Wu Di, Navrotsky Alexandra
Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616.
Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616
Proc Natl Acad Sci U S A. 2015 Apr 28;112(17):5314-8. doi: 10.1073/pnas.1505874112. Epub 2015 Apr 13.
Knowing the nature of interactions between small organic molecules and surfaces of nanoparticles (NP) is crucial for fundamental understanding of natural phenomena and engineering processes. Herein, we report direct adsorption enthalpy measurement of ethanol on a series of calcite nanocrystals, with the aim of mimicking organic-NP interactions in various environments. The energetics suggests a spectrum of adsorption events as a function of coverage: strongest initial chemisorption on active sites on fresh calcite surfaces, followed by major chemical binding to form an ethanol monolayer and, subsequently, very weak, near-zero energy, physisorption. These thermochemical observations directly support a structure where the ethanol monolayer is bonded to the calcite surface through its polar hydroxyl group, leaving the hydrophobic ends of the ethanol molecules to interact only weakly with the next layer of adsorbing ethanol and resulting in a spatial gap with low ethanol density between the monolayer and subsequent added ethanol molecules, as predicted by molecular dynamics and density functional calculations. Such an ordered assembly of ethanol on calcite NP is analogous to, although less strongly bonded than, a capping layer of organics intentionally introduced during NP synthesis, and suggests a continuous variation of surface structure depending on molecular chemistry, ranging from largely disordered surface layers to ordered layers that nevertheless are mobile and can rearrange or be displaced by other molecules to strongly bonded immobile organic capping layers. These differences in surface structure will affect chemical reactions, including the further nucleation and growth of nanocrystals on organic ligand-capped surfaces.
了解有机小分子与纳米颗粒（NP）表面之间相互作用的本质，对于从根本上理解自然现象和工程过程至关重要。在此，我们报告了乙醇在一系列方解石纳米晶体上的直接吸附焓测量，旨在模拟各种环境中的有机-NP相互作用。能量学表明，吸附事件的频谱是覆盖度的函数：在新鲜方解石表面的活性位点上最初有最强的化学吸附，随后主要是化学结合形成乙醇单分子层，接着是非常弱的、接近零能量的物理吸附。这些热化学观察结果直接支持了一种结构，即乙醇单分子层通过其极性羟基与方解石表面结合，使得乙醇分子的疏水端仅与下一层吸附的乙醇发生微弱相互作用，从而在单分子层与随后添加的乙醇分子之间形成乙醇密度较低的空间间隙，正如分子动力学和密度泛函计算所预测的那样。乙醇在方解石NP上的这种有序组装类似于在NP合成过程中有意引入的有机封端层，尽管结合力较弱，并且表明表面结构会根据分子化学性质持续变化，从很大程度上无序的表面层到有序但仍可移动且能被其他分子重新排列或取代的层，再到强结合的固定有机封端层。这些表面结构的差异将影响化学反应，包括纳米晶体在有机配体封端表面上的进一步成核和生长。
