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PubMed2002

Proton and hydroxide ion mobility in capillary electrophoresis.

A capillary electrophoresis method for measuring the effective mobilities of proton and hydroxide ions was developed. Photodiode array detection coupled with pH-indicating dyes enabled the detection of proton and hydroxide ions using a conventional CE instrument. The effects of ionic strength and pH of the background electrolyte on the measurement of effective mobility of the proton were first investigated. Mobility measurements were found to be independent of applied voltage. Hydroxide ion mobility was also measured and found to be in agreement with literature values. The effects of pH indicator and proton source on the ion mobility were determined and are described herein.

Journal Article↗
PubMed2003

Synergistic pseudo-hydroxide extraction: synergism and anion selectivity in sodium extraction using a crown ether and a series of weak lipophilic acids.

The nature of the weak lipophilic acid used in synergistic combination with a model crown ether cation host was shown to have a strong effect on the strength and selectivity of sodium hydroxide separation from alkaline aqueous salt solutions. Sodium ion-pair extraction employing only cis-syn-cis-dicyclohexano-18-crown-6 (1) in nitrobenzene (NB) was correlated with the standard Gibbs energy (deltaG(p)o) of anion partitioning into NB and was notably weak and nonselective for the hydroxide ion, in accord with Hofmeister bias. The Hofmeister order can be selectively overcome for NaOH by utilization of acid-base chemistry coupled with complexation of sodium ion in the NB phase. Upon addition of a lipophilic organic acid into the solution of 1 in NB, sodium extraction was selectively enhanced due to the initiation of an exchange reaction between the aqueous sodium ion and the ionizable proton of the organic acid. A series of weak lipophilic hydroxy acids (HA) including fluorinated alcohols and phenols was tested. The resulting synergistic pseudo-hydroxide extraction correlates with the pKa of the employed HA; the most acidic cation exchangers provide the greatest synergism. The synergistic factor obtained using a fluorinated benzyl alcohol 7 was as high as 256. Ion-pair extraction of neutral sodium salts was not changed or only mildly enhanced by addition of HA into the NB solution of 1. This enhancement was explained by hydrogen bonding of HA with the anion as related to the hardness of the anion and the acidity of HA. In comparison with the synergism observed for NaOH, this enhancement was weak and unable to overcome the Hofmeister effect. Examination of extraction selectivity revealed that the combination of 1 and 7 preferentially extracted NaOH over all other sodium salts, including the normally preferred nitrate and perchlorate salts. Quantitative recovery of NaOH from the NB phase was demonstrated via hydrolysis of the organic acid upon a single contact of the loaded solvent with water.

Journal Article↗
PubMed2001

Implications of aqueous silica sorption to iron hydroxide: mobilization of iron colloids and interference with sorption of arsenate and humic substances.

This work highlighted practical implications of aqueous silica sorption to iron hydroxide in natural and engineered systems. Two types of surfaces were prepared by exposing 10 mg/L preformed Fe(OH)3 to aqueous silica (0-200 mg/L as SiO2) for periods of 1.5 h or 50 days. After 1.5 h, the concentration of iron passing through a 0.45 microm pore size filter at pH 6.0-9.5 was always negligible, but if zeta potential < or =-15 mV as much as 35% of the iron passed through filters after 50 days of aging. When arsenate was added to 10 mg/L iron hydroxide particles equilibrated with aqueous silica for 1.5 h, percentage arsenate removals were high. In contrast, if silica was preequilibrated with iron for 50 days, arsenate removals decreased markedly at higher pH and aqueous silica concentrations. Similar trends were observed for humic substances, although their removal was nearly completely prevented at pH 8.5 at SiO2 concentrations above 50 and 10 mg/L at 1.5 h and 50 days exposure, respectively. The mechanism of interference was hindered sorption to the iron hydroxide surface.

Arsenates↗
PubMed2002

Dissolution of plutonium(IV) hydroxide by desferrioxamine siderophores and simple organic chelators.

Plutonium is thought to exist mostly as low soluble plutonium(IV) species in the environment and, therefore, has low potential of becoming mobile. Due to their prevalence and high solution stability constants for Pu(IV), microbial siderophores could significantly affect plutonium solubility and mobility. In this study, the ability of trihydroxamate desferrioxamine siderophores to solubilize Pu(IV) solids was investigated. Both desferrioxamine B and E (DFB and DFE) are far less effective at solubilizing amorphous Pu(IV) hydroxide in neutral solution than are simple chelators, such as EDTA, citrate, and tiron, despite the fact that these chelators have smaller solution stability constants for Pu(IV) than do DFE and DFB. Positively charged, linear DFB is less effective than the neutral cyclic DFE. Hydroxamate siderophores may, in fact, passivate Pu(IV) hydroxide surfaces, thereby inhibiting solubilization by other chelators. PuO(2) solubilization under these conditions is far slower than that of Pu(IV) hydroxide.

Journal Article↗
PubMed2006

Tetrahedral Co(II) coordination in alpha-type cobalt hydroxide: Rietveld refinement and X-ray absorption spectroscopy.

We report a Rietveld refinement analysis and X-ray absorption study on a green-color Cl(-)-intercalated alpha-type cobalt hydroxide phase. The refinement clearly demonstrated that one-fifth to one-sixth of the Co(II) at octahedral sites was replaced by pairs of tetrahedrally coordinated Co(II) on each side of the hydroxide plane, represented by a structural formula of [Co(octa)(0.828)Co(tetra)(0.348)(OH)2](0.348+)Cl(0.348).0.456H2O. X-ray absorption spectroscopy also indicated that the divalent cobalt were in local neighboring environments of both octahedral and tetrahedral coordination. Furthermore, UV-vis spectroscopic measurements elucidate the typical green/blue color of an alpha-type cobalt hydroxide.

Journal Article↗
PubMed2002

First-principles study of aqueous hydroxide solutions.

Car-Parrinello molecular dynamics simulations have been carried out for aqueous NaOH and KOH solutions under ambient conditions over a wide range of concentrations. From these simulations, we have observed a continuous change of the water structure with added hydroxide, characterized by a significant shift of the second peak of the OO radial distribution functions to shorter distances. At the highest concentration investigated, the normal tetrahedral coordination of pure water is completely missing, a result that is consistent with a recent neutron diffraction experiment. The added hydroxide also gives rise to some unique spectroscopic features, including a "free" O-H stretch, a broadening of the normal water OH stretching band, and a large blue shift of both the librational band and the low-frequency translation. These results are in good agreement with the experimental data. Finally, it was demonstrated that the structural and dynamical behavior is inextricably linked to the formation of compact hydroxide-water complexes.

Journal Article↗
PubMed2003

The hydroxide complex of Pseudomonas aeruginosa heme oxygenase as a model of the low-spin iron(III) hydroperoxide intermediate in heme catabolism: 13C NMR spectroscopic studies suggest the active participation of the heme in macrocycle hydroxylation.

13C NMR spectroscopic studies have been conducted with the hydroxide complex of Pseudomonas aeruginosa heme oxygenase (Fe(III)-OH), where OH(-) has been used as a model of the OOH(-) ligand to gain insights regarding the elusive ferric hydroperoxide (Fe(III)-OOH) intermediate in heme catabolism at ambient temperatures. Analysis of the heme core carbon resonances revealed that the coordination of hydroxide in the distal site of the enzyme results in the formation of at least three populations of Fe(III)-OH complexes with distinct electronic configurations and nonplanar ring distortions that are in slow exchange relative to the NMR time scale. The most abundant population exhibits a spin crossover between S = (1)/(2) and S = (3)/(2) spin states, and the two less abundant populations exhibit pure, S = (3)/(2) and S = (1)/(2), (d(xy)())(1) electronic configurations. We propose that the highly organized network of water molecules in the distal pocket of heme oxygenase, by virtue of donating a hydrogen bond to the coordinated hydroxide ligand, lowers its ligand field strength, thereby increasing the field strength of the porphyrin (equatorial) ligand, which results in nonplanar deformations of the macrocycle. This tendency to deform from planarity, which is imparted by the ligand field strength of the coordinated OH(-), is likely reinforced by the flexibility of the distal pocket in HO. These findings suggest that if the ligand field strength of the coordinated OOH(-) in heme oxygenase is modulated in a similar manner, the resultant large spin density at the meso carbons and nonplanar deformations of the pophyrin ring prime the macrocycle to actively participate in its own hydroxylation.

Biomimetic Materials↗
PubMed2005

Electrochemical deposition of mesoporous nickel hydroxide films from dilute surfactant solutions.

A series of micelle-templated mesoporous nickel hydroxide films were prepared by electrochemical deposition from dilute surfactant solutions by using different types of template and by varying plating solvent composition. Lamellar mesostructured Ni(OH)2 films are obtained with only anionic surfactant sodium dodecyl sulfate (SDS) as the template. In particular, a unique cooperative assembly fashion, that is, the combination between Ni2+ and a complex composed of the primary template SDS and a cosurfactant, such as triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers and poly(ethylene glycol), was explored, by which two-dimensional hexagonal mesoporous Ni(OH)2 films were electrodeposited. Meanwhile, the deposition medium also plays a crucial role in determining the mesostructure of Ni(OH)2 films. For the composite nickel hydroxide films deposited from aqueous solution or dilute aqueous solution of ethylene glycol (<20 wt %) in the presence of SDS or the SDS-poly(alkylene oxide) polymer complexes, a mixed lamellar phase with d(001) = 37.4 A and d(001) = 28.5 A was obtained. However, single lamellar phase with d(001) = 37.4 A was electrodeposited from concentrated aqueous solutions of ethylene glycol (> or = 20 wt %). Furthermore, such deposition baths have access to hexagonal mesoporous nickel hydroxide films with d(100) = 37.4 A at 70 degrees C with the SDS-poly(alkylene oxide) polymer complexes as the templates. Within the potential window for Ni(OH)2, the morphology and quality of mesostructured films are significantly dependent on the deposition potential, while the mesostructures of the composite films always remain unchanged.

Journal Article↗
PubMed2005

Physisorption of hydroxide ions from aqueous solution to a hydrophobic surface.

We present results from detailed molecular dynamics simulations revealing a counterintuitive spontaneous physical adsorption of hydroxide ions at a water/hydrophobic interface. The driving force for the migration of the hydroxide ions from the aqueous phase is the preferential orientation of the water molecules in the first two water layers away from the hydrophobic surface. This ordering of the water molecules generates an electrical potential gradient that strongly and favorably interacts with the dipole moment of the hydroxide ion. These findings offer a physical mechanism that explains intriguing experimental reports indicating that the interface between water and a nonionic surface is negatively charged.

Journal Article↗
PubMed2004

Unidirectionally aligned copper hydroxide crystalline nanorods from two-dimensional copper hydroxy nitrate.

We were able to synthesize unidirectionally aligned copper hydroxide nanorods from two-dimensional copper hydroxide nitrate by anion exchange reaction using NaOH without any membrane or template. The structural conversion from Cu2(OH)3NO3 to Cu(OH)2 was confirmed by XRD and FT-IR data. The synthesized copper hydroxide nanorods showed unidirectionally aligned and tightly bound arrays on a hexagonal plate, leading to giant nanorod bundles.

Journal Article↗
PubMed2006

Determination of ionic conductivity and its impact on proton diffusion model for nickel hydroxide.

The ionic conductivity is an important but previously ignored aspect for the nickel hydroxide used in alkaline batteries. With a specially designed device, the ionic conductivity is determined for single beads of spherical nickel hydroxide in KOH solutions. The apparent ionic conductivity is found on the order of 10(-3)-10(-2) S cm-1 in 6 M KOH and to change with the conductivity of the solution in which the bead is immersed. The ionic conductivity of the bead can be mainly attributed to the electrolyte absorbed in the bead. On the basis of these findings, the dual structure model for proton diffusion in spherical nickel hydroxide is refined by specifying nanoparticles to be the component showing a large apparent proton diffusion coefficient (on the order of 10(-7) cm2 s-1). This refined model is able to interpret the main features of the diffusion coefficients reported in the literature, including the unusually large scattering (up to 6 orders of magnitude) and inconsistency in the dependence of proton diffusion coefficient on the state of the charge. Besides, this refined model is supported by the influence of bulk KOH concentration on chronoamperometry and transmission electron microscopy observations.

Journal Article↗
PubMed2006

Hydration, swelling, interlayer structure, and hydrogen bonding in organolayered double hydroxides: insights from molecular dynamics simulation of citrate-intercalated hydrotalcite.

Molecular dynamics (MD) simulation of the Mg/Al (3:1) layered double hydroxide (LDH), hydrotalcite (HT), containing citrate, C6H5O7(3-), as the charge balancing interlayer anion provides new molecular scale insight into the interlayer structure, hydrogen bonding, and energetics of the hydration and consequent swelling of LDH compounds containing organic and biomolecules. Citrate-HT exhibits affinity for water up to very high hydration levels, in contrast to the preferred low hydration states of most LDHs intercalated with small, inorganic anions. This result is consistent with the recent experimental observation of the delamination of lactate-HT. The high water affinity is rationalized in terms of the preference of citrate ion for hydrogen bonds (H-bonds) donated from water molecules rather than from the hydroxyl groups of the metal hydroxide layer and the need to develop an integrated interlayer H-bond network among the citrate ions, water, and -OH groups of the hydroxide layers. The changes in the orientation of citrate molecules with progressive hydration are also intimately related to its preference to accept hydrogen bonds from water.

Journal Article↗
PubMed2005

Photochemical Smiles rearrangement and Meisenheimer complex formation catalyzed by hydroxide ion via electron hole transfer catalysis.

[reactions: see text] Photochemical para-to-nitro Smiles rearrangement and para-to-nitro Meisenheimer complex formation occurs for nitrophenoxyethylamines with high concentrations of hydroxide ion in aqueous solution. Both photoreactions show first-order dependence on hydroxide ion concentration, but the mechanism involving hydroxide ion does not involve acid-base catalysis. The reactions take place from the triplet excited states of the nitrophenyl ethers. Analysis of quantum yields and kinetics is consistent with an electron hole transfer catalysis mechanism.

Journal Article↗
PubMed2002

The nature and transport mechanism of hydrated hydroxide ions in aqueous solution.

Compared to other ions, protons (H(+)) and hydroxide ions (OH(-)) exhibit anomalously high mobilities in aqueous solutions. On a qualitative level, this behaviour has long been explained by 'structural diffusion' the continuous interconversion between hydration complexes driven by fluctuations in the solvation shell of the hydrated ions. Detailed investigations have led to a clear understanding of the proton transport mechanism at the molecular level. In contrast, hydroxide ion mobility in basic solutions has received far less attention, even though bases and base catalysis play important roles in many organic and biochemical reactions and in the chemical industry. The reason for this may be attributed to the century-old notion that a hydrated OH(-) can be regarded as a water molecule missing a proton, and that the transport mechanism of such a 'proton hole' can be inferred from that of an excess proton by simply reversing hydrogen bond polarities. However, recent studies have identified OH(-) hydration complexes that bear little structural similarity to proton hydration complexes. Here we report the solution structures and transport mechanisms of hydrated hydroxide, which we obtained from first-principles computer simulations that explicitly treat quantum and thermal fluctuations of all nuclei. We find that the transport mechanism, which differs significantly from the proton hole picture, involves an interplay between the previously identified hydration complexes and is strongly influenced by nuclear quantum effects.

Journal Article↗
PubMed2003

Synthesis and structure of [Na11(OtBu)10(OH)]: 1H NMR shift of a hydroxide ion encapsulated in a 21-vertex alcoholate cage.

[Na11(OtBu)10(OH)], a hydroxide enclosing 21-vertex cage compound, was found to crystallize from mixtures of sodium tert.butanolate with sodium hydroxide. Its structure can be derived from the known (NaOtBu)6-hexaprismane by replacing one butanolate unit with OH- and capping the latter with five additional units of NaOtBu. The hydroxide shows a signal at -3.21 ppm in the 1H NMR spectrum.

Journal Article↗
PubMed2004

The reduction of tris-dithiolene complexes of molybdenum(VI) and tungsten(VI) by hydroxide ion: kinetics and mechanism.

The kinetic study of the spontaneous reduction of some neutral tris-dithiolene complexes [ML3] of molybdenum(VI) and tungsten(VI), (L = S2C6H4(2-), S2C6H3CH3(2-) and S2C2(CH3)2(2-); M = Mo or W) by tetrabutylammonium hydroxide in tetrahydrofuran-water solutions demonstrates that OH- is an effective reductant. Their reduction is fast, clean and quantitative. Depending upon both the molar ratio in which the reagents are mixed and the amount of water present, one- or two-electron reductions of these tris-dithiolene complexes were observed. If Bu4NOH is present in low concentration or/and at high concentrations of water, the total transformation of the neutral M(VI) complex into the monoanionic M(V) complex is the only observed process. Stopped-flow kinetic data for this reaction are consistent with the rate law: -d[ML3]/dt = d[ML3-]/dt = k[ML3][Bu4NOH]. The proposed mechanism involves nucleophilic attack of OH- to form a mono-anionic seven-coordinate intermediate [ML3OH]-, which interacts with another molecule of [ML3] to generate the monoanionic complex [ML3]- transfering the oxygen from coordinated OH- to water. Hydrogen peroxide was identified as the reaction product. The molybdenum complexes are more difficult to reduce than their corresponding tungsten complexes, and the values of k obtained for the molybdenum and tungsten series of complexes increase as the ene-1,2-dithiolate ligand becomes more electron-withdrawing (S2C6H4(2-) > S2C6H3CH3(2-) > S2C2(CH3)2(2-)). This investigation constitutes the only well-established interaction between hydroxide ion and a tris(dithiolene) complex, and supports a highly covalent bonding interaction between the metal and the hydroxide ion that modulates electron transfer reactions within these complexes.

Journal Article↗
PubMed1981

Ageing of alpha-chymotrypsin: Cannibalistic and hydroxide ion reactions.

The loss of catalytic activity (irreversible denaturation) of alpha-chymotrypsin (EC 3.4.21.1) as a function of pH between 6 and 13 is described. Around neutrality the irreversible denaturation is second order with respect to enzyme. At high pH, the irreversible denaturation is first order with respect to enzyme and also with respect to hydroxide ion. The first- or second-order dependency on enzyme concentration has been shown both by the time course of denaturation and by the effect of enzyme concentration on the rate of denaturation. The first-order dependency on hydroxide ion was shown by a plot of the logarithm of the rate constant vs. pH. These enzyme inactivations are peptide bond cleavages at all pH values. From the kinetics, the irreversible denaturation can be described as a cannibalistic reaction of two enzyme molecules around neutrality or as a reaction of one enzyme molecule plus one hydroxide ion at high pH.

Journal Article↗
PubMed2001

Microwave-assisted synthesis of Zn/Al layered double hydroxide-(anthraquinone-2,6-disulfonate) nanocomposite.

Nanocomposites of Zn/Al-layered double hydroxide(anthraquinone-2,6-disulfonate) were synthesized by spontaneous direct assembly of inorganic and organic phases from aqueous solution. Powder X-ray diffraction patterns showed that a pure, single nanocomposite phase of good crystallinity was obtained using 1.0 M antraquinone-2,6-disulfonate ion (AQ26) and aging at 80 degrees C using conventional heating for 7 days or 0.5 h under microwave radiation, and these samples are denoted as ZAAN26C or ZAAN26MH, respectively. Zn/Al-nitrate-layered double hydroxide synthesized by a conventional method (ZANLC) showed a basal spacing of 8.3 A while both the nanocomposites showed 18.8 A as a result of AQ26 intercalation. FTIR study showed that the resulting nanocomposites are free from nitrate, the co-anion present in the mother liquor, indicating that only AQ26 is preferred during intercalation for the formation of the nanocomposite. The Brunauer, Emmet and Teller (BET) and micropore surface areas for ZAAN26C decreased relative to the ZANLC from 16.2 to 4.7 and 1.6 to 1.3 m2/g, respectively. These results indicate that AQ26 can be rapidly interdcalated in layered double hydroxide using microwave-aging resulting in a nanocomposite.

Aluminum Compounds↗