High-pressure liquid-liquid immiscibility in aqueous solutions of tetra-n-butylammonium bromide studied by a diamond anvil cell technique

By use of a diamond anvil cell, we show that at high pressures aqueous solu tions of tetra-n-butylammonium bromide form a closed liquid-liquid immiscib ility loop with a critical concentration of about 2.0 mol-kg(-1). We report data for a near-critical isopleth, which describes the pressure dependence of the upper and lower consolute points. At about 700 MPa and 373 K both c onsolute points coincide to form a hypercritical point, which characterizes the minimum pressure to achieve immiscibility. We show that this immiscibi lity can be rationalized in terms of Fitter's ion-interaction theory. We de termine the ion-interaction parameters of Fitter's theory up to 423 K at no rmal pressure. Inclusion of volumetric data gives the correct trend toward a high-pressure immiscibility. We discuss the results in terms of the inter play between ionic and hydrophobic forces in this system.