Energetic Origin of Proton Affinity to the Air/Water Interface

Abstract

Recent experimental and theoretical studies showed the preference of the hydronium ion for the vapor/water interface. To investigate the mechanism responsible for the surface propensity of this ion, we performed a series of novel quantum chemical simulations combined with the theory of solutions. The solvation free energy of the H(3)O(+) solute placed at the interface was obtained as -97.9 kcal/mol, being more stable by 3.6 kcal/mol than that of the solute embedded in the bulk. Further, we decomposed the solvation free energies into contributions from the water molecules residing in the oxygen and the hydrogen sides of the solute to clarify the origin of the surface preference. When the solute was displaced from the bulk to the interface, it was shown that the free energy contribution from the oxygen side is destabilized by ∼10 kcal/mol because of a reduction of the number of surrounding solvent water molecules. It was observed, however, that the free energy contribution due to the hydrogen side of the solute is unexpectedly stabilizing and surpasses the destabilization in the opposite side. We found that the stabilization in the hydrogen side originates from the solute-solvent interaction in the medium range beyond the nearest neighbor. It was also revealed that the free energy contribution due to the solute's electronic polarization amounts to about the half of the total free energy change associated with the solute displacement from the bulk to the interface.

Thermodynamics of hydronium and hydroxide surface solvation

“…In summary we find that the hydroxide surface affinity is approximately +4 kJ/mole -hence preferring the bulkwhere it is -3 kJ/mole for hydronium -preferring the surface), (see 3). These findings are in agreement with a large body of evidence [11][12][13][14][15][16][17][18][19][20]. This surface enhancement of H 3 O + in a neat water droplet corresponds to a slightly lower surface p[H], which coincides with an increased surface p[OH] (3A, right yaxis).…”

“…The inside (bulk) of the droplet is taken as the reference point where the energy was defined to zero. 3A confirms earlier experimental [11][12][13][14][15] and theoretical [12,[16][17][18][19][20] work showing that hydronium is preferentially solvated at the water surface. The enthalpic contribution to the PMF for hydronium solvation is stronger than the unfavorable entropic contribution (3B,C), demonstrating that the surface preference of hydronium is an enthalpic effect.…”

“…They estimated that the hydroxide surface affinity should be on the order of 20 k B T, or 50 kJ/mol at room temperature. These values are an order of magnitude larger than the effects seen here and in previous experiments [11,[13][14][15] and calculations [16][17][18][19][20]. Mundy et al have performed ab initio MD simulations [53] and find a surface preference of OH − of just 1k B T. While ab initio simulations are in principle more general than classical simulations, they cannot provide the same rigorous sampling.…”

Brønsted basicity of the air–water interface

“…Some surface-specific nonlinear spectroscopic studies (9, 61), most theoretical calculations (7,51,62), and the ion partitioning analysis of surface tension data on electrolyte solutions (19) have predicted the accumulation of H 3 O + at and the exclusion of OH − from the air-water interface. On the basis of such evidence it has been argued that water surface is acidic (7,25).…”

The Structure of (H3O)2B12F12·6H2O – a CCP Lattice of B12F122– Anions Intercalated with a Nonplanar Network of O–H···O Connected O6 Rings

“…at the air/water interface, where H 3 O + ions may be concentrated relative to the bulk solution), [22,23] and (iii) as free H 3 O + ions [24] or as [(…”