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

The heat capacities of aqueous solutions of methyldiethanolamine (MDEA) and 2-amino2-methyl-l-propanol (AMP) were measured in a heat-flux differential scanning calorimeter at temperatures 278.15 K less than or equal to T less than or equal to 368.15 K, to yield excess molar heat capacities C-p,m(E) over the whole mole fraction range. Densities of aqueous AMP were determined with a vibrating tube densimeter over the temperature range 293.15 K less than or equal to T less than or equal to 353.15 K, and excess molar thermal expansibilities and used to derive excess molar volumes V-m(E) E-m(E) = (partial derivativeV(m)(E)/partial derivativeT)(p). A modified Redlich-Kister treatment was employed to describe the M excess functions. The standard partial molar properties C-p,2(o) and V-2(o) at T = 298.15 1 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 C-p,2(o) and V-2(o) for AMP from measurements at fractions x(2)(MDEA) less than or equal to 0.03. Values of C-p,2(o) low mole fractions {x(2)(AMP) less than or equal to 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 c(p,m)(E) (water + MDEA) and C-p,m(E)(water + AMP), while the behaviour of v(m)(E) for the two aqueous alkanolamines is similar. (C) 2002 Elsevier Science Ltd. All rights reserved.