Solubilization ability of star-shaped trimeric quaternary ammonium bromide surfactant

Abstract

The solubilization ability of surfactants is closely related to their molecular design. The number of alkyl chains in a surfactant molecule accordingly affects the forming of micellar structures. In the present study, the solubilization ability of cationic surfactants was studied by changing number of alkyl chains by using one of these three types of surfactants: star-shaped trimeric quaternary ammonium bromides (tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine bromides (3CntrisQ, n = 10, 12, and 14 is the number of carbon atoms in the alkyl chains)), dimeric surfactants (1,2-bis(alkyldimethylammonio)ethane dibromide (n-2-n, n = 10, 12, and 14)), and dodecyltrimethylammonium bromide (DTAB). The solubilization abilities of the above surfactants were studied using naphthalene and stearic acid at 298.2 K. Judging from the value of the molar solubilization ratios (MSR) for naphthalene and stearic acid, the solubilization ability generally improved as the number of alkyl chains of the surfactant molecules increased, as follows: DTAB <12-2-12 < 3C12trisQ. The Gibbs free energy change (ΔG0) values for the solubilization of naphthalene and stearic acid into 3C12TrisQ solutions were relatively large negative numbers, −28.3 and −59.4 kJ mol−1, respectively. Moreover, small angle X-ray scattering analysis indicated that electron density at the ellipsoidal micellar surface of 3C12trisQ was decreased by solubilizing naphthalene and stearic acid, however, the micellar shape was not drastically changed from ellipsoidal micelle. These results indicated that the trimeric surfactant presented better potential as solubilizer compared to typical monomeric surfactants.