Experimental and theoretical studies of the kinetics of the reactions of OH radicals with acetic acid, acetic acid-d3 and acetic acid-d4 at low pressure

The kinetics of the reactions of OH with acetic acid, acetic acid-d3 and acetic acid-d4 were studied from 2 to 5 Torr and 263-373 K using a discharge flow system with resonance fluorescence detection of the OH radical. The measured rate constants at 300 K for the reaction of OH with acetic acid and acetic acid-d4 (CD3C(O)OD) were (7.42+/-0.12)x10(-13) and (1.09+/-0.18)x10(-13) cm3 molecule-1 s-1 respectively, and the rate constant for the reaction of OH with acetic acid-d3 (CD3C(O)OH) was (7.79+/-0.16)x10(-13) cm3 molecule-1 s-1. These results suggest that the primary mechanism for this reaction involves abstraction of the acidic hydrogen. Theoretical calculations of the kinetic isotope effect as a function of temperature are in good agreement with the experimental measurements using a mechanism involving the abstraction of the acidic hydrogen through a hydrogen-bonded complex. The rate constants for the OH+acetic acid and OH+acetic acid-d4 reactions display a negative temperature dependence described by the Arrhenius equations kH(T)=(2.52+/-1.22)x10(-14) exp((1010+/-150)/T) and kD(T)=(4.62+/-1.33)x10(-16) exp((1640+/-160)/T) cm3 molecule-1 s-1 for acetic acid and acetic acid-d4, respectively, consistent with recent measurements that suggest that the lifetime of acetic acid at the low temperatures of the upper troposphere is shorter than previously believed.     

Reactive uptake of acetic acid on calcite and nitric acid reacted calcite aerosol in an environmental reaction chamber

The heterogeneous chemistry of gas-phase acetic acid with CaCO(3)(calcite) aerosol was studied under varying conditions of relative humidity (RH) in an environmental reaction chamber. Infrared spectroscopy showed the loss of gas-phase reactant and the appearance of a gaseous product species, CO(2). The acetic acid is observed to adsorb onto the calcite aerosol through both a fast and a slow uptake channel. While the fast channel is relatively independent of RH, the slow channel exhibits enhanced uptake and reaction as the RH is increased. In additional experiments, the calcite aerosol was exposed to both nitric and acetic acids in the presence of water vapor. The rapid conversion of the particulate carbonate to nitrate and subsequent deliquescence significantly enhances the uptake and reaction of acetic acid. These results suggest a possible mechanism for observed correlations between particulate nitrate and organic acids in the atmosphere. Calcium rich mineral dust may be an important sink for simple organic acids.     

Chelating agents related to ethylenediamine bis(2-hydroxyphenyl)acetic acid (EDDHA): synthesis,characterization, and equilibrium studies of the free ligands and their Mg2+, Ca2+, Cu2+, and Fe3+ chelates

Iron chelates such as ethylenediamine-N,N'-bis(2-hydroxyphenyl)acetic acid (EDDHA) and their analogues are the most efficient soil fertilizers to treat iron chlorosis in plants growing in calcareous soils. EDDHA, EDDH4MA (ethylenediamine-N,N'-bis(2-hydroxy-4-methylphenyl)acetic acid), and EDDCHA (ethylenediamine-N,N'-bis(2-hydroxy-5-carboxyphenyl)acetic acid) are allowed by the European directive, but also EDDHSA (ethylenediamine-N,N'-bis(2-hydroxy-5-sulfonylphenyl)acetic acid) and EDDH5MA (ethylenediamine-N,N'-bis(2-hydroxy-5-methylphenyl)acetic acid) are present in several commercial iron chelates. In this study, these chelating agents as well as p,p-EDDHA (ethylenediamine-N,N'-bis(4-hydroxyphenyl)acetic acid) and EDDMtxA (ethylenediamine-N,N'-bis(2-metoxyphenyl)acetic acid) have been obtained following a new synthetic pathway. Their chemical behavior has been studied to predict the effect of the substituents in the benzene ring on their efficacy as iron fertilizers for soils above pH 7. The purity of the chelating agents has been determined using a novel methodology through spectrophotometric titration at 480 nm with Fe(3+) as titrant to evaluate the inorganic impurities. The protonation constants were determined by both spectrophotometric and potentiometric methods, and Ca(2+) and Mg(2+) stability constants were determined from potentiometric titrations. To establish the Fe(3+) and Cu(2+) stability constants, a new spectrophotometric method has been developed, and the results were compared with those reported in the literature for EDDHA and EDDHMA and their meso- and rac-isomers. pM values have been also determined to provide a comparable basis to establish the relative chelating ability of these ligands. The purity obtained for the ligands is higher than 87% in all cases and is comparable with that obtained by (1)H NMR. No significant differences have been found among ligands when their protonation and stability constants were compared. As expected, no Fe(3+) complexation was observed for p,p-EDDHA and EDDMtxA. The presence of sulfonium groups in EDDHSA produces an increase in acidity that affects their protonation and stability constants, although the pFe values suggest that EDDHSA could be also effective to correct iron chlorosis in plants.     

A path integral molecular dynamics study on intermolecular hydrogen bond of acetic acid–arsenic acid anion and acetic acid–phosphoric acid anion clusters

We apply ab initio path integral molecular dynamics simulation employing ωB97XD as the quantum chemical calculation method to acetic acid–arsenic acid anion and acetic acid–phosphoric acid anion clusters to investigate the difference of the hydrogen bond structure and its fluctuation such as proton transfer. We found that the nuclear quantum effect enhanced the fluctuation of the hydrogen bond structure and proton transfer, which shows treatment of the nuclear quantum effect was essential to investigate these systems. The hydrogen bond in acetic acid–arsenic acid anion cluster showed characters related to low-barrier hydrogen bonds, while acetic acid–phosphoric acid anion cluster did not. We found non-negligible distinction between these two systems, which could not be found in conventional calculations. We suggest that the difference in amount of atomic charge of the atoms consisting the hydrogen bond is the origin of the difference between acetic acid–arsenic acid and acetic acid–phosphoric acid anion cluster. © 2018 Wiley Periodicals, Inc.     

Thermal evolution of acetic acid nanodeposits over 123-180 K on noncrystalline ice and polycrystalline ice studied by FTIR reflection-absorption spectroscopy: hydrogen-bonding interactions in acetic acid and between acetic acid and ice

Acetic acid vapor-deposited on ultrathin noncrystalline ice (NCI) and polycrystalline ice (PCI) films (less than 6 nm thick) under ultrahigh vacuum conditions has been investigated by using Fourier Transform Infrared Reflection-Absorption Spectroscopy. Pristine acetic acid deposited at 123 K (on a copper support) appears as an amorphous solid, which undergoes an irreversible phase transformation to a more structurally ordered (polycrystalline) form upon annealing to 153 K. Acetic acid is found to adsorb on NCI and PCI films initially through hydrogen bonding between C=O and dangling OH (of ice), followed by the formation of multilayers at 123 K. Thermal evolution studies of a low exposure of acetic acid on the ultrathin NCI and PCI films show that acetic acid undergoes coevaporation with water likely as an acetic acid hydrate at 155 K, which continues until the entire ice film has been exhausted at 165 K. Above 165 K, the remaining acetic acid solid appears to evaporate without undergoing the phase transformation, in contrast to the case of a high acetic acid exposure. Coevaporation of acetic acid with water is also found to proceed at a faster rate than the subsequent evaporation of acetic acid, which is consistent with the weaker interactions observed in the H-bonded acetic acid hydrate than that in acetic acid solid.     

Kinetics of beta-haematin formation from suspensions of haematin in aqueous benzoic acid

Kinetics of beta-haematin (synthetic malaria pigment) formation from haematin have been studied in the presence of aqueous benzoic acid and derivatives of benzoic acid. Formation of the beta-haematin product is demonstrated by X-ray diffraction and IR spectroscopy. Reactions were followed by determining the fraction of unreacted haematin at various time points during the process via reaction of extracted aliquots with pyridine. The kinetics can be fitted to the Avrami equation, indicating that the process involves nucleation and growth. Reaction kinetics in stirred benzoic acid are similar to those previously observed in acetic acid, except that benzoic acid is far more active in promoting the reaction than acetic acid. The reaction reaches completion within 2 h in the presence of 0.050 M benzoic acid (pH 4.5, 60 degrees C). This compares with 1 h in the presence of 4.5 M acetic acid and 4 h in the presence of 2 M acetic acid. The reaction rate in benzoic acid is not affected if the stirring rate is decreased to zero, but very vigorous stirring appears to disrupt nucleation. The rate constant for beta-haematin formation in benzoic acid has a linear dependence on benzoic acid concentration and follows Arrhenius behaviour with temperature. There is a bell-shaped dependence on pH. This suggests that the haematin species in which one propionate group is protonated and the other is deprotonated is optimal for beta-haematin formation. When the reaction is conducted in para-substituted benzoic acid derivatives, the log of the rate constant increases linearly with the Hammett constant. These findings suggest that the role of the carboxylic acid may be to disrupt hydrogen bonding and pi-stacking in haematin, facilitating conversion to beta-haematin. The large activation energy for conversion of precipitated haematin to beta-haematin suggests that the reaction in vivo most likely involves direct nucleation from solution and probably does not occur in aqueous medium.     

Photodissociation spectroscopy of the Mg+-acetic acid complex

We have studied the structure and photodissociation of Mg(+)-acetic acid clusters. Ab initio calculations suggest four relatively strongly bound ground state isomers for the [MgC(2)H(4)O(2)](+) complex. These isomers include the cis and trans forms of the Mg(+)-acetic acid association complex with Mg(+) bonded to the carbonyl O atom of acetic acid, the Mg(+)-acetic acid association complex with Mg(+) bonded to the hydroxyl O atom of acetic acid, or to a Mg(+)-ethenediol association complex. Photodissociation through the Mg(+)-based 3p<--3s absorption bands in the near UV leads to direct (nonreactive) and reactive dissociation products: Mg(+), MgOH(+), Mg(H(2)O)(+), CH(3)CO(+), and MgCH(3) (+). At low energies the dominant reactive quenching pathway is through dehydration to Mg(H(2)O)(+), but additional reaction channels involving C-H and C-C bond activation are also open at higher energies.     

Measurement of the fourth O-H overtone absorption cross section in acetic acid using cavity ring-down spectroscopy

We report the absolute absorption cross sections of the fourth vibrational O-H (5ν(OH)) overtone in acetic acid using cavity ring-down spectroscopy. For compounds that undergo photodissociation via overtone excitation, such intensity information is required to calculate atmospheric photolysis rates. The fourth vibrational overtone of acetic acid is insufficiently energetic to effect dissociation, but measurement of its cross section provides a model for other overtone transitions that can affect atmospheric photochemistry. Though gas-phase acetic acid exists in equilibrium with its dimer, this work shows that only the monomeric species contributes to the acetic acid overtone spectrum. The absorption of acetic acid monomer peaks at ～615 nm and has a peak cross section of 1.84 × 10(-24) cm(2)·molecule(-1). Between 612 and 620 nm, the integrated cross section for the acetic acid monomer is (5.23 ± 0.73) × 10(-24) cm(2)·nm·molecule(-1) or (1.38 ± 0.19) × 10(-22) cm(2)·molecule(-1)·cm(-1). This is commensurate with the integrated cross section values for the fourth O-H overtone of other species. Theoretical calculations show that there is sufficient energy for hydrogen to transition between the two oxygen atoms, which results in an overtone-induced conformational change.     

Biohydrogen Production from Cheese Whey Wastewater in a Two-Step Anaerobic Process

Cheese whey-based biohydrogen production was seen in batch experiments via dark fermentation by free and immobilized Enterobacter aerogenes MTCC 2822 followed by photofermentation of VFAs (mainly acetic and butyric acid) in the spent medium by Rhodopseudomonas BHU 01 strain. E. aerogenes free cells grown on cheese whey diluted to 10 g lactose/L, had maximum lactose consumption (～79%), high production of acetic acid (1,900 mg/L), butyric acid (537.2 mg/L) and H(2) yield (2.04 mol/mol lactose; rate,1.09 mmol/L/h). The immobilized cells improved lactose consumption (84%), production of acetic acid (2,100 mg/L), butyric acid (718 mg/L) and also H(2) yield (3.50 mol/mol lactose; rate, 1.91 mmol/L/h). E. aerogenes spent medium (10 g lactose/L) when subjected to photofermentation by free Rhodopseudomonas BHU 01 cells, the H(2) yield reached 1.63 mol/mol acetic acid (rate, 0.49 mmol/L/h). By contrast, immobilized Rhodopseudomonas cells improved H(2) yield to 2.69 mol/mol acetic acid (rate, 1.87 mmol/L/h). The cumulative H(2) yield for free and immobilized bacterial cells was 3.40 and 5.88 mol/mol lactose, respectively. Bacterial cells entrapped in alginate, had a sluggish start of H(2) production but outperformed the free cells subsequently. Also, the concomitant COD reduction for free cells (29.5%) could be raised to 36.08% by immobilized cells. The data suggest that two-step fermentative H(2) production from cheese whey involving immobilized bacterial cells, offers greater substrate to- hydrogen conversion efficiency, and the effective removal of organic load from the wastewater in the long-term.     

Partition and determination of cadmium, copper, lead and zinc in marine suspended particulate matter

A method for the determination of Cd, Cu, Pb, and Zn in the total and acetic acid (25%) soluble fraction of suspended particulate matter collected on Nuclepore membranes is described. The acetic acid leaching was carried out in a modified Millipore vacuum filtration apparatus. The total sample and the acetic insoluble fraction were decomposed in teflon bombs with HF/aqua regia. Flame or graphite furnace AAS was used for metal determinations. Sequential acetic acid extractions (3) show that on the average 76.4-92.0% of the metals are removed with the first extraction. Cadmium is potentially most easily available to biota with 90.4% of the total metal concentration located in the acetic acid soluble fraction.     

Acetic acid permeation through photovoltaic backsheets: Influence of the composition on the permeation rate

Acetic acid, which is formed during degradation of the most frequently used photovoltaic (PV) encapsulant ethylene vinyl acetate (EVA), is linked to several PV module failure mechanisms like corrosion of interconnectors, cells or potential induced degradation (PID). An evaluation of a new measurement technique and data analysis for acetic acid permeation through photovoltaic backsheet films is described. The influence of the layer composition of the multilayer backsheets on the acetic acid permeation rates (AATR) was determined by investigating the permeation properties of the individual layers as well as the whole polymeric multilayer composites. Various polymeric backsheet types based on PET-core layers showing a very high barrier for acetic acid permeation were compared to fully polyamide material stacks which allow for a much higher transmission of acetic acid through the backsheet out of the module. Incorporated Al-layers were found to not affect the AATRs of PET based backsheets.     

Electrochemically induced mineralization of organics by molecular oxygen on boron-doped diamond electrode

The complete mineralization of organic pollutants present in wastewater is usually achieved via thermally activated oxygen/air. This process occurs at high temperatures and pressures (300 °C, 200 atm) and often gives small amount of acetic acid as a final product. In this work, we demonstrate using acetic acid as a model compound that organic molecules can be activated electrochemically such that they react at room temperature with oxygen, resulting in mineralization of even acetic acid present in 1 M HClO₄ supporting electrolyte. This electrochemically induced activation occurs during anodic polarization of boron-doped diamond electrodes (BDD) in air/oxygen-saturated solutions. The direct evidence for this process was found during electro-oxidation of acetic acid saturated with isotopically labelled ¹⁸O₂ resulting in evolution of C¹⁸O₂ and C¹⁶O¹⁸O. We suggest that the mechanism of activation on BDD is initiated by hydroxyl radicals formed on the electrode surface.     

A vibrational sum frequency spectroscopy study of the liquid-gas interface of acetic acid-water mixtures: 1. Surface speciation

Aqueous acetic acid solutions have been studied by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. The concentration range 0-100% acetic acid has been studied in the CH/OH and the C-O/C=O regions, and in order to clarify peak assignments, experiments with deuterated acetic acid and water have also been performed. Throughout the whole concentration range, the acetic acid is proven to be protonated. It is explicitly shown that the structure of a water surface becomes disrupted even at small additions of acetic acid. Furthermore, the spectral evolution upon increasing the concentration of acetic acid is explained in terms of the different complexes of acetic acid molecules, such as the hydrated monomer, linear dimer, and cyclic dimer. In the C=O region, the hydrated monomer is concluded to give rise to the sum frequency (SF) signal, and in the CH region, the cyclic dimer contributes to the signal as well. The combination of results from the CH/OH and the C-O/C=O regions allows a thorough characterization of the behavior of the acetic acid molecules at the interface to be obtained.     

The effects of gold and Co-adsorbed carbon on the adsorption and thermal decomposition of acetic acid on Pd{111}

The growth and annealing behavior of ultrathin Au films on Pd{111} were monitored with scanning tunneling microscopy (STM) and medium energy ion scattering (MEIS). The adsorption of acetic acid on both clean and deliberately carbon-contaminated bimetallic surfaces was investigated with reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). We report that the surface chemistry of acetic acid is strongly modified by the presence of Au in the bimetallic surface which acts both to stabilize adsorbed acetate and to decrease the tendency of acetic acid to decompose on adsorption to produce adsorbed carbon. The adsorption of acetic acid at 300 K is found to cause measurable segregation of Pd to the surface for all surface compositions tested.     

Degradation of polymer mixtures. VIII. Blends of poly(vinyl acetate) with poly(methyl methacrylate)

The degradation of blends of PVA and PMMA in the form of films cast from a common solution of the polymers has been studied by TVA, TG, and EGA (evolved gas analysis) for acetic acid. Volatile degradation products have been characterized by spectroscopic and GLC techniques. Molecular weight, spectral and thermal stability changes in PMMA extracted from partially degraded blends have been examined. These blends behave in a closely analogous manner to PVC-PMMA blends already investigated. The results suggest that the PMMA component of the heterogeneous blends is modified in two ways: (1) in a destabilization reaction series initiated by attack of acetate radicals generated in the PVA phase which migrate into the PMMA phase, and (2) in a stabilization reaction involving conversion of ester side groups to acid and subsequently to anhydride ring structures which act as blocking points for depolymerization. The rate of acetic acid production in the blend is less than in PVA degraded alone. The mechanism of degradation of PVA is reconsidered in the light of these results.     

Monitoring of substrate and product concentrations in acetic fermentation processes for onion vinegar production by NIR spectroscopy: value addition to worthless onions

Wastes and by-products of the onion-processing industry pose an increasing disposal and environmental problem and represent a loss of valuable sources of nutrients. The present study focused on the production of vinegar from worthless onions as a potential valorisation route which could provide a viable solution to multiple disposal and environmental problems, simultaneously offering the possibility of converting waste materials into a useful food-grade product and of exploiting the unique properties and health benefits of onions. This study deals specifically with the second and definitive step of the onion vinegar production process: the efficient production of vinegar from onion waste by transforming onion ethanol, previously produced by alcoholic fermentation, into acetic acid via acetic fermentation. Near-infrared spectroscopy (NIRS), coupled with multivariate calibration methods, has been used to monitor the concentrations of both substrates and products in acetic fermentation. Separate partial least squares (PLS) regression models, correlating NIR spectral data of fermentation samples with each kinetic parameter studied, were developed. Wavelength selection was also performed applying the iterative predictor weighting–PLS (IPW-PLS) method in order to only consider significant spectral features in each model development to improve the quality of the final models constructed. Biomass, substrate (ethanol) and product (acetic acid) concentration were predicted in the acetic fermentation of onion alcohol with high accuracy using IPW-PLS models with a root-mean-square error of the residuals in external prediction (RMSEP) lower than 2.5% for both ethanol and acetic acid, and an RMSEP of 6.1% for total biomass concentration (a very satisfactory result considering the relatively low precision and accuracy associated with the reference method used for determining the latter). Thus, the simple and reliable calibration models proposed in this study suggest that they could be implemented in routine applications to monitor and predict the key species involved in the acetic fermentation of onion alcohol, allowing the onion vinegar production process to be controlled in real time.     

Alternative mobile phases for the reversed-phase high-performance liquid chromatography of peptides and proteins

The use of a high content of acetic acid as mobile phase additive for the reversed-phase high-performance liquid chromatography (RP-HPLC) of several proteins and extracts of biological tissues was evaluated for a divinylbenzene (DVB)-based stationary phase, and the separations obtained with acetic acid gradients in acetonitrile, isopropanol or water were compared with classical polypeptide RP-HPLC on silica C4 with trifluoroacetic acid (TFA)-acetonitrile. The separation patterns for recombinant derived interleukin-1 beta (IL-1 beta) on the C4 column eluted with TFA-acetonitrile and the DVB column eluted with acetic acid-acetonitrile were similar, but only the polymeric column was able to separate the components present in an iodinated IL-1 beta preparation. Neither eluent had any harmful effect on the biological activity of IL-1 beta isolated after RP-HPLC. Several standard proteins could be separated when the polymeric column was eluted with acetic acid gradients in acetonitrile, isopropanol or water and, although the separation efficiency with acetic acid in water was lower than that in combination with classical organic modifiers, insulin, glucagon and human growth hormone (hGH) were eluted as sharp, symmetrical peaks. The recoveries of insulin and hGH were comparable for all three mobile phases (80-90%). The separation patterns obtained from a crude acetic acid extract of a normal and a diabetic, human pancreas analysed using acetic acid gradients with or without organic modifiers were found to be similar and comparable to those obtained on a silica C4 column eluted with an acetonitrile gradient in TFA. The principal differences resulted from the use of different UV wavelengths (215 nm for TFA-acetonitrile, 280 nm for acetic acid). Acetic acid extracts of recombinant derived hGH-producing Escherichia coli were separated on the DVB column eluted with an acetic acid gradient in water. Although the starting material was a highly complex mixture, the hGH isolated after this single-step purification was surprisingly pure (as judged by sodium dodecyl sulphate-polyacrylamide gel electrophoresis). Consequently several (pure) polypeptides and complex biological samples were separated on a polymeric stationary phase eluted with acetic acid gradients in water without the use of organic modifiers.     

Adsorption of acetic acid on ice: experiments and molecular dynamics simulations

Adsorption study of acetic acid on ice surfaces was performed by combining experimental and theoretical approaches. The experiments were conducted between 193 and 223 K using a coated wall flow tube coupled to a mass spectrometric detection. Under our experimental conditions, acetic acid was mainly dimerized in the gas phase. The surface coverage increases with decreasing temperature and with increasing concentrations of acetic acid dimers. The obtained experimental surface coverages were fitted according to the BET theory in order to determine the enthalpy of adsorption deltaH(ads) and the mololayer capacity N(M(dimers)) of the acetic acid dimers on ice: deltaH(ads) = (-33.5 +/- 4.2) kJ mol(-1), N(M(dimers)) = (l1.27 +/- 0.25) x 10(14) dimers cm(-2). The adsorption characteristics of acetic acid on an ideal ice I(n)(0001) surface were also studied by means of classical molecular dynamics simulations in the same temperature range. The monolayer capacity, the configurations of the molecules in their adsorption sites, and the corresponding adsorption energies have been determined for both acetic acid monomers and dimers, and compared to the corresponding data obtained from the experiments. In addition, the theoretical results show that the interaction with the ice surface could be strong enough to break the acetic acid dimers that exist in the gas phase and leads to the stabilization of acetic acid monomers on ice.     

Self-association of acetic acid in dilute deuterated chloroform. Wide-range spectral reconstructions and analysis using FTIR spectroscopy, BTEM, and DFT

The binary solution of acetic acid in CDCl(3) was studied at room pressure on the interval T = 293-313 K with a series of acetic acid concentrations up to 0.16 M. In-situ Fourier transform infrared (FTIR) spectroscopy measurements on the interval of 400-3800 cm(-1) were utilized as the analytical method to monitor the spectral changes due to self-association of acetic acid. The band-target entropy minimization (BTEM) algorithm was employed to reconstruct the underlying pure component spectra. Analysis successfully provided two major spectral estimates of acetic acid, namely, the monomer (primarily in the form of monomer-CDCl(3) complex) and the centrosymmetric cyclic dimer. In addition, analysis provided one minor spectral estimate containing signals from both noncyclic dimers and higher aggregates. Also, spectral estimates were obtained for phosgene and water which were present at trace levels even though considerable precaution was taken to conduct the experiments under anhydrous and anaerobic conditions. Density functional theory (DFT) calculation was performed to assign the acetic acid structures corresponding to the BTEM spectral estimates. Since the structure of dilute acetic acid has been the subject of numerous studies, the present investigation helps to resolve some issues concerning the speciation of acetic acid at low concentrations in low polarity solvents. In particular, the present study provides for the first time, wide-range spectral reconstructions of the species present.