Studies of the Association Mechanism and Diluent Effect Using Excess Volume

Systems of Tri-n-octylamine and lower carboxylic acids

Abstract

To investigate the association mechanism of tri-n-octylamine (TOA) with organic acids as well as the diluent effect on the association equilibrium of the systems, excess volumes different (υ^E) of the mixtures of TOA with four lower carboxylic acids and mixtures of TOA and acetic acid with five different diluents were measured at 30°C. For the ternary mixtures, evaluation of the diluent effect was based on an approximation of the pseudo-binary mixture of TOA and acetic acid.
The findings in the present investigation are as follows.
1) For the binary and pseudo-binary mixtures, the composition (X_B^min), which gives the minimum valume of υ^E varies according to the kinds of acids and diluents.
2) The dominant associating species of TOA with an acid can be estimated from the X_B^min. And the equilibrium constants of the association are obtainable from the model proposed in this study by fitting the observed υ^E values of the binary and pseudobinary mixtures.
3) There is a significant relationship between X_B^min and the acceptor number of the diluent. The stronger in acceptor characteristics the diluent is, the smaller the number of the acid associated with TOA becomes.