The Surface and Solution Properties of Dihexadecyl Dimethylammonium Bromide

Abstract

The surface adsorption behavior and solution aggregate microstructure of the dichain cationic surfactant dihexadecyl dimethylammonium bromide (DHDAB) have been studied using small angle neutron scattering (SANS), light scattering, neutron reflectivity (NR), and surface tension (ST). Using a combination of surface tension and neutron reflectivity, the DHDAB equilibrium surface excess at saturation adsorption has been measured as 2.60 +/- 0.05 x 10 (-10) mol.cm (-2). The values obtained by both methods are in good agreement and are consistent with the values reported for other dialkyl chain surfactants. The critical aggregation concentration (CAC) values obtained from both methods (NR and ST) are also in good agreement, with a mean value for the CAC of 4 +/- 2 x 10 (-5) M. The surface equilibrium is relatively slow, and this is attributed to monomer depletion in the near surface region, as a consequence of the long monomer residence times in the surfactant aggregates. The solution aggregate morphology has been determined using a combination of SANS, dynamic light scattering (DLS), cryogenic transmission electron microscopy (CryoTEM), and ultrasmall angle neutron scattering (USANS). Within the concentration range 1.5-80 mM, the aggregates are in the form of bilamellar vesicles with a lamellar " d-spacing" of the order of 900 A. The vesicles are relatively polydisperse with a particle size in the range 2000-4000 A. Above 80 mM, the bilamellar vesicles coexist with an additional L beta lamellar phase.