Accurate quantification of hydration number for polyethylene glycol molecules

Hydration water can even decide the physicochemical properties of hydrated organic molecules. However, by far the most important hydration number for organic molecules, in particular polyethylene glycol which we are concerned with here, usually suffers from a large discrepancy. Here, we provide a scheme for accurate and unambiguous quantification of the hydration number based on the universal water-content dependence of glass transition temperature for aqueous solutions, testified by experimental results for polyethylene glycol molecules of a molar weight ranging from 200 to 20000.Moreover, we also clarify the fundamental misunderstanding lying in the definition and quantification of hydration water for PEG molecules in the literature, therein the hydration number for PEG in water-rich solutions has been determined at a critical concentration, across which the properties of the solution display obviously distinct water-content dependence.

Effects of Concentration and Freeze-Thaw on the First Hydration Shell Structure of Zn^(2+) Ions

To investigate the effects of salt concentration and freeze-thaw (FT) on the first hydration shell of Zn2+ ions in Zn(NO_3)_2 aqueous solutions, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to examine Zn K-edge EXAFS spectra of Zn(NO_3)_2 aqueous solutions with various concentrations before and after FT treatment. The influences of salt concentration and freeze-thaw on the structural parameters of the first hydration shell of Zn2+ ions, including hydration number, Zn-O distance and thermal disorder, were analyzed. The results show that Zn2+ ions have 3.2―6.8 nearest oxygen neighbors with the Zn-O distance being 0.202―0.207 nm. In highly concen-trated solutions, Zn2+ ions are hydrated with four water molecules in a tetrahedral form. The dilution of Zn(NO_3)_2 aqueous solutions increases the number of water molecules in the first hydration shell of Zn2+ ions to six with their octahedral arrangement. Both the hydration number in the first hydration shell of Zn2+ ions and the degree of thermal disorder increase when the FT treatment is operated in Zn(NO_3)_2 aqueous solutions.

Ultrasonic Studies of 4-Aminobutyric Acid in Aqueous Salbutamol Sulphate Solutions at Different Temperatures

Ultrasonic speeds of 4-aminobutyric acid in 0.0041,0.0125 and 0.0207 mol·kg^-1 aqueous salbutamol sulphate（SBS） solutions are measured at 308.15,313.15 and 318.15 K.Isentropic compressibility kS,change in isentropic compressibility△k S,relative change in isentropic compressibility（ △kS /kS^0） ,apparent molal compressibility kφ,limiting apparent molal compressibility kφ^0 ,transfer limiting apparent molal compressibility k φ^0,hydration number nH,pair and triplet interaction parameters kAS,kASS are estimated.The above parameters are used to interpret the solute-solute and solute-solvent interactions of 4-aminobutyric acid in the aqueous salbutamol sulphate solutions.

Effect of Temperature on Volumetric and Viscometric Properties of Homologous Amino Acids in Aqueous Solutions of Metformin Hydrochloride

Density（ρ）and viscosity（η）are measured for glycine,DL-α-alanine DL-α-valine,and DL-α-leucine in 0.05,0.10,0.15 and 0.20 mol·L^-1aqueous metformin hydrochloride at 308.15,313.15 and 318.15 K.The measured values are used to estimate some important parameters,such as partial molal volume Vφ,standard partial molal volume Vφ^0,transfer volume ΔVφ^0,hydration number nH,the second derivative of infinite dilution of partial molal volume with respect to temperature,viz., ^2 Vφ^0 /T^2,viscosity B-coefficient,variation of B with temperature,viz., dB/dT,free energy of activation per mole of solvent Δμ1^＊0 and solute Δμ2^0＊ of the amino acids.These parameters are interpreted in terms of solute-solute and solute-solvent interactions and structure making/breaking ability of solutes in the given solution.In addition,Vφ^0,0 ΔVφ^0,viscosity B-coefficient,ΔB and Δμ2 ^0＊ are split into group contributions（NH3^＋ COO ^-）and -CH2 of the amino acids using their linear correlation and their behavior is discussed.