Equilibrium, nonequilibrium, and nonlinear enthalpy relaxation in a supercooled ionic liquid [Ca(NO3)(2)](0.4)(KNO3)(0.6)

We investigated extensively equilibrium, nonequilibrium, and nonlinear aspe cts of the enthalpy relaxation in a supercooled liquid [Ca(NO3)(2)](0.4)(KN O3)(0.6). The equilibrium properties of relaxation, which are described in the framework of linear response theory, were determined from the frequency -dependent heat capacity measurements. The nonequilibrium and nonlinear asp ects of relaxation were investigated by differential scanning calorimetry a nd time-domain dynamic calorimetry. It was found that the nonequilibrium re laxation can be fully accounted for in terms of the equilibrium one if the latter is properly extended, and that the successful extension of the equil ibrium linear relaxation function must include both nonstationariness and t hermorheological complexity. No evidence of genuine nonlinear enthalpy resp onse, aside from the nonequilibrium effects, was seen in the time-domain dy namic calorimetric data, even when a temperature jump as large as 8 K was i mposed. (C) 2001 Elsevier Science B.V. All rights reserved.