Article pubs.acs.org/est
Aggregation Kinetics and Transport of Single-Walled Carbon Nanotubes at Low Surfactant Concentrations Dermont Bouchard,*,† Wei Zhang,§ Tremaine Powell,† and U-sa Rattanaudompol‡ †
USEPA Office of Research and Development, National Exposure Research Laboratory, 960 College Station Road, Athens, Georgia 30605, United States ‡ Student Services Contractor, Athens, Georgia 30605, United States § Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, United States S Supporting Information *
ABSTRACT: Little is known about how low levels of surfactants can affect the colloidal stability of single-walled carbon nanotubes (SWNTs) and how surfactant-wrapping of SWNTs can impact ecological exposures in aqueous systems. In this study, SWNTs were suspended in water with sodium dodecylsulfate (SDS) as a surface-active dispersing agent. The effect of SDS concentration on SWNT suspension stability was investigated with time-resolved dynamic light scattering (TRDLS) initial aggregation studies utilizing both monovalent (Na+) and divalent (Ca2+) cations. The critical coagulation concentration (CCC) values increased with SDS concentration for the Na+ treatments, but the Ca2+ treatments were less sensitive to SDS concentration changes. Longer term stability studies with SDS concentrations orders of magnitude below the SDS critical micelle concentration demonstrated that SWNTs remained suspended for over six weeks in a surface water. Transport studies in a freshwater sediment similarly showed a SDS concentration-dependent mobility of SDS−wrapped SWNTs in that SWNTs showed a relatively greater retention at lower SDS concentrations (0.001%−0.05% w/v) than at a higher SDS concentration (0.1%). It is hypothesized that the stability and mobility of SWNT suspensions is directly related to the surface coverage of SDS on the SWNT surface that simultaneously increases electrosteric repulsion and decreases surface chemical heterogeneity. Overall, these studies demonstrate that low levels of surfactant are effective in stabilizing and mobilizing SWNTs in environmental media.
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INTRODUCTION There are currently more than 1300 consumer products on the market that contain nanoscale materials,1 and the development and commercial production of nanomaterials is expected to continue to grow rapidly. The use of carbon-based nanomaterials in consumer products is second only to that of nanoscale silver.1 Of the carbon-based materials, carbon nanotubes (CNTs) in particular have garnered a lot of attention due to their unique electronic, mechanical, and structural properties2−4 as well as their potential in drug delivery and other biomedical applications.5,6 As more nanomaterials are produced, the possibility of accidental release and subsequent human and ecological exposures increases.7,8 The potential for accidental release into the environment and subsequent adverse effects underscores the importance of understanding the colloidal stability of nanoparticles dispersed in water as well as the retention of nanoparticles in environmental media. Many CNT applications require that the CNT bundles be separated into individual tubes for maximal performance. However, separating CNT bundles into CNT singlets is problematic as individual CNTs readily form parallel bundles due to strong van der Waals interaction energies estimated to be approximately 500 eV per micrometer of tube−tube contact.9 To facilitate CNT dispersion in water, CNTs are commonly functionalized (usually through strong acid © 2012 American Chemical Society
oxidation) or ultrasonicated in the presence of surface-active stabilizing agents such as surfactants. While effective at solubilizing CNTs, covalent functionalization has the undesirable side effect of creating defects in the CNT surface which alter the CNT electrical and optical properties,10,11 rendering them less useful for high value applications. CNT debundling and stabilization with surfactants circumvents this problem and can yield stable, high concentration CNT aqueous suspensions. Ultrasonication with both anionic and nonionic surfactants has been observed to be effective in debundling and stabilizing CNTs in aqueous suspension, indicating that both electrostatic and steric effects are operative in stabilizing CNTs.12−14 Using single-walled carbon nanotubes (SWNTs) and anionic surfactants, Duque et al.15 observed that smaller diameter tubes dispersed at higher concentrations than larger diameter tubes. Other studies have concluded that the most important parameter for CNT dispersion is the CNT-surfactant ratio,14 and that for nonionic surfactants, higher molecular weight species are more effective at dispersing CNTs.16 Most CNTsurfactant studies have used high concentrations of surfactants, Received: Revised: Accepted: Published: 4458
December March 19, March 23, March 23,
22, 2011 2012 2012 2012
dx.doi.org/10.1021/es204618v | Environ. Sci. Technol. 2012, 46, 4458−4465
Environmental Science & Technology
Article
Table 1. Key Properties of SWNT Suspensionsa SDS conc. (% w/v)
pH
0.001 0.01 0.05 0.10
7.10 7.33 7.40 7.51
mass conc. (mg L−1) 10.6 11.3 16.4 27.9
± ± ± ±
0.4 1.2 0.9 1.2
Dh (nm) 197 197 170 164
± ± ± ±
4 2 6 2
EPM (μm cm V−1 s−1)
PDI 0.254 0.245 0.216 0.192
± ± ± ±
0.015 0.016 0.044 0.025
−3.49 −4.10 −3.96 −3.83
± ± ± ±
0.33 0.49 0.73 0.50
ζ-potential (mV) −44.5 −52.4 −50.5 −48.9
± ± ± ±
4.3 6.2 9.3 6.4
Dh is the hydrodynamic diameter, PDI is a polydispersity index, and EPM is the electrophoretic mobility. ζ-potential values were calculated from the EPM values using the Smoluchowski equation. Means and 95% confidence limits are from measurements made at initial suspension creation and at 4 and 17 days later. a
then sonicated with a probe sonicator (Sonics & Materials, Newton, CT, U.S.) for 10 min at an energy level of ∼30 W. The SWNT suspensions were then centrifuged at 40 000 RCF for 4 h (Sorvall RC 5C plus, Thermo-Fisher Waltham, MA, U.S.) to remove impurities and minimize the presence of bundled SWNTs. The supernatants were then carefully decanted off to form the working stock suspensions which were stored at room temperature for the duration of the experiments. Aliquots of the stock suspensions were adjusted to specific electrolyte concentrations with NaCl and CaCl2. All treatments were sonicated at low power (Branson 1210, Branson Ultrasonics, Danbury, CT, U.S.) for 5 min prior to size, mass, and ζ-potential measurements, and prior to introduction into porous media columns for transport studies. SWNT size in aqueous suspension was measured using dynamic light scattering (DLS) with a ZetaSizer Nano ZS (Malvern Instruments, Worcestershire, U.K.). The intensity averaged (Z-average) hydrodynamic diameter (Dh) was calculated from measured diffusivities using the Stokes− Einstein equation. ζ-potentials of the suspensions were also measured using a ZetaSizer Nano ZS instrument which employs phase analysis light scattering (PALS) to measure the electrophoretic mobility of charged particles. The Smoluchowski equation was used to calculate ζ-potential from electrophoretic mobility. Instrument performance was verified using NIST-traceable latex microsphere and polystyrene microsphere standards. Suspended SWNT mass was determined using visible light absorbance at 633 nm (Enspire Multimode Plate Reader, Perkin-Elmer, Inc., Waltham MA, U.S.). Since particle size distribution can affect absorbance, separate calibration curves were developed for SDS-SWNT suspensions that deviated more than 10% from standards. Atomic force microscopy (AFM) materials and methods are provided in Supporting Information (SI1). SWNT Initial Aggregation Kinetics and Long-Term Stability Studies. Time-resolved dynamic light scattering (TRDLS) was used to measure the change of SWNT intensityaveraged hydrodynamic diameter (Dh) with time and as a function of background solution SDS concentrations and ionic strength. In these experiments, 500 μL of SDS-SWNT suspension was pipetted into a quartz cuvette and an additional 500-μL aliquot of NaCl or CaCl2 stock solution added to yield a specific electrolyte and SDS concentration. The cuvette was then immediately vortexed for 1 s and placed in the DLS instrument. The Dh measurements were conducted over time periods ranging from 30 to 60 min. The initial aggregation period was defined as the time period from experiment initiation (t0) to the time when measured Dh values exceeded 1.50Dh,initial. The initial aggregation rate constants (ka) for the SWNTs are proportional to the initial rate of increase of Dh with time:31,32
above their critical micelle concentrations (CMC), but studies that have used lower surfactant concentrations have demonstrated that CNTs are readily dispersed by surfactants below their CMC.13,17 The effects of lower surfactant concentrations on CNT dispersibility are particularly important for environmental studies as dilution of the surfactant will occur as soon as a surfactant-stabilized CNT suspension is released into surface or ground waters. Furthermore, quantifying CNT stability and transport under environmentally relevant solution conditions is requisite for understanding the distribution of CNTs in environmental media and for estimating environmental exposures. Reports on the aggregation and transport of functionalized CNTs,18−21 CNTs suspended in water via exhaustive sonication,22,23 and surfactant-wrapped CNTs,24−26 have indicated that these materials form stable aqueous suspensions, but that CNT transport in porous media is dependent on the stabilization mechanism.27 It appears, for example, that at surfactant concentrations above the CMC surfactant-wrapped CNTs are more mobile in porous media25,26,28 than functionalized CNTs.18,21 However, there is little information available on the relationship between surfactant concentration and CNT stability and mobility in environmental systems. The need for data of this type is particularly acute given that surfactant concentrations will vary in any surfactant-wrapped CNT environmental release scenario, and that physical-chemistry computational studies have indicated that stable CNT suspensions can be formed below the surfactant CMC.29,30 The objectives of this work were to quantify the effects of a common SWNT dispersant, sodium dodecyl sulfate (SDS), at concentrations below the CMC on SWNT suspended mass concentration, particle size, and ζ-potential; to characterize the initial aggregation kinetics and longer term stability of SDSwrapped SWNTs; and to examine the transport of these materials in a freshwater sediment. This investigation is the first detailed quantitative investigation of the effects of surfactant concentration on these key processes that determine the environmental fate of SWNTs in natural porous media and aqueous systems.
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MATERIALS AND METHODS Preparation and Characterization of SDS-SWNT Suspensions. The SWNTs were purchased from Cheap Tubes Inc. (Brattleboro, VT, USA) with reported 90% wt. purity and an average outside diameter of 1.1 nm. The sodium dodecyl sulfate (SDS, purity >99%) was purchased from Thermo-Fisher (Waltham, MA, U.S.). SDS stock solutions were prepared using double deionized (DDI) water (resistivity >18 MΩ-cm) and SDS to create 0.10, 0.05, 0.01, and 0.001% (w/v) solutions. SWNT were added to the SDS solutions to yield an initial concentration of 100 mg/L, and the material was 4459
dx.doi.org/10.1021/es204618v | Environ. Sci. Technol. 2012, 46, 4458−4465
Environmental Science & Technology
ka ∝
1 ⎛ d D h (t ) ⎞ ⎜ ⎟ N0 ⎝ dt ⎠t → 0
Article
breakthrough experiments were conducted in a downward flow mode. The background influents were initially flowed through the columns at a Darcy velocity (U) of 2.8 cm/min and then at a lower velocity (i.e., U = 0.18 cm/min and average pore water velocity [v] = 0.41 cm/min) that was used in the column experiments. The experimental flow velocity is within the ranges reported in prior studies.18,20,21 At least 72 pore volumes of background influents were flushed through the media until the absorbance of the background effluents were stable and minimal, whereupon the inflow was switched to the second syringe pump containing the SWNT input suspensions or 3 H2O tracer using a two-position Rheodyne valve (IDEX Health & Science, Oak Harbor, WA). The input injection was continued for 20 min (i.e., about 1.1 pore volumes), followed by flushing with the background influents for 60 min. The effluent samples were collected every 4 min by a fraction collector (RETRIEVER 500, TELEDYNE ISCO, Lincoln, NE) and analyzed by a plate reader at 633 nm for the SWNT experiments, and by a liquid scintillation analyzer (PerkinElmer, Waltham, MA) for the 3H2O tracer experiment. Two to five replicates were conducted for each SWNT experiment and one replicate for the tracer experiment. The breakthrough curves (BTCs) were plotted as the normalized effluent concentrations (C/C0) versus pore volumes. Effluent mass recovery (MBTC) was determined by integrating the BTCs using the trapezoidal rule and then dividing the recovered mass by the input mass. The SWNT mass loss to the experimental apparatus was less than 2% as estimated by conducting a media-free column experiment for the SWNT suspension at 0.001% SDS concentration. The transport of colloids, including nanoparticles, under a steady-state flow condition may be described by a convectiondispersion equation with a first-order kinetic deposition term.34,35
(1)
where N0 is the initial particle concentration. The particle attachment efficiency α (or inverse stability ratio, 1/W) is used to quantify particle aggregation kinetics; it is defined as the initial aggregation rate constant (ka) normalized by the aggregation rate constant measured under diffusion-limited (fast) conditions time:31,32 k 1 α= = a = W ka,fast
1 N0
D h (t ) dt
N0,fast
D h (t ) dt
1
( ) ( )
t→0
t → 0,fast
(2)
Using the sample preparation technique described above, the concentration of each SDS-SWNT suspension (half that reported in Table 1 due to 1:1 dilution) remained constant for each measurement made at varying electrolyte concentrations. This simplifies eq 2 (i.e., N0 drops out) so that α can be determined directly by normalizing the initial slope of the aggregation profile for a specific background solution chemistry by the initial slope under diffusion-limited (fast) conditions. In this study, a minimum of nine data points were used to determine initial slopes. To determine the long-term stability of the SDS-SWNTs in representative environmental systems, the suspended mass concentrations of SWNTs in 0.01 and 0.001% (w/v) SDS and in 10 mM NaCl, 1.0 mM CaCl2 and in Calls Creek water, a small stream near Athens GA, U.S., were measured over a 6week period. Calls Creek water parameters were pH 7.9, conductivity 26 μS/cm, total dissolved solids 27 ± 5.2 mg/L, and total organic carbon (TOC) 2.8 ± 0.57 mg/L) after filtration through a 0.45 μm filter. Column Transport Studies. A fresh water sediment collected from Calls creek was used as a porous medium in the transport studies. Prior to use the sediment was sieved to the size fraction of 250−500 μm, thoroughly rinsed with DDI water, oven-dried at 42 °C, and stored in a closed container. The sediment mainly contained quartz, small amounts of biotite, albite, orthoclase, and a very minor abundance of hematite, as identified by X-ray diffraction, as well as amorphous Al hydroxides (i.e., 146 mg/kg hydroxylamine extractable Al). The sediment also had 0.057% (by weight) organic carbon, 10 meq/kg cation exchange capacity, and 0.789 m2/g BET surface area (roughness factor 126). Detailed description of the sediment properties was provided elsewhere.33 The sediment was dry-packed into Omnifit glass columns of 1.5-cm inner diameter and 7.5-cm length (L) (Diba Industries, Danbury, CT) to a porosity of 0.45. Two 125-μm brass mesh screens were placed at both ends of the columns to support the sediment and disperse the flow, and then the columns were sealed by two end pieces with O-rings. The SDS-wrapped SWNT stock suspensions at 0.001%, 0.01%, 0.05%, and 0.1% SDS were adjusted with concentrated NaCl solutions to achieve a total ionic strength of 10 mM (Na+) and sonicated prior to injecting into the columns. The SDS solutions free of SWNTs were used as background influents. The packed media were first saturated by flowing deaerated background influents upward through the columns with a syringe pump (Model 500D, TELEDYNE ISCO, Lincoln, NE), and then the columns were turned upside down with the tubing attached so that the
∂C ∂ 2C ∂C =D 2 −v − kdC ∂t ∂z ∂z
(3)
where C (mg/L) is the liquid phase particle concentrations, t (min) is the elapsed time, D (cm2/min) is the hydrodynamic dispersion coefficient, z (cm) is the travel distance, v (cm/min) is the pore water velocity, and kd is the deposition rate coefficient (min−1). In a convection-dominant flow regime with a high Peclet number (Pe = vL/D > 50, where L is the column length), the dispersion term in eq 3 is ignored, and kd is estimated as follows:34,36,37 v kd = − ln(MBTC) (4) L This approach was justified as the fitting of the tracer BTC to the convection-dispersion equation (CDE) gave a dispersion coefficient of 0.0176 cm2/min and subsequently a Peclet number of 173. This kd term includes all of the retention processes occurring in the media under a clean-bed condition, and is used along with the BTCs to distinguish the transport behavior of SDS-wrapped SWNTs at varying SDS concentrations.
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RESULTS AND DISCUSSION Dispersal of SWCNT in SDS. Table 1 contains key physical and chemical parameters characterizing the SDS-SWNT suspensions used in this study. The prepared SDS-SWNT stock suspensions were very stable with respect to mass
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suspended, Dh, and ζ-potential over the course of the experimental period for each suspension. Measurements made on a representative stock suspension over a 17-day period indicated that mass suspended, Dh, and ζ-potential varied by
