V13G-01 INVITED
Very Tiny Rocks: Site-Specific, Size-Dependent Reaction Kinetics at Nanoparticle-Water Interfaces
One of the most fundamental challenges in geochemistry is to be able to understand the rates and mechanisms of elementary reactions that describe chemical processes occurring at mineral-water interfaces. One of the reasons for the primitive conceptual state of reaction kinetics in solid earth geochemistry is that it is very difficult to identify defensible elementary reactions where theoretical predictions can be made and the results can tested experimentally at the same length and time scale of the prediction. For example, the most fundamental predictor of complexation kinetics in aqueous solution is the characteristic water exchange rate, which are well known for the aquo ions and vary by 20 orders of magnitude even for simple trivalent ions. In contrast, for interfacial reactions, it was not even known whether water exchange rates were faster or slower than equivalent metal sites in solution, prohibiting any quantitive understanding of mineral reaction kinetics at the molecular level. Recent advances in synthesis and characterization of materials at nanometer length scales has been able to bridge the gap in scale, and nanometer-sized minerals have given us our first quantitative understanding of elementary reaction rates for fundamental processes involving water and hydroxide exchange reactions. I describe the results of molecular dynamics calculation and experimental measurement of the rates of water, hydroxide, and proton exchange reactions on nanoparticle surfaces. The calculations already show that transition state theory is completely inadequate to understand the rates of even the simplest elementary reactions. Furthermore, the mechanistic implications of rate parameters such as activation volume and activation enthalpy may be different in moving from aquo ions to interfaces. Is a molecular understanding of geochemical processes really needed? One might have asked a biologist at the turn of the century whether studying the structure of proteins would ever be useful for curing disease. True molecular level understanding of interfacial interactions has the potential to revolutionize geology, allowing unprecedented detail and accuracy in such important contexts as climate reconstruction and tectonic history. Geology has an inevitable molecular future.
V13G-02
Crystals size and surface chemistry dependent phase diagram for nanocrystals of rutile and anatase: Experimental studies and computer modeling
It is well known that rutile is the thermodynamically stable phase of TiO2 under ambient conditions at the macroscale, and that anatase is the thermodynamically stable phase at the nanoscale. Both anatase and rutile have superior performance in a range of advanced photochemical applications. It is important for our understanding of the stability of nanostructures in different chemical and physical environments, because both rutile and anatase nanocrystals are used in different chemical and engineering environments. Using a size–, shape– and temperature–dependent thermodynamic model we have generated the first phase diagram for anatase and rutile nanocrystals by incorporating more experimentally relevant parameters (both the equilibrium shape and surface chemistry). Results from hydrothermal synthesis and DFT-based computer modeling show acidic environment favors rutile formation. The acidic solution also favors OH2–terminated surfaces of both anatase and rutile. The boundary between rutile an danatase ranges from ~ 10 nm to ~ 50 nm, depending on temperature and surface composition. The calculated phase map indicates that the equilibrium boundary between anatase and rutile nano-crystals is surface charge chemistry dependent, which relates to both their formation and post-synthesis environments.
V13G-03 INVITED
Molecular simulations of the swelling of clays
The particularly interesting subject of the swelling behavior of clay minerals is addressed by a combination of molecular dynamics and Monte Carlo sampling techniques. The introduced algorithm essentially mimics the experimental determination of the water adsorption isotherm and quantitatively predicts clay swelling for montmorillonite-type clays including such details as the occurrence of hydrated states and hysteresis. Furthermore, important insights into the underlying mechanism of clay swelling from the one-layer to the two- layer hydrate are derived. It turns out that, for this case, clay-swelling proceeds via the migration of counterions that are initially bound to the mineral surface to the central interlayer plane where they become fully hydrated. The extent of clay swelling strongly depends on the charge locus. We demonstrate that this might be the source of the observed hysteresis in clay swelling.
V13G-04
Nano-confined water in the interlayers of hydrocalumite: Reorientational dynamics probed by neutron spectroscopy and molecular dynamics computer simulations
Layered double hydroxides (LDHs, anionic clays) represent excellent model systems for detailed molecular- level studies of the structure, dynamics, and energetics of nano-confined water in mineral interlayers and nano-pores, because LDH interlayers can have a well-defined structures and contain H2O molecules and a wide variety of anions in structurally well-defined positions and coordinations. [Ca2Al(OH)6]Cl·2H2O, also known as hydrocalumite or Friedel's salt, has a well- ordered Ca,Al distribution in the hydroxide layer and a very high degree of H2O,Cl ordering in the interlayer. It is also one of the only LDH phase for which a single crystal structure refinement is available. Thus, it is currently the best model compound for understanding the structure and dynamical behavior of interlayer and surface species in other, less-ordered, LDHs. We investigated the structural and dynamic behavior of water in the interlayers of hydrocalumite using inelastic (INS) and quasielastic (QENS) neutron scattering and molecular dynamics computer simulations. The comperehensive neutron scattering studies were performed for one fully hydrated and one dehydrated sample of hydrocalumite using several complementary instruments (HFBS, DCS and FANS at NCNR; HRMECS and QENS at IPNS) at temperatures above and below the previously discovered order-disorder interlayer phase transition. Together the experimental and molecular modeling results capture the important details of the dynamics of nano-confined water and the effects of the orientational ordering of H2O molecules above and below the phase transition. They provide otherwise unobtainable experimental information about the transformation of H2O librational and diffusional modes across the order-disorder phase transition and significantly add to our current understanding of the structure and dynamics of water in LDH phases based on the earlier NMR, IR, X-ray, and calorimetric measurements. The approach can now be extended to probe the dynamics of nano-confined and interfacial water in more disordered phases (LDH, clays, cement, etc.), for which much less initial structural information is available.
V13G-05
Chemical and Structural Dependences of the Fractionation of Oxygen and Silicon Isotopes Between Rock-forming Minerals: a First-Principles Density Functional Study
Ab initio methods based on density functional theory have proven to be successful in reproducing the physical and chemical properties of complex systems. Within this framework, we have recently developed a methodology to predict equilibrium fractionation factors as a function of temperature (1). We use PBE functionals, combined with the use of pseudopotentials and planewave basis sets. Our previous work focused on the effect of the polymerization of the silicate network on Si-isotope fractionation (2), which had previously been predicted to be a determining factor. Our work does not confirm this assumption. In particular, a large fractionation was estimated between kaolinite and lizardite, despite identical polymerization structures. To investigate the origin of this fractionation, we studied minerals with structures closely related to lizardite and kaolinite, such as talc, pyrophyllite, muscovite and phlogopite. In terms of Si-isotope fractionation, muscovite is very similar to kaolinite (with Δ qtz-musc = 0.18 ‰ at 700°C) whereas talc and pyrophyllite appear significantly heavier than lizardite and kaolinite, respectively, despite their similar structures and cation contents (Δ qtz-talc = 0.35 ‰, Δ qtz-pyro = 0.08 ‰ at 700°C). In terms of O-isotopes fractionation, calculated quartz-muscovite and muscovite-water fractionations agree well with existing estimates at high temperature, based on experimental and empirical data. Interestingly, talc is calculated to be very similar to muscovite, whereas pyrophyllite will be significantly heavier (Δ qtz-pyro = 3.3 ‰ at 200°C ). Those similarities and differences will be discussed in terms of structures and cation contents. In previous studies, oxygen fractionation systematics have been related to the nature of the first neighbors of oxygen atoms (bond-type models), or to more precise structural features, through the modified increment method for example (3). We will critically analyze these concepts to help understand how oxygen and silicon isotope systematics are affected by structural features. References: 1. Meheut et al 2007, GCA, v. 71, p. 3170. 2. Meheut et al, 2008, doi:10.1016/j.chemgeo.2008.06.051. 3. Zheng, 1993, GCA, v. 57, p. 1079.
V13G-06 INVITED
Protein Adhesion and Ion Substitution (on/in)to Minerals
Arsenic and pathogenic prion protein-scrapie (PrPsc) are important contaminants which may soil and water for decades, unless they are removed by sorption. Two sorption mechanisms will be discussed, namely the organics (Prp and single aminoacid) adsorption on clay and the arsenic substitution in gypsum. The elucidation of these contrasted mechanisms will be shown to request complementary molecular-mechanical simulations with experimental spectroscopic investigations. As first example, structural studies performed at ILL/ESRF on As-doped gypsum (CaSO4 2H2O) using neutron and X-ray diffraction data and EXAFS were performed to determine how As fits into the bulk of gypsum structure. The combined Rietveld analysis of neutron and X-ray diffraction data shows an expansion of the unit cell volume proportional to the As concentration within the samples. to-sulfate substitution mechanisms were used as simulation starting hypotheses. DFT-based simulations (Mulliken analysis) were used to interpret charge distribution and to show that among the possible mechanisms, a sulphate substitution by either protonated, or fully deprotonated, arsenate ion, only the protonated arsenate substitution could best fit the EXAFS data. In the second example, we used Molecular Dynamics to understand the mechanism of strong binding of the pathogenic PrP peptide with clay mineral surfaces. We modeled only the infectious moiety, PrP92-138, of the whole PrPsc structure, with explicitly solvating water molecules in contact with the cleavage plane of pyrophillite, as a model for montmorillonite without any cationic substitution. Partial residual negative charges on the cleavage plane were balanced with K+ ions. The peptide anchored to the clay surface via up to 10 hydrogen bonds from lysine and histidine residues to oxygen atoms of the siloxane cavities, and a total adsorption energy of 3465 KJ.mol-1 was obtained. Our results were compared to the one obtained by chemical and thermal analysis, 23Na, 1H, 13C solid state NMR and MD computation on sorption of single lysine amino acid on model synthetic Na-montmorillonite. Our data provide further insight about interactions between lysine and montmorillonite which depend strongly on lysine concentration.
V13G-07
Dynamic simulations of polypeptide templates to promote Ca-carbonate nuclei
Over the years, there have been a number of attempts to form synthetic organic templates that mimic
dynamic processes at the interface between organic matter and mineral surfaces. One approach has been
to isolate the templating matrix from mineralized tissues and examine the growth of calcium salts in the
presence of this matrix. Other experiments have focused on synthetic (bio-)organic templates, such as
polymers, macromolecular complexes, phospholipid vesicles, pleated polyamino acids entrapped in gelatin
self-assembled monolayers on gold substrates, and Langmuir films. In the case of Langmuir monolayers, the
amphiphilic molecules can be designed in such a way that they act as artificial two-dimensional nuclei for the
promotion of crystal nucleation. Such films have been used as templates to direct the crystal nucleation and
growth of calcium carbonate. For example, Buijnsters et al. (2001)1 used Langmuir films of amide-containing
phospholipids in the presence of calcium ions to form well-defined two-dimensional domains at the air-water
interface.
This is the starting point of our molecular dynamics simulations. After deriving a pure-core potential set for
fast molecular dynamics simulations, we have created different two-dimensional networks of amide-containing
phospho¬lipids that serve as templates for Ca carbonate seed formation. We can vary the distance and
structural arrangement of the functional groups to control adsorption and seed formation. The molecular
dynamics runs in these calculations contain water with different concentrations of Ca2+ and CO32- ions.
We have chosen a slightly different approach for polypeptide chains as template formers. Hybrids of two and
three-dimensional networks of these chains with varying connectivities (chemically and structurally) were
used to simulate interfaces for early seed formation. Charged (mostly negatively) functional groups on the
networks allow polar carbonate surfaces to be exposed at the interface whereas in air or water (without a
template), typically the non-polar surface such as (104) is the most stable one.
The ultimate goal of this project is to provide systematic insight into template and, thus, seed formation
control from a theoretical point of view. Ultimately, we want to understand which carbonate will form with
which surface at the interface depending on the template provided.
http://www.geo.lsa.umich.edu/compmin/
V13G-08
Speciation in Aqueous MgSO4 Fluid at High Pressures and Temperatures Studied by First-Principles Modeling and Raman Spectroscopy
Aqueous fluids play an essential role in mass and energy transfer in the lithosphere. Their presence has also a large effect on physical properties of rocks, e.g. the electrical conductivity. Many chemical and physical properties of aqueous fluids strongly depend on the speciation, but very little is known about this fundamental parameter at high pressures and temperatures, e.g. at subduction zone conditions. Here we use a combined approach of first-principles molecular dynamics simulation and Raman spectroscopy to study the molecular structure of aqueous 2~mol/kg MgSO4 fluids up to pressures of 3~GPa and temperatures of 750~°C. MgSO4-H2O is selected as a model system for sulfate bearing subduction zone fluids. The simulations are performed using Car-Parrinello dynamics, a system size of 120 water and four MgSO4 molecules with production runs of at least 10~ps at each P and T. Raman spectra were obtained in situ using a Bassett-type hydrothermal diamond anvil cell with external heating. Both simulation and spectroscopic data show a dynamic co-existence of various associated molecular species as well as dissociated Mg2+ and SO42- in the single phase fluid. Fitting the Raman signal in the frequency range of the ν1-SO42- stretching mode yields the P-T dependence of the relative proportions of different peaks. The latter can be assigned to species based on literature data and related to the species found in the simulation. The dominant associated species found in the P-T range of interest here are Mg-SO4 ion pairs with one (monodentate) and two (bidentate) binding sites. At the highest P and T, an additional peak is identified. At low pressures and high temperature (T>230~°C), kieserite, MgSO4·H2O, nucleated in the experiment. At the same conditions the simulations show a clustering of Mg, which is interpreted as a precursor of precipitation. In conclusion, the speciation of aqueous MgSO4 fluid shows a complex behavior at high P and T that cannot be extrapolated from ambient conditions. The combination of molecular modeling and in situ spectroscopic experiments is a promising approach towards quantitative understanding of geochemical processes in subduction zones.