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26
2022
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09
Does adding a certain amount of electrolyte (such as salt or sodium carbonate) to the detergent improve the washing effect?
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Adding an electrolyte with an opposite charge to anionic surfactants can generally improve surface activity and reduce the critical micelle concentration (CMC). This is because increasing the concentration of counterions for ionic surfactants affects the diffusion double layer of surfactant micelles, reducing the repulsive forces between surfactant ions, which in turn decreases the thickness of the double layer, making it easier for micelles to form, thus lowering surface tension and CMC. For example, in a Cl2H25·OSO3Na solution, increasing the NaCl concentration, such as adding 0.02 mol/L concentration, can reduce the CMC from the original 0.0081 mol/L to 0.0038 mol/L, and with a concentration of 0.2 mol/L, the CMC drops to 0.00083 mol/L. Additionally, adding a small amount of salt (NaCl) to liquid detergents can also increase the concentration of the detergent, but the amount should not be excessive, or it may lead to precipitation. Generally, liquid detergents can add about 1% NaCl, while powdered detergents can add 20% to 40% Na2SO4.
The effect of electrolytes on nonionic surfactants is relatively small, but when electrolytes are added to the solution, they can lower the cloud point of nonionic surfactants, which is the "salting-out" effect of electrolytes. The "salting-out" effect causes nonionic surfactants to aggregate into larger micelles, and at a certain point, a new phase separates out. If the detergent is a mixture of anionic surfactants and nonionic surfactants, then after adding NaCl, on one hand, the addition of nonionic surfactants to anionic surfactants can increase the cloud point; on the other hand, it can also lower the cloud point. In this case, it is better not to add NaCl.
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