Phase diagrams of acetonitrile or (acetone plus water plus EPPS) buffer phase separation systems at 298.15 K and quantum chemical modeling

We found that addition of 3-[4-(2-hydroxyethyl)-1-piperazinyl]propane sulfonic acid (EPPS) induces phase separation of (acetonitrile + water) mixtures and (acetone + water) mixtures at T = 298 K, although these organic solvents are miscible with water at any mixing ratio. The phase diagrams of (acetonitrile + water + EPPS) and (acetone + water + EPPS) systems showed that the phase separation takes place in a wide range of the organic solvent mass fractions, 0.25 < w < 0.95. The solubility limits of EPPS buffer before the phase separations were determined by densimetry. The (liquid + liquid) equilibrium (LLE) and (solid + liquid + liquid) equilibrium (SLLE) data were measured for these ternary systems at T = 298.15 K under atmospheric pressure. The EPPS can be used as an auxiliary agent to pass over the (acetonitrile + water) azeotrope. The phase boundary data of the different specific regions in the phase diagrams were fitted to an empirical equation and the effective excluded volume (EEV) model. The experimental LLE data (tie-lines) were found to be thermodynamically consistent according to Othmer-Tobias equation. The NRTL activity coefficient model was used to correlate the experimental tie-lines. Molecular mechanical and semi-empirical quantum-mechanical calculations were used to study the intermolecular interactions between EPPS buffer and acetonitrile/acetone in gas phase and in water under the periodic boundary conditions. Then, we proposed a possible mechanism of EPPS-induced phase separation of the investigated systems. (C) 2012 Elsevier Ltd. All rights reserved.