A Different Look at Shigeru's Latest
The paper that came out from Shigeru’s laboratory earlier this year in Analyitical Chemistry (Ishimatsu et al. 2010 downloadable here) demonstrated that the permeability of pnc-Si membranes to small monovalent ions in 0.1 M salt is predictable from membrane geometry alone. The paper also shows that multivalent and/or large ions do not follow theory as they experience hindrance from steric and electrostatic effects in the same buffer. Note that the ‘theory’ in the case of the Ishimatsu paper simply predicts diffusion based on average membrane pore sizes, porosity, and membrane thickness; it does not include the hindrance factors that Jess included in her paper. Since the plotted data didn’t explicitly describe hindrance as a function of molecular size, I used the molecular sizes in Table 1 and the permeabilities reported in Figure 5 and created the plot below. The plot shows that small ions that pass through the membrane without hindrance are < 1/10th of pore size, whereas those experiencing hindrance are highly charged or >30% of pore size.
