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Crystallization of ions in aqueous micro-droplet or nano-droplet on solid surfaces is ubiquitous, with applications ranging from inkjet printing to pesticide spraying. The substrates involved are typically nonpolar. Yet, the atomistic mechanism of crystallization within sessile droplets on such nonpolar substrates is still unclear. Here, we employ molecular dynamics simulations to investigate the crystallization of sodium chloride inside an aqueous nano-droplet on a nonpolar face-centered-cubic (111) surface. Crystallization occurs inside the droplet rather than at the liquid-gas or solid-liquid interface, when the concentration of the sodium chloride in the droplet exceeds 3.76 mol/kg. The phenomenon originates from the spatial distributions of water molecules and ions: a dense interfacial water layer forms at the solid-liquid interface, whereas ions accumulate in the droplet interior, increasing the local concentration. The ion-water hydration caused by the electrostatic interaction is dominant in ion-solid interaction. The spatial confinement provided by the solid, rather than the physical properties of the solid, enriches ions inside the nano-droplet, thereby triggering the crystallization. We further apply this mechanism to the separated aqueous sodium chloride nanodroplets, in which the gas phase destroys the continuous spatial distribution of ions in the droplet. Analogous crystallization is observed in the sessile droplets of potassium chloride solution on nonpolar solid surfaces, indicating the generality of crystallization in nano-droplets. These findings provide atomic-scale guidance for controlling crystallization in nano-droplets related to microelectronics, inkjet printing, and related technologies.
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Keywords:
- solid-liquid interface /
- crystallization /
- hydration /
- nano-droplet /
- molecular dynamics
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图 1 (a) 模拟初始构型(t = 0, 液滴的尺寸为4 nm×4 nm×4 nm, 离子浓度为4.50 mol/kg, 衬底在xy方向的尺寸分别为10.182 nm和10.224 nm)的侧视图, 其中红色、白色、蓝色和灰色分别表示氧、氢、钠、氯; (b) 模拟结果(t = 80 ns)侧视图, 80 ns后液滴中的离子形成了氯化钠晶体; (c) 不同模拟时间对应的离子径向分布函数(RDF); (d) 体相溶液的初始构型(液滴的尺寸为4 nm×4 nm×4 nm, 浓度为4.50 mol/kg, 采用三维周期性边界条件)和80 ns后液滴的状态和离子分布侧视图(NPT系统, 虚线框表示周期性边界); (e) 体相溶液中离子的RDF, 分别采用NVT系综和NPT系综进行计算, 均没有出现结晶
Figure 1. (a) Side-view of the simulation setup, the aqueous droplet with a size of 4 nm×4 nm×4 nm is placed on the substrate of 10.182 nm×10.224 nm; (b) after 80 ns simulation, the ions in the droplet form the NaCl crystal, the red sphere represents oxygen atom, white hydrogen, blue sodium and grey chloride, respectively; (c) the radial distribution function (RDF) of the crystal for different simulation times; (d) side-view of initial structure of the bulk aqueous solution with size of 4 nm×4 nm×4 nm together with the distribution of ions, and final structure of bulk solution with ions after 80-ns simulations, the NPT ensemble is used, the dashed line means the periodic boundary; (e) the RDF of the ions in bulk solution after simulation, both NVT and NPT ensembles are applied but no crystal is noticed after 80 ns simulations.
图 2 纳米液滴在不同固体上的结晶行为 (a) 氯化钠浓度分别为4.00, 3.76和3.64 mol/kg液滴中形成的晶体对应的RDF; (b) 在280, 300, 340和360 K下, 氯化钠溶液中得到的晶体的RDF, 插图为300 K下结晶的侧视图; (c) 在不同作用强度参数的原子构成的固体上, 液滴中出现的晶体对应的RDF; (d) 增大固体厚度至0.835 nm后, 氯化钠溶液中得到的晶体的RDF; (e) 增大固体xy方向的尺寸至13.281 nm×13.291 nm后形成的晶体的RDF; (f) 增加固体表面纳米结构(沟道在x方向的宽度为0.89 nm, y方向的宽度为0.77 nm)后形成的晶体的RDF; (g) 采用双层石墨烯(尺寸为10.226 nm×10.332 nm)作为固体衬底; 氯化钠溶液中形成的晶体的RDF
Figure 2. Crystallization on different solid surfaces: (a) The RDF of crystals in the droplet with concentrations of 4.00, 3.76, 3.64 mol/kg; (b)the RDF of crystals at 280, 300, 340 and 360 K, the inset is the side view of crystal formed at 300 K; (c) the side views of water droplets on solids composed of atoms with different Lennard-Jones parameters ε and the RDF of crystals formed in the droplet on solid surfaces; (d) the RDF of crystals on the solid with thickness of 0.835 nm (five atom layers); (e) the RDF of crystals on the solid with size of 13.281 nm×13.291 nm; (f) the RDF of crystals on the solid with structures, the size of channel is 0.89 nm (x-axis) and 0.77 nm (y-axis); (g) the RDF of crystals on the two-layer graphene with size of 10.226 nm×10.332 nm in x-axis and y-axis.
图 3 (a) 纳米液滴在不同时刻截面侧视图(t = 4 ns和t = 80 ns), 水分子层位于离子与固体表面之间(黑虚线以下); (b) 液滴内部的水分子和离子在竖直方向上的空间分布, 插图为离子浓度的分布; (c) 非极性固体原子不同$ {\varepsilon }_{\mathrm{S}} $数值对应的钠离子空间分布变化情况; (d) 不同浓度的孤立液滴内部离子在78 ns时的径向分布函数, 当浓度为4.50 mol/kg和4.00 mol/kg时, 液滴内部的离子发生结晶, 插图为浓度为4.50 mol/kg的孤立纳米液滴内部晶体的截面侧视图
Figure 3. (a) The side view of the cross-section of the nano-droplet at 4 ns and 80 ns, the water molecule layer occupies the space between ions and the solid surface as denoted by the dashed black line; (b) the spatial distribution of water molecules and Na+ ions, the position of Z = 0 is defined as the top surface of the solid, inset represents the distribution of the concentration of NaCl; (c) the spatial distribution of Na+ ion for different $ {\varepsilon }_{\mathrm{S}} $ in Eq. (1); (d) the RDF of ions in the isolated droplets with different concentrations at 78 ns, the ions inside the droplet with concentrations of 4.50 mol/kg and 4.00 mol/kg form the NaCl crystal, inset represents the side view of the cross-section of the crystal formed in the isolated nano-droplet with concentration of 4.50 mol/kg.
图 4 改变离子电荷密度后, 水分子(a)和钠离子(b)在竖直方向的空间分布, 对于液滴内部的离子而言, 其数密度随着电荷密度的增大而增大; (c) 液滴中离子浓度最大值随钠离子电荷密度的变化; (d) 钠离子-水分子(以氧原子代替)的径向分布函数, 插图为水分子在钠离子周围的分布图
Figure 4. (a) The spatial distribution of water molecules mediated by the charge density of the ions; (b) the number density of ions inside the nano-droplet increase as the charge density of ions increases; (c) maximum spatial density of Na+ ions as a function of ion charge density; (d) RDFs of Na-O in the solution as the charge density, the inset is the water molecules around the sodium ions, inset represents water molecules around Na+ ion.
图 5 非极性固体表面的氯化钾液滴和体现氯化钾溶液的结晶情况对比 (a) 经过80 ns的模拟后, 固体表面的氯化钾纳米液滴(浓度为5.50 mol/kg)内的离子分布; (b) 相同浓度体相溶液中的离子分布的侧视图, 其中青色表示钾离子, 虚线框表示周期性边界; (c) 固体表面液滴与体相溶液内部离子的RDF在不同时刻的比较, 固体表面的纳米液滴2 ns(黑)、78 ns(红)和体相溶液(蓝)的RDF
Figure 5. Comparison of the crystallization in the nano-droplet KCl solution on the solid and that in the bulk KCl solution: (a) The side view of ions in the nano-droplet of KCl solution on solid surface; (b) bulk solution with the same concentration of 5.50 mol/kg, the cyan spheres denote potassium ions, the dashed line means the periodic boundary; (c) the RDF of ions in the nano-droplet on solid at 2 ns (black) and 78 ns (red), compared to that in the bulk solution at 78 ns (blue).
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