1. Overview
Carboxymethyl cellulose (CMC) is a water-soluble anionic polysaccharide widely used in food, pharmaceuticals, cosmetics, oilfield extraction and papermaking. A key property of CMC is its viscosity, but in practical applications, its viscosity often needs to be regulated to meet specific processing and performance requirements.

2. Structure and viscosity characteristics of CMC
CMC is a carboxymethylated derivative of cellulose, and its molecular structure determines its viscosity characteristics in solution. The viscosity of CMC depends on its molecular weight, degree of substitution (DS), and the temperature and pH of the solution. High molecular weight and high DS usually increase the viscosity of CMC, while elevated temperature and extreme pH conditions may reduce its viscosity.

3. Mechanisms of the effect of additives on CMC viscosity

3.1 Electrolyte effect
Electrolytes, such as salts (NaCl, KCl, CaCl?, etc.), can reduce the viscosity of CMC. Electrolytes dissociate into ions in water, which can shield the charge repulsion between CMC molecular chains, reduce the extension and entanglement of molecular chains, and thus reduce the viscosity of the solution.
Ionic strength effect: Increasing the ionic strength in the solution can neutralize the charge on the CMC molecules, weaken the repulsion between molecules, make the molecular chains more compact, and thus reduce the viscosity.
Multivalent cation effect: For example, Ca2?, by coordinating with negatively charged groups on multiple CMC molecules, can more effectively neutralize the charge and form intermolecular crosslinks, thereby significantly reducing the viscosity.

3.2 Organic solvent effect
Adding low-polar or non-polar organic solvents (such as ethanol and propanol) can change the polarity of the aqueous solution and reduce the interaction between CMC molecules and water molecules. The interaction between solvent molecules and CMC molecules can also change the conformation of the molecular chain, thereby reducing the viscosity.
Solvation effect: Organic solvents can change the arrangement of water molecules in the solution, so that the hydrophilic part of the CMC molecules is wrapped by the solvent, weakening the extension of the molecular chain and reducing the viscosity.

3.3 pH Changes
CMC is a weak acid, and changes in pH can affect its charge state and intermolecular interactions. Under acidic conditions, the carboxyl groups on the CMC molecules become neutral, reducing charge repulsion and thus reducing viscosity. Under alkaline conditions, although the charge increases, extreme alkalinity may lead to depolymerization of the molecular chain, thereby reducing viscosity.
Isoelectric point effect: Under conditions close to the isoelectric point of CMC (pH ≈ 4.5), the net charge of the molecular chain is low, reducing charge repulsion and thus reducing viscosity.

3.4 Enzymatic hydrolysis
Specific enzymes (such as cellulase) can cut the molecular chain of CMC, thereby significantly reducing its viscosity. Enzymatic hydrolysis is a highly specific process that can precisely control viscosity.

Mechanism of enzymatic hydrolysis: Enzymes hydrolyze the glycosidic bonds on the CMC molecular chain, so that the high molecular weight CMC is broken down into smaller fragments, reducing the length of the molecular chain and the viscosity of the solution.

4. Common additives and their applications

4.1 Inorganic salts
Sodium chloride (NaCl): widely used in the food industry to adjust the texture of food by reducing the viscosity of CMC solution.

Calcium chloride (CaCl?): used in oil drilling to adjust the viscosity of drilling fluid, which helps to carry drill cuttings and stabilize the well wall.

4.2 Organic acids
Acetic acid (acetic acid): used in cosmetics to adjust the viscosity of CMC to adapt to different product textures and sensory requirements.

Citric acid: commonly used in food processing to adjust the acidity and alkalinity of the solution to control viscosity.

4.3 Solvents
Ethanol: used in pharmaceuticals and cosmetics to adjust the viscosity of CMC to obtain suitable product rheological properties.

Propanol: used in industrial processing to reduce the viscosity of CMC solution for easy flow and processing.

4.4 Enzymes
Cellulase: used in textile processing to reduce the viscosity of slurry, making coating and printing more uniform.

Amylase: sometimes used in the food industry to adjust the viscosity of CMC to adapt to the processing needs of different foods.

5. Factors Affecting the Effectiveness of Additives

The effectiveness of additives is affected by many factors, including the molecular weight and degree of substitution of CMC, the initial concentration of the solution, temperature, and the presence of other ingredients.
Molecular weight: CMC with high molecular weight requires higher concentrations of additives to significantly reduce viscosity.
Degree of substitution: CMC with high degree of substitution is less sensitive to additives and may require stronger conditions or higher concentrations of additives.
Temperature: Increased temperature generally enhances the effectiveness of additives, but too high a temperature may cause degradation or side reactions of additives.
Mixture interactions: Other ingredients (such as surfactants, thickeners, etc.) may affect the effectiveness of additives and need to be considered comprehensively.

6. Future Development Directions
The research and application of reducing the viscosity of CMC is moving towards a green and sustainable direction. Developing new additives with high efficiency and low toxicity, optimizing the conditions for the use of existing additives, and exploring the application of nanotechnology and smart responsive materials in CMC viscosity regulation are all future development trends.
Green additives: Look for naturally derived or biodegradable additives to reduce environmental impact.
Nanotechnology: Use the efficient surface and unique interaction mechanism of nanomaterials to precisely control the viscosity of CMC.
Smart responsive materials: Develop additives that can respond to environmental stimuli (such as temperature, pH, light, etc.) to achieve dynamic regulation of CMC viscosity.

Additives play an important role in regulating CMC viscosity. By rationally selecting and applying additives, the needs of different industries and consumer products can be effectively met. However, in order to achieve sustainable development, future research should focus on the development of green and efficient additives, as well as the application of new technologies in viscosity regulation.