Cellulose ethers are a class of water-soluble polymers derived from cellulose, a natural polysaccharide found in plant cell walls. These polymers exhibit a range of physicochemical properties that make them suitable for a variety of applications in the food, pharmaceutical, cosmetic and construction industries. One of the most notable properties of cellulose ethers is their ability to thicken aqueous solutions and form gels through thixotropy.
Chemistry of cellulose ethers
Cellulose ethers are cellulose derivatives obtained by modifying the hydroxyl groups of cellulose molecules. The three most common cellulose ethers are methylcellulose, hydroxypropylmethylcellulose (HPMC), and carboxymethylcellulose (CMC). The chemical structures of these polymers are shown in Figure 1 .
Figure 1 Chemical structures of: (a) methylcellulose, (b) hydroxypropylmethylcellulose, and (c) carboxymethylcellulose
In methylcellulose, some of the hydroxyl groups on the anhydroglucose units of cellulose are replaced by methyl groups (-CH3). In HPMC, some hydroxyl groups are substituted with methyl and hydroxypropyl (-CH2-CH(OH)-CH3) groups. In CMC, some hydroxyl groups are replaced by carboxymethyl (-CH2-COOH) groups. These substitutions change the solubility and rheological properties of the cellulose molecule, making it more soluble in water and easier to thicken.
Thickening of cellulose ethers
The thickening properties of cellulose ethers are due to their ability to form hydrogen bonds with water molecules. A hydrogen bond is a weak interaction between a hydrogen atom connected to an electronegative atom and another electronegative atom, such as oxygen or nitrogen. When cellulose ether is dissolved in water, the hydroxyl groups on the polymer chains form hydrogen bonds with water molecules. This results in the formation of a hydrated polymer network that traps water and slows its movement. The viscosity of the solution increases and becomes thicker.
The degree of thickening depends on several factors, including polymer concentration, degree of substitution, and temperature. As the polymer concentration increases, the viscosity of the solution also increases. This is because there are more polymer chains available to form a hydrated network. The degree of substitution also affects thickening. Polymers with a higher degree of substitution have more hydroxyl groups substituted, which increases their ability to form hydrogen bonds and thickens the solution. Temperature also plays a role in thickening. As the temperature increases, the viscosity of the solution decreases due to the decrease in hydrogen bond strength.
Thixotropy of cellulose ethers
Thixotropy is a property of certain materials that causes their viscosity, or resistance to flow, to decrease over time when they are subjected to stress, such as shaking or stirring. This property is due to the reversible formation and breaking of weak intermolecular bonds such as hydrogen bonds or van der Waals forces. Thixotropic materials become more fluid when stressed, but return to their original state when the stress is removed.
Cellulose ethers exhibit thixotropy due to the reversible formation and breaking of hydrogen bonds between polymer chains and water molecules. When subjected to pressure (such as shaking or stirring), hydrogen bonds break and the hydrated polymer network is destroyed. This causes the viscosity and flow resistance to decrease and the solution to become more fluid. However, when the stress is relieved, hydrogen bonds are re-formed and the hydrated polymer network is re-established. The solution returns to its original state, and the viscosity and flow resistance increase.
Applications of cellulose ethers
Cellulose ethers have a wide range of applications due to their thickening and thixotropic properties. In the food industry, they are used as thickeners, emulsifiers and stabilizers in products such as sauces, dressings and ice cream. In the pharmaceutical industry, they are used as binders, disintegrating agents and sustained-release agents in tablet formulations. In the cosmetics industry, they are used as emulsifiers, thickeners and film-forming agents in products such as lotions and creams. In the construction industry, they are used as additives in cement and mortar formulations to improve workability and reduce water penetration.