Dynamic surface tensions of aqueous surfactant mixtures: Experimental investigation

We present an experimental investigation of the interfacial behavior of aqu eous solutions of two nonionic surfactants and their binary mixture. Specif ically, we measure both the equilibrium surface tensions (using the Wilhelm y plate method) and the dynamic surface tensions (using the pendant bubble technique) of aqueous solutions of two nonionic alkyl ethoxylate surfactant s, dodecyl penta(ethylene oxide) (C12E5) and decyl octa(ethylene oxide) (C1 0E8), as well as of their binary mixture. These measurements are then compa red with the predictions made using recently developed molecular-thermodyna mic theories of the equilibrium and the dynamic interfacial behaviors of no nionic surfactant mixtures and are found to be in good agreement. The main advantage of the equilibrium and the dynamic interfacial theories adopted h ere is that they are based on the molecular characteristics of the surfacta nts, and as such, the required number of experimentally determined paramete rs can be reduced significantly. Indeed, no experimental measurements, neit her of the equilibrium nor of the dynamic type, need to be conducted on the mixed surfactant solutions. In addition to the complete prediction of the dynamic surface tensions and the dynamic surface concentrations, the theore tical framework also includes a simplified time scale analysis of the dynam ic interfacial behavior that allows "quick" insight into the relationship b etween the molecular structure of a surfactant and its dynamic interfacial behavior. The experimental measurements also show that the simplified time scale analysis of the dynamic interfacial behavior provides an approximate, yet quantitatively accurate, description of the rate of adsorption, measur ed through the dynamic surface tension, of the single nonionic surfactants as well as of a surfactant component in the binary mixture of nonionic surf actants.