(282a) Identifying Synergism in Multicomponent Colloidal and Interfacial Systems

Synergism is defined as an interaction of discrete agents such that the total effect is greater than the sum of the individual effects of agents acting alone. In chemical or pharmaceutical systems, the definition is normally refined to specify that the combined effect is greater than the effect of either agent at the same total concentration as was present in the multi-agent system. A well-known example in interfacial science is the principle of interfacial tension synergism, whereby the same degree of interfacial tension reduction is achieved in a mixed surfactant system at a lower total concentration than that required for any of the surfactants acting alone. Synergism is routinely exploited, by design or serendipitously, in the formulation of multicomponent commercial complex fluid products with enhanced functionality or more desirable physical attributes. The mechanisms underlying macroscopic performance enhancement in multicomponent formulations are often unknown. It is often difficult to predict the physical chemical behavior of a multicomponent system based on the behaviors of any of the system components acting in isolation. This is particularly problematic when synergism is present. This serves as a practical motivation for the current presentation, which will focus on less commonly noted examples of synergism in fundamental colloidal and interfacial phenomena. An equally important motivation is to challenge our natural reductionist tendency in fundamental investigations to focus on the effects of one critical system component. While the reductionist approach is needed to fully understand fundamental phenomena, when the purpose of the research area is to facilitate the design of new formulated systems, it is important to step back up in complexity to begin to approach the complexity of practical systems. Taking a “partially reductionist” approach can anticipate behaviors in complex systems while still revealing fundamental mechanistic insights. This presentation will include recent research to illustrate conditions that yield synergism in colloidal forces, particularly colloidal depletion forces in mixtures of water-soluble polymers and surfactants. It will also introduce the concept of solutal Marangoni transport synergism, which is related to interfacial tension synergism, but emphasizes the development in a synergistic manner of stronger interfacial flows in the presence surfactant mixtures. A final example will be interfacial tension synergism at the oil/water interface when the two bulk liquids introduce their own surfactants to the interface.