Surface tension and dilation rheology of DNA solutions in mixtures with azobenzene-containing cationic surfactant

Abstract

The surface tension and dilational surface visco-elasticity of the individual solutions of the biopolymer DNA and the azobenzene-containing cationic surfactant AzoTAB, as well as their mixtures were measured using the drop profile analysis tensiometry. The negatively charged DNA molecules form complexes with the cationic surfactant AzoTAB. Mixed DNA+AzoTAB solutions exhibit high surface activity and surface layer elasticity. Extremes in the dependence of these characteristics on the AzoTAB concentration exist within the concentration range of 3×10−6–5×10−5M. The surface tension of the mixture shows a minimum with a subsequent maximum. In the same concentration range the elasticity shows first a maximum and then a subsequent minimum. A recently developed thermodynamic model was modified to account for the dependence of the adsorption equilibrium constant of the adsorbed complex on the cationic surfactant concentration. This modified theory shows good agreement with the experimental data both for the surface tension and the elasticity values over the entire range of studied AzoTAB concentrations.