Use of HS-SPME for analysis of the influence of salt concentration and temperature on the activity coefficient at infinite dilution of ethanol-water-salt systems

Accurate knowledge of thermodynamic properties is of great importance in optimizing industrial processes of chemical separation. However, traditional techniques have limitations that hinder the use in complex systems, especially involving strong electrolytes. Solid phase micro-extraction (SPME) was used to determine infinite dilution activity coefficients of ethanol in water-salt systems. Different concentrations of inorganic salts were used to verify the influence on the activity coefficient at temperatures of 323.15, 333.15, 343.15, and 353.15 K. Salts evaluated in SPME include potassium chloride (KCl), calcium chloride (CaCl2), and aluminum chloride (AlCl3) at concentrations of 1.125, 1.800, and 2.571 mol/Kg. Comparisons between data obtained using SPME and the theoretical basis and literature data showed good agreement. Thermodynamic parameters such as partial molar excess enthalpy at infinite dilution, partial molar excess entropy at infinite dilution, and partial molar excess Gibbs energy were calculated from the infinite dilution activity coefficient. AlCl3 produced the highest activity coefficient for ethanol at infinite dilution, indicating a strong influence on the separation of ethanol. The results obtained indicate that SPME technique can be considered an alternative method on determination of activity coefficients at infinite dilution.