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

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.本研究结合量子化学（quantum chemistry）与团簇尺寸分布动力学方法，针对不同正丁醇（n-butanol）饱和比与相对湿度条件，探究了正丁醇与水在氯化钠（sodium chloride，NaCl）₁₀晶种表面的非均相成核过程。此外，本研究通过分别对比(NaCl)₅、(NaCl)₁₀与(NaCl)₂₅晶种以考察晶种尺寸的影响，同时通过对比(Na₁₀Cl₉)⁺、(NaCl)₁₀与(Na₉Cl₁₀)⁻晶种，解析了晶种电荷对正丁醇非均相成核过程的调控效应。正丁醇是大气气溶胶（atmospheric aerosol）研究领域中凝聚粒子计数器（condensation particle counters）的常用工作流体，而氯化钠晶种则常被用于实验校准，同时也可作为模型体系（例如海盐气溶胶（sea spray aerosol））。总体而言，本研究的模拟结果复现了NaCl–正丁醇–H₂O体系的实验观测规律，例如在正丁醇过饱和度恒定的条件下，成核速率随相对湿度与温度升高而增大的趋势。本研究结果还为驱动非均相成核初始步骤的气-晶种相互作用提供了分子层面的深入认知。本工作的核心目标在于证明：理论研究可揭示非均相成核初始步骤的分子机制，且能够寻找到兼具成本效益与足够精度的构型采样与能量评估方法组合，从而成功模拟多组分体系的非均相成核过程。展望未来，我们预计此类模拟可进一步拓展至化学组成更为复杂的晶种体系。