Hydrophilic vs Hydrophobic: What's The Difference and How To Select

Author: Cavon Cormack - Application Engineer, Saint-Gobain Life Sciences 

A filter’s hydrophilic or hydrophobic nature is one typical property that aids in determining the applications in which it can most readily be employed. Therefore, understanding the difference between these two characteristics will greatly assist you with any filtration project you’re about to start.

Hydrophilic materials are water attractive or water “loving”, therefore hydrophilic filters are typically used in liquid filtration processes. The hydrophilicity of a filter allows polar liquids, like water, to interact more efficiently and maximize the filtration process. An example of such an application within the medical industry involves removing bacteria from aqueous intravenous (IV) solutions, such as 0.9% Normal Saline.  In this application, the IV solution which is composed of sodium chloride has a surface tension of ~70 mN/m and this is approximately 96 percent that of water (72.86 mN/m), thus making it easier for the filter to wet out. Wettability is an important parameter for efficient filtration processes because it indicates the ability of a liquid to maintain contact with a solid surface. A hydrophilic membrane is the ideal selection for this type of application as continuous flow of 0.9% Normal Saline is extremely important when treating a severely dehydrated patient.

Conversely, hydrophobic materials are water “hating” or repelling, therefore hydrophobic filters are typically used in gas filtration processes. The hydrophobicity of a filter allows non-polar fluids, like air, to interact more efficiently and prevent liquids, like water, from passing through the filter. Hydrophobic filters are commonly used in gas vent functions within medical devices. These filters allow you to vent out air or other gases while retaining the water based fluids that will be used to aid in the treatment of patients.

It’s also worth noting that hydrophobic membranes can be employed in liquid filtration processes. Firstly, hydrophobic membranes will wet out with liquids of a lower density and surface tension than water. Additionally, there are instances when your filtration liquid and a typical hydrophilic membrane (i.e. Polyethersulfone or PES) are chemically incompatible, which will to the fluid degrading the membrane thus making a hydrophobic membrane a better choice.

Similarly, hydrophilic membranes can be employed in gas filtration processes. You may have a system that over time develops condensation which can allow liquid to build up into the filter. You would want to address a situation such as this by employing a hydrophilic membrane to avoid any performance failures or to prevent any type of liquid contamination.

A wettability test is a simple procedure used to demonstrate if a membrane is hydrophilic vs. hydrophobic. When a liquid droplet comes into contact with a surface, the droplet has the tendency to spread out on the surface or to remain roughly spherical; the angle formed between the liquid and surface is called the contact angle. If the liquid of interest is water, a low contact angle (less than 90 degrees) indicates that the surface is hydrophilic, while a high contact angle (90 degrees or higher) indicates that the surface is hydrophobic. By performing this simple test, you can immediately check and verify the membrane’s wettability and its affinity towards a liquid, which will help you understand the degree of hydrophilicity vs. hydrophobicity.

Lastly, it is extremely important to remain mindful of the fluid properties (surface tension, viscosity, density, and specific weight – to name a few) that may be used in your filtration project as those properties will influence your operational parameters by affecting chemical compatibility, pore size, life span, maintenance frequency, temperature, pressure, and flow rate.  Saint-Gobain Life Sciences offers a wide variety of standard and custom filtration solutions.  Their engineers are ready to help you with your next filtration project.

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