Solubilization of polycyclic aromatic compounds into supralong-chain surfactants with double quaternary ammonium micelles

The dissolution of poorly water-soluble compounds in micelles via hydrophobic interactions is termed solubilization. The solubilization of polycyclic aromatic compounds, namely, naphthalene, anthracene, and pyrene, was studied using supralong-chain surfactants with two consecutive cationic hydrophilic groups [CnH2n+1N+(CH3)2-(CH2)2-N+(CH3)3 2Br-: Cn-2Am (n = 18, 20, and 22)] and typical cationic surfactants [CnH2n+1N+(CH3)3 Br-: Cn-Am (n = 12, 14, and 16)]. Long alkyl chains in hydrophobic surfactants are advantageous for forming a large micellar core. The maximum quantity of solubilized polycyclic aromatic compounds increased as the alkyl chain length of the surfactant increased. Therefore, the molar solubilization ratio (MSR), which represents the solubilization ability of the surfactant, was maximized when the alkyl chain length was C22-2Am. Similarly, the Gibbs free energy (ΔG0) associated with the solubilization of polycyclic aromatic compounds was also lowest for C22-2Am. Surfactants with long alkyl chains functioned as excellent solubilizers. The solubilization sites of the polycyclic aromatic compounds were determined using two-dimensional NMR and small-angle X-ray scattering (SAXS) measurements. As the molecular size of the polycyclic aromatic compounds increased, the solubilization positions in the micelles shifted from the palisade layer to the vicinity of the hydrophilic groups. Extension of the alkyl chain length of the surfactant resulted in an increase in the palisade region of the micelles, which was advantageous for solubilization.