Two types of liquid phase separation induced by soft centrifugation in aqueous ethyl acetate using ethanol as co-solvent

Abstract

Water-ethyl acetate-ethanol is widely used as “green” extractant system. We show that two different types of phase separation can be induced upon centrifugation in this ternary system using ethanol as a co-solvent of water and ethyl acetate (EA): centrifuge-induced criticality and centrifuge-induced emulsification. The expected composition profiles of samples after centrifugation can be represented by bent lines in a ternary phase diagram when gravitational energy is added to the free energy of mixing. The experimental equilibrium composition profiles behave qualitatively as expected and can be predicted using a phenomenological theory of mixing. The concentration gradients are small except near the critical point, as expected for small molecules. Nevertheless, they are usable when accompanied by temperature cycles. These findings open new possibilities of centrifugal separation, even if control is delicate during temperature cycles. These schemes are accessible even at relatively low centrifugation speed for molecules that float and sediment with apparent molar masses several hundred times larger than the molecular mass.