This International Collaboration in Chemistry award, co-funded in France by the Agence Nationale de la Recherche (ANR) and in the United States by the National Science Foundation, through the Macromolecular, Supramolecular, and Nanochemistry Program of the Chemistry Division, supports an international collaboration between Drs. Laurence Romsted, David Case, and Ronald Sauers, all from Rutgers University, New Brunswick, and Drs. Reiko Oda, Michel Laguerre (both from the Institut Europeen de Chimie et de Biologie, France), and Dario Bassani (from the Institut des Science Molecularier CNRS, France). The fundamental goal of the project is the characterization of ion-specific effects on the properties of aqueous solutions of primarily gemini amphiphiles. The team will systematically vary amphiphile structure and use a wide variety of counterions to identify the shifts in the balance of forces controlling aggregate structure from the molecular level to bulk solution properties. Cationic gemini amphiphiles with decyl and dodecyl tails will be synthesized, with fully and partially methylated headgroups and a variety of counterions, e.g., the typical inorganic counterions of the Hofmeister series, but also organic counterions, such as alkylcarboxylates and alkylphosphates of various chain lengths. By using a combination of physical, chemical, and simulation methods, the researchers will determine correlations between physical properties (e.g., critical micelle concentration, ionization degree, and aggregation number and interfacial concentrations of the counterions and water), as determined by chemical trapping and by simulation using a combined molecular dynamics/density functional theory approach. The simulations will also provide new information on the overall organization of the tails, headgroups, counterions, and water within the aggregate. Parallel studies on single chain analogs will highlight the importance of the methylene bridge of the gemini surfactants. Because chemical trapping and molecular dynamic simulations make no assumptions about aggregate structure, correlations discovered between amphiphile structure, counterion type and aggregate physical properties will deepen current understanding and provide new insight into the delicate balance of forces that controls aggregation behavior of association colloids.
One of the oldest, unresolved challenges in colloid and surface chemistry is a coherent explanation for ion-specific effects on the properties of colloids and biointerfaces, first demonstrated in 1888 by Hofmeister as a specific salt effect on the solubility of a protein. Since then, many empirically observed orders have been published including ion-specific effects on a variety of aqueous solution properties of self-assembling ionic association colloids such as micelles, microemulsions and vesicles. This collaborative project offers a systematic approach toward understanding the interplay of intermolecular forces controlling amphiphile aggregate structures. Results should have real-life impact by aiding in selection of amphiphiles for particular applications (currently a trial-and-error procedure) and should enhance the utility of soft materials, e.g., by tuning their structures simply by changing the counterion. This project's novel multidisciplinary approach benefits from the complementary expertise of six collaborators in two countries: Drs. Oda and Bassani will obtain precise physical property measurements on the aggregation; Dr. Romsted will use his group's chemical trapping method to determine interfacial water and counterion concentrations; simulations by Drs. Laguerre, Case, and Sauers should provide new insight into specific interactions between headgroups, counterions, and water at gemini micelle interfaces. This research collaboration will also provide advanced training for undergraduate and graduate students and postdoctoral fellows. They will be immersed in a fundamental and intellectually challenging project that includes experience with a variety of experimental approaches combined with simulations in laboratories in the US and France, and in discussions on characterizing amphiphile solutions at the aggregate and molecular levels.
