Measured Adsorption Enthalpies Research Articles

Molecular insights from theoretical calculations explain the differences in affinity and diffusion of airborne contaminants on surfaces of hBN and graphene

Exposed surfaces of two-dimensional (2D) materials are susceptible to the adsorption of various molecules including airborne contaminants, which can affect their performance in real applications. Hexagonal boron nitride (hBN) is structurally the closest relative to graphite and its single layer form to graphene. The adsorption of organic molecules to graphene was subject of extensive research, however, little is known about interaction ofadsorbates to hBN surface. We studied the affinity of organic molecules to the surface of hBN by inverse gas chromatography. The adsorption enthalpies of polar molecules including acetonitrile, nitromethane, ethanol, and acetone exhibited strong coverage dependency up to 20 % of a monolayer. Density functional theory and molecular dynamics calculations were employed to understand and interpret experimentally measured adsorption enthalpies. The calculations revealed that the strong affinity of polar molecules at low coverage was due to adsorption on steps and edges of hBN. The calculated surface diffusion barriers of all molecules were rather low, i.e., below 0.5 kcal/mol (except for benzene and cyclohexane), and molecules adsorbed on the surface behaved like a 2D gas. The results demonstrated that coupling inverse gas chromatography with computer simulations can provide vital insights into the mechanism of adsorption at the molecular level.

Silver as a capturing material for iodine released from lead\u2013bismuth eutectic in various conditions

The usage of silver as a filtering material for removal of iodine from the gas phase of a lead–bismuth eutectic based nuclear reactor was investigated in various atmospheres representing normal operation as well as accident conditions. Thermochromatography experiments were performed to quantify the retention experienced on a silver surface by iodine species evaporated from a lead–bismuth eutectic sample. Measured adsorption enthalpies ranged from −171 to − 208 kJ mol−1 with observed differences attributed to various surface effects rather than a change in iodine speciation. The postulated adsorption mechanism is chemisorption of iodine atoms on the silver surface. Metallic silver fulfills the desired criteria for a capturing material in water-free filtering systems to be used as an alternative to traditional alkaline scrubbers commonly used in LWR systems.

Relativistic Effects on the Properties of Lr: A Periodic DFT Study of the Adsorption of Lr on Surfaces of Ta in Comparison with Lu and Tl.

With the aim of finding out whether the predicted 7s27p1/2 ground-state electron configuration of Lr will have an effect on its reactivity, calculations of the adsorption properties of Lr(7s27p), its homologue Lu(6s25d), and the related p element Tl(6s26p) on the surface of Ta were performed using the relativistic periodic ADF BAND suite. The obtained adsorption energies, Eads(M), are in excellent agreement with the measured adsorption enthalpies of Lu and Tl, showing that Lr adsorbs on the surface of Ta similarly to Lu and much differently (215 kJ/mol more strongly) from Tl. An AO population analysis reveals that Lr interacts with the Ta surface preferentially via the 7s AO, with some participation of the 6d as well as 7p1/2 and 7p3/2 AOs. In contrast, Eads(Tl) is governed mainly by the 6p(Tl) AOs. Thus, the present investigations show that Lr should behave like Lu but not like the p element Tl on transition-metal surfaces.

Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite.

Understanding strength and nature of noncovalent binding to surfaces imposes significant challenge both for computations and experiments. We explored the adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas chromatography and theoretical calculations, providing new insights into the strength and nature of adsorption of small organic molecules on these surfaces. The measured adsorption enthalpies on fluorographite range from −7 to −13 kcal/mol and are by 1–2 kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity of organic adsorbates to fluorographene than to graphene. The dispersion-corrected functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86b-vdW) achieved the best agreement with the experimental data. Computations show that the adsorption enthalpies are controlled by the interaction energy, which is dominated by London dispersion forces (∼70%). The calculations also show that bonding to structural features, like edges and steps, as well as defects does not significantly increase the adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.

Immobilisation of lead(II) ions from aqueous solutions on natural coal

The immobilisation of lead(II) ions from aqueous solutions on natural coal was investigated to compare calculated and measured adsorption enthalpies. For this purpose, adsorption isotherms were measured at temperatures of 303, 333 and 353 K. Adsorption enthalpy ΔH was evaluated from temperature dependence of the equilibrium constant of adsorption using the van‘t Hoff equation. Thus, the value of ΔH = 27 kJ mol−1 was obtained manifesting endothermic effect of lead(II) immobilisation on the coal. However, based on the flow and immersion calorimetric measurements, the exothermic character of lead(II) adsorption on the studied coal was proven with a value of about −7 kJ mol−1.

Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry

The adsorption and desorption of the system CO/Pt(111) and C 6H 6/Pt(111) at 300 K has been investigated with a pulsed molecular beam method in combination with a microcalorimeter. For benzene the sticking probability has been measured in dependence of the coverage θ. For coverages θ > 0.8 transient adsorption is observed. From an analysis of the time-dependence of the molecular beam pulses the rate constant for desorption is determined to be 5.6 s − 1 . With a precursor-mediated kinetic adsorption model this allows to obtain also the hopping rate constant of 95.5 s − 1 . The measured adsorption enthalpies could be best described by (199 − 77 θ − 51 θ 2) kJ/mol, in good agreement with the literature values. For CO on Pt(111) also transient adsorption has been observed for θ > 0.95 at 300 K. The kinetic analysis yields rate constants for desorption and hopping of 20 s −1 and 51 s −1, respectively. The heats of adsorption show a linear dependence on coverage (131 − 38 θ) kJ/mol between 0 ≤ θ ≤ 0.3, which is consistent with the desorption data from the literature. For higher coverage (up to θ = 0.9ML) a slope of −63 kJ/mol describes the decrease of the differential heat of adsorption best. This result is only compatible with desorption experiments, if the pre-exponential factor decreases strongly at higher coverage. We found good agreement with recent quantum chemical calculations made for ( θ = 0.5ML).

Theoretical predictions of adsorption behavior of elements 112 and 114 and their homologs Hg and Pb

Fully relativistic (four-component) density-functional theory calculations were performed for elements 112 and 114 and their lighter homologs, Hg and Pb, interacting with gold systems, from an atom to a Au(n) cluster simulating the Au(111) surface. Convergence of the adatom-metal cluster binding energies E(b) with cluster size was reached for n>90. Hg, Pb, and element 114 were found to preferably adsorb at the bridge position, while element 112 was found to preferably adsorb at a hollow site. Independently of the cluster size, the trend in E(b) is Pb>>114>Hg>112. The obtained E(b) for Pb and element 112 are in good agreement with the measured adsorption enthalpies of these elements on gold, while the Hg value is obviously underestimated, confirming the observation that adsorption takes place not on the surface but in it. A comparison of chemical bonding in various systems shows that element 114 should be more reactive than element 112: A relative inertness of the latter is caused by the strong relativistic stabilization of the 7s atomic orbital. On the contrary, van der Waals bonding in element 114 systems should be weaker than in those of element 112 due to its larger radius.

Isothermal titration microcalorimetric studies of the effect of salt concentrations in the interaction between proteins and hydrophobic adsorbents

The effect of (NH 4) 2SO 4 concentrations on the interaction mechanism between myoglobin and two hydrophobic adsorbents, butyl- and octyl-Sepharose was investigated by the equilibrium binding analysis and by directly measured adsorption enthalpies. The result obtained from the isotherms demonstrated that the affinities of myoglobin adsorption onto the both adsorbents were increased with salt concentrations. Furthermore, the adsorption enthalpies measured by using isothermal titration calorimetry (ITC) were decreased with an increment of salt concentrations, and these findings were explained by the reduction in the dehydration heat and the enhancement of heat released by the hydrophobic interaction as salt concentrations increase. Additionally, the adsorption enthalpies of myoglobin with both the resin increased as the amount of bound protein, and the increment in the variation of enthalpy value at 0 M appeared to be steeper than that at 1.0 M (NH 4) 2SO 4. This result implies that protein–protein repulsion was stronger in the absence of the salt in these experiments. The thermodynamic parameters presented herein have important implication, both for providing further insight into the binding mechanism of protein adsorption and for improving theoretical approaches to HIC.

A “Site Creation” Model for Specific Adsorption of Aqueous Nucleobases, Nucleosides, and Nucleotides on Compost-Derived Humic Acid

Aqueous nucleic acid constituent solutes cytosine (1), cytidine (2), cytidine 5‘-monophosphate (3), thymine (4), thymidine (5), thymidine 5‘-monophosphate (6), uracil (7), uridine (8), uridine 5‘-monophosphate (9), adenine (10), adenosine (11), adenosine 5‘-monophosphate (12), guanosine (13), and guanosine 5‘-monophosphate (14) adsorb on compost-derived humic acid in sequential steps A, B, and C. Langmuir analysis of the isotherms gives a site capacity vi and equilibrium constant Ki for a given solute and adsorption step i that increases vA < vB < vC and decreases KA > KB > KC at 25.0 °C. Data at six temperatures in the range 10.0−40.0 °C indicate that structurally related solutes like 7, 8, and 9 adsorb through their nucleobase units. Adsorption of a solute in step A, B, or C can be endothermic, thermoneutral, or exothermic. This paper uses the site capacity vi at 25.0 °C, the average site capacity over the experimental temperature range 〈vi〉, the adsorption equilibrium constant Ki at 25.0 °C, and the adsorption enthalpy ΔHi and entropy ΔSi for each solute to propose a model consisting of four components for each step i = A or B or C: (a) solute adsorption on a specific HA site A, B, or C involved in respective step A, B, or C; (b) as a result of primary adsorption on site A or B or C for many solutes, freeing up or “creation” of more sites A or B or C, respectively, from interactions A ... X, Y, etc. with other humic acid (HA) structural components X, Y, etc.; (c) solute adsorption at the newly available sites A or B or C; and (d) site hydration by water. The model accounts for the wide range of measured adsorption enthalpies and entropies. Among several conclusions is that adenine has an outstanding ability to interact with A and B sites of compost-derived HA and that B sites are harder for other solutes to access than A and C sites. Humic acids change their dimensions and state of aggregation to suit their environment. It now appears that selectivity for a solute is related to HA's ability to make more adsorption sites available near the point of primary solute interaction.

Caffeine adsorption from aqueous solutions onto polymeric sorbents: The effect of surface chemistry on the adsorptive affinity and adsorption enthalpy

Measurements of the adsorption equilibria and the adsorption enthalpies are used to characterize the thermodynamics of caffeine adsorption from aqueous solutions onto three polymeric sorbents which vary in their ability to donate π electrons for adsorptive interactions. Measured adsorption enthalpies, by both the temperature dependence of the adsorption affinities and calorimetric methods, are in qualitative agreement and demonstrate that the sorbents with the highest adsorption affinity for caffeine also display the largest adsorption enthalpy. These observations support the contention that caffeine adsorption is driven predominantly by specific enthalpic interactions of the solute with the delocalized π electrons on the surface of the sorbents.

Calorimetric investigation of anion adsorption onto goethite

Titration calorimetry was employed to investigate adsorption of fluoride, iodate, phosphate, and salicylate anions onto goethite at constant temperature (25 o C), pH (4.0), ionic strength (0.05 M), and solid concentration (10 g/L). In addition, adsorption data were collected at three temperatures (10,25, and 40 o C) and proton stoichiometry values were monitored as well. Measured adsorption enthalpies were exothermic and decreased with increasing surface coverage for all anions