How does the tiny pore size help RO Membrane achieve seawater desalination?

1. Extremely tiny pore size: the precision scale of the microscopic world
The pore size of the RO Membrane can be called the ultimate precision scale in the microscopic world. Usually, its pore size is only about 0.0001 microns, which is so small that it is almost unimaginable. In order to feel this scale more intuitively, we can compare it with common substances. Most viruses have a diameter between 0.02 and 0.3 microns, and the pore size of the RO Membrane is much smaller than that of most viruses. With such a tiny pore size, only very small molecules such as water molecules have a chance to pass through.
The diameter of a water molecule is about 0.276 nanometers. In comparison, the diameter of salt in seawater, such as sodium chloride ion pairs, is about 0.5 nanometers, and the size of microorganisms such as Escherichia coli is even at the micron level. This huge size difference makes the RO Membrane like a well-designed super sieve in the process of seawater desalination. Salt and impurities cannot pass through the micropores on the membrane due to their large size, while water molecules can relatively easily pass through these micropore channels and transfer from the seawater side to the freshwater side due to their tiny size.
From the perspective of physical principles, this screening mechanism based on size differences is consistent with fluid mechanics and molecular diffusion theory. When seawater flows through the RO Membrane under external pressure, water molecules follow the law of Brownian motion under pressure and find and pass through the membrane pores on a microscopic scale. However, salt and impurities cannot participate in this microscopic "crossing journey" because their size exceeds the range of membrane pores, and are effectively intercepted. This precise screening effect provides the most basic guarantee for seawater desalination and is one of the key elements for RO Membranes to become precise "screens".

2. Unique composition of materials: material basis of microstructure
The reason why the microstructure of the RO Membrane can be so precise is inseparable from the unique characteristics of the polymer materials that constitute it. There are many types of polymer materials that constitute RO Membranes, among which polyamide, cellulose acetate, etc. are more common materials. These polymer materials have unique chemical structures and physical properties, providing a material basis for building precise microstructures.
Taking polyamide composite membranes as an example, they are usually composed of an ultra-thin polyamide active separation layer and a support layer. The polyamide active separation layer is the core part for achieving the separation of salt and impurities. It forms an extremely fine microporous structure on the surface of the support layer through advanced technologies such as interfacial polymerization. The polyamide material itself has good chemical stability and mechanical strength, and can withstand various pressures and chemical effects in the desalination process while maintaining the stability of the microstructure.
In the microstructure of the polyamide active separation layer, the molecular chains interact with each other through covalent bonds and hydrogen bonds, forming a tight and orderly arrangement. This arrangement not only determines the pore size and distribution of the membrane, but also affects the affinity and repulsion of the membrane to different substances. For example, some functional groups on the polyamide molecular chain, such as amide groups, have certain polarity and can form hydrogen bonds with water molecules, thereby promoting the transmission of water molecules in the membrane pores. For charged salt ions, the charge distribution on the surface of the polyamide membrane will produce electrostatic repulsion, further preventing salt from passing through the membrane pores.
Cellulose acetate membranes also have unique microstructural properties. Cellulose acetate is a cellulose derivative that contains a large number of hydroxyl and acetyl groups in its molecular structure. These functional groups give cellulose acetate membranes good hydrophilicity and selectivity. At the microscopic scale, cellulose acetate molecules interact through intermolecular van der Waals forces and hydrogen bonds to form a membrane structure with a specific pore size and porosity. This structure can effectively block salt and impurities in seawater while allowing water molecules to pass through to achieve seawater desalination.

3. Complex morphology of microscopic surface: "rugged terrain" of the microscopic world
At the microscopic scale, the surface of the RO Membrane is not flat and smooth, but presents a complex terrain full of tiny grooves and pores. This complex surface morphology further enhances the ability of the RO Membrane to act as a precision "sieve".
The micropores on the surface of the RO Membrane are not simple circular holes, but have complex shapes and irregular distributions. These micropores may be elliptical, polygonal or even some irregular shapes that are difficult to describe. Moreover, their distribution on the membrane surface is not uniform, but rather random. This irregular shape and distribution increases the difficulty for salt and impurities to pass through the membrane pores.
When salt and impurities try to pass through the membrane pores, they not only face the limitation of pore size, but also the challenges brought by the shape and distribution of the membrane pores. Due to the irregularity of the membrane pores, salt and impurities may be blocked when approaching the membrane pores because they cannot perfectly match the membrane pores. For example, an irregularly shaped colloidal particle may be intercepted when trying to pass through an elliptical membrane pore because some parts of the particle cannot pass through the narrow part of the membrane pore.
In addition, the microscopic grooves and pores on the surface of the RO Membrane also affect the flow and diffusion of water molecules on the membrane surface. Before passing through the membrane pores, water molecules need to diffuse and migrate to a certain extent on the membrane surface. The complex morphology of the membrane surface can increase the contact area between water molecules and the membrane surface, promote the diffusion of water molecules, and thus increase the water flux of the membrane. At the same time, this complex surface morphology also helps to reduce the deposition of salt and impurities on the membrane surface, reduce the risk of membrane pollution, and ensure the long-term stable operation of the RO Membrane.

4. Synergistic effect of microstructure: the overall efficiency of precision "screen"
The microstructure of the RO Membrane is not a simple addition of each component, but through the synergistic effect of the extremely fine pore size, the unique composition of the material and the complex morphology of the microscopic surface, it has jointly created its powerful efficiency as a precision "screen".
The tiny size of the pore size provides the most basic physical barrier for the separation between water molecules and salt and impurities. The unique chemical structure and physical properties of the material determine the selectivity and stability of the membrane, allowing the RO Membrane to maintain good performance in a complex seawater environment. The complex morphology of the microscopic surface further enhances the separation ability and anti-pollution performance of the membrane.
In the actual desalination process, these microstructural elements cooperate with each other and work together. When seawater flows to the RO Membrane under pressure, first of all, salt and impurities are initially intercepted on the membrane surface due to the huge difference in size and membrane pores. Then, the chemical properties of the material and the surface charge distribution adsorb or repel salt and impurities, further preventing them from passing through the membrane pores. At the same time, water molecules diffuse and migrate in the complex topography of the membrane surface, find and pass through the membrane pores, and achieve the transfer from seawater to fresh water.