Dispersing Carbon Nanotubes in Water with Amphiphiles: Dispersant

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C: Physical Processes in Nanomaterials and Nanostructures

Dispersing Carbon Nanotubes in Water With Amphiphiles: Dispersant Adsorption, Kinetics and Bundle Size Distribution as Defining Factors Jing Dai, Ricardo M.F. Fernandes, Oren Regev, Eduardo F. Marques, and Istvan Furo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06542 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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The Journal of Physical Chemistry

Dispersing Carbon Nanotubes in Water with Amphiphiles: Dispersant Adsorption, Kinetics and Bundle Size Distribution as Defining Factors Jing Dai1, Ricardo M. F. Fernandes1,2, Oren Regev3, Eduardo F. Marques2, and István Furó1,*

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Division of Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

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CIQUP, Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal 3

Department of Chemical Engineering and the Ilse Katz Institute for Nanotechnology, BenGurion University of Negev, 84105 Beer-Sheva, Israel *Email address: [email protected]

ACS Paragon Plus Environment

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Abstract Debundling and dispersing single-walled carbon nanotubes (SWNTs) is essential for applications but the process is not well understood. In this work, aqueous SWNT dispersions were produced by sonicating pristine SWNT powder in the presence of an amphiphilic triblock copolymer (Pluronic F127) as dispersant. Upon centrifugation, one obtains a supernatant with suspended individual tubes and thin bundles and a precipitate with large bundles (and impurities). In the supernatant, that constitutes the final dispersion, we determined the dispersed SWNT concentration by thermogravimetric analysis (TGA) and UV-Vis spectroscopy and the dispersant concentration by 1H NMR. The fraction of dispersant adsorbed at the SWNT surface was obtained by 1H diffusion NMR. Sigmoidal dispersion curves recording the concentration of dispersed SWNTs as a function of supernatant dispersant concentration were obtained at different SWNT loadings and sonication times. As SWNT bundles are debundled into smaller and smaller ones, the essential role of the dispersant is to sufficiently quickly cover the freshly exposed surfaces created by shear forces induced during sonication. Primarily kinetic reasons are behind the need for dispersant concentrations required to reach a substantial SWNT concentration. Centrifugation sets the size-threshold below which SWNT particles are retained in the dispersion, and consequently determines the SWNT concentration as a function of sonication time.

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1. Introduction Since the breakthrough of carbon nanotubes (CNTs) by Iijima,1 many thousands of studies have been carried out on CNTs. Owing to their unique mechanical, thermal and electric properties, applications of CNTs on such diverse areas as molecular electronics, composite reinforcement and energy storage have emerged.2-3 Most of those applications require CNTs to be processed, individually dispersed and preferably sorted in liquids, which is often water.4-7 However, due to their aromatic structure, CNTs tend to bundle via strong cohesive van der Waals interactions and are thus insoluble in water.8-9 Hence, two main strategies have been devised to disperse CNTs in aqueous media: (i) covalent functionalization by attaching functional groups on CNT surface and (ii) non-covalent functionalization by physical adsorption of amphiphilic molecules (dispersants).10 Non-covalent functionalization is often preferred since the sp2 hybridization of carbon atoms in CNTs is not changed and thus the intrinsic properties of the material (e.g., conductivity and strength) are preserved. In particular, the dispersibility of single-walled carbon nanotubes (SWNTs) in aqueous media by different amphiphiles, such as surfactants,5, 11-14 synthetic15-21 and natural22-26 polymers has been extensively studied in the past years. However, many of the reported results, such as the obtained SWNT concentrations suffer from huge discrepancies (sometimes two orders of magnitude for the same nominal features),27 assumedly because the CNTs from particular manufacturers, their intrinsic purity and other features but also (seemingly identical) preparation procedures all must have exhibited a significant scatter. Scarce systematic studies of the dispersion process have yielded dispersibility curves in which the SWNT concentration is measured as a function of the amphiphile concentration. The typical emerging picture from those studies is an increase of the dispersed SWNT concentration upon increasing amphiphile concentration until a

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plateau is reached.11-13, 26, 28-32 At higher surfactant concentration, a depletion-driven effect is sometimes observed, due to a high volume fraction of free micelles, which results in a loss of dispersed CNTs to the precipitate.11, 33-34 Re-aggregation35-37 into looser structures is one factor that limits long-time stability of the created dispersion. In a recent work,28 we analyzed CNT dispersibility curves by various ionic surfactants, and proposed some quantitative characterization that allowed one to discuss the influence of assorted surfactant molecular properties on the dispersion process, under given preparation conditions that were carefully controlled. The main goal of the current study is to improve further the understanding of the dispersion process at a molecular level, which may be critical for optimizing the process or rationally using noncovalent functionalization in designing nanocomposite materials. For this, we focus on dispersions prepared with a nonionic block copolymer, Pluronic F127, which shows a convenient feature – on the time scale of NMR diffusion experiments (in the order of 10-2 s), this dispersant exchanges slowly between two states: free in bulk solution and adsorbed on the CNT surfaces.18, 38-39 Hence, with the help of 1H diffusion NMR we can measure the variation of amount of adsorbed dispersant along the dispersion curve. We explore this behavior at different SWNT loadings and at different sonication times. As will be shown, the picture that comes out is that CNT dispersibility is mainly controlled by kinetic factors rather than by tube-tube or tube-dispersant interactions, provided that sufficient energy density is transferred to the CNT/dispersant mixture.

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2. Experimental Section 2.1. Materials CoMoCat single-walled carbon nanotubes of mainly (6,5) chirality and 0.7-0.9 nm outer diameter (SouthWest NanoTechnologies Inc.) were investigated. Pluronic F127 (~ 12.5 kD), a tri-block (PEOx-PPOy-PEOx) polymer with hydrophilic polyethylene oxide (PEO) and hydrophobic polypropylene oxide (PPO) blocks with nominal composition of x = 100 and y = 65 (see inset in Figure 1) and heavy water (99.9 atom % D) were purchased from Sigma-Aldrich. All materials were used as received.

2.2 Preparation and quantification of the CNT dispersions The CNT dispersions were prepared by a procedure presented in detail previously.28 The initial state was dry SWNT powder mixed at a loading of either 0.5, 1.0 or 1.5 mg·mL-1 into the polymer solution with selected F127 concentrations cF127-init. The sample was sonicated using a Qsonica Q500 tip sonicator equipped with a 3 mm microtip. The microtip was always carefully placed in the center of the vial, inserted 1 cm below the liquid surface in a vial with a spherical bottom, 1.4 cm inner diameter and 3.8 cm length. During sonication, the vial was continually kept close (within 1-2 degrees) to room temperature by immersing it in a circulating cold water bath. Typically, the samples were sonicated for 10 minutes (that is, with the exception for data in Fig. 5), that correspond to a total sonication energy density of approx. 1.1 kJ/mL. After sonication, the samples were transferred to 5 mL cryogenic tubes (VWR) and centrifuged during 30 min at 4000 g. After centrifugation, 1 mL of the supernatant (uppermost 1/3 of the volume) was separated by decantation. The experiments below were performed on this supernatant fraction (termed 5



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henceforth as the dispersion) at room temperature. The dispersion was stable for more than few weeks with no visible precipitation. The carbon nanotube concentration present in the supernatant were determined using a combined thermogravimetric analysis (TGA)/UV-Vis method (as detailed in the Supporting Information SI).40 In short, the washed and dried precipitate was analyzed by TGA: since SWNTs do not decompose at the conditions used, the mass loss observed has been assigned (through mass balance) to the dispersant fraction in the dried supernatant.

2.3. NMR measurements The 1H NMR experiments were carried out on a Bruker Avance III 500 MHz spectrometer equipped with a Bruker GREAT 60 gradient unit and a z-gradient probe DIFF 30. The diffusion experiments41 were performed at 20 ºC using the stimulated echo sequence. The 90º pulse length was 7 µs, the gradient pulse length, δ, was set to 2 ms, the gradient stabilization delay to 1 ms, and the diffusion time, Δ, to 20 ms. All other experimental details were as previously published.38-39 A short description of the NMR diffusion method, essential for following the results and discussion shown further, is herein presented. In typical 1H NMR diffusion experiments, one records the intensity of a particular 1H peak in the spectrum as it varies upon increasing strength, g, of the applied magnetic field gradient pulses. For molecules in simple liquids, this decay is single exponential and is described by the Stejskal-Tanner equation: !(#) !(%)

= exp (−𝑏𝐷)

(1)

where the variable b collects the pulse program parameters — with b = (gdg)2-(D-d)/3 and all the parameters as defined above, and g the gyromagnetic ratio of 1H nuclei — and D denotes the selfdiffusion coefficient.

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As shown in Fig. 1, in the F127-based dispersions of SWNTs the NMR diffusional decays of the F127 signal are two-component. Those two components have been identified to arise from the free (in bulk solution) and bound (adsorbed to the SWNT surface) fractions of F127.18, 38-39 Hence, the two-exponential function !(#) !(%)

= 𝑓 exp(−𝑏𝐷#/012 ) + (1 − 𝑓) exp5−𝑏𝐷6788 9

(2)

was fitted to the experimental data with f as the fraction of SWNT-bound F127 molecules and Dbound and Dfree denoting the self-diffusion coefficients in the bound and free molecular states, respectively. The values of f were obtained by least-square fitting (using the Levenberg-Marquardt algorithm) of a two-component diffusional decay to the experimental data. The relative 1H peak intensities in Figure 1 arise from the hydrophilic PEO side chains of F127 (for reasons previously discussed,38-39 the 1H NMR signal from the adsorbed hydrophobic PPO units becomes undetectable).

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Figure 1. 1H NMR diffusional decays observed for the signal from the ethyleneoxide moieties of F127 molecules (inset) in the SWNT dispersions, prepared at cSWNT-init = 1.0 mg/mL with different initial dispersant (F127) concentrations.

3. Results and discussion 3.1. The SWNT dispersibility curves at different SWNT loadings The dispersibility curves display the concentration of dispersed nanotubes in the supernatant, cSWNT-sup, as a function of the amphiphile concentration in the initial dispersing mixture, cF127-init. The curves were recorded at different initial loadings of SWNT powder (0.5, 1.0 and 1.5 mg/mL) and are displayed in Fig. 2. For each curve, the initial SWNT loading is the same for all samples, and the abscissa presents the initial dispersant concentrations for each sample prepared. Following sonication and centrifugation (see Experimental section 2.2), the concentration of the NT in the supernatant is measured for each sample. All the F127-SWNT dispersibility curves exhibit a sigmoidal shape, in line with previous results for other dispersants.13, 28, 30, 34 This behavior will be further discussed in the following section.

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Figure 2. The concentration of dispersed SWNT, cSWNT-sup, as a function of the initial F127 concentration, cF127-init, recorded at different initial SWNT loadings (0.5, 1.0 and 1.5 mg/mL). Lines are for visual guidance.

3.2. SWNT dispersibility curves depending on the final dispersant concentration in the dispersion As mentioned above, the SWNT dispersibility curves in Fig. 2 are presented as a function of cF127init,

the concentration of F127 initially present prior to the sonication-centrifugation step. However,

during this process not all dispersant molecules are freely available because some adsorb instead on SWNT particles that later precipitate upon centrifugation. These particles could be large SWNT bundles that did not debundle into sufficiently small components during the dispersion process, carbonaceous impurities or, possibly, re-aggregated nanotubes/bundles. We estimate the F127 concentration in the supernatant cF127-sup from the intensity of the 1H NMR peak that arises from the ethylene oxide moieties within the F127 relative to the intensity of the same peak in a neat aqueous solution of known concentration. Based on those data, two figures highlight some features in the dispersion process. In Fig. 3a we present the ratio between the

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dispersant concentration in supernatant, cF127-sup, and the initial dispersant concentration, cF127-init, as a function of the initial concentration, cF127-init. In Fig. 3b we re-plot the dispersed nanotube concentration, cSWNT-sup, as it depends on cF127-sup, and not on cF127-init as in Fig. 2. We note that the curves in Fig. 3b are still sigmoidal, with a take-off value and a plateau in dispersibility. Also significantly, the different curves seem to coalesce at low cF127-sup, setting a common take-off value at about 0.1-0.3 mg/mL.

Figure 3. (a) The ratio between the dispersant concentration in the supernatant, cF127-sup, and the initial dispersant concentration, cF127-init, plotted as a function of cF127-init, with cF127-sup as estimated from by 1H NMR intensity of the ethylene oxide signal. The lines are exponential fits to guide the eye, and the inset presents a semi-log plot of the same data. (b) The dispersibility curves in Fig. 2 re-plotted as a function of the dispersant concentration in the supernatant cF127sup.

The legend in (b) applies also to (a).

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As shown by the inset in Fig. 3a, at low initial dispersant concentration (roughly, cF127-init < 1 mg/mL) most of the dispersant molecules do not get into the supernatant but adsorb instead on those SWNT particles that precipitate. The different initial behaviors observed in Fig. 3a (for cF127init

roughly < 15 mg/mL) at varying SWNT loadings are well explained then by the fact that, at the

particular preparation conditions here and irrespective of the actual loading value, approx. about 50% or more of the SWNT load does not become dispersed (as easily inferred from Fig. 3b). Hence, the higher the loading, the more precipitate surface to adsorb to is available. Upon increasing cF127-init, cF127-sup increases and reaches, irrespective of loading, its final value at about 80-90% indicating that only a small fraction (10-20 %) of dispersant is lost when maximum dispersibility of SWNT is attained. (If the sole reason of this loss were surfaces in the precipitate saturated by the dispersant, the plateau values in Fig. 3a should be higher at lower initial SWNT loading. No such clear trend is present for reasons that remain unresolved at this stage.) New insights about the dispersion process arise when comparing further the dispersibility curves plotted as a function of cF127-sup (Fig. 3b) instead of cF127-init (Fig. 2). To get to those insights, we start by re-capitulating the model that was proposed for the SWNT debundling (a.k.a exfoliation) process.42-43 Hence, sonication induces high shear forces by acoustic cavitation, which open up clefts in the SWNT bundles. In those clefts, pristine SWNT surfaces become exposed to the aqueous solution. Presumably, since the shear forces are random and fluctuating, those clefts can re-close; this would also be favored by the hydrophobic nature of the SWNT surface. Yet, such re-closing becomes hindered if dispersant molecules become adsorbed from the liquid phase onto the freshly exposed SWNT surfaces. A repetition of this process may lead to debundling into individual nanotubes and/or thin bundles. Hence, the essential role of the dispersant is its adsorption on the SWNT surface. Yet, an equally essential but seldom mentioned feature is the

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kinetics of this adsorption – namely, the fact that it must happen quickly enough before the cleft can re-close. In this context, the crucial feature of Fig. 3b is that, irrespective of the SWNT loading, the dispersion process takes off at roughly the same cF127-sup, as mentioned above. Hence, the essential control parameter for dispersibility is not cF127-init but cF127-sup. From another angle, cF127-init cannot be the control parameter since part of the dispersant initially added gets depleted from the solution after having adsorbed on the added and then precipitated SWNT particles. The larger the loading, the larger is this effect as is illustrated by the difference of the initial slopes in Fig. 3a. In the next sub-section, we shall analyze the mechanism by which cF127-sup becomes the control parameter of the dispersibility.

3.3. The polymer fraction adsorbed on the surface of debundled SWNTs in the dispersion F127 molecules in the supernatant exchange between two available states: one where they are free in solution and one where they are adsorbed on the surface of the debundled SWNT species that are either individual nanotubes or thin bundles. For reasons previously discussed,38-39 F127 molecules in both of those states contribute to the 1H NMR signal of ethylene oxide molecules. Hence, both of those dispersant fractions are part of cF127-sup as determined above. If conditions are set so that the exchange is slow with regard to the relevant time scale of NMR diffusion experiments, such experiments yield diffusional signal decays that are bi-exponential, as shown previously in Figure 1. In such bi-exponential decays, the fast component arises from quickly diffusing free F127 molecules while the slow component is yielded by slowly diffusing adsorbed F127 molecules. From the experimental bi-exponential decays, the free (1- f ) and SWNT-bound ( f ) fractions of cF127-sup can be obtained as the relative amplitudes of the two decay components

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extracted by least-square fitting. This has been explored previously to study surface coverage and competitive adsorption at SWNT surfaces.18, 38-39 In particular, in samples prepared at a fixed cSWNT-init = 1 mg/mL and cF127-init = 3 mg/mL (that yielded cSWNT-sup = 0.24 mg/mL and cF127-sup = 2.1 mg/mL) we have demonstrated that f was rather low, a few percent, and complete surface coverage corresponded to f·cF127-sup / cSWNT-sup ≈ 0.5 mg(dispersant)/mg(SWNT).38

Figure 4. The fraction f of F127 polymers adsorbed at SWNT surfaces available in the dispersion as obtained by 1H NMR diffusion experiments for different initial SWNT loadings and plotted on linear (a) and logarithmic (b) scale. The legend in (a) applies also to (b).

Here, we present in Fig. 4 the SWNT-bound dispersant fraction f vs the total amount of dispersant in the supernatant, cF127-sup as followed up all along the dispersibility curve. Primarily, this figure demonstrates convincingly that the fraction of adsorbed dispersant is small, a few percent, all along the dispersibility curve. Secondly, we consider the data in Fig. 3b and Fig. 4b for cSWNT-init = 1 mg/mL. At cF127-sup ≈ 0.2 mg/mL, that is roughly the take-off concentration, f ≈ 2-3%. With cSWNT-sup ≈ 0.05 mg/mL at that point, we obtain f·cF127-sup / cSWNT-sup ≈ 0.1 mg (dispersant)/mg(SWNT). This value is much below complete surface coverage that, as can be

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recalled, is ≈ 0.5 mg(dispersant)/mg(SWNT).38 Jointly, these results indicate that the dispersant concentration needed to achieve significant debundling is much higher than that required for a high surface coverage. This in turn implies that the primary reason for having a cF127-sup at which the amount of debundled and dispersed nanotubes takes off (Fig. 3b) is not connected to the need to achieve a threshold of high surface coverage. Neither is it so that the dispersion process is stopped at a particular degree of debundling because those newly created surfaces deplete the dispersant in the solution. Having excluded those reasons, the requirement of having cF127-sup to achieve dispersion must arise from kinetic considerations, namely, that a high enough coverage of the freshly-exposed surfaces must be attained quickly enough. There are two secondary points to make with regard to Fig. 4. At high cF127-sup values, f scales roughly with the SWNT content. This supports the notion that f, indeed, measures the amount of surface-adsorbed dispersant. Secondly, f exhibits a maximum (for 1.0 and 1.5 mg/mL loadings, shown clearly on the logarithmic scale in Fig. 4b). A possible explanation is that, on one hand, as we increase dispersant concentration from low initial values, debundling commences and more SWNTs surface becomes exposed by debundling whereupon more dispersant becomes adsorbed. In that regime (visible for 1.0 and 1.5 mg/mL loadings, but not detectable for 0.5 mg/mL loading), an incremental increase in cF127-sup seems to lead to a relatively large increase in the amount of exposed SWNT surfaces. On the other hand, as one approaches the dispersant concentration at which the amount of dispersed SWNT converges to its plateau value cSWNT-max, adding more dispersant creates little or no more surface and thereby the fraction of surface adsorbed dispersant must decrease. These opposing trends cause appearance of a maximum value for f. As there is more surface to be covered at higher SWNT loading, that maximum value is shifted to higher dispersant concentration, as shown indeed in Fig. 4b.

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A few questions remain. Why do different dispersants yield different dispersibility curves, as observed28 under similar conditions? What is the origin of the different plateau values, cSWNTmax?

And why are different dispersant concentrations required for different dispersants for the

debundling to commence? To re-phrase these questions: if the kinetics of adsorption to the SWNT surface is the limiting factor, why do different surfactants exhibit such different quantitative behavior, yet roughly with the same generic shape of their dispersibility curve? We offer two hypotheses. First, bulk concentration and bulk diffusion determine how quickly dispersant is available for adsorption at the surface. Thereby bulk concentration plays a major role in kinetics. Yet, the molecular interaction between the adsorbing moiety of the dispersant and the SWNT surface determines the rate for the actual step where the dispersant leaves the bulk phase and binds to the surface. This interaction must be significantly different for different surfactants because of differences in, e.g., hydrophobic chain length. Another possible contributing mechanism is that the adsorbed state is disfavored by forces driving clefts to re-close. A less favored surface-adsorbed state requires a higher (by a factor that depends on the relative change in free energy for the adsorbed dispersant) bulk concentration in order to maintain full coverage.

3.4. Sonication energy as a limiting factor for the amount of dispersed SWNT In the previous sub-sections, we analyzed the initial part of the dispersibility curve and argued that the observed concentration dependences are connected to the kinetics of dispersant adsorption. However, we also observe in Fig. 3b that cSWNT-sup (i) reaches an apparent plateau value cSWNT-max irrespective of the initial loading and (ii) under conditions studied so far, those plateau values are well below the initial loading cSWNT-init, being roughly (and, as we argue below, incidentally) at the half of it. In this subsection, we investigate the underlying reason for that behavior. 15



Re-aggregation of once-dispersed bundles and individual SWNTs cannot explain the plateau behavior, since that should lead to lower cSWNT-max/cSWNT-init ratios at high loadings which is clearly not the case (Fig. 1). To get an insight about the physical mechanism that, under the conditions explored so far, sets cSWNT-max/cSWNT-init ≈ 0.5 irrespective of the initial loading, we performed dispersion studies with different sonication energy densities (obtained using different sonication times). To the best of our knowledge (and to our surprise), a systematic and detailed study of the concerted influence of ultrasonication energy and the dispersant concentration has not been reported. Clearly, delivered sonication energy affects both cSWNT-sup as well as the bundle size and the fraction of individual NTs,12,

28, 43-45

but sonication also breaks the NTs46-47 and thereby

diminishes their electrical/thermal transport and mechanical performance.12, 39, 43 Hence, from the point of view of applications one should stop sonication at minimum CNT damage.43 From the basic point of view, at high sonication energy densities the nanotubes may get so much shorter that their dispersion changes physical character. Hence, we limited our sonication energy density to maximum 1.1 kJ/mL (in our case, corresponding to 10 minutes sonication), in agreement with previous reports.47

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Figure 5. Sonication time effect on SWNT dispersions, with the initial SWNT loading set to cSWNTinit

= 1 mg/mL, and for constant centrifugation conditions (g = 4000, t = 30 mins). The sonication power density was 1.8 W/mL.

Recall that the dispersed SWNTs in our supernatant at concentration cSWNT-sup are individual tubes and thin bundles that are smaller than a given size threshold. This threshold is not a general material feature but rather set by our centrifugation procedure, its duration and the gfactor applied.48 Hence, the plateau values cSWNT-max obtained at different initial loadings cannot be general material features, either. Yet, two questions remain – why do we get a plateau and why do we get it at a particular value? We propose that the feature that defines cSWNT-sup and, ultimately, cSWNT-max is how the centrifugation-specific size threshold is placed in relation to the size distribution of the dispersed SWNT species created by ultrasonication in the presence of dispersants. Clearly, more extensive ultrasonication leads to a continually decreasing particle/bundle size in other similar particle

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systems,49-50 in particular carbon particles51 and aggregates of multi-walled carbon nanotube (MWNT).50, 52 Our dispersions are dilute and particle-particle collisions should not contribute to the debundling processes. This dictates the same size distribution after the same ultrasonication process at the same dispersant concentration cF127-sup. If cF127-sup is sufficiently high and all opened clefts remain open (recall the previous subsections), one reaches maximum debundling effectivity and adding more dispersant has no effect. Hence the plateau behavior at each initial SWNT loading. Finally, the value of the plateau is, as indicated, set by the position of the bundle size distribution obtained by maximally effective sonication in relation to the centrifugation-dependent threshold, as observed in a recent report.53 This predicts increasing plateau value cSWNT-max upon increasing sonication time/energy density that is, indeed, shown in Fig. 5. In this model, in an ideal SWNT system with no impurities one should obtain cSWNT-max ≈ cSWNT-init at long enough sonication times (that is, high enough sonication energy densities). Of course, the latter may be impractical because of nanotube breakage.

4. Conclusions Even though the dispersing process of SWNTs into water has been taken up by hundreds of publications, detailed and reproducible dispersibility curves seem to be scarce. Perhaps partly for that reason, there are several open questions. In this work, we obtained and presented such curves depicting how the amount of dispersed SWNT depends on the actual dispersant concentration in the final dispersion.

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Our first finding is that, regarding the dispersant, the control parameter is indeed not the initial dispersant concentration but instead the dispersant concentration that remains in the dispersion after adsorption to non-dispersed particles (removed by centrifugation) depletes the solution. The amount of that available dispersant required to disperse sizable amounts of SWNT is much higher than the dispersant required to completely cover the solution-exposed SWNT surfaces created by sonication. This clearly shows that the surfactant concentration required for dispersion constitutes primarily a kinetic demand. That is, enough dispersant must be available to immediately cover the freshly exposed SWNT surfaces created by shear forces during sonication and thereby prevent those surfaces to re-close. The obtained dispersibility curves are sigmoidal. The plateau of those curves is then argued to appear at the point where the dispersant concentration is higher than that sufficient to ensure a rapid enough coverage of the fresh SWNT surfaces. During the sonication step of the preparation process, the original SWNT particles are debundled into smaller and smaller bundles, yielding a bundle size distribution that (at a particular dispersant concentration) depends on the invested sonication energy density. The centrifugation step used during the preparation process of the dispersion then simply sets the size threshold below which the bundles remain in the dispersion (while bundles above the size threshold precipitate – in the limit of ultrasonication, only individual SWNTs are below the size threshold). The way dispersibility curves depend on the sonication energy density simply reflects how the size distribution is moved relative to the fixed centrifugation-set size threshold – more sonication yields smaller bundles and thereby higher SWNT content in the dispersion. Besides some basic insights, our findings should have some relevance on the application of debundled CNTs in industry.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. UV-Vis spectrum of F127-SWNT dispersion and the extinction coefficients at different concentrations. Details of the combined TGA/UV-Vis quantification of the nanotube concentration. (PDF)

Acknowledgments This work has been supported by the Swedish Research Council VR. CIQUP acknowledges financial support from FEDER/COMPETE and FCT through grants UID/QUI/00081/2013, POCI-01-0145-FEDER-006980 and NORTE-01-0145-FEDER-000028.

References 1. Iijima, S., Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56-58. 2. Popov, V. N., Carbon Nanotubes: Properties and Application. Mater. Sci. Engin. Rep. 2004, 43, 61-102. 3. De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J., Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339, 535-539. 4. Ichinose, Y.; Eda, J.; Yomogida, Y.; Liu, Z.; Yanagi, K., Extraction of High-Purity Single-Chirality Single-Walled Carbon Nanotubes through Precise Ph Control Using Carbon Dioxide Bubbling. J. Phys. Chem. C 2017, 121, 13391-13395. 5. Di Crescenzo, A.; Cambré, S.; Germani, R.; Di Profio, P.; Fontana, A., Dispersion of Swcnts with Imidazolium-Rich Surfactants. Langmuir 2014, 30, 3979-3987. 6. Subbaiyan, N. K.; Cambré, S.; Parra-Vasquez, A. N. G.; Hároz, E. H.; Doorn, S. K.; Duque, J. G., Role of Surfactants and Salt in Aqueous Two-Phase Separation of Carbon Nanotubes toward Simple Chirality Isolation. ACS Nano 2014, 8, 1619-1628.

20


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7. Liang, L.; Xie, W.; Fang, S.; He, F.; Yin, B.; Tlili, C.; Wang, D.; Qiu, S.; Li, Q., High-Efficiency Dispersion and Sorting of Single-Walled Carbon Nanotubes Via Non-Covalent Interactions. J. Mater. Chem. C 2017, 5, 11339-11368. 8. Huang, Y. Y.; Terentjev, E. M., Dispersion of Carbon Nanotubes: Mixing, Sonication, Stabilization, and Composite Properties. Polymers-Basel 2012, 4, 275-295. 9. Angelikopoulos, P.; Bock, H., The Science of Dispersing Carbon Nanotubes with Surfactants. Phys. Chem. Chem. Phys. 2012, 14, 9546-57. 10. Kharissova, O. V.; Kharisov, B. I.; de Casas Ortiz, E. G., Dispersion of Carbon Nanotubes in Water and Non-Aqueous Solvents. RSC Advances 2013, 3, 24812-24852. 11. Blanch, A. J.; Lenehan, C. E.; Quinton, J. S., Optimizing Surfactant Concentrations for Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solution. J. Phys. Chem. B 2010, 114, 9805-9811. 12. Blanch, A. J.; Lenehan, C. E.; Quinton, J. S., Parametric Analysis of Sonication and Centrifugation Variables for Dispersion of Single Walled Carbon Nanotubes in Aqueous Solutions of Sodium Dodecylbenzene Sulfonate. Carbon 2011, 49, 5213-5228. 13. Clark, M. D.; Subramanian, S.; Krishnamoorti, R., Understanding Surfactant Aided Aqueous Dispersion of Multi-Walled Carbon Nanotubes. J. Colloid Interface Sci. 2011, 354, 144-51. 14. Dobies, M.; Iżykowska, J.; Wilkowska, M.; Woźniak-Braszak, A.; Szutkowski, K.; Skrzypczak, A.; Jurga, S.; Kozak, M., Dispersion of Water Proton Spin–Lattice Relaxation Rates in Aqueous Solutions of Multiwall Carbon Nanotubes (Mwcnts) Stabilized Via Alkyloxymethylimidazolium Surfactants. J. Phys. Chem. C 2017, 121, 11839-11850. 15. Liang, C.; Wang, B.; Chen, J.; Yong, Q.; Huang, Y.; Liao, B., Dispersion of Multi-Walled Carbon Nanotubes by Polymers with Carbazole Pendants. J. Phys. Chem. B 2017, 121, 8408-8416. 16. Dror, Y.; Pyckhout-Hintzen, W.; Cohen, Y., Conformation of Polymers Dispersing Single-Walled Carbon Nanotubes in Water: A Small-Angle Neutron Scattering Study. Macromolecules 2005, 38, 78287836. 17. Florent, M.; Shvartzman-Cohen, R.; Goldfarb, D.; Yerushalmi-Rozen, R., Self-Assembly of Pluronic Block Copolymers in Aqueous Dispersions of Single-Wall Carbon Nanotubes as Observed by Spin Probe Epr. Langmuir 2008, 24, 3773-9. 18. Frise, A. E.; Pagès, G.; Shtein, M.; Pri Bar, I.; Regev, O.; Furó, I., Polymer Binding to Carbon Nanotubes in Aqueous Dispersions: Residence Time on the Nanotube Surface as Obtained by Nmr Diffusometry. J. Phys. Chem. B 2012, 116, 2635-2642. 19. Granite, M.; Radulescu, A.; Pyckhout-Hintzen, W.; Cohen, Y., Interactions between Block Copolymers and Single-Walled Carbon Nanotubes in Aqueous Solutions: A Small-Angle Neutron Scattering Study. Langmuir 2011, 27, 751-759. 20. Han, Y.; Ahn, S. K.; Zhang, Z.; Smith, G. S.; Do, C., Tunable Encapsulation Structure of Block Copolymer Coated Single-Walled Carbon Nanotubes in Aqueous Solution. Macromolecules 2015, 48, 3475-3480. 21. Samanta, S. K.; Fritsch, M.; Scherf, U.; Gomulya, W.; Bisri, S. Z.; Loi, M. A., Conjugated PolymerAssisted Dispersion of Single-Wall Carbon Nanotubes: The Power of Polymer Wrapping. Acc. Chem. Res. 2014, 47, 2446-56. 22. Antonucci, A.; Kupis-Rozmysłowicz, J.; Boghossian, A. A., Noncovalent Protein and Peptide Functionalization of Single-Walled Carbon Nanotubes for Biodelivery and Optical Sensing Applications. ACS Appl. Mater. Interfaces 2017, 9, 11321-11331. 23. Ao, G.; Streit, J. K.; Fagan, J. A.; Zheng, M., Differentiating Left- and Right-Handed Carbon Nanotubes by DNA. J. Am. Chem. Soc. 2016, 138, 16677-16685. 24. Edri, E.; Regev, O., Ph Effects on Bsa-Dispersed Carbon Nanotubes Studied by SpectroscopyEnhanced Composition Evaluation Techniques. Anal. Chem. 2008, 80, 4049-54.

21



25. Frise, A. E.; Edri, E.; Furó, I.; Regev, O., Protein Dispersant Binding on Nanotubes Studied by Nmr Self-Diffusion and Cryo-Tem Techniques. J. Phys. Chem. Lett. 2010, 1, 1414-1419. 26. Mougel, J.-B.; Adda, C.; Bertoncini, P.; Capron, I.; Cathala, B.; Chauvet, O., Highly Efficient and Predictable Noncovalent Dispersion of Single-Walled and Multi-Walled Carbon Nanotubes by Cellulose Nanocrystals. J. Phys. Chem. C 2016, 120, 22694-22701. 27. Duan, W. H.; Wang, Q.; Collins, F., Dispersion of Carbon Nanotubes with Sds Surfactants: A Study from a Binding Energy Perspective. Chem. Sci. 2011, 2, 1407-1413. 28. Fernandes, R. M. F.; Abreu, B.; Claro, B.; Buzaglo, M.; Regev, O.; Furó, I.; Marques, E. F., Dispersing Carbon Nanotubes with Ionic Surfactants under Controlled Conditions: Comparisons and Insight. Langmuir 2015, 31, 10955-10965. 29. Bai, Y.; Lin, D.; Wu, F.; Wang, Z.; Xing, B., Adsorption of Triton X-Series Surfactants and Its Role in Stabilizing Multi-Walled Carbon Nanotube Suspensions. Chemosphere 2010, 79, 362-367. 30. Grossiord, N.; van der Schoot, P.; Meuldijk, J.; Koning, C. E., Determination of the Surface Coverage of Exfoliated Carbon Nanotubes by Surfactant Molecules in Aqueous Solution. Langmuir 2007, 23, 3646-3653. 31. Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.; Coleman, J. N., Quantitative Evaluation of Surfactant-Stabilized Single-Walled Carbon Nanotubes: Dispersion Quality and Its Correlation with Zeta Potential. J. Phys. Chem. C 2008, 112, 10692-10699. 32. Wang, Q.; Han, Y.; Wang, Y.; Qin, Y.; Guo, Z.-X., Effect of Surfactant Structure on the Stability of Carbon Nanotubes in Aqueous Solution. J. Phys. Chem. B 2008, 112, 7227-7233. 33. Tardani, F.; La Mesa, C., Attempts to Control Depletion in the Surfactant-Assisted Stabilization of Single-Walled Carbon Nanotubes. Colloids Surf. A 2014, 443, 123-128. 34. Rastogi, R.; Kaushal, R.; Tripathi, S. K.; Sharma, A. L.; Kaur, I.; Bharadwaj, L. M., Comparative Study of Carbon Nanotube Dispersion Using Surfactants. J. Colloid Interface Sci. 2008, 328, 421-428. 35. Fernandes, R. M. F.; Buzaglo, M.; Regev, O.; Furo, I.; Marques, E. F., Mechanical Agitation Induces Counterintuitive Aggregation of Pre-Dispersed Carbon Nanotubes. J. Colloid Interface Sci. 2017, 493, 398-404. 36. Devre, R. D.; Budhlall, B. M.; Barry, C. F., Enhancing the Colloidal Stability and Electrical Conductivity of Single-Walled Carbon Nanotubes Dispersed in Water. Macromol. Chem. Phys. 2016, 217, 683-700. 37. Ramos, E.; Pardo, W. A.; Mir, M.; Samitier, J., Dependence of Carbon Nanotubes Dispersion Kinetics on Surfactants. Nanotechnology 2017, 28, 135702. 38. Fernandes, R. M. F.; Buzaglo, M.; Regev, O.; Marques, E. F.; Furó, I., Surface Coverage and Competitive Adsorption on Carbon Nanotubes. J. Phys. Chem. C 2015, 119, 22190-22197. 39. Fernandes, R. M. F.; Buzaglo, M.; Shtein, M.; Pri Bar, I.; Regev, O.; Marques, E. F.; Furó, I., Lateral Diffusion of Dispersing Molecules on Nanotubes as Probed by Nmr. J. Phys. Chem. C 2014, 118, 582-589. 40. Shtein, M.; Pri-bar, I.; Regev, O., A Simple Solution for the Determination of Pristine Carbon Nanotube Concentration. Analyst 2013, 138, 1490-1496. 41. Price, W. S., Nmr Studies of Translational Motion; Cambridge University Press: Cambridge, 2009. 42. Strano, M. S.; Moore, V. C.; Miller, M. K.; Allen, M. J.; Haroz, E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E., The Role of Surfactant Adsorption During Ultrasonication in the Dispersion of SingleWalled Carbon Nanotubes. J. Nanosci. Nanotechnol. 2003, 3, 81-86. 43. Dassios, K. G.; Alafogianni, P.; Antiohos, S. K.; Leptokaridis, C.; Barkoula, N.-M.; Matikas, T. E., Optimization of Sonication Parameters for Homogeneous Surfactant-Assisted Dispersion of Multiwalled Carbon Nanotubes in Aqueous Solutions. J. Phys. Chem. C 2015, 119, 7506-7516. 44. Matarredona, O.; Rhoads, H.; Li, Z.; Harwell, J. H.; Balzano, L.; Resasco, D. E., Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solutions of the Anionic Surfactant Naddbs. J. Phys. Chem. B 2003, 107, 13357-13367. 22


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Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45. Grossiord, N.; Regev, O.; Loos, J.; Meuldijk, J.; Koning, C. E., Time-Dependent Study of the Exfoliation Process of Carbon Nanotubes in Aqueous Dispersions by Using Uv-Visible Spectroscopy. Anal. Chem. 2005, 77, 5135-9. 46. Hilding, J.; Grulke, E. A.; Zhang, Z. G.; Lockwood, F., Dispersion of Carbon Nanotubes in Liquids. J. Disp. Sci. Technol. 2003, 24, 1-41. 47. Fuge, R.; Liebscher, M.; Schröfl, C.; Oswald, S.; Leonhardt, A.; Büchner, B.; Mechtcherine, V., Fragmentation Characteristics of Undoped and Nitrogen-Doped Multiwalled Carbon Nanotubes in Aqueous Dispersion in Dependence on the Ultrasonication Parameters. Diam. Relat. Mater. 2016, 66, 126-134. 48. In this context, our preparation procedure defers from studies where only individual SWNT are left in the supernatant (due to the use of stronger centrifugation, i.e., ultracentrifugation, see reference 53). Since we used only 4000 g centrifugation, our supernatant contains both individual and thin SWNT bundles. Therefore, the concentration of the SWNT in the supernatant is much higher. 49. Kaiser, M.; Berhe, A. A., How Does Sonication Affect the Mineral and Organic Constituents of Soil Aggregates?-a Review. J. Plant Nutr. Soil Sci. 2014, 177, 479-495. 50. Peters, D., Ultrasound in Materials Chemistry. J. Mater. Chem. 1996, 6, 1605-1618. 51. Kumar, P.; Bohidar, H. B., Aqueous Dispersion Stability of Multi-Carbon Nanoparticles in Anionic, Cationic, Neutral, Bile Salt and Pulmonary Surfactant Solutions. Colloids Surf. A 2010, 361, 13-24. 52. Krause, B.; Mende, M.; Pötschke, P.; Petzold, G., Dispersability and Particle Size Distribution of Cnts in an Aqueous Surfactant Dispersion as a Function of Ultrasonic Treatment Time. Carbon 2010, 48, 2746-2754. 53. Zheng, Y.; Sanchez, S. R.; Bachilo, S. M.; Weisman, R. B., Indexing the Quality of Single-Wall Carbon Nanotube Dispersions Using Absorption Spectra. J. Phys. Chem. C 2018, 122, 4681-4690.

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Dispersing Carbon Nanotubes in Water with Amphiphiles: Dispersant

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