Question

The pour size of reverse osmosis membrane is about

• A. 1 nm
• B. 0.2 nm
• C. 0.4 nm
• D. 0.8 nm

Hint:

RO membranes have an effective pore size of approximately 0.2 nm (2 \AA). This is small enough to reject hydrated sodium and chloride ions, letting only pure water pass through.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Reverse osmosis (RO) is the tightest membrane separation process used in the food and dairy industries, primarily for concentration and water removal.
The transport of water through an RO membrane is described by a solution-diffusion mechanism rather than simple pore filtration.
The effective pore size of these dense polymeric membranes is at the atomic scale, allowing them to reject even small hydrated ions.

Step 2: Detailed Explanation:
Reverse osmosis membranes are typically made from thin-film composites of polyamide or cellulose acetate.
The free volume spaces between the polymer chains act as temporary pores during water transport.
The effective pore diameter of these membranes is approximately 0.1 to 0.3 nm.
This corresponds to a standard average value of 0.2 nm (or 2 \AA ngstroms).
This extremely small pore size is slightly larger than the kinetic diameter of a water molecule (approximately 0.26 nm) but is smaller than the hydrated radii of common ions like sodium ($Na^+$, 0.36 nm) and chloride ($Cl^-$, 0.33 nm).
This size difference allows the membrane to reject dissolved salts, lactose (diameter approximately 0.8 nm), and larger proteins while letting water pass through.
Thus, 0.2 nm is the correct pore size range.

Step 3: Final Answer
The pore size of a reverse osmosis membrane is approximately 0.2 nm.