Structure, dynamics and solvation in heterogeneous environments

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Prof. Branka Ladanyi is supported by an award from the Chemical Theory, Models and Computational Methods program for research dealing with systems in which fluid interfaces or microheterogeneities play an important role. Several such systems are considered. One set includes fluids confined either in surfactant assemblies on in silica nanopores. Another concerns the properties of water in solvation shells of molecules of biological relevance. The methods of approach involve statistical mechanical theory and computer simulation. The work on fluids in surfactant assemblies deals with two types of systems. One of these is reverse micelles (RMs) in which surfactant-coated water droplets are dispersed in a continuous nonpolar phase. Atomistic RM models are being developed and their properties are being investigated as functions of water content w0 = [H2O]/[surfactant], a parameter closely related to water droplet size. Following up on their work on model development of RMs formed by anionic surfactant AOT, the research group is working on developing an atomistic model for RMs formed by the cationic surfactant CTAB (cetyltrimethylammonium bromide), determining the structure and dynamics as a function of w0 and comparing the results to relevant experimental data. The next task is to investigate solvation of molecular ions in RMs, determining how solute-surfactant electrostatic interactions influence solute location within the RM and how they are reflected in the solute time-resolved infrared spectra. The other type of system under study contains water and perfluorooctane sulfonic acid (PFOS) surfactant and serves as a model for fuel cell membrane materials. Water and ion mobility is being investigated in these systems as water content varies and the surfactant undergoes transitions between lamellar and hexagonal phases. The work on fluids in silica nanopores builds on earlier studies of the properties of water confined in approximately cylindrical pores of varying diameter. Research is also being made into calculating and analyzing the observable in quasi-elastic neutron scattering (QENS) of water confined in nanopores and investigating how pore hydrophilicity and anisotropy influence molecular rotational and translational dynamics detectable by QENS. Studies are also being made into the interfacial structure and dynamics of liquids composed of molecules that are anisotropic in shape and polarizability, starting with benzene and its fluorinated analogs. Contact with experiment is made by modeling collective polarizability anisotropy dynamics observable by optical Kerr effect. The work is in collaboration with research groups at the University of Perugia in the study of aqueous oligosaccharide solutions using depolarized light scattering (DLS) and molecular dynamics (MD) simulation. The goal of this work is to map out and develop a better understanding of the dynamics of water in solvation shells of biomolecules. The approach includes measuring DLS spectra of solutions at varying concentrations of biomolecules, modeling the spectra via MD and molecular theory and analyzing them in terms of contributions from different species, their interactions and correlations. In addition to oligosaccharides, the project includes studies of aqueous solutions of oligopeptides and proteins.These studies deal with liquids confined in nanopores and surfactant assemblies as well as with complex aqueous mixtures, using methods of theoretical and computational chemistry. Understanding the properties of these systems at the molecular level is important in areas beyond physical chemistry. Aqueous interfaces with ionic surfactants are a feature of biological systems such as proteins and cell membranes. Understanding such systems is of technological importance, related to electrochemistry, fuel cells, oil recovery, and environmental clean-up, among others. Understanding the properties of liquids confined in nanopores is important in heterogeneous catalysis, separations, lubrication, and microfluidics. This research in the properties of aqueous solutions provides information on water interactions with biomolecules and about the molecular basis for biopreservation. Students and postdocs working on this project are learning theoretical and computational methods of statistical mechanics of many-body systems and applying them to systems and phenomena relevant to chemistry, materials science, and biochemistry. Since the proposal includes collaborative work with experimental groups, they have the opportunity of learning how their work applies to real systems and how the quantities that they are calculating are measured.