Calorimetric investigation of the complex formation reaction of 18-crown-6 ether with d,l-alanine in water-ethanol mixtures

The standard thermodynamic parameters (Delta(r) GA degrees, Delta(r) HA degrees, and T Delta(r) SA degrees) of the reaction of molecular complex formation of 18-crown-6 ether (18C6) with d,l-alanine (Ala), [Ala18C6], have been obtained from calorimetric titration experiments carried out using the microcalorimetric system TAM III (TA Instruments, USA) at T = 298.15 K in water-ethanol (H2O-EtOH) solvents at X (EtOH) = 0 A center dot 0.6 mol fractions. Results show that the increase of the EtOH concentration in solvent brings about an increase of the [Ala18C6] complex stability and of the exothermicity of the reaction of complex formation. The solvation contributions of 18C6, Ala, [Ala18C6] to Delta(r) GA degrees and Delta(r) HA degrees at various X (EtOH) values are also analyzed.