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 v olumetric properties of aqueous solutions, and a fixed-cell scanning calori meter to measure aqueous solution heat capacities. Solutions investigated i nclude sugars, neutral weak acids and bases, neutralized acids and bases, n eutral and protonated amino acids, surfactants, and both strong and ion-pai red 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 vol umes V-phi and apparent molar heat capacities C-p phi of the solutions and fit them by regression to equations that describe the surfaces (V-phi, T, m ) and (C-p,C-phi, T, m). These results reflect changes and differences in b oth solute-solute and solute-solvent interactions. Standard state partial m olar volumes V(2)degrees and heat capacities C(p,2)degrees were estimated b y extrapolation to the m = 0 mol(.)kg(-1) plane of the fitted surfaces. We have calculated (Delta C-r(p,m), T, m) surfaces for various proton dissocia tion reactions, and we have created surfaces representing (Delta H-r(m), T, m) and (pQ(a), T, m) by integration of our (Delta C-r(p,m), T, m) surfaces using values for (Delta H-r(m), m) and (pK(a), m) at a reference T as inte gration constants. We have also created surfaces representing (Delta S-r(m) , T, m) and (Delta V-r(m), T, m) for these dissociation reactions. (C) 2001 Elsevier Science B.V. All rights reserved.