1. Principle of salt resistance
1.1 Molecular structure characteristics: Sodium carboxymethyl cellulose (CMC) is an anionic linear polymer compound with a large number of carboxymethyl groups (-CH₂COONa) in its molecular chain. In salt solutions, these carboxymethyl groups play a vital role. When encountering salt ions, the carboxymethyl groups can bind to the salt ions through electrostatic action. For example, in sodium chloride (NaCl) solution, the carboxymethyl groups of CMC molecules attract sodium ions (Na⁺) to form a relatively stable structure. This structure helps to reduce the damage of salt ions to the CMC molecular chain.
1.2 Maintenance of hydration layer: There is a hydration layer around the CMC molecular chain. In a salt solution environment, although salt ions easily cause the CMC molecular chain to shrink, CMC can maintain the hydration layer by virtue of its own structural characteristics. This is because the hydrophilicity of carboxyl groups makes water molecules tightly surround the molecular chain like a protective film. The existence of this hydration layer can prevent salt ions from getting too close to the CMC molecular chain, thereby maintaining the stability of CMC and making it have a certain anti-salination effect.
2. Factors affecting salt resistance
2.1 The influence of degree of substitution: The degree of substitution of CMC refers to the average number of hydroxyl groups replaced by carboxyl groups on each glucose unit. The higher the degree of substitution, the more carboxyl groups there are on the CMC molecular chain. This means that in a salt solution, there are more carboxyl groups that can interact with salt ions, thereby enhancing its salt resistance. For example, at the same salt concentration, CMC with a degree of substitution of 1.2 generally has better salt resistance than CMC with a degree of substitution of 0.8.
2.2 The influence of degree of polymerization: The degree of polymerization reflects the length of the CMC molecular chain. The higher the degree of polymerization, the longer the molecular chain, and the greater the degree of entanglement between the molecular chains. In a salt solution, this entangled structure can increase the stability of CMC. Longer molecular chains can better resist the interference of salt ions and reduce the shrinkage and precipitation of molecular chains. However, too high a degree of polymerization may lead to poor fluidity of CMC in the solution, and the relationship between the degree of polymerization and salt resistance needs to be balanced according to the specific application scenarios.
3. Application scenarios of salt resistance
3.1 Oil drilling field: In drilling fluid, CMC is widely used to adjust the performance of drilling fluid. In the formation environment with high salt concentration, CMC with good salt resistance can maintain the viscosity and stability of drilling fluid. It can prevent the drilling fluid from becoming thinner or losing the ability to suspend rock cuttings due to the influence of salt, ensuring the smooth progress of drilling operations. For example, on offshore oil drilling platforms, the high salinity of seawater poses a challenge to drilling fluid, and salt-resistant CMC can effectively solve this problem.
3.2 Food industry: Some food processing processes involve the addition of salt, such as in the process of pickling food. In the processing and preservation of these foods, the addition of salt-resistant CMC can play a role in thickening, stabilization, and water retention, so that the food can still maintain a good texture and taste in the presence of salt, and prevent excessive loss of water in the food due to the effect of salt. For example, in the process of marinating meat products, CMC can better retain the meat juice in the meat while maintaining the taste and appearance of the product.
3.3 Papermaking industry: In the papermaking process, CMC is added to improve the performance of paper. When using salt-containing papermaking additives or processing salt-containing fiber raw materials, salt-resistant CMC can effectively play its role, while maintaining the uniformity of the paper, while improving the strength, tensile strength and flexibility of the paper. For example, when processing recycled fiber papermaking, since the recycled fiber may contain salt, the use of salt-resistant CMC helps to produce high-quality paper.
