Electrostriction around colloid molecules and interfacial action of inhalation anesthetic: Volume function

Poly( -lysine) exists as a polyelectrolyte in an aqueous solution with charged -NH₃⁺ of the side-chain terminals at pH values below 10.5, while it loses the charges above this pH. Due to the electrostatic repulsion, the conformation of the charged form is random coil while that of the uncharged form is helix. The densities of each form were measured by an oscillation densimeter at several poly( -lysine · HBr) concentrations and the apparent molal volumes were estimated. By extrapolating the apparent molal volumes to infinite dilution, the partial molal volumes of each form at infinite dilution were obtained. When expressed by the partial molal volume per residue at infinite dilution, the values were 125.3 cm³ residue⁻¹ for the uncharged form and 112.8 cm³ residue⁻¹ for the charged form at 298.15°K. From the temperature dependence of the partial molal volume, the partial molal expansibilities were found to be 0.070 cm³ residue⁻¹ deg⁻¹ for the uncharged form and 0.106 cm³ residue⁻¹ deg⁻¹ for the charged form. The smaller partial molal expansibility of the uncharged form compared to the charged form is in agreement with the general pattern that hydrophobic macromolecules show smaller expansibility than hydrophilic macromolecules. An inhalation anesthetic, methoxyflurane, did not alter the volume of the uncharged form and expanded only the charged form. At the anesthetic concentration of 1.7 × 10⁻³ m, the partial molal volume of the charged poly( -lysine · HBr) was expanded by 0.27%. The partial molal volume of methoxyflurane in aqueous solution was 108.5 cm³ mole⁻¹ at 278.15°K while that of the pure liquid state was 113.1 cm³ mole⁻¹. The decrease of the partial molal volume of methoxyflurane in aqueous solution is attributable to the structuring of water molecules around the anesthetic. The partial molal volume of the anesthetic in the 1.0 × 10⁻⁴ m charged poly( -lysine · HBr) solution was 110.9 cm³ mole⁻¹. This increase of the partial molal volume of methoxyflurane in the peptide solution indicates that the anesthetic-water contact is partially destroyed by the binding.