twenty three. Can anionic surfactants be mixed with cationic surfactants? Why is this?
Answer: The mixture of positive and negative ion surfactants with opposite ionic properties has always been considered a “forbidden zone” in the past. It was believed that mixing anionic and cationic surfactants will interact and precipitate, thus losing effectiveness and cannot be used. But the shackles of this traditional concept are being broken. According to research, under appropriate conditions, anionic surfactants and cationic surfactants can be mixed and can significantly reduce surface tension. However, it should be pointed out that when anionic and cationic surfactants are mixed, precipitation must be avoided. Practice has proved that under certain conditions, anionic and cationic surfactants interact strongly to form a composite or molecular compound, which has higher surface activity and greatly reduces the surface tension of the solution. For example: C8H17SO4Na and C8H17N The surface tension of the equimolar mixture of (CH3)3Br is 0.23mN/m, while the anionic surfactant C8H17 SO4Na is 41mN/m, and the cationic surfactant alone C8H17N ( The surface tension of CH3)3Br is 38mN/m.
Non-equimolar mixtures of anionic and cationic surfactants also undergo this change to varying degrees, and it is also reflected in the CMC. For example: the CMC of sodium lauryl sulfate is 8.7×10-3mol/L, add the cationic surfactant C2H5N+(CH3)3Cl–, its CMC is 4.2×10-3 mol/L; add C4H9N+(CH3 )3Cl–, its CMC is 2.38×10-3mol/L; add C6H13 N+(CH3)3Cl–, CMC It is 1.25×10-3 mol/L; add C8H17 N+ (CH3)3Cl–, the CMC is 0.40×10-3mol/L; add C10H21N+(CH3)3Cl-, the CMC is 0.21×10-3mol/L. It can be seen that as long as a small amount of counter-ionic surfactant is added to an anionic (or cationic) ionic surfactant, the surface tension and CMC of the solution can be reduced. However, not all anionic and cationic surfactants can be mixed. Only when the hydrophobic groups of the anionic and cationic surfactants are large and the number of carbon atoms of the two are close, can they have high surface activity. This high activity is also reflected in other properties of the solution, such as wettability, foam stability, etc.
It can be seen that there is a strong interaction between anionic and cationic surfactants in the mixed solution. The essence of this interaction is mainly the electrostatic interaction between surfactant ions with opposite electrical properties and the hydrophobic carbon chain. Interaction. Compared with a single surfactant, not only does it not have the repulsion of the same charge, but it increases the attraction between opposite charges, which greatly promotes the association between two different charged ions, making it easier to form micelles in the solution, and on the surface ( interface) are more easily adsorbed, resulting in higher surface activity.
In summary, as long as through appropriate methods and under appropriate conditions, anionic and cationic surfactants can be mixed, and can improve surface activity and have outstanding colloidal chemical properties, which are worthy of attention. However, if the conditions and methods are improper, precipitation will occur and precipitate, such as:
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