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DETERMINATION OF SATURATION VAPOR PRESSURE OF LOW VOLATILE SUBSTANCES THROUGH THE STUDY OF EVAPORATION RATE BY THERMOGRAVIMETRIC ANALYSIS

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Subject of Study.Research of vapor pressure of low volatile substances is a complicated problem due to both direct experimental implementation complexity and, most significantly, the issues faced correctness of the analysis and processing of experimental data. That is why it is usually required engaging the reference substances (with vapor pressures well studied). The latter drastically reduces the effectiveness of the experimental methods used and narrows their applicability. The paper deals with an approach to the evaporation process description (sublimation) of low volatile substances based on molecular kinetic description in view of diffusive and convection processes. The proposed approach relies on experimental thermogravimetricfindingsina wide range of temperatures, flow rates ofthe purge gas and time. Method. A new approach is based on the calculation of the vapor pressure and uses the data about the speed of evaporation by thermogravimetric analysis depending on the temperature, the flow rate of the purge gas, and the evaporation time. The basis for calculation is the diffusion-kinetic description of the process of evaporation (mass loss) of the substance from the exposed surface. The method is applicable to determine the thermodynamic characteristics for both the evaporation (the equilibrium liquid - vapor) and sublimation (the equilibrium solid - vapor). We proposed the appropriate method of the experiment and analysis of its data in order to find the saturated vapor pressure of individual substances of low volatility. Main Results. The method has been tested on substances with insufficiently reliable and complete study of the thermodynamic characteristics but, despite this, are often used (because of the other data limitations) as reference ones. The vaporization process (liquid-vapor) has been studied for di-n-butyl phthalate C16H22O4 at 323,15–443,15 К, and sublimation for benzoic acid C7H6O2at 303,15–183,15 К. Both processes have been carried in a stream of nitrogen N2 (20-250 ml·min-1); the duration of evaporation-sublimation (each TGA experiment) is 10 hours. As a result, the vapor pressure of these substances has been determined in a wide temperature range; analysis of the dependence for the evaporation coefficients on TGA experiment conditions has been carried out; recommendations on their choice for determination of the enthalpy of vaporization and sublimation of the evaporation rate have been given. Practical Relevance. The presented theoretical and experimental apparatus allows determining the vapor pressure by TGA method for wide classes of compounds with varying volatility (including low volatility). The proposed method requires only necessary data on isothermal evaporation (sublimation) and no standards. It is advisable to use this approach for the study of a wide range of high boiling compounds, such as pharmacologically active substances, oils, "green solvents", including ionic liquids, and others.

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