Permittivity at 70 GHz of water in aqueous solutions of some amino acids and related compounds

Abstract

The effects of some 15 organic solutes in aqueous solution, including urea, acetamide, simple amino acids and peptides, upon the dielectric dispersion of water have been studied by measuring the complex permittivity at 70 GHz and 20 °C. The results are expressed by two quantities derived from a simple model according to which water is the only significant contributor to the dielectric loss factor ε″. These quantities are the apparent dielectric relaxation frequency of water, ω₁, and the limiting high-frequency permittivity of the solute, ε_{2 a ∞}. In general, with exceptions discussed, ω₁ is decreased in presence of the solutes, a result of practical significance in increasing the already dominant role of modified water in the heating of biological materials by microwaves. A correlation with the apolar composition of the solute is noted. The values of ε_{2a ∞} are generally not much different from that of pure water. It seems therefore that, like water itself, these solutes must traverse a further dispersion so as to attain the optical refractive index. A few measurements at different temperatures suggest that ε-aminocaproic acid (6-aminohexanoic acid) and bovine plasma albumin cause a small decrease in the activation entropy of water. An analogy between dielectric relaxation and thermal migration is noted and illustrated by analysis of the Soret coefficients of amino acids in aqueous solution.