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Molecular-level insight into the dissociation of nitric acid in water is obtained from X-ray photoelectron spectroscopy and first-principles molecular dynamics (MD) simulations. Our combined studies reveal surprisingly abrupt changes in solvation configurations of undissociated nitric acid at approximately 4 M concentration. Experimentally, this is inferred from shifts of the N1s binding energy of HNO(3)(aq) as a function of concentration and is associated with variations in the local electronic structure of the nitrogen atom. It also shows up as a discontinuity in the degree of dissociation as a function of concentration, determined here from the N1s photoelectron signal intensity, which can be separately quantified for undissociated HNO(3)(aq) and dissociated NO(3)(-)(aq). Intermolecular interactions within the nitric acid solution are discussed on the basis of MD simulations, which reveal that molecular HNO(3) interacts remarkably weakly with solvating water molecules at low concentration; around 4 M there is a turnover to a more structured solvation shell, accompanied by an increase in hydrogen bonding between HNO(3) and water. We suggest that the driving force behind the more structured solvent configuration of HNO(3) is the overlap of nitric acid solvent shells that sets in around 4 M concentration.

Aqueous amine-based solutions are the cheapest currently employed solvents to capture CO 2 emitted into the atmosphere from industrial processes, as occurring for instance in coal-operated power plants.[1] CO 2 capture involves flowing flue gas through an aqueous solution containing a relatively nontoxic compound such as monoethanolamine (MEA) that reacts selectively with CO 2 . The technique has been used in the chemical industry for more than 60 years, [2] and will likely continue to play an important role for some time. However, the overall process is costly because of the energy required for CO 2 recovery, [1b] and poses several technical problems, for example, solvent disposal. [1b, 3] Moreover, the properties that are essential for efficient capture are not well understood, [4] and there is an especially high demand to better understand the CO 2 -amine solution interactions at the molecular level. This will be of great relevance for the development of more energy-efficient processes and the design of capture solutions. The reactivity of the ternary mixture MEA + H 2 O + CO 2 , including the complex chemical equilibria between the various species that are formed in solution at ambient conditions, has been the subject of intensive research. Several kinetic models and reaction mechanisms have been proposed, [1a, 5] and thermodynamic properties determined. [1d, 4] Surprisingly, very few studies have explicitly addressed the role of the solution-gas interface, although it is of key importance for properly modeling absorption kinetics. This fact has motivated the present experimental study of the electronic structure and composition of the aqueous MEA and CO 2 -treated MEA gas-solution interface. Our experiments show that neutral MEA exhibits a relatively enhanced concentration at the surface of the aqueous solution, whereas the products of the reaction of MEA with CO 2 , carbamate and carbamic acid, show a preference for the bulk solution. These observations indicate that a detailed understanding of the behavior of CO 2 at the liquid/vapor interface and the interface-to-bulk transport of the products will be important for understanding CO 2 capture.We report soft-X-ray photoelectron spectroscopy (PE) measurements from a vacuum liquid microjet of aqueous solutions of pure MEA (HOC 2 H 4 NH 2 ) and CO 2 -treated MEA (CO 2 loading 0.24 mol mol À1 MEA), probing both the solution surface and into bulk solution. Surface-sensitive PE measurements require that the inelastic mean free path (IMFP) of the detected photoelectrons is minimal, which is the case when electron kinetic energies (eKEs) are near 60-100 eV. [6] We acquired N 1s and C 1s spectra from the solution surfaces with 500 and 378 eV photon energies, respectively, corresponding to around 90 eV eKE, where the IMFP is 10 , resulting in a very interface-sensitive experiment.[7] Higher photon energies up to 1017 eV for N 1s and 898 eV for C 1s were also applied and correspond to probing predominantly the bulk aqueous solution. [8] At low to mid CO 2 loading,...

In the work described here, the electronic structure of sulfuric acid in water is explored by liquid-jet photoelectron spectroscopy. From the S2p photoelectron spectra of H 2 SO 4 (aq), measured over a large concentration range and aided by previously reported HSO 4 − /SO 4 2and HSO 4 − /H 2 SO 4 concentration ratios in the bulk solution, we obtain detailed electronic structure information of each species. Comparing our results with previous studies on the dissociation of nitric acid, we argue that the solvation structure of H 2 SO 4 (aq) changes around 5−7 M concentration, at which point a dramatic change in both the HSO 4 − photoelectron peak width and binding energy occurs.

Photoelectron angular distributions (PADs) from aqueous solution surfaces reveal details on the spatial arrangement of solute molecules at the solution -gas-phase interface. This is demonstrated here for mixed equimolar aqueous solutions of dimethyl sulfoxide / dimethyl sulfone ((CH3)2SO / (CH3)2SO2), and dimethyl sulfoxide / dimethyl sulfite ((CH3)2SO / (CH3)2SO3, all molecules having a propensity to reside near the solution surface. Although the surface-active molecules coexist at the surface, (CH3)2SO2 yields a more intense sulfur 2p surface photoelectron signal than for (CH3)2SO, and for (CH3)2SO3) the effect is even larger. To understand this behavior we have for one of the solutions mixtures, (CH3)2SO / (CH3)2SO2, performed PAD measurements. Surprisingly, both molecules exhibit almost identical PADs implying that the emitted photoelectrons have experienced similar (limited) amount of scattering interactions. Hence, the molecules reside at the same distance with respect to the solution -vacuum interface rather than (CH3)2SO2 being closer to the surface than (CH3)2SO, as one may have assumed based on the relative photoelectron signal intensities. Instead, the relative surface and bulk concentrations of the two compounds differ. We also report S 2p photoelectron spectra from single-component dimethyl sulfide, (CH3)2S, aqueous solutions measured at a single detection angle. The exceptionally large surface propensity of (CH3)2S is recognized by a narrow, gas-phase-like photoelectron spectrum revealing that (CH3)2S experiences very little hydration interactions. Experimentally observed trends in surface activity for the different molecules, which are complemented here by molecular dynamics simulations, agree with findings obtained with other surface sensitive techniques. New information on the surface structure of mixed solutions is uniquely obtained from the anisotropic angular distributions of the photoelectrons.