Kwan Victor, Consta Styliani
Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7.
J Am Soc Mass Spectrom. 2021 Jan 6;32(1):33-45. doi: 10.1021/jasms.0c00053. Epub 2020 Jul 13.
The surface excess charge layer (SECL) in droplets has often been associated with distinct chemistry. We examine the effect of the nature of ions in the composition and structure of SECL by using molecular dynamics. We find that in the presence of simple ions the thickness of SECL is invariant not only with respect to droplet size but also with respect to the nature of the ions. In the presence of simple ions, this layer has a thickness of ∼1.5-1.7 nm but in the presence of macroions it may extend to ∼2.0 nm. The proportion of ions contained in SECL depends on the nature of the ions and the droplet size. For the same droplet size, I and model HO ions show considerably higher concentration than Na and Cl ions. We identify the maximum ion concentration region, which, in nanodrops, may partially overlap with SECL. As the relative shape fluctuations decrease when microdrop size is approached, the overlap between SECL and maximum ion concentration region increases. We suggest the extension of the bilayer droplet structure assumed in the equilibrium partitioning model of Enke to include the maximum ion concentration region that may not coincide with SECL in nanodrops. We compute the ion concentrations in SECL, which are those that should enter the kinetic equation in the ion-evaporation mechanism, instead of the overall drop ion concentration that has been used.
液滴中的表面过剩电荷层（SECL）通常与独特的化学性质相关。我们通过分子动力学研究了离子性质对SECL组成和结构的影响。我们发现，在存在简单离子的情况下，SECL的厚度不仅相对于液滴大小不变，而且相对于离子性质也不变。在存在简单离子的情况下，该层的厚度约为1.5 - 1.7纳米，但在存在大离子的情况下，它可能延伸至约2.0纳米。SECL中所含离子的比例取决于离子的性质和液滴大小。对于相同的液滴大小，I和模型HO离子的浓度比Na和Cl离子高得多。我们确定了最大离子浓度区域，在纳米液滴中，该区域可能与SECL部分重叠。随着接近微滴大小时相对形状波动减小，SECL与最大离子浓度区域之间的重叠增加。我们建议扩展恩克平衡分配模型中假设的双层液滴结构，以包括在纳米液滴中可能与SECL不一致的最大离子浓度区域。我们计算了SECL中的离子浓度，这些浓度是应进入离子蒸发机制动力学方程的浓度，而不是一直使用的总液滴离子浓度。
