The world of surfactants is so vast and well-known that it would be useless to reproduce it entirely in this blog, we will therefore briefly illustrate the most important types and characteristics of the surfactants commonly used in the industry for the treatment of metals.
We can certainly begin to divide surfactants into 3 major categories: Anionic, non-ionic and cationic. The differences in ionic charge allows their compatibility in certain pH environments, but this is not an always valid rule. The compatibility of all the surfactants in a given formulation is imposed by many factors, such as the presence of salts, bases or strong acids, solvents with different polarity, etc. Usually the cationics are well tolerated in acidic or extreme pH and are used, for example, in pickling agents, where they also manage to perform a double function of degreasing and mild inhibition of corrosion. Anionics can be used in many areas and have the characteristic of being reactive against the particulate present on the metals to be cleaned, they are widely used but have a labeling usually more unfavorable than cationics. Non-ionics represent the majority of surfactants, are used for all degreasing processes and have different peculiarities to each other, for example the degree of ethoxylation or propoxylation. Ethoxylates are excellent foaming agents and oil emulsifiers but they cannot always be used in industry, where foam is often a problem. They are then coupled with propoxylates which, being not very similar to water, tend to rise to the surface, especially when hot, by removing the foam.
The foam problem should not be underestimated, especially in an alkaline environment, where they can create by-products or even saponifying with a consequent increase in foam. When this latter situation occurs, it becomes complicated to keep the foam volume under control and the only surfactants that manage to deaerate even in this difficult scenario are precisely those with a high degree of propoxylation. However, the abuse of this kind of surfactants is not appropriate since both the wettability and the stability of the formulations would be compromised.
The molecular weight of surfactants is another important feature, surfactants of very high molecular weight are never used in industry, such as in cosmetics, except for hydrotroping complex saline systems or other surfactants among them. There are also other types of surfactants, which can have different ionic charges, but with the function of hydrotropes, which allow to "combine" solvents insoluble with water, or make the saline / alkaline systems more stable. Some of these hydrotropes are modified quaternary ammonium salts or sulphonated salts, such as sodium xylene sulfonate or sodium cumene sulfonate.
Another category of surfactants are amphoteric, that is, they have a dual behavior, cationic in acidic environments and anionic in alkaline environments; their use is centered in more targeted sectors.
There are also particular surfactants (such as modified siloxanes), so technical as to be defined as real Additives, they are used in sectors other than detergents, such as those of paints or professional coatings. Their function is not to emulsify the oil but to wet the substrate and level the coating and at the same time deaerate the product. These additives are ideally soluble in hybrid water-solvent systems.
In the table below you can view some types of surfactants and hydrotropes commonly used in metal cleaning.