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摘要: 为从原子层面揭示沥青热老化与紫外老化的底层机制，基于从头算分子动力学和密度泛函理论分析了沥青质在多种温度及紫外辐射条件下的老化反应路径与反应势能参数；基于傅里叶变换红外光谱试验，分析了原样沥青、热老化沥青和紫外老化沥青试样表面化学官能团的变化规律，比较了它们的老化程度。研究结果表明：沥青质老化涉及的亚反应包括由氧气或自由基攫氢所触发的环烷芳构化与含氧基团形成，以及直接的侧链均裂；沥青质老化机理可归纳为沥青质在氧气分子或自由基的侵袭下不断失去氢原子并转化为具有高反应性的不稳定结构，因而经由分子异构化或吸附氧原子等后续反应来降低自身能量，由此引发了沥青质老化行为的持续进展；温度提升不仅加快老化反应速率，还使更多类型的老化反应得以发生；芳构化反应的能垒最低，因此，在较低温度下即可发生，含氧基团的形成次之，而侧链均裂反应的能垒最高，只能在较高温度下才发生；在紫外线辐射下，沥青质分子跃迁至激发态，其反应能垒相比基态显著降低，能大幅加快老化反应；傅里叶变换红外光谱测试结果表明紫外老化沥青试样的老化程度远高于热老化沥青试样，验证了理论计算结果。Abstract: In order to reveal the underlying mechanisms of thermal and ultraviolet (UV) aging of asphalt at the atomic level, the aging reaction paths and corresponding potential energy parameters of asphaltenes under various temperature and UV radiation conditions were analyzed based on ab initio molecular dynamics and density functional theory. The change rules of chemical functional groups on the surfaces of virgin, thermal-aged, and UV-aged asphalt specimens were analyzed based on Fourier transform infrared spectroscopy tests, and their aging degrees were compared. Research results indicate that the involved subreactions of asphaltene aging include the cycloalkane aromatization and the formation of oxygen-containing groups triggered by O2 or radicals induced hydrogen abstraction, as well as direct homolytic cleavage on the side chains. The aging mechanism of asphaltenes can be summarized as follows: asphaltenes lose hydrogen atoms constantly and transform into highly reactive and unstable structures under the attack of O2 molecules or radicals, and thus their energy reduces through subsequent reactions such as molecular isomerization or adsorption of oxygen atoms. As a result, the continuous progress of asphaltene aging behavior is triggered. The increase in temperature not only accelerates the rate of aging reactions, but also triggers more types of aging reactions. The aromatization reaction has the lowest energy barrier and thus can occur at lower temperatures, followed by the formation of oxygen-containing groups, and the homolytic cleavage reaction on the side chains has the highest energy barrier and can only occur at higher temperatures. Under UV radiation, the asphaltene molecule transitions to the excited state, and its reaction energy barrier is significantly lower than that under the ground state. Therefore, it can significantly accelerate the aging reaction. Fourier transform infrared spectroscopy tests show that the aging degree of UV aging asphalt specimens is much higher than that of thermal-aged asphalt specimens, which verifies the theoretical calculation results.
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图 2 采用UVA-340光源对太阳光紫外线区域的模拟
Figure 2. Simulation of ultraviolet region of sunlight by UVA-340 light source
图 3 2 000 K温度下AIMD模拟得到的沥青质老化期间分子结构演变过程
Figure 3. Molecular structure evolution process during asphaltene aging from AIMD simulation at 2 000 K
图 4 2 000 K温度下模拟体系能量随老化时间的演化
Figure 4. System energy evolution with aging time in simulation at 2 000 K
图 6 沥青质老化前后分子间结合能变化
Figure 6. Changes in intermolecular binding energy before and after asphaltene aging
图 7 不同温度下的沥青质老化反应过程
Figure 7. Asphaltene aging reaction processes at different temperatures
图 8 沥青质老化反应体系势能面及潜在老化路径
Figure 8. Potential energy surface and potential aging paths of asphaltene aging reaction system
图 9 沥青质不同位置氧气攫氢及均裂反应的自由能垒
Figure 9. Free energy barriers of H-abstraction by O2 and homolysis reactions at different locations in asphaltene
图 10 基态及激发态下沥青质发生氧气攫氢和均裂反应的自由能垒
Figure 10. Free energy barriers of H-abstraction by O2 and homolysis reactions on asphaltenes in ground and excited states
图 11 基态和激发态下沿沥青质老化路径的自由能面与能垒
Figure 11. Free energy surfaces and energy barriers along asphaltene aging paths in ground and excited states
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