-assembled monolayers (SAMs) with different compositions, ranging from oil-wet to water-wet. We also compared our predictions to literature data for wetting of a polydimethylsilane surface. By explicitly including deprotonation for silica surfaces and carboxylic acid SAMs, very good agreement was obtained with experimental data for nearly all surfaces. Poor agreement was found for amine-terminated SAMs, which could be the result of both method and model insufficiencies and impurities known to be present for such surfaces. Solid-liquid IFT cannot be measured directly, making predictions such as from our method all the more important.

A Microkinetic Model of Calcite Step Growth.

In spite of decades of research, mineral growth models based on ion attachment and detachment rates fail to predict behavior beyond a narrow range of conditions. Here we present a microkinetic model that accurately reproduces calcite growth over a very wide range of published experimental data for solution composition, saturation index, pH and impurities. We demonstrate that polynuclear complexes play a central role in mineral growth at high supersaturation and that a classical complexation model is sufficient to reproduce measured rates. Dehydration of the attaching species, not the mineral surface, is rate limiting. Density functional theory supports our conclusions. The model provides new insights into the molecular mechanisms of mineral growth that control biomineralization, mineral scaling and industrial material synthesis.

Specific ion effects on the hydrophobic interaction of benzene self-assembled monolayers.

The interaction of aromatic compounds with various ions in aqueous solutions plays a role in a number of fields, as diverse as protein folding and enhanced oil recovery, among others. Therefore, we have investigated the effect of the four electrolytes, KCl, NaCl, MgCl2 and CaCl2, on the hydrophobic interaction of benzene self-assembled monolayers. Using the jump to contact phenomenon of an atomic force microscope (AFM) tip as an indicator of attractive forces between the surfaces of a sample and the tip, we discovered lower frequencies in the snap in as well as narrower distributions for the snap in distance for the monovalent ions, especially for K(+), compared with the behaviour for the divalent ions. These observations are explained by the accumulation of charge at the surface by cation-π interactions and an influence of the ions on the formation of capillaries that bridge the tip to the surface. Bridging capillaries, i.e. nanometre scale gas bubbles, are some of the factors contributing to the long range hydrophobic interaction. The results demonstrate how ions influence the attraction of hydrophobic entities in aqueous solutions.

Mechanistic insight into biopolymer induced iron oxide mineralization through quantification of molecular bonding.

Microbial production of iron (oxyhydr)oxides on polysaccharide rich biopolymers occurs on such a vast scale that it impacts the global iron cycle and has been responsible for major biogeochemical events. Yet the physiochemical controls these biopolymers exert on iron (oxyhydr)oxide formation are poorly understood. Here we used dynamic force spectroscopy to directly probe binding between complex, model and natural microbial polysaccharides and common iron (oxyhydr)oxides. Applying nucleation theory to our results demonstrates that if there is a strong attractive interaction between biopolymers and iron (oxyhydr)oxides, the biopolymers decrease the nucleation barriers, thus promoting mineral nucleation. These results are also supported by nucleation studies and density functional theory. Spectroscopic and thermogravimetric data provide insight into the subsequent growth dynamics and show that the degree and strength of water association with the polymers can explain the influence on iron (oxyhydr)oxide transformation rates. Combined, our results provide a mechanistic basis for understanding how polymer-mineral-water interactions alter iron (oxyhydr)oxides nucleation and growth dynamics and pave the way for an improved understanding of the consequences of polymer induced mineralization in natural systems.

Numerical simulations of NMR relaxation in chalk using local Robin boundary conditions.

The interpretation of nuclear magnetic resonance (NMR) data is of interest in a number of fields. In Ögren (2014) local boundary conditions for random walk simulations of NMR relaxation in digital domains were presented. Here, we have applied those boundary conditions to large, three-dimensional (3D) porous media samples. We compared the random walk results with known solutions and then applied them to highly structured 3D domains, from images derived using synchrotron radiation CT scanning of North Sea chalk samples. As expected, there were systematic errors caused by digitalization of the pore surfaces so we quantified those errors, and by using linear local boundary conditions, we were able to significantly improve the output. We also present a technique for treating numerical data prior to input into the ESPRIT algorithm for retrieving Laplace components of time series from NMR data (commonly called T-inversion).

The mechanisms of crystal growth inhibition by organic and inorganic inhibitors.

Understanding mineral growth mechanism is a key to understanding biomineralisation, fossilisation and diagenesis. The presence of trace compounds affect the growth and dissolution rates and the form of the crystals produced. Organisms use ions and organic molecules to control the growth of hard parts by inhibition and enhancement. Calcite growth in the presence of Mg2+ is a good example. Its inhibiting role in biomineralisation is well known, but the controlling mechanisms are still debated. Here, we use a microkinetic model for a series of inorganic and organic inhibitors of calcite growth. With one, single, nonempirical parameter per inhibitor, i.e. its adsorption energy, we can quantitatively reproduce the experimental data and unambiguously establish the inhibition mechanism(s) for each inhibitor. Our results provide molecular scale insight into the processes of crystal growth and biomineralisation, and open the door for logical design of mineral growth inhibitors through computational methods.

Controlling biomineralisation with cations.

The production of polymers for controlling calcite growth is a well-known approach in biomineralising organisms. Numerous studies have shown that polymers significantly influenced the growth rate and morphology of CaCO3 but little is known about how the polymers are actually controlled by the organisms. Here we show that cations control the effect of polysaccharides and that these processes have been in place for at least 60 million years. We studied the interaction between cleaved samples of pure calcite and ancient coccolith associated polysaccharides (aPS) that we had extracted from the samples of Cretaceous chalk, in solutions that contained one of the common seawater cations, K+, Ca2+, Mg2+ and Sr2+. With atomic and chemical force microscopy (AFM and CFM), we showed that K+, Ca2+ and Sr2+ complex aPS through a weak, outer sphere bonding, giving the aPS affinity to sites on steps and terraces. In contrast, Mg2+ enhanced the formation of stronger and longer aPS complexes, resulting in low affinity to calcite terraces and strong affinity to steps. It is known that adsorption is influenced by ionic potential and ionic strength. Our results show that cation-polysaccharide complexing can modify the effectiveness of the polymer. Thus, creating organic molecules with cation complexing ability is an effective strategy for regulating mineral growth, both now and in the past.