Heats of transport of aqueous tetraalkylammonium hydroxides and the electrophoretic effect

Abstract

A conductivity cell has been used to measure the Soret coefficients of dilute aqueous tetraalkylammonium hydroxides (R₄NOH; R = methyl, ethyl, propyl, butyl) at 25 °C. The heats of transport calculated from the Soret coefficients decrease as the concentration is increased. In contrast to this ‘normal’ behaviour, the heats of transport of dilute aqueous Bu₄NCl, Bu₄NBr and Bu₄NI decrease and then increase as the concentration is raised. Owing to the large differences in the radii and heats of transport of the NBu₄⁺ and halide ions, an abnormally strong electrophoretic effect is predicted for the tetrabutylammonium halides. The nearly identical heats of transport of the NBu₄⁺ and OH^– lead to a weak electrophoretic effect for aqueous Bu₄NOH, which may account for the normal concentration dependence of its heat of transport.