Thermodynamics of (water + hexamethylphosphoramide) mixtures: Heat capacity properties in the temperature range between 283.15 K and 298.15 K at ambient pressure

[Display omitted] •Molar and excess molar heat capacities of binary (water + hexamethylphosphoramide) mixtures were derived calorimetrically.•Experimental measurements were carried out at T = (283.15 to 298.15) K and ambient pressure over a whole composition range.•Three main composition regions whose interaction-related behavior are largely temperature-dependent were argued by results.•Excess heat capacities are positive up to HMPA mole fraction (xh) ~ 0.9 and their temperature trend is changed at xh ~ 0.33.•Unlike standard heat capacity of HMPA in water, this quantity for water in HMPA is negative and increasing with temperature. The specific (cp) and molar (Cp) heat capacities of the mixtures of hexamethylphosphoramide (HMPA, or h) with water (w) have been measured using an adiabatic calorimetry at T = (283.15, 288.15, 293.15, and 298.15) K and ambient pressure. Based on the results obtained, the excess molar heat capacities of the mixtures, CpE, as well as apparent molar heat capacities of each component, Cp,ϕ,i (down to its infinite dilution, Cp,i°), have been calculated. The CpE values are positive up to HMPA mole fractions being xh ≈ 0.9 and the dCpE/dT derivative undergoes a positive-to-negative sign inversion at xh = 0.33. Both concentration and temperature dependences of Cp,ϕ,i have also a complex character. The correlations of molar heat capacity properties with volume-related those for (H2O + HMPA) at the same temperatures have been discussed, too.