Despite their hazardous nature, carbonaceous particles (CPs) own very appealing properties, which make them a leading material in numerous applications. A systematic work on the properties of aqueous dispersions containing CPs, cetylpyridinium chloride (CPyCl), and sodium salicylate (NaSal) is here presented. Being CPs not soluble in water, an effective preparation method to obtain stable and homogeneous solutions was developed. Furthermore, a full characterization of the systems was performed by means of UV-Visible absorption spectroscopy, rheology, and interfacial tension measurements. Hydrophobic CPs are hard to disperse in a water matrix. The adoption of CPyCl as solubility enhancer was a sound strategy to finely disperse high concentration of CPs in an aqueous environment. The high CPs concentration involved conferred to the solutions a dark color and peculiar properties, such as UV and visible light radiation absorbance. The addition of NaSal to the CPyCl-CPs water solutions donated to the system robust viscoelasticity. We investigated 100 mM CPyCl aqueous solutions, with 60 and 70 mM NaSal, containing various amounts of CPs (0–9500 ppm). CPs at concentrations beneath 9500 ppm do not influence the system rheological properties. The well-known effect of NaSal on CPyCl solutions remains unaltered even in the presence of CPs at concentrations below 9500 ppm. On the other hand, the addition of 9500 ppm CPs provokes a moderate change in the rheological properties and microstructure of the systems. At a such high concentration, CPs seem to mimic the effect of NaSal on the micellar solutions, acting as a full-fledged hydrophobic salt.

Carbonaceous particles can be produced during the incomplete combustion process of all carbon containing fuels and are largely known to represent an impacting and persistent pollutant, harmful to human health and environment.1–3 Mostly made by carbon in elemental or organic form, these particles are identified as soot or carbon soot. Carbon soot is the unwanted by-product from carbon-based material combustion4 and, therefore, can be collected through the burning of oils, fuels, wood, plastics, rubber, etc. Its physical and chemical properties heavily depend on the source nature and process conditions.5 Conversely, carbon black as a material composed of carbonaceous particles shares with soot most of the chemico-physical features. Carbon black finds its usage in several applications, including pneumatics industry, pigments production, and cathode catalysts trade.6 Also, similarly to soot, carbon black chemico-physical properties, including composition morphology and surface reactivity, can significantly vary upon the reaction parameters of production. Despite that carbonaceous particles, even in the form of carbon black, can be produced through different approaches (e.g., plasma, laser arc discharge chemical vapor deposition, etc.), combustion in furnaces remains the most adopted method of production. The distinction between carbon soot and carbon black lies in the final application field of interest—environmental science or materials science—being potentially identical just looking at their chemico-physical properties. As such, a generic term of combustion generated carbonaceous particles (CPs) will be here adopted.

To delve into chemico-physical features of CPs, scientific works rely on sophisticated techniques, such as electron microscopy, scattering methodologies, transmission electron photomicrographs, etc., to access the organization and morphology of the material, often coupled to numerical investigations to support the system description.7–14 In recent years, advancements in materials science have witnessed increasing interest in the development of tailored nanocomposite materials, based on the introduction of micro/nanoparticles into simple or complex liquid matrices.15,16 In particular, the combination of nanometric particles and proper surfactants into liquid media stands as a hot subject in many emerging scientific works.17–19 Combustion generated CPs well fit in with such a kind of applications, which aim to the manufacture of carbon-based porous materials.20,21

The dispersion of CPs within an aqueous matrix has the benefit of avoiding the use of toxic and costly solvents that—in many scientific works—are habitually employed for a complete particle dispersion.14,22 However, fresh CPs are extremely hydrophobic, and their introduction into water is a nontrivial and challenging task. To exploit their unique properties (e.g., UV and visible light radiation absorbance, high conductivity, etc.) to a significant extent, CPs have to be adequately dispersed within the solvent environment, preventing the formation of undesirable huge carbonaceous agglomerates. The dispersion of hydrophobic particles within an aqueous media may be addressed by using amphiphilic molecules as solubility enhancers. Surfactants and micellar solutions are well-known in an enormous number of technological applications and may be adopted as dispersing and solubilizing agents and rheology modifiers.23 For example, Santini et al.24 investigated carbon black and carbon soot aqueous dispersions with hexadecyltrimethylammonium bromide (CTAB), which is a cationic surfactant. Zabiegaj et al.12 explored the properties of water dispersions of CPs in the presence of different ionic and cationic surfactants by means of surface tension, scattering, and zeta potential. Other studies proved that, while untreated CPs—also referred to as fresh soot—own a high hydrophobicity, oxidative process of aging donates to CPs levels of hydrophilicity; in fact, aged soot is able to lure and retain water.25 However, soot aging requires expensive treatments, and the dispersion of byproducts of combustion processes remains the true challenge. A good method to tweak the microstructural evolution of micellar solutions consists in the addition of salts, which cause the increase in environmental ionic strength and the reduction of repulsion force between the surfactant heads.26 The control of the morphological features of micellar systems allows to obtain desired system properties and performances.

We herein report on dispersions of CPs collected from lab-scale controlled combustion environment within aqueous environment, whose properties were studied through UV-Visible (UV-Vis) absorption spectroscopy, rheology, and interfacial tension measurements. The preparation of stable and uniformly dispersed water systems of CPs was attempted by employing a suitable formulation protocol, which successfully allowed for obtaining samples with not only an optimal dispersion of CPs but also a long-lasting stability, even for the highly concentrated systems.

Different aqueous solutions containing CPs, cetylpyridinium chloride (CPyCl), and sodium salicylate (NaSal) were prepared. CPs were collected from a laminar premixed flame of ethylene and air. The investigated burner and flame were analogous to those of Sirignano et al.27 The combustion reactor consists of a porous sintered bronze McKenna type burner, with a water-cooling system at a flow rate of 1 l/min. The porous plate is surrounded by a porous bronze anulus through which nitrogen can flow to isolate the flame from the external environment and avoid diffusion phenomena on the flame edge. To stabilize the flame, a steel disk with a thickness of 4 mm was placed at a distance of 21 mm from the burner rim. The operative C/O ratio was equal to 0.82 (i.e., an equivalent ratio of 2.46). CPs were collected on a 75 × 25 × 1 mm quartz slide at a specific height of the flame (10 mm above the burner). The sampling slides covered with CPs were pretreated using dichloromethane, in order to remove the soluble organic fraction from CPs samples collected in the flame. CPs were then mechanically removed from the substrate and analyzed.

First, CPs were dispersed in N-methylpyrrolidone (NMP), a strong solvent largely used to create stable dispersions of CPs.14 These systems were prepared weighting CPs before dispersion and used as reference to measure the CPs content of all the other dispersions.

Water dispersions of CPs at different concentrations were then prepared. CPs were weighted to have a nominal concentration ranging from 10 to 60 ppm. CPs were put in water and then sonicated for 8 h. After that, a filtration was performed by letting the dispersions pass by gravity through a paper filter with micrometric pores in order to eliminate the eventual presence of large agglomerates and make the samples suitable for spectroscopic analysis. Water dispersions were prepared as reference for testing the dispersibility of CPs. Specifically, the protocol for the preparation of water-CPs samples can be summarized as follows:

  • Dispersion of CPs in water through sonication in an ultrasound bath (Elma, S40H, power 340 W, frequency 37 kHz, ambient temperature; the time of sonication—from 2 up to 8 h—was chosen according to the CPs concentration: the higher the CPs concentration, the higher the sonication time);

  • Filtration of the samples by using a paper filter with micro-sized pores to remove particles agglomerates;

  • Second sonication cycle to enhance the system stability and uniformity.

Successively, we adopted a similar preparation protocol for other base solution containing the surfactant and, eventually, the salt. The surfactant exploited in association with the CPs was CPyCl (Sigma-Aldrich, St. Louis, MO). We prepared various water dispersions with a fixed CPs concentration and different CPyCl contents. CPyCl water solutions without CPs were also prepared to allow for the comparison. Furthermore, water dispersions containing CPs, CPyCl, and NaSal (Sigma-Aldrich, St. Louis, MO) were also prepared. The following protocol for the preparation of samples containing CPyCl and NaSal was adopted:

  • Preparation of solutions with 100 mM CPyCl through magnetic stirring;

  • Addition of CPs to the CPyCl-based solution;

  • Sonication of the samples for a few hours (the higher the CPs concentration, the higher the sonication time);

  • Dissolution of NaSal in the samples through magnetic stirring.

We report in Table I the composition of the investigated samples.

TABLE I.

Composition of the investigated samples.

Samples group Effective CPs concentration (ppm) CPyCl concentration (mM) NaSal concentration (mM)
Water + CPs  1–27 
Water + CPyCl  0.1–100 
Water + CPs + CPyCl  0–5.4  0.1–100 
Water + CPs + CPyCl + NaSal  0–14  100  0–100 
Water + CPs + CPyCl + NaSal  0–9500  100  60–70 
Samples group Effective CPs concentration (ppm) CPyCl concentration (mM) NaSal concentration (mM)
Water + CPs  1–27 
Water + CPyCl  0.1–100 
Water + CPs + CPyCl  0–5.4  0.1–100 
Water + CPs + CPyCl + NaSal  0–14  100  0–100 
Water + CPs + CPyCl + NaSal  0–9500  100  60–70 

It is worth remarking that the concentration values of CPs in the CPs-based solutions reported in this manuscript refer to the quantity of particles actually dispersed in the investigated systems after the filtration and sonication cycles and, hence, once obtained homogeneous and stable solutions (see below). Measurements on samples were performed after waiting some days to check their stability and equilibrium. We show in the supplementary material a picture of some CPs-based samples, showing the coloring effect of different CPs concentrations.

1. UV-Visible (UV-Vis) absorption spectroscopy

Optical measurements were carried out on a UV-Vis spectrophotometer (Agilent 8453) in the wavelength (λ) range 200–1100 nm to exploit the optical properties of the flame-produced CPs for characterizing the dispersion characteristics. First, three solutions with fixed concentrations of CPs (5, 10, and 20 ppm) were easily homogeneously dispersed in NMP. The UV-Vis spectra of the NMP-CPs solutions were used as a “calibration reference” to evaluate the effective CPs mass concentration of the water-CPs samples. The estimation of CPs concentration in water and NMP has been made according to the best practice adopted in the scientific community.28 NMP can be used as the reference solvent to disperse large content of CPs finely and stably, and the obtained absorption values can be used in water dispersions. Hence, UV-Vis analysis allowed for the evaluation of the effective quantity of CPs dispersed in the solutions. The effective mass concentration of CPs finely dispersed in the investigated systems does not reflect the amount of CPs added to water during the samples preparation. In fact, the formation of large particle aggregates leads to the presence of supernatant and particle sedimentation. Moreover, absorption spectroscopy allowed to understand the homogeneity of the solutions. The dispersions were considered homogeneous when it was possible to apply subsequent dilutions (1:10 up to 1:1000) preserving the mass concentration evaluated by absorption through different dilutions. Absorption is also helpful in evaluating the sonication effect on the CPs dispersions and identifying, thus, the sonication time needed to attain stable and homogeneous systems. The system stability was assessed by checking the constancy of CPs concentration over time by absorption.

2. Rheology

The rheological properties of the samples were studied through a rotational stress-controlled rheometer (DHR-2, Discovery Hybrid Rheometer, TA Instruments, USA), equipped with parallel plates geometry (diameter 40 mm) for dynamic tests and cone-plate geometry (diameter 60 mm and cone angle 0.5°) for steady measurements. Specifically, frequency sweeps were performed to study the system linear viscoelasticity. Strain sweeps detected the linear viscoelastic regime (a strain value of 10% was used for all dynamic tests). Moreover, flow curve tests were conducted to study the flow properties of the investigated systems. A Peltier unit was used for the control of temperature.

3. Interfacial tension measurements

Interfacial tension was measured through the pendant drop method.29 The experimental apparatus used for interfacial tension measurements consisted of a needle, a digital camera, and a light source.

As stated before, CPs dispersion within an aqueous matrix is a hard task because of CP hydrophobic nature. Indeed, when introduced in water, CPs form agglomerates, giving rise to non-homogeneous dispersions. Although we adopted a suitable preparation protocol (see Sec. II) for the formulation of stable and uniformly dispersed CPs water dispersions, the sample preparation was not efficient due to the significant loss of material. Indeed, the amount of CPs in the dispersions after filtration and sonication was much lower than the one added during the preparation.

1. UV-Visible (UV-Vis) absorption spectroscopy

UV-Vis analysis allowed for the evaluation of the effective quantity of CPs dispersed in the solutions. As an example, Fig. 1 shows the UV-Vis spectra of the water solution with a final CPs concentration of 27 ppm, recorded after the filtration and sonication at three different times (0, 48, and 120 h). It is worth underlining that the starting nominal CPs concentration (before filtration and sonication) of the sample was 60 ppm. Furthermore, Fig. 1 reports the UV-Vis spectra of the water solution with a final CPs concentration of 10 ppm, at different times. Specifically, this solution was prepared by dilution of the sample with 60 ppm CPs (after its sonication). The starting nominal CPs concentration of this solution was equal to the final one (10 ppm), with no change in the actual homogeneously dispersed quantity of CPs. The successful preparation by dilution of samples at specific (minor) CPs concentrations starting from CPs aqueous solutions stock represents a proof of the stability and good dispersion degree of the stock solution itself.

FIG. 1.

UV-Vis spectra (after filtration and sonication) of 10 and 27 ppm CPs aqueous solutions, at different time (nominal concentrations equal to 10 and 60 ppm, respectively).

FIG. 1.

UV-Vis spectra (after filtration and sonication) of 10 and 27 ppm CPs aqueous solutions, at different time (nominal concentrations equal to 10 and 60 ppm, respectively).

Close modal

The UV-Vis spectra in Fig. 1 disclose an absorbance peak at 250 nm, which is distinctive of CPs spectral properties. Clearly, absorbance increases with increasing CPs concentration. Overall, it appears clear that CPs can be finely dispersed in water, reaching a configuration stable over time. However, the fraction of CPs effectively dispersed indicates a significant loss of material (on the order of 50% and more) without following a specific trend with CPs concentration.

Starting from the considerations on CPs-water dispersions, the attention has been focused on the possibility to use a low amount of surfactants to form micellar complexes able to help in CPs dispersion. In fact, to exploit their unique properties to a significant extent, hydrophobic CPs were dispersed in water with the aid of an amphiphilic molecule as dispersibility enhancer, i.e., CPyCl. The introduction of CPyCl in the water solutions aims to prevent the formation of undesirable huge carbonaceous agglomerates and allow for an efficient preparation of finely CPs dispersions, avoiding loss of material and reaching high CPs concentrations. The dispersion control of CPs in surfactant solutions remains qualitative, being impossible to perform scattering techniques on the prepared solutions, which are dark and characterized by a tight network of wormlike micelles.

1. UV-Visible (UV-Vis) absorption spectroscopy

As per CPs-water dispersions, the CPyCl-based systems were analyzed through absorption spectroscopy. Figure 2 displays the comparison between the UV-Vis spectra of the 10 mM CPyCl water solution (with no CPs) and the 10 mM CPyCl water solution with 5.4 ppm CPs.

FIG. 2.

UV-Vis spectra of 10 mM CPyCl aqueous solution and 10 mM aqueous solution with 5.4 ppm CPs. The black curve is the summation spectrum of the 10 mM CPyCl and 5.4 ppm CPs aqueous solutions.

FIG. 2.

UV-Vis spectra of 10 mM CPyCl aqueous solution and 10 mM aqueous solution with 5.4 ppm CPs. The black curve is the summation spectrum of the 10 mM CPyCl and 5.4 ppm CPs aqueous solutions.

Close modal

The presence of CPs does not modify the optical properties of the micellar solution, and results in just an additive effect. Indeed, the UV-Vis spectrum of the CPyCl-CPs water solutions equals the sum of the spectra of the corresponding CPyCl water solution (without CPs) and the CPs-water solution (without CPyCl).

As previously discussed, spectroscopy investigation allows us to compute the effective amount of CPs that are homogeneously dispersed in the solutions. Figure 3 reports the UV-Vis spectra of NMP-CPs samples at three different CPs concentrations, namely, 5, 10, and 20 ppm.

FIG. 3.

UV-Vis spectra of NMP-CPs solutions, at different CPs concentrations.

FIG. 3.

UV-Vis spectra of NMP-CPs solutions, at different CPs concentrations.

Close modal
At λ = 500 nm, CPs absorbance changes linearly with CPs concentration. We report in the supplementary material the absorbance values at 500 nm wavelength as a function of CPs concentration for the NMP-CPs samples, as obtained from the UV-Vis spectra of Fig. 3. The absorbance values at 500 nm wavelength change linearly with CPs concentration as follows:
A = m c .
(1)
Equation (1) is the linear regression and presents one fitting parameters, m, which is equal to 0.0349 ± 0.0005 ppm−1. A and c represent the absorbance and CPs concentration, respectively. The effective CPs mass concentration of the water-based samples—given their absorbance at λ = 500 nm—can be evaluated by considering Eq. (1) as a “calibration line.” It is worth underlining that NMP has been largely used in the literature as solvent able to finely disperse large amount of CPs; however, its toxicity represents a limitation in the use on large scale.14 As already mentioned, the values of CPs concentration that we report in this manuscript refer to the effective amount of CPs homogeneously dispersed in the water-based samples.

2. Rheology

After assessing the possibility of finely disperse CPs in CPyCl water systems, we experimentally studied the rheological behavior of CPyCl-CPs aqueous solutions to unveil the influence of CPs on the system structural properties. We report in Fig. 4 the comparison between the flow curves of the 1 mM/100 mM CPyCl aqueous samples and the 1 mM/100 mM CPyCl aqueous samples with 5.4 ppm CPs. In particular, the viscosity, η, is reported as a function of the shear rate. The macroscopic rheological measurements reveal a liquid-like Newtonian behavior, and, in general, the presence of CPs does not affect both the linear and non-linear rheology of the CPyCl water samples.

FIG. 4.

Flow curves of the 1 mM/100 mM CPyCl aqueous samples and the 1 mM/100 mM CPyCl aqueous samples with 5.4 ppm CPs.

FIG. 4.

Flow curves of the 1 mM/100 mM CPyCl aqueous samples and the 1 mM/100 mM CPyCl aqueous samples with 5.4 ppm CPs.

Close modal

1. Rheology

Micellar systems are well-known to be affected by the addition of external agents. In particular, the addition of sodium salicylate (NaSal) significantly modifies the rheological behavior. Aqueous solutions of CPyCl, NaSal, and CPs were investigated, considering that NaSal—once added to CPyCl-based solutions—donates to the systems robust viscoelastic properties.26 We aimed to examine not only the effect of NaSal on the CPyCl-CPs water solutions but also the role of CPs when added to CPyCl-NaSal water solutions.

Figure 5 reports the linear frequency sweeps of the 100 mM CPyCl aqueous solutions with various amounts of NaSal (40, 55, 70, and 100 mM), without and with (14 ppm) CPs. The viscoelastic moduli, G′ and G″, are reported as a function of frequency.

FIG. 5.

Linear frequency responses of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal: (a) 40 mM NaSal; (b) 55 mM NaSal; (c) 70 mM NaSal; and (d) 100 mM NaSal.

FIG. 5.

Linear frequency responses of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal: (a) 40 mM NaSal; (b) 55 mM NaSal; (c) 70 mM NaSal; and (d) 100 mM NaSal.

Close modal

At low NaSal concentrations (20–40 mM), the system frequency response implies the existence of disordered micellar structures (spherical or possibly slightly elongated cylindrical micelles), as suggested by a viscous rheological behavior [Fig. 5(a)—the elastic modulus being negligible]. At the investigated concentration, the addition of CPs does not generate any change of the rheology. At 50–55 mM NaSal, the action of the binding salt causes the transition from spherical to rod-like micelles. Contour length increase allows for entanglements formation, resulting in appearance of viscoelastic properties. At the investigated concentration, CPs do not anticipate this transition, which occurs at the same binding salt concentration for both samples with and without CPs [Fig. 5(b)]. NaSal concentration increase (60–70 mM) results in the formation of very long and well-entangled wormlike micelles, as for polymer melts. The rheological response is characterized by a marked viscoelastic behavior [Fig. 5(c)], which can be described by the Maxwell mechanical model (the system can be defined as purely Maxwellian). A further increase in binding salt concentration rests the purely Maxwellian response, generating a liquid-like rheological behavior [Fig. 5(d)].

Figure 6 describes the zero-shear viscosity, η 0, as a function of NaSal concentration, of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal.

FIG. 6.

Zero-shear viscosity vs NaSal concentration of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal.

FIG. 6.

Zero-shear viscosity vs NaSal concentration of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal.

Close modal

The zero-shear viscosity presents a non-monotonic behavior with increasing NaSal concentration, due to the penetrating action of the binding salt in changing the morphology of the micellar structures.26 At low NaSal concentrations (20–40 mM), the zero-shear viscosity is that of the solvent, i.e., water. As the system salinity increases, η 0 abruptly increases of five orders of magnitude, reaching its maximum between 60 and 70 mM NaSal; a further addition of salt rapidly lowers the overall system viscosity. The presence of 14 ppm CPs does not alter the zero-shear viscosity trend. Hence, the binding salt is the only component owning an evident effect on the system rheology. Also, for the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal, containing various amounts of CPs (0–9500 ppm), despite the high CPs concentration involved, CPs do not play any crucial role in the evolution of the zero-shear viscosity. The same results can be drawn also in non-linear regime by looking at the steady viscosities reported in Fig. 7.

FIG. 7.

Flow curves of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal.

FIG. 7.

Flow curves of the 100 mM CPyCl aqueous solutions—without and with (14 ppm) CPs—containing different amounts of NaSal.

Close modal

To investigate the effect of CPs at different concentrations, the attention has been focused on the two CPyCl-NaSal aqueous solutions that exhibit the highest G′ modulus, namely, 60 or 70 mM NaSal. Figure 8 illustrates the effect of CPs on CPyCl-NaSal aqueous solutions. Specifically, it shows the linear frequency responses of the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal, containing various amounts of CPs, namely, 0, 70, 800, and 9500 ppm.

FIG. 8.

Linear frequency responses of the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal, containing various amounts of CPs (0–9500 ppm). Insets of the trends of G N 0 and τ with CPs concentration are shown.

FIG. 8.

Linear frequency responses of the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal, containing various amounts of CPs (0–9500 ppm). Insets of the trends of G N 0 and τ with CPs concentration are shown.

Close modal

Up to 800 ppm, the presence of CPs does not strongly affect the linear rheological viscoelastic response of the systems. However, the addition of 9500 ppm CPs causes a slight decrease in the plateau modulus ( G N 0 ) for both systems and, only for the sample with 60 mM NaSal, a lower frequency value ( ω min) at which the loss modulus has its minimum ( G min), i.e., a higher breaking and reforming relaxation time ( τ break).

The breaking and reforming time of the Cates model, τ break, can be estimated as
τ break = 1 ω min .
(2)
According to the Cates theory, the reptation time may be computed from the main relaxation time, τ, as
τ rep = τ 2 τ break .
(3)
We report in Tables SI and SII the values of G N 0, G min, τ, τ break, and τ rep. Tables SI and SII show the values of the parameters characterizing the linear rheological response of the viscoelastic aqueous solutions as a function of the CPs concentration.
We report in Tables SIII and SIV the microstructural parameters of the investigated samples that were calculated from the rheological data. In particular, the microstructure parameters of wormlike entangled micellar solutions can be estimated through the Larson equation:
G N 0 = k B T ξ 3 k B T l e 9 / 5 l p 6 / 5 ,
(4)
where ξ is the network mesh size, l e is the average length between two entanglement points, and l p is the micellar persistence length. Equation (4) allows for the calculation of l e; we used a value of l p equal to 20 nm.26 In turn, the contour length ( L ¯) and the number of entanglements in the wormlike micellar network ( n v) can be obtained from the following equations:
l e L ¯ G min G N 0 ,
(5)
n v = L ¯ l e .
(6)
Microstructure lengths confirm that CPs does not systematically affect the network if their concentration is below 9500 ppm. Only the addition of 9500 ppm of CPs in the systems generates a change in the microstructural parameters, although moderate, in both datasets. At 70 mM NaSal, the high amount of CPs seems to provide shorter wormlike micelles, which, hence, entangle less, with an increase in the network mesh size.

We show in Fig. 9 the flow curves of the 100 mM CPyCl aqueous solutions with 60/70 mM NaSal, containing various amounts of CPs (0–9500 ppm). The presence of CPs at high concentration in the system with 70 mM NaSal, as said, generates a slight change in the system flow behavior, with a measurable decrease in the zero-shear viscosity when the content of CPs exceeds a certain value. A possible explanation of this behavior is the combined presence of NaSal and CPs, both working as hydrophobic additives for the micellar palisade. It has been proven that salts with a complex structure (at least one benzene ring in the molecular structure) can penetrate the palisade layer and effectively change the packing surfactant parameter. The latter drives the self-assembly of surfactant molecules. The molecular formula of CPs can contain many benzene rings, being as such a sort of giant penetrating additive. CPs synergically work with NaSal, both owning a hydrophobic nature. In other words, the presence of CPs could help the salicylate to induce the transitions shown in Fig. 6, thanks to a combined effect within the palisade layer: when the concentration of CPs is too small, there is no significant change in the zero-shear viscosity; when, instead, the content of CPs is high enough, the viscosity overcomes the maximum (see Fig. 6), and the measured zero-shear viscosity drops to a smaller value.

FIG. 9.

Flow curves of the 100 mM CPyCl aqueous solutions, with 60 (a) or 70 (b) mM NaSal, containing various amounts of CPs (0–9500 ppm).

FIG. 9.

Flow curves of the 100 mM CPyCl aqueous solutions, with 60 (a) or 70 (b) mM NaSal, containing various amounts of CPs (0–9500 ppm).

Close modal

2. Interfacial tension measurements

We performed interfacial tension measurements in order to disclose the influence of CPs on air-water interface. Figure 10 reports the interfacial tension as a function of CPs concentration of the 100 mM CPyCl aqueous solutions, with 60 (a) or 70 (b) mM NaSal. In particular, CPs do not affect the interfacial tension properties of the investigated systems.

FIG. 10.

Interfacial tension vs CPs concentration of the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal. The error bars are evaluated as standard deviations of multiple experiments.

FIG. 10.

Interfacial tension vs CPs concentration of the 100 mM CPyCl aqueous solutions, with 60 or 70 mM NaSal. The error bars are evaluated as standard deviations of multiple experiments.

Close modal

In this work, we attempted to finely disperse high concentration of CPs in an aqueous environment. An effective preparation protocol allowed for obtaining very stable and uniform solutions, although the high CPs concentrations involved. A good strategy to tackle such challenge is the use of amphiphilic compounds. To exploit their unique properties to a significant extent, hydrophobic CPs were dispersed in water with the aid of an amphiphilic molecule as solubility enhancer, i.e., CPyCl. The introduction of CPyCl in the solutions prevented the formation of undesirable huge carbonaceous agglomerates and allowed for an efficient preparation of adequately dispersed CPs solutions, with no material loss, up to 9500 ppm.

This work aimed to grasp the interaction between CPs particles and a suitable surfactant, and the system morphological transitions induced by the presence of a salt. The properties of aqueous dispersions of CPs in association with the cationic surfactant cetylpyridinium chloride and sodium salicylate were investigated. UV-Visible absorption spectroscopy, rheology, and interfacial tension measurements provided a systematic characterization of such systems, revealing that a significant amount of CPs is required to observe some change in the properties of the CPyCl-NaSal systems. Furthermore, the main parameters describing the microstructure of the micellar systems were estimated as a function of CPs concentration, at the NaSal concentrations at which entangled wormlike micelles form.

We found that the CPs are working in a synergic way with the salicylate: they are both binding additives for the palisade structure of the micelles created by the surfactant molecules when the concentration overcomes a specific value. This could explain the behavior, non-monotonic only in some cases, of the zero-shear viscosity of the solutions, by keeping fixed the salt and the surfactant concentration, and varying CPs content. We studied, in particular, the 100 mM CPyCl solution with 60 and 70 mM NaSal and various amounts of CPs. The CPs, from a rheological point of view, seem to behave similar to NaSal, actually acting as a binding additive for the surfactant aggregates, although the highest content of CPs changes only in minimal way the resulting rheology.

In conclusion, these results offer new insights into the dispersions of carbonaceous material within an aqueous environment and their properties. Micellar solutions can be used to incorporate huge quantities of CPs, which, on the one hand, are able to change the color of the final system but, on the other hand, keep all the other properties, i.e., rheology and interfacial tension, unaltered.

See the supplementary material for details on rheological and microstructural parameters.

The authors have no conflicts to disclose.

Nicola Antonio Di Spirito: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Roberta Minopoli: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Rossana Pasquino: Conceptualization (equal); Data curation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal). Mariano Sirignano: Conceptualization (equal); Data curation (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material