Degradation mechanism of CMC under highly alkaline conditions

Carboxymethylcellulose (CMC) is a derivative of natural cellulose, formed by replacing part of the hydroxyl groups in the cellulose molecule with carboxymethyl groups. Due to its good solubility, thickening and stability properties, CMC is widely used in food, medicine, petrochemical and other fields. However, under highly alkaline conditions, CMC will undergo degradation reactions, and this process has a significant impact on its application performance.

1. Drivers of degradation
Under highly alkaline conditions, the degradation of CMC is mainly driven by the following two factors:

Effect of alkali on cellulose backbone
The main chain of CMC is a cellulose structure composed of glucose units connected by β-1,4-glycosidic bonds. In a strong alkaline environment, the base can attack these glycosidic bonds and initiate a base-catalyzed hydrolysis reaction. The hydrolysis reaction can cause long-chain CMC molecules to break into shorter chain segments and even produce oligosaccharides or monosaccharides.

Effect of bases on carboxymethyl groups
The carboxymethyl group is a characteristic functional group of CMC, and its chemical properties are more active under alkaline conditions. In a highly alkaline environment, the carboxymethyl group may undergo a decarboxylation reaction (i.e., the COO⁻ group is converted into carbon dioxide and released), resulting in the loss of functional groups in the CMC molecule. This change may reduce the solubility and other properties of CMC.

2. Specific degradation pathways
Under highly alkaline conditions, the degradation of CMC mainly involves the following mechanisms:

2.1 Breaking of glycosidic bonds
Hydroxide ions (OH⁻) in an alkaline environment will destroy the β-1,4-glycosidic bond in the CMC main chain through nucleophilic attack. The fracture reaction is mainly divided into two steps:

Hydroxide ions attack the oxygen atoms of the glycosidic bond, causing the sugar ring to crack;
Water molecules participate in the reaction to form broken oligosaccharides or simple sugars.
2.2 Degradation of carboxymethyl groups
Carboxymethyl groups are easily further degraded under alkaline conditions, which may appear as:

Decarboxylation reaction: High-concentration hydroxide ions react with carboxymethyl groups to generate carbon dioxide and release methyl alcohols;
Oxidation: Under strong alkaline conditions, the carboxymethyl group may be oxidized into other forms of derivatives, resulting in changes in CMC properties.
2.3 β-elimination reaction
Secondary hydroxyl groups in CMC molecules may undergo β-elimination reactions under highly alkaline conditions. This reaction will cause the breakage of the CMC molecular chain and generate some unsaturated compounds or intermediates.

3. Influencing factors
The rate and extent of CMC degradation under highly alkaline conditions are affected by many factors, including:

Alkali concentration: The higher the alkali concentration, the faster the degradation rate;
Temperature: High temperature can accelerate the degradation reaction;
Reaction time: The longer the reaction time, the higher the degree of degradation;
Degree of substitution of CMC: CMC with a higher degree of substitution is more resistant to degradation due to the protective effect of the carboxymethyl group.

4. Practical impact and solutions
The degradation of CMC under highly alkaline conditions will significantly reduce its thickening, suspension and other properties, thereby affecting its effectiveness in practical applications. To slow down the rate of degradation, the following measures can be taken:

Control base concentration and reaction time;
Compound CMC with other stabilizers before use to improve the alkali resistance of the system;
Choose CMC with a higher degree of substitution to increase stability against alkaline environments.

The degradation mechanism of CMC under highly alkaline conditions mainly involves alkali-catalyzed hydrolysis of the cellulose backbone, decarboxylation and oxidation reactions of carboxymethyl groups, and β-elimination of secondary hydroxyl groups. These reactions jointly lead to the breakage of CMC molecular chains and structural changes, thereby significantly affecting its physical and chemical properties. In-depth understanding of these degradation mechanisms and improving the stability of CMC through optimizing use conditions and modification techniques are important directions for future research.