Thermodynamics for self-association of caffeine in water: apparent molar volumes and apparent molar heat capacities of aqueous caffeine at temperatures from 278.15 to 393.15 K and at the pressure 0.35 MPa

摘要

We have determined the apparent molar volumes V_(phi) and the apparent molar heat capacities C_(p,phi) for aqueous solutions of caffeine at temperatures T from 278.15 K to 393.15 K, at the pressure 0.35 MPa, and at molalities m from 0.01 mol·kg~(-1) to 0.1 mol·kg~(-1). We used a vibrating tube densimeter (DMA 512 P, Anton PAAR Austria) to determine the densities and volumetric properties, and a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences, American Fork, UT, USA) to obtained the heat capacities. We have assumed that aqueous caffeine is described by a monomer-dimer equilibrium, and we have applied Young's Rule to fit our results with regression functions that are thermodynamically self-consistent and that include the equilibrium constant and the changes in enthalpy DELTA_rH_m, heat capacity, DELTA_rC_(p,m), volume DELTA_rV_m, and entropy DELTA_rS_m for the monomerdimer reaction over the ranges of temperature and molality of our experiments.