Eq. ( ) can in turn be used to estimate Γ, provided the value of 𝐷 for Br⁻ in our experiment can be assessed. An estimated value of 𝐷 can be derived from the selfdiffusion coefficients in water for Br⁻ (2.01 × 10 [𝑚 /𝑠]) and the major counterion Na + (1.33 × 10 [𝑚 /𝑠]) tabulated in . Since major ion association can be neglected at the low ionic strength involved in our experiment, 𝐷 can be set equal to the mean of the two tracer diffusion coefficient values (see Figure in ) yielding 𝐷 = 1.67 × 10 [𝑚 /𝑠] and Γ = 0.949. This value is close to 1 implying that the ratio between tortuosity and constrictivity is close to unity, as would be expected for a molecular mesh material such as silica-gel.