Theoretical and experimental investigation of the equilibrium and dynamic interfacial behavior of mixed surfactant solutions

Abstract

In many commercial applications involving surfactants, the desired properties are controlled by both the equilibrium and the dynamic interfacial behavior. In particular, surfactant adsorption at air-water interfaces causes the surface tension to decrease, which, for example, can control the spreading properties of a liquid, and hence, is important in practical applications involving the use of paints and pesticides, as well as in the manufacturing of photographic films. Similarly, surfactant adsorption at oil-water interfaces causes the interfacial tension to decrease, which, for example, can enhance the ability of surfactant solutions to remove oily soil from dirty surfaces (fabric, hair, skin, etc.) during cleaning applications. A predictive, molecular-thermodynamic theory capable of modeling the behavior of surfactants at solution interfaces would help minimize the need for costly and time-consuming experimentation associated with the development of surfactant products based on a trial-and-error approach. Furthermore, this theory should encompass mixtures of surfactants, since their use in industrial applications is widespread, whether intentionally, to take advantage of synergism between the surfactant components in a mixture, or simply because it is too costly to mass produce a single, pure surfactant. With this as motivation, a molecularly-based theoretical framework to model both the equilibrium and the dynamic adsorption of surfactant mixtures at both the air-water interface and the oil-water interface has been developed. The equilibrium air-water surface equation of state is based on a two-dimensional, nonideal gas-like monolayer model of the adsorbed surfactant molecules. For non-ionic surfactants, two types of interactions were accounted for:

(cont.) (i) repulsive, steric interactions, which were modeled using a hard-disk treatment, and (ii) attractive, van der Waals interactions, which were modeled using a virial expansion, truncated to second order in surfactant surface concentration. Since both the hard-disk size and the second-order virial coefficients characterizing these interactions can be deduced from the known molecular structures of the surfactants, this surface equation of state contains no experimentally determined parameters. This nonionic surface equation of state was subsequently modified to incorporate electrostatic effects associated with charged surfactants. For mixtures that contain only a single ionic surfactant species, the electrostatic contribution to the surface equation of state was computed using a Gouy-Chapman based approach, which also included a Stern layer of counterion exclusion. This electrostatic description assumes that all of the electrostatic charge on the adsorbed surfactant molecules is located on a single two-dimensional charge layer. The Gouy-Chapman model was then extended to mixtures that contain multiple ionic surfactants, or surfactants that contain multiple charged groups (such as zwitterionic surfactants) by treating the case of multiple, two-dimensional charge layers at the interface. This extended theoretical framework is capable of treating any number of surfactant components containing any number of charged groups ...