Pahlavan Farideh, Mousavi Masoumeh, Hung Albert, Fini Ellie H
Center for Innovation in Materials, Methods and Management, North Carolina A&T State University, 1601 E. Market St, Greensboro, North Carolina 27405, USA.
Phys Chem Chem Phys. 2016 Apr 7;18(13):8840-54. doi: 10.1039/c5cp07180a. Epub 2016 Feb 15.
The nature and origin of bee-like microstructures (bees) in asphalt binders and their impact on asphalt oxidation have been the subject of extensive discussions in recent years. While several studies refer to the bees as solely surface features, some others consider them to be bulk microcrystalline components that are formed due to co-precipitation of wax and asphaltene molecules. In this study, we use a rigorous theoretical and experimental approach to investigate the interplay of asphalt components (mainly asphaltene and wax) and their impact on bee formation. In the theoretical section, quantum-mechanical calculations using density functional theory (DFT) are used to evaluate the strength of interactions between asphaltene unit sheets in the presence and absence of a wax component, as well as the mutual interactions between asphaltene molecules (monomers and dimers) and paraffin wax. The results of this section reveal that paraffin waxes not only do not reinforce the interaction between the asphaltene unit sheets, they destabilize asphaltene assembly and dimerization. AIM (Atom in Molecules) analysis shows the destabilizing effect of wax on asphaltene assembly as a reduction in the number of cage and bond critical points between asphaltenes. This destabilization effect among interacting systems (asphaltene-asphaltene and wax-asphaltene) does not support the hypothesis that interaction between paraffin waxes and non-wax components, such as asphaltene, is responsible for their co-precipitation and bee formation. To further examine the effect of wax component on asphalt microstructure experimentally, we used atomic force microscopy (AFM) to study the surface morphology of an asphalt sample doped with 1% to 25% paraffin wax. In agreement with the conclusions drawn from the DFT approach, our experiments indicate that paraffin wax tends to crystallize separately and form lamellar paraffin wax crystal inclusions with 10 nm thickness. Moreover, the addition of 3% wax into asphalt results in a significant increase in surface roughness from 0.5 nm to 4.1 nm and an increase in bee wavelength from 651 nm to 1038 nm.
近年来，沥青结合料中类蜂状微观结构（蜂状物）的性质、起源及其对沥青氧化的影响一直是广泛讨论的主题。虽然一些研究将蜂状物仅视为表面特征，但另一些研究则认为它们是由于蜡和沥青质分子的共沉淀而形成的块状微晶成分。在本研究中，我们采用了严谨的理论和实验方法来研究沥青成分（主要是沥青质和蜡）之间的相互作用及其对蜂状物形成的影响。在理论部分，使用密度泛函理论（DFT）进行量子力学计算，以评估在有蜡成分和无蜡成分的情况下沥青质单元片之间相互作用的强度，以及沥青质分子（单体和二聚体）与石蜡之间的相互作用。这部分的结果表明，石蜡不仅不会增强沥青质单元片之间的相互作用，反而会使沥青质聚集和二聚化不稳定。分子中的原子（AIM）分析表明，蜡对沥青质聚集的去稳定作用表现为沥青质之间笼状和键临界点数量的减少。相互作用体系（沥青质-沥青质和蜡-沥青质）之间的这种去稳定作用并不支持石蜡与非蜡成分（如沥青质）之间的相互作用导致它们共沉淀和形成蜂状物的假设。为了通过实验进一步研究蜡成分对沥青微观结构的影响，我们使用原子力显微镜（AFM）研究了掺杂1%至25%石蜡的沥青样品的表面形态。与从DFT方法得出的结论一致，我们的实验表明，石蜡倾向于单独结晶并形成厚度为10 nm的层状石蜡晶体夹杂物。此外，向沥青中添加3%的蜡会导致表面粗糙度从0.5 nm显著增加到4.1 nm，蜂状物波长从�51 nm增加到1038 nm。
