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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Correlation of transverse and rotational diffusion coefficient: A probe of chemical composition in hydrocarbon oils
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Correlation of transverse and rotational diffusion coefficient: A probe of chemical composition in hydrocarbon oils

机译：横向和旋转扩散系数的相关性：烃油中化学成分的探讨

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摘要

Measurements of relaxation time and diffusion coefficient by nuclear magnetic resonance are well-established techniques to study molecular motions in fluids. Diffusion measurements sense the translational diffusion coefficients of the molecules, whereas relaxation times measured at low magnetic fields probe predominantly the rotational diffusion of the molecules. Many complex fluids are composed of a mixture of molecules with a wide distribution of sizes and chemical properties. This results in correspondingly wide distributions of measured diffusion coefficients and relaxation times. To first order, these distributions are determined by the distribution of molecular sizes. Here we show that additional information can be obtained on the chemical composition by measuring two-dimensional diffusion-relaxation distribution functions, a quantity that depends also on the shape and chemical interactions of molecules. We illustrate this with experimental results of diffusion-relaxation distribution functions on a series of hydrocarbon mixtures. For oils without: significant amounts of asphaltenes, the diffusion-relaxation distribution functions follow a power-law behavior with an exponent that depends on the relative abundance of saturates and aromatics. Oils with asphaltene deviate from this trend, as asphaltene molecules act as relaxation contrast agent for other molecules without affecting their diffusion coefficient significantly. In waxy oils below the wax appearance temperature a gel forms. This is reflected in the measured diffusion-relaxation distribution functions, where the restrictions due to the gel network reduce the diffusion coefficients without affecting the relaxation rates significantly.
机译：通过核磁共振测量弛豫时间和扩散系数是研究流体中分子运动的成熟技术。扩散测量可感知分子的平移扩散系数，而在低磁场下测得的弛豫时间则主要探测分子的旋转扩散。许多复杂的流体是由大小和化学性质分布广泛的分子混合物组成的。这导致测得的扩散系数和弛豫时间相应地分布较宽。首先，这些分布取决于分子大小的分布。在这里，我们表明可以通过测量二维扩散-松弛分布函数来获得有关化学成分的其他信息，该量还取决于分子的形状和化学相互作用。我们用一系列烃混合物上的扩散松弛弛豫分布函数的实验结果来说明这一点。对于不含以下物质的油：大量的沥青质，扩散松弛弛豫分布函数遵循幂律行为，其指数取决于饱和物和芳烃的相对丰度。含沥青质的油偏离了这一趋势，因为沥青质分子在不显着影响其扩散系数的情况下充当其他分子的弛豫造影剂。在蜡出现温度以下的蜡质油中会形成凝胶。这反映在测得的扩散松弛分布函数中，其中由于凝胶网络引起的限制会降低扩散系数，而不会显着影响弛豫率。

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