Thermodynamics of aqueous amines: excess molar heat capacities, volumes, and expansibilities of {water + methyldiethanolamine (MDEA)} and {water + 2-amino-2-methyl-1-propanol (AMP)}

Abstract

The heat capacities of aqueous solutions of methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) were measured in a heat-flux differential scanning calorimeter at temperatures 278.15K⩽T⩽ 368.15 K, to yield excess molar heat capacities Cp,mEover the whole mole fraction range. Densities of aqueous AMP were determined with a vibrating tube densimeter over the temperature range 293.15 K⩽T⩽ 353.15 K, and used to derive excess molar volumesVmE and excess molar thermal expansibilitiesEmE= (∂VmE/∂T)p. A modified Redlich–Kister treatment was employed to describe the excess functions. The standard partial molar propertiesCp,2o and V2oatT= 298.15 K for MDEA, as determined by extrapolating the Redlich–Kister expressions to infinite dilution, are consistent with results from a Picker calorimeter and densimeter at mole fractionsx2 (MDEA) ⩽ 0.03. Values ofCp,2o and V2ofor AMP from measurements at low mole fractions { x2(AMP) ⩽ 0.03} differ from the Redlich–Kister extrapolations, suggesting that specific interactions take place in dilute solutions. The results are consistent with Lumry’s model (Faraday Symp. Chem. Soc. 1982, 17, 93–108), with MDEA being less hydrophobic than AMP. The mole-fraction dependence of the reduced excess heat capacities is most pronounced at T< 298.15 K, with clear differences in the behaviour of Cp,mE(water+MDEA) and Cp,mE(water+AMP), while the behaviour ofVmE for the two aqueous alkanolamines is similar.