Effect of temperature on the limiting excess volumes of amines in aqueous solution

Abstract

Limiting partial molal volumes (V^°₂) from measurements of density in the concentration range 0.002–0.1 mol dm^–3 for aqueous solutions of NH₃, MeNH₂, EtNH₂, n-PrNH₂, N-BuNH₂, t-BuNH₂, (Me)₂NH and (Me₃)N have been obtained at 5, 15 and 25 °C. The method of Cabani and coworkers for applying hydrolysis corrections in dilute solutions of amines had to be modified slightly in order to make it applicable to the lower concentration range used in this work. It is shown that the modified method applies equally well to both the low (0.002–0.1 mol dm^–3) and slightly higher (0.1–0.3 mol dm^–3) concentration ranges, yielding identical results. The present results show that limiting excess volumes become less negative with increasing temperature for lower amines, but higher amines show an opposite trend. Comparison of volumetric properties of amines with those of alcohols indicates that in monofunctional solutes the functional group plays an important role in solute–solvent interaction. It is found in particular that the —NH₂ group tends to offset the structure promoted by the hydrophobic group in amines.