Buffer interactions: Densities and solubilities of some selected biological buffers in water and in aqueous 1,4-dioxane solutions

Abstract

The densities of aqueous 1,4-dioxane solutions of tris(hydroxymethyl)aminomethane (TRIS), N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS), N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic acid (TAPSO), and N-tris[hydroxymethyl]-4-amino-butanesulfonic acid (TABS) were measured at T = 298.15, 303.15, 310.15, and 318.15 K, using a high precision vibrating tube digital densitometer. On the basis of these data, the isobaric thermal expansibilities, α( m, T), have been evaluated. The solubilities of these buffers in water and at several concentrations of 1,4-dioxane at 298.15 K have also been determined from the experimental results of density measurements. Two-liquid phase behavior was found from the binary system containing the TABS buffer over a given composition range. The solubility data were further used to calculate the free energy of transfer ( Δ G ′ tr ) of these buffers from water to aqueous 1,4-dioxane solutions at 298.15 K. We observed positive values of ( Δ G ′ tr ) for the buffers from water to 1,4-dioxane, indicating that the interactions between these biological buffers and 1,4-dioxane are unfavorable. The contribution of transfer free energies ( Δ g ′ tr ) of various functional groups from water to the aqueous 1,4-dioxane solutions, have been estimated.