Determination of asphaltene structural parameters by Raman spectroscopy

Abstract

The objective of this study is to determine the aromatic sheet diameter (L a ) and the number of aromatic sheets per stack (n) for asphaltene molecules from Raman spectra by investigating and optimizing empirical equations proposed in the literature for both crystalline and amorphous carbon materials. The results clearly indicated that the best empirical equation for determining the aromatic sheet diameter for asphaltene was the equation proposed by Tuinstra and Koenig, but only when the graphitization indicator I D /I G would have been determined from the peak integrated ratio, rather than from the peak intensity ratio. The best results of L a were obtained when the Raman spectra were deconvoluted and fitted with three peaks using Gaussian function. The number of aromatic sheets per stack for asphaltene was also determined by examining various empirical equations proposed for crystalline carbon materials. The results indicated that these equations could be applied on asphaltene if the first-order region of Raman spectrum were to be fitted with two peaks, representing the G and D bands. Fitting the spectra with more peaks could result in splitting the G band into two bands (i.e., G and D2 bands), which would shift the G-band position to a wavenumber below 1581.6 cm À1 , and, subsequently, prevent the utilization of these empirical equations.

Formation and Stability of Heterogeneous Organo–Ionic Surface Layers on Geological Carbonates

“…Raman spectra of all aged minerals present D1 (1344–1357 cm –1 ) and G bands (1565–1580 cm –1 ). These vibrational modes represent the graphitic ordering of polyaromatic hydrocarbons (Figure S1), also previously reported for molecular organizations of polyaromatic oil components such as asphaltenes − and kerogens . We do not consider the presence of these Raman bands as an absolute determinant of the molecular identity of the surface-active agent but rather defer to solubility testing and other analytical methods (later section).…”

“…Confocal Raman microscopy is an established method to characterize the molecular organizations of graphitic materials such as asphaltenes. ,,− Here we apply CRM to elucidate the low-salinity effect, investigating the desorption and structural transitions of the surface-active asphaltene assemblies in various brine compositions (Figure a).…”

“…To address the impacts of brine salinity on the supramolecular organization of the mineral-attached asphaltenes, we consider the Raman peaks at ∼1350 and ∼1580 cm –1 , respectively, known as the D and G bands characteristic for polyaromatic materials. Using these spectral features, the degree of organization of polyaromatic materials is often investigated via an empirical relation for the aromatic sheet diameter ( L a ) defined as where E l is the excitation laser energy (eV) and A G and A D are the integrated areas of the D and G bands, respectively. ,, Previous studies show that L a corelates with aromatic sheet diameter and number of sheets in an asphaltene stack. , L a is also reported to inversely scale with the hydrogen-bonding capability of asphaltenes . Considering the chemical heterogeneity of the carbonate-bound asphaltene material, we utilize L a distributions only as a qualitative probe for the structural (re)­organization of mineral-attached asphaltenes under different salinity conditions.…”

“…Using these spectral features, the degree of organization of polyaromatic materials is often investigated via an empirical relation for the aromatic sheet diameter (L a ) defined as where E l is the excitation laser energy (eV) and A G and A D are the integrated areas of the D and G bands, respectively. 63,131,133 Previous studies show that L a corelates with aromatic sheet diameter and number of sheets in an asphaltene stack. 63,64 L a is also reported to inversely scale with the hydrogen-bonding capability of asphaltenes.…”

“…63,131,133 Previous studies show that L a corelates with aromatic sheet diameter and number of sheets in an asphaltene stack. 63,64 L a is also reported to inversely scale with the hydrogen-bonding capability of asphaltenes. 134 Considering the chemical heterogeneity of the carbonate-bound asphaltene material, we utilize L a distributions only as a qualitative probe for the structural (re)organization of mineral-attached asphaltenes under different salinity conditions.…”