Interactions between large organic ions of opposite charge: III. The Effect of Molecular Structure on the Interaction between Sodium Alkyl Sulfates and Phosphonium Salts. The Alkyl Chain

Abstract

The effect of alkyl chain length on the interaction between sodium alkyl sulfates (anions) and various phosphonium compounds (cations) has been investigated by conductimetric titration. The logarithm of the stoichiometric solubility product (log K s ′) is linearly related to alkyl chain length and the free energy of transfer (ΔG F 0) of a CH 2 group from aqueous environment to the complex is found to be −3.2 KJ mole −1 for alkyl groups attached to the anion but only −1.0 kJ mole −1 for groups attached to the cation. The thermodynamic quantities ΔH F 0 and ΔS F 0 have been derived from studies on the temperature variation of log K s ′. The derived data support the view that the separated phase resulting from anion-cation interaction is liquid-like in nature (a coacervate). Enthalpy-entropy compensation effects and the variation of the temperature minimum in log K s ′ versus temperature plots with alkyl chain length are also described and discussed.