Heterogeneous Nucleation of Butanol on NaCl: A Computational Study of Temperature, Humidity, Seed Charge, and Seed Size Effects

Abstract

Using a combination of quantum chemistry and cluster size distribution dynamics, we study the heterogeneous nucleation of n-butanol and water onto sodium chloride (NaCl)₁₀ seeds at different butanol saturation ratios and relative humidities. We also investigate how the heterogeneous nucleation of butanol is affected by the seed size through comparing (NaCl)₅, (NaCl)₁₀, and (NaCl)₂₅ seeds and by seed electrical charge through comparing (Na₁₀Cl₉)⁺, (NaCl)₁₀, and (Na₉Cl₁₀)^- seeds. Butanol is a common working fluid for condensation particle counters used in atmospheric aerosol studies, and NaCl seeds are frequently used for calibration purposes and as model systems, for example, sea spray aerosol. In general, our simulations reproduce the experimentally observed trends for the NaCl-BuOH-H₂O system, such as the increase of nucleation rate with relative humidity and with temperature (at constant supersaturation of butanol). Our results also provide molecular-level insights into the vapor-seed interactions driving the first steps of the heterogeneous nucleation process. The main purpose of this work is to show that theoretical studies can provide molecular understanding of initial steps of heterogeneous nucleation and that it is possible to find cost-effective yet accurate-enough combinations of methods for configurational sampling and energy evaluation to successfully model heterogeneous nucleation of multicomponent systems. In the future, we anticipate that such simulations can also be extended to chemically more complex seeds.

Figure 1

Example of the NaCl seed with condensing BuOH and H₂O molecules.

Figure 2

Diagram of the set of simulated clusters describing the seed–butanol–water nucleation process. For example, a cluster containing the NaCl seed, two butanol molecules, and three water molecules is denoted 1S2B3W.

Figure 3

Global minimum structures of the butanol clusters at T = 25 °C and the DLPNO-CCSD(T)/aug-cc-pVTZ//LC-ωHPBE/def2TZVP level of theory.

Figure 4

Gibbs free energy profiles of pure butanol cluster formation at different conditions, at the DLPNO-CCSD(T)/aug-cc-pVTZ//LC-ωHPBE/def2TZVP level of theory. The free energies are computed using the actual butanol monomer concentration, see eq 5. The green line illustrates how large the saturation ratio would need to be for the critical cluster to lie within the simulated set of clusters.

Figure 5

Gibbs free energy profiles for butanol clustering onto a (NaCl)₁₀ seed at the DLPNO-CCSD(T)/aug-cc-pVTZ//LC-ωHPBE/def2TZVP level of theory. The free energies are computed using the actual butanol monomer concentration, see eq 5.

Figure 6

Illustrations of seed structural changes and butanol orientation during butanol clustering onto the NaCl seed.

Figure 7

Heterogeneous nucleation rate as a function of temperature for the butanol saturation ratio of 1 (blue line) and ratio 5 (green line). The rates are plotted relative to that obtained at S = 1 and 25 °C (red point).

Figure 8

Butanol saturation ratio as a function of temperature for constant nucleation rates of J/J_ref = 1 (blue line) and J/J_ref = 10 (green line). J_ref corresponds to the nucleation rate at S = 1 and T = 25 °C (red point).

Figure 9

Example of Gibbs free energies of formation at T = 25 °C, butanol saturation ratio of S = 1, and humidity of RH = 10% at the DLPNO-CCSD(T)/aug-cc-pVTZ//LC-ωHPBE/def2TZVP level of theory. Each box corresponds to one seed–butanol–water clusters with the given numbers of water and butanol molecules attached to the seed. The free energies have been computed using the actual vapor concentrations.

Figure 10

Structural changes of the seed as well as butanol and water orientation during butanol–water condensation on the NaCl seed.

Figure 11

Heterogeneous nucleation rate as a function of RH for butanol saturation ratios of 1 (blue line) and 5 (green line). The nucleation rate is shown with respect to the rate at T = 25 °C, S = 1, and RH = 0% (red point).

Figure 12

Seed size and charge variations during butanol condensation on different NaCl seeds.

Figure 13

Effect of seed size and seed charge on the first steps of heterogeneous seed–butanol nucleation. In all figures, we used the GFN2-xTB level (low level of theory) and temperature T = 25 °C. The left figures show Gibbs free energies of formation computed using actual vapor concentrations. Full lines correspond to saturation ratios of 1 and dashed lines correspond to saturation ratios of 5. The right figures show the nucleation rates of butanol on NaCl seeds for butanol saturation ratios of 1 (blue) and 5 (green). The nucleation rates are shown with respect to the (NaCl)₁₀ seed at S = 1 (red point). (a) Formation Gibbs free energy profiles with varied seed size (y). (b) Nucleation rates with varied seed size (y). (c) Formation Gibbs free energy profiles with varied seed size charge (q). (d) Nucleation rates with varied seed size (q).