On the Uptake of Ammonia by the Water/Vapor Interface M. A. Carignano* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 M. M. Jacob and E. E. Avila Fa.M.A.F., UniVersidad Nacional de Co ´ rdoba, Co ´ rdoba, Argentina, and CONICET-Argentina, Buenos Aires, Argentina ReceiVed: December 7, 2007; In Final Form: January 29, 2008 The passage of a single ammonia molecule from an infinitely dilute gas through the water/vapor interface is studied by constrained molecular dynamics simulations. The free energy of the system as a function of the distance between the ammonia and the interface has a minimum in the interfacial region. It is found that the preference of the ammonia for the interface is mainly due the disruption of the solvent structure caused by the ammonia in the bulk region, which results in an increase of the solvent internal energy. Introduction The general problem of the uptake of small molecules by water has been the subject of several recent theoretical and experimental works. 1-6 The importance of this subject stems mainly from the role that the water/vapor interface plays in atmospheric chemistry. 7 Ammonia is one of the most important trace gases found in the atmosphere. Since it is released by natural and anthropogenic sources, it is considered a primary pollutant and is certainly not innocuous. 8,9 Particularly, ammonia is a reactive and variable gas, and its atmospheric cycle is linked to liquid water. Because it is basic and also highly soluble in water, ammonia compounds play a key role in the acid-base exchange processes in clouds. The concentration of NH 3 in the atmosphere varies between 0.1 and 10 ppbv; 7 it can be higher over polluted cities or agricultural fields and much lower over remote oceans. Absorp- tion of ammonia by water spray is also an important cleaning strategy in some technological processes, especially in the semiconductor industry. 10 The interfacial properties of ammonia-water mixtures have been recently studied by Paul and Chandra 11 using molecular dynamics simulations. They have considered several (finite) ammonia concentrations and studied the surface tension, density profiles, molecular orientation, and diffusion behavior in the bulk and the surface region among other properties. It was found that the ammonia molecules have a higher preference to occupy the interfacial region than the water molecules. The dependency of the surface tension with the ammonia concentration was found to be in qualitative agreement with the equilibrium surface tension measurement performed by Donaldson 2 and also with the older results of O. K. Rice. 12,13 This surface tension behavior is consistent with an accumulation of ammonia molecules at the surface and led to the idea of surface-bound states. 1,14 This idea was further developed by Donaldson, 2 who also provides a discussion of the thermodynamic and kinetic theory used to analyze the adsorption process and applies it to the ammonia case. By using Henry’s law and extrapolating the results to zero vapor pressure, the standard free energy for ammonia adsorption from the gas phase was determined to be -19.1 kJ/mol at 298 K and -20.6 kJ/mol at 278 K. Supported by the experimental results and ab initio calculation of the NH 3 -H 2 O and NH 3 - (H 2 O) 2 complexes, Donaldson concludes that ammonia is bound to a small number (two or three) of water molecules forming a surface-bound state. Moreover, this state is stabilized by a more favorable ammonia-water interaction at the surface than that in the bulk region. Sum frequency generation experiments performed by Shultz et al. 15 have provided the first experimental picture of NH 3 at the water/vapor interface. Their findings indicate that the ammonia docks to the dangling OH bonds in such a way that the C 3 molecular axis forms an average angle of 25-38° with the normal to the interfacial plane. Besides the formation of the surface-bound states stabilized by a low enthalpy of adsorption, another possible origin of the lower free energy of the adsorbed state with respect to the fully solvated one is the energetic cost to rearrange the solvent molecules around the solute. To the best of our knowledge, this possibility has not been explored with molecular simulations yet, and as it will be shown below, this is the main reason for the occurrence of the surface minimum in the free-energy profile. From the atomistic simulation point of view, a practical methodology to study the passage of small molecules through the water/vapor interface is constrained molecular dynamics. This method has been well described by Somasundaram et al., 3 and a recent review 5 summarizes the findings of several groups. Dang and Garrett 4 have applied this technique to the particular case of the ammonia/water. In their study, they have used polarizable model potentials to describe the water-water and water-ammonia interactions. The free energy of the system as a function of the position of the solute molecule is calculated. The results of Dang and Garrett 4 show no minimum at the water/ vapor interface; instead, the free energy decreases monotonically as the ammonia enters the water phase. In this paper, we use constrained molecular dynamics simulations to determine the free-energy profile for ammonia uptake. We use the same pairwise additive model potentials used by Paul and Chandra 11 and employ a similar methodology to the one used by Dang and Garrett. 4 The simulations were run * To whom correspondence should be addressed. E-mail: cari@purdue.edu. 10.1021/jp7115593 CCC: $40.75 © xxxx American Chemical Society PAGE EST: 4 Published on Web 03/12/2008