Surface Thermodynamics of Aqueous Solutions of Morpholine and Methylmorpholine

Abstract

The surface tension of aqueous solutions of morpholine and methylmorpholine has been measured at temperatures from (298.15 to 328.15) K, covering the whole range of concentration. The surface tension of the solutions decreases with increasing morpholine concentrations at the water-rich side, and it remains practically constant for the rest of solutions. The surface enthalpy and surface entropy have been obtained from the temperature variation of the surface tension at constant mole fraction. The surface entropy of these aqueous solutions shows a nonrandom behavior at very low concentrations of morpholine and of methylmorpholine. There are two maxima (at x2 = 0.025 and x2 = 0.085) in the surface entropy of morpholine (2) + water (1) mixtures; methylmorpholine (3) + water (1) mixtures show one (at x3 = 0.048). The surface enthalpies are similar to the surface entropies. The binding constants of morpholine and methylmorpholine on the water surface were calculated, and in both cases, the binding constants are temperature dependent. For morpholine, it increases with temperature; for methylmorpholine, it decreases.