Small-angle neutron scattering study of micellar structures of dimeric surfactants

Dimeric or gemini surfactants consist of two hydrophobic chains and two hydrophilic head groups covalently connected by a hydrocarbon spacer. Small-angle neutron scattering measurements from bis-cationic ${\mathrm{C}}_{16}{\mathrm{H}}_{33}{\mathrm{N}}^{+}({\mathrm{CH}}_3{)}_2-({\mathrm{CH}}_2{)}_m-{\mathrm{N}}^{+}({\mathrm{CH}}_3{)}_2{\mathrm{C}}_{16}{\mathrm{H}}_{33}\hspace{0.5em}2{\mathrm{Br}}^{-}$ dimeric surfactants, referred to as $16-m-16$, for different length of hydrocarbon spacers $m$= 3–6, 8, 10, and 12, are reported. The measurements have been carried out at various concentrations: $C=2.5\mathrm{}$ and 10 mM for all $m$ and $C=30\mathrm{}$ and 50 mM for $m>~5$. It is found that micellar structure depends on the length of the spacer. Micelles are disks for $m=3\mathrm{}$, cylindrical for $m=4\mathrm{}$, and prolate ellipsoidals for other values of $m$. These structural results are in agreement with the theoretical predictions based on the packing parameter. It has also been observed that conformation of the spacer and the hydrophobic chains in the interior of the micelle change as the length of the spacer is increased. The concentration dependence for $m>~5$ shows that the effect of surfactant concentration on the size of the micelle is more pronounced for $m=5\mathrm{}$ and 12 than for the intermediate spacers. The fractional charge on the micelle increases with the increase in spacer length and decreases when the concentration is increased