Energy Institute, City College of New York, New York, New York 10031, United States.
Langmuir. 2012 Jul 3;28(26):9986-95. doi: 10.1021/la301423c. Epub 2012 Jun 22.
Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.
沥青质是原油中高分子量的组成部分，不溶于正庚烷而溶于甲苯。它们有助于稳定在原油回收过程中形成的油包水乳状液，并阻碍液滴聚结。因此，沥青质对油水分离过程和油的产量产生不利影响。鉴于此，需要更好地了解沥青质在油水界面的物理化学效应。本研究基于悬挂滴实验中此类体系的界面张力的新数据，阐明了这些效应的某些方面，这些数据得到了核磁共振（NMR）和动态光散射（DLS）研究结果的支持。使用不同沥青质浓度（质量分数）和溶剂粘度的悬挂滴实验表明，短时间内界面张力降低动力学受单体存在的沥青质部分的体扩散控制。在低质量分数下，大部分沥青质似乎以单体形式存在，但在质量分数大于约 80ppm 时，它们似乎聚集为更大的结构，这与 NMR 和 DLS 结果一致。在较长时间内，界面张力降低动力学较慢且不再受扩散控制。为了研究后期阶段的控制机制，悬挂滴实验以类似于朗缪尔槽的方式进行，在各种条件下老化的液滴扩展过程中测量界面张力。观察到界面张力取决于表面覆盖率而不是时间。所有观察结果表明，后期阶段的转变是到吸附势垒控制的阶段，而不是构象弛豫的阶段。
