Micellar size, shape and counterion binding of N-(1,1-Dihydroperfluoroalkyl)-N,N,N-trimethylammonium chloride in aqueous solutions

Abstract

Under the limiting conditions, light-scattering method was applied to determine the size and shape of fluorinated micellar structure in aqueous solution. The hydrodynamic radii (rh) of molecular aggregates were investigated for four amphiphiles with different alkyl chain length: N-(1,1-dihydroperfluorooctyl)-, N-(1,1-dihydroperfluorodecyl)-, N-(1,1-dihydroperfluorododecyl)-, and N-(1,1-dihydroperfluorotetradecyl)-N,N,N-trimethylammonium chloride by dynamic light scattering. Size of these molecular aggregates drastically increased with increasing carbon number of alkyl chain and the concentration. For example, the rh values were 35, 63, 95, and 136 nm for C8, C10, C12, and C14 of the alkyl chain, respectively, at the concentration of 15× critical micelle concentration (cmc) and at 298.2 K. On the other hand, the mean radius of gyration (rg) and the apparent molecular weight of these aggregates were measured using static light scattering. The rg value was found to be 34, 73, and 125 nm for C8, C10, and C12 of the alkyl chain, respectively, at the same condition above. The shape of aggregate was analyzed by comparing the dependence of rg/rh with theoretical values. The plots of rg against rh almost corresponded with the theoretical line of oblate ellipsoid (disk-like), and which was also expected from calculation of packing parameter value. The measurement for chloride ion activity was made by a chloride ion selective electrode to know the degree of counterion binding to the aggregates, whose values increased with increasing total concentration and with the carbon number of alkyl chain. The chloride ion binding to aggregates neutralized cationic head groups under their electrostatic repulsion and partly promoted the growth of aggregates.