Structure of and Competitive Adsorption in Alkyl Dicarbamate Two-Dimensional Crystals

The potential for relatively minor structural changes to dramatically impact materials properties
is one of the primary obstacles to achieving the rational design of functional materials. For example, having
an odd versus an even number of carbons between functional groups in polymers can cause large variation
in melting point and mechanical properties. This odd−even effect is especially pronounced in hydrogen-bonded polymers and oligomers. To shed light on the structural basis of this phenomenon, physisorbed
monolayers and single crystals of alkyl dicarbamates were investigated by scanning tunneling microscopy
and X-ray diffraction, respectively. The related two- and three-dimensional crystal structures both
demonstrated a clear odd−even effect in packing geometry. The differing accommodation of intermolecular
interactions between odd and even packing motifs was directly related to the melting point trends and
further dissected through computation. In addition, these oligomers displayed unusual competitive adsorption
behavior; the relative preference for adsorption of a smaller species from a binary solution was increased
compared to alkanes. These results were explained in the context of hydrogen bond density effects that
arise due to competition for a limited substrate surface area. This study provides a model for understanding
oligourethane surface coatings and demonstrates the importance of molecular structure and hydrogen
bonding in determining adsorption behavior.

细微的结构调整即可对材料性能产生显著影响，这是实现功能材料理性设计的主要障碍之一。例如，聚合物官能团之间的碳原子数为奇数与偶数时，会导致熔点和力学性能出现大幅差异。这种奇偶效应（odd-even effect）在氢键结合型聚合物与低聚物中尤为显著。
为阐明该现象的结构本源，研究人员分别借助扫描隧道显微镜（scanning tunneling microscopy, STM）与X射线衍射（X-ray diffraction, XRD），对烷基二氨基甲酸酯的物理吸附单层及单晶开展了表征研究。相关二维与三维晶体结构在堆积几何维度均呈现出清晰的奇偶效应。
奇偶堆积模式下分子间相互作用的适配性差异，与熔点变化趋势直接相关，并可通过计算分析进一步解析。此外，这类低聚物展现出反常的竞争吸附行为：相较于烷烃，二元溶液中对小分子物种的吸附相对偏好性有所提升。
上述结果可通过氢键密度效应得到合理解释——该效应源于有限基底表面积的竞争机制。本研究为理解低聚氨基甲酸酯（oligourethane）表面涂层提供了理论模型，并证实了分子结构与氢键作用在决定吸附行为中的重要性。