Mutina Albina R, Hürlimann Martin D
Kazan State University, Kremlevskaya Str., 18, 420008 Kazan, Russia.
J Phys Chem A. 2008 Apr 17;112(15):3291-301. doi: 10.1021/jp710254d. Epub 2008 Mar 12.
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.
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动扩散系数，而在低磁场下测量的弛豫时间主要探测分子的转动扩散。许多复杂流体由大小和化学性质分布广泛的分子混合物组成。这导致测量的扩散系数和弛豫时间相应地分布广泛。一阶近似下，这些分布由分子大小分布决定。在此，我们表明通过测量二维扩散 - 弛豫分布函数可以获得关于化学成分的额外信息，该量还取决于分子的形状和化学相互作用。我们用一系列烃类混合物的扩散 - 弛豫分布函数的实验结果对此进行说明。对于不含大量沥青质的油，扩散 - 弛豫分布函数遵循幂律行为，其指数取决于饱和烃和芳烃的相对丰度。含沥青质的油偏离此趋势，因为沥青质分子作为其他分子的弛豫对比剂，而不显著影响它们的扩散系数。在蜡析出温度以下的含蜡油中会形成凝胶。这在测量的扩散 - 弛豫分布函数中有所体现，其中凝胶网络造成的限制降低了扩散系数，而对弛豫速率影响不大。
