Non-Linear Taft Relationship applied to surface tensions of aliphatic acids: Inter-molecular hydrogen bonding versus intra-molecular hydrogen bonding

Hammett and Taft equations in their most basic forms are linear free energy relationships between standard free energies of two reaction series. Therefore any chemical or physical property inherently associated with any of the thermodynamic property like enthalpy H, free energy G, internal energy E or entropy S are subjected to substituent effects. One such physical property is the surface tension. Surface tension is a kind of surface free energy (G) of any liquid and is also supposed to be effected by the structure of liquids. Hence the present study has been selected to see the effect of substituents on surface tension of some aliphatic acids. The quick glance at the present study ended up with a non-linearity of Taft equation to the surface tension data of aliphatic acids. Hence Non-Linear Taft Relationship (NLTR) was applied to surface tensions (γ) of some aliphatic acids. The non-linearity of log γ versus Taft σ* correlation was explained in terms of inter-molecular hydrogen bonding versus intra-molecular hydrogen bonding and in terms of steric effects.