Dielectric properties of urea and acetamide in aqueous solution

Abstract

The dielectric dispersion of urea and acetamide in aqueous solution can be reproduced fairly well by either two Debye relaxations or one Cole–Cole relaxation, with very little room for deciding in favour of one alternative or the other. There appears to be a small additional high-frequency relaxation in urea, revealed only in the measurements at 70 GHz. The amplitudes of the two Debye dispersions are compatible with a two-component water/solute model without ‘irrotational’ water or solute hydration. In keeping with this, the static relative permittivities of urea, acetamide and other amides (based on data from other sources) are analysed in terms of partial quantities whose values indicate changes in the electrical contributions of water and solute. In addition, a simple group-additivity model splitting the solute into apolar ‘holes’ and polar radicals provides a degree of rationalization in the static dielectric behaviour of various substituted amides. It is seen that concentrated urea, simple amides and N,N-dialkyl amides are all characterized by the very high permittivity of water, proportionately much larger than the accompanying changes of partial specific volume, enthalpy and entropy. It is suggested that the pronounced electrostatic modification of water may play a role in some of the disruptive effects of amides on proteins and viruses.