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

Highlights: • Binary mutual diffusion coefficients of Na₄BRA solutions at 25.00 °C. • Ternary mutual diffusion coefficients of aqueous NH₄VO₃(1) + Na₄BRA(2) at 25.00 °C. • Coupled diffusion in aqueous NH₄VO₃(1) + Na₄BRA(2) solutions analysed by using Nernst equations. 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.