# Gemini Surfactant as a Template Agent for the Synthesis of More Eco-Friendly Silica Nanocapsules
## 1 Introduction  
Engineered nanomaterials (ENMs) have been proposed as new solutions for encapsulation of active compounds, such as corrosion inhibitors, antifouling agents, pH indicators, drugs or dyes [1; 2; 3; 4; 5; 6; 7]. Nanocapsules are core-shell structures with diameter generally ranging between 100-500 nm [8], and one of the most used nanocapsules are based on mesoporous silica, typically with a diameter of 100-200 nm [9]. This cutting-edge immobilization technique has assumed a relevant role in the field of smart coatings, by providing a controlled release of encapsulated molecules, reducing their toxicity (compared to the free forms), preventing direct interaction with coating matrices, reducing leakages and ultimately increasing the coating's service life [3; 10; 11]. However, state-of-the-art information shows that even empty silica nanocapsules show some degree of toxicity [10]. They have been found to be more toxic towards selected marine species than other engineered nanomaterials, namely Zn-Al layered double hydroxides (LDH) [12; 13]. Recent studies have revealed that the silica nano-form or the formation of agglomerates in artificial saltwater, over time, is not necessarily thesource of silica nanocapsules toxicity [10; 11; 12; 14]. Indeed, it is related to the presence of a cationic surfactant, \(N\)-hexadecyl-\(N\),\(N\),\(N\)-trimethylammonium bromide (CTAB) which is used as a template during nanomaterials' synthesis. Replacing CTAB with a greener surfactant has been suggested previously in order to reduce capsules' toxicity [10]. There are strategies that allow the complete removal of CTAB from capsules' surface such as extraction with solvents solutions, washing with a solution of ethanolic HCl or thermal treatment (calcination) [9; 15; 16]. Although these steps are effective for removal of synthesis remnants from empty silica nanocapsules, they may be undesirable if the entrapment of active molecules inside the nanomaterials is the main goal: calcination leads to complete degradation of all organic molecules, while using other solvents may lead to leaching of the encapsulated agents as well. Therefore, replacing CTAB with a more eco-friendly surfactant would prevent toxicological issues and may avoid the implementation of additional steps. Cationic gemini surfactants are a group of innovative quaternary ammonium salts that consist of two monomeric moieties linked by a spacer. Each moiety is made of a hydrophilic part (positively charged nitrogen atom) and a hydrophobic part (alkyl chain) [17]. Due to this dimeric structure, gemini surfactants show unique surface properties, such as critical micelle concentrations (CMC), which is lower comparing with conventional cationic surfactants, with higher efficacy in lowering surface tension (for surfactants with the same number of carbon atoms in hydrophobic parts). CMC is the concentration at which micelles start forming [18], and the ability of cationic surfactants to form stable oil in water emulsion allows their use as a template in silica nanocapsules synthesis [14]. For a conventional monomeric surfactant, \(N\)-dodecyl-\(N\),\(N\),\(N\)-trimethylammonium bromide (DTAB), the CMC is 16 mM whereas for a dimeric surfactant, with a rigid spacer, such as 1,4-bis-[\(N\)-(1-dodecyl)-\(N\),\(N\)-dimethylammoniumiummethyl]benzene dibromide (QSB2-12) the CMC is 1.21 mM [18]. For CTAB the CMC is 0.907 mM [19], lower than for QSB2-12. It has been reported that by modifying the surfactants' structure, aggregation behavior may be changed. Increasing the number of hydrophilic parts decreases the CMC (when the hydrophobic part is fixed), which is also observed when an increase in the number of carbon chains of the hydrophobic part occurs (when hydrophilic part is fixed) [17]. Gemini surfactants are known to be less toxic to freshwater organisms [20]. As an example, and looking at the key freshwater species _Daphnia magna_, CTAB was found to be 28 times more toxic than QSB2-12: LC\({}_{50}\) of 0.026 mg/L [21] vs. 0.73 mg/L [20], respectively.