The effect of molecular structure on the surface properties of selected quaternary ammonium salts

Equilibrium and dynamics of adsorption at the air/water interface of cationic surfactants belonging in the group of quaternary ammonium bromides was studied. Static surface tension of aqueous surfactants' solutions was measured in a wide range of surfactant concentrations using du Noüy ring method. Quaternary ammonium salts with different alkyl chain numbers and hydrophobicities were investigated. The Frumkin equation and reorientation model were used for a quantitative description of the process of their adsorption. The adsorption dynamics of the considered salts was described as a supplement to the equilibrium study. It was found that four short alkyl chains in tetraalkylammonium salt build a molecule which is weakly surface active. The number of alkyl chains as well as the symmetry of the molecules are important factors, determining the behavior of the surfactants at the interface and in the bulk solution. Surfactants with asymmetrical structure, like TTABr or MTOABr, show a strong tendency to reorientation, in opposite to tetraalkylammonium bromides and DDABr. Measurements of the dynamic surface tension revealed also strong dependence of the dynamics of adsorption on the chemical structure of the molecules. Measurements of the particle size by the dynamic light scattering method enabled the presence or lack of the micelles in the selected solutions to be confirmed.