The use of densimetry and fixed-cell scanning calorimetry to probe solute and solution structure and reactions from 270 K to 400 K

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 V_φ and apparent molar heat capacities C_p,φ of the solutions and fit them by regression to equations that describe the surfaces (V_φ, T, m) and (C_p,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 C_p,2° were estimated by extrapolation to the M = 0 mol·kg⁻¹ plane of the fitted surfaces. We have calculated (Δ_r,C_p,m, T, m) surfaces for various proton dissociation reactions, and we have created surfaces representing (Δ_rH_m, T, m) and (pQ_a, T, m) by integration of our (Δ_rC_p,m, T, m) surfaces using values for (Δ_rH_m, m) and (pK_a, m) at a reference T as integration constants. We have also created surfaces representing (Δ_rS_m, T, m) and (Δ_rV_m, T, m) for these dissociation reactions.