Paper title:
The effect of thickness, pore size and structure of a nanomembrane on the flux and selectivity in reverse osmosis separations: a molecular dynamics study

Journal:
Chemical Physics Letters 397 (2004) 211?215

Author/s:
Sohail Murad *, Ludwig C. Nitsche
Department of Chemical Engineering, University of Illinois at Chicago, 810 S Clinton Street, Chicago IL 60607, USA

Summary: 
The paper used molecular dynamics (MD) simulations to investigate the effects of pore size, structure, and membrane thickness on the flux and selectivity of RO membranes.

Two types of membranes (“straight”-structured and “tortuous”-structured) were modeled using layers of barrier molecules that were tethered harmonically to periodic lattice sites. Membrane pore sizes ranged from 0.3-0.5 nm. Thickness of membranes varied from 0.5-1.0 nm (corresponding to 2-4 layers).

Results from the simulations indicate that fluxes for straight and tortuous membranes don’t have significant differences, which is contrary to what is usually known for microscopic and macroscopic membranes. Aside from this, the study also found out that the flux is linearly related to pore radius, similar to the characteristics of Knudsen regime. On the other hand, it was revealed that membrane thickness is inversely proportional to flux (flux versus 1/N, where N is number of membrane layers). 

Selectivity was affected by pore size and membrane thickness. It was shown that within a narrow range of less than 0.1 nm, selectivity can range from 0% to 100%. This sensitivity of results means that membrane designs should give big considerations to pore size. In addition, membrane thickness affects selectivity in an indirect way since it is related to pore size first then to transitional selectivity. Pore radius is inversely proportional to membrane thickness.

Contribution and application:
This work confirms that MD may be used to understand what happens in membrane separation processes. Because of the findings of this paper, membrane manufacturers and researchers may focus more on the parameters (pore size, etc.) that have bigger and more pronounced effects on membrane processes.



By: Hannah Ebro
hannah@gist.ac.kr