Surface Tension and the Scaled Particle Approach

About the earliest study of molten organic salts was the measurement of surface tensions by Walden of a number of alkylammonium nitrates, thiocyanates, and halides. Most recently the chlorides have been studied by Kisza and Hawranek. The surface tensions of these salts are similar to those for organic liquids and they range from 20 to 50 dyn/cm. These 

Walden s early study related the structure of the salt melts to the degree of alkylation. The negative of the temperature coefficient of the molar surface tension is the surface entropy and it had been shown empirically to be inversely related to the degree of association. Walden s surface tensions for salts with cations of the same molecular weight showed that the primary ammonium salts are the most highly associated, with the tertiary and quaternary the least associated. 

Recently Bloom and Reinsborough have measured the surface tension of pyrdinium chloride as well as tra nsport properties and used it to interpret micelle formation of this salt with cetyl and myristyl cationic soaps. Earlier Coleman and Prideaux studied mixtures of carboxylic acids with diethylamine and piperidine, but these are complicated systems which are too far afield of this discussion. 

The scaled particle solution to the hard-sphere model permits the calculation of the surface tension if the density and the size of the molecules are known. Mayer has shown the theory to apply to both nonelectrolytes and salts. The same solution of the hard-sphere model permits the calculation of the various thermodynamic properties such as the thermal expansion coefficient. Since the thermal expansion coefficient is known for most salts for which there are surface tension data, the former can be used to calculate the latter. 

Walden and Birr showed that for isomeric picrates the thermal expansion coefficients of the primary, secondary, and tertiary ammonium salts are greater than for the quaternary salts. Presumably the larger expansion coefficients occur in sustems which are less ionic because of the dissociation of the salt into amine and picric acid. This explanation is in agreement with the comparison of the isoelectronic salts and hydrocarbons in Section 2.2. For them the nonelectrolyte has the greater thermal expansion coefficient when the comparison is made at constant pressure and temperature, in agreement with Walden and Birr, while for a comparison at constant volume and temperature the salt has a greater coefficient. Thus nonelectrolyte components are in equilibrium with the primary through tertiary ammonium salts and the shifts in this equilibrium will be reflected in the thermal expansion coefficient and produce unreasonable estimates of the surface tension. 

---