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Reducing Surface Tension - Surfactants in Coatings

Mar. 03, 2022

While the general topic of surfactants has not changed in the last 6 years, some new chemicals have surfaced and there was less emphasis on bio-based or green chemistry then than there is today. Natural surfactants must have both head and tail groups to come from a truly natural source. Personal care surfactants often have the same chemical composition as paint surfactants, but may have different names or slightly different functions.


Surfactants are materials that reduce the surface tension between two liquids or between a liquid and a solid. In a general sense, any material that affects the interfacial surface tension can be considered a surfactant, but in a practical sense, surfactants can act as wetting agents, emulsifiers, foaming agents, dispersants, etc.

calcium dodecyl benzene sulfonate

calcium dodecyl benzene sulfonate



Surfactants play an important role as dispersants, emulsifiers, detergents, wetting agents, foaming agents and defoamers in many practical applications and products, including: paints, emulsion binders, inks, fungicides (disinfectants), shampoos, toothpastes, firefighting (foams), detergents, insecticides, deinking of recycled paper, ski waxes, and spermicides.


Surfactant adsorption kinetics are important for practical applications, such as in emulsification or coating processes and foaming processes where bubbles or droplets are rapidly created and need to be stabilized. As interfaces form, the diffusion of surfactant to the interface impedes adsorption, which may lead to kinetic limitations. These energy potentials may be due to spatial or electrostatic repulsion; spatial repulsion is the basis for the working principle of dispersants. The surface rheology of the surfactant layer is important for the stability of foams and emulsions.


Classification and examples

There are four types of surfactants and a brief review of each type is provided below. These classifications are based on the polar composition of the head group: nonionic, anionic, cationic, and amphoteric.


Nonionic Surfactants

Nonionic surfactants have no charge groups in the head. Ionic surfactants have a head with a net charge. If the charge is negative, the surfactant is more specifically referred to as an anionic surfactant; if the charge is positive, the surfactant is referred to as cationic. If the surfactant contains a head with two oppositely charged groups, it is referred to as an amphoteric surfactant.


Phenethyl phenol formaldehyde resin pesticide emulsifier monomer

Phenethyl phenol formaldehyde resin pesticide emulsifier monomer


 

Anionic surfactants

Anionic surfactants contain anionic functional groups in their heads, such as sulfonates, phosphates, sulfates and carboxylates. These are the most common surfactants and include alkyl carboxylates (soaps), such as sodium stearate. Stearates account for more than 50% of global surfactant use. Many of these can be used in emulsion polymerization.

 

Cationic surfactants

Cationic surfactants consist of positively charged heads. Most cationic surfactants are used as anti-microbial agents, anti-fungal agents, etc. in household, institutional and industrial cleaners (benzalkonium chloride (BAC), cetyl pyridinium chloride (CPC), benzethonium chloride (BZT)). The cationic nature of surfactants is usually inconsistent with the world of nonionic and anionic charges, which can damage the cell membranes of bacteria and viruses.


Amphoteric surfactants

Amphoteric (amphoteric) surfactants have both cationic and anionic centers on the same molecule. The anionic portion can be variable and include sulfonates. Amphoteric surfactants are usually pH sensitive and will behave as anionic or cationic depending on pH.


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