Quantum state resolved scattering from room-temperature ionic liquids: the role of cation versus anion structure at the interface.
Quantum state resolved scattering from room-temperature ionic liquids: the role of cation versus anion structure at the interface.
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量子态解析室温离子液体的散射：界面处阳离子与阴离子结构的作用。
DOI：
10.1021/jp2033802
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发表时间：
2011
期刊：
影响因子：
--
通讯作者：
D. Nesbitt
中科院分区：
文献类型：
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作者：
J. Roscioli;D. Nesbitt
我们给出了一系列室温离子液体(RTIL)对种子CO(2)超音速冷却分子束(E(INC)=61.9(40)kJ/mol)的态分辨散射研究结果。这些RTIL是由C(N)-甲基咪唑阳离子与BF(4)(-)或Tf(2)N(-)反阴离子组成。用高分辨率半导体激光吸收光谱监测了这些非平衡表面散射碰撞的最终振动量子态分布，作为(I)阳离子烷基链长和(Ii)阴离子大小的函数，并分析了热脱附(TD)与脉冲散射(IS)动力学的倾向。对于固定的BF(4)(-)或Tf(2)N(-)反离子，分布表明Td分数(α)随着烷基侧链中的C原子个数(N)的增加而增加，这为非极性基团在气液界面上的选择性择优提供了证据。相反，对于短碳链(n=4)，当阴离子从致密且较难极化的BF(4)(-)改变为较大且较易极化的Tf(2)N(-)时，热分数减少，而对于较长的烷基链(n=8，12)，对阴离子一致性的任何敏感性基本上消失。这些综合数据说明了界面位置上阴阳离子竞争的一些有趣的趋势，特别是(I)足够短的烷基头基的界面层上存在界面阴离子，(Ii)由于平均表面质量增加而导致的惯性“僵硬”，以及(Iii)界面区域阴离子尺寸较大的倾向。最后，TD几率遵循与阴离子大小完全相反的“大体积”亨利定律溶解度常数的趋势，这进一步突出了气液界面上的超热碰撞所采样的内在非平衡动力学。
We present results on state-resolved scattering studies for seeded CO(2) supersonically cooled molecular beams (E(inc) = 61.9(40) kJ/mol) from a series of room-temperature ionic liquids (RTILs). These RTILs are composed of C(n)-methylimidazolium cations with BF(4)(-) or Tf(2)N(-) counteranions. The final rovibrational quantum state distributions from these nonequilibrium surface scattering collisions are monitored by high-resolution diode laser absorption spectroscopy as a function of (i) cation alkyl chain length and (ii) anion size, and analyzed to yield the propensity for thermal desorption (TD) versus impulsive scattering (IS) dynamics. For a fixed BF(4)(-) or Tf(2)N(-) counteranion, the distributions reveal an increase in the TD fraction (α) with the C atom number (n) in the alkyl side chain, which provides evidence for selective preference of nonpolar groups at the gas-liquid interface with increasing chain length. Conversely, for short carbon chains (n = 4), the thermal fraction decreases when the anion is changed from a compact and less polarizable BF(4)(-) to the bulkier and more polarizable Tf(2)N(-), whereas any sensitivity to anion identity essentially vanishes for longer alkyl chains (n = 8, 12). These combined data illustrate a number of interesting trends in anion versus cation competition for interfacial sites, specifically (i) the presence of interfacial anions at the surface layer for sufficiently short alkyl headgroups, (ii) inertial "stiffening" due to increasing average surface mass, as well as (iii) a propensity for larger anion sizes in the interfacial region. Finally, the TD probabilities follow the exact opposite trend in "bulk" Henry's Law solubility constants with respect to anion size, which further highlights the intrinsically nonequilibrium dynamics sampled by hyperthermal collisions at the gas-liquid interface.