Dissociation of Strong Acid Revisited: X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations of HNO₃ in Water

Abstract

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.

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

“…25 Both theory and experiment suggest that nitric acid is not fully dissociated at the interface of aqueous solutions. [26][27][28][29][30][31][32][33][34] Indeed, recent results suggest that undissociated HNO 3 interacts very weakly with water. 33,34 Because of the presence of significant concentrations of water vapor in the lower atmosphere, there is an abundance of water available on surfaces such as roads, buildings, etc., as well as in airborne particles.…”

“…[26][27][28][29][30][31][32][33][34] Indeed, recent results suggest that undissociated HNO 3 interacts very weakly with water. 33,34 Because of the presence of significant concentrations of water vapor in the lower atmosphere, there is an abundance of water available on surfaces such as roads, buildings, etc., as well as in airborne particles. [35][36][37] Surfaces in the tropospheric boundary layer also hold a variety of adsorbed organic compounds.…”

“…The fact that HNO 3 intercalates in this manner is consistent with the reported interactions of HNO 3 with CCl 4 43 and with condensed phase organics, 4 and with the recent proposal that undissociated HNO 3 interacts weakly with water. 33,34 C. Molecular dynamics simulations of the interaction of gaseous HNO 3 and water vapor with a self-assembled monolayer Fig. 8 shows density profiles of HNO 3 and water (if present) as a function of distance (z) away from the substrate.…”

Ion Spatial Distributions at the Air– and Vacuum–Aqueous K2CO3 Interfaces

“…In this regard, three prominent analytical techniques have recently proved noteworthy complements to the well established macroscopic techniques of surface tension [8][9] and surface potential [10][11] for the study of air (vacuum)-water interfaces: second harmonic generation (SHG), [12][13][14][15][16][17][18][19][20][21][22] sum-frequency generation (SFG) spectroscopy [23][24][25][26][27][28][29][30][31] and liquid based X-ray photoelectron spectroscopy (XPS). [32][33][34][35][36][37][38][39][40][41][42][43][44][45] All three of these methods are capable of interrogating the microscopic structure of the air (vacuum)-water interface, and often provide complementary information due to the different properties probed. For instance, SFG is a second order non-linear vibrational spectroscopy typically used to investigate the fundamental OH stretching region (between 3100−3500 cm −1 ), which provides detailed information on the structure and orientation of water within the non-centrosymmetric region at the interface.…”

Dissociative Adsorption of Nitric Acid at the Surface of Amorphous Solid Water Revealed by X-ray Absorption Spectroscopy

“…27−31 At the surface of nitric acid solutions, the ionic dissociation of HNO 3 was reported to be inhibited compared to the bulk, 1−11 an effect that was attributed to incomplete solvation at aqueous interfaces. Generally speaking, the stability of molecular nitric acid with respect to ionic dissociation appears to be promoted by increasing temperatures 9,32 and by higher HNO 3 concentrations, 9−11 both in the bulk of nitric acid solutions 11,32 as well as at their surface. 10,11 However, the extent of this weaker acidity is still being debated because it varies with these parameters as well as with the inherent surface specificity of the technique.…”