The use of densimetry and fixed-cell scanning calorimetry to probe solute and solution structure and reactions from 270 K to 400 K
Résumé
Abstract We have used a vibrating-tube densimeter to investigate the densities and volumetric properties of aqueous solutions, and a fixed-cell scanning calorimeter to measure aqueous solution heat capacities. Solutions investigated include sugars, neutral weak acids and bases, neutralized acids and bases, neutral and protonated amino acids, surfactants, and both strong and ion-paired electrolytes. We have made measurements at molalities m from 0.005 mol·kg−1 to 0.5 mol·kg−1, at temperatures T from 278.15 K to 393.15 K, and at the pressure P = 0.35 MPa. We have calculated the apparent molar volumes Vφ and apparent molar heat capacities Cp,φ of the solutions and fit them by regression to equations that describe the surfaces (Vφ, T, m) and (Cp,φ, T, m). These results reflect changes and differences in both solute-solute and solute-solvent interactions. Standard state partial molar volumes V2° and heat capacities Cp,2° were estimated by extrapolation to the M = 0 mol·kg−1 plane of the fitted surfaces. We have calculated (Δr,Cp,m, T, m) surfaces for various proton dissociation reactions, and we have created surfaces representing (ΔrHm, T, m) and (pQa, T, m) by integration of our (ΔrCp,m, T, m) surfaces using values for (ΔrHm, m) and (pKa, m) at a reference T as integration constants. We have also created surfaces representing (ΔrSm, T, m) and (ΔrVm, T, m) for these dissociation reactions.