Gaikwad Ravi, Hande Aharnish, Das Siddhartha, Mitra Sushanta K, Thundat Thomas
Department of Chemical and Materials Engineering, University of Alberta , Edmonton, Alberta T6G 2V4, Canada.
Langmuir. 2015 Jan 20;31(2):679-84. doi: 10.1021/la503968v. Epub 2015 Jan 5.
In this paper, we provide measurement of charge of asphaltene nanoaggregates in air using electrostatic force microscopy. We obtain the average surface charge density of the nanoaggregates as 43.7 nC/cm(2). Among the different aspects of asphaltene, one of the least known is its charge and the effect of solvent and compositional variability (of asphaltene) in dictating this charge. For aqueous systems, asphaltene charge demonstrates a strong dependence on the pH and the salt concentration, indicating that a possible ionization of the surface groups leads to this charging. On the contrary, for asphaltene in nonpolar media (e.g., toluene and heptane), it is believed that asphaltene native charge is central in dictating this charging. This native charge is the solvent-independent charge or the asphaltene charge in air. Our measurements, therefore, provide the first direct quantification (i.e., a quantification of charge not from the measurement of the asphaltene mobilities, which in turn requires specification of the nonuniform asphaltene size distribution) of this asphaltene native charge by conducting the measurements in air. Similar measurements in a solvent may introduce a solvent-dependent value, thereby forbidding not only the exact quantification of this native charge but also the understanding of the specific role of the solvent. This measurement, therefore, will provide a useful starting point to quantify the mechanism of asphaltene charging in nonpolar solvents with important ramifications in deciphering the role of asphaltene in transport and handling of crude and heavy oils.
在本文中，我们使用静电力显微镜对空气中沥青质纳米聚集体的电荷进行了测量。我们获得纳米聚集体的平均表面电荷密度为43.7 nC/cm²。在沥青质的不同方面中，人们了解最少的之一是其电荷以及溶剂和（沥青质的）组成变化对这种电荷的影响。对于水性体系，沥青质电荷对pH值和盐浓度有很强的依赖性，这表明表面基团的可能电离导致了这种带电现象。相反，对于非极性介质（如甲苯和庚烷）中的沥青质，人们认为沥青质的固有电荷是决定这种带电现象的关键。这种固有电荷是与溶剂无关的电荷或空气中沥青质的电荷。因此，我们的测量通过在空气中进行测量，首次直接量化了这种沥青质固有电荷（即不是通过测量沥青质迁移率来量化电荷，而测量迁移率又需要确定非均匀的沥青质尺寸分布）。在溶剂中进行类似测量可能会引入与溶剂有关的值，从而不仅无法准确量化这种固有电荷，也无法理解溶剂的具体作用。因此，这种测量将为量化非极性溶剂中沥青质带电机制提供一个有用的起点，这对于解读沥青质在原油和重油运输与处理中的作用具有重要意义。
