Estimation of thermophysical properties of binary organic solutions- surface tensions

Abstract

Thermophysical properties present an interesting field of research to understand molecular nature and behaviour. The primary reason is that these properties mirror any change at the molecular level. One such property is surface tension. The definition of surface tension as the surface free energy per unit area is seemingly innocuous, but it has much depth and, in some sense, is the catalyst for this investigation. Earlier work on this property focused on developing methods of measurement. For a long time, the emphasis was on the accuracy of measurement and an understanding of the forces that affect surface tension. It was also realised that interactions between groups (such as –CH?–, –CH?) affect surface tension and could therefore be used as a means of predicting it. This led to a stream of research on group contributions, which yielded established methods such as UNIQUAC, ASOG, and UNIFAC. It was felt that there was a need to study molecular interactions through the measurement of surface tension. The development of a correlation to predict surface tension was the second part of this problem. A homologous series was chosen for this study. Solutions of benzene, toluene, and xylene (BTX) with n paraffinic alcohols (methanol to octanol) were considered. It was observed that these components are miscible in all proportions. The surface tension was measured by well established methods such as the drop weight method and the du Noüy tensiometric method. From the plot of experimental surface tension data vs. mole fraction of solute (BTX), there was clearly a gradation of properties, with ethanol being an exception owing to its high degree of hydrogen bonding. The curves for methanol and ethanol showed minimum deviation from ideality. The addition of BTX only went so far as to break the degree of association (hydrogen bonding); this could perhaps explain the near linearity. The behaviour of the other solutions showed a sluggishness at low BTX concentrations, indicating the effect of steric hindrance as well. Using group contribution methods (UNIFAC), the activity coefficients were calculated, from which the Gibbs excess free energy (?G?) was obtained. The curves of excess free energy vs. surface tension showed interesting trends. It was also observed that the surface tension corresponding to the maximum ?G? was nearly proportional to the chain length of the alcohol concerned, from pentanol onwards. A correlation was developed based on the UNIFAC group contribution method for n alcohols–BTX systems at 25?°C. The calculated activity coefficients were correlated with the surface tensions of the alcohols. It is seen that there is an excellent correspondence between the experimental data and calculated data. This correlation may be considered a pseudo theoretical correlation-simple but effective. It is quite general in its ability to handle binary solutions where one component is an n paraffinic alcohol. It can be claimed that this correlation considerably reduces the tedium of long calculations.