When the molecular weight is similar, linear molecules tend to have lower retention compared to branched or spherical molecules, which are retained more effectively. This is due to differences in how these structures interact with the membrane during the separation process.
Several factors influence the rejection efficiency during membrane filtration:
1. **Relative Molecular Weight**
The size of the molecule plays a key role. Larger molecules are generally more easily rejected by the membrane than smaller ones.
2. **Molecular Properties**
(1) Even when molecular weights are equal, the shape of the molecule significantly affects retention. Linear molecules have lower retention, while branched and spherical molecules are retained more.
(2) For charged membranes, the rejection of ions or molecules with opposite charges is typically lower, as electrostatic attraction can reduce the resistance. Conversely, same-charged molecules face higher rejection.
(3) If the membrane has an adsorptive property, it can enhance the rejection of certain solutes by binding them to the surface.
3. **Presence of Other Polymer Solute**
When multiple polymers are present together, the rejection rate for one solute may increase compared to when it's alone. This is often caused by concentration polarization, where the solute accumulates near the membrane surface, increasing the effective concentration and thus the rejection.
4. **Operating Conditions**
(1) As temperature increases, the viscosity of the solution decreases, leading to a reduction in rejection efficiency.
(2) Higher flow velocity at the membrane surface reduces concentration polarization, which in turn lowers the rejection rate.
(3) At the isoelectric point of a protein, its net charge becomes zero, reducing electrostatic repulsion between particles. This leads to a thicker gel polarization layer on the membrane surface, increasing resistance and resulting in higher rejection of the solute compared to other pH values.
This phenomenon highlights the importance of understanding both the properties of the solute and the operating conditions to optimize membrane performance.
Source: http://news.chinawj.com.cn
Editor: Hardware Business Network Information Center
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