Nita Sahai

Nanominerals, Mineral Nanoparticles, and Earth Systems

Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.

Evaluation of internally consistent parameters for the triple-layer model by the systematic analysis of oxide surface titration data

Systematic analysis of surface titration data from the literature has been performed for ten oxides (anatase, hematite, goethite, rutile, amorphous silica, quartz, magnetite, δ-MnO2, corundum, and γ-alumina) in ten electrolytes (LiNo3, NaNO3, KNO3, CsNO3, LiCl, NaCl, KCl, CsCl, NaI, and NaClO4) over a wide range of ionic strengths (0.001 M–2.9 M) to establish adsorption equilibrium constants and capacitances consistent with the triple-layer model of surface complexation. Experimental data for the same mineral in different electrolytes and data for a given mineral/ electrolyte system from various investigators have been compared. In this analysis, the surface protonation constants (K,, and Ks,2) were calculated by combining predicted values of ΔpK(log Ks,2 − log Ks,1) (Sverjensky and Sahai, 1996) with experimental points of zero charge; site-densities were obtained from tritium-exchange experiments reported in the literature, and the outer-layer capacitance (C2) was set at 0.2 F·m−2. This scheme permitted us to retrieve consistent sets of values for the inner layer capacitance (C1), and for the electrolyte adsorption constants (Ks,L− and Ks,M+) corresponding, respectively, to the equilibria >SOH2+ + Laq− = >SOH2+ − L− and >SO− + Maq+ = >SO− − M+ Aqueous activity coefficients were calculated using the extended Debye-Huckel equation (Helgeson et al., 1981), which is valid to high ionic strengths (>0.5 M). Systematic analysis of the data reveals important trends and differences between triple-layer model predictions and experimental data and between data for the same mineral/ electrolyte from different investigators. Furthermore, the analysis yields an internally consistent set of triple-layer parameters which will be used in developing a predictive model for electrolyte adsorption based on Born solvation and electrostatic theory (Sahai and Sverjensky, 1997a).

Solvation and electrostatic model for specific electrolyte adsorption

A solvation and electrostatic model has been developed for estimating electrolyte adsorption from physical and chemical properties of the system, consistent with the triple-layer model. The model is calibrated on experimental surface titration data for ten oxides and hydroxides in ten electrolytes over a range of ionic strengths from 0.001 M-2.9 M (Sahai and Sverjensky, 1997a). The model assumes the presence of a single type of surface site, >SOH. It is proposed that for a 1:1 electrolyte of the type M+L−, the logarithms of the adsorption constants (Ks,M+and Ks,L−) representing the equilibria >SO− + Maq+ = >SO− − M+and>SOH2+ + Laq− = >SOH2+ − L− contain contributions from an ion-intrinsic component and a solvation component. According to Born solvation theory, log Ks,M+ and log Ks, L− can be linearly correlated with inverse dielectric constant of the k-th mineral (1ϵk) resulting in the equations log Ks,M+ = − δωM+2.303RT1ϵk + log Kii,M+″and log Ks,L− = − δωL−2.303RT1ϵk + log Kii,L+″ The ion-intrinsic part (log Kii″) is a linear function of the inverse electrostatic radius (1re,j) of the j-th aqueous ion, where, in general, j = M+ or L−. The interfacial solvation coefficient (Δ, Ωj) associated with the solvation component is linearly related to the inverse effective radius (1Re,j) of the adsorbed ion and to the charge (Zj) on the ion. The model is consistent with surface protonation constants (Ks,1and Ks,2) calculated from experimental points of zero charge and values of ΔpK predicted from the Pauling bond-strength per unit bond-length (sr>S−OH) of the bulk mineral (Sahai and Sverjensky, 1997a), site-densities (Ns) from isotopic-exchange data, and outer-layer capacitance (C2) equal to 0.2 F m−2. As a first approximation, we also find an empirical trend between capacitance (C1) of the inner-layer and 1(re,ML·ωML) where re,ML is the electrostatic radius and ωML is the solvation coefficient of the aqueous electrolyte. Taken together, these correlations enable the calculation of surface protonation and electrolyte adsorption at equilibrium from the properties of the mineral/ solution system.

Effects of pseudowollastonite (CaSiO3) bioceramic on in vitro activity of human mesenchymal stem cells

We report the effects of two pseudowollastonite (beta-CaSiO(3)) substrates on the attachment, viability, proliferation and osteogenic differentiation of human mesenchymal stem cells (hMSCs), and provide detailed mechanistic links of surface texture, soluble factors and culture media to cell activities. Cell attachment and viability were lower for psWf (fine-grained, roughness 0.74 microm) than for psWc (coarse-grained, roughness 1.25 microm) surface, and were ascribed to the greater specific area of the finer psWf particles resulting in higher release rate of Si, which is cytotoxic at high levels. Interestingly, proliferation was greater on psWf. Osteogenic differentiation occurred on both surfaces, indicated by calcium phosphate bone nodule formation and by osteocalcin, osteopontin and core-binding factor alpha-1 gene expression. Gene levels were lower on psWf than on psWc at day 8 in growth medium, explained by differences in Ca and/or Si concentrations between the two surfaces. Similar gene expression on both surfaces at day 16 in both growth and osteogenic induction media was attributed to pro-osteogenic effects of Ca and P at specific concentrations and complementary Ca and P levels on the two surfaces. In summary, soluble factors from substrates may be more important for osteogenic differentiation in growth medium than small surface roughness variations within a factor of 2. Optimum concentration ranges exist for individual soluble factors to balance cell toxicity/growth versus osteogenic differentiation, and soluble factors together have complex, cooperative or opposing, effects on a given cell activity.

How Does Bone Sialoprotein Promote the Nucleation of Hydroxyapatite? A Molecular Dynamics Study Using Model Peptides of Different Conformations

Bone sialoprotein (BSP) is a highly phosphorylated, acidic, noncollagenous protein in bone matrix. Although BSP has been proposed to be a nucleator of hydroxyapatite (Ca(5)(PO(4))(3)OH), the major mineral component of bone, no detailed mechanism for the nucleation process has been elucidated at the atomic level to date. In the present work, using a peptide model, we apply molecular dynamics (MD) simulations to study the conformational effect of a proposed nucleating motif of BSP (a phosphorylated, acidic, 10 amino-acid residue sequence) on controlling the distributions of Ca(2+) and inorganic phosphate (Pi) ions in solution, and specifically, we explore whether a nucleating template for orientated hydroxyapatite could be formed in different peptide conformations. Both the alpha-helical conformation and the random coil structure have been studied, and inorganic solutions without the peptide are simulated as reference. Ca(2+) distributions around the peptide surface and interactions between Ca(2+) and Pi in the presence of the peptide are examined in detail. From the MD simulations, although in some cases for the alpha-helical conformation, we observe that a Ca(2+) equilateral triangle forms around the surface of peptide, which matches the distribution of Ca(2+) ions on the (001) face of the hydroxyapatite crystal, we do not consistently find a stable nucleating template formation in general for either the helical conformation or the random coil structure. Therefore, independent of conformations, the BSP nucleating motif is more likely to help nucleate an amorphous calcium phosphate cluster, which ultimately converts to crystalline hydroxyapatite.

Calculating the acidity of silanols and related oxyacids in aqueous solution

Abstract Ab initio molecular orbital theory was used to calculate deprotonation energies and enthalpies (ΔE d , ΔH d ) of oxyacid monomers and oligomers. Results were interpreted with reference to current phenomenological models for estimating metal-oxide surface acidities. The ultimate goal is to predict surface acidities using the ab initio method. We evaluated contributions to ΔE d and ΔH d from the electrostatic potential at the proton, electronic relaxation, geometric relaxation, solvation, and polymerization for the neutral-charge gas-phase molecules H 2 O, CH 3 OH, HCOOH, SiH 3 OH, Si(OH) 4 , Si 2 O 7 H 6 , H 3 PO 4 , P 2 O 7 H 4 , H 2 SO 3 , H 2 SO 4 , HOCl, HClO 4 , Ge(OH) 4 , As(OH) 3 , and AsO(OH) 3 . ΔE d, gas calculated at the modest 6-31G∗ HF of theory level correlates well with experimental pK a in solution, because hydration enthalpies for the acid anions (ΔH hyd, A− ) are closely proportional to ΔE d, gas . That is, anion interaction energies with water in aqueous solution and with H + in the gas phase are closely correlated. Correction for differential hydration between an acid and its conjugate base permits generalization of the ΔE d, gas – pK a correlation to deprotonation reactions involving charged acids. Thus, stable protonated, neutral, and deprotonated species Si(OH) 3 (OH 2 ) 1+ , Si(OH) 4 0 , Si(OH) 3 O 1− , and Si(OH) 2 O 2 2− have been characterized, and solution pK a ’s for Si(OH) 3 (OH 2 ) 1+ and Si(OH) 3 O 1− were estimated, assuming that the charged species (Si(OH) 3 (OH 2 ) 1+ , Si(OH) 3 O −1 ) fit into the same ΔE d, gas – pK a correlation as do the neutral acids. The correlation yields a negative pK a (∼ −5) for Si(OH) 3 (OH 2 ) +1 . Calculated ΔE d, gas also correlates well with the degree of O under-bonding evaluated using Brown’s bond-length based approach. ΔE d, gas increases along the series HClO 4 – Si(OH) 4 mainly because of increasingly negative potential at the site of the proton, not because of differing electronic or geometric relaxation energies. Thus, pK a can be correlated with underbondings or local electrostatic energies for the monomers, partially explaining the success of phenomenological models in correlating surface pK a of oxides with bond-strengths. Accurate evaluation of ΔH d, gas requires calculations with larger basis sets, inclusion of electron correlation effects, and corrections for vibrational, rotational, and translational contributions. Density functional and 2nd-order Moller-Plesset results for deprotonation enthalpies match well against higher-level G2(MP2) calculations. Direct calculation of solution pK a without resorting to correlations is presently impossible by ab initio methods because of inaccurate methods to account for solvation. Inclusion of explicit water molecules around the monomer immersed in a self-consistent reaction field (SCRF) provides the most accurate absolute hydration enthalpy (ΔH hyd ) values, but IPCM values for the bare acid (HA) and anion (A − ) give reasonable values of ΔH hyd, A − – ΔH hyd, HA values with much smaller computational expense. Polymers of silicate are used as model systems that begin to approach solid silica, known to be much more acidic than its monomer, Si(OH) 4 . Polymerization of silicate or phosphate reduces their gas-phase ΔE d, gas relative to the monomers; differences in the electrostatic potential at H + , electronic relaxation and geometric relaxation energies all contribute to the effect. Internal H-bonding in the dimers results in unusually small ΔE d,gas which is partially counteracted by a reduced ΔH hyd . Accurate representation of hydration for oligomers persists as a fundamental problem in determining their solution pK a , because of the prohibitive cost involved in directly modeling interactions between many water molecules and the species of interest. Fortunately, though, the local contribution to the difference in hydration energy between the neutral polymeric acid and its anion seems to stabilize for a small number of explicit water molecules.

Amine-Catalyzed Biomimetic Hydrolysis and Condensation of Organosilicate

Biogenic silica production occurs at mild conditions with greater control of pore size, shape, and micropatterning than is possible with typical industrial sol -gel methods, providing inspiration for potential alternative routes to silica synthesis. Researchers have implicated the amine moieties, histidine and polylysine, on proteins isolated from sponges and diatoms as catalysts for biogenic silica precipitation. Different mechanistic roles have been ascribed to the amines, but few systematic, quantitative studies isolating one effect from another have been conducted. In the present study, we use 29Si NMR spectroscopy to systematically examine the different possible mechanistic roles of monoand polyamines in catalyzing silica synthesis at mildly acidic pH ( ∼5) from an organosilicate starting compound, trimethylethoxysilane (TMES). TMES has a single organosilicate bond, so there are no competing reactions and the reaction progress can be followed with little ambiguity. Hydrolysis and condensation (dimerization) of TMES lead to the products trimethylsilanol (TMSiOH) and hexamethyldisiloxane (HMD). The Refocused Insensitive Nuclei Enhanced by Polarization Transfer pulse sequence (RINEPT +) provides unambiguous, quantitative29Si NMR spectra from which the hydrolysis and condensation rates in the presence of each amine can be obtained. For both monoand polyamines, the catalytic efficiency scales with the concentration of conjugate base form and inversely with p Ka. Thus, catalysis is most efficient with more acidic monoamines, such as pyridine and imidazole, as well as for the longer polyamines, where the most acidic protonation constant is lower than the experimental pH ( ∼5). We postulate a nucleophilecatalyzed hydrolysis mechanism where the conjugate base of the amine attacks Si to form a pentacoordinate intermediate with TMES. Condensation is interpreted as an acid-catalyzed S N2 mechanism. Our findings potentially explain the evolutionary selection of histidine-containing proteins for biogenic silica synthesis by sponges and address the chemical mechanisms at work for the precipitation of silica by polylysinecontaining proteins in diatoms. Along with the physical mechanisms suggested by other research groups, the systematic results from the present study indicate that amines may be employed in more than one type of mechanistic strategy for catalyzing biogenic and biomimetic silica polymerization.

Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water

Abstract Surface protonation is the most fundamental adsorption process of geochemical interest. Yet remarkably little is known about protonation of mineral surfaces at temperatures greater than 25°C. Experimentally derived standard enthalpies of surface protonation, ΔHr,1°, ΔHr,2°, and ΔHr,ZPC°, correspond to the reactions >SOH+H + =>SOH 2 + >SO − +H + =>SOH >SO − +2H + =>SOH 2 + respectively, and provide a starting point for evaluating the role of surface protonation in geochemical processes at elevated temperatures. However, the experimental data for oxides do not have a theoretical explanation, and data are completely lacking for silicates other than SiO2. In the present study, the combination of crystal chemical and Born solvation theory provides a theoretical basis for explaining the variation of the enthalpies of protonation of oxides. Experimental values of ΔHr,1°, ΔHr,2°, and ΔHr,ZPC° consistent with the triple layer model can be expressed in terms of the inverse of the dielectric constant (1/e) and the Pauling bond strength per angstrom (s/rM-OH) of each mineral by equations such as ΔH r,ZPC ° =ΔΩ r,Z [(1/e)−(T/e) 2 (∂e/∂T)]−B ′ Z (s/r M-OH )+H ′ Z . The Born solvation coefficient ΔΩr,Z was taken from a prior analysis of surface equilibrium constants. The coefficients BZ′ and HZ′ were derived by regression of experimental enthalpies for rutile, γ-alumina, magnetite, hematite, and silica. This approach permits widespread prediction of the enthalpies of surface protonation. Predicted standard enthalpies of surface protonation for oxides and silicates extend over the ranges (in kcal.mole−1): ΔHr,1° ≈ −3 to −15; ΔHr,2° ≈ −0.5 to −18; ΔHr,ZPC° ≈ −4 to −33. Minerals with the largest values of s/rM-OH (e.g., quartz and kaolinite) are predicted to have weakly negative enthalpies and a weak temperature dependence for their protonation equilibrium constants. Conversely, minerals with the smallest values of s/rM-OH (e.g., garnets and olivines) should have strong negative enthalpies and a strong temperature dependence for their protonation equilibrium constants.

Estimating adsorption enthalpies and affinity sequences of monovalent electrolyte ions on oxide surfaces in aqueous solution

Abstract A new expression is developed for estimating the adsorption enthalpy of aqueous, monovalent ions on charged surfaces of solid oxides up to about 70°C. For sorption of the M-th cation and L-th anion represented as: >SO− + M+ = >SO− − −M+and >SOH2+ + L− = >SOH2+ −−L−the enthalpy at 25°C is given by: ΔH i,k 0 =ΔΩ i T[ 1 e k 2 ( ∂e k ∂T )− 1 e w 2 ( ∂e w ∂T )]+ΔG i,k 0 , where i = M+ or L−, >SO− and >SOH2+ are charged surface sites, ΔΩi is the interfacial Born solvation coefficient of the i-th monovalent ion, ϵk and ϵw are the dielectric constants of the k-th solid and of bulk water, respectively, T is the absolute temperature, and ΔGi, k0 is the free energy of ion adsorption. The small values predicted for enthalpies suggest weak temperature dependence for electrolyte affinities. The reaction enthalpy is negative for all oxides considered, and is the major contribution to the free energy of adsorption. Reactions are less exothermic for solids with smaller dielectric constants. Ion-specific trends are also noted, with exothermicity of enthalpy decreasing as Li+ > Na+ > K+ > Rb+ = NH4+ > Cs+ > TMA+ (tetramethylammonium) for all oxides except quartz and amorphous SiO2 where the reverse trend is predicted. Similarly, exothermicity decreases as F− > Cl− > Br− > I− for all oxides excluding quartz and amorphous SiO2. The entropic contribution to free energy is small, and is negative for all the oxides considered including quartz, but is positive for amorphous SiO2, suggesting an intriguing difference between the surfaces of quartz and amorphous SiO2. In order to determine the temperature dependence of surface-complexation, ΔHM+, k0 and ΔHL−, k0 are combined with the enthalpies for deprotonation and protonation of the neutral surface site (−ΔHH+,20, ΔHH+,10), respectively, yielding ΔHM+, k0∗ and ΔHL−, k0∗ which correspond to the reactions: >SOH + M+ = >SO− − −M+ + H+and >SOH + H+ + L− = >SOH2+ − −L− Positive values of ΔHM+, k0∗ (endothermic reaction) are obtained for all oxides considered (except pyrolusite and quartz) implying that M+ complexation should increase with temperature. Amorphous silica differs from quartz in that reactions are slightly endothermic to thermoneutral. Negative values of ΔHL−, k0∗ (exothermic reaction) are obtained for all oxides considered, suggesting that L− complexation decreases with temperature. ΔHM+, k0∗ and ΔHL−, k0∗ vary only slightly with ion-identity because their values are dominated by −ΔHH+, 20 and ΔHH,10+. Also, ΔHM+, k0∗ and ΔHL−, k0∗ do not vary systematically with ϵk because −ΔHH+, 20 and ΔHH+, 10 depend not only on ϵk but also on the Pauling bond strength per metal–oxygen bond length (s/r) of the metal constituting the solid oxide.

Formation energies and NMR chemical shifts calculated for putative serine-silicate complexes in silica biomineralization

Abstract We have used ab initio Hartree–Fock (HF) theory to determine the thermodynamic feasibility for the formation of H-bonded and covalently bonded serine-silicic acid complexes, with Si in quadra- and penta-coordination ( Q Si and P Si, respectively). Such complexes have been suggested previously to play a role in silica biomineralization, a process that controls silicon fluxes in the oceans, and thus, global silicon cycling. Geometries and energies were obtained at the HF/6–31G∗ level, where solvation was represented by a spherical cavity in a dielectric continuum. 29 Si, 13 C, and 17 O NMR shifts at the HF/6–311+G(2d,p) level, and 17 O nuclear quadrapole coupling constants (NQCC) at the HF/6 to 31G∗ level are also reported to aid in experimental identification of the complexes. Our results show that if H-bonded and/or covalently bonded serine- Q Si complexes did exist in biogenic silicification, they would not be detected by 29 Si-NMR because their predicted isotropic shifts are similar to inorganic Q Si. The penta-coordinated complex, [serO P Si(OH) 4 ] 1− , would be detectable because of large 29 Si isotropic (−121 to −140.1 ppm) and anisotropic shifts (110–142 ppm) and diagnostically long P Si–O bond lengths (1.7–1.9 A). 13 C shifts are found to be insensitive to the type of bonding. 17 O shifts are the most sensitive to bond type and Si coordination number, because it is the most directly involved nucleus in the C–O–Si bonding. The formation of the covalent quadra-coordinated complex, serO Q Si(OH) 3 is 10 kcal mol −1 more exothermic than the H-bonded complex, serOH… Q Si(OH) 4 . The penta-coordinated complex, is thermodynamically unfavorable at the acidic pH of the silica deposition vesicle within diatoms. Silicic acid is energetically favored over organic silicon complexes as the form of dissolved silicon taken up by the organism at the basic pH of seawater.