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New Answers to the Optical Interrogation of Asphaltenes. Complex States of Primary Aggregates from Steady State Fluorescence Studies Igor N. Evdokimov, Aleksey A. Fesan, and Aleksandr P. Losev Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01943 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016
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New Answers to the Optical Interrogation of Asphaltenes. Complex States of Primary Aggregates from Steady State Fluorescence Studies Igor N. Evdokimov,* Aleksey A. Fesan, and Aleksandr P. Losev Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, Moscow B-296, GSP-1, 119991, Russia
ABSTRACT: This study examined the states of primary asphaltene aggregates in 0.2-27 mg/L toluene solutions by steady state fluorescence emission (SSFE) techniques. The experimental results do not support the conventional models of “consecutive aggregation” with interdependent states of aggregates and with monotonic increase of the complexity of aggregates with increasing concentration. The observed concentration dependencies of SSFE spectra were strongly nonmonotonic, with several intervals of “apparent reentrance” when the measured properties in more concentrated solutions returned to those in less concentrated ones. Literature analysis revealed qualitatively similar nonmonotonic/”reentrant” effects of asphaltene concentration in experimental results of other research groups. We suggest that the complex effects of concentration are consistent with autonomous kinetic routes to the independent states of molecular aggregates observed in most experiments with asphaltenes in good solvents.
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1. INTRODUCTION For several decades, the dominant conceptual model for aggregation of asphaltenes in native petroleum and in solutions (cf. a critical review in ref. 1) has been based on compact clusters (“micelles”, “nanoaggregates”) formed by assembly of some basic molecules, conventionally referred to as “asphaltene monomers”.2,3 Presumably, the distinctive structural units of these “monomers” are fairly large sheets of 7-10 condensed aromatic rings which facilitate aggregation into parallel stacks via π-π interactions at asphaltene contents above a specific “critical micellar concentration” (CMC) of 2-10 g/L2,3 or a “critical nanoaggregate concentration” (CNAC) of ca. 100 mg/L.4,5 However, a number of publications by various research teams challenge this popular paradigm for asphaltene behavior.1,6-15 In particular, recent steady state fluorescence emission (SSFE) experiments14,15 demonstrated that aggregating asphaltene “monomers” appear to be much smaller molecular species (predominantly containing 1-3 ring aromatic fluorophores). On the other hand, these SSFE data showed that more complex asphaltene molecules with multi-ring aromatic fluorophores (e.g. like those imaged by atomic force microscopy5) are apparently inactive during aggregation over the studied concentration range (≤27 mg/L). Moreover, now there is ample experimental evidence that some primary asphaltene aggregates are formed at concentrations as low as 0.05-0.5 mg/L even in “good” solvents (benzene, toluene, toluene/methanol, etc.).11-15 Preliminary SSFE data15 also indicated that mono-, di- and triaromatic “monomers” may contribute to primary aggregates at somewhat different asphaltene concentrations. As a result, the complexity of primary asphaltene aggregates may be a non-monotonic function of asphaltene content in solutions. This paper presents new SSFE data on the complex behavior of primary aggregates in toluene solutions with small increments of asphaltene concentration, and then draws on the recent literature to support a new view of the structure of primary asphaltene aggregates.
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2. EXPERIMENTAL 2.1. Materials. n-C7 (heptane insoluble) solid asphaltenes were separated from a heavy Russian crude oil (oil #5 in Table 3) according to the standard ASTM D 6560 method.16 A stock solution of solid asphaltenes in toluene (“chemically pure” grade) was prepared with an asphaltene concentration of 1880 mg/L. This stock solution was stored in a hermetically sealed glass vessel in the dark and at ambient pressure and temperature (24.5-25.6 oC). In each sample of dilute solution the concentration of asphaltenes was verified by weighing ca. 50 ml of pure toluene before and after mixing with a pre-determined volume of the stock solution (weighting accuracy of 0.1 mg). Note, that asphaltene dissolution/aggregation have recently been proven to be kinetic phenomena and can take several hours,17 days or weeks in some cases.18 Consequently, optical tests were performed at various times (from 1 hour to 7 days) after preparation of representative samples, to ensure that the asphaltene aggregates in the studied dilute solutions no longer evolved. The results virtually stabilized after a period of three days; hence, asphaltenes in solutions that aged over this standard period were regarded to be in their equilibrium states. 2.2 Measurements of Steady State Fluorescence Emission (SSFE) Spectra. SSFE measurements were conducted by using a Cary Eclipse fluorescence spectrophotometer with classic 90o geometry and 10-mm quartz sample cuvettes. The excitation wavelength λex was set to 265 nm to ensure emission from the smallest asphaltene fluorophores.14,15 The SSFE spectra were recorded with 5-nm excitation and emission slits in a 265-550 nm emission wavelength λem range with 1-nm intervals and a scan rate of 120 nm/min. The measured spectra were corrected for “inner filter effects” (IFE) which can be segregated into “primary IFEs” from the attenuation of the excitation light with λex and “secondary IFEs” that attenuate the fluorescence emission with λem.19 In this study, we employed the absorbance-based approach for IFE correction,19 where the observed fluorescence intensity ( I λobs ex , λem ) is converted into
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the corrected fluorescence intensity ( I λcorr ex , λem ) for each pair of excitation and emission wavelengths by the following transformation: obs {0.5×[ A ( λex ) + A ( λem )]} I λcorr ex ,λem = I λex , λem × 10
The factor 0.5 in the power exponent appears in the 90o geometry because absorbance (excitation) and fluorescence (emission) are assumed to occur at the midpoint of the cuvette. Optical absorbance spectra A(λ), which have been employed for IFE correction, were measured at the 270-1100 nm range under an SF-56 Spectrophotometer (ZAO “OKB SPECTR”, Saint-Petersburg, Russia). Ten-millimeter sample and reference (toluene) quartz cuvettes were used, and the measurements were performed at 24±0.5 oC. Large short-wavelengths absorbance precluded the possibility of IFE correction in samples with asphaltene concentrations >27 mg/L. Finally, the corrected SSFE data for the asphaltene solutions were further processed by subtracting the background solvent fluorescence emission, measured with pure solvent (toluene) in the sample cell.
3. RESULTS AND DISCUSSION 3.1. Characteristic Fluorescence Emission Peaks from Basic Asphaltene Molecules and Primary Aggregates. Representative SSFE spectra for some asphaltene solutions are shown in Figure 1. All spectra are normalized by the maximum intensity in the common group of overlapping peaks at 270-315 nm range of emission wavelengths (for further analysis delimited by the left dashed line in the figure). In our previous publications14,15 these peaks we attributed to basic (individual, non-aggregated) asphaltene molecules with 1-2 ring aromatic fluorophores. This conclusion was supported by comparison with standard SSFE spectra of individual hydrocarbon molecules20-22 (Table 1) and of molecules with hetero/metal atoms which are known to influence the aggregation behavior of asphaltenes1 (Table 2). ACS Paragon Plus Environment
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Figure 1. Fluorescence emission spectra from representative asphaltene solutions, obtained with 265 nm excitation. The data sets are labeled with the respective asphaltene concentrations (in mg/L). The vertical dashed lines delimit characteristic ranges of emission wavelengths (cf. text). The spectra are normalized to the maximum intensities of the short-wavelengths peaks.
Table 1. Positions of fluorescence emission peaks in standard spectra of some simple 1-4 ring hydrocarbon molecules and their aggregates.
No. of Compound
Emission peak wavelength (nm),
rings
[Reference] Individual
Two-membered
molecules
aggregates
1
styrene
306 [20]
335 [23]
2
naphthalene
320 [22]
405 [24]
3
phenanthrene
365 [20]
415 [25]
3
anthracene
389 [20]
468 [26]
4
pyrene
391 [20]
483 [27]
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Table 2. Positions of fluorescence emission peaks in standard spectra of some molecules with hetero/metal atoms and their aggregates. No. of Hetero/ Compound rings
Emission peak wavelength (nm),
metal
[Reference]
atoms
1
N
1,4-dicyanobenzene
1
S
thiophene
Individual
Two-membered and
molecules
higher aggregates
290 [21]
378 [21]
285 (weak) [28]
408-522 [29] 540-670 [30-32]
2
S
benzothiophene
300-310 [33]
460-490 [34]
3
S
dibenzothiophene
328 [35]
400-495 [34,36]
2
N
quinoline
384 [37]
430-480 [38,39]
3
N
benzoquinoline
351-367 [40]
440 [40]
V, Ni
metalloporphyrins
No fluorescence [41,42]
Figure 1 shows that in SSFE spectrum for solution with asphaltene content of 0.2 mg/L the above 1-2 ring peaks are dominant. There also are several low-intense peaks which may be attributed to basic asphaltene molecules with 2-3 ring aromatic fluorophores (with a possible presence of some 4-ring aromatic systems). In particular, according to Tables 1 and 2, peaks at 320325 nm are characteristic for simple 2-ring aromatic systems (like in naphthalene) and some 3-ring molecules with heteroatoms (like in dibenzothiophene). Furthermore, peaks at 355-365 nm are indicative of non-linear 3-ring fluorophores (like in phenanthrene and benzoquinoline). The broad peak near 370-390 nm most probably is a signature of linear 3-ring fluorophores (like in anthracene) although the same wavelengths range is characteristic for emission from 2-ring quinoline fluorophores and from compact pericondensed 4-ring aromatic systems (like in pyrene). It should be emphasized that the intensities of fluorescence peaks are not directly proportional to the relative abundances of the molecules with respective fluorophores in the studied samples because of differences in individual extinction coefficients, quantum yields, etc.19 Hence, without additional
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information from other experimental techniques, the quantitative frequency distribution of basic asphaltenes versus the size of their fluorophores cannot be derived solely from fluorescence measurements.43 Furthermore, as noted before, the positions of SSFE peaks indicate only a number of aromatic rings in asphaltene fluorophores, while any conclusions regarding the nature of particular substituents in basic asphaltene molecules would be merely speculative.15 Finally, a distinctive feature of 0.2 mg/L solution is the absence of any longer-wavelength (≥400 nm) characteristic emission peaks which may have been attributed to non-aggregated asphaltene molecules with larger fluorophores. For example, non-compact linear 4-ring fluorophores (like in tetracene) would emit at 480-510 nm and all fluorophores with ≥5 aromatic rings would emit at wavelengths above 410 nm.20,22 However, SSFE spectra at large wavelengths are continuous and for all studied asphaltene concentrations exhibit the exponential behavior.15 The qualitatively new feature which appears in SSFE spectra from samples with asphaltene concentrations ≥ 0.75 mg/L is a group of overlapping intense peaks at 320-500 nm range of emission wavelengths (delimited by the first dashed line in Figure 1), earlier attributed15 to some asphaltene molecular aggregates with tightly linked aromatic fluorophores. In particular, literature analysis (cf. Tables 1 and 2) shows that the characteristic SSFE peaks for molecular complexes of monoaromatics with various substituents, like styrene and 1,4-dicyanobenzene, are observed at wavelengths below 380 nm.21-23 However, the presence sulfur in 1-ring thiophene shifts the peaks from thiophene dimers above 400 nm29 and from polymeric aggregates above 540 nm30-32. SSFE peaks from tight complexes of simple 2-ring fluorophores typically are observed at wavelengths not exceeding 410 nm (e.g., at 404 nm for aggregates of naphthalene24). As follows from Table 1, SSFE peaks at long wavelengths (>410 nm) are more typical for emission from tight aggregates of the largest (3-4 ring) simple fluorophores observed in basic asphaltene molecules. Table 2 shows that for 2-ring and 3-ring molecules with heteroatoms characteristic peaks from aggregates appear in the
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range of 400-490 nm. Finally, it should be noted that SSFE spectra are not expected to provide any information on the presence of metalloporphyrins in asphaltene solutions. It is well known that the vanadium and nickel petroporphyrins can be identified and quantified by their optical absorption spectra, with an intense Soret band around 400 nm energy; however these compounds do not exhibit fluorescence.41,42 For further discussion of experimental data it is also important that significantly red-shifted SSFE peaks are observed after the appearance of the nearest neighbor fluorophore in the simplest two-membered aggregates of 1-4 ring fluorophores (Table 1 and 2) while the positions of these peaks only marginally depend on a number of new fluorophores that are attached to the initial nearest neighbors.15 Hence the exact sizes (aggregation numbers) of emerging asphaltene aggregates can not be deduced solely from SSFE measurements. However, the above analysis shows that in the discussed 320-500 nm range SSFE peaks from aggregates of smaller (1-2 ring) fluorophores typically appear at lower wavelengths than peaks from aggregates of larger (3-4 ring) fluorophores. Somewhat arbitrary, the boundary between peaks from aggregates from smaller and larger aggregates was drawn at 410 nm (as indicated by the second dashed line in Figure 1). The presented experimental data show that the intensity of emission from 1-2 ring aggregates (at 320-410 nm) is not a straightforward function of asphaltene concentration. Namely, in this wavelengths range the normalized intensity significantly increases with concentration up to 1.9 mg/L, while at 4.3 mg/L it becomes notably lower and returns to the previous high level only at 7.7 mg/L (for a more detailed data analysis cf. figures 2 and 3). Concentration dependence of emission intensity from 3-4 ring aggregates also exhibits some non-monotonic features for solutions with asphaltene concentrations close to 4.3 mg/L (for details cf. figure 2). The growing peaks of asphaltene aggregates in SSFE spectra of figure 1 mask the behavior of low-intense peaks from non-aggregated basic molecules with 2-4 ring fluorophores. However, ACS Paragon Plus Environment
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characteristic peaks from individual single-ring fluorophores (at emission wavelengths 10) basic asphaltene molecules. Furthermore, Figure 6 presents additional evidence for complicated nonmonotonic effects of asphaltene concentration on the properties of aggregates revealed by fluorescence measurements. In particular, the specific significance of ∼ 4.3 mg/L (cf. Figures 2 and 3) is confirmed by maxima of both relative intensity and position of the high-MW peak. To conclude this Section, it should be noted that one of the reviewers insisted that the results of Figure 6 are not appropriate for comparison with our SSFE data, because the experimental conditions are absolutely different. In the reviewer’s opinion, the molecular aggregation in FTICR MS, which must be controlled by the ionization process under vacuum condition, is not the same as that in toluene solution under ambient condition. In support of the above comparison, we may point out that the authors of ref.11 are a group of highly qualified specialists in mass-spectrometry and in petroleum science, headed by Professor Alan G. Marshall, founder of “Petroleomics”.3 In this paper and in other publications these specialists present multiple arguments that the measured massspectra reflect aggregation states of asphaltenes in the studied solutions. True, they acknowledge that in mass-spectrometry not all molecules are probed because “only a subset (the most aromatic component) of asphaltenes is ionized”.11 However, the same limitations are inherent for our fluorescence methods as only aromatic asphaltenes are fluorescent.
3.5. Possible Nature of the Observed Nonmonotonic Effects of Asphaltene Concentration in Solutions. Absence of “Consecutive Aggregation” in Asphaltene Experiments. In solvents of constant compositions (including native oils at unchanging ambient ACS Paragon Plus Environment
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conditions) a key parameter in controlling the aggregation state of asphaltenes is their concentration. As described in Introduction, in the dominant conceptual models1-5 asphaltenes exist as “monomers” below a certain critical concentration (CMC/CNAC), while above CMC/CNAC the basic aggregating species are micelles/nanoaggregates which form more complex clusters at higher concentration (Figure 7A). Also, some authors suggest multi-step concepts6-14 in which aggregates of increasing complexity are formed by sequential addition of asphaltene “monomers” with increasing concentration (Figure 7B). The latter concepts are more close to the well-known and mathematically well-developed theories of step-wise molecular aggregation.55-58 These theories are based on the sequential addition of monomer X1 to oligomer Xi−1 containing i − 1 molecules: Ki X i −1 + X 1 ←→ Xi
(1)
In practice the dependence of equilibrium constant Ki on the number of molecules in an aggregate (referred to as “the profile of equilibrium constant”57) is always chosen by an investigator depending on the system under investigation. The two most important cases employed in the majority of applications (cf. Refs. 55,56 and references therein) are: (1) equal values of Ki at all stages of aggregation (the “EK model” or the “isodesmic model”) and (2) the profile Ki = f(i) is introduced empirically as a function of the number of molecules, i, in aggregate.
Figure 7. “Consecutive aggregation” schemes with parental relationship between asphaltene aggregates at increasing concentration.
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The important common assumption of all these concepts/theories is a necessary parental relationship between more and more complex states of asphaltene aggregates arising with increasing concentration. Hence, neither of such widely employed descriptions of asphaltene aggregation can account for experimentally observed nonmonotonic/reentrant concentration effects illustrated in Figures 2-6. However, this does not mean that the above descriptions are wrong or should be fundamentally revised. The simple, but usually overlooked, answer to the apparent experimentaltheoretical controversy is that aggregation of asphaltenes in solvents of constant composition is never observed experimentally. In particular, it is not technically possible to realize the sequential aggregation mechanisms of Figure 7 or Eq. 1. Addition of a very dilute solution of “monomers” to a solution of some aggregates would always decrease the net asphaltene concentration. In practice, as illustrated in Figure 8, samples with varying contents of asphaltenes are commonly prepared by independent procedures of diluting some stock sample (concentrated solution or solid asphaltenes). Consequently, there is no direct parental relationship between molecular/aggregate systems in any samples with close asphaltene concentrations. Furthermore, the observed states of asphaltenes in the studied samples obviously are achieved by mechanisms of dissolution and not of aggregation. Of major importance are strong kinetic effects – as noted in section 2.1, equilibrium states of asphaltenes may be observed only after aging the freshly prepared samples for several days. On the basis of our recent and previous experimental results12-15 we suggest that the aging process proceeds via two different stages, labeled as “fragmentation” and “dissolution” in Figure 8. In the first stage which lasts at least for several hours, initial large asphaltene aggregates from stock samples are broken into smaller, but still complex, fragments with fairly close individual properties. Hence, in course of this stage there are only quantitative differences (in the numbers of fragments) between various samples and the registered effects of asphaltene concentration would
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appear fairly continuous. This situation is illustrated in Figure 4 by experimental data on refractive index increments in toluene solutions of asphaltenes aged for 3 hours.14
Figure 8. Scheme with autonomous routes to independent systems of asphaltene aggregates at different concentrations.
Apparently, in the next stage of aging which lasts for several days, fairly similar fragments of stock asphaltene aggregates in each sample further evolve by the individual successions of dissolution events specific for the particular molecular environment (i.e. for the particular asphaltene concentration). Note, that in theoretical models which account for kinetics of aggregation/dissolution (cf. refs. 55,57 and references therein) constants Ki in specific molecular events (like that described by Eq. 1) were also shown to depend on overall molecular concentration. Eventually in all samples, as schematically illustrated in the upper part of Figure 8, autonomous structural states S1, S2, etc. of asphaltenes with qualitatively different properties may be formed. In this scheme, there is no reason to expect that the succession of equilibrium states in samples with decreasing concentration S4→S3→S2→S1 would be repeated in the course of any nonequilibrium dissolution route (as illustrated in the upper left corner of Figure 8). Furthermore, in the scheme of Figure 8 there is also no ban on the above discussed “apparent reentrance” (cf. Figures 2 and 3), i.e. on situation where the equilibrium properties of solution S4, more concentrated than solution S3, are ACS Paragon Plus Environment
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more close to the properties of much less concentrated solution S2. As an additional supporting illustration, consider the experimental data set in Figure 4 for toluene solutions of asphaltenes equilibrated/aged for 10 days,14 where nonmonotonic concentration effects with well-resolved “reentrant” minima become clearly seen, in contrast to the non-equilibrium (3 hours) data set.
3.6. On the possible role of adsorption and evaporation effects in our experiments. Some critical scientists suggest that fluorescence studies in extremely dilute solutions are strongly affected by removal of multi-ring components of asphaltenes via adsorption onto the quartz surface of the cuvettes and glass vials used for sample preparation and analysis. Such conclusions they support by references to experimental data of Acevedo et al.59-61. In particular, Acevedo et al. report that as a result of adsorption onto silica surfaces, asphaltene content in studied solutions decreased from 400 to 238 mg/L61 and from 20 to 13.5 mg/L60, i.e. by ∼40% and by ∼33%, respectively. However, these large numerical values do not indicate a crucial role of adsorption in our fluorescent studies. In particular, in their studies Acevedo et al. employed silica in the form of layers of fine powders on flat substrates, namely, commercial “silica 60” with a pore diameter of 60 Å, pore volume of 0.8 mL/g, mean diameter size of 10−12 µm, and very high specific areas of 32-44 m2/g.60,61 Hence, the total area of their silica surfaces approached 106 cm2. On the other hand, in our fluorescence studies the quartz/glass areas in contact with any dilute sample (in 1 mL quartz measurement cell and in 50 mL glass vials employed for sample preparation) was ca. 60 cm2. Acevedo et al. concluded that in extremely dilute asphaltene solutions, like those employed in our studies, a monolayer of adsorbed asphaltenes is formed.60,61 Hence, the total mass of the absorbed asphaltenes MA may be estimated with literature data62 for a typical radius (∼1 nm) and a typical mass (∼500 Da = 8⋅10-19 mg) of a multi-ring asphaltene molecule. For random close packing in a monolayer of such molecules at 60 cm2 surfaces, MA≈9⋅10-4 mg. In our most dilute samples (∼1
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mg/L) the initial content of asphaltenes in a 50 mL vial is ca. 0.05 mg and the relative changes due to adsorption would be below 2%. If the absence of characteristic SSFE peaks from multi-ring asphaltenes is explained by selective adsorption, it would indicate a negligible quantity (
