Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “DISSOCIATION”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Dissociation of internal energy-selected methyl bromide ion revealed from threshold photoelectron-photoion coincidence velocity imaging

Dissociative photoionization of methyl bromide (CH{sub 3}Br) in an excitation energy range of 10.45–16.90 eV has been investigated by using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. The coincident time-of-flight mass spectra indicate that the ground state X{sup 2}E of CH{sub 3}Br{sup +} is stable, and both A{sup 2}A{sub 1} and B{sup 2}E ionic excited states are fully dissociative to produce the unique fragment ion of CH{sub 3}{sup +}. From TPEPICO 3D time-sliced velocity images of CH{sub 3}{sup +} dissociated from specific state-selected CH{sub 3}Br{sup +} ion, kinetic energy release distribution (KERD) and angular distribution of CH{sub 3}{sup +} fragment ion are directly obtained. Both spin-orbit states of Br({sup 2}P) atom can be clearly observed in fast dissociation of CH{sub 3}Br{sup +}(A{sup 2}A{sub 1}) ion along C–Br rupture, while a KERD of Maxwell-Boltzmann profile is obtained in dissociation of CH{sub 3}Br{sup +}(B{sup 2}E) ion. With the aid of the re-calculated potential energy curves of CH{sub 3}Br{sup +} including spin-orbit coupling, dissociation mechanisms of CH{sub 3}Br{sup +} ion in A{sup 2}A{sub 1} and B{sup 2}E states along C–Br rupture are revealed. For CH{sub 3}Br{sup +}(A{sup 2}A{sub 1}) ion, the CH{sub 3}{sup +} + Br({sup 2}P{sub 1/2}) channel is occurred via an adiabatic dissociation by vibration, while the Br({sup 2}P{sub 3/2}) formation is through vibronic coupling to the high vibrational level of X{sup 2}E state followed by rapid dissociation. C–Br bond breaking of CH{sub 3}Br{sup +}(B{sup 2}E) ion can occur via slow internal conversion to the excited vibrational level of the lower electronic states and then dissociation.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Dissociation of H{sub 2} on carbon doped aluminum cluster Al{sub 6}C

The dissociation of H{sub 2} molecule is the first step for chemical storage of hydrogen, and the energy barrier of the dissociation is the key factor to decide the kinetics of the regeneration of the storage material. As a light element, aluminum is an important candidate component for storage materials with high gravimetric density. This paper investigates the adsorption and dissociation of H{sub 2} on carbon doping aluminum cluster Al{sub 6}C. The study shows that doping carbon into aluminum cluster can significantly change the electronic structure and increase the stability. Al{sub 6}C has a few stable isomers with close energies and their structures are quite flexible. The molecular adsorption of H{sub 2} on Al{sub 6}C is very weak, but the H{sub 2} molecule can be dissociated easily on this cluster. The stable product of the dissociated adsorption is searched and the different paths for the dissociation are investigated. During the dissociation of H{sub 2}, the structure of the cluster adjusts accordingly, and strong orbital interaction between the hydrogen and the cluster occurs. The calculated energy barrier for the dissociation is only 0.30 eV, which means the dissociation can take place at moderate temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Dissociation path competition of radiolysis ionization-induced molecule damage under electron beam illumination

Radiolysis ionization under electron beam illumination induces dissociation and damage of organic and biological molecules; thus, it is impossible to image the related materials by transmission electron microscopy (TEM). To understand the atomistic mechanism of radiolysis damage, we developed a systematical procedure based on real-time time-dependent density functional theory (rt-TDDFT) for simulating the radiolysis damage processes of molecules; this procedure can describe the ionization cross sections of the electronic states and the fast dissociation processes caused by hot carrier cooling and the Auger decay on deep levels. For the radiolysis damage of C 2 H 6 O 2 , our simulation unexpectedly showed that there is strong competition among three different dissociation paths, including fast dissociation caused by nonadiabatic cooling of the hot carrier; fast dissociation caused by Auger decay, which induces double ionization and Coulomb explosion; and slow dissociation caused by increased kinetic energy. As the energy of the incident electron beam changes, the time scales of these dissociation paths and their relative contributions to the molecule damage change significantly. These simulation results explain the measured mass spectra of the C 2 H 6 O 2 dissociation fragments and also provide clear competition mechanisms for blocking these dissociation paths in the TEM imaging of organic and biological materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Infrared Laser-Induced Isotope-Selective Dissociation of Molecular van der Waals (SF{sub 6}){sub m}Ar{sub n} Clusters

The results of studies on the isotope-selective dissociation of homogeneous and mixed van der Waals (SF{sub 6}){sub m}Ar{sub n} clusters induced by infrared laser radiation are presented. The method is based on the selective vibrational excitation of clusters by an infrared laser, which leads to their heating and dissociation. An experimental setup and procedure are described. The results of measurements of the efficiency and selectivity of cluster dissociation at different pressures and gas compositions above the nozzle, as well as various parameters of laser radiation, are presented. The dependences of the efficiency and selectivity of cluster dissociation on the frequency and power of the exciting laser radiation on the composition and pressure of gas above the nozzle, as well as on the distance of the irradiation zone of the particles on the nozzle exit are obtained. It is shown that the frequency of laser radiation and the composition and pressure of gas above the nozzle significantly affect the parameters of selective infrared dissociation of clusters. Experiments on isotope-selective dissociation of both homogeneous (SF{sub 6}){sub m} clusters and mixed (SF{sub 6}){sub m}Ar{sub n} clusters are performed. It was found that, with the dissociation of (SF{sub 6}){sub m}Ar{sub n} clusters, a rather high selectivity can be reached. Thus, in the case of an SF{sub 6}/Ar mixture at a pressure ratio of 1/200, for the selectivity of dissociation of ({sup 32}SF{sub 6}){sub 2} clusters with respect to {sup 34}SF{sub 6}{sup 32}SF{sub 6} clusters, the values ≥10–20 are obtained under conditions of irradiation by a beam on the 10P(32) laser line (at a frequency of 932.96 cm{sup –1}). Conditions are found when the values of efficiency and selectivity of dissociation of homogeneous (SF{sub 6}){sub m} clusters and mixed (SF{sub 6}){sub m}Ar{sub n} clusters are optimal.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Quantifying the Relevance of Prompt Dissociations of Resonance Stabilized Free Radicals in Flames

In prior studies from our laboratory we have shown that weakly-bound free radicals have a propensity to promptly dissociate at elevated temperatures relevant to flames. Dissociation can occur so rapidly at high temperatures in such weakly-bound radicals that it interferes with collisional relaxation of internal energies leading to prompt dissociation. While prompt dissociation is a ubiquitous feature in such free radical dissociations, we have hypothesized that its relevance to global combustion observables such as flame speeds depends on satisfying specific criteria. Two criteria that motivate the present study are that the free radicals must play a central role in the oxidation mechanism and that their thermal dissociation rate coefficients should be small enough. Resonance stabilized radicals have bond strengths ~ 50 kcal/mol (or larger) and consequently are known to dissociate slowly and persist even in high temperature environments. In this work we have calculated prompt dissociation probabilities for some simple allylic (allyl, methyl allyl, and dimethyl allyl) radicals. Flame simulations were performed to quantify the relevance of including prompt dissociations of these allylic radicals.

Allyl Radicals↗
DOE OSTI2022

Mechanisms of Complete Dissociation of CO 2 on Iron Clusters

Dissociation of CO 2 on iron clusters was studied by using semilocal density functional theory and basis sets of triple-zeta quality. Fe 2 , Fe 4 , and Fe 16 clusters were selected as the representative host clusters. When searching for isomers of Fe n CO 2 , n=2, 4 and 16 corresponding to carbon dioxide attachment to the host clusters, its reduction to O and CO, and to the complete dissociation, it was found that the total spin magnetic moments of the lowest energy states of the isomers are often quenched with respect to those of initial reagents Fe n +CO 2 . Dissociation pathways of the Fe 2 +CO 2 , Fe 4 +CO 2 , and Fe 16 +CO 2 reactions contain several transition states separated by the local minima states; therefore, a natural question is where do the spin flips occur? Since lifetimes of magnetically excited states were shown to be of the order of 100 fs, the search for the CO 2 dissociation pathways was performed under the assumption that magnetic deexcitation may occur at the intermediate local minima. Two dissociation pathways were obtained for each Fe n +CO 2 reaction using the gradient-based methods. It was found that the Fe 2 +CO 2 reaction is endothermic with respect to both reduction and complete dissociation of CO 2 , whereas the Fe 4 +CO 2 and Fe 16 +CO 2 reactions are exothermic to both reduction and complete dissociation of carbon dioxide. Finally, the CO 2 reduction was found to be more favorable than its complete dissociation in the Fe 4 case.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Water adsorption and dissociation on Ni(110): How is it different from its close packed counterparts?

Water adsorption and dissociation on Ni(110) surface is studied in detail and compared with its close packed counterparts using density functional theory calculations. Water adsorption occurs on the top site as found on Ni(100) and Ni(111) but the adsorption is stronger on Ni(110). H and OH preferably adsorb on the short bridge sites (brgshort) opposed to hollow sites on (100) and (111) surfaces. Energy barriers for water molecule dissociation on Ni(110) as obtained from the transition state (TS) calculations were low compared to other Ni low indexed surfaces. TS geometries at different positions of the lattice coordinate, Q, were obtained to study the effect of surface temperature on dissociation of H{sub 2}O molecules. These calculations revealed that second layer atoms were also involved in the TS. Dissociation probabilities are obtained using a semi-classical approximation by sampling Q for a Boltzmann distribution at different temperatures. Results showed that the increasing surface temperature significantly increases the dissociation probabilities at lower energies and saturates near the barrier for dissociation. Although the contribution from both top and second layers is similar at low surface temperatures, motion of top layer atoms contribute more towards dissociation probability at higher surface temperatures. Dissociation probabilities obtained are more than one order of magnitude higher than that on Ni(100) and Ni(111) surfaces suggesting Ni(110) to be more reactive among the low indexed Ni surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The role of energy transfer and competing bimolecular reactions in characterizing the unimolecular dissociations of allylic radicals

Resonantly stabilized radicals (RSR's) such as allylic radicals typically have bond energies exceeding 45 kcal/mol and are therefore reasonably long-lived up to quite high temperatures. Consequently, kinetics of the unimolecular dissociations and bimolecular reactions of these long-lived RSR's assume significance in high temperature chemically reacting systems such as in flames. Our recent analyses (J. Cho et al., Proc. Comb. Inst., 2022) of the uncertainty propagated by the energy transfer parameters for 1-methylallyl (1MA) dissociation to the laminar flame speed, indicates an intricate coupling between the kinetics of 1MA dissociation (chain propagation) and its reaction with H-atoms (chain-termination). Here, this work extends such analyses to other C 3 – C 5 allylic radicals: allyl, 2-methylallyl, 1,1-dimethylallyl, 1,2-dimethylallyl, and 1,3-dimethylallyl. Theoretical kinetics for the C 3 – C 4 allylic radicals were taken from prior literature studies (J.A. Miller et al., J. Phys. Chem., 2008; J. Cho et al., Proc. Comb. Inst., 2022; R.S. Tranter et al., Proc. Comb. Inst., 2017). Potential energy surfaces were newly computed for the dissociations of the larger C5 allylic radicals and the reactions of these radicals with H-atoms. These PES's were then used to calculate rate constants for the relevant unimolecular and bimolecular reactions. Energy transfer has a crucial role to play in these pressure-dependent reactions and therefore the present kinetics calculations relied on a-priori predictions of collisional energy transfer parameters to best characterize the branching between chain propagating unimolecular dissociations of these allylic radicals and chain terminating bimolecular reactions of these RSR's with H-atoms. Prompt dissociations are now established to be a universal feature of free radical dissociations. Here we have also characterized the propensity for prompt dissociations in these more-stable allylic RSR's and their role in combustion simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Modeling dissociation of ionic liquids with electrolyte NRTL model

Dissociation of ionic liquids (ILs) is critical to electrochemical process performance by affecting ionic conductivity. Prior studies of thermodynamic modeling either assumed no dissociation or complete dissociation that were not consistent with experimental observations. This work presented an eNRTL model for three aqueous imidazolium-based IL systems: [Emim][EtSO 4 ], [Emim][TFA], and [Emim][TfO]. The model accurately correlates the IL dissociation and VLE data in the entire concentration range from pure IL to infinite dilution aqueous solution. The system non-ideality is dominated by the dissociation chemistry. Dissociation behavior is driven by strong short-range interaction of the molecular IL-water pair relative to the dissociated IL-water pair, consistent with prior studies indicating that molecular IL and water can form an energetically favorable complex. The interaction energy parameters calculated in this work qualitatively agrees with the hydrogen bonding ability of the anions previously reported. This research not only provides a model that well describes the IL dissociation but also demonstrates the physical significance of eNRTL model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Weakly Bound Free Radicals in Combustion: "Prompt" Dissociation of Formyl Radicals and Its Effect on Laminar Flame Speeds

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, timescales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational-rotational relaxation process. Here we characterize this “prompt” dissociation of formyl (HCO), an important combustion radical, using direct dynamics calculations for OH + CH2O and H + CH2O (key HCO-forming reactions). For all other HCO-forming reactions, presumption of a thermal incipient HCO distribution was used to derive prompt dissociation fractions. Inclusion of these theoretically derived HCO prompt dissociation fractions into combustion kinetics models provides an additional source for H-atoms that feeds chain branching reactions. Simulations using these updated combustion models are therefore shown to enhance flame propagation in 1,3,5-trioxane and acetylene. The present results suggest that HCO prompt dissociation should be included when simulating flames of hydrocarbons and oxygenated molecules and that prompt dissociations of other weakly bound radicals may also impact combustion simulations

Dynamics↗
DOE OSTI2017

The dissociative chemisorption of CO 2 on Ni(100): A quantum dynamics study

A quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore the dissociative chemisorption of CO 2 on Ni(100). The largest barrier to reaction corresponds to the formation of a bent anionic molecular precursor, bound to the surface by about 0.24 eV. The barrier to dissociation from this state is small. In our computed dissociative sticking probabilities on Ni(100) for molecules, the ground states are in very good agreement with available experimental data, reasonably reproducing the variation in reactivity with collision energy. Vibrational excitation of the incident CO 2 can enhance reactivity, particularly for incident energies at or below threshold, and there is clear mode specific behavior. Both the vibrational enhancement and the increase in dissociative sticking with surface temperature are much weaker than that found in recent studies of methane and water dissociative chemisorption. The energetics for CO 2 adsorption and dissociation on the stepped Ni(711) surface are found to be similar to that on Ni(100), except that the barrier to dissociation from the anionic precursor is even smaller on Ni(711). Here, we predict that the dissociative sticking behavior is similar on the two surfaces.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2006

Generating Breathable Air Through Dissociation of N2O

A nitrous oxide-based oxygen-supply system (NOBOSS) is an apparatus in which a breathable mixture comprising 2/3 volume parts of N2 and 1/3 volume part of O2 is generated through dissociation of N2O. The NOBOSS concept can be adapted to a variety of applications in which there are requirements for relatively compact, lightweight systems to supply breathable air. These could include air-supply systems for firefighters, divers, astronauts, and workers who must be protected against biological and chemical hazards. A NOBOSS stands in contrast to compressed-gas and cryogenic air-supply systems. Compressed-gas systems necessarily include massive tanks that can hold only relatively small amounts of gases. Alternatively, gases can be stored compactly in greater quantities and at low pressures when they are liquefied, but then cryogenic equipment is needed to maintain them in liquid form. Overcoming the disadvantages of both compressed-gas and cryogenic systems, the NOBOSS exploits the fact that N2O can be stored in liquid form at room temperature and moderate pressure. The mass of N2O that can be stored in a tank of a given mass is about 20 times the mass of compressed air that can be stored in a tank of equal mass. In a NOBOSS, N2O is exothermically dissociated to N2 and O2 in a main catalytic reactor. In order to ensure the dissociation of N2O to the maximum possible extent, the temperature of the reactor must be kept above 400 C. At the same time, to minimize concentrations of nitrogen oxides (which are toxic), it is necessary to keep the reactor temperature at or below 540 C. To keep the temperature within the required range throughout the reactor and, in particular, to prevent the formation of hot spots that would be generated by local concentrations of the exothermic dissociation reaction, the N2O is introduced into the reactor through an injector tube that features carefully spaced holes to distribute the input flow of N2O widely throughout the reactor. A NOBOSS includes one or more "destroyer" subsystems for removing any nitrogen oxides that remain downstream of the main N2O-dissociation reactor. A destroyer includes a carbon bed in series with a catalytic reactor, and is in thermal contact with the main N2O-dissociation reactor. The gas mixture that leaves the main reactor first goes through a carbon bed, which adsorbs all of the trace NO and most of the trace NO2. The gas mixture then goes through the destroyer catalytic reactor, wherein most or all of the remaining NO2 is dissociated. A NOBOSS can be designed to regulate its reactor temperature across a range of flow rates. One such system includes three destroyer loops; these loops act, in combination with a heat sink, to remove heat from the main N2O-dissociation reactor. In this system, the N2O and product gases play an additional role as coolants; thus, as needed, the coolant flow increases in proportion to the rate of generation of heat, helping to keep the main-reactor temperature below 540 C.

Zubrin, Robert↗
DOE OSTI2017

What Hinders Electron Transfer Dissociation (ETD) of DNA Cations?

Radical activation methods, such as electron transfer dissociation (ETD), produce structural information complementary to collision-induced dissociation. Herein, electron transfer dissociation of 3-fold protonated DNA hexamers was studied to gain insight into the fragmentation mechanism. The fragmentation patterns of a large set of DNA hexamers confirm cytosine as the primary target of electron transfer. The reported data reveal backbone cleavage by internal electron transfer from the nucleobase to the phosphate linker leading either to a•/w or d/z• ion pairs. This reaction pathway contrasts with previous findings on the dissociation processes after electron capture by DNA cations, suggesting multiple, parallel dissociation channels. However, all these channels merely result in partial fragmentation of the precursor ion because the charge-reduced DNA radical cations are quite stable. Two hypotheses are put forward to explain the low dissociation yield of DNA radical cations: it is either attributed to non-covalent interactions between complementary fragments or to the stabilization of the unpaired electron in stacked nucleobases. MS{sup 3} experiments suggest that the charge-reduced species is the intact oligonucleotide. Moreover, introducing abasic sites significantly increases the dissociation yield of DNA cations. Consequently, the stabilization of the unpaired electron by π–π-stacking provides an appropriate rationale for the high intensity of DNA radical cations after electron transfer. .

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Dissociation behavior of protons incorporated in yttrium doped barium zirconate

The dissociation behavior of protons incorporated in yttrium-doped barium zirconate (BZY20) was investigated via combination of in-situ diffuse reflectance Fourier transform infrared spectroscopy (FT-IR) and evolved gas analysis. The vicinities of dopant or host ions were considered as sites of proton dissolution. The temperature dependence of the dissociation of protons from each site was evaluated. Protons which were considered to have dissolved in the vicinity of dopant ions were easily removed and dissociated as water. On the other hand, those which dissolved in the vicinity of host ions were dissociated as hydrogen at high temperatures. Such dissociation behavior was also detected after using D{sub 2}O saturated gas for pre-treatment. These results suggest that some protons which dissolved in the perovskite oxide could dissociate as hydrogen without forming oxygen vacancies. (131words) - Graphical abstract: Different dissociation reactions of protons from yttria doped barium zirconate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Effects of Field Strength and Silver Nanowire Size on Plasmon-Enhanced N 2 Dissociation

Dissociation of the nitrogen molecule via plasmon-enhanced catalysis using noble metal nanoparticles has been investigated both experimentally and computationally in recent years. However, the mechanism of plasmon-enhanced nitrogen dissociation is still not very clear. In this work, we apply theoretical approaches to examine the dissociation of a nitrogen molecule on atomically-thin Ag n nanowires (n = 6, 8, 10, 12) and a Ag 19 + nanorod. Ehrenfest dynamics provides information about the motion of nuclei during the dynamics process and real-time TDDFT calculations show the electronic transitions and population of electrons over the first 10s of fs time scale. The activation and dissociation of nitrogen is typically enhanced when the electric field strength increases. However, the enhancement is not always monotonic with field strength. As the length of the Ag wire increases, nitrogen is typically easier to dissociate and thus requires lower field strengths, even though the plasmon frequency is lower. The Ag 19 + nanorod leads to faster dissociation of N 2 than the atomically-thin nanowires. Altogether, our detailed study yields insights into the mechanisms involved in plasmon-enhanced N 2 dissociation, as well as provides information about factors that can be used to improve adsorbate activation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Investigating the Elusive Nature of Atomic O from CO 2 Dissociation on Pd(111): The Role of Surface Hydrogen

CO 2 dissociation is a key step in CO 2 conversion reactions to produce value-added chemicals typically through hydrogenation. In many cases, the atomic O produced from CO 2 dissociation can potentially block adsorption sites or change the oxidation state of the catalyst. Here, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations to investigate the presence of surface species from the dissociation of CO 2 on Pd(111). AP-XPS results show that CO 2 was dissociated to produce adsorbed CO, but dissociated atomic O was not observed at room temperature. We were only able to observe atomic O when CO 2 was introduced at 500 K. Further investigations of O-covered Pd(111) revealed that chemisorbed O could be easily removed by low pressures of CO and H 2 . Notably, the effect of H 2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 × 10 -9 Torr, and the presence of H2 at ambient pressure prevented CO 2 dissociation. DFT calculations showed that in the presence of background H 2 , facile CO 2 dissociation took place via the reverse water–gas shift (rWGS) reaction, which resulted in the formation of adsorbed CO and removal of O by H 2 . DFT also identified the possible variation of surface species on simultaneous exposure of CO 2 and H 2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO 2 hydrogenation catalysis by controlling the reaction conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Challenges in Transitioning Cocrystals from Bench to Bedside: Dissociation in Prototype Drug Product Environment

We report ablets containing a theophylline–glutaric acid (TG) cocrystal dissociated rapidly forming crystalline theophylline (20–30%), following storage at 40 °C/75% RH for 2 weeks. Control tablets of TG cocrystal containing no excipients were stable under the same conditions. The dissociation reaction was water-mediated, and the theophylline concentration (the dissociation product), monitored by synchrotron X-ray diffractometry, was strongly influenced by the formulation composition. Investigation of the binary compacts of the TG cocrystal with each excipient revealed the influence of excipient properties (hydrophilicity, ionizability) on cocrystal stability, providing mechanistic insights into a dissociation reaction. Ionizable excipients with a strong tendency to sorb water, for example, sodium starch glycolate and croscarmellose sodium, caused pronounced dissociation. Microcrystalline cellulose (MCC), while a neutral but hydrophilic excipient, also enabled solution-mediated cocrystal dissociation in intact tablets. Magnesium stearate, an ionizable but hydrophobic excipient, interacted with the cocrystal to form a hygroscopic product. The interaction is believed to be initiated in the disordered cocrystal–excipient particle interface. In contrast, the cocrystal was stable in the presence of lactose, a neutral excipient with no tendency to sorb water. The risk of unintended cocrystal dissociation can be mitigated by avoiding contact with water both during processing and storage.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2011

Does Nitric Acid Dissociate at the Aqueous Solution Surface?

Nitric acid is a prevalent component of atmospheric aerosols, and the extent of nitric acid dissociation at aqueous interfaces is relevant to its role in heterogeneous atmospheric chemistry. Several experimental and theoretical studies have suggested that the extent of dissociation of nitric acid near aqueous interfaces is less than in bulk solution. Here, dissociation of HNO3 at the surface of aqueous nitric acid is quantified using X-ray photoelectron spectroscopy of the nitrogen local electronic structure. The relative amounts of undissociated HNO3(aq) and dissociated NO3-(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we determine the degree of dissociation, αint, in the interface (the first ~3 layers of solution) as a function of HNO3 concentration. Our measurements show that dissociation is decreased by approximately 20% near the solution interface compared with bulk, and furthermore that dissociation occurs even in the top-most solution layer. The experimental results are supported by first-principles MD simulations, which show that hydrogen-bonds between HNO3 and water molecules at the solution surface stabilize the molecular form at low concentration, in analogy to the stabilization of molecular HNO3 that occurs in bulk solution at high concentration. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

54 ENVIRONMENTAL SCIENCES↗