Neutral molecules, diffusion-limited transport

Theory and Mechanism. In Chapter 3, Reinhoudt and coworkers review recent mechanistic aspects of carrier-assisted transport through supported liquid membranes. Carriers for selective transport of neutral molecules, anions, cations, or zwitterionic species have been developed. Transport is described in terms of partitioning, complexation, and diffusion. Most of the mechanistic studies were focused on diffusion-limited transport, in which diffusion of the solute-carrier complex through the membrane phase is the rate-limiting step for transport. However, for some new carriers, the rate-limiting step was found to be decomplexation at the membrane phase-receiving phase interface. 

Another situation arises when the transport of the anion is assisted. In a later section, the transport of salts using an anion carrier, a mixture of a cation and anion carrier, or a ditopic carrier is discussed. Finally, the transport of neutral molecules will be described in the section on Diffusion-Limited Transport of Neutral Molecules. This is illustrated with our recent investigations on the transport of urea. 

In fact, this equation is similar to the flux equation for neutral molecules (see section on Diffusion-Limited Transport of Neutral Molecules). 

In other experiments, the photon-gated transport of Fe(bpy)3 + at conical recessed nanopores, functionalized with a spiropyran (SP) moiety, has been described. Upon exposure to UV light, SP is converted in the presence of a weak acid to the protonated merocyanine, MEH+. MEH+ is converted back to SP by shining visible light on the nanopore orifice. The effect of the photon-generated charges on the diffusion-limited oxidation of Fe(bpy)3 + is significant. In the dark (i.e., with the surface attached molecule in the electrically neutral form, SP), for Fe(bpy)3 oxidation is ca. 8... 

The simulation models also correctly predicted the diffusivities of hydronium and methanol in a wide range of temperature (Fig. 19). Methanol is a neutral species and weakly interacts with Nation backbone. It is not surprising that the present MD models that do not consider chemical interaction between the molecules can still correctly evaluate the diffusivity of methanol. Because the present experimental setup is limited for liquid samples, whether or not the permeability of diffusivity is strongly depends on water content has not been examined. In summary, this work provided benchmark for the atomistic simulation of the transport processes in Nation at water content above 3 although at some points, the errors can be 100%. 

---