Coupled mutual diffusion in aqueous (ammonium monovanadate + butyl-substituted sulfonated resorcinarene) solutions: An experimental and theoretical approach

Abstract

Binary mutual diffusion coefficients (D) of aqueous solutions of the macrocyclic salt tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(butyl)resorcin[4]arene (Na4BRA) have been measured at 298.15 K by the Taylor dispersion method. The limiting diffusion coefficient of Na4BRA obtained by extrapolation of the data is used to evaluate the limiting conductivity and hydrodynamic radius of the BRA4− ion. The change in the hydrodynamic radius of sulfonated resorcinarenes with increasing the length of alkyl substituents is analysed. In the second part of this work, the effect of ammonium monovanadate on the diffusion of aqueous Na4BRA was investigated by measuring apparent ternary mutual diffusion coefficients for aqueous NH4VO3(1) + Na4BRA(2) solutions. In binary aqueous Na4BRA solutions (no added NH4VO3), the BRA4− ions diffuse at the same speed as the smaller and relatively mobile Na+ counterions to maintain electroneutrality. As the NH4VO3 concentration is raised, the main diffusion coefficient of NH4BRA drops to the tracer diffusion coefficients of BRA4− ions in supporting NH4VO3 solutions. Coupled diffusion of NH4VO3 and Na4BRA is significant. In particular, Na4BRA concentration gradients produce significant counter-current coupled flows of NH4VO3. Nernst equations are used to provide an electrostatic interpretation of the strongly coupled diffusion of Na4BRA and NH4VO3.