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William I. Higuchi - One of the best experts on this subject based on the ideXlab platform.
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Laboratory Investigations Metastable Equilibrium Solubility Behavior of Carbonated Apatite in the Presence of Solution Strontium
2020Co-Authors: D D Heslop, Arif Ali Baig, Y Bi, William I. HiguchiAbstract:The purpose of this study was to use the concept of Metastable Equilibrium solubility (MES) to describe the anomalous solubility behavior of carbon- ated apatite (CAP) in the presence of solution stron- tium. A CAP sample (4.8 wt% CO3, synthesized at 70� C) was prepared by precipitation. Baseline MES distribu- tions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. In order to assess the influence of strontium, MES profiles were then determined in a similar fashion with 20, 30, 40, 50, 60, 70, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. From the compositions of the equilibrating buffer solutions, ion activity products (IAPs) of the form Ca10-nSrn (PO4)6(OH)2 (n = 0-10) were calculated in an attempt to determine the correct function governing the disso- lution of the CAP preparation. The results demonstrate the following important findings: (a) at high solution strontium/calcium ratios (i.e., when 60% or more of the solution calcium was replaced by strontium), the MES profiles in all the experiments were found to be essen- tially superimposable when the solution IAPs were cal- culated using the stoichiometry of Ca6Sr4(PO4)6(OH)2, and (b), at low solution strontium/calcium ratios (i.e., when 40% or less of the solution calcium was replaced by strontium), the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapa- tite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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influence of crystallite microstrain on surface complexes governing the Metastable Equilibrium solubility behavior of carbonated apatites
Journal of Colloid and Interface Science, 2008Co-Authors: Kongnara Papangkorn, Arif Ali Baig, Makoto Otsuka, D D Heslop, Kunikazu Moribe, William I. HiguchiAbstract:Abstract This study was on the influence of the mineral phase crystallite microstrain (CM) on the nature of the surface complex (SC) governing the Metastable Equilibrium solubility (MES) behavior of carbonated apatites (CAPs) in aqueous acidic media (0.10 M acetate buffers, with and without fluoride, 0.50 M ionic strength maintained with NaCl). The MES behavior of a set of four CAPs (synthesized at 85 °C by a precipitation method) of increasing CM and therefore of increasing MES (CAP4 > CAP3 > CAP2 > CAP1) was quantified. The following were the findings. For CAP1 and CAP2, the SCs deduced were Ca10(PO4)6(OH)2 and Ca10(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. For CAP3 and CAP4, the SCs deduced were Ca9.5(PO4)6OH or Ca9.5(HPO4)(PO4)5(OH)2 and NaCa9.5(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. These results together with that from an earlier limited study show that the Ca/P ratio of the SC decreases from 1.67 to 1.58 to 1.50 with increasing CM of the CAPs; this relationship inversely correlates with the chemistry of maturation of aqueously precipitated defective apatites. Also the SCs do not appear to exist as a continuous series and only a few SCs may account for the MES behavior over a wide range of CAP preparations.
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a comparative study of the Metastable Equilibrium solubility behavior of high crystallinity and low crystallinity carbonated apatites using ph and solution strontium as independent variables
Journal of Colloid and Interface Science, 2005Co-Authors: D D Heslop, Arif Ali Baig, Makoto Otsuka, Y Bi, William I. HiguchiAbstract:Abstract Using solution strontium and pH as independent variables, the Metastable Equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 °C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 °C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca10–nSrn(PO4)6(OH)2 ( n = 0 – 10 )) and the calcium/hydroxide deficient hydroxyapatite template (Ca9–nSrn(HPO4)(PO4)5OH ( n = 0 – 9 )). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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Metastable Equilibrium solubility behavior of carbonated apatite in the presence of solution strontium
Calcified Tissue International, 2003Co-Authors: D D Heslop, Arif Ali Baig, Y Bi, William I. HiguchiAbstract:The purpose of this study was to use the concept of Metastable Equilibrium solubility (MES) to describe the anomalous solubility behavior of carbonated apatite (CAP) in the presence of solution strontium. A CAP sample (4.8 wt% CO3, synthesized at 70°C) was prepared by precipitation. Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. In order to assess the influence of strontium, MES profiles were then determined in a similar fashion with 20, 30, 40, 50, 60, 70, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. From the compositions of the equilibrating buffer solutions, ion activity products (IAPs) of the form Ca10-nSrn(PO4)6(OH)2 (n = 0–10) were calculated in an attempt to determine the correct function governing the dissolution of the CAP preparation. The results demonstrate the following important findings: (a) at high solution strontium/calcium ratios (i.e., when 60% or more of the solution calcium was replaced by strontium), the MES profiles in all the experiments were found to be essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2, and (b), at low solution strontium/calcium ratios (i.e., when 40% or less of the solution calcium was replaced by strontium), the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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relationships involving Metastable Equilibrium solubility surface complexes and crystallite disorder with carbonated apatites
Calcified Tissue International, 2001Co-Authors: Hong Zhuang, Arif Ali Baig, Anil Chhettry, N Zhang, William I. HiguchiAbstract:Previous studies have shown that the Metastable Equilibrium solubility (MES) behavior of carbonated apatites (CAPs) may be described by a surface complex with the hydroxyapatite (HAP) stoichiometry in the absence of solution fluoride and by that with the fluorapatite (FAP) stoichiometry when appreciable solution fluoride is present. Studies have also shown that the magnitude of the MES is directly related to the crystallinity of the CAP. The aim of the present investigation was to examine the relationship between the CAP MES determined in the presence of solution fluoride and CAP crystallinity and to examine the effect of the change in the stoichiometry of the surface complex (from that of HAP to that of FAP) upon the relationship of the CAP MES to crystallinity. CAP samples were prepared by methods based on the precipitation of CAP from calcium phosphate solutions and the hydrolysis of dicalcium phosphate dihydrate in bicarbonate solutions. From X-ray diffraction experiments, the crystallite microstrain, and the full width at half maximum (FWHM) of the 002 reflection were determined for the CAPs, From CAP MES experiments conducted in the presence of solu tion fluoride, linear plots of the mean MES (i.e., mean pK F A P ) values vs the crystallinity parameter (i.e., microstrain and FWHM) were obtained that yielded slopes that were essentially the same as those obtained in the absence of solution fluoride (i.e., mean pK H A P values vs crystallinity). This parallel finding suggests that the CAP crystallite disorder affects the energetics of the two surface complexes essentially to the same extent and provides new insight into the nature of CAP surface complexes.
Everett L. Shock - One of the best experts on this subject based on the ideXlab platform.
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Metastable Equilibrium of substitution reactions among oxygen and nitrogen bearing organic compounds at hydrothermal conditions
Geochimica et Cosmochimica Acta, 2020Co-Authors: Kirtland J Robinson, Kristopher M Fecteau, Ian R Gould, Hilairy E Hartnett, Lynda B Williams, Everett L. ShockAbstract:Abstract Measured abundances of organic compounds can reveal information about the environments in which they formed. Since organic compounds can be mobilized and released from geologic and planetary settings, they have the potential to provide insights into environments that are difficult to observe directly. To advance our understanding of these environments, this study identifies organic reactions that approach Metastable Equilibrium in experiments, so that future studies can predict geochemical conditions in remote settings (e.g., deep Earth, extraterrestrial bodies) by monitoring reaction ratios of compounds involved in similar organic reactions. At high temperatures organic redox reactions can equilibrate, which allows comparisons with thermodynamic properties to yield estimates of reaction conditions. However, redox reactions may equilibrate too slowly to be applicable to lower temperature systems. To explore Metastable equilibria at lower temperatures, we studied substitution reactions that tend to be faster than redox reactions. In this study, we demonstrate that oxygen- and nitrogen-bearing organic compounds at hydrothermal conditions undergo a series of simultaneous substitution reactions that rapidly approach steady-state ratios indicative of Metastable Equilibrium. Four sets of aqueous experiments were performed at 250 °C and 40 bar, each beginning with a single organic reactant: benzyl alcohol, benzylamine, dibenzylamine, or tribenzylamine. All reactant solutions were prepared with identical bulk elemental compositions by adjusting the concentrations of the starting organic compounds and adding ammonia as needed. After 2 h at hydrothermal reaction conditions, all of the model compounds were detectable in all four sets of experiments, evidence for reversibility of reactions among the compounds. After 72 h, reaction ratios between the model compounds converged in all four sets of experiments, consistent with approaches toward Metastable Equilibrium. Reaction ratios for ether and aldehyde formation reactions were also observed to group within a relatively small range, but without a clear convergence pattern, suggesting other non-redox reactions may approach Metastable Equilibrium. The approach to Metastable Equilibrium among the initial organic reactants could be observed and quantified even in the presence of competing redox reactions whose mechanisms are less understood, including dibenzylimine and toluene formation, which did not appear to reach steady-states. These findings identify classes of organic compounds and reactions that can reflect the conditions at which they last equilibrated and should be targeted for analysis in natural systems.
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a Metastable Equilibrium model for the relative abundances of microbial phyla in a hot spring
PLOS ONE, 2013Co-Authors: Jeffrey M Dick, Everett L. ShockAbstract:Many studies link the compositions of microbial communities to their environments, but the energetics of organism-specific biomass synthesis as a function of geochemical variables have rarely been assessed. We describe a thermodynamic model that integrates geochemical and metagenomic data for biofilms sampled at five sites along a thermal and chemical gradient in the outflow channel of the hot spring known as “Bison Pool” in Yellowstone National Park. The relative abundances of major phyla in individual communities sampled along the outflow channel are modeled by computing Metastable Equilibrium among model proteins with amino acid compositions derived from metagenomic sequences. Geochemical conditions are represented by temperature and activities of basis species, including pH and oxidation-reduction potential quantified as the activity of dissolved hydrogen. By adjusting the activity of hydrogen, the model can be tuned to closely approximate the relative abundances of the phyla observed in the community profiles generated from BLAST assignments. The findings reveal an inverse relationship between the energy demand to form the proteins at equal thermodynamic activities and the abundance of phyla in the community. The distance from Metastable Equilibrium of the communities, assessed using an equation derived from energetic considerations that is also consistent with the information-theoretic entropy change, decreases along the outflow channel. Specific divergences from Metastable Equilibrium, such as an underprediction of the relative abundances of phototrophic organisms at lower temperatures, can be explained by considering additional sources of energy and/or differences in growth efficiency. Although the metabolisms used by many members of these communities are driven by chemical disequilibria, the results support the possibility that higher-level patterns of chemotrophic microbial ecosystems are shaped by Metastable Equilibrium states that depend on both the composition of biomass and the environmental conditions.
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petroleum oil field waters and authigenic mineral assemblages are they in Metastable Equilibrium in hydrocarbon reservoirs
Geochimica et Cosmochimica Acta, 1993Co-Authors: Harold C. Helgeson, Annette M. Knox, Christine E. Owens, Everett L. ShockAbstract:Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface.† To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming Metastable Equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (ƒO2(g)).‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO2 (aCO2(aq)). The values of log ƒO2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log ƒO2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous Equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in Metastable Equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of aCO2(aq) in the waters, to calculate Equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6–15, depending on the activity of aqueous CO2. However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous Metastable Equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca2+, H+, CO2, CH3COOH, CH3COO−, and other aqueous species in oil field waters may be in Metastable Equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs.§ If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by Metastable Equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H2O at the oil-water interface to form lighter paraffins and CO2(aq), CH3COOH(aq), and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria.∥ However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs.
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Petroleum, oil field waters, and authigenic mineral assemblages Are they in Metastable Equilibrium in hydrocarbon reservoirs
Geochimica et Cosmochimica Acta, 1993Co-Authors: Harold C. Helgeson, Annette M. Knox, Christine E. Owens, Everett L. ShockAbstract:Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface. † † The term oil field waters is used in the present communication as a generic designation of saline formation waters in hydrocarbon reservoirs, regardless of their salinity. To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming Metastable Equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (f{hook} O 2(g) ). ‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO 2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO 2 (a CO 2(aq) ). The values of log f{hook} O 2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log f{hook} O 2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous Equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in Metastable Equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of a CO 2(aq) in the waters, to calculate Equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6-15, depending on the activity of aqueous CO 2 . However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous Metastable Equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca 2+ , H + , CO 2 , CH 3 COOH, CH 3 COO - , and other aqueous species in oil field waters may be in Metastable Equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs. § § Note in this regard that mere recognition of a given Equilibrium state carries no necessary causal implication with respect to mass transfer processes that may have led to the state. If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by Metastable Equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H 2 O at the oil-water interface to form lighter paraffins and CO 2(aq) , CH 3 COOH (aq) , and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria. ∥ ∥ As indicated above, the term hydrolytic disproportionation is used in the present communication to refer to the reaction of a given hydrocarbon in crude oil with H 2 O to form a lighter hydrocarbon and oxidized carbon-bearing aqueous species. In contrast, the term disproportionation is used by Tissot and Welte (1984) to refer to production of low and high molecular weight hydrocarbons from those of intermediate molecular weight with accompanying "decarboxylation dehydration, and desulfurization [of the crude oil to yield] carbon dioxide, water, and hydrogen sulfide." However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs. © 1993.
Arif Ali Baig - One of the best experts on this subject based on the ideXlab platform.
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Laboratory Investigations Metastable Equilibrium Solubility Behavior of Carbonated Apatite in the Presence of Solution Strontium
2020Co-Authors: D D Heslop, Arif Ali Baig, Y Bi, William I. HiguchiAbstract:The purpose of this study was to use the concept of Metastable Equilibrium solubility (MES) to describe the anomalous solubility behavior of carbon- ated apatite (CAP) in the presence of solution stron- tium. A CAP sample (4.8 wt% CO3, synthesized at 70� C) was prepared by precipitation. Baseline MES distribu- tions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. In order to assess the influence of strontium, MES profiles were then determined in a similar fashion with 20, 30, 40, 50, 60, 70, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. From the compositions of the equilibrating buffer solutions, ion activity products (IAPs) of the form Ca10-nSrn (PO4)6(OH)2 (n = 0-10) were calculated in an attempt to determine the correct function governing the disso- lution of the CAP preparation. The results demonstrate the following important findings: (a) at high solution strontium/calcium ratios (i.e., when 60% or more of the solution calcium was replaced by strontium), the MES profiles in all the experiments were found to be essen- tially superimposable when the solution IAPs were cal- culated using the stoichiometry of Ca6Sr4(PO4)6(OH)2, and (b), at low solution strontium/calcium ratios (i.e., when 40% or less of the solution calcium was replaced by strontium), the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapa- tite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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influence of crystallite microstrain on surface complexes governing the Metastable Equilibrium solubility behavior of carbonated apatites
Journal of Colloid and Interface Science, 2008Co-Authors: Kongnara Papangkorn, Arif Ali Baig, Makoto Otsuka, D D Heslop, Kunikazu Moribe, William I. HiguchiAbstract:Abstract This study was on the influence of the mineral phase crystallite microstrain (CM) on the nature of the surface complex (SC) governing the Metastable Equilibrium solubility (MES) behavior of carbonated apatites (CAPs) in aqueous acidic media (0.10 M acetate buffers, with and without fluoride, 0.50 M ionic strength maintained with NaCl). The MES behavior of a set of four CAPs (synthesized at 85 °C by a precipitation method) of increasing CM and therefore of increasing MES (CAP4 > CAP3 > CAP2 > CAP1) was quantified. The following were the findings. For CAP1 and CAP2, the SCs deduced were Ca10(PO4)6(OH)2 and Ca10(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. For CAP3 and CAP4, the SCs deduced were Ca9.5(PO4)6OH or Ca9.5(HPO4)(PO4)5(OH)2 and NaCa9.5(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. These results together with that from an earlier limited study show that the Ca/P ratio of the SC decreases from 1.67 to 1.58 to 1.50 with increasing CM of the CAPs; this relationship inversely correlates with the chemistry of maturation of aqueously precipitated defective apatites. Also the SCs do not appear to exist as a continuous series and only a few SCs may account for the MES behavior over a wide range of CAP preparations.
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a comparative study of the Metastable Equilibrium solubility behavior of high crystallinity and low crystallinity carbonated apatites using ph and solution strontium as independent variables
Journal of Colloid and Interface Science, 2005Co-Authors: D D Heslop, Arif Ali Baig, Makoto Otsuka, Y Bi, William I. HiguchiAbstract:Abstract Using solution strontium and pH as independent variables, the Metastable Equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 °C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 °C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca10–nSrn(PO4)6(OH)2 ( n = 0 – 10 )) and the calcium/hydroxide deficient hydroxyapatite template (Ca9–nSrn(HPO4)(PO4)5OH ( n = 0 – 9 )). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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Metastable Equilibrium solubility behavior of carbonated apatite in the presence of solution strontium
Calcified Tissue International, 2003Co-Authors: D D Heslop, Arif Ali Baig, Y Bi, William I. HiguchiAbstract:The purpose of this study was to use the concept of Metastable Equilibrium solubility (MES) to describe the anomalous solubility behavior of carbonated apatite (CAP) in the presence of solution strontium. A CAP sample (4.8 wt% CO3, synthesized at 70°C) was prepared by precipitation. Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. In order to assess the influence of strontium, MES profiles were then determined in a similar fashion with 20, 30, 40, 50, 60, 70, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. From the compositions of the equilibrating buffer solutions, ion activity products (IAPs) of the form Ca10-nSrn(PO4)6(OH)2 (n = 0–10) were calculated in an attempt to determine the correct function governing the dissolution of the CAP preparation. The results demonstrate the following important findings: (a) at high solution strontium/calcium ratios (i.e., when 60% or more of the solution calcium was replaced by strontium), the MES profiles in all the experiments were found to be essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2, and (b), at low solution strontium/calcium ratios (i.e., when 40% or less of the solution calcium was replaced by strontium), the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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relationships involving Metastable Equilibrium solubility surface complexes and crystallite disorder with carbonated apatites
Calcified Tissue International, 2001Co-Authors: Hong Zhuang, Arif Ali Baig, Anil Chhettry, N Zhang, William I. HiguchiAbstract:Previous studies have shown that the Metastable Equilibrium solubility (MES) behavior of carbonated apatites (CAPs) may be described by a surface complex with the hydroxyapatite (HAP) stoichiometry in the absence of solution fluoride and by that with the fluorapatite (FAP) stoichiometry when appreciable solution fluoride is present. Studies have also shown that the magnitude of the MES is directly related to the crystallinity of the CAP. The aim of the present investigation was to examine the relationship between the CAP MES determined in the presence of solution fluoride and CAP crystallinity and to examine the effect of the change in the stoichiometry of the surface complex (from that of HAP to that of FAP) upon the relationship of the CAP MES to crystallinity. CAP samples were prepared by methods based on the precipitation of CAP from calcium phosphate solutions and the hydrolysis of dicalcium phosphate dihydrate in bicarbonate solutions. From X-ray diffraction experiments, the crystallite microstrain, and the full width at half maximum (FWHM) of the 002 reflection were determined for the CAPs, From CAP MES experiments conducted in the presence of solu tion fluoride, linear plots of the mean MES (i.e., mean pK F A P ) values vs the crystallinity parameter (i.e., microstrain and FWHM) were obtained that yielded slopes that were essentially the same as those obtained in the absence of solution fluoride (i.e., mean pK H A P values vs crystallinity). This parallel finding suggests that the CAP crystallite disorder affects the energetics of the two surface complexes essentially to the same extent and provides new insight into the nature of CAP surface complexes.
Makoto Otsuka - One of the best experts on this subject based on the ideXlab platform.
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influence of crystallite microstrain on surface complexes governing the Metastable Equilibrium solubility behavior of carbonated apatites
Journal of Colloid and Interface Science, 2008Co-Authors: Kongnara Papangkorn, Arif Ali Baig, Makoto Otsuka, D D Heslop, Kunikazu Moribe, William I. HiguchiAbstract:Abstract This study was on the influence of the mineral phase crystallite microstrain (CM) on the nature of the surface complex (SC) governing the Metastable Equilibrium solubility (MES) behavior of carbonated apatites (CAPs) in aqueous acidic media (0.10 M acetate buffers, with and without fluoride, 0.50 M ionic strength maintained with NaCl). The MES behavior of a set of four CAPs (synthesized at 85 °C by a precipitation method) of increasing CM and therefore of increasing MES (CAP4 > CAP3 > CAP2 > CAP1) was quantified. The following were the findings. For CAP1 and CAP2, the SCs deduced were Ca10(PO4)6(OH)2 and Ca10(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. For CAP3 and CAP4, the SCs deduced were Ca9.5(PO4)6OH or Ca9.5(HPO4)(PO4)5(OH)2 and NaCa9.5(PO4)6F2 for the nonfluoride and the fluoride cases, respectively. These results together with that from an earlier limited study show that the Ca/P ratio of the SC decreases from 1.67 to 1.58 to 1.50 with increasing CM of the CAPs; this relationship inversely correlates with the chemistry of maturation of aqueously precipitated defective apatites. Also the SCs do not appear to exist as a continuous series and only a few SCs may account for the MES behavior over a wide range of CAP preparations.
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a comparative study of the Metastable Equilibrium solubility behavior of high crystallinity and low crystallinity carbonated apatites using ph and solution strontium as independent variables
Journal of Colloid and Interface Science, 2005Co-Authors: D D Heslop, Arif Ali Baig, Makoto Otsuka, Y Bi, William I. HiguchiAbstract:Abstract Using solution strontium and pH as independent variables, the Metastable Equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 °C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 °C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca10–nSrn(PO4)6(OH)2 ( n = 0 – 10 )) and the calcium/hydroxide deficient hydroxyapatite template (Ca9–nSrn(HPO4)(PO4)5OH ( n = 0 – 9 )). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.
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Metastable Equilibrium solubility distribution of carbonated apatite as a function of solution composition
Joint International Conference on Information Sciences, 1999Co-Authors: Anil Chhettry, Zeren Wang, Arif Ali Baig, Makoto Otsuka, A M Barry, Hong Zhuang, William I. HiguchiAbstract:Abstract Previous studies have shown that carbonated apatites (CAPs) exhibit the phenomenon of Metastable Equilibrium solubility (MES) in weak acid media. The purpose of the present investigation was to examine two questions: first, whether the MES concept is applicable to a broader range of solution conditions and, second, whether a driving force function associated with a surface complex having a constant stoichiometry governs the dissolution of CAP and, if so, what is this stoichiometry. CAP preparations with carbonate contents of 1.8–5.7 wt% (synthesized by hydrolysis of dicalcium phosphate anhydrate in solutions of varying bicarbonate levels or by direct precipitation from supersaturated calcium/phosphate/carbonate solutions) were studied as follows. MES distributions for each of the CAP preparations were determined by equilibrating the CAP under stirred conditions in a series of acetate buffers (0.10 M) containing various levels of calcium and phosphate in the pH range 4.5–6.5 and a solution calcium/phosphate ratio in the range 0.1–10. The amount dissolved in each instance was regarded as the fraction of the CAP possessing an MES value greater than that corresponding to the ion activity product (IAP) of the equilibrating solution. The solution IAPs were calculated from the solution compositions using plausible calcium phosphate stoichiometries, viz., dicalcium phosphate dihydrate, octacalcium phosphate, tricalcium phosphate, hydroxyapatite, carbonated apatite (based on the bulk composition of the particular CAP involved in the experiment), and tetracalcium phosphate. The fraction of CAP dissolved was plotted against the solution IAPs for each experimental set using each of the six assumed stoichiometries for the surface complex. The results demonstrated that the MES concept was applicable to all of the CAP preparations in media of various solution compositions and different pH levels. The most important new outcome of this study was that MES profiles for each of the CAP preparations in all of the experiments were found to be superimposable when solution IAPs were calculated using the hydroxyapatite stoichiometry, while such was not the case when other stoichiometries were used to calculate the solution IAPs.
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Metastable Equilibrium solubility behavior of bone mineral
Calcified Tissue International, 1999Co-Authors: Arif Ali Baig, Zeren Wang, William I. Higuchi, S C Miller, A M Barry, Makoto OtsukaAbstract:Previous studies have shown that carbonated apatites with a range of carbonate contents and crystallinities exhibit the phenomenon of Metastable Equilibrium solubility (MES) distributions. The purpose of the present study was to investigate the solubility behavior of bone mineral using the concepts of MES and MES distributions and, together with crystallinity and chemical composition data, examine the similarity of bone mineral to carbonated apatite (CAP). Bone samples were harvested from 1-, 5-, and 8-month-old rats. The organic components of the bone samples were removed by hydrazine deproteination. Carbonated apatite was synthesized by the hydrolysis of dicalcium phosphate dihydrate (DCPD) in a NaHCO3-containing media at 50°C. The MES distributions of bone mineral and CAP were determined by equilibrating predetermined amounts of CAP or bone mineral in a series of 0.1 M acetate buffers containing calculated levels of calcium and phosphate and maintained at essentially constant pHs of 5.0, 5.3, 5.7, and 6.5. From the compositions of the equilibrating buffer solutions, ion activity products based upon the stoichiometries of octacalcium phosphate, hydroxyapatite, and carbonated apatite were calculated in an attempt to determine the function governing the dissolution of CAP and bone mineral. The results of this study demonstrated that the MES distribution phenomenon appeared to hold for bone mineral and that the changes in crystallinity of bone mineral with age correlated well with changes in the MES values. A CAP sample was prepared that was found to be an excellent synthetic prototype closely mimicking the physicochemical behavior of bone mineral from an 8-month-old rat. Another finding of this study was that the ion activity product function based upon the hydroxyapatite stoichiometry well described the MES results obtained with both CAP and bone mineral. The interpretation that a surface complex with hydroxyapatite stoichiometry governs the solubility behavior of bone mineral is, therefore, consistent with the experimental data. Other calcium phosphate stoichiometries for the surface complex showed systematic variations in the MES profiles when the pH of the equilibrating solution was varied.
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effect of carbonate content and crystallinity on the Metastable Equilibrium solubility behavior of carbonated apatites
Joint International Conference on Information Sciences, 1996Co-Authors: Arif Ali Baig, Zeren Wang, Makoto Otsuka, William I. Higuchi, Racquel Z LegerosAbstract:Abstract The purpose of this investigation was to assess the applicability of the Metastable Equilibrium solubility (MES) concept, previously developed in our laboratory, over wide ranges of two independent variables, carbonate content and crystallinity, and also to examine the influences of these variables on the MES behavior of carbonated apatites (CAPs). The CAP samples were prepared by dicalcium phosphate dihydrate (DCPD) hydrolysis for 48 h in NaHCO3-containing media at 95, 70, and 50°C. This method of preparation gave CAP samples with varying carbonate contents and crystallinities. A previously developed technique with slight modification was used to determine the MES distribution for each of the CAP samples. The equilibration solutions were prepared both with and without extraction of fluoride with hydroxyapatite (HAP). From X-ray diffraction, the full width at half-maximum (FWHM) of the 002 reflection was used as a measure of crystallinity. The findings of this study showed that each of these preparations possessed an MES distribution and therefore provided further support that the MES distribution is a common phenomenon describing the dissolution behavior of CAPs, regardless of their carbonate content and crystallinities. The crystallinities of the CAPs decreased and the MES values increased with increasing carbonate content and decreasing temperature of synthesis. A plot of the mean MES value against CAP crystallinity revealed that the mean MES was a single-valued function of crystallinity; i.e., when crystallinity was taken into account, there was no additional effect of carbonate on the MES. When fluoride was not extracted from the equilibration solutions, the MES shifted to lower values. The concept of MES distribution and its dependence on the crystallinity of CAP may provide insight into the mechanism of dissolution of biological apatites, which may be considered imperfect crystalline substances.
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petroleum oil field waters and authigenic mineral assemblages are they in Metastable Equilibrium in hydrocarbon reservoirs
Geochimica et Cosmochimica Acta, 1993Co-Authors: Harold C. Helgeson, Annette M. Knox, Christine E. Owens, Everett L. ShockAbstract:Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface.† To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming Metastable Equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (ƒO2(g)).‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO2 (aCO2(aq)). The values of log ƒO2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log ƒO2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous Equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in Metastable Equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of aCO2(aq) in the waters, to calculate Equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6–15, depending on the activity of aqueous CO2. However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous Metastable Equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca2+, H+, CO2, CH3COOH, CH3COO−, and other aqueous species in oil field waters may be in Metastable Equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs.§ If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by Metastable Equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H2O at the oil-water interface to form lighter paraffins and CO2(aq), CH3COOH(aq), and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria.∥ However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs.
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Petroleum, oil field waters, and authigenic mineral assemblages Are they in Metastable Equilibrium in hydrocarbon reservoirs
Geochimica et Cosmochimica Acta, 1993Co-Authors: Harold C. Helgeson, Annette M. Knox, Christine E. Owens, Everett L. ShockAbstract:Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface. † † The term oil field waters is used in the present communication as a generic designation of saline formation waters in hydrocarbon reservoirs, regardless of their salinity. To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming Metastable Equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (f{hook} O 2(g) ). ‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO 2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO 2 (a CO 2(aq) ). The values of log f{hook} O 2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log f{hook} O 2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous Equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in Metastable Equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of a CO 2(aq) in the waters, to calculate Equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6-15, depending on the activity of aqueous CO 2 . However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous Metastable Equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca 2+ , H + , CO 2 , CH 3 COOH, CH 3 COO - , and other aqueous species in oil field waters may be in Metastable Equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs. § § Note in this regard that mere recognition of a given Equilibrium state carries no necessary causal implication with respect to mass transfer processes that may have led to the state. If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by Metastable Equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H 2 O at the oil-water interface to form lighter paraffins and CO 2(aq) , CH 3 COOH (aq) , and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria. ∥ ∥ As indicated above, the term hydrolytic disproportionation is used in the present communication to refer to the reaction of a given hydrocarbon in crude oil with H 2 O to form a lighter hydrocarbon and oxidized carbon-bearing aqueous species. In contrast, the term disproportionation is used by Tissot and Welte (1984) to refer to production of low and high molecular weight hydrocarbons from those of intermediate molecular weight with accompanying "decarboxylation dehydration, and desulfurization [of the crude oil to yield] carbon dioxide, water, and hydrogen sulfide." However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs. © 1993.
