The key difference between molecular sieve and silica gel is that a molecular sieve is a material containing pores of similar size, whereas silica gel is a substance can be used to prepare a porous material with pores of different sizes.
Both molecular sieve and silica gel are important materials in chemical separations. They are porous materials having pores that allow some analytes to pass through while retaining others. By choosing the pore size, we can separate the compound of desire from a mixture.
What is Molecular Sieve?
Molecular sieve is a material with very small holes (pores) with uniform size. This means that pores have similar diameters. Therefore, we can use molecular sieves for the separation of small particles from large particles. When a mixture of differently sized particles passes through a molecule sieve, large particles leave the sieve first followed by medium-sized particles. There are two major uses of molecular sieves: used in chromatography as a separation technique and used as desiccants, e.g. activated charcoal.
Generally, the size of pores of a molecular sieve ranges from the nanometer scale to the angstrom scale. There are two major types of molecular sieves based on the size of pores: microporous sieves and macroporous sieves. The size of pores in microporous sieves is typically less than 2 nm. Macroporous sieves have pores typically higher than 50 nm. There is also another category of molecular sieves as mesoporous sieves, which has a pore size ranging from 2 to 50 nm.
Some examples of microporous sieves include zeolites, activated carbon, clays, and porous glass. A common example of mesoporous materials is silica gel. An example of a macroporous material is mesoporous silica.
The major advantage of molecular sieves is that we can regenerate these materials for further applications. There are few methods for this regeneration, including, changing pressure, heating, purging with a carrier gas, and heating under high vacuum.
What is Silica Gel?
Silica gel is a type of molecular sieve containing an irregular pattern of silicon and oxygen atoms with non-uniform pores. This material is an amorphous form of silicon dioxide. It contains nanometer-scale voids and pores. These voids may contain either water or any other fluid we are using in the preparation of the silica gel. E.g. gas, vacuum, other solvents, etc. Since the pore sizes are non-uniform, we can say that this molecular sieve has an average pore size of 2.4 nm.
Silica gel has a strong affinity towards water; thus, we can use it as a desiccant. This material is very hard and translucent. However, it is considerably softer than silica glass or quartz. When silica gel is saturated with water, it remains in a hard state.
In commercial grade, we can find silica gel in the form of granules or beads. These beads have a diameter of few millimetres. Sometimes, these beads also contain some amount of an indicator reagent that can change the colour of the beads when water is absorbed. As a desiccant, these beads are included in food packages as small packets to absorb water vapour inside the package.
What is the Difference Between Molecular Sieve and Silica Gel?
The key difference between molecular sieve and silica gel is that a molecular sieve is a material containing pores of similar size, whereas silica gel is a substance we can use to prepare a porous material which has pores with different sizes. Moreover, molecular sieves are mainly used in chromatography as a separation technique, and as desiccants while silica gel is mainly used as a desiccant.
Below infographic summarizes the difference between molecular sieve and silica gel.
Summary – Molecular Sieve vs Silica Gel
Molecular sieve and silica gel are important materials in chemical analysis. The key difference between molecular sieve and silica gel is that a molecular sieve is a material containing pores of similar size whereas silica gel is a substance that we can use to prepare a porous material which has pores with different sizes.
1. “MSN vials” By Victor Lin Group /Iowa State University – Self-photographed (Public Domain) via Commons Wikimedia
2. “Silica gel bag open with beads” By Clemente assumed – No machine-readable source provided. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia