Heiles Sven, Cooper Richard J, DiTucci Matthew J, Williams Evan R
Department of Chemistry , University of California , Berkeley B42 Hildebrand Hall , Berkeley , California 94720-1460 , USA . Email:
Chem Sci. 2017 Apr 1;8(4):2973-2982. doi: 10.1039/c6sc04957e. Epub 2017 Jan 26.
Sequential water molecule binding enthalpies, Δ, are important for a detailed understanding of competitive interactions between ions, water and solute molecules, and how these interactions affect physical properties of ion-containing nanodrops that are important in aerosol chemistry. Water molecule binding enthalpies have been measured for small clusters of many different ions, but these values for ion-containing nanodrops containing more than 20 water molecules are scarce. Here, Δ values are deduced from high-precision ultraviolet photodissociation (UVPD) measurements as a function of ion identity, charge state and cluster size between 20-500 water molecules and for ions with +1, +2 and +3 charges. The Δ values are obtained from the number of water molecules lost upon photoexcitation at a known wavelength, and modeling of the release of energy into the translational, rotational and vibrational motions of the products. The Δ values range from 36.82 to 50.21 kJ mol. For clusters containing more than ∼250 water molecules, the binding enthalpies are between the bulk heat of vaporization (44.8 kJ mol) and the sublimation enthalpy of bulk ice (51.0 kJ mol). These values depend on ion charge state for clusters with fewer than 150 water molecules, but there is a negligible dependence at larger size. There is a minimum in the Δ values that depends on the cluster size and ion charge state, which can be attributed to the competing effects of ion solvation and surface energy. The experimental Δ values can be fit to the Thomson liquid drop model (TLDM) using bulk ice parameters. By optimizing the surface tension and temperature change of the logarithmic partial pressure for the TLDM, the experimental sequential water molecule binding enthalpies can be fit with an accuracy of ±3.3 kJ mol over the entire range of cluster sizes.
连续水分子结合焓Δ对于详细理解离子、水和溶质分子之间的竞争相互作用，以及这些相互作用如何影响在气溶胶化学中具有重要意义的含离子纳米液滴的物理性质至关重要。已经测量了许多不同离子的小团簇的水分子结合焓，但对于含有超过20个水分子的含离子纳米液滴，这些值却很稀少。在此，通过高精度紫外光解离（UVPD）测量推导出Δ值，该值是离子种类、电荷态以及20 - 500个水分子之间团簇尺寸的函数，且适用于带 +1、+2和 +3电荷的离子。Δ值是通过在已知波长下光激发时损失的水分子数量以及对产物平动、转动和振动运动中能量释放的建模获得的。Δ值范围为36.82至50.21 kJ/mol。对于含有超过约250个水分子的团簇，结合焓介于体相汽化热（44.8 kJ/mol）和体相冰的升华焓（51.0 kJ/mol）之间。对于少于150个水分子的团簇，这些值取决于离子电荷态，但在较大尺寸时依赖性可忽略不计。Δ值存在一个最小值，它取决于团簇尺寸和离子电荷态，这可归因于离子溶剂化和表面能的竞争效应。实验得到的Δ值可以使用体相冰参数拟合到汤姆逊液滴模型（TLDM）。通过优化TLDM的表面张力和对数分压的温度变化，实验得到的连续水分子结合焓在整个团簇尺寸范围内可以以±3.3 kJ/mol的精度拟合。
