Aggregation properties of supralong-chain surfactants with double or triple quaternary ammonium head groups

Abstract

Double or triple quaternary ammonium head groups were designed to improve the solubility of supralong alkyl chain surfactants. In the surfactant head group, quaternary ammonium groups are connected by an ethylene spacer. Micellar shapes of divalent surfactants [CnH2n+1N+(CH3)2–(CH2)2–N+(CH3)3 2Br−: Cn-2Am (n=18, 20, and 22)] and trivalent surfactants [CnH2n+1N+(CH3)2–(CH2)2–N+(CH3)2–(CH2)2–N+(CH3)3 3Br−: Cn-3Am (n=18, 20, and 22)] were studied in aqueous solutions by means of dynamic light scattering (DLS) and transmission electron microscopy (TEM). Changes in the surfactant concentration have a small influence on the apparent hydrodynamic radii (rh) of the molecular aggregates in both surfactant series. Average values of rh of aggregates are 60–90nm for Cn-2Am (n=18, 20, and 22) and 2–40nm for Cn-3Am (n=18, 20, and 22). TEM micrographs showed that aggregates of Cn-2Am (n=18, 20, and 22) typically formed rod-like micelles. In contrast, trivalent surfactants of Cn-3Am (n=18, 20, and 22) formed spherical (C18-3Am) or ellipsoidal micelles (C20-3Am and C22-3Am). Moreover, the degree of micellar counterion binding for these surfactants was determined by using a bromide ion-selective electrode, which indicated relatively high values (0.8–0.9) for Cn-2Am (n=18, 20, and 22) and more common values (0.5–0.8) for Cn-3Am (n=18, 20, and 22). The size of the aggregates is closely related to the degree of counterion binding.