## 3 Results and Discussion  
### Synthesis and Spectroscopic Characterization of Gemini Surfactants  
Gemini surfactants tested in this paper as capping agents for AgNPs were received by alkylation of \(N\),\(N\)-dimethyl-\(N\)-hexadecylamine with 1,6-dibromohexane or bis(2-bromoethyl) ether, for 16-6-16, and 16-O-16 respectively (Figure 1). A synthetic procedure developed in our laboratory assumes carrying out the reaction without a solvent at room temperature [50,75]. These reactions proceed according to the nucleophilic substitution mechanism S\({}_{\text{N}}\)2. The rate of these reactions depends on the substrates concentrations. In case of the reaction without a solvent, the concentration of the reactants is the highest possible. Such reactions take place with high efficiency without waste. These reactions are in accordance with the greenolution approach, because they reduce the use of solvent to a minimum and minimize costs.  
The structure and purity of gemini surfactants obtained were confirmed by \({}^{1}\)H NMR, \({}^{13}\)C NMR, FTIR, and elemental analysis.  
The **16-6-16**: mp 222-223 \({}^{\circ}\)C; elemental analysis (%) calcd. for C\({}_{42}\)H\({}_{90}\)Br\({}_{2}\)N\({}_{2}\): C 64.43, H 11.58, N 3.58; found C 64.72, H 11.68, N 3.40; \({}^{1}\)H NMR (403 MHz, CDCl\({}_{3}\)) 3.73 (bs, 4H, N\({}^{+}\)CH\({}_{2}\)(spacer)), 3.49 (m, 4H, -N\({}^{+}\)CH\({}_{2}\)-), 3.39 (s, 12H, -N\({}^{+}\)(CH\({}_{3}\))\({}_{2}\)), 2,01 (bs, 4H, -N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-(spacer)), 1.72 (bs, 4H, -N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-),1.57 (bs, 4H, -N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)CH\({}_{2}\)(spacer)), 1.36-1.25 (m, 52H, -CH\({}_{2}\)(CH\({}_{2}\))\({}_{13}\)CH\({}_{3}\)), 0.88 (t, 6H, -CH\({}_{2}\)CH\({}_{3}\)); \({}^{13}\)C NMR (101 MHz, CDCl\({}_{3}\)): 65.9 (-N\({}^{+}\)CH\({}_{2}\)spacer), 64.06 (-N\({}^{+}\)CH\({}_{2}\)-), 50.98 (-N\({}^{+}\)CH\({}_{3}\)), 31.89 (-CH\({}_{2}\)CH\({}_{2}\)CH\({}_{3}\)), 29.61, 29.59, 29.56, 29.34, 29.27, 29.45, 29.37, 29.12 (-N\({}^{+}\)(CH\({}_{2}\))\({}_{11}\)-), 26.86 (-N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-), 24.37 (-N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-(spacer)), 22.6 (-N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-CH\({}_{2}\)(spacer)), 21.61 (-CH\({}_{2}\)CH\({}_{3}\)), 14.11 (-CH\({}_{2}\)CH\({}_{3}\)).  
The 16-O-16: mp 240-242 \({}^{\circ}\)C; elemental analysis (%) calcd. for C\({}_{40}\)H\({}_{86}\)Br\({}_{2}\)N\({}_{2}\)O: C 62.32, H 11.24, N 3.63; found C 61.41, H 11.68, N 3.20; \({}^{1}\)H NMR (403 MHz, CDCl\({}_{3}\)) 4.36 (bs, 4H, OCH\({}_{2}\)), 4.05 (bs, 4H, N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)O-), 3.67-3.62 (m, 4H, -N\({}^{+}\)CH\({}_{2}\)-), 3.46 (s, 12H, -N\({}^{+}\)(CH\({}_{3}\))\({}_{2}\)), 1.73 (bs, 4H, -N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)-), 1.47-1.10 (m, 52H, -CH\({}_{2}\)(CH\({}_{2}\))\({}_{13}\)CH\({}_{3}\)), 0.88 (t, 6H, -CH\({}_{2}\)CH\({}_{3}\)); \({}^{13}\)C NMR (101 MHz, CDCl\({}_{3}\)): 65.92(-N\({}^{+}\)CH\({}_{2}\)CH\({}_{2}\)O-), 64.59 (-OCH\({}_{2}\)-), 64.03 (-N\({}^{+}\)CH\({}_{2}\)-), 51.57 (-N\({}^{+}\)CH\({}_{3}\)), 31.82 (-CH\({}_{2}\)CH\({}_{2}\)CH\({}_{3}\)), 29.61, 29.58, 29.56, 29.54, 29.37, 29.25, 26.25 (-N\({}^{+}\)(CH\({}_{2}\))\({}_{11}\)-), 28.83 (-CH\({}_{2}\)CH\({}_{2}\)CH\({}_{2}\)-), 22.83 (-CH\({}_{2}\)CH\({}_{3}\)), 14.00 (-CH\({}_{2}\)CH\({}_{3}\)).  
FTIR spectra for tested dimeric alkylammonium salts (Figure 2) show regular bands for the asymmetric (\(\upnu_{\text{as}}\)) and symmetric (\(\upnu_{\text{s}}\)) stretching vibrations of methyl and methylene groups at 2916-2846 cm\({}^{-1}\), as well as bands for the deformation vibration (\(\updelta\)) of methyl groups at 1472-1464 cm\({}^{-1}\). The 16-O-16 spectra shows usual peaks of C-O stretching vibration at 1146 cm\({}^{-1}\). Due to hydrophilicity (ability to absorb water) of gemini surfactants the strong absorption at the 3500-3350 cm\({}^{-1}\) region from the -OH stretching vibrations of water molecule is detected in FTIR spectra.  
Figure 1: Synthesis of 16-6-16 and 16-O-16.