Article pubs.acs.org/JPCB
Molecular Dynamics Simulations of the Interaction between Water Molecules and Aggregates of Acetic or Propionic Acid Molecules Bastien Radola,† Sylvain Picaud,*,† Delphine Vardanega,†,‡ and Pál Jedlovszky§,∥,⊥ †
Institut UTINAM - UMR 6213, CNRS, Univ. Bourgogne Franche-Comté, F-25000 Besançon Cedex, France PhLAM - UMR8523, CNRS, Univ. Lille 1, F-59655 Villeneuve d’Ascq, France § Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eötvös Loránd University, Pázmány Péter stny, 1/a, H-1117 Budapest, Hungary ∥ HAS Research Group of Technical Analytical Chemistry, Szt. Gellért tér 4, H-1111 Budapest, Hungary ⊥ EKF Department of Chemistry, Eszterházy tér 1, H-3300 Eger, Hungary ‡
ABSTRACT: Water adsorption around small acetic and propionic acid aggregates has been studied by means of molecular dynamics simulation in the temperature range of 100−265 K as a function of the water content. Calculations have shown that acetic and propionic acid molecules behave similarly and that both the temperature and the water content have a strong influence on the behavior of the corresponding systems. Two situations have been evidenced for the acid−water aggregates, corresponding either to water adsorption on large acid grains at very low temperatures or to the formation of droplets consisting of acid molecules adsorbed at the surface of water aggregates at higher temperatures and high water content. At low water content and high temperature, only a partial mixing between water and acid molecules has been observed. The results of the present simulations emphasize the need for further experimental and simulation works to achieve a better characterization of the effects of both temperature and humidity on the behavior of organic aerosols in the troposphere. constituents of fine aerosols,7 it is now clear that water condensation and heterogeneous ice nucleation on organic species cannot be ignored.8 Among these organic compounds, carboxylic acids are one of the most abundant species. Indeed, these molecules are emitted directly to the atmosphere via several biogenic (biomass burning) and anthropogenic (pharmaceutical and chemical industries, biofuels) sources, and they can also be formed secondarily via photo-oxidation of hydrocarbons in both gas and aqueous phases of clouds.9 Focusing on the interaction between carboxylic acids and water is thus very important in the context of tropospheric physico-chemistry, and combination of field measurements, laboratory experiments, and modeling studies appears mandatory to achieve a better characterization of the corresponding systems. Because of the high variability of the organic aerosol phase in terms of sources, composition, and evolution, the modeling of ideal systems by computer simulations appears to be appealing. Such modeling can be the first step toward better understanding aerosol behavior, using information on the molecular scale. More specifically, interactions between the organic phase and the surrounding water molecules can be investigated in detail on the basis of a rigorous description of the corresponding interactions; however, even simplified, a realistic
1. INTRODUCTION Carbonaceous aerosols constitute a significant subgroup of atmospheric aerosols that consists of elemental carbon (black carbon, freshly emitted soot), organic carbon, and, often, a mixture of both elemental and organic carbon (brown carbon).1 These aerosols can be primarily emitted or secondarily formed in the atmosphere, for instance, as oxidation products of gaseous organic precursors with low vapor pressure;2 however, because of the complexity of their chemical nature, the significance of carbonaceous aerosols in driving physical and chemical atmospheric processes is still very poorly understood. Carbonaceous aerosols play an important role in climatic processes, having both direct and indirect effects.3−5 Direct effects on climate come from scattering and absorbing solar and terrestrial radiations, whereas indirect effects are directly connected to the ability of these particles to act as cloud condensation (CCN) and ice formation (IN) nuclei.6 CCN are particles that in the presence of supersaturated water vapor initiate condensation and become centers of cloud or fog droplets, whereas IN are the necessary ingredients for heterogeneous nucleation processes, leading to the formation of ice particles.7 Thus, understanding the mechanisms that drive the interaction between carbonaceous aerosols and surrounding water molecules is of fundamental interest to better assess the role that these CCN and IN play in atmospheric processes. In addition, because chemical characterization of these aerosols has revealed that organic compounds are the major © XXXX American Chemical Society
Received: August 20, 2015 Revised: November 24, 2015
A
DOI: 10.1021/acs.jpcb.5b08110 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
Table 1. Lennard-Jones Parameters (σ in nm and ε in kJ/mol) and Fractional Charges (q/e) Located on the Different Atoms of the Acetic,34 Propionic,35 and Water Molecules39 in the Potential Models Used acetic acid
propionic acid
water
atom
q/e
σ
ε
atom
q/e
σ
ε
atom
q/e
σ
ε
C (CO) O (OC) O (O−H) H (H−O) C (CH3) H1 H2 H3
0.55 −0.5 −0.58 0.45 0.08 0 0 0
0.375 0.296 0.307 0 0.391 0 0 0
0.439 0.879 0.711 0 0.669 0 0 0
C (CO) O (OC) O (O−H) H (H−O) C (CH2) H1 H2 C (CH3) H3 H4 H5
0.52 −0.44 −0.53 0.45 −0.12 0.06 0.06 −0.18 0.06 0.06 0.06
0.375 0.296 0.300 0 0.350 0.242 0.242 0.350 0.242 0.242 0.242
0.439 0.879 0.711 0 0.276 0.063 0.063 0.276 0.063 0.063 0.063
O H1 H2 M1 M2
0 0.241 0.241 −0.241 −0.241
0.312 0 0 0 0
0.669 0 0 0 0
2. COMPUTATIONAL DETAILS Molecular dynamics (MD) simulations of acetic (CH3COOH) and propionic acid (CH3CH2COOH) aggregates interacting with water molecules have been carried out for different compositions using the GROMACS 4.5.4 open-source program package.33 The potential energy of the systems investigated has been calculated as the sum of the atom−atom pairwise interaction energies between the interacting species. These interaction energies have been represented by combination of Lennard-Jones (6−12) and Coulombic terms, whose parameters for the different atoms are given in Table 1. Lennard-Jones parameters and partial charges for the acid molecules were taken from the OPLS-AA library,34,35 and their geometries were determined using the Automated Topology Builder (ATP) tool.36−38 To be consistent with our previous studies,24−26 water molecules have been represented by the five-site TIP5P model.39 These molecules have been kept rigid in the simulations by means of the SETTLE algorithm,40 whereas no constraint has been applied to acid molecules.34,35 (The parameters for the internal interaction potentials have been taken from the OPLS-AA database.) The Lennard-Jones interactions have been cut beyond 1.4 nm, and the Particle Mesh Ewald (PME) method41 has been used to take into account the long-range electrostatic interactions. The geometry of the acid and water molecules is shown in Figure 1.
modeling of these systems has to consist of at least hundreds of molecules. As a consequence, quantum calculations will certainly not be feasible; instead, classical approaches based on a simplified, but nonetheless relevant, description of the organic aerosol−water interactions is the next best thing. For instance, classical interaction models have been shown to correctly reproduce experimental results when characterizing the interaction between small volatile organic compounds and ice surfaces using either molecular dynamics simulations10−13 or Monte Carlo calculations.14−23 Then, classical approaches have been used to characterize the interaction between large organic aggregates and water molecules as a function of the temperature and of the water content in the corresponding simulations.24−26 Similarly, a few molecular dynamics simulation studies have been recently devoted to the characterization of the reverse situation, that is, when big water droplets are coated by various organic molecules.27−32 These two approaches, namely, water droplets interacting with organic matter and organic aggregates interacting with water, led to similar conclusions, in particular, regarding the state of mixture of the aerosol particles. Indeed, water and organic molecules have been found to form either mixed or demixed phases depending on the temperature, the water/acid ratio, and, more importantly, the type of the organic molecules. Thus, using the O:C ratio as a proxy for characterizing the organic aerosol hydrophilicity, as recently suggested by Schill and Tolbert,8 could not be an oversimplification. In contrast, a systematic study of a large series of carboxylic acid molecules should be undertaken before any global conclusion (if any) can be drawn. As a consequence, we complement our previous works on oxalic,24 malonic,25 and formic26 acids by considering here the case of aggregates formed by acetic and propionic acid molecules. Indeed, after characterizing the influence of the internal conformation of the acid molecule (difference between oxalic and malonic acids) and the influence of the chemical type of acid (mono- vs dicarboxylic acid), the present study aims at investigating the effect of the length of the hydrophobic tail that could also play a significant role in the interaction between the acid and water molecules. The computational details of our simulation study are given in Section 2, whereas the corresponding results are detailed in Section 3. These results are then discussed in Section 4, and the main conclusions of the present study are summarized in Section 5.
Figure 1. Molecule models for acetic, propionic, and water molecules used in the simulations. O, C, and H atoms are represented by red, light-blue, and white balls. Note that M1 and M2 (represented by rose balls) are two additional sites used in the TIP5P model of water to better represent the charge distribution of the water molecule.39
Simulations have been performed in the (N,V,T) ensemble with a time step of 0.1 fs. The temperature has been controlled by means of the Berendsen thermostat42 during equilibration and using the Nosé−Hoover algorithm43,44 for production runs. Indeed, although the Berendsen method allows quick convergence of the temperature value, which is of great interest B
DOI: 10.1021/acs.jpcb.5b08110 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
Figure 2. Equilibrium snapshots of (a) acetic and (b) propionic acid aggregates at 150, 200, and 250 K (left, middle, and right) and (from top to bottom) for 0, 1:1, 4:1, and 6:1 water/acid ratios. (The results for additional simulations at 10:1 water/acid ratio are also shown for acetic acid.) For the acid molecules, O, C, and H atoms are represented by red, light-blue, and white balls, whereas all of the atoms of water molecules are represented in dark blue for clarity. Note that snapshots have been taken at the end of the production runs, as an illustration.
malonic acids.25,26 We have thus chosen to work with aggregates made of 120 acid molecules, which appeared as a good compromise between the relevance and the time cost of the simulations performed with large numbers of additional water molecules. Indeed, to simulate the effect of increasing humidity on the behavior of the acid aggregates, five different systems in the case of acetic, and four in the case of propionic acid, have been created by adding a different amount of water molecules, being randomly scattered in the neighborhood of the stabilized acid aggregate surface and by performing further equilibration runs of 1 ns. Thus, water/acid number ratios of 1:1, 4:1, and 6:1 have been considered with both carboxylic acid molecules, and, in addition, the number ratio of 10:1 has also been considered in the case of the acetic acid aggregates. Note that when adding the water molecules, the size of the simulation box has been
for stabilization runs, it does not yield a correct thermodynamical ensemble. In contrast, the Nosé−Hoover thermostat converges more slowly but corresponds to simulations in the correct thermodynamic ensemble. First, MD simulations of neat acid aggregates have been performed, by randomly placing 120 acid molecules in a cubic simulation box with periodic boundary conditions; this cubic box had an edge length of 6 nm in the case of acetic and 7 nm in the case of propionic acid. The system was then equilibrated at 100 K during 1 ns, which was sufficiently long to obtain the formation of a stable, large, and compact aggregate composed of the 120 acid molecules, being completely isolated from its periodic images. Further simulations with larger numbers of acid molecules in the box showed the same clustering behavior, at least until 240 molecules (the maximum of molecules that was considered here), as already observed for formic and C
DOI: 10.1021/acs.jpcb.5b08110 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
3. RESULTS OF THE SIMULATIONS 3.1. Structure of Acid Aggregates without Water. As previously stated, simulations of acetic and propionic acid molecules initially scattered randomly in the simulation box resulted in the formation of one single big aggregate irrespective of (i) the number of molecules and (ii) the final temperature of the simulations. This aggregate gathers together almost all acid molecules present in the simulation box, as already observed in previous simulations devoted to formic and malonic acids25,26 and in contrast with what was obtained for oxalic acid molecules that were found to form smaller aggregates.24 This behavior, evidenced by a careful examination of the snapshots issued from the simulations, has been confirmed by the analysis of the size distribution P(n) of the clusters formed by the acid molecules in the box. Indeed, only one single peak at high n values was always obtained in P(n) distribution for each system (not shown), corresponding to the formation of a single aggregate containing almost all the acid molecules. Some equilibrium snapshots of the acetic and propionic acid aggregates are shown in Figure 2 (top) as an illustration. Note that in the temperature range of the simulations, acetic and propionic acids are in the solid phase and should exhibit a crystalline structure that could have been evidenced by the study of some radial distribution functions and by careful examination of the hydrogen bonds network in the results of our molecular dynamics simulations; however, the aggregates formed here are characterized by a high ratio of surface to bulk molecules due to their quite small size, thus preventing the formation of well-ordered crystals. Nevertheless, these aggregates showed some local ordering due to hydrogen bonding between acid molecules (see later). 3.2. Phase Behavior of the Binary Aggregates. Acid aggregates have then been placed in a humid environment by adding water molecules to the simulation box with 1:1, 4:1, and 6:1 (and even 10:1 in the case of acetic acid) water/acid ratios. Simulations have been performed at various temperatures varying between 100 and 250 K. (Some simulations have also been performed at 265 K for acetic acid molecules.) A first analysis of the results has been done simply by looking at selected equilibrium snapshots taken from the simulations, some of them being shown in Figure 2 as an illustration of the equilibrated systems under investigation. These snapshots clearly show that at low water content (1:1 water/acid ratio) acetic acid molecules (Figure 2a) tend to form big aggregates on which surrounding water molecules are adsorbed, irrespective of the temperature. A similar situation has also been observed at higher water contents but only at low temperatures, typically
