Reactive transport modelling as a toolbox to compare remediation strategies for aquifers impacted by uranium in situ recovery.

More than 60% of worldwide uranium production is based on the In Situ Recovery mining technique. This exploitation method directly falls within the scope of the applications of reactive transport modelling to optimize uranium production and limit its associated environmental impact. We propose a modelling approach which is able to represent the natural evolution of an aquifer impacted by an ISR test performed using sulfuric acid. The model is calibrated on a 12 year-long data series obtained from 12 monitoring wells surrounding an ISR pilot cell. Through this process-based approach, we simulate the impact of several remediation strategies that can be considered in these contexts. In particular, we model the impact of Pump & Treat combined with reverse osmosis, as well as the circulation of non-impacted fluids through the reservoir with different operating strategies. Our approach allows to compare the effectiveness of these strategies. For this small-scale ISR pilot, monitored natural attenuation constitutes an interesting approach due to its faster pH recovery time with respect to Pump & Treat (5-10 years to pH ∼ 6), whose efficiency can be improved by the addition of exchangeable cations. Circulation of unimpacted fluids can reduce pH recovery times if performed for periods longer than the ISR exploitation and/or deployed with a delay. Combined with an economic evaluation of their deployment, this modelling approach can help the mining operator select and design optimal remediation strategies from an environmental and economical standpoint.

GaPO4 Single Crystals: Growth Condition by Hydrothermal Refluxing Method.

Bulk GaPO4 is an advanced piezoelectric material operating under high temperatures according to the α-ß phase transition at 970 °C. This work presents the technological development of a hydrothermal refluxing method first applied for GaPO4 single crystal growth. Crystals of 10-20 g were grown in mixtures of aqueous solutions of low- and high-vapor-pressure acids (H3PO4/HCl) at 180-240 °C (10-20 bars). The principal feature of the refluxing method is a spatial separation of crystal growth and nutrient dissolution zones. This leads to a constant mass transportation of the dissolved nutrient, even for materials with retrograde solubility. Mass transport is carried out by dissolution of GaPO4 nutrient in a dropping flow of condensed low-vapor-pressure solvent. This method allows an exact saturation at temperature of equilibrium and avoids spontaneous crystallization as well loss of seeds. Grown crystals have a moderate OH- content and reasonable structural uniformity. Moreover, the hydrothermal refluxing method allows a fine defining of GaPO4 concentration in aqueous solutions of H3PO4, H2SO4, HCl and their mixtures at set T-P parameters (T < 250 °C, p = 10-30 bars). The proposed method is relatively simple to use, highly reproducible for crystal growth of GaPO4 and probably could applied to other compounds with retrograde solubility.

Assessment of electrocatalytic hydroxylase activity of cytochrome P450 3A4 (CYP3A4) by means of derivatization of 6ß-hydroxycortisol by sulfuric acid for fluorimetric assay.

We used rapid one-step derivatization of 6ß-hydroxylated hydrocortisone by sulfuric acid for fluorimetric determination of CYP3A4-dependent hydroxylase reaction in the electrochemical system. We have shown that CYP3A4 substrate - hydrocortisone - and its 6ß-hydroxylated product have different emission wavelengths at an excitation λex = 365 nm after treatment with sulfuric acid:ethanol (3:1) mixture (λem = 525 ±â€¯2 nm and λem = 427 ±â€¯2 nm, respectively). The detection limit for 6ß-hydroxycortisol was estimated to be 0.32 µM (corresponding to 0.095 nmol in 300 µL sample) (S/N = 3). Using the fluorimetric method of 6ß-hydroxycortisol detection following the electrolysis of hydrocortisone with CYP3A4 immobilized on a screen-printed graphite electrode modified by didodecyldimethylammonium bromide we have calculated the steady-state kinetic parameters of CYP3A4 for hydrocortisone: the maximal rate of the reaction (Vmax) as 89 ±â€¯5 pmol of product per min per pmol of electroactive enzyme and the Michaelis constant (KM) as 10 ±â€¯2 µM. In our system, ketoconazole inhibited hydroxylase activity of CYP3A4 towards hydrocortisone with the IC50 value of 70 ±â€¯5 nM. The approach proposed for determination of the CYP3A4 electrocatalytic activity can be used for throughput screening of different modulators of this cytochrome P450 isozyme during drug development.

Optimization of metals extraction from spent lithium-ion batteries by sulphuric acid and sodium metabisulphite through a techno-economic evaluation.

The main factors that affect the extraction of metals from spent lithium-ion batteries by acid leaching using H2SO4, and sodium metabisulphite, were evaluated and optimized through a set of experiments, framed by a techno-economic approach. The maximum value of the profit response was obtained with the highest possible values of acid concentration (2.5 M) and time (2 h), a liquid/solid ratio of 5 L/kg, and the lowest possible value of temperature (40 °C). After leaching, the electrodes active material contained in the metals decreased, while it was still significant in the graphite, as observed by scanning electron microscopy-energy dispersive spectrometry and x-ray powder diffraction. Even though the performed economic evaluation was a summarized outline it can be considered suitable to compare different leaching conditions and to determine the possible best combinations of factors that can optimize the profit response.

A critical assessment of transmethylation procedures for n-3 long-chain polyunsaturated fatty acid quantification of lipid classes.

Lipid transmethylation methods described in the literature are not always evaluated with care so to insure that the methods are effective, especially on food matrix or biological samples containing polyunsaturated fatty acid (PUFA). The aim of the present study was to select a method suitable for all lipid species rich in long chain n-3 PUFA. Three published methods were adapted and applied on individual lipid classes. Lipid (trans)methylation efficiency was characterized in terms of reaction yield and gas chromatography (GC) analysis. The acid-catalyzed method was unable to convert triglycerides and sterol esters, while the method using an incubation at a moderate temperature was ineffective on phospholipids and sterol esters. On the whole only the method using sodium methoxide and sulfuric acid was effective on lipid classes taken individually or in a complex medium. This study highlighted the use of an appropriate (trans)methylation method for insuring an accurate fatty acid composition.

Assessment of the Effects Exerted by Acid and Alkaline Solutions on Bone: Is Chemistry the Answer?

The treatment of corpses with extremely acid or basic liquids is sometimes performed in criminal contexts. A thorough characterization by chemical analysis may provide further help to macroscopic and microscopic analysis; 63 porcine bone samples were treated with solutions at different pH (1-14) for immersion periods up to 70 days, as well as in extremely acidic sulfuric acid solutions (9 M/18 M) and extremely basic sodium hydroxide. Inductively coupled optical emission spectrometry (ICP-OES)/plasma mass spectrometry (ICP-MS), Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRPD), and scanning electron microscopy (SEM) showed that only the sulfuric acid solution 18 M was able to completely dissolve the sample. In addition, chemical analysis allowed to recognize the contact between bone and substances. Hydrated calcium sulfate arose from extreme pH. The possibility of detecting the presence of human material within the residual solution was demonstrated, especially with FT-IR, ICP-OES, and EDX.

Toxicity assessment and feasible recycling process for amorphous silicon and CIS waste photovoltaic panels.

End-of-Life (EoL) photovoltaic (P/V) modules, which are recently included in the 2012/19/EU recast, require sound and sustainable treatment. Under this perspective, this paper deals with 2nd generation P/V waste modules, known as thin-film, via applying chemical treatment techniques. Two different types of modules are examined: (i) tandem a-Si:H/µc-Si:H panel and, (ii) Copper-Indium-Selenide (CIS) panel. Panels' pretreatment includes collection, manual dismantling and shredding; pulverization and digestion are further conducted to identify their chemical composition. A variety of elements is determined in the samples leachates' after both microwave-assisted total digestion and Toxicity Characteristic Leaching Procedure (TCLP test) using Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) analysis. The analysis reveals that several elements are detected in the two of panels, with no sample exceeds the TCLP test. Concentrations of precious and critical metals are also measured, which generates great incentives for recovery. Then, further experiments, for P/V recycling investigation, are presented using different acids or acid mixtures under a variety of temperatures and a stable S/L ratio, with or without agitation, in order to determine the optimal recycling conditions. The results verify that chemical treatment in P/V shredded samples is efficient since driving to ethylene-vinyl acetate (EVA) resin's dissolution, as well as valuable structural materials recovery (P/V glass, ribbons, cells, P/V intermediate layers). Among the solvents used, sulfuric acid and lactic acid demonstrate the most efficient and strongest performance on panels' treatment at gentle temperatures providing favorably low energy requirements.

Fermentative production of high titer gluconic and xylonic acids from corn stover feedstock by Gluconobacter oxydans and techno-economic analysis.

High titer gluconic acid and xylonic acid were simultaneously fermented by Gluconobacter oxydans DSM 2003 using corn stover feedstock after dry dilute sulfuric acid pretreatment, biodetoxification and high solids content hydrolysis. Maximum sodium gluconate and xylonate were produced at the titer of 132.46g/L and 38.86g/L with the overall yield of 97.12% from glucose and 90.02% from xylose, respectively. The drawbacks of filamentous fungus Aspergillus niger including weak inhibitor tolerance, large pellet formation and no xylose utilization were solved by using the bacterium strain G. oxydans. The obtained sodium gluconate/xylonate product was highly competitive as cement retarder additive to the commercial product from corn feedstock. The techno-economic analysis (TEA) based on the Aspen Plus modeling was performed and the minimum sodium gluconate/xylonate product selling price (MGSP) was calculated as $0.404/kg. This study provided a practical and economic competitive process of lignocellulose utilization for production of value-added biobased chemicals.

Novel integrated mechanical biological chemical treatment (MBCT) systems for the production of levulinic acid from fraction of municipal solid waste: A comprehensive techno-economic analysis.

This paper, for the first time, reports integrated conceptual MBCT/biorefinery systems for unlocking the value of organics in municipal solid waste (MSW) through the production of levulinic acid (LA by 5wt%) that increases the economic margin by 110-150%. After mechanical separation recovering recyclables, metals (iron, aluminium, copper) and refuse derived fuel (RDF), lignocelluloses from remaining MSW are extracted by supercritical-water for chemical valorisation, comprising hydrolysis in 2wt% dilute H2SO4 catalyst producing LA, furfural, formic acid (FA), via C5/C6 sugar extraction, in plug flow (210-230°C, 25bar, 12s) and continuous stirred tank (195-215°C, 14bar, 20min) reactors; char separation and LA extraction/purification by methyl isobutyl ketone solvent; acid/solvent and by-product recovery. The by-product and pulping effluents are anaerobically digested into biogas and fertiliser. Produced biogas (6.4MWh/t), RDF (5.4MWh/t), char (4.5MWh/t) are combusted, heat recovered into steam generation in boiler (efficiency: 80%); on-site heat/steam demand is met; balance of steam is expanded into electricity in steam turbines (efficiency: 35%).

Recovery of cobalt from spent lithium-ion batteries using supercritical carbon dioxide extraction.

Continuing technological development decreases the useful lifetime of electronic equipment, resulting in the generation of waste and the need for new and more efficient recycling processes. The objective of this work is to study the effectiveness of supercritical fluids for the leaching of cobalt contained in lithium-ion batteries (LIBs). For comparative purposes, leaching tests are performed with supercritical CO2 and co-solvents, as well as under conventional conditions. In both cases, sulfuric acid and H2O2 are used as reagents. The solution obtained from the supercritical leaching is processed using electrowinning in order to recover the cobalt. The results show that at atmospheric pressure, cobalt leaching is favored by increasing the amount of H2O2 (from 0 to 8% v/v). The use of supercritical conditions enable extraction of more than 95wt% of the cobalt, with reduction of the reaction time from 60min (the time employed in leaching at atmospheric pressure) to 5min, and a reduction in the concentration of H2O2 required from 8 to 4% (v/v). Electrowinning using a leach solution achieve a current efficiency of 96% and a deposit with cobalt concentration of 99.5wt%.

Quantitation of sugar content in pyrolysis liquids after acid hydrolysis using high-performance liquid chromatography without neutralization.

A rapid method for the quantitation of total sugars in pyrolysis liquids using high-performance liquid chromatography (HPLC) was developed. The method avoids the tedious and time-consuming sample preparation required by current analytical methods. It is possible to directly analyze hydrolyzed pyrolysis liquids, bypassing the neutralization step usually required in determination of total sugars. A comparison with traditional methods was used to determine the validity of the results. The calibration curve coefficient of determination on all standard compounds was >0.999 using a refractive index detector. The relative standard deviation for the new method was 1.13%. The spiked sugar recoveries on the pyrolysis liquid samples were between 104 and 105%. The research demonstrates that it is possible to obtain excellent accuracy and efficiency using HPLC to quantitate glucose after acid hydrolysis of polymeric and oligomeric sugars found in fast pyrolysis bio-oils without neutralization.

A high-performance direct transmethylation method for total fatty acids assessment in biological and foodstuff samples.

Isolation is the main bottleneck in the analysis of fatty acids in biological samples and foods. In the last few decades some methods described direct derivatization procedures bypassing these steps. They involve the utilization of methanolic HCL or BF3 as catalysts, but several evidences from previous works suggest these reagents are unstable, lead to the formation of artifacts and alter the distribution of specific compounds as hydroxy fatty acids or CLA. However, the main issue is that they are excellent esterification reagents but poor in transterification, being not suitable for the analysis of all lipid classes and leading to erroneous composition quantitations. The present research work is a comprehensive comparison of six general methylation protocols using base, acid or base/acid catalysts plus a proposed method in the analysis of total fatty acids in lipid standards mixtures, foodstuff and biological samples. The addition of aprotic solvents to the reaction mixture to avoid alterations was also tested. Results confirmed that procedures solely involving acid catalyst resulted in incomplete derivatizations and alteration of the fatty acid profile, partially corrected by addition of the aprotic solvent. The proposed method combining sodium methoxyde and sulfuric acid showed absence of alteration of the FAME profile and the best values for response factors (short chain fatty acids to PUFA), accuracy in the determination of total cholesterol and derivatization performance, thus showing a high reliability in the determination of the total fatty acid composition in biological samples and foods.

Formation and growth of molecular clusters containing sulfuric acid, water, ammonia, and dimethylamine.

The structures and thermochemistry of molecular clusters containing sulfuric acid, water, ammonia, and/or dimethylamine ((CH3)2NH or DMA) are explored using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Clusters are of the general form [(BH(+))n(HSO4(-))n(H2O)y], where B = NH3 or DMA, 2 ≤ n ≤ 8, and 0 ≤ y ≤ 10. Cluster formulas are written based on the computed structures, which uniformly show proton transfer from each sulfuric acid molecule to a base molecule while the water molecules remain un-ionized. Cluster formation is energetically favorable, owing to strong electrostatic attraction among the ions. Water has a minor effect on the energetics of cluster formation, lowering the free energy of formation by ∼ 10% depending on the cluster size and number of water molecules. Cluster growth (addition of one base molecule and one sulfuric acid molecule to a pre-existing cluster) and base substitution (substituting DMA for ammonia) are also energetically favorable processes for both anhydrous and hydrated clusters. However, the effect of water is different for different bases. Hydrated ammonium bisulfate clusters have a more favorable free energy for growth (i.e., incrementing n with fixed y) than anhydrous clusters, while the reverse is observed for dimethylammonium bisulfate clusters, where the free energy for growth is more favorable for anhydrous clusters. The substitution of DMA for ammonia in bisulfate clusters is favorable but exhibits a complex water dependence. Base substitution in smaller bisulfate clusters is enhanced by the presence of water, while base substitution in larger bisulfate clusters is less favorable for hydrated clusters than that for anhydrous clusters. While DMA substitution can stabilize small clusters containing one or a few sulfuric acid molecules, the free energy advantage of forming amine clusters relative to ammonia clusters becomes less pronounced at larger sizes, especially when the effect of water is considered.

Effect of temperature on the low-linear energy transfer radiolysis of the ceric-cerous sulfate dosimeter: a Monte Carlo simulation study.

The stochastic modeling of the (60)Co γ/fast-electron radiolysis of the ceric-cerous chemical dosimeter has been performed as a function of temperature from 25-350°C. The system used is a dilute solution of ceric sulfate and cerous sulfate in aqueous 0.4 M sulfuric acid. In this system, H(•) (or HO2(•) in the presence of dissolved oxygen) and H2O2 produced by the radiolytic decomposition of water both reduce Ce(4+) ions to Ce(3+) ions, while (•)OH radicals oxidize the Ce(3+) present in the solution back to Ce(4+). The net Ce(3+) yield is given by G(Ce(3+)) = g(H(•)) + 2 g(H2O2) - g((•)OH), where the primary (or "escape") yields of H(•), H2O2 and (•)OH are represented by lower case g's. At room temperature, G(Ce(3+)) has been established to be 2.44 ± 0.8 molecules/100 eV. In this work, we investigated the effect of temperature on the yield of Ce(3+) and on the underlying chemical reaction kinetics using Monte Carlo track chemistry simulations. The simulations showed that G(Ce(3+)) is time dependent, a result of the differences in the lifetimes of the reactions that make up the radiolysis mechanism. Calculated G(Ce(3+)) values were found to decrease almost linearly with increasing temperature up to about 250°C, and are in excellent agreement with available experimental data. In particular, our calculations confirmed previous estimated values by Katsumura et al. (Radiat Phys Chem 1988; 32:259-63) showing that G(Ce(3+)) at ∼250°C is about one third of its value at room temperature. Above ∼250°C, our model predicted that G(Ce(3+)) would drop markedly with temperature until, instead of Ce(4+) reduction, Ce(3+) oxidation is observed. This drop is shown to occur as a result of the reaction of hydrogen atoms with water in the homogeneous chemical stage.

A theoretical study of hydrated molecular clusters of amines and dicarboxylic acids.

Amines and carboxylic acids have been recognized as important precursor species in atmospheric new particle formation. In this study, the interaction between dimethylamine and succinic acid is investigated using Basin Paving Monte Carlo (BPMC) sampling with the classical force field to obtain low energy conformers of dimethylamine and succinic acid hydrated molecular clusters. Geometry optimization and frequency calculations are further performed on the basis of the BPMC results using density functional theory. At standard temperature and pressure, dimethylamine binds to succinic acid with a bonding energy of 14.2 kcal mol(-1), smaller than that of dimethylamine with sulfuric acid (21.1 kcal mol(-1)). Hydration promotes proton transfer from succinic acid to dimethylamine and consequently increases the interaction strength, while proton transfer from sulfuric acid to dimethylamine occurs without hydration. On the other hand, the reactivity of sulfuric acid with dimethylamine decreases with the degree of hydration of sulfuric acid. The free energies of formation for hydrated clusters consisting of dimethylamine and succinic acid reveal that the interaction between amines and dicarboxylic acids likely exerts a synergetic effect on atmospheric aerosol nucleation by formation of aminium carboxylate ion pairs.

3-step approach towards evaluation and elimination of acid use in pre-treatment for a brackish water reverse osmosis process.

To control carbonate scale formation on reverse osmosis (RO) membrane surfaces, it is common practice to add acid as a pre-treatment chemical. However, advancements in the formulation of proprietary scale inhibitors have resulted in a re-evaluation of the need to suppress the pH of RO feedwater. In this study, a 3-step approach was taken to eliminate sulfuric acid pre-treatment at a 4.5 MGD (17,000 m(3)/day) brackish water RO plant operating for over 7 years without previous membrane replacement. The 3-Step approach adopted in this study to evaluate and eliminate use of acid in pre-treatment process involved first pilot testing the plan to reduce the dependence on acid. Secondly, implementing the plan on the full-scale system with conservative pH increments and thirdly continuously screening for scale formation potential using a "canary" monitoring device. This 3-step approach resulted in the successful elimination of sulfuric acid pre-treatment at the brackish water RO plant, with an estimated $105,000 minimum annual cost savings.

Influence of twin-screw extrusion on soluble arabinoxylans and corn fiber gum from corn fiber.

BACKGROUND: The effect of feed moisture content and screw speed in the extrusion process with and without chemical pretreatment of corn fiber was investigated. Different chemical pretreatment methods (NaOH and H2 SO4 solution) were compared. The improvement of reducing sugar, soluble arabinoxylans (SAX) content and the yield of corn fiber gum was measured. RESULTS: A high reducing sugar content was obtained in the filtrate fraction from the extruded destarched corn fiber (EDCF) with H2SO4 pretreatment. Feed moisture content most effectively improved both reducing sugar and SAX content of filtrate. Increasing feed moisture content and screw speed resulted in a higher SAX content in the filtrate of the EDCF with NaOH pretreatment. The SAX content of the residual solid from the EDCF with NaOH pretreatment was higher compared to H2SO4 pretreated and unpretreated samples and significantly increased with decreasing feed moisture content. The screw speed did not have a major impact after enzyme hydrolysis. The yield of corn fiber gum was increased by 12% using NaOH pretreatment combined with extrusion process as compared to the destarched corn fiber. CONCLUSION: The results show the great potential of the extrusion process as an effective pretreatment for disruption the lignocelluloses of corn fiber, leading to conversion of cellulose to glucose and hemicelluloses to SAX and isolation of corn fiber gum.

Decontamination of metals and polycyclic aromatic hydrocarbons from slag-polluted soil.

Metallurgy is an industrial activity that is one of the largest contributors to soil contamination by metals. This contamination is often associated with organic compound contamination; however, little research has been aimed at the development of simultaneous processes for decontamination as opposed to treatments to heavy metals or organic compounds alone. This paper presents an efficient process to decontaminate the soils polluted with smelting by-products rich in Cu, Zn and polycyclic aromatic hydrocarbons (PAHs). A simultaneous treatment for metals and PAHs was also tested. The process is mainly based on physical techniques, such as crushing, gravimetric separation and attrition. For the finest particle size fractions, an acid extraction with H2SO4 was used to remove metals. The PAH removal was enhanced by adding surfactant during attrition. The total metal removals varied from 49% to 73% for Cu and from 43% to 63% for Zn, whereas a removal yield of 92% was measured for total PAHs. Finally, a technical-economic evaluation was done for the two processes tested.

Quantitative assessment of the sulfuric acid contribution to new particle growth.

The Nano Aerosol Mass Spectrometer (NAMS) was deployed to rural/coastal and urban sites to measure the composition of 20-25 nm diameter nanoparticles during new particle formation (NPF). NAMS provides a quantitative measure of the elemental composition of individual, size-selected nanoparticles. In both environments, particles analyzed during NPF were found to be enhanced in elements associated with inorganic species (nitrogen, sulfur) relative to that associated with organic species (carbon). A molecular apportionment algorithm was applied to the elemental data in order to place the elemental composition into a molecular context. These measurements show that sulfate constitutes a substantial fraction of total particle mass in both environments. The contribution of sulfuric acid to new particle growth was quantitatively determined and the gas-phase sulfuric acid concentration required to incorporate the measured sulfate fraction was calculated. The calculated values were compared to those calculated by a sulfuric acid proxy that considers solar radiation and SO(2) levels. The two values agree within experimental uncertainty. Sulfate accounts for 29-46% of the total mass growth of particles. Other species contributing to growth include ammonium, nitrate, and organics. For each location, the relative amounts of these species do not change significantly with growth rate. However, for the coastal location, sulfate contribution increases with increasing temperature whereas nitrate contribution decreases with increasing temperature.

Structure and energetics of nanometer size clusters of sulfuric acid with ammonia and dimethylamine.

The structures of positively and negatively charged clusters of sulfuric acid with ammonia and/or dimethylamine ((CH(3))(2)NH or DMA) are investigated using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Positively charged clusters of the formula [(NH(4)(+))(x)(HSO(4)(-))(y)](+), where x = y + 1, are studied for 1 ≤ y ≤ 10. These clusters exhibit strong cation-anion interactions, with no contribution to the hydrogen-bonding network from the bisulfate ion protons. A similar hydrogen-bonding network is found for the [(DMAH(+))(5)(HSO(4)(-))(4)](-) cluster. Negatively charged clusters derived from the reaction of DMA with [(H(2)SO(4))(3)(NH(4)(+))(HSO(4)(-))(2)](-) are also studied, up to the fully reacted cluster [(DMAH(+))(4)(HSO(4)(-))(5)](-). These clusters exhibit anion-anion and ion-molecule interactions in addition to cation-anion interactions. While the hydrogen-bonding network is extensive for both positively and negatively charged clusters, the binding energies of ions and molecules in these clusters are determined mostly by electrostatic interactions. The thermodynamics of amine substitution is explored and compared to experimental thermodynamic and kinetic data. Ammonia binds more strongly than DMA to sulfuric acid due to its greater participation in hydrogen bonding and its ability to form a more compact structure that increases electrostatic attraction between oppositely charged ions. However, the greater gas-phase basicity of DMA is sufficient to overcome the stronger binding of ammonia, making substitution of DMA for ammonia thermodynamically favorable. For small clusters of both polarities, substitutions of surface ammonium ions are facile. As the cluster size increases, an ammonium ion becomes encapsulated in the center of the cluster, making it inaccessible to substitution.