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Per E M Siegbahn - One of the best experts on this subject based on the ideXlab platform.

  • structures and energetics for o2 formation in photosystem ii
    Accounts of Chemical Research, 2009
    Co-Authors: Per E M Siegbahn
    Abstract:

    Water oxidation, forming O2 from water and sunlight, is a fundamental process for life on earth. In nature, the enzyme photosystem II (PSII) catalyzes this reaction. The oxygen evolving complex (OEC), the complex within PSII that catalyzes the actual formation of the O−O bond, contains four manganese Atoms and one Calcium Atom connected by oxo bonds. Seven amino acid side chains in the structure, mostly carboxylates, are ligated to the metal Atoms. In the study of many enzyme mechanisms, theoretical modeling using density functional theory has served as an indispensable tool. This Account summarizes theoretical research to elucidate the mechanism for water oxidation in photosynthesis, including the most recent findings. The development of successively larger models, ranging from 50 Atoms in the active site up to the present model size of 170 Atoms, has revealed the mechanism of O2 formation with increasing detail. The X-ray crystal structures of PSII have provided a framework for optimizing the theoretica...

  • structures and energetics for o2 formation in photosystem ii
    Accounts of Chemical Research, 2009
    Co-Authors: Per E M Siegbahn
    Abstract:

    Water oxidation, forming O2 from water and sunlight, is a fundamental process for life on earth. In nature, the enzyme photosystem II (PSII) catalyzes this reaction. The oxygen evolving complex (OEC), the complex within PSII that catalyzes the actual formation of the O−O bond, contains four manganese Atoms and one Calcium Atom connected by oxo bonds. Seven amino acid side chains in the structure, mostly carboxylates, are ligated to the metal Atoms. In the study of many enzyme mechanisms, theoretical modeling using density functional theory has served as an indispensable tool. This Account summarizes theoretical research to elucidate the mechanism for water oxidation in photosynthesis, including the most recent findings. The development of successively larger models, ranging from 50 Atoms in the active site up to the present model size of 170 Atoms, has revealed the mechanism of O2 formation with increasing detail. The X-ray crystal structures of PSII have provided a framework for optimizing the theoretica...

Hongbo Zeng - One of the best experts on this subject based on the ideXlab platform.

  • probing anisotropic surface properties and surface forces of fluorite crystals
    Langmuir, 2018
    Co-Authors: Zhiyong Gao, Lei Xie, Xin Cui, Wei Sun, Hongbo Zeng
    Abstract:

    Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, Atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin-Landau-Verwey-Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the Atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface Atom (especially Calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each Calcium Atom on the {100} surface with four dangling bonds is more active than the Calcium Atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes.

  • Probing Anisotropic Surface Properties and Surface Forces of Fluorite Crystals
    2018
    Co-Authors: Zhiyong Gao, Lei Xie, Xin Cui, Wei Sun, Hongbo Zeng
    Abstract:

    Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, Atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin–Landau–Verwey–Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the Atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface Atom (especially Calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each Calcium Atom on the {100} surface with four dangling bonds is more active than the Calcium Atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes

G H Nancollas - One of the best experts on this subject based on the ideXlab platform.

  • Calcium orthophosphates crystallization and dissolution
    ChemInform, 2009
    Co-Authors: Lijun Wang, G H Nancollas
    Abstract:

    Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all Calcium orthophosphates consist of three major chemical elements: Calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (oxidation state −2).1 The orthophosphate group (PO43−) is structurally different from meta (PO3−), pyro (P2O74−), and poly (PO3)nn−. In this review, only Calcium orthophosphates will be discussed. The chemical composition of many Calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO42− or H2PO4−, and/or incorporated water as in diCalcium phosphate dihydrate (CaHPO4 · 2H2O).1 Most Calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the Calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the Calcium phosphate phase.2 It is now generally recognized that the crystallization of many Calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in vivo presence of small peptides, proteins, and inorganic additives other than Calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form.3 Studies of apatite mineral formation are complicated by the possibility of forming several Calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octaCalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of Calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in vitro crystallization, in vivo studies of bone formation rarely show the presence of these acidic Calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in vivo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous Calcium phosphate phase (ACP) followed by transformation to apatite. Table 1 Ca/P Molar Ratios, Chemical Formulas, and Solubilitiesa of Some Calcium Orthophosphate Minerals1,3,4 Table 2 Crystallographic Data of Calcium Orthophosphates1,4,5 In this review, we will discuss some important parameters related to crystal nucleation and growth/dissolution including the supersaturation/undersaturation, pH, ionic strength and the ratio of Calcium to phosphate activities (Table 3). We then focus on the dynamics of crystallization/dissolution in the presence of additive molecules pertinent to biogenic Calcium phosphate minerals. Table 3 Crystal Growth Controls and Their Effect on the Bulk Solution and the Crystal Surfaces6 2. Biologically Related Calcium Phosphate Phases 2.1. Structure, Composition, and Phase Stability 2.1.1. Amorphous Calcium Phosphate (ACP) During the synthesis of HAP crystals through the interaction of Calcium and phosphate ions in neutral to basic solution, a precursor amorphous phase is formed that is structurally and chemically distinct from HAP.7 However, calculations have shown that the phase consisted of individual or groups of HAP unit cells.8 Chemical analysis of the precursor phase indicated this noncrystalline phase to be a hydrated Calcium phosphate (Ca3(PO4)2 · xH2O) with a Ca/P ratio 1.50,8 consisting of roughly spherical Ca9(PO4)6 “Posner’s clusters” (PC) close-packed to form larger spherical particles with water in the interstices.9 PCs appeared to be energetically favored in comparison to alternative candidates including Ca3(PO4)2 and Ca6(PO4)4 clusters.10 The structure of PCs in isolated form is notably different from that in a HAP environment.11 In particular, the chirality feature of PCs found in the HAP environment is suggested to disappear in an isolated form and in aqueous solution. The reconsideration of PCs as possible components in the actual structural model of ACP resulted from the cluster growth model of the HAP crystal.12 Ab initio calculations confirmed that stable isomers exist on the [Ca3(PO4)2]3 potential energy surface (PES).12,13 These isomers correspond to compact arrangements, i.e., arrangements in which the Ca and PO4 are disposed closely together. Their geometries are compatible with the terms “roughly spherical” used in Posner’s hypothesis. The calculations performed on the monomer and dimer PES revealed that the relative energies of the different isomers are governed by a specific bonding pattern in which a Calcium Atom interacts with two PO43− groups, forming four CaO bonds.12,13 The compact isomers on the trimer PES are energetically favored in comparison to monomer or dimer isomers. This is rationalized by the appearance of a specific bonding pattern for the trimer case in which a Calcium forms six CaO bonds with six different PO4 groups. This type of bonding in encountered in HAP.13 It is now generally agreed that, both in vitro and in vivo, precipitation reactions at sufficiently high supersaturation and pH result in the initial formation of an amorphous Calcium phosphate with a molar Calcium/phosphate ratio of about 1.18–2.50. The chemical composition of ACP is strongly dependent on the solution pH: ACP phases with Ca/P ratios in the range of 1.18:1 precipitated at pH 6.6 to 1.53:1 at pH 11.7 and even as high as 2.5:1.4 Two amorphous Calcium phosphates, ACP1 and ACP2, have been reported with the same composition, but differing in morphology and solubility.14,15 The formation of ACP precipitate with little long-range order tends to consist of aggregates of primary nuclei (roughly spherical clusters) with composition Ca9(PO4)65 dependent on the conditions of formation. It hydrolyzes almost instantaneously to more stable phases. These amorphous clusters served as seeds during HAP crystallization via a stepwise assembly process12 and were presumed to pack randomly with respect to each other,16 forming large 300–800 A spheres. Recent experimental studies found that ACP has definite local Atomic microcrystalline order rather than a random network structure. NMR of thoroughly dried ACP suggests that the tightly held water resides in the interstices between clusters,17 but these are probably not of intrinsic importance in the structure of ACP. It is well-known that ACP contains 10–20% by weight of tightly bound water, which is removed by vacuum drying at elevated temperature.9 However, drying does not alter the Calcium and phosphorus Atomic arrangement. The side band intensities of dried ACP suggest that its chemical shift anisotropy is similar to or identical with that of normal ACP.17 ACP has an apatitic short-range structure, but with a crystal size so small that it appears to be amorphous by X-ray analysis. This is supported by extended X-ray absorption fine structure (EXAFS) on biogenic and synthetic ACP samples.18–20 The CaP amorphous phase transforms to HAP microcrystalline in the presence of water. The lifetime of the metastable amorphous precursor in aqueous solution was reported to be a function of the presence of additive molecules and ions, pH, ionic strength, and temperature.21 The transformation kinetics from ACP to HAP, which can be described by a ”first-order” rate law, is a function only of the pH of the mediating solution at constant temperature. The solution-mediated transformation depends upon the conditions which regulate both the dissolution of ACP and the formation of the early HAP nuclei.22 Tropp et al. used 31P NMR to demonstrate that the strength of ACP side bands is due to a characteristic structural distortion of unprotonated phosphate and not to a mixture of protonated and unprotonated phosphates,17 suggesting that ACP could contain substantial amounts of protonated phosphate not in the form of any known phase of Calcium phosphate crystals. Yin and Stott suggested that, in the transformation from ACP to HAP, ACP need only dissociate into clusters rather than undergo complete ionic solvation. The cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of Posner’s cluster with sodium ions and especially with protons leads to a considerable stability increase, and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar Atomic arrangement that it has as a structural unit in the HAP crystal. This may be a key factor in the transformation from ACP to HAP crystal.23 In general, ACP is a highly unstable phase that hydrolyzes almost instantaneously to more stable phases. In the presence of other ions and macromolecules or under in vivo conditions, ACP may persist for appreciable periods3 and retain the amporphous state under some specific experimental conditions.24

  • Calcium orthophosphates crystallization and dissolution
    Chemical Reviews, 2008
    Co-Authors: Lijun Wang, G H Nancollas
    Abstract:

    Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all Calcium orthophosphates consist of three major chemical elements: Calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (oxidation state −2).1 The orthophosphate group (PO43−) is structurally different from meta (PO3−), pyro (P2O74−), and poly (PO3)nn−. In this review, only Calcium orthophosphates will be discussed. The chemical composition of many Calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO42− or H2PO4−, and/or incorporated water as in diCalcium phosphate dihydrate (CaHPO4 · 2H2O).1 Most Calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the Calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the Calcium phosphate phase.2 It is now generally recognized that the crystallization of many Calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in vivo presence of small peptides, proteins, and inorganic additives other than Calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form.3 Studies of apatite mineral formation are complicated by the possibility of forming several Calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octaCalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of Calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in vitro crystallization, in vivo studies of bone formation rarely show the presence of these acidic Calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in vivo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous Calcium phosphate phase (ACP) followed by transformation to apatite. Table 1 Ca/P Molar Ratios, Chemical Formulas, and Solubilitiesa of Some Calcium Orthophosphate Minerals1,3,4 Table 2 Crystallographic Data of Calcium Orthophosphates1,4,5 In this review, we will discuss some important parameters related to crystal nucleation and growth/dissolution including the supersaturation/undersaturation, pH, ionic strength and the ratio of Calcium to phosphate activities (Table 3). We then focus on the dynamics of crystallization/dissolution in the presence of additive molecules pertinent to biogenic Calcium phosphate minerals. Table 3 Crystal Growth Controls and Their Effect on the Bulk Solution and the Crystal Surfaces6 2. Biologically Related Calcium Phosphate Phases 2.1. Structure, Composition, and Phase Stability 2.1.1. Amorphous Calcium Phosphate (ACP) During the synthesis of HAP crystals through the interaction of Calcium and phosphate ions in neutral to basic solution, a precursor amorphous phase is formed that is structurally and chemically distinct from HAP.7 However, calculations have shown that the phase consisted of individual or groups of HAP unit cells.8 Chemical analysis of the precursor phase indicated this noncrystalline phase to be a hydrated Calcium phosphate (Ca3(PO4)2 · xH2O) with a Ca/P ratio 1.50,8 consisting of roughly spherical Ca9(PO4)6 “Posner’s clusters” (PC) close-packed to form larger spherical particles with water in the interstices.9 PCs appeared to be energetically favored in comparison to alternative candidates including Ca3(PO4)2 and Ca6(PO4)4 clusters.10 The structure of PCs in isolated form is notably different from that in a HAP environment.11 In particular, the chirality feature of PCs found in the HAP environment is suggested to disappear in an isolated form and in aqueous solution. The reconsideration of PCs as possible components in the actual structural model of ACP resulted from the cluster growth model of the HAP crystal.12 Ab initio calculations confirmed that stable isomers exist on the [Ca3(PO4)2]3 potential energy surface (PES).12,13 These isomers correspond to compact arrangements, i.e., arrangements in which the Ca and PO4 are disposed closely together. Their geometries are compatible with the terms “roughly spherical” used in Posner’s hypothesis. The calculations performed on the monomer and dimer PES revealed that the relative energies of the different isomers are governed by a specific bonding pattern in which a Calcium Atom interacts with two PO43− groups, forming four CaO bonds.12,13 The compact isomers on the trimer PES are energetically favored in comparison to monomer or dimer isomers. This is rationalized by the appearance of a specific bonding pattern for the trimer case in which a Calcium forms six CaO bonds with six different PO4 groups. This type of bonding in encountered in HAP.13 It is now generally agreed that, both in vitro and in vivo, precipitation reactions at sufficiently high supersaturation and pH result in the initial formation of an amorphous Calcium phosphate with a molar Calcium/phosphate ratio of about 1.18–2.50. The chemical composition of ACP is strongly dependent on the solution pH: ACP phases with Ca/P ratios in the range of 1.18:1 precipitated at pH 6.6 to 1.53:1 at pH 11.7 and even as high as 2.5:1.4 Two amorphous Calcium phosphates, ACP1 and ACP2, have been reported with the same composition, but differing in morphology and solubility.14,15 The formation of ACP precipitate with little long-range order tends to consist of aggregates of primary nuclei (roughly spherical clusters) with composition Ca9(PO4)65 dependent on the conditions of formation. It hydrolyzes almost instantaneously to more stable phases. These amorphous clusters served as seeds during HAP crystallization via a stepwise assembly process12 and were presumed to pack randomly with respect to each other,16 forming large 300–800 A spheres. Recent experimental studies found that ACP has definite local Atomic microcrystalline order rather than a random network structure. NMR of thoroughly dried ACP suggests that the tightly held water resides in the interstices between clusters,17 but these are probably not of intrinsic importance in the structure of ACP. It is well-known that ACP contains 10–20% by weight of tightly bound water, which is removed by vacuum drying at elevated temperature.9 However, drying does not alter the Calcium and phosphorus Atomic arrangement. The side band intensities of dried ACP suggest that its chemical shift anisotropy is similar to or identical with that of normal ACP.17 ACP has an apatitic short-range structure, but with a crystal size so small that it appears to be amorphous by X-ray analysis. This is supported by extended X-ray absorption fine structure (EXAFS) on biogenic and synthetic ACP samples.18–20 The CaP amorphous phase transforms to HAP microcrystalline in the presence of water. The lifetime of the metastable amorphous precursor in aqueous solution was reported to be a function of the presence of additive molecules and ions, pH, ionic strength, and temperature.21 The transformation kinetics from ACP to HAP, which can be described by a ”first-order” rate law, is a function only of the pH of the mediating solution at constant temperature. The solution-mediated transformation depends upon the conditions which regulate both the dissolution of ACP and the formation of the early HAP nuclei.22 Tropp et al. used 31P NMR to demonstrate that the strength of ACP side bands is due to a characteristic structural distortion of unprotonated phosphate and not to a mixture of protonated and unprotonated phosphates,17 suggesting that ACP could contain substantial amounts of protonated phosphate not in the form of any known phase of Calcium phosphate crystals. Yin and Stott suggested that, in the transformation from ACP to HAP, ACP need only dissociate into clusters rather than undergo complete ionic solvation. The cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of Posner’s cluster with sodium ions and especially with protons leads to a considerable stability increase, and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar Atomic arrangement that it has as a structural unit in the HAP crystal. This may be a key factor in the transformation from ACP to HAP crystal.23 In general, ACP is a highly unstable phase that hydrolyzes almost instantaneously to more stable phases. In the presence of other ions and macromolecules or under in vivo conditions, ACP may persist for appreciable periods3 and retain the amporphous state under some specific experimental conditions.24

Zhiyong Gao - One of the best experts on this subject based on the ideXlab platform.

  • probing anisotropic surface properties and surface forces of fluorite crystals
    Langmuir, 2018
    Co-Authors: Zhiyong Gao, Lei Xie, Xin Cui, Wei Sun, Hongbo Zeng
    Abstract:

    Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, Atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin-Landau-Verwey-Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the Atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface Atom (especially Calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each Calcium Atom on the {100} surface with four dangling bonds is more active than the Calcium Atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes.

  • Probing Anisotropic Surface Properties and Surface Forces of Fluorite Crystals
    2018
    Co-Authors: Zhiyong Gao, Lei Xie, Xin Cui, Wei Sun, Hongbo Zeng
    Abstract:

    Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, Atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin–Landau–Verwey–Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the Atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface Atom (especially Calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each Calcium Atom on the {100} surface with four dangling bonds is more active than the Calcium Atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes

Lijun Wang - One of the best experts on this subject based on the ideXlab platform.

  • Calcium orthophosphates crystallization and dissolution
    ChemInform, 2009
    Co-Authors: Lijun Wang, G H Nancollas
    Abstract:

    Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all Calcium orthophosphates consist of three major chemical elements: Calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (oxidation state −2).1 The orthophosphate group (PO43−) is structurally different from meta (PO3−), pyro (P2O74−), and poly (PO3)nn−. In this review, only Calcium orthophosphates will be discussed. The chemical composition of many Calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO42− or H2PO4−, and/or incorporated water as in diCalcium phosphate dihydrate (CaHPO4 · 2H2O).1 Most Calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the Calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the Calcium phosphate phase.2 It is now generally recognized that the crystallization of many Calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in vivo presence of small peptides, proteins, and inorganic additives other than Calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form.3 Studies of apatite mineral formation are complicated by the possibility of forming several Calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octaCalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of Calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in vitro crystallization, in vivo studies of bone formation rarely show the presence of these acidic Calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in vivo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous Calcium phosphate phase (ACP) followed by transformation to apatite. Table 1 Ca/P Molar Ratios, Chemical Formulas, and Solubilitiesa of Some Calcium Orthophosphate Minerals1,3,4 Table 2 Crystallographic Data of Calcium Orthophosphates1,4,5 In this review, we will discuss some important parameters related to crystal nucleation and growth/dissolution including the supersaturation/undersaturation, pH, ionic strength and the ratio of Calcium to phosphate activities (Table 3). We then focus on the dynamics of crystallization/dissolution in the presence of additive molecules pertinent to biogenic Calcium phosphate minerals. Table 3 Crystal Growth Controls and Their Effect on the Bulk Solution and the Crystal Surfaces6 2. Biologically Related Calcium Phosphate Phases 2.1. Structure, Composition, and Phase Stability 2.1.1. Amorphous Calcium Phosphate (ACP) During the synthesis of HAP crystals through the interaction of Calcium and phosphate ions in neutral to basic solution, a precursor amorphous phase is formed that is structurally and chemically distinct from HAP.7 However, calculations have shown that the phase consisted of individual or groups of HAP unit cells.8 Chemical analysis of the precursor phase indicated this noncrystalline phase to be a hydrated Calcium phosphate (Ca3(PO4)2 · xH2O) with a Ca/P ratio 1.50,8 consisting of roughly spherical Ca9(PO4)6 “Posner’s clusters” (PC) close-packed to form larger spherical particles with water in the interstices.9 PCs appeared to be energetically favored in comparison to alternative candidates including Ca3(PO4)2 and Ca6(PO4)4 clusters.10 The structure of PCs in isolated form is notably different from that in a HAP environment.11 In particular, the chirality feature of PCs found in the HAP environment is suggested to disappear in an isolated form and in aqueous solution. The reconsideration of PCs as possible components in the actual structural model of ACP resulted from the cluster growth model of the HAP crystal.12 Ab initio calculations confirmed that stable isomers exist on the [Ca3(PO4)2]3 potential energy surface (PES).12,13 These isomers correspond to compact arrangements, i.e., arrangements in which the Ca and PO4 are disposed closely together. Their geometries are compatible with the terms “roughly spherical” used in Posner’s hypothesis. The calculations performed on the monomer and dimer PES revealed that the relative energies of the different isomers are governed by a specific bonding pattern in which a Calcium Atom interacts with two PO43− groups, forming four CaO bonds.12,13 The compact isomers on the trimer PES are energetically favored in comparison to monomer or dimer isomers. This is rationalized by the appearance of a specific bonding pattern for the trimer case in which a Calcium forms six CaO bonds with six different PO4 groups. This type of bonding in encountered in HAP.13 It is now generally agreed that, both in vitro and in vivo, precipitation reactions at sufficiently high supersaturation and pH result in the initial formation of an amorphous Calcium phosphate with a molar Calcium/phosphate ratio of about 1.18–2.50. The chemical composition of ACP is strongly dependent on the solution pH: ACP phases with Ca/P ratios in the range of 1.18:1 precipitated at pH 6.6 to 1.53:1 at pH 11.7 and even as high as 2.5:1.4 Two amorphous Calcium phosphates, ACP1 and ACP2, have been reported with the same composition, but differing in morphology and solubility.14,15 The formation of ACP precipitate with little long-range order tends to consist of aggregates of primary nuclei (roughly spherical clusters) with composition Ca9(PO4)65 dependent on the conditions of formation. It hydrolyzes almost instantaneously to more stable phases. These amorphous clusters served as seeds during HAP crystallization via a stepwise assembly process12 and were presumed to pack randomly with respect to each other,16 forming large 300–800 A spheres. Recent experimental studies found that ACP has definite local Atomic microcrystalline order rather than a random network structure. NMR of thoroughly dried ACP suggests that the tightly held water resides in the interstices between clusters,17 but these are probably not of intrinsic importance in the structure of ACP. It is well-known that ACP contains 10–20% by weight of tightly bound water, which is removed by vacuum drying at elevated temperature.9 However, drying does not alter the Calcium and phosphorus Atomic arrangement. The side band intensities of dried ACP suggest that its chemical shift anisotropy is similar to or identical with that of normal ACP.17 ACP has an apatitic short-range structure, but with a crystal size so small that it appears to be amorphous by X-ray analysis. This is supported by extended X-ray absorption fine structure (EXAFS) on biogenic and synthetic ACP samples.18–20 The CaP amorphous phase transforms to HAP microcrystalline in the presence of water. The lifetime of the metastable amorphous precursor in aqueous solution was reported to be a function of the presence of additive molecules and ions, pH, ionic strength, and temperature.21 The transformation kinetics from ACP to HAP, which can be described by a ”first-order” rate law, is a function only of the pH of the mediating solution at constant temperature. The solution-mediated transformation depends upon the conditions which regulate both the dissolution of ACP and the formation of the early HAP nuclei.22 Tropp et al. used 31P NMR to demonstrate that the strength of ACP side bands is due to a characteristic structural distortion of unprotonated phosphate and not to a mixture of protonated and unprotonated phosphates,17 suggesting that ACP could contain substantial amounts of protonated phosphate not in the form of any known phase of Calcium phosphate crystals. Yin and Stott suggested that, in the transformation from ACP to HAP, ACP need only dissociate into clusters rather than undergo complete ionic solvation. The cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of Posner’s cluster with sodium ions and especially with protons leads to a considerable stability increase, and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar Atomic arrangement that it has as a structural unit in the HAP crystal. This may be a key factor in the transformation from ACP to HAP crystal.23 In general, ACP is a highly unstable phase that hydrolyzes almost instantaneously to more stable phases. In the presence of other ions and macromolecules or under in vivo conditions, ACP may persist for appreciable periods3 and retain the amporphous state under some specific experimental conditions.24

  • Calcium orthophosphates crystallization and dissolution
    Chemical Reviews, 2008
    Co-Authors: Lijun Wang, G H Nancollas
    Abstract:

    Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all Calcium orthophosphates consist of three major chemical elements: Calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (oxidation state −2).1 The orthophosphate group (PO43−) is structurally different from meta (PO3−), pyro (P2O74−), and poly (PO3)nn−. In this review, only Calcium orthophosphates will be discussed. The chemical composition of many Calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO42− or H2PO4−, and/or incorporated water as in diCalcium phosphate dihydrate (CaHPO4 · 2H2O).1 Most Calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the Calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the Calcium phosphate phase.2 It is now generally recognized that the crystallization of many Calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in vivo presence of small peptides, proteins, and inorganic additives other than Calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form.3 Studies of apatite mineral formation are complicated by the possibility of forming several Calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octaCalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of Calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in vitro crystallization, in vivo studies of bone formation rarely show the presence of these acidic Calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in vivo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous Calcium phosphate phase (ACP) followed by transformation to apatite. Table 1 Ca/P Molar Ratios, Chemical Formulas, and Solubilitiesa of Some Calcium Orthophosphate Minerals1,3,4 Table 2 Crystallographic Data of Calcium Orthophosphates1,4,5 In this review, we will discuss some important parameters related to crystal nucleation and growth/dissolution including the supersaturation/undersaturation, pH, ionic strength and the ratio of Calcium to phosphate activities (Table 3). We then focus on the dynamics of crystallization/dissolution in the presence of additive molecules pertinent to biogenic Calcium phosphate minerals. Table 3 Crystal Growth Controls and Their Effect on the Bulk Solution and the Crystal Surfaces6 2. Biologically Related Calcium Phosphate Phases 2.1. Structure, Composition, and Phase Stability 2.1.1. Amorphous Calcium Phosphate (ACP) During the synthesis of HAP crystals through the interaction of Calcium and phosphate ions in neutral to basic solution, a precursor amorphous phase is formed that is structurally and chemically distinct from HAP.7 However, calculations have shown that the phase consisted of individual or groups of HAP unit cells.8 Chemical analysis of the precursor phase indicated this noncrystalline phase to be a hydrated Calcium phosphate (Ca3(PO4)2 · xH2O) with a Ca/P ratio 1.50,8 consisting of roughly spherical Ca9(PO4)6 “Posner’s clusters” (PC) close-packed to form larger spherical particles with water in the interstices.9 PCs appeared to be energetically favored in comparison to alternative candidates including Ca3(PO4)2 and Ca6(PO4)4 clusters.10 The structure of PCs in isolated form is notably different from that in a HAP environment.11 In particular, the chirality feature of PCs found in the HAP environment is suggested to disappear in an isolated form and in aqueous solution. The reconsideration of PCs as possible components in the actual structural model of ACP resulted from the cluster growth model of the HAP crystal.12 Ab initio calculations confirmed that stable isomers exist on the [Ca3(PO4)2]3 potential energy surface (PES).12,13 These isomers correspond to compact arrangements, i.e., arrangements in which the Ca and PO4 are disposed closely together. Their geometries are compatible with the terms “roughly spherical” used in Posner’s hypothesis. The calculations performed on the monomer and dimer PES revealed that the relative energies of the different isomers are governed by a specific bonding pattern in which a Calcium Atom interacts with two PO43− groups, forming four CaO bonds.12,13 The compact isomers on the trimer PES are energetically favored in comparison to monomer or dimer isomers. This is rationalized by the appearance of a specific bonding pattern for the trimer case in which a Calcium forms six CaO bonds with six different PO4 groups. This type of bonding in encountered in HAP.13 It is now generally agreed that, both in vitro and in vivo, precipitation reactions at sufficiently high supersaturation and pH result in the initial formation of an amorphous Calcium phosphate with a molar Calcium/phosphate ratio of about 1.18–2.50. The chemical composition of ACP is strongly dependent on the solution pH: ACP phases with Ca/P ratios in the range of 1.18:1 precipitated at pH 6.6 to 1.53:1 at pH 11.7 and even as high as 2.5:1.4 Two amorphous Calcium phosphates, ACP1 and ACP2, have been reported with the same composition, but differing in morphology and solubility.14,15 The formation of ACP precipitate with little long-range order tends to consist of aggregates of primary nuclei (roughly spherical clusters) with composition Ca9(PO4)65 dependent on the conditions of formation. It hydrolyzes almost instantaneously to more stable phases. These amorphous clusters served as seeds during HAP crystallization via a stepwise assembly process12 and were presumed to pack randomly with respect to each other,16 forming large 300–800 A spheres. Recent experimental studies found that ACP has definite local Atomic microcrystalline order rather than a random network structure. NMR of thoroughly dried ACP suggests that the tightly held water resides in the interstices between clusters,17 but these are probably not of intrinsic importance in the structure of ACP. It is well-known that ACP contains 10–20% by weight of tightly bound water, which is removed by vacuum drying at elevated temperature.9 However, drying does not alter the Calcium and phosphorus Atomic arrangement. The side band intensities of dried ACP suggest that its chemical shift anisotropy is similar to or identical with that of normal ACP.17 ACP has an apatitic short-range structure, but with a crystal size so small that it appears to be amorphous by X-ray analysis. This is supported by extended X-ray absorption fine structure (EXAFS) on biogenic and synthetic ACP samples.18–20 The CaP amorphous phase transforms to HAP microcrystalline in the presence of water. The lifetime of the metastable amorphous precursor in aqueous solution was reported to be a function of the presence of additive molecules and ions, pH, ionic strength, and temperature.21 The transformation kinetics from ACP to HAP, which can be described by a ”first-order” rate law, is a function only of the pH of the mediating solution at constant temperature. The solution-mediated transformation depends upon the conditions which regulate both the dissolution of ACP and the formation of the early HAP nuclei.22 Tropp et al. used 31P NMR to demonstrate that the strength of ACP side bands is due to a characteristic structural distortion of unprotonated phosphate and not to a mixture of protonated and unprotonated phosphates,17 suggesting that ACP could contain substantial amounts of protonated phosphate not in the form of any known phase of Calcium phosphate crystals. Yin and Stott suggested that, in the transformation from ACP to HAP, ACP need only dissociate into clusters rather than undergo complete ionic solvation. The cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of Posner’s cluster with sodium ions and especially with protons leads to a considerable stability increase, and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar Atomic arrangement that it has as a structural unit in the HAP crystal. This may be a key factor in the transformation from ACP to HAP crystal.23 In general, ACP is a highly unstable phase that hydrolyzes almost instantaneously to more stable phases. In the presence of other ions and macromolecules or under in vivo conditions, ACP may persist for appreciable periods3 and retain the amporphous state under some specific experimental conditions.24