Volumetric investigation of the hydrotropic action of aqueous sodium propionate solutions on pentanol at 298 K

The miscibility range for water-sodium propionate-pentanol mixtures has been determined at 298 K. The phase equilibria are compared with 'normal' and 'detergentless' microemulsion systems. Sodium propionate imposes a very slight salting out action (if any) on pentanol in dilute aqueous solutions. However, pentanol gains full miscibility with concentrated salt solutions. Volumetry was chosen to detect the interactions between the components since it has been found to monitor both changes in the environment of the hydrocarbon chains as well as large shifts in the polar association equilibria. According to the apparent molar volumes the enhanced solubility of pentanol is apparently obtained through dissociation of the alcohol complexes. Interestingly the concentrated hydrotrope solvent seems to be apolar in character. The state of the hydrotrope is almost entirely determined by the relative amount of (hydration) water. The interaction between pentanol and sodium propionate also seems to be of importance in dilute pentanol solutions. The absence of extended association structures in the system explains the difference between hydrotropy and normal solubilization, which is also evidenced by the volumetric behaviour of pentanol. On the other hand, the restriction of the mutual miscibility of the hydrotrope and the alcohol to the region where all the water is considered to be bound as hydration water of the ions marks the distinction between these systems and 'detergentless microemulsions'.