# Biocompatible Dispersants for Carbon Nanomaterials
## 6 Biocompatible Dispersants
### Alkyl Chain Surfactants  
Perhaps the most well-studied class of dispersants is alkyl chain surfactants (ACSs). As seen in Figure 5, they may be anionic or cationic. They consist of a hydrophilic ionic head group attached to an alkyl chain tail, which binds to the CNMs surface. The ionic head group can be either cationic, such as in the case of CTAB or anionic, as SDS is. Gemini surfactants, also known as di-cationic surfactants, consist of two cationic groups linked together with a spacer, whilst each group also has an alkyl chain attached to it. These head groups are typically quaternary ammonium [54] or imidazolium [55] in nature. Although they may not be as efficient as other classes of dispersants, these molecules have a variety of uses. Once bound to a CNM's surface, ACSs cause nanoparticles to repel each other through electrostatic interactions. ACSs with small, highly charged head groups excel at de-bundling CNTs due to their ability to penetrate the dense fibres and instigate the unzipping process. The main disadvantage of ACSs is that they are toxic in high doses [13], which is not optimal for in-vivo studies. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Num.** & **Dispersant** & **CNM** & **Description** & **Application** & **Ref.** \\ \hline
1 & SDC & CNO & CNOs non-covalently & Stability studies; analysis of & [48] \\  & & & functionalised with SDC & surfactant efficiency & \\
2 & SDC & CQD & SDC as both a dispersant and & Formation of CQD containing & \\  & & & EPD agent & composite films via EPD for & [49] \\  & & & Graphene functionalised with & & Production of Palladium \\
3 & SDC, SC, sodium & Graphene & a range of BAS via graphene & nanoparticle-based catalysts & \\  & & & oxide reduction & for the electrooxidation of & \\
4 & SC & SWCNT & SWCNTs non-covalently & & \\  & & & functionalised with SC & & Chiral purification of \\  & & & SDC is used to exfoliate and & & SWCNTs via two-phase & [50] \\
5 & SDC & SWCNT & disperse SWCNTs to form & & aqueous extraction & \\  & & & stable dispersions & Determination of dispersion & \\  & & & SWCNTs non-covalently & & quality and its correlation & \\
6 & SC, SDC, sodium & SWCNT & functionalised with BAS; & & \\  & & & aromatic derivatives of these & & \\  & & & salts were also used & & \\ \hline \end{tabular}
\end{table} 
Table 3: BAS-based CNM dispersions. ACSs are capable of dispersing CNOs, as demonstrated by our group using hexadecyltrimethylammonium bromide (CTAB) and SDBS [48] (Table 4 (1)). This study also found that ACSs with negatively charged head groups are more effective at dispersing CNOs than their cationic counterparts. This effectiveness is due to the electrostatic repulsion between the negatively charged CNM surface and the head group of the surfactant, forcing anionic ACSs to take a tail-first approach when binding to the CNO surface--thus resulting in more efficient packing of surfactant molecules and better coverage of the CNM's surface. [PERSON] et al. have also found success dispersing CNOs with ACSs, producing stable dispersions using CTAB, sodium dodecyl sulfate (SDS), and SDBS [56]. A wide range of CNO: surfactant mass ratios were investigated, which is beneficial as the surfactants optimal efficiency can be determined. Unlike many other studies, in vitro antimicrobial assays were performed. It was found that only the CNO/CTAB dispersion negatively affected cell viability. This was likely due to the dissociation of CTAB from the CNO surface. As discussed earlier, this is an unavoidable problem with ACSs as they form dynamic dispersions [56] (Table 4 (2)). [PERSON] et al. further demonstrated that ACSs could be utilised to disperse MWCNTs and graphene; SDS, SDBS, and lauryl betaine were used [5] (Table 4 (3)). These dispersions were tested for use as heat transfer fluids due to the CNM's conductivity. Unfortunately, many of these dispersions suffered from poor stability, likely because the CNM concentration was too high.