Journal of Colloid and Interface Science 299 (2006) 740–746www.elsevier.com/locate/jcis

Stability of the aqueous suspensions of nanotubes in the presenceof nonionic surfactant

Milana O. Lisunova a,b, Nikolai I. Lebovka a,b,∗, Olexander V. Melezhyk c, Yurie P. Boiko b

a Faculty of Applied Physics, National Aviation University, Kiev, Ukraineb Institute of Biocolloid Chemistry, Ukrainian National Academy of Sciences, Kiev, Ukraine

c TM Spetsmash Ltd., Kiev, Ukraine

Received 21 January 2006; accepted 6 March 2006

Available online 5 April 2006

Abstract

Stability of aqueous suspensions of multiwalled carbon nanotubes (MWNTs) and their percolation behavior are investigated. Nanotubes ofaqueous suspensions show a strong tendency to aggregation and networking into electroconductive clusters. The percolation threshold of theelectrical conductivity is rather low and of order ϕ ≈ 0.01 (where ϕ is the volume fraction), which can be explained by the high aspect ratioof MWNTs. Strong influence of the nonionic surfactant Triton X-305 on the colloidal stability of aqueous suspensions of MWNTs is observed.Addition of surfactant exerts a stabilizing effect at surfactant concentration Cs proportional to the weight concentration C of MWNTs, Cs ≈C mol/dm3. The transient behavior of electrical conductivity in the aqueous suspensions is explained by fractal aggregation processes. The fractaldimension is shown to be sensitive to the surfactant concentration Cs.© 2006 Elsevier Inc. All rights reserved.

Keywords: Nanotubes; Aqueous suspensions; Electrical conductivity; Percolation; Fractal dimension

1. Introduction

Carbon nanotubes (CNTs) exhibit unique electronic, ther-mal, optical, and mechanical properties, which can be usefulfor many technologically important purposes [1]. One of theirapplications seeming to be rather promising in the near futureis as nanoscale filler for production of composite materials withattractive conductivity and tensile strength properties [2–8].

Multiwalled carbon nanotubes (MWNTs) are reported tohave a rather high electrical conductivity of about 1.85 ×103 S/cm along the long axis, which allows them to conductcurrent of enormously high density, exceeding 107 A cm−2 [9].Owing to the high aspect ratio (100–1000) of carbon nanotubesand their high specific surface area, only a very small concen-tration of nanotubes (≈0.1–5 vol% of CNTs) is required forconductive path formation in a composite [10–18]. CNTs pos-

* Corresponding author. Fax: +380 (44) 424 08 78.E-mail address: nikolai.lebovka@utc.fr (N.I. Lebovka).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.03.012

sess one of the highest thermal conductivities known [19], andthey are very promising as a composite filler in materials des-ignated for thermal management applications [20,21]. CNTspossess high tensile strength and modulus and they are signifi-cantly stiffer than currently available carbon fibers [22,23]. SoCNTs are expected to be useful as conductive and mechanicalreinforcements for low-weight composite materials for variousapplications, such as sensors, electromagnetic shielding mate-rials and antistatic films [24,25].

However, CNTs are poorly soluble in most solvents [26].Due to strong van der Waals attraction, they exhibit a tendencyto aggregation and formation of the large entangled agglomer-ates. Recently proposed large-scale catalytic methods for pro-duction of CNTs result in highly entangled aggregates of nan-otubes [27]. In many important applications, CNTs can realizetheir unique properties only if they are in a highly homoge-neous dispersed state, and in most cases their aggregation isan obstacle to most of their applications. Sometimes CNT so-lution handling would be exceptionally useful because their

M.O. Lisunova et al. / Journal of Colloid and Interface Science 299 (2006) 740–746 741

processing in other forms exhibits many difficulties. Some stud-ies of CNTs suspended in organic solvents have been reported[26,28–31]. Behavior of CNTs in aqueous medium is also ofgreat interest because of their potential applications for prob-ing biomolecules in water and at surfaces, including biologicalmembranes, using atomic force microscopy [32–34] and theiruse as biosensor devices [35–37]. Because of the hydrophobicnature of the external surface of carbon tubes, their solubilityin water is expected to be very low. The theoretical analy-sis of water structure at the water–carbon nanotube interfacemakes it possible to expect properties similar to those foundfor water at the idealized graphite surface [38], though resultsof [39] evidence drying of the interstices, or water expulsionfrom spaces between the CNTs, when they are less than 0.9–1.0 nm.

A stable colloidal aqueous suspension of CNTs can beachieved in the presence of surfactants [40–45]. Such a suspen-sion permits, for example, sorting of carbon tubes by length, butfurther surfactant removal is problematic [46]. Nonionic sur-factants seem to be rather promising for stabilization of aque-ous suspensions of CNTs, because this type of surfactants wasfound to interact strongly with the graphite surface [47].

The aim of our work is to study stability of aqueous suspen-sions of nanotubes and their electrical conductivity percolationin the presence of the nonionic surfactant Triton X-305.

2. Experimental

2.1. Synthesis of MWNTs and their properties

The MWNTs were prepared from ethylene using the chem-ical vapor deposition (CVD) method as described elsewhere[48]. FeAlMo0.07 was used as a catalyst for production of theCNTs. For purification of the MWNTs from mineral additivesand for elimination of the effect of catalyst, the samples weretreated with concentrated alkali (NaOH) and acid (HCl) solu-tions. After that, the samples were filtered and the excess acidwas removed. The remaining black powder was watered repeat-edly by distilled water until the pH value of the filtrate becamethe same as that of the distilled water. The residual mass contentof the mineral additives was <1%.

An example of a transmission electron micrograph (TEM) ofthe MWNTs is presented in Fig. 1. The MWNTs can be consid-ered as a collection of concentric nanotubes with an interlayerspacing of 0.34 nm. The presented TEM picture clearly showsthe difference between the MWNT diameters and tangling ofthe nanotubes.

Typically used MWNTs have outer diameter de about 12–20 nm and length about tens of micrometers, which result inan aspect ratio of up to 100–1000. The inner diameter di ofthe MWNTs varies from 1 to 3 nm. The specific surface areaS is 190 m2/g (±10%). The specific electric conductivity σ

of the powder of compressed MWNTs (at pressure 15 TPa) is10.0 S/cm along the axis of compression and appears to be sig-nificantly lower than that of the graphite (σ = 103–106 S/cm[49]). The density of the MWNT walls was assumed to be thesame as the density of pure graphite, ρc = 2045 kg/m3.

Fig. 1. Electron micrograph of MWNTs.

Fig. 2. Structure of Triton X-305 (polyoxyethylene (30)4-(1,1,3,3-tetramethyl-butyl) phenyl ether).

2.2. Preparation of MWNT suspensions

The water used in this study was double-distilled. It hadconductivity 5 µS/cm at 20 ◦C and the background electrolyte(0.01 M KCl) was present in all the cases. The MWNTs weresuspended in water solution in the presence of the nonionicsurfactant Triton X-305 (MW = 1527.87 g), Sigma–Aldrich(Fig. 2). Triton X-305 was shown to be effective for stabiliza-tion of the aqueous suspension of colloidal graphite [50]. Itscritical micelle concentration (CMC) in water at 20 ◦C is on theorder of 10−3 mol/dm3 [52].

Ultrasonication was used for preparation of mixed aqueousMWNT suspensions. To keep suspensions from overheating,they were sonicated in a cold-water bath maintained at 0–4 ◦C.The MWNT suspensions were sonicated for 30 min at a fre-quency of 22 kHz and an output power of 150 W. Ultrasonica-tion is an accepted technique for dispersing the highly entan-gled or aggregated nanotube samples [51,52], but longer timesof high-energy sonication can introduce defects and can de-crease the lengths of nanotubes [53,54]. After cooling to roomtemperature the suspensions were stirred carefully for 5 minand put in a conductivity cell thermostated at 25 ◦C, and con-ductivity measurements were immediately started. Each experi-ment was repeated at least five times to calculate the mean valueof the experimental data.

The volume fraction of MWNT suspensions was estimatedas

(1)ϕ = (1 + ρc/ρw(1/C − 1)

)−1,

where ρc = 1000 kg/m3 is the water density and C is the weightconcentration of nanotubes in an aqueous suspension.

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2.3. Instrumentation

The structure of MWNTs was examined using the trans-mission electron microscopy (TEM, JEM 100 ◦C, Jeol, Tokyo,Japan, operating at accelerating voltage 100 kV). The ultra-sonication was conducted using a sonifier equipped with a cuphorn UZDN-2T, Russia, which was operated at a frequency of22 kHz with output power 150 W. The electrical conductiv-ity measurements were made using P-5068 (Tochelectropribor,Kiev, Ukraine) at frequency f = 1 kHz, in a cell with two ver-tical platinum electrodes with surface area 1 cm2.

3. Results and discussion

Fig. 3 presents the electrical conductivity σ versus weightconcentration of the MWNTs C in an aqueous suspension.Here, σo = 1.2 × 10−3 S/cm is the electrical conductivity ofbackground 0.01 M KCl electrolyte. The electrical conductivityσ was measured immediately after suspension placement intothe conductometric cell and its thorough stirring. The electricalconductivity of the MWNT particles, σp (≈10 S/cm), is consid-erably higher than that of the background electrolyte, σp � σo.As concentration of the MWNTs, C, increases, the distance be-tween the nanotubes diminishes and the number of intersectionsincreases, and it results in increased electrical conductivity ofsuspension σ .

The Maxwell model [55] predicts the following dependenceof the suspension conductivity σ versus the disperse phase vol-ume fraction ϕ,

(2)σ = σo

(1 + 3ϕ

(α + 2)/(α − 1) − ϕ

),

where α = σp/σo is the dimensionless ratio of conductivities.

Fig. 3. Electrical conductivity σ versus mass concentration of MWNTs C inaqueous suspensions. Here σo = 1.2 × 10−3 S/cm is electrical conductivityof background 0.01 M KCl electrolyte, and error bars are standard error ofthe means. Insert shows σ/σo − 1 versus volume fraction of nanoparticles ϕ′;dashed line corresponds to a slope of 1000.

In the limit of small concentration, ϕ → 0, and α = σp/σo� 1 (conductive particles in an insulating liquid), Eq. (3) re-duces to the linear equation

(3)σ/σo − 1 ≈ 3ϕ.

The insert in Fig. 3 presents σ/σo −1 versus volume fractionof nanoparticles ϕ, estimated from Eq. (1). The experimentaldata evidence that the theoretically predicted slope 3 is mate-rially exceeded and is of order 1000 (estimated in concentra-tion interval 0 < ϕ < 0.01). This is a result of imperfection ofMaxwell’s model, which is based on the assumption that par-ticles are spherical and randomly distributed in suspensions,there exist no aggregations, and distances between the particlesare much larger than their diameters [55]. In fact, the MWNTscan be regarded as elliptical particles with a very large aspect ra-tio, of order 100–1000, and the Maxwell model fails to describethe concentration dependence of the electrical conductivity ifparticles are elongated [56,57].

Extension of the Maxwell model to elongated particles [58]predicts the following dependence of the suspension conductiv-ity σ versus volume fraction ϕ of the disperse phase

(4)σ = σo

(1 + 3ϕ

r(1 − ϕ + 3/r(α − 1))

),

where r is the ratio of the surface area of a spherical particle tothe surface area of an elongated particle of equivalent volume.

In the limit of small concentration, ϕ → 0, and α � 1 (inour case, α ≈ 104), Eq. (4) reduces to

(5)σ = σo(1 + 3ϕ/(r + 3/α)

).

This equation reduces to Maxwell’s equation (3) for thespherical particles at r = 1, but r � 1 and the slope 3/(r +3/α)

can be rather high for elongated particles with a large aspect ra-tio, which is in accordance with our experimental observations.

The insert in Fig. 3 evidences also that electrical conduc-tivity begins to rise appreciably at ϕ � 0.01. This indicatesthe presence of a strong tendency to aggregation, beginning ofpercolation, and networking of MWNTs into electroconductiveclusters. In this case, the percolation approach is more reliablefor description of the electrical conductivity behavior [59].

The percolation threshold at very low MWNT loadings, ϕ ≈0.01, is typical for MWNT composites [10–18] as far as thepercolation threshold decreases rapidly with the particle axialratio increase and particle size decrease [59].

The investigations show that the electrical conductivity ofan aqueous suspension of MWNTs is not stable and continu-ously increases with time t after intensive stirring of the sus-pension. Fig. 4 presents the relative conductivity σ − σi versustime t after intensive stirring at different mass concentrationsof MWNTs C. The observed transient electrical conductivitybehavior reflects structural changes in the aqueous suspension.Because of the hydrophobic nature of the external surface,the carbon nanotubes exhibit a strong tendency to form largeentangled agglomerates in an aqueous suspension and to set-tling down from suspension. Fig. 5 shows the photographs ofthe MWNT suspensions at different times after thorough stir-ring. They evidence rapid sedimentation at a concentration of

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Fig. 4. The kinetics of relative electrical conductivity (σ −σi) changes at differ-ent mass concentrations of MWNTs, C = 10−4 (1), 10−3 (2), 10−2 (3). Hereσi is an initial electrical conductivity measured after intensive mixing of sus-pension. Insert shows the enlarged portion of figure for the time period wherefractal dimension was estimated.

Fig. 5. Photographs of MWNT suspensions at different times after thoroughstirring: t = 0 s (a), 30 s (b), 60 s (c), 24 h (d), C = 10−3.

C = 10−3. The interparticle interactions can suppress sedimen-tation at higher concentrations of MWNTs, and aqueous sus-pensions are sedimentationally stable visually at concentrationsC � 10−2. It is remarkable that noticeable changes of σ − σiwith time t are observed for long-time intervals (t ≈ 1000 s)after intensive stirring of the suspension even for sedimenta-tionally stable suspensions (at concentrations C = 0.01–0.05).

The double logarithmic σ − σi versus t presentation givenin Fig. 4 clearly reveals different types of behavior for timescales 0 < t < 100 s and t > 100 s. The initial time period0 < t < 100 s probably corresponds to the phase of rapidsedimentation (for C < 0.01) or fast rebuilding of contactsbetween micrometer-sized MWNT aggregates after intensivestirring (for C � 0.01). The long-time behavior observed att > 100 s may reflect the compacting processes in the sed-iment of MWNTs at the bottom of the conductometric cell(for C < 0.01), or the large-scale morphological changes in

Fig. 6. Photographs of MWNT suspensions in the absence (a) and in the pres-ence (b) of nonionic surfactant Triton X-305. Concentration of MWNTs isC = 10−3, and concentration of surfactant is Cs = 10−3 mol/dm3.

the structure of aggregates and their agglomeration into frac-tal clusters (for C � 0.01).

The morphological changes in the structure of suspensionscan influence the electrical conductivity of suspensions. Our ex-perimental data evidence the existence of a strong tendency ofMWNTs to aggregation and networking into electroconductiveclusters for the sedimentationally stable suspensions at weightconcentrations C � 10−2. In the presence of fractal aggrega-tion the theoretical estimate predicts the following kinetics ofelectrical conductivity [60],

(6)σ − σi ∝ t3/df−1,

where df is the fractal dimension of the clusters.The value of df as estimated from the slope of the σ − σi

versus t curve in the double logarithmic presentation is ≈2.5(curve 3 in Fig. 4; the estimation was made in the interval of100 < t < 2000 s), and approximately the same values wereobtained for the sedimentationally stable suspensions withinC = 0.01–0.05.

Formation of networks with fractal-like structures in di-lute aqueous suspensions of colloidal single-walled carbonnanotubes (SWNT) was previously observed using the light-scattering measurement technique [42]. Formation of fractal ag-glomerates in short-chain diamine suspensions of SWNTs wasalso reported [61]. It was noted that SWNTs were not dispersedto the single-tube level and were highly agglomerated into frac-tal clusters with an average radius of gyration of 1.7 µm.

The sedimentation stability of an aqueous colloidal suspen-sion of hydrophobic particles may be enhanced by additionof surfactants [43,62]. The surfactant molecules get adsorbedonto the exterior carbon surface by their hydrophobic segmentswith hydrophilic tail directed to the exterior. Fig. 6 shows theMWNT suspension photographs in the absence (a) and in thepresence (b) of the nonionic surfactant Triton X-305.

Nonionic surfactants can adsorb strongly to graphite [63–65]and addition of nonionic surfactant to the nanotube suspensionswas previously shown to enhance suspension stability [42]. Ourexperimental data for different concentrations of MWNTs showthat the sedimentationally unstable suspension becomes morehomogeneous and visually stable at surfactant concentration ex-ceeding Cs ≈ C mol/dm3 (for example Cs = 10−3 mol/dm3

744 M.O. Lisunova et al. / Journal of Colloid and Interface Science 299 (2006) 740–746

Fig. 7. The kinetics of relative electrical conductivity (σ − σi) changes at dif-ferent concentrations of nonionic surfactant Triton X-305: Cs = 0 mol/dm3

(1), 10−3 mol/dm3 (2), 5 × 10−3 mol/dm3 (3), and 10−2 mol/dm3 (4). Theconcentration of MWNTs in aqueous suspension is C = 0.01. Insert shows thereduced conductivity Z = (σ −σi)/(σmax −σi) versus the time t . Here σi is aninitial electrical conductivity measured after intensive mixing of the sample att = 0, and σmax is measured at t = 2000 s.

at C = 10−3 in Fig. 6). If we imagine that all the surfactantmolecules are distributed only at the carbon tube surface, thensurfactant concentration required for formation of the adsorp-tion monolayer will be equal to

(7)Cs,mono ≈ 103SC/SsNa ≈ 0.2C mol/dm3,

where S is the specific surface area of nanotubes (≈190 m2/g),Ss is surface area per surfactant molecule in the adsorbed layer(≈1.65 nm2 for parallel orientation of the Triton X-305 mole-cules to the surface as estimated for the graphite–water interface[63,64]), and Na is Avogadro’s number.

These estimates of Cs,mono are very rough, because mecha-nisms of nonionic surfactant absorption at carbon surfaces arestill debated [66,67]. Adsorption of a nonionic surfactant atthe carbon surface increases with concentration increase up tothe CMC (≈10−3 mol/dm3 Triton X-305 [51]), and further onit remains fairly constant. In any case, we may conclude thatMWNT suspension stabilization requires formation of severalsurfactant monolayers near the nanotube surface, and higherconcentration of a MWNT suspension will require higher sta-bilizing concentration of the surfactant.

Kinetics of the electrical conductivity changes at differentconcentrations of the nonionic surfactant Triton X-305 is shownin Fig. 7. The MWNT concentration in the aqueous suspensionis C = 0.01, and this suspension is sedimentationally stableeven in the absence of the surfactant. Addition of the surfactantresults in decreasing electrical conductivity of the suspension.This probably reflects decrease of the number of intersectioncontacts and increase of the energy barrier for electron transferbetween nanotube intersections in the presence of surfactant.

Differences in the electrical conductivity kinetics at differ-ent surfactant concentrations are more distinct in the insert in

Fig. 8. The fractal dimension df as function of nonionic surfactant concentra-tions Cs. The concentration of MWNTs in aqueous suspension is C = 10−2.

Fig. 7, where the reduced conductivity Z = (σ − σi)/(σmax −σi) versus time t is presented. Here, σi is the initial electricalconductivity measured after intensive stirring of the sample att = 0, and σmax is measured at t = 2000 s.

With increasing of surfactant concentration the long-timetransient conductivity behavior at t > 100 diminishes and theslope of the σ −σi versus t curve in double logarithmic presen-tation decreases. As follows from Eq. (6), it corresponds to theincreasing of fractal dimension df with increasing of Cs.

Fig. 8 shows the fractal dimension df versus nonionic surfac-tant concentration Cs for the sedimentationally stable MWNTsuspension of C = 0.01. The fractal dimension df continu-ally increases with surfactant concentration Cs increase: fromdf ≈ 2.5 at Cs = 0 to df ≈ 3 at Cs = 10−2 mol/dm3.

Different spatial arrangements of the nanotubes in an aque-ous suspension are schematically presented in Fig. 8. The rami-fied loose-packing structures with strong intersection contactsbetween nanotubes are observed at small surfactant concen-trations (Cs). The Cs increase results in formation of a morehomogeneous structure, but the number of intersections andelectrical contacts between nanotubes decreases. It is interest-ing to note that formation of a homogeneous structure withdf ≈ 3 is observed at a surfactant level corresponding to its sta-bilizing concentration Cs ≈ C mol/dm3.

4. Summary

The nanotubes dispersed in an aqueous medium show astrong tendency to aggregation and networking into electro-conductive clusters. The percolation threshold at rather lowvalues of the MWNT loading, ϕ ≈ 0.01, may reflect the highaspect ratio of the elongated nanotube particles. The observedtransient electrical conductivity behavior was shown to reflectsedimentation and structural changes in the MWNT suspen-sions. Such 1 wt% suspensions are sedimentationally stable,and transient behavior of electrical conductivity observed in

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such systems makes it possible to suggests formation of afractal structure during aggregation in the vicinity of the per-colation threshold. Addition of the nonionic surfactant TritonX-305 contributes to suspension stabilization at a surfactantconcentration Cs proportional to the weight concentration ofMWNTs, Cs ≈ C mol/dm3. The observed effect is similar tothat previously obtained for aqueous hydrophobic suspensionsof graphite [50]. The nonionic surfactant is known to effectnoticeably different electrical connectivity and rigidity (or sol–gel) percolation transitions in micrometer-sized graphite sus-pensions [50]. The present work shows the important effectsthe surfactant exerts on the structural organization of aqueoushydrophobic suspensions of nanotubes. The surfactant addi-tion changes spatial arrangement of the nanotubes, causes in-crease of the fractal dimension df, and affects the transientelectrical conductivity behavior. In aqueous hydrophobic sus-pensions of nanosized MWNTs, the electrical connectivity andrigidity percolation transitions are likely to display the no-ticeable dependence versus the concentration and type of sur-factant stabilizer, and we shall look into this in our futurework.

Acknowledgments

This work was partially funded by the NAS of Ukraine underProjects 2.16.1.4, 2.16.1.7, and 2.16.2.1. The authors thank Dr.N.S. Pivovarova for her help with the manuscript preparation.

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