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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
The Structure of Asphaltenes During Precipitation Investigated by Ultra-Small-Angle X-ray Scattering Yuan Yang, Wattana Chaisoontornyotin, and Michael Paul Hoepfner Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01873 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018
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The Structure of Asphaltenes During Precipitation Investigated by Ultra-Small-Angle X-ray Scattering Yuan Yang†, Wattana Chaisoontornyotin‡, and Michael P. Hoepfner∗† †
Department of Chemical Engineering, The University of Utah, 50 South Central Campus Drive, Salt Lake City, UT 84112 ‡ Center of Innovation for Flow through Porous Media, University of Wyoming, 1000 E University Ave, Laramie, WY 82071 ABSTRACT Time-resolved size and structure measurements of asphaltenes while in the process of precipitating were monitored for the first time using ultra-small-angle X-ray scattering (USAXS). The results revealed that asphaltenes precipitating from a heptane-toluene mixture demonstrate a hierarchical structure of an asphaltene-rich phase (e.g., droplet) that further agglomerates into fractal flocs. The fractal flocs that form by the agglomeration of the asphaltene-rich phase are what is commonly detected by optical microscopy above the precipitation detection point. The surface of the asphaltene-rich phase is initially rough and transitions to a smooth interface, as would be expected for a highly viscous liquid. Simultaneous SAXS measurements were also performed to investigate the structure of soluble asphaltenes, providing comprehensive structural characterization from the nanometer to micrometer length scales as a function of time. Further, the results demonstrate that the size and concentration of asphaltenes remaining in solution (e.g., soluble asphaltenes) does not change during precipitation, while the structure of insoluble asphaltenes varies. The ability to measure the properties of asphaltenes as they undergo precipitation opens new opportunities for understanding the fundamental mechanisms of asphaltene deposition and aggregation and the impact of chemical inhibitors to alter these processes. The universality of these conclusions and how specific properties vary as a function of asphaltene source and solution properties can provide valuable insight into asphaltene behavior.
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1. Introduction The destabilization of asphaltenes can cause numerous problems in the processing of petroleum products, including deposition on surfaces and stabilization of water-in-oil emulsions.1–4 Asphaltenes are a polydisperse fraction in crude oil whose precipitation mechanism is typically studied in the laboratory by the addition of a precipitant, such as n-heptane.2,5–8 Extensive study of asphaltene phase behavior has been performed previously; however, the highly associating nature of asphaltenes complicates thermodynamic stability and kinetic aggregation predictions.3,9–11 Asphaltenes in good solvents and in crude oil assemble into “nanoaggregates” (aggregates of 5-8 molecules with a radius less than 5 nm12–15) and further associate into stable fractal clusters with larger sizes (radius of gyration, Rg, less than approx. 40 nm) and a loose internal structure (i.e., extensively solvated).16,17 When a precipitant is added to an asphaltene solution, nanometer-scale asphaltene fractal clusters can be destabilized and aggregate into larger (>50 nm) and denser structures (i.e., less internal solvation), previously called insoluble clusters/asphaltenes.15 Experiments have demonstrated that asphaltene deposits can form before insoluble particles are detected by optical microscopy, revealing that submicron insoluble asphaltenes can deposit.6 Furthermore, modeling investigations of asphaltene deposition obtain good agreement with experimental results by assuming that asphaltenes deposit as colloidal particles with a diameter on the order of 100 nm.18,19 However, verification of the size or structure of insoluble asphaltenes during the process of destabilization and aggregation on the submicron length-scale has not be previously obtained. In order to understand the asphaltene precipitation mechanism, it is important to study the transition from nanometer-sized soluble asphaltene clusters to micrometer-length precipitated material during the destabilization and aggregation process.
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Slow kinetics of precipitation for asphaltenes were uncovered by Maqbool et al. and other researchers, and demonstrate that the time required to transition from the nano-scale to the macroscale can vary between seconds to months based on the solution conditions.20,21 After adding a precipitant to an asphaltene system, the mass of precipitated asphaltenes increases as a function of mixing time until the equilibrium quantity is obtained.7,20,22,23 Numerous mechanisms have been applied to model asphaltene precipitation and phase behavior: liquid-solid or liquid-liquid phase transition,
10,24–27
colloidal aggregation,9,28–31 and heterogeneous nucleation.32 However, the
microscopic mechanism of asphaltene destabilization is still poorly understood3,15 and the lack of direct observation on how asphaltenes arrange in solvents limits the validation of modeling approaches. Numerous methods have been applied to study asphaltene structural changes during destabilization. Time-resolved asphaltene aggregation that is initiated by precipitants (e.g., n-heptane) was previously studied using dynamic light scattering,33,34 confocal microscopy,35 optical microscopy,20,36,37 and small-angle scattering (SAS).15–17,29,31,38,39 Dynamic light scattering results show the time-resolved radius of asphaltene aggregates increasing from 300nm to more than 4 microns,33,34 but only provide size measurements based on a spherical particle shape without insight into the shape, internal structure, or interface properties. Microscopy is commonly used to visualize the structure of asphaltenes during precipitation but has a minimum detectable size of approx. 500 nm.20,36,37 Both of these approaches are unable to directly probe the internal structure and organization of asphaltenes at length scales below a few hundred nanometers. Detailed structural characterization
