Article pubs.acs.org/Langmuir
Strongly Bound Sodium Dodecyl Sulfate Surrounding Single-Wall Carbon Nanotubes Jia Xu,† Robert Mueller,‡ Eric Hazelbaker,‡ Yang Zhao,‡ Jean-Claude J. Bonzongo,§ Justin G. Clar,∥ Sergey Vasenkov,‡ and Kirk J. Ziegler*,†,‡ †
Department of Materials Science and Engineering, ‡Department of Chemical Engineering, and §Engineering School of Sustainable Infrastructure and Environment, Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida 32611, United States ∥ Department of Chemistry, Elon University, Elon, North Carolina 27244, United States S Supporting Information *
ABSTRACT: NMR techniques have been widely used to infer molecular structure, including surfactant aggregation. A combination of optical spectroscopy, proton NMR spectroscopy, and pulsed field gradient NMR (PFG NMR) is used to study the adsorption number for sodium dodecyl sulfate (SDS) with single-wall carbon nanotubes (SWCNTs). Distinct transitions in the NMR chemical shift of SDS are observed in the presence of SWCNTs. These transitions demonstrate that micelle formation is delayed by SWCNTs due to the adsorption of SDS on the nanotube surface. Once the nanotube surface is saturated, the free SDS concentration increases until micelle formation is observed. Therefore, the adsorption number of SDS on SWCNTs can be determined by the changes to the apparent critical micelle concentration (CMC). PFG NMR found that SDS remains strongly bound onto the nanotube. Quantitative analysis of the diffusivity of SDS allowed calculation of the adsorption number of strongly bound SDS on SWCNTs. The adsorption numbers from these techniques give the same values within experimental error, indicating that a significant fraction of the SDS interacting with nanotubes remains strongly bound for as long as 0.5 s, which is the maximum diffusion time used in the PFG NMR measurements.
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INTRODUCTION SWCNTs have attracted tremendous attention over the past two decades due to their special electronic, thermal, optical, and mechanical properties.1−4 However, the aggregation or bundling of SWCNTs may impede their use in many applications.5−10 To overcome this obstacle, surfactants and other polymers are often employed to suspend SWCNTs in aqueous solutions11 for applications in biomedical,5,6 electronic,7,8 and environmental fields. 9,10 It is becoming increasingly clear that modulating the surface properties of surfactant coatings on SWCNTs has a significant impact on their functionality.12−18 For example, the separation of SWCNTs appears to be driven by differences in the surfactant structure surrounding the nanotube species in both the selective adsorption14−16 and the aqueous two-phase extraction methods.17,18 The structure of the surfactant or other molecules around the SWCNTs could also have important implications in toxicology19 and drug delivery.20 Therefore, it is important to develop characterization strategies to understand the formation and morphology of surfactant structures around SWCNTs, especially differences between specific (n, m) types. Several approaches have been used to indirectly probe the surfactant structure in SWCNT suspensions. Since all carbon atoms are distributed on the surface of SWCNTs, the variation of optical signatures in photoluminescence (PL) and Raman spectroscopy indicate changes to the environment around the © 2017 American Chemical Society
nanotube, indirectly providing information about how the nanotube interacts with other materials. Therefore, PL can help identify SWCNT aggregation state,3 surfactant coverage through quenching effects,21,22 and the interaction of specific molecules through solvatochromic effects.23−27 An alternative approach, small-angle neutron scattering (SANS), can be used to probe the dimensions of nanoscale structures in a suspension. This information can be used to understand the aggregation behavior of surfactants and has been applied to SWCNTs suspended in SDS,28 Pluronic,29,30 and Triton X100.31 However, this technique requires structural models to fit the data so that dimensional and aggregation behavior can be determined. In many cases, the signal from the surfactant around the nanotubes must be separated from the signal of free surfactant or micelles, complicating analysis. While direct imaging techniques, such as cryo-TEM, provide visual pictures of the surrounding environment of SWCNTs,11 the freezing and unfreezing (due to beam heating during imaging) can alter the structure from its original state. While each of these methods can provide some insight into surfactant structure, additional methods are needed to directly probe the structure or interactions of the surfactant. Received: March 6, 2017 Revised: May 3, 2017 Published: May 5, 2017 5006
DOI: 10.1021/acs.langmuir.7b00758 Langmuir 2017, 33, 5006−5014
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lyzer (Houston, TX) with excitation from 662 and 784 nm diode lasers. The final concentration of the SWCNT suspension after ultracentrifugation was calculated by Beer’s law using an extinction coefficient of 0.043 L mg −1 cm −1 at 763 nm.47 Typically, concentrations of SDS−SWCNT suspensions were in the range of 0−40 mg/L. To convert SWCNT concentrations to mM, the average length of SWCNTs was assumed to be 500 nm with an average molecular weight of 1 080 000 g/mol. CMC of Aqueous SDS Solutions by Conductivity. A benchtop conductivity meter (CON 510) was used to determine the CMC for SDS in both H2O and D2O. Initial SDS concentrations of 17.5 mM in water and 27 mM in D2O were sequentially diluted while measuring the conductivity, as shown in Figure S1. The CMCs of SDS in H2O and D2O were similar and matched known literature values, as shown in Table S1. Proton Nuclear Magnetic Resonance (NMR) Measurements. SDS solutions and SWCNT suspensions in D2O were prepared at the desired SDS concentration. In a typical experiment, additional SDS powder was added to the as-prepared SWCNT suspension with an initial SDS concentration of 34 mM to achieve higher SDS concentrations up to 340 mM. This approach was needed to simultaneously control the amount of SWCNTs and surfactant during dilution, enabling the ratio of surfactant:SWCNT (R) to be adjusted in the range of 106−107 (mM/mM). The SWCNT solutions were then diluted incrementally with D2O and mixed using a vortex stirrer (2000 rpm) for 30 s. After mixing, 0.6 mL of solution was placed into a 1 cm diameter NMR tube. Proton NMR spectra were recorded using a Varian Mercury 300 MHz spectrometer at 298 K. The spectra have been referenced to the HDO peak (around 4.8 ppm).48 A full spectrum of the chemical shift data for SDS is shown in Figure S2. To ensure that the dynamics associated with the dilution process did not affect the structure being probed around the SWCNTs, control experiments were conducted by preparing suspensions at specific concentrations and comparing the chemical shifts to suspensions at the same concentrations that were prepared by either concentration or dilution. As shown in Figure S3, the chemical shifts measured for the two approaches were within the experimental error at multiple concentrations of SDS and SWCNTs. Determining the CMC from NMR Data and Error Analysis. A detailed description of the method to determine the CMC and quantify the errors associated with its measurement is presented in the Supporting Information. Briefly, the chemical shift data are first plotted as a function of the reciprocal concentration of SDS. Three distinct regions are observed with two transitions points. The first transition is known to be associated with the CMC. Linear expressions are used to characterize the chemical shift data above and below the CMC. The CMC and variables associated with the linear expressions are regressed from the data, as shown in Figure S4. For each SDS concentration during dilution, the measurement errors are propagated to determine the uncertainty of that concentration. Therefore, subsequent dilution steps result in the propagation of more error, leading to higher uncertainty in 1/c values at low SDS concentrations. Assessing the Impact of Metal Impurity and SWCNTs on the Chemical Shift of SDS. The presence of SWCNTs and magnetic particles, such as the catalyst used to synthesize SWCNTs,49 could influence the measured chemical shift of SDS. As shown in Figure S5a, the presence of a high concentration of iron ions leads to significant changes to the chemical shift of the α-proton in SDS. However, the chemical shift does not deviate significantly from those of pure SDS when the Fe concentration is below ∼6 mg/L. In a similar manner, the addition of iron ions to the SWCNT suspensions did not significantly impact the measurements. Under these conditions, the well-known NMR line broadening effect due to the presence of ferromagnetic species does not interfere with the chemical shift determination. Collectively, the data suggest that the iron content in the SDS− SWCNT system should be kept below ∼6 mg/L in order to investigate the structure of SDS by proton NMR. The concentration of iron impurities remaining in the final SDS− SWCNT suspensions was determined by ICP-OES (PerkinElmer Plasma 3200RL), where the SWCNT suspensions were diluted with
Many surfactants, such as SDS, consist of a hydrophilic headgroup and a hydrophobic tail. In aqueous systems, the hydrophobic tails aggregate into ordered molecular structures (micelles) once the surfactant reaches the critical micelle concentration (CMC). A further increase in surfactant concentration results in the morphological change from spherical to ellipsoidal micelles and eventually to rod-like micelles.32,33 These morphological changes in surfactant-only systems can be observed by measuring changes in the physical and chemical properties of the surfactant solution at these transitions, including surface tension, conductivity, and NMR chemical shifts.33−35 Among these methods, proton NMR is considered to be advantageous over other techniques because it detects the microenvironmental change around every single proton type in the surfactant.36−40 For this reason, NMR can probe specific proton types in a molecule to determine their role in various processes.41 NMR can also simultaneously detect different types of surfactant because of their unique NMR spectra.42 Therefore, NMR has the potential to follow the interactions among single or mixed surfactant solutions.43,44 Pulsed field gradient (PFG) NMR enables this approach to be extended to molecular diffusion measurements. For example, PFG NMR was used to determine the diffusivity of the free monomer and micelles of SDS as well as micelle stabilization by the addition of C12TAB.36 There are only a few examples of the use of NMR to measure surfactant structure or interactions around SWCNTs. Proton NMR was used to identify the structure and dynamics of surfactant on functionalized SWCNTs45 while diffusion NMR measurements probed SWCNT−surfactant interactions for mixtures of SDS and sodium cholate surfactants.44 Herein, a combination of optical and NMR techniques is used to investigate the structure and dynamics of the anionic surfactant SDS as it interacts with SWCNTs in aqueous solutions near the CMC of pure SDS. PL and absorbance spectroscopy provide direct information about what the nanotube is experiencing while NMR yields direct information about the environment surrounding the surfactant.
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EXPERIMENTAL SECTION
Preparation of Aqueous SWCNT Suspensions. HiPco SWCNTs were obtained from Rice University (HPR 164.1), and all other chemicals were purchased from Sigma-Aldrich (St. Louis, MO) and used as received unless otherwise stated. SWCNTs were suspended in D2O by SDS according to previously reported methods.46 In a typical procedure, SWCNT suspensions were prepared by mixing approximately 15 mg of raw SWCNTs with 60 mL of 34 mM SDS−D2O solution using high-shear homogenization (IKA T-25 Ultra-Turrax) at 8000 rpm for 1 h and then tip ultrasonication (Misonix S3000) for 10 min with 50% amplitude. SWCNT bundles as well as a significant amount of the metal catalyst were removed by centrifugation (Beckman SW-28 rotor) at 20 000 rpm (75 536g) for 4 h. During homogenization and sonication, the solution was covered to limit D2O exchange with atmospheric H2O. To verify that the concentration of surfactant was not altered during the preparation of the SWCNT suspension, atomic absorbance measurements for sodium were conducted on a PerkinElmer Analyst 300. Pure SDS and SDS−SWCNT suspensions (both at 34 mM SDS) were digested with nitric acid and placed in a water bath held at 60 °C for 24 h. The samples and calibration standards were diluted to 5% nitric acid content for analysis. The deviation in the final amount of SDS in the SWCNT suspension was found to be less than ±2.5%. Optical Characterization. The SWCNT suspensions were characterized by absorption (0.4 cm path) and fluorescence (1 cm path) spectroscopy on an Applied NanoFluorescence Nanospectra5007
DOI: 10.1021/acs.langmuir.7b00758 Langmuir 2017, 33, 5006−5014
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Langmuir D2O in an acid background of 5% HCl and 3% HNO3 by volume. Figure S5b shows that SDS−SWCNT suspensions have adequate removal of iron catalyst below the 6 mg/L threshold after 1 h of ultracentrifugation. All suspensions used in this study were centrifuged for 4 h, resulting in iron concentrations of ∼2 mg/L (about 1/3 of the limit). A high concentration of SWCNTs may also cause broadening of the proton NMR lines due to magnetic susceptibility effects, obscuring changes in the chemical shift of the α-proton. As shown in Figure S5c, the peaks in the NMR spectra of SDS−SWCNT suspensions were broader than in pure SDS. A detailed description of the SWCNT broadening effect in NMR is discussed in the Supporting Information. As shown in Figure S6a, the broadening effect of the NMR spectra in SDS decreases when the concentration of SWCNTs is diluted. In this study, the CMC was determined only from NMR spectra that have minimal peak broadening, such as those shown in Figure S6b−d. Changes to the chemical shift as a function of SWCNT concentration are plotted for several concentrations of SDS in Figure S7. A comparison of these chemical shifts to those of pure SDS at the same concentration is shown in Figure S8. The data indicate that there are only statistical differences between these two when the concentration of SDS is near the CMC. Pulsed Field Gradient (PFG) NMR Measurements. Proton PFG NMR measurements are performed using an Avance III HD spectrometer (Bruker Bio-Spin) at a magnetic field strength of 17.6 T. Sine-shaped magnetic field gradients with the effective duration between 0.3 and 0.5 ms and amplitude up to 13 T/m were generated using a diff60 diffusion probe (Bruker BioSpin) and Great60 gradient amplifier (Bruker BioSpin). A 20 mM SDS−SWCNT suspension containing 24 mg/L of SWCNTs in D2O was diluted with D2O to obtain a 10 mM SDS−SWCNT suspension with 12 mg/L of SWCNTs. For comparison, 10 and 20 mM solutions of SDS in D2O were also prepared. All the liquid samples were put in 5 mm NMR tubes. At the time of measurement, the gradient chiller temperature and the sample air temperature were set to room temperature to minimize potential convection effects. PFG NMR measurements were conducted at diffusion times of 9.8, 120, and 500 ms using the stimulated echo with the longitudinal eddy current delay (STELED) pulse sequence.50 The spectrum of each sample was recorded using the same acquisition parameters. Self-diffusivities were obtained by measuring PFG NMR signal attenuation with increasing magnetic field gradient amplitude, g, and keeping all other pulse sequence parameters constant. For normal diffusion with a single diffusion coefficient (D), PFG NMR attenuation curves can be described using
Ψ = exp[− (γgδ)2 tD]
Figure 1. (a) Chemical structure and (b) corresponding NMR spectrum of sodium dodecyl sulfate (SDS). Specific protons in the chemical structure are identified in the spectrum.
influenced by its surrounding due to the deshielding effect of oxygen atoms, which leads to larger changes in the chemical shift with increasing SDS concentration.51 Therefore, the variation in the chemical shift of the α-proton in SDS is used to identify the structural changes associated with the interaction of SDS with SWCNTs. Structural Configuration of SDS in Aqueous Solution. Figure 2a shows distinct transitions in the chemical shift of the α-proton at specific SDS concentrations. These three distinct regions are similar to those observed in prior NMR studies of SDS aggregation.33,34 At low SDS concentrations (first region), the chemical shift is at a constant value since the monomers do not have significant interaction with one another. As the SDS concentration increases, an obvious transition appears in the chemical shift data, indicating the self-assembly of SDS into spherical micelles. This transition is known as the first critical micelle concentration (CMC1) of SDS.52−55 Around this transition point, the solution contains a large fraction of SDS monomer and a small fraction of SDS micelles. In the second region, the change in the chemical shift after the formation of aggregate structures can be attributed to the new local microenvironment of SDS. The chemical shift continues to decrease as the fraction of micelles in the solution increases. Further increases to the SDS concentration lead to a change in aggregate structure from spherical to ellipsoidal micelles. This structural change is evident by the steady increase in the chemical shift, which corresponds to CMC2. NMR has been shown to accurately identify the CMC of several surfactants,35,38,39,41 including SDS.33,34 A comparison of the CMCs of SDS obtained by various techniques is shown in Table S1. Structural Configuration of SDS around SWCNTs in Aqueous Solution. Changes to the chemical shift are also observed in SDS−SWCNT suspensions (the full NMR spectrum is shown in Figure S2). Although it would be ideal to obtain direct information about the structure surrounding the SWCNTs, care should be exercised since the NMR signal represents the signal coming from all SDS in the solution. Because the nanotube suspensions are in the dilute regime, a
(1)
where Ψ is the NMR signal attenuation, γ is the gyromagnetic ratio, g is the gradient amplitude, δ is the effective gradient pulse length, and t is the diffusion time. It was verified that the 1H PFG NMR attenuation curves measured for each line of the proton NMR spectrum of SDS shown in Figure 1 are the same within experimental uncertainty. It was also verified that changing the time interval between the first and second π/2 radio-frequency pulses of the STELED sequence between 1.25 and 1.82 ms while keeping the diffusion time constant did not change the measured diffusion coefficient. These results indicate that there are no measurement artifacts in the reported PFG NMR data. The attenuation curves shown in this work correspond to the signal equal to the area under the line of the tail group protons, which is the strongest line in the SDS spectrum (Figure 1).
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RESULTS AND DISCUSSION Figure 1 presents the proton NMR spectrum for SDS at a concentration of 34 mM and its corresponding molecular structure. The chemical shifts of the β-proton, tail group, and terminal group protons in SDS are close to each other and insensitive to changes in SDS concentration in pure solutions.33 On the other hand, the chemical shift of the α-proton is easily 5008
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suspension was stable even when it was diluted beyond CMC1, suggesting that a large amount of surfactant remains adsorbed onto the surface of the nanotube. The normalized absorbance and PL shown in Figures 3a and 3b, respectively, show that the spectra have not changed significantly while diluting the suspension to SDS concentrations lower than the CMC of pure SDS. The lack of peak broadening or red-shifting of the
Figure 2. NMR chemical shifts of the α-proton in (a) pure SDS and (b) SDS−SWCNT suspensions as a function of the reciprocal concentration of SDS. Distinct transitions are observed that are associated with changes to the aggregation state of SDS. The dark lines are the fitted expressions described in the Supporting Information.
majority of the SDS might not be associated with the nanotubes. Two regimes can be discussed with respect to the one-pulse NMR measurement of SDS signal in the nanotube suspensions: (i) fast exchange and (ii) slow or no exchange between the SDS associated with the nanotubes and the remaining SDS in the suspension. In the former regime, the chemical shift of the SDS line(s) sensitive to the association with the nanotubes is expected to represent the weighted average of the contributions from the SDS associated and not associated with the nanotubes. In contrast, the recorded SDS chemical shifts in the latter regime can be equal to those not associated with the nanotubes because of a significant line broadening expected for the SDS associated with the nanotubes. This line broadening will render the NMR lines of the nanotube-associated SDS to be invisible in the solution NMR measurements reported here. As Figure 2b illustrates, the general shape of the chemical shift dependence as a function of SDS concentration is the same as that in the pure SDS solution. Two distinct transitions are still observed in the SWCNT suspension. The CMC2 transition in the presence of SWCNTs occurs at identical concentrations as pure SDS. The similarity of these transitions is most likely due to the very low fraction of SDS associated with SWCNTs at these conditions. Therefore, this transition can only be attributed to the change in shape from spherical to ellipsoidal micelles in the bulk solution. Although there is no difference in the chemical shift data of Figure 2b at high 1/c values below CMC1 (see Figures S7a and S8 as well), a prior small-angle neutron scattering (SANS) study showed that surfactant is randomly adsorbed on the surface even at these concentrations.28 Indeed, the SWCNT
Figure 3. Dispersion quality of SDS−SWCNT suspension. Normalized (a) absorbance and (b) PL spectra (excited at 662 nm) measured for SWCNT suspensions in 6 and 34 mM of SDS. Absorbance and PL spectra were normalized by concentration and intensity at 1123 nm, respectively. (c) Effect of dilution on the ratio of PL (measured at 1123 nm) to absorbance (measured at 662 nm), which is a measure of dispersion quality. The initial SWCNT suspension is in 34 mM of SDS. The dashed line is the average value of PL/Abs. The vertical line indicates the CMC1 for pure SDS. (d) Role of surfactant structure on the quenching behavior of the solvent. If there is too little surfactant coating the SWCNTs (left), then the solvent interacts directly with the exciton, quenching the PL. However, a tightly bound conformal coating (right) provides a barrier to the interaction of the solvent with the nanotube surface. 5009
DOI: 10.1021/acs.langmuir.7b00758 Langmuir 2017, 33, 5006−5014
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Langmuir spectra indicates that the SWCNTs are well suspended under CMC1. Figure 3c shows the ratio of the SWCNT PL to absorbance as the suspension is diluted. This ratio was previously shown to be a measure of dispersion quality.3,11 If the nanotube surface is exposed or bundling occurs, there are significant changes to the PL/Abs ratio. However, the ratio in Figure 3c remains constant throughout the dilution process. As Figure 3d illustrates, a significant layer of surfactant must remain adsorbed onto the surface of the SWCNT to provide a barrier to the quenching effect of the bulk phase. It is anticipated that the surfactant on the nanotubes is exchanging with free monomer in solution. Figure 4a shows the possible states where SDS can exist in the SWCNT suspension. The free monomer in solution can aggregate into micelle structures in the bulk solution or they can adsorb onto the surface of the SWCNT. The lack of chemical shift changes at high 1/c in Figure 2b could indicate that slow or no exchange is occurring on the time scale of one-pulse NMR experiment (hundreds of microseconds). In other words, the measured NMR signal is solely associated with free SDS in solution. PFG NMR can provide dynamic information about the exchange between various states in surfactant systems.34,36 The PFG NMR diffusion data shown in Figure 4b indicate that the presence of SWCNTs in SDS solutions at concentrations near CMC1 (10 mM) have little effect on the measured diffusivity of (3.74 ± 0.03) × 10−10 m2/s at 298 K, which was obtained from least-squares fitting of the attenuation curves using eq 1. This diffusivity was found to be independent of diffusion time in the range between 10 and 500 ms. SDS diffusivity in nanotube-free SDS solutions can be presented as a weighted sum of the diffusivities of free monomeric SDS (f) and SDS micelles (m)34
D = p f D f + pm D m
(2)
where p and D are the fraction and diffusivities, respectively. Equation 2 also applies to the case where slow or no exchange of SDS with nanotubes occurs; i.e., only free monomeric SDS and micelles would contribute to the measured PFG NMR signal. Therefore, the sum of free monomeric SDS and SDS micelle fractions in eq 2 is 1 (pf + pm = 1) for both pure SDS solutions and nanotube suspensions. However, the values of pf and pm depend on the presence of SDS associated with the nanotubes. For any given total concentration of SDS between CMC1 and CMC2, the number of SDS ions forming micelles is decreased in the SWCNT suspension in comparison to the pure SDS solution because of the presence of nanotubes that shift the equilibrium distribution (see Figure 4a). Since the diffusivity of SDS micelles, Dm = (6.1 ± 0.9) × 10−11 m2/s, and free monomers, Df = (4.7 ± 0.08) × 10−10 m2/s, are wellknown,56 the fraction of free SDS can be calculated at low SDS concentration (10 mM) using eq 2.36 This fraction was found to be equal to 0.77 and 0.76 ± 0.03 with and without SWCNTs, respectively. As Figure 4c illustrates, the distribution between free monomer and micelles was expected to be ∼0.7, which is very close to both calculated values from PFG NMR. Therefore, the observed lack of the diffusivity difference in Figure 4b due to the presence of SWCNTs at low SDS concentration (10 mM) is associated with the low concentration of SWCNTs (12 mg/L) and low concentration of micelles, resulting in small amounts of SDS associated with nanotubes and, consequently, small changes in the distribution of free monomer and micelles. At higher SDS concentrations, the fraction of free SDS decreases from ∼0.7 to ∼0.3 when the concentration of SDS
Figure 4. (a) SDS can exist as either free monomer, micelles, or adsorbed thermodynamic equilibrium states. (b) PFG NMR attenuation curves at different diffusion times for SDS in pure aqueous solutions (red) and SWCNT suspensions (blue). The SWCNT concentrations in SDS were 12 and 24 mg/L in 10 and 20 mM SDS, respectively. Error bars for these measurements are about ±0.0001, which are smaller than the data points plotted. (c) Change in the concentration of free monomer and micelles as a function of the total concentration of SDS based on literature data.56 Note that the fractions of free monomer at 10 and 20 mM SDS are ∼0.7 and ∼0.3, respectively.
increased from 10 to 20 mM (see Figure 4c). Therefore, the measured diffusivity at 20 mM for pure SDS solutions is expected to be lower due to the contribution from a larger fraction of slowly diffusing species, as observed by the flatter slope in Figure 4b. However, the addition of SWCNTs (24 mg/ L) at the higher SDS concentration (20 mM) leads to an increase in the SDS diffusivity in the range of a few percent, i.e., from (1.95 ± 0.03) × 10−10 to (2.05 ± 0.03) × 10−10 m2/s at 298 K. The diffusivities were obtained from least-squares fitting 5010
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are only measuring the delayed transition of SDS to micelles (apparent CMC), the saturation of surfactant on the nanotube is related to the shift in the CMC through the overall mass balance. This phenomenon was further studied by examining changes to the apparent CMC at different ratios of surfactant:SWCNT (R). Figure 5a shows that all data points
of the attenuation curves in Figure 4b using eq 1. The diffusivities of both did not change when the diffusion time was changed between 10 and 500 ms. The diffusivity change due to the nanotube addition at the higher SDS concentration (20 mM) is demonstrated by the higher slope of the attenuation curves in Figure 4b. The observed small diffusivity change can be detected with a high precision owing to the possibility to measure the attenuation curves for SDS in the range of more than 3 orders of magnitude. The applied PFG NMR technique has the unique capability to achieve this attenuation range because of large gradients. If fast exchange of SDS between the nanotube surface and the free monomer were occurring, the measured SDS diffusivity would be anticipated to be slower than in the nanotube-free solution since SDS monomers would be expected to sample the nanotube surface during the time scale of the diffusion measurements. Therefore, an SDS diffusivity increase can be explained by the existence of strong binding between SDS and the nanotube surface (case ii discussed above). This binding reduces the total number of SDS available for micelle formation, shifting the equilibrium distribution shown in Figure 4a to fewer micelles. Hence, the fraction of the PFG NMRvisible SDS that aggregate into micelles (pm) decreases, and the corresponding fraction of the PFG NMR-visible free SDS (pf = 1 − pm) increases as a result of the nanotube addition. Their relation to SDS adsorbed onto the surface of the nanotube is attained by correcting the concentration of SDS through the overall mass balance. This shift in the distribution of SDS states explains the observed increase of the SDS diffusivity (D), which is measured by PFG NMR under the conditions of slow or no exchange of the nanotube-bound SDS and fast exchange between monomeric and aggregated (into micelles) SDS, i.e., conditions used to develop eq 2. Therefore, an increase in the fraction of the PFG NMR-visible monomeric SDS is associated with the fraction of strongly adsorbed SDS on SWCNTs. At the same time, there is a corresponding decrease in the fraction of SDS micelles. The distribution of SDS among these different states could be calculated at 20 mM SDS concentrations. Analysis of the PFG NMR diffusivities obtained for SDS and SDS−SWCNTs indicates that the corresponding number of strongly adsorbed surfactant is estimated to be approximately 0.06 ± 0.03 mol of SDS per gram of SWCNT (see Supporting Information). Both the PFG NMR and PL/Abs data indicate that there is at least a small fraction of surfactant tightly bound to the nanotube surface even at concentrations below CMC1. A zetapotential study also showed that a significant charge associated with the surfactant could be observed for SWCNT suspensions under CMC1.57 This tightly bound surface state helps explain why dispersion stability can be observed for several days below the CMC1. Phillips et al. came to a similar conclusion recently when analyzing SWCNT adsorption onto hydrogels.58 The only region of Figure 2b that shows significant deviations from pure SDS solutions is the intermediate region around CMC1. As shown in Figure 2b, the NMR data show that the transition near CMC1 has shifted to a higher SDS concentration in the presence of SWCNTs. This increase in the transition of SDS−SWCNT suspensions indicates that the addition of SWCNTs delays the formation of micelle structures in solution because of the shift in distribution to the nanotube surface. Similar to the adsorption of surfactant on polymers,59,60 these changes indicate that saturation of the surfactant on the surface of the nanotube has occurred. Although the NMR data
Figure 5. (a) Shift in the apparent CMC as a function of the ratio of surfactant to SWCNT (R). The error bar shows the 90% confidence interval of the apparent CMC (±0.139 mM). (b) Effect of SWCNT concentration on the apparent CMC of SDS. The line is the leastsquares fit, which approaches the CMC of pure SDS.
are above CMC1 and follow a consistent trend. At low R (high SWCNT concentrations), the deviations are significant due to the larger amount of SDS that adsorbs onto the nanotubes. As the SWCNT concentration is reduced, the apparent CMC approaches CMC1 of pure SDS. Figure 5b shows that there is a linear relationship between SWCNT concentration and the apparent CMC. At the apparent CMC, micelles have only begun to form. Therefore, the concentration of SDS is only attributed to free SDS or SDS adsorbed on the SWCNT surface. Because the saturation line is only shifted from the apparent CMC (i.e., parallel),59,60 the average adsorption number of SDS per SWCNT can be calculated from the reciprocal of the linear slope in Figure 5b and was found to be 0.064 ± 0.014 mol/g SWCNT. The adsorption numbers of SDS calculated from both proton and PFG NMR (0.064 ± 0.014 and 0.06 ± 0.03 mol/g SWCNT, respectively) are the same within experimental error. The adsorption number has been calculated by dynamic simulation studies;3,61,62 however, it is not clear that the reported values correspond to saturated adsorption, which would be the conditions measured by this study. Only one molecular dynamics simulation study appeared to focus on saturated SDS surrounding a SWCNT, finding an adsorption number of 0.056 mol/g,3 which is very close to the values obtained experimentally. The adsorption number of another 5011
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surfactant, sodium dodecylbenzenesulfonate (SDBS), was found experimentally to be about 0.061−0.075 mol/g SWCNT,63 which is also close to the values measured from our study. While the proton NMR approach only measures the total amount of surfactant strongly bound to the nanotubes on the time scale of one-pulse NMR experiments (hundreds of microseconds), PFG NMR measures dynamic processes that can distinguish between loosely and strongly bound states on the time scale of the diffusion measurement.36,64 In this study, the latter time scale was between 10 and 500 ms, and only strongly bound surfactant was observed by PFG NMR. A good agreement between the adsorption numbers from proton NMR chemical shift measurements and proton PFG NMR indicates that the majority of surfactant ions interacting with the nanotubes in the studied systems remain strongly bound for at least 0.5 s.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]fl.edu (K.J.Z.). ORCID
Kirk J. Ziegler: 0000-0001-6996-2791 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Professor Dinesh Shah for helpful discussions, Professor Yiider Tseng for access to the ultracentrifuge, and the Richard Smalley Institute at Rice University for supplying the nanotubes. A portion of this work was performed in the McKnight Brain Institute at the National High Magnetic Field Laboratory’s AMRIS Facility, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490 and the State of Florida. We especially thank Dan Plant at AMRIS for his help with NMR measurements.
SUMMARY AND CONCLUSIONS
NMR techniques have been successfully employed to measure the adsorption number for SDS around SWCNTs. By observing the changes to the chemical shift of the α-proton of SDS, proton NMR found that the formation of micelles was delayed by the presence of SWCNTs. These transitions can be related to the interaction of adsorbed SDS on the surface of SWCNTs and the formation of micelles in the bulk solution after saturation of SDS on the nanotube surface. Only free SDS monomer is observed by NMR chemical shift measurements at concentrations below these apparent CMC values. However, optical measurements indicate that there is strongly bound SDS on SWCNTs even at concentrations below these apparent CMC values. The proton NMR signal of strongly bound SDS is expected to be undetectable in the NMR measurements reported in this study because of the line broadening and the corresponding short T2 NMR relaxation times of strongly bound SDS. At any given total SDS concentration, the presence of strongly bound SDS on SWCNTs alters the distribution between free SDS and those aggregated into micelles. Analysis of the PFG NMR data also showed changes in the distribution of SDS between free and micelle states. These changes to SDS distribution were used to calculate the adsorption number from both proton and PFG NMR data, which was found to be the same value in both techniques. These data indicate that nearly all of the SDS interacting with the SWCNTs is strongly bound and not exchanging with free SDS on the time scale up to 0.5 s, which is the maximum diffusion time used in the PFG NMR measurements.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00758. Comparison of CMC measurements of SDS by various methods, control group measurements, CMC determination from NMR data, error propagation and analysis, discussion of metallic catalysts and susceptibility effects on NMR spectra, and calculations of adsorption numbers (PDF) 5012
DOI: 10.1021/acs.langmuir.7b00758 Langmuir 2017, 33, 5006−5014
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Langmuir
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