Paper title:

Influence of Ion Size and Charge on Osmosis

 

Journal:

J. Phys. Chem. B, 116, 4206?4211 (2012)

 

Author/s:

James Cannon[1]*, Daejoong Kim[2], Shigeo Maruyama[1], and Junichiro Shiomi[1]*

 [1] Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

[2] Department of Mechanical Engineering, Sogang University, 505 Adam Schall Hall, 1 Shinsu-dong, Mapo-gu, Seoul, 121-742, Republic of Korea

 

 

Summary:

By using molecular dynamics (MD) simulations, this paper studies about the effects of size and charge of ions on the extent of osmosis of water through carbon nanotube (CNT) membranes.

The system studied was composed of a pure water chamber and a saltwater chamber, separated by two sets of 2x2 array CNT membranes. The salt-water chamber had 5 M concentration. Water was modeled using SPC/E while interactions were governed by LJ potential and Lorentz-Berthelot mixing rules. Parameters for salt were patterned after Na and Cl and, except for the radius, other properties were kept the same in all the simulations. Equilibration time was 4 ns while production lasted for 32 ns to ensure accuracy for the system that was sensitive to thermal fluctuations. Timestep was 2 fs, ensuring good energy conservation. Other simulation parameters used for LAMMPS runs were indicated in the paper as well.

Hypothetical ions of varying sizes were used in the study. Ionic radius was varied by varying σ, which is the LJ parameter that represents finite distance at which interparticle potential is zero. It was observed that smaller ions are held at a distance from the membrane because of their strongly bound hydration shell. Moreover, as ions become larger, they become less hydrophilic because their coulombic interaction with water weakens (in other words, larger separation for ion and water). Overall, results show that osmosis is strongest if ions having a radius of 3-4 ? are used. This is due to the placement of ions relative to water near the membrane wall and the presence of strong ion-water hydration.

In summary, “ion hydrophilicity”, which is what determines osmotic strength is affected by both the charge and size of the ion. Results revealed that a small change in these two properties can alter ion hydrophilicity substantially.

 

Contribution and application:

This paper clearly shows the importance of MD in studying osmosis. Even though we can observe osmosis macroscopically, understanding its process requires a detailed look into the interactions between molecules.

In addition, the importance of selecting solute for forward osmosis was highlighted in this paper. By describing which solute characteristics affect osmotic strength, this paper can be a basis for selecting draw solutes for higher efficiency in the forward osmosis process.

 

By: Hannah Ebro

hannah@gist.ac.kr