# Biocompatible Dispersants for Carbon Nanomaterials
## 6 Biocompatible Dispersants
### Alkyl Chain Surfactants
These dispersions were mainly used as a comparison to cellulose-based dispersions, which were found to be more effective. [PERSON] et al. have also demonstrated the ability of ACSs to disperse MWCNTs, using SDBS, SDS, lithium dodecyl sulfate (LDS), and tetradecyl trimethyl ammonium bromide (TTAB) to form aqueous dispersions [51]. The quality of these dispersions was investigated using a range of metrics determined from physiochemical characterisation, such as mean nanotube length and bundle diameter. The quality of these dispersions ranked as follows: SDS > LDS > SDBS > TTAB. The position of TTAB as the molecule with the lowest quality dispersions is due to its cationic hydrophilic group, as discussed above. SDS, LDS, and SDBS all have anionic head groups. However, it is important to note that the dispersant efficiency was not considered in this study and, therefore, the concentration-dependent dispersing abilities of these surfactants need to be examined (Table 4 (4)). A perhaps under-utilised use for this class of surfactants is the solubilisation of CNMs by a surfactant substitution mechanism. As mentioned above, these small molecules are ideal for de-bundling CNTs yet suffer from poor dispersion stability due to poor adhesion. Larger surfactants with better dispersion stability, particularly steric and polymer surfactants, often have poor de-bundling abilities due to steric repulsion at the bundle surface [16]. One way around this is to use a surfactant substitution solubilisation method, where the de-bundled CNT fibres are stabilised by replacing the head/tail surfactant molecules bound to its surface with larger surfactant. [PERSON] et al. successfully leveraged this method, using a combination of SDS/Surfhorpe 1216 on MWCNTs, to produce a dispersion with a high proportion of individual CNTs [16] (Table 4 (5)). One avenue of increasing the efficiency of ACSs is through Gemini surfactants. These biocompatible, di-cationic molecules are proven to be more efficient than their single-chained counterparts [55; 57], even at very low concentrations and even though they are  
Figure 5: Examples of (**A**), anionic, (**B**), cationic and (**C**), Gemini ACS. cationic. This is due to two main factors (i) their double-tailed structure allows for stronger adsorption to CNMs due to increased non-polar surface area for hydrophobic interactions and (ii) enhanced packing of surfactant molecules due to the two anionic head groups being confined to a small area by the spacer groups. For CNT systems, the structures of Gemini surfactants allow for more efficient exfoliation of CNT bundles and enhanced stability of the resulting individual CNT dispersions, as demonstrated by [PERSON] et al. in 2017 [55]. A range of imidazolium-based n-s-n Gemini surfactants was synthesised and used to disperse and exfoliate MWCNTs. These were produced by first synthesising a 1,1'-(1,6-hexanediol) bis (1H-imidazole) head group then reacting it with the appropriate chloromethyl alkyl ether to form a collection of n-6-n Gemini surfactants, where n = 8, 10, 12, and 14 (denoted IMIC6C8, IMIC6C10, IMIC6C12, and IMIC6C14, respectively). IMIC6C12 was found to be the most efficient surfactant, as its tail length was long enough to promote adequate CNM binding yet short enough to allow efficient packing of surfactants on the CNM surface (Table 4 (6)). [PERSON] et al. found that by increasing the s value of a Gemini surfactant, more efficient MWCNT dispersants could be obtained [54]. This improvement was noticed after testing a range of n-s-n bis(quaternary ammonium) surfactants. It was found that the 16-2-16 compound (the one with the longest tail/shortest spacer) was the most effective due to its tight packing and CNT de-bundling efficiency. Therefore, it produced the most concentrated CNM dispersion. Conversely, the 12-12-12 compound (shortest tail/longest spacer) was the most efficient, as it had the highest \(\eta^{*}\) value.