Properties of the Polar Fraction of Hassi-Messaoud Asphaltenes

Jul 24, 2012 - University of Sciences and Technology of Oran, Post Office Box ... and Heterogeneous Media by UV-Vis and Fluorescence Spectroscopic Stu...
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Properties of the Polar Fraction of Hassi-Messaoud Asphaltenes Mortada Daaou,§,‡ Dalila Bendedouch,‡ Ali Modarressi,† and Marek Rogalski*,† †

LCP- A2MC, Université de Lorraine, 1, bd Arago, 57070 Metz, France LCPM, University of Oran, Post Office Box 1524, 31000 El-M’naouer, Algeria § University of Sciences and Technology of Oran, Post Office Box 1505, 31000 El-M’naouer, Algeria ‡

ABSTRACT: Polar fractions of Hassi-Messaoud asphaltenes were obtained by two different extraction/precipitation procedures. Properties of these fractions were studied using elemental analysis, LDI-TOF mass spectroscopy, and solid state 13C NMR. The aggregation and the flocculation onset of polar fractions were determined. It was demonstrated that the most polar fraction of asphaltenes that is soluble in highly polar solvents, such as N-methyl pyrrolidone and nonsoluble in mildly polar or nonpolar solvents is composed of small molecules, containing a high oxygen ratio. This fraction plays an important role in asphaltene aggregate formation. The understanding of this role would be helpful for designing new antiflocculation agents.

1. INTRODUCTION Asphaltenes constitute the most complex fraction of the crude oil. They are conventionally defined as a fraction of the crude oil that is insoluble in n-alkanes but soluble in toluene.1 Because of their natural tendency to form aggregates, they may flocculate/precipitate and cause severe problems in oil production, transportation, and refining.2−5 The propensity of asphaltenes to flocculate is a complex function of the crude oil composition and does not depend directly on the asphaltene content.6 The Algerian oil from Hassi-Messaoud fields (less than 1 wt % of asphaltenes) displays severe precipitation problems, but the rich in asphaltenes, 17.2 wt %, Venezuelan oil from the Boscan field is stable. Recently,7 we have studied the separation of asphaltenes in fractions composed of asphaltene molecules differing in polarity. It was demonstrated that the aromaticity and the content of heteroatoms such as sulfur, nitrogen, and oxygen as well as traces of metals like nickel, iron, and vanadium are key parameters necessary to understand the mechanism of asphaltene aggregation. These elements are mostly contained in the polar asphaltene fractions. The widely accepted mechanism of asphaltene aggregation involves π−π overlapping between aromatic sheets and formation of hydrogen and charge transfer bonding between functional polar groups.8,9 According to Leon et al.,10 the unstable asphaltenes are characterized by high aromaticity, low hydrogen content, and high condensation ratio of aromatic rings. Maruska and Rao11 demonstrated that interactions between heteroatoms and particularly acid−base interactions lead to aggregation of asphaltenes and to formation of high molar mass oligomers. Wattana and Fogler12 found that asphaltenes of unstable crude oils and of solid deposits contain much more polar molecules as compared to asphaltenes of stable crude oils. Juyal et al.13 showed that blocking of polar asphaltene moieties by methylation or trimethyl silylation lowers the intrinsic polarity and increases the stability of asphaltenes. In the previous work,7 we fractionated a Hassi-Messaoud asphaltenes by successive flocculation of asphaltenes dissolved in toluene, dichloromethane (DCM), or tetrahydrofuran (THF) with n-heptane. The fraction flocculated from THF, called (AS-D)THF, represented 11.5% (w/w) of the initial © 2012 American Chemical Society

amount of asphaltenes and was insoluble in nonpolar and mildly polar organic solvents but soluble in highly polar ones, such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). This result indicates that the (AS-D)THF fraction is highly polar and is present in the crude oil as a dispersion stabilized by other crude components.7 In the present work, the chemical and structural characterization of this fraction was presented with results of elemental analysis, LDI-TOF mass spectroscopy and solid state 13C NMR. Next, a new fractionation of the same asphaltenes was done using successively NMP, DMF, and DMSO. These polar solvents used for asphaltene fractionation may be characterized using their propensity to form hydrogen and polar bonding according to the three-dimension solubility parameter theory proposed by Hansen.14 All three compounds display hydrogen bonding contribution similar to that observed with aromatic compounds. The polar contribution of solvent molecules is high and rises in the series: NMP, DMF, and DMSO. The dielectric constant of three solvents, being 32.2, 36.7, and 46.4, respectively, at 25 °C, follows the same order. The solvent polarity determines the affinity in respect to different families of compounds present in the crude oil. While, NMP is a very good solvent for aromatics, fair for cyclanic, and poor for alkanes, DMSO is only a fairly good solvent for aromatic and poor for cyclanic and alkanes. Similar differences are observed with oxygen compounds: NMP is a better solvent for ethers and ketones than is DMSO. Three solvents display the strong interaction with alcohols and acids. Properties of DMF are intermediate. Fractions obtained by extraction with these solvents contain the most polar molecules of asphaltenes. The residue obtained after the last extraction corresponds to molecules nonsoluble in polar solvents and represents 26.9% (w/w) of the total amount of asphaltenes. The aggregation pattern of these polar fractions was studied and the aggregation and flocculation onsets were determined. Received: April 4, 2012 Revised: July 20, 2012 Published: July 24, 2012 5672

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Figure 1. (a) Scheme of asphaltene fractionation using polar solvents. (b) Scheme of asphaltene fractionation using nonpolar and mildly polar solvents according to previously described protocol.7 asphaltenes were dissolved in toluene, and then, they were partly flocculated with n-heptane. Asphaltenes contained in the solid and liquid phases were recovered. Asphaltenes recovered from the liquid were dissolved in the next solvent and the procedure was repeated. The (AS-D)THF fraction corresponds to the residue obtained after the last flocculation from THF and contains the most polar compounds of asphaltenes. This procedure is inverse to the one applied in the present work. Previously,7 the asphaltene fractionation was done by partial flocculation from nonpolar and mildly polar solvents. The method used in the present study consisted in partial dissolution of asphaltenes in polar solvents. 2.4. Characterization Techniques. 2.4.1. Elemental Analysis. The carbon, hydrogen, nitrogen, and sulfur content of (AS-D)THF were determined using a Thermo Finnigan EA 1112 analyzer. The repeatability of these measurements was of 0.2%. The oxygen content was not measured directly but was taken as the percentage completing the mass balance of the sample. As the fraction processing involves several dissolution/precipitation steps, we considered that the content of mineral materials in the samples was negligible. 2.4.2. Solid State 13C NMR Spectroscopy. Analyses were conducted using a Bruker solid-state NMR spectrometer operating at 400 MHz for the 1H Larmor frequency and at 100 MHz for the 13C Larmor frequency. The spectrometer was equipped with a double resonance magic angle spinning (MAS) probe. The spectra were referenced to tetramethylsilane. All experiments were conducted at a spinning rate of 14 kHz. The relaxation delay was equal to 120 s. All cross-polarization (CP) experiments were done with a 1H 90° pulse and a contact time of 1 ms. Both the 1H spin-lock and decoupling frequencies were fixed at 25 kHz. Simple and variable CP contact time experiments performed at times ranging from 0 to 1.5 ms were used to determine the optimum contact time. 2.4.3. Mass Spectroscopy. The average molecular weight of polar fractions was determined by mass spectroscopy (Reflex VI Bruker Daltonics) using laser desorption/ionization time-of-flight (LDI-TOF) technique in a reflector mode and optimized nitrogen laser (λ = 337 nm) energy at 80% of the full power. All samples were prepared in toluene at concentration of 0.001 g·L−1.

2. EXPERIMENTAL SECTION 2.1. Chemicals. NMP, DMF, and DMSO were from Fisher Chemicals with 99% purity, while the n-heptane was from Prolabo Chemicals, 98% purity. 2.2. Extraction of Asphaltenes. Asphaltenes used in this study were obtained from the crude oil of the Hassi-Messaoud field using the usual procedure. An n-heptane/crude oil mixture at 40:1 volume ratio was gently shaken over 24 h at ambient temperature to precipitate the total asphaltenes present in the oil. Then, precipitated asphaltenes called in the text “total asphaltenes” and noted as (As-D)tot were filtered through a 0.22 μm pore size paper filter and washed with 10 mL of n-heptane, until the solvent was colorless (usually five times was sufficient). 2.3. Polar Fractions of Asphaltenes. 2.3.1. Fractionation of (As-D)tot with Polar Solvents. The total asphaltenes, (As-D)tot, obtained as previously described, were fractionated according to the procedure summarized in Figure 1a. One gram of total asphaltenes was added to 10 mL of NMP. The solution was gently stirred, at ambient temperature, until dissolution of components soluble in NMP, and then centrifuged for 18 min at 1400 rpm. After evaporation of NMP, the first polar fraction of asphaltenes named ASNMP was obtained. This fraction corresponded to 38.2% (w/w) of the initial amount of total asphaltenes. The centrifuged residue was dissolved in 10 mL of DMF and then stirred and centrifuged for 18 min at 1400 rpm DMF was evaporated and the residue corresponding to 21.2% (w/w) of total asphaltenes was identified as the second polar fraction, named ASDMF. The third fraction, ASDMSO was obtained by dissolving the residue in DMSO and then using the same separation protocol. This operation yielded the ASDMSO fraction (13.7% (w/w)), and the final nonpolar residue named ASNP (26.9% (w/w)) corresponding to the asphaltene fraction insoluble in polar solvents. 2.3.2. Fractionation of the (As-D)tot using Nonpolar and Mildly Polar Solvents: Obtaining the (AS-D)THF Fraction. In the previous work,7 the detailed protocol of fractionation was described. The scheme given in Figure 1b illustrates this procedure. The fractionation consists on the partial flocculation of asphaltenes from toluene, dichloromethane, and tetrohydrofuran solutions. At first, the (As-D)tot 5673

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2.4.4. Fluorescence Spectroscopy. Aggregation of total asphaltenes and polar asphaltene fractions either in toluene or in toluene/nheptane mixture was followed by fluorescence spectroscopy. Analysis of resulting spectra made it possible to determine the critical aggregation concentration (CAC) and the flocculation onset (FO). A fluorescence spectroscopy setup Fluoromax-3 from Jobin Yvon Horiba SAS was used in emission mode in the range from 350 to 700 nm with an excitation wavelength of 256 nm, an excitation slit width of 15.0 nm, a scan duration of 1800 s, and a data time interval of 0.10 s. The CAC of asphaltene fractions was determined studying changes of spectra of toluene solutions as function of concentration. The FO of the fractions were determined by studying changes of spectra of toluene solutions during titration with n-heptane. All measurements were conducted at the room temperature.

alphatic chains is similar to these obtained previously with the fractions of the storage tank deposit.7 Therefore, all fractions obtained according to the described fractionation method13 display molar masses in the narrow interval. 3.1.3. Solid State 13C NMR Spectroscopy. The 13C NMR spectrum of (AS-D)THF asphaltene fraction shown in Figure 3

3. RESULTS AND DISCUSSION 3.1. Characterization of (AS-D)THF Fraction. 3.1.1. Elemental Analysis. Results of the elemental analysis of the (ASD)THF fraction are presented in Table 1. The value of the Table 1. Elemental Analysis of (AS-D)THF, (w/w) %a

a

C

H

N

S

O

H/C

74.5

7.6

0.6

3.7

13.6

1.22

H/C is the atomic ratio of hydrogen and carbon. Figure 3. Solid state 13C-NMR spectra of (AS-D)THF fraction: (a) Lorentizien fit of the aliphatic range.

atomic ratio H/C being 1.22 indicates a low aromaticity of the (AS-D)THF fraction15 as compared to 0.95 obtained with asphaltenes from Hassi-Messaoud field analyzed in our previous study.7 However this result is comparable to H/C = 1.22, as found with (AS-E)THF and other fractions obtained by extraction of the storage tank solid deposit.7 The content of sulfur in all fractions does not change with fractionation and is as found with total asphaltenes.16 The most significant difference was found with the oxygen content being 7.2% with (AS-D)tot,13 and 13.6% with (AS-D)THF. The high polarity of the latter fraction suggests that the (AS-D)THF oxygen is present as the acid or ketone moieties. 3.1.2. Molecular Weight Distribution by LDI-TOF Mass Spectroscopy. Figure 2 shows the LDI-TOF spectra corresponding to (AS-D)THF fraction. Mass experiments indicate the monomodal distribution with the maximum of ion abundance found at around m/z = 500. This result indicating that the (ASD)THF fraction contains few aromatic rings connected with

are similar to spectra presented in the literature.16,17 The resonance observed in the ranges of 0−90 ppm corresponds to aliphatic carbons. The resonance of aromatic carbon is observed in the range 100−180 ppm. The aliphatic resonance was fitted with five Lorentzian peaks (see Figure 3ainset in Figure 3) at 14.3, 22.2, 29.0, 32.0, and 39.8 ppm in order to determine n, the average number of carbons per alkyl side chain. In the range of frequencies corresponding to the aromatic carbon two peaks centered at 125 and 167 ppm respectively were observed. The former was attributed to protonated and carbon substituted aromatic carbons whereas the latter was attributed to oxygen substituted aromatic carbons. The content of carbon in different chemical environments obtained from 13C NMR spectrum is given in Table 2. These results indicate that a relative abundance of Table 2. Average Molecular Parameters and Atomic Percentage of the Aliphatic and Aromatic Carbon in (ASD)THF According to 13C NMR Spectraa Cα %

Cβ %

Cγ %

Cal %

Car %

fa

Ra

Φ

n

7.5

27.8

17.5

52.8

47.2

0.47

3.7

0.63

7.1

Cα, Cβ, and Cγ correspond, respectively, to aliphatic carbons in α, β, and γ positions. Cal and Car are total aliphatic and total aromatic carbons, respectively. a

aliphatic and aromatic structures was 52.8% and 47.2%, respectively. Combining 13C NMR data with mass spectroscopy and elemental analysis results made it possible to determine structural parameters such as the aromatic carbon fraction (fa), the average number of aromatic rings (Ra), the shape factor of the aromatic sheet (Φ), and the average number of carbon atoms per alkyl side chain (n), as outlined by Calemma et al.18 and other authors.19−22 Results listed in Table 2 suggest that the (AS-D)THF fraction has a rather low aromaticity of 47% and

Figure 2. LDI-TOF mass spectra of the fraction (AS-D)THF. 5674

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distribution goes to zero at 600−800 amu. The result obtained in this case is significantly lower comparing to 500 amu obtained with asphaltenes fractionated with nonpolar and mildly polar solvents.7 This “desegregation” of asphaltenes in the presence of polar solvents suggests that the cohesion of elemental aggregates that are stable in the crude oil is due to non covalent bonding and to the presence of small but polar molecules. 3.2.2. Fluorescence Spectroscopy. Studying the fluorescence spectra as a function of concentration provides information about aggregation of asphaltene in organic solvent.26,27 We examined the shift of fluorescence spectral peak of asphaltenes as function of the solvent concentration with solvent being either toluene or toluene/n-heptane mixture. Results obtained made it possible to evaluate the critical aggregation concentration (CAC) or flocculation onset (FO). CAC corresponds to the onset of asphaltene aggregation in toluene solution while FO determines the minimal volume of nheptane necessary to start the flocculation process. CAC Determination. Figure 5 shows emission fluorescence spectra of total asphaltenes and of polar fractions, ASNMP, AS DMF, and AS DMSO in toluene solutions at different concentrations going from 0.1 to 2.0 g·L−1. The observed fluorescence spectra are similar to those reported by other authors for asphaltenes of different origin.26,27 All spectra exhibit a broad band with a local maximum that may be attributed to the fluorophore moieties of asphaltenes molecules. An increasing shift to smaller emission energies (higher emission wavelength at the maximum of intensity, λem) was observed when asphaltene concentration has been increased. Figure 6 shows the variation of the wavelength at the maximum of the fluorescence emission intensity, λem (determined by integration of the spectra of Figure 5) versus asphaltene concentration in toluene. The shift increase with increasing concentration became lower when the asphaltene concentration has been higher than the aggregation onset (CAC).28 That can be explained by decreasing concentration of fluorescent moieties due to the asphaltene aggregation involving the π−π bonds. The inverse behavior was observed with ASDMSO fraction that is due to the more complex aggregation mechanism occurring in the case of this polar fraction. The aggregation onset, CAC, corresponds to the slope change on λem versus concentration plot as shown at Figure 6. The numerical CAC value was found at the intersection of two straight lines drawn through experimental points. In certain cases, the straight line model was not sufficient to fit experimental data. The nonlinearity (especially in low concentration range) or the three step mechanism (in the case of ASNMP) was observed. However, at this stage of our knowledge concerning the aggregation mechanism, it was impossible to take into account these phenomena and it was decided to eliminate certain data and to use a linear model. The CAC values of asphaltenic fractions are reported in Table 3. It may be noted that the CAC value of ASDMSO is much higher comparing to other asphaltene fractions: (AS-D)tot, ASNMP, and ASDMF. FO Determination. The emission fluorescence spectra of asphaltenes and polar asphaltene fractions in toluene solutions at 2 g·L−1 versus the volume of n-heptane are shown in Figure 7a−d. The increasing n-heptane content induces a progressive asphaltene aggregation with an important shift to a smaller wavelength (higher intensity) of emission signal due to the aggregation of asphaltene molecules.28

that the average length of the alkyl side chains is of seven carbon atoms. The values of Ra (3.7) and of Φ (0.63) indicate that the average molecule of (AS-D)THF has four aromatic rings with a low condensation rate. 3.2. Characterization of (AS-D)tot, ASNMP, ASDMF, and ASDMSO Fractions. 3.2.1. Molecular Weight Distribution by LDI-TOF Mass Spectroscopy. Figures 4 shows the LDI-TOF

Figure 4. LDI-TOF mass spectra of asphaltenes and polar asphaltene fractions: (a) AStot, (b) ASNMP, (c) ASDMF, (d) ASDMSO.

spectra corresponding to total asphaltenes, (AS-D)totFigure 4a, and to polar asphaltene fractions, Figures 4b−d. All experiments were performed with very dilute asphaltene solutions (∼0.001 g·L−1 of toluene) to reduce aggregation of asphaltenes. Results obtained with (AS-D)tot indicate a significant polydispersity of masses, with three abundance maxima at about 500, 750, and 1400 amu and with a broad distribution of masses going up to 3000 amu. All results obtained with asphaltene from different origins7,16,23−25 show a broad distribution of masses that seems to be a general property of asphaltenes defined as a solubility class. On the other hand, mass spectra of polar asphaltene fractions, Figures 4b−d, exhibit a monomodal, narrow distribution with a surprisingly low maximum abundance of about 300 amu. This 5675

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Figure 5. Emission fluorescence spectra of polar asphaltene fractions in toluene with varying concentration: (a) AStot, (b) ASNMP, (c) ASDMF, (d) ASDMSO.

Table 3. CAC and FO Values of Total Aspahltenes and Asphaltene Fractions asphaltenes samples

CAC

FO

(AS-D)tot ASNMP ASDMF ASDMSO

0.66 0.63 0.60 1.28

0.37 0.40 0.35 0.29

of the added n-heptane volume. The break point observed in all plots of λem versus n-heptane volume can be attributed to the flocculation onset, FO (flocculent volume, like as n-heptane, added to asphaltenic solution necessary to starting the flocculation process29). The optical microscope inspection of mixtures corresponding to the break point on λem versus nheptane volume plot showed the presence of asphaltene aggregates. Moreover, the break point observed with (AS-D)tot was close to FO value (0.32) obtained by UV−visible spectrophotometery.7 Therefore, the n-heptane volume corresponding to this break point converges with the FO. Data reported in Table 3 indicate that the FO of (AS-D)tot = 0.37 is close to FO of polar fractions being 0.35 and 0.40 with ASDMF and ASNMP, respectively. FO of ASDMSO is significantly lower, 0.29, that confirms the low stability of this fraction in respect to the asphaltene flocculation. It may be observed that λem versus n-heptane volume plots are different with every fraction studied. This behavior (as previously in the case of CAC

Figure 6. CAC determined using the wavelength at the maximum of the emission fluorescence (λem) variation versus concentration of asphaltene fraction (g·L−1): (■) ASNMP, (●) ASDMF, (▲) ASDMSO, and (⧫) AStot. Full lines correspond to the linear fit.

Figure 8 shows the emission wavelength at maximum intensity of asphaltenes, λem, in toluene (2 g·L−1) as a function 5676

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Figure 7. Emission fluorescence spectra of polar asphaltene fractions in toluene (2 g·L−1) as function of the volume of n-heptane added: (a) AStot, (b) ASNMP, (c) ASDMF, and (d) ASDMSO.

4. CONCLUSION In this study, we were concerned with the structure and the chemical properties of the most polar fraction of HassiMessaoud asphaltenes which was obtained using two methods. The first one (F1) consisted in successive partial flocculation from toluene, DCM, and THF. The last fraction non soluble in non polar or mildly polar solvents represented 11.5% of the total amount of asphaltenes and was soluble in DMSO and DMF. The average molecular weight of this fraction was of 500 amu. The second fractionation (F2) consisted in a consecutive dissolution of total asphaltenes in NMP, DMF, and DMSO. The sum of three polar fractions of asphaltenes (ASPOLAR = ASDMF (38.2%) + ASNMP (21.2%) + ASDMSO (13.7%)) represents 73.1% (w/w) of the total asphaltenes. The average molecular weight of every one of these fractions was about 300 amu. The remaining 26.9% (w/w) of (AS-D)tot corresponds to the most aliphatic fraction of asphaltenes characterized by high average molecular weight (ASNP). The total asphaltene sample displayed one maximum at masses lower than 500 amu, two abundance maxima at about 750 and 1400 amu with a broad distribution of masses going up to 3000 amu. The molecular weight of 700 is largely accepted as the value corresponding to an average asphaltene molecule.

Figure 8. Determination of FO using the changes of the wavelength at maximum emission fluorescence (λem) of asphaltenes in toluene (2 g·L−1) as a function of the volume of n-heptane added: (■) ASNMP, (●) ASDMF, (▲) ASDMSO, and (⧫) AStot. Full lines correspond to the linear fit.

determination) may be explained by differences of fraction concentrations resulting in different aggregation mechanisms. 5677

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We noticed a significant difference of the yield and of the average molecular weight of polar fractions obtained by two separation techniques. The use of highly polar solvents (F2) yields the fraction ASPOLAR corresponding to 73.1% (w/w) of AStot with the average molecular weight of 300 amu. This result indicates that the remaining 26.9% (w/w) of asphaltenes would have the average molecular weight of about 1800 amu if M = 700 amu is taken as the average molar weight of the total asphaltenes. This is in agreement with mass spectroscopy results indicating the presence in AStot of heavy molecules with masses ranging from 1400 to 3000 amu. The high oxygen content in ASPOLAR may suggest that the polar fractions contain naphthenic acids. Naphthenic acids correspond to mixtures of cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to 700 amu. However, this hypothesis is not confirmed by CAC and FO values. Indeed, the AStot, ASDMF, and ASNMP fractions display very close values of CAC and FO. Significant differences were observed with ASDMSO displaying CAC significantly higher and FO significantly lower. This finding suggests that ASDMSO comparing to other fractions has more aromatic character. As naphthenic acids have mainly aliphatic character, they should shift CAC and FO in the opposite way. All ASPOLAR fractions are well soluble in toluene which confirms their aromatic character. It may be concluded that the ASPOLAR fraction composed of small and polar asphaltenes plays an important role in asphaltene ordering. Strong hydrogen bonding and/or acid− base interaction between molecules of ASPOLAR and between ASPOLAR molecules with other components of asphaltenes leads to the formation of small molecular complexes facilitating the solubilization of both classes of asphaltenes. During extraction with nonpolar and mildly polar solvents a part of these complexes broke and the fraction flocculated from THF is insoluble excepting in very polar solvents. The average molecular weight of 500 amu of this fraction representing 11.5% (w/w) of total asphaltenes indicates that it mainly contains the same molecules as present in ASPOLAR. However, (AS-D)THF is insoluble in toluene contrary to ASPOLAR. It may be concluded that the solubility in toluene would depend on the ratio of ASPOLAR components to other components of asphaltenes. Accordingly to classical theories of asphaltene ordering in petroleum fluids,30 the asphaltenes form a kind of inverse micelles stabilized in the crude with nonpolar moieties at the peripheries. The cohesion of the micelle core would be provided by interaction between asphaltene sheets. Results of the present work suggest that this cohesion of small aggregates is due to the interaction between main components of fractions ASPOLAR−ASPOLAR and ASPOLAR−ASNP. The mass spectroscopy showed that the atomic weight of basic components of asphaltenes is about 300 amu. Therefore, the species with atomic weight of 700−1000 amu are rather small aggregates of strongly interacting monomers. This is the most significant finding of this work that would be important for designing new antiflocculation agents.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5678

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