No abstract

Densities and osmotic coefficient measurements for dilute aqueous solutions of glycine, l-leucine, and glycylglycine have been reported at 298.15 K. The partial molar volumes and activity coefficients of solute as well as solvent have been estimated using the density and osmotic coefficient data, respectively. Excess and mixing thermodynamic properties, such as Gibbs free energy, enthalpy, and entropy changes, have been obtained using the activity data from this study and the heat data reported in the literature. The concentration enthalpy-entropy compensation effects have been observed for the studied systems, and the compensation temperatures are reported. It has been observed that the excess free energy change for all the studied systems is almost the same over the studied concentration range, showing that the differences in properties of such solutions are largely decided by the enthalpy-entropy effects. These results, along with partial entropy data, show the effects of the presence of hydrophobic interactions and water structure making effect in the case of aqueous solutions of l-leucine. The application of the Starikov-Norden enthalpy-entropy compensation model yielded information about a "hidden Carnot cycle" and the existence of multiple microphases. Application of the Kirkwood-Buff (KB) theory of solutions for the studied systems yields pair correlation functions between the components. The variation of Kirkwood-Buff integrals with concentration further signifies the concentration dependence of the hydrophobic hydration and interactions in the solution phase. The osmotic second virial coefficients have also been obtained using the KB theory and show good agreement with those obtained using the McMillan-Mayer theory of solutions. The mean square concentration fluctuations is estimated using the KB theory, which gives information about the microheterogeneity in the solution phase, which further reflects the presence of hydration and solute association.

Purine and pyrimidine nucleobases are of central attraction in biological and chemical sciences as these bases or their derivative are primary components in genetic materials. The solubility of these components is the main concern and puts certain limitations on their use in laboratory synthesis of medicinally important molecules. In this work, efficient conversions of purine and pyrimidine nucleobases (namely uracil, thymine, adenine, guanine, xanthine and hypoxanthine) into corresponding completely water soluble salts are reported. All the reported salts are room temperature ionic liquids except xanthine salts which have melting points higher than 100 °C and hence cannot be called as ionic liquids. Synthesized room temperature ionic liquids from nucleobases were well characterized using spectral and thermal analysis. Experimental data of some physicochemical properties such as density, conductivity, glass transition temperature, melting temperature as well as decomposition temperature reported which shows that the reported ionic liquids are stable upto 200 °C.

In the present study, we have deposited hierarchical flower-like microstructured zinc oxide (ZnO) thin films directly on a glass substrate by using the simplistic aqueous chemical route for different concentrations of triethanolamine (TEA) which acted like a complexing agent. The as-synthesized ZnO thin films were subsequently annealed at 300 °C and are characterized with characterization techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), photoluminescence (PL), and electrical resistivity. The hexagonal wurtzite crystal structure of as-synthesized ZnO thin films was confirmed by their XRD patterns and the well-resolved ZnO flowers-like morphology was revealed from the FESEM micrographs. From FESEM images it can be seen that the ZnO flower is composed of dozens of nanorods originating from the same core in a symmetric fashion with an average diameter of around 180-300 nm. The flower-like morphology was obtained at 0.3 M TEA concentration. Due to its hierarchical structure, the deposited ZnO thin films were employed for multiple applications such as gas sensing and anti-microbial activity. The ZnO thin films with micro-flowers like morphology showed the maximum gas sensor sensitivity ∼64.50 at 150 °C for 100 ppm of NO2 gas. Moreover, the bacteria were completely destroyed in the presence of as-deposited ZnO thin films.

Measurements for osmotic pressure and densities for lysozyme in aqueous and aqueous solutions containing urea have been reported at 298.15 K using a vapor pressure osmometer and vibrating tube digital density meter, respectively. The partial molar volumes and activity coefficients of both the components and the osmotic pressure of the solutions have been calculated. The molar mass and second virial coefficient of lysozyme have been obtained using the McMillan–Mayer theory of solutions and are compared with the values obtained in aqueous salt solutions. The Guggenheim–Stokes equation for osmotic coefficient has been used to obtain the protein molecular diameter in an aqueous medium. The estimated diameter of lysozyme is similar in magnitude to that in the pure state, confirming the retaining of the native state of the protein in aqueous solutions over the studied concentration region. These studies have been extended to aqueous–urea (∼2.0 mol·kg–1) solutions at 298.15 K. Data of water activity and activity coefficient of all the three components in ternary solutions as a function of lysozyme concentration have been determined by the methodology developed by us earlier. The data have been further analyzed to obtain Gibbs free energy changes of transfer of lysozyme as well as that urea from corresponding aqueous binary to aqueous ternary solutions through which information about the binding of denaturant to protein can be derived. The analysis further reveals that the binding of urea to the protein molecules occurs through the breakdown of protein–water H–bonds leading to the opening of a coiled structure of lysozyme. All these results are discussed in terms of protein hydration, protein–protein hydrophobic interactions, and protein denaturant binding equilibria for the solutions of lysozyme.