Interaction-component analysis of the urea effect on amino acid analogs.

Transfer energetics from pure water to a urea-water mixture is examined for a set of amino acid analog solutes by using molecular dynamics simulation and free-energy calculation. The free energy of transfer from pure-water solvent to 8 M urea-water mixed solvent is calculated for each solute, and the urea-water mixture is shown to be a more favorable solvent than pure water. The correlation of the transfer free energy is then examined against the corresponding changes upon transfer of the average sum of solute-solvent interaction energy and its electrostatic and van der Waals components. A strong correlation is observed against the change in solute-solvent energy, with a dominant contribution from the van der Waals component for neutral solutes. The electrostatic component exhibits a weak correlation due to the compensation between the contributions from urea and water. The transfer free energy is further decomposed into the contributions from urea and water within an approximate framework of the energy-representation theory of solvation. It is found that urea makes a favorable contribution to the transfer free energy. The contribution from water depends on the hydrophobicity/hydrophilicity of the solute. Urea and water act cooperatively for hydrophobic solute, and are competitive against each other for hydrophilic solute. The effect of excluded volume is also addressed, and is seen to be minor in the transfer energetics due to the compensation between the contributions from urea and water

Unraveling the glass-like dynamic heterogeneity in ring polymer melts: From semi-flexible to stiff chain

Ring polymers are an intriguing class of polymers with unique physical properties, and understanding their behavior is important for developing accurate theoretical models. In this study, we investigate the effect of chain stiffness and monomer density on static and dynamic behaviors of ring polymer melts using molecular dynamics simulations. Our first focus is on the non-Gaussian parameter of center of mass displacement as a measure of dynamic heterogeneity, which is commonly observed in glass-forming liquids. We find that the non-Gaussianity in the displacement distribution increases with the monomer density and stiffness of the polymer chains, suggesting that excluded volume interactions between centers of mass have a stronger effect on the dynamics of ring polymers. We then analyze the relationship between the radius of gyration and monomer density for semi-flexible and stiff ring polymers. Our results indicate that the relationship between the two varies with chain stiffness, which can be attributed to the competition between repulsive forces inside the ring and from adjacent rings. Finally, we study the dynamics of bond-breakage virtually connected between the centers of mass of rings to analyze the exchanges of inter-molecular networks of bonds. Our results demonstrate that the dynamic heterogeneity of bond-breakage is coupled with the non-Gaussianity in ring polymer melts, highlighting the importance of bond-breaking method in determining the inter-molecular dynamics of ring polymer melts. Overall, our study provides insights into the fundamental mechanism of ring polymers and sheds light on the factors that govern their dynamic behavior.Comment: 10 pages, 7 figure

NMR Study of Water Structure in Supercritical States (INTERFACE SCIENCE-Solutions and Interfaces)

The proton chemical shift of water is measured at temperatures up to 400°C and densities of 0.19, 0.41, 0.49, and 0.60 g/cm3. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in the gas phase. Comparison of the observed chemical shift to that of a solitary water molecule in an organic solvent shows that the hydrogen bonding persists in the supercritical water. At each density, the strength of the hydrogen bonding is found to reach a plateau value at high temperatures

A unified perspective on preferential solvation and adsorption based on inhomogeneous solvation theory

How cosolvents affects solvation has been revealed through the independent determination of solute-solvent and solute-cosolvent interactions guaranteed by the phase rule. Based on the first principles of inhomogeneous solvation theory, we present here a general matrix theory encompassing both preferential solvation and surface adsorption. The central role of the stability conditions that determine how many excess numbers (surface excesses) are independently determinable, have been clarified from the first principles. The advantage of the inhomogeneous approach has been demonstrated to be in its ease in treating solvation and adsorption in a unified manner, while its disadvantage, for example in membrane dialysis experiments, can be overcome by the inhomogeneous-homogeneous conversion