The Organic and Macromolecular Chemistry Program in the Chemistry Division at the National Science Foundation supports Professor Laurence S. Romsted at Rutgers University in New Brunswick NJ. He proposes to do three projects that combine the logic of pseudophase models with the power of his arenediazonium probe method. These projects will determine the concentrations of weakly basic nucleophiles, including water, in the interfacial regions of surfactant based association colloids, and rate constants for reactions with antioxidants in opaque emulsions. The results should give new insight into the contribution of ion-pairing and hydration to the balance of forces controlling aggregate morphology; new information on the conformations and orientations of polypeptides at membrane mimetic surfaces; and a new understanding of previously unattainable determinations of antioxidant reactivity and distribution in emulsions. Completion of the projects will deepen current understanding of the balance of forces controlling surfactant aggregate structure. They will aid in refining the "tuning" of the structures of soft materials and enhance their utility as encapsulating agents, thickeners, and fluids for bulk transport. The conformation and orientation of the HIV-1 surface glycoprotein, gp41, which is important in the cell fusion mechanism leading to AIDS, will be viewed at interfaces from a new chemical perspective. The creation of a scale of antioxidant efficiency will be of general utility and enhance both nutrition and health.
In a broader sense, Professor Romsted's research program includes: (a) advanced training for undergraduate and graduate students and postdoctoral fellows; (b) scientific collaborations, including student and faculty exchanges, with colleagues in the US, France, Sweden, and Brazil; (c) continued participation in national and international meetings and publication in international journals; (d) strengthening the teaching of a graduate course in "Supramolecules and Assemblies" and undergraduate organic chemistry; and (e) laying the conceptual basis for soft materials technologies. Thus, providing new insight into morphologies of membrane bound proteins, and improving safety and shelf-life of emulsified foods.
Introduction. Surfactant molecules are combinations of antipodal solubilities, a long, water insoluble tail, bonded to a polar, water soluble headgroup but with a wide range of structural possiblities, charged or uncharged polar organic groups such as polyoxyethylene chains or sugars. In aqueous solution, surfactants spontaneously self-assemble to form aggregates of various sizes such as micelles, vesicles, microemulsions and emulsions. All these surfactant structures enhance the solubility of organic molecules in water and also speed and inhibit chemical reactions. The structures of the aggregates formed depend on the concentration of the surfactant in water, but also on other additives such as oil, salts, and organic alcohols. The structures are sensitive to changes in solution compositions, e.g., the types of counterions whose affects often follow the Hofmeister series, but also depend on the type and amount of oil and alcohol. A delicate balance-of-forces, e.g., electrostatic, dipole, dispersion, hydrogen bonding and hydration, govern aggregate size and shape and substantial current research is aimed at understanding these interactions. Our approach is unique because we probe the properties of surfactant aggregates using chemical reactions. My group has developed a kinetic model for describing aggregate effects on chemical reactivity, the pseudophase ion exchange model, and during this grant period we demonstrated unequivocally that our chemical kinetic method, which is grounded in the pseudophase model, works on stirred or kinetically stable emulsions and provides a robust method for determining the distributions of antioxidants between the oil, interfacial and aqueous regions of emulsions. We also continued to develop the chemical trapping method in which an aggregate bound hydrophobic arenediazonium ion reacts with nucleophiles in the interfacial region to make stable products that can be analyzed by HPLC. The product yields can be converted to estimates of the interfacial molarity of a variety of ions and molecules such as halides, water, alcohols, amide bonds and nonionic surfactant headgroups. Chemical Kinetic Method in Emulsions. We, with Carlos Bravo-Diaz’ group in Spain, demonstrated for the first time that pseudophase models provide a robust method for determining: (a) the distributions of the important antioxidants, a-tocopherol, and, t-butylhydroquinone (TBHQ), in model food emulsions; (b) the thermodynamics of a-tocopherol, gallic acid and catechin distributions in model food emulsions; (c) the first interpretation of chemical reactivity in ionic emulsions using pseudophase emulsions; and (d) an interpretation of ion specific effects on chemical reactivity of TBHQ with the arenediazonium ion probe in zwitterionic emulsions. The immediate aim of our work is solving an important problem in food chemistry, understanding the distributions of antioxidants in emulsions. The results demonstrate that pseudophase models provide a natural explanation for the effect of antioxidant hydrophobicity on the distributions of antioxidants and that the distribution correlates with their efficiency. Esters of gallic and caffeic acids with alkyl groups of short to long lengths were prepared and their distributions were determined by the chemical kinetic method and compared with their bulk efficiencies as determined by a standard method. Both their distributions and the efficiencies pass through maximum values, i.e., the molarity in the interfacial region, for the same gallate and caffeate ester in each series. These results clearly demonstrate that the model and method provide a complete understanding of how antioxidants distribute in emulsions. Chemical Trapping Method and Related Work • Together with Reiko Oda’s group at Bordeaux University, we demonstrated gas phase specific ion effects on competing substitution and elimination reactions and that the results correlate with the predictions of density functional theory. This was a fundamental investigation into structure-reactivity relations in the gas phase. • Results from a chemical trapping study in anionic surfactants with amino acid headgroups show that the arenediazonium ion probe cleaves the amino acid from the tail at the amide bond. This result will support the application of the method to the cleavage of peptide bonds of proteins located in the interfacial region of membrane mimics and the determination of the topology and orientation. Journal Reviews, Book Chapters and Book In 2012 Romsted wrote an introduction to surfactant self-assembly for an encyclopedia, edited a book on surfactant science and technology, and in 2013 organized a special issue of Current Opinion in Colloid and Interface Science on chemical reactivity in colloidal systems, and contributed a chapter on modeling chemical reactivity in emulsions. These general publications are spreading the word on modeling chemical reactivity in surfactant systems. In Preparation Two manuscripts are focused on demonstrating that absence of a relationship between emulsion droplet size and chemical reactivity and another is on a unique observation of surface tension minima that is not caused by an impurity. Romsted is writing a Langmuir Feature Article with Carlos Bravo-Diaz that is a detailed treatment of the pseudophase model as applied to emulsions and a justification for why the model works.
