| The osmotic and activity coefficients of the solution composition are an important part of the research of solution thermodynamics. Activity is the foundation of solution chemistry theory research. Amino acid is the basic component of protein and polypeptide, moreover glycine is a kind of amino acids, so the study of thermodynamic properties of glycine solution system will provide important basic datas in order to make further explorations of the structure and properties of amino acids and peptides. For glycine solution system, due to the lack of a reliable experimental method. It is so difficult to obtain the accurate osmotic coefficients data, especially when the concentration is less than 0.20 mol/kg, that the glycine solution system is lack of accurate activity data for a long time, eventually leading to the various thermodynamic properties of the glycine solution research limited.On the application of a new type of isopiestic method, this paper attempts to obtain the osmotic coefficient of low concentration (less than 0.20 mol/kg) of glycine solution system. The main research contents are as follows:(1)Using the isopiestic method, equilibrium concentration and water activity at 298.15 K and 308.15 K were measured for the Glycine solution system with the concentration range from 0.020 mol/kg to 1.279 mol/kg. And calculated the osmotic coefficient at 298.15 K, the calculated results in the average standard deviation was 0.0078,308.15 K the standard deviation was 0.0096.(2)Based on the simplified form of Pitzer equations, the experimental determination of activity datas were fitted under 298.15 K and 308.15 K, the fitting parameters were as follows:at 298.15 K, CФ=0.01943, BФ=-0.09819, R2=0.98491; at 308.15 K, CФ=0.01753,BФ=-0.08378, R2=0.94515. We compared the fitting calculated values with the experimental values using the Pitzer parameters, the maximum relative deviation was less than 1%. It found that the results coincided well. At 298.15 K and 308.15 K, the Pitzer model can better describe the thermodynamic properties of glycine solution system. Compared the fitting calculation value with the literature value, the maximum relative deviation was less than 0.020, the results showed that the experimental data as a whole was relatively reliable. In order to reasearch the interaction between glycine molecules and water molecules, the activity data of the glycine system was fitted by the normal model, the fitting parameters were as follows:At 298.15 Kω=0.38 kJ/mol; at 308.15 K,ω=0.41 kJ/mol.(3)We calculated the dissolving balance constant Ks, excess gibbs free energy GE, standard gibbs free energy △rGmθ and the heat enthalpy change △rHmθ under 298 K to 308 K. At 298.15K, we calculated △fGmθ、△fHmθ and △fSmθ of NH+CH2COO-. The results showed that the dissolved constant increased with the temperature rises, indicated that raising the temperature could promote glycine dissolving reaction. The excess gibbs free energy of the system was negative, and its values decreased with the concentration increasing of glycine solution, the phenomenon could be explained by the electrostatic interaction model and the properties of the mixed solvent. Migration of standard gibbs free energy was less than zero. We concluded that glycine was in a state of lower free energy in aqueous solution. Enthalpy change value △H>0, indicated that the glycine dissolving reaction were endothermic.(4)Analyzed and discussed the interaction between glycine molecules in Glycine-aqueous solution system. Using the Ф～m and y-m diagrams, found that the activity coefficient and osmotic coefficient value decreased with the increase of glycine solution concentration; In order to further study the interaction of amino acid molecules. The dy/dm-m was plotted, found that the activity coefficient of the glycine solution at 298.15 K was varies with the change of concentration degree more than 308.15K. |
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