Change in Chirality of Semiconducting Single-Walled Carbon Nanotubes Can Overcome Anionic Surfactant Stabilization: A Systematic Study of Aggregation Kinetics

Abstract

Single-walled carbon nanotubes' (SWNT) effectiveness in applications is enhanced by debundling or stabilization. Anionic surfactants are known to effectively stabilize SWNTs. However, the role of specific chirality on surfactant-stabilized SWNT aggregation has not been studied to date. The aggregation behavior of chirally enriched (6,5) and (7,6) semiconducting SWNTs, functionalized with three anionic surfactants-sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and sodium deoxycholate (SDOCO)-was evaluated with time-resolved dynamic light scattering. A wide range of mono- (NaCl) and di-valent (CaCl₂) electrolytes as well as a 2.5 mg TOC/L Suwannee River humic acid (SRHA) were used as background chemistry. Overall, SDBS showed the most effectiveness in SWNT stability, followed by SDOCO and SDS. However, the relatively larger diameter (7,6) chiral tubes compromised the surfactant stability, compared to (6,5) chiral enrichment, due to enhanced van der Waals interaction. The presence of di-valent electrolytes overshadowed the chirality effects and resulted in similar aggregation behavior for both the SWNT samples. Molecular modeling results enumerated key differences in surfactant conformation on SWNT surfaces and identified interaction energy changes between the two chiralities to delineate aggregation mechanisms. The stability of SWNTs increased in the presence of SRHA under 10 mM monovalent and mixed electrolyte conditions. The results suggest that change in chirality can overcome surfactant stabilization of semiconducting SWNTs. SWNT stability can also be strongly influenced by the anionic surfactant structure.

Keywords: Chirality; aggregation kinetics; anionic surfactants; molecular dynamic simulation; single-walled carbon nanotube; stability.

Figure 1

High resolution TEM micrographs of SWNTs: top row for SG65 and bottom row for SG76 SWNTs. Pristine, SDS, SDBS, and SDOCO functionalized SWNTs are placed from left to right, i.e., (a) SG65-Pristine, (b) SG65-SDS, (c) SG65-SDBS, (d) SG65-SDOCO, (e) SG76-Pristine, (f) SG76-SDS, (g) SG76-SDBS, and (h) SG76-SDOCO. Functionalized SWNTs are debundled and declustered in comparison to respective pristine tubes.

Figure 2

EPM of SG65 SWNTs as a function of (a) NaCl and (b) CaCl₂ salt concentration. At least three separate experiments were performed for each condition and data presented here represent the mean of three independent experiments with one standard deviation. Measurements were carried out at a pH of ~6.5 and a temperature of 20±0.5 °C.

Figure 3

Aggregation profiles of (a) SDS, (b) SDBS, and (c) SDOCO modified SG65 SWNTs under a wide range of NaCl concentration. Aggregation experiments were conducted at a temperature of 20±0.5 °C and at least 2 duplicate samples were tested to obtain significant reproducibility.

Figure 3

Aggregation profiles of (a) SDS, (b) SDBS, and (c) SDOCO modified SG65 SWNTs under a wide range of NaCl concentration. Aggregation experiments were conducted at a temperature of 20±0.5 °C and at least 2 duplicate samples were tested to obtain significant reproducibility.

Figure 4

Stability plots of (a) SG65 and (b) SG76 SWNTs as a function of NaCl salt concentration, and (c) SG65 and (d) SG76 SWNTs as a function of CaCl₂ salt concentration. The attachment efficiencies are calculated by normalizing the actual aggregation rate with the favorable (fast) aggregation rate. All the rates are calculated from corresponding aggregation profiles: Figures S6 (SG65-NaCl), S7 (SG76-NaCl), S8 (SG65-CaCl₂) and S9 (SG76-CaCl₂), respectively. Aggregation experiments were conducted at a temperature of 20±0.5 °C.

Figure 4

Stability plots of (a) SG65 and (b) SG76 SWNTs as a function of NaCl salt concentration, and (c) SG65 and (d) SG76 SWNTs as a function of CaCl₂ salt concentration. The attachment efficiencies are calculated by normalizing the actual aggregation rate with the favorable (fast) aggregation rate. All the rates are calculated from corresponding aggregation profiles: Figures S6 (SG65-NaCl), S7 (SG76-NaCl), S8 (SG65-CaCl₂) and S9 (SG76-CaCl₂), respectively. Aggregation experiments were conducted at a temperature of 20±0.5 °C.

Figure 5

Front and side views of representative gas phase optimized geometries for surfactants adsorbed on SWNTs. Left column is for SG65 and right column is for SG76 SWNTs. Top to bottom: SWNTs with (a–b) SDS, (c–d) SDBS, and (e–f) SDOCO surfactants. The interaction energy (I.E.) for each SWNT-surfactant is shown below the side view snapshot for each case. The I.E. of (a) SG65-SDS, (b) SG76-SDS, (c) SG65-SDBS, (d) SG76-SDBS, (e) SG65-SDOCO, and (f) SG76-SDOCO are −39.6, −42.7, −33.7, −35.7, −38.3, and −40.8 kcal/mol, respectively.