Classification of membranes according to pore size

There are four different types of membranes according to the pore size and the molecular weight of the solute it can reject. Though there are no absolute criteria that divide those four membrane types, but following can be considered a general classification.

  • MF (microfiltration) – Pore size generally spans 0.1 micron to 1 micron, which are measured by porometers. The principle of the instrument is described here, but it measures gas flow rate through the membranes of which pores are filled up (or wetted) with the liquids of known surface tension. Under the rising TMP conditions, the biggest pore with the lowest capillary pressure will be opened up first while smaller pores follow in the order of their pore size. By analyzing the relation between the gas permeation rate and the TMP, pore size distribution can be calculated.
  • UF (ultrafiltration) – Pore size generally spans 0.01 micron to 0.1 micron, which is measured by prometers. But the pore sizes are often expressed as molecular weight cut off (MWCO) that is measured by filtering surrogate molecules that have known molecular weights. MWCO ranges 1,000 Da to 300,000 Da.
  • NF (nanofiltration) – Pore size might be between 1 nm and 10 nm, but it is not determinable easily since prometer is no longer effective for this pore size range. MWCO may range 200Da – 1,000 Da depending on operational condition. NF can effectively remove divalent ions at relatively high efficiency, e.g. 70-99%, but it’s efficiency of removing monovalent ions such as Na+, K+, Cl, etc. are typically low at 30-80%.
  • RO (reverse osmosis) – No pores can be identified by scanning micron microscopy (SEM). The high chemical potential imposed by the high pressure in feed side dissolves water into the membrane material followed by water desorption from the permeate side where chemical potential is low. As a result the solubility of molecules in membrane material plays a major role in rejection efficiency in addition to the sieving mechanisms. For instance, the rejection efficiency of lithium ions (6.9 Da) is much higher than that of much larger ethanol molecules (46.1 Da). It is mainly because lithium ions becomes very unstable when they are dissolved in membrane materials due to the charge repulsion enhanced by the low electric permittivity of membrane material. On the other hand, since ethanol molecules do not have net charge, they do not experience high repulsions among others when they are dissolved in membrane materials.  The most popular RO membrane material, polyamide, has a relative electric permittivity of ~5 while water has 78. This suggests that as soon as ions dissolved in membrane materials, repulsion among ions increases around 15.6 times. Due to the stronger destabilization of divalent ions in membrane materials, rejection efficiencies of divalent ions are much higher than those of monovalent ions. Rejection efficiencies of small organics without charge such as methanol, ethanol, propanol, acetone, etc. are typically lower than 30%. A few theoretical models exist to describe the phenomena and are described here.

Pore s12
Fig. 1. Classification of membrane filtration as a function of molecular weight cut off and pore size (Koch Membrane Systems).

MF and UF typically run at low pressure, e.g. <6 bar, but NF and RO runs at relatively high pressure, e.g. >8 bar. The following diagram shows the solutes that are rejected by each membrane. As discussed here, regardless of the membrane pore size, actual solute sizes rejected by MF and UF are approximately same since the cake layer formed on membrane surface acts as a dynamic membrane in actual filtration process. However, tiny molecules with low molecular weight perhaps less than a few tens of thousand Dalton can be better rejected by UF than by MF regardless of the existence of dynamic membrane.

NF branes and RO membranes are used to remove trace organic molecules and ions in water filtration. Due to the looser skin layer structure, NF membranes tend to pass mono-valent ions (Li+, Na+, K+, etc.), but not di- and tri-valent ions (Ca2+, Mg2+, Fe2+, Fe3+, etc.). Typical rejection efficiency of mono- and di-valent ions by NF is 30-80% and 70-95%, respectively. RO membranes reject mono-valent ions at 90-99.9% while rejection di-valent ions at higher efficiency. It is noteworthy that NF and RO are not solely rely on the size exclusion mechanism, but also rely on the solution-diffusion mechanism that essentially affected by how easily the solutes can dissolve in the membrane material. Since low-molecular-weight charge-neutral solvents, such as methanol, ethanol, acetone, etc., easily dissolve into polyamide, rejection efficiency of such solvents tends to be low, e.g. 10-50%.

Fig. 2 illustrates what kind of particles can be rejected by each membrane.

Pore s10
Fig. 2. The particles rejected by membrane as a function of pore size: An approximate relation.

The “rough” relation between pore size and MWCO is plotted in Fig. 3 based on the relation between approximate protein diameter and molecular weight. Since the effective molecular diameter can vary depending on experimental condition, this relation should be used only to obtain a rough idea on the pore size and MWCO relation. In fact, many different versions of similar graphs are available on public domain. One example is here, which is quite different from the relation shown in Fig. 3.

Pore s1
Fig. 3. Relation between pore size and molecular weight cut off (Raw data from von Recum, 1999)


© Seong Hoon Yoon