The original article on “Surfactants” (a contraction of the three words “Surface Active Agents”) was written in the fall of 2015i. Although the general topic of surfactants has not changed in nearly 6 years, several new chemistries have surfaced, and there wasn’t nearly as much emphasis on bio-based or green chemistry back then, as there is today. In addition, if you are new to coatings you should also be looking at ancillary markets such as personal care and cosmetics. Prospector has an excellent expert in Belinda Carli. See her article entitled “How to Select Natural Surfactants”ii. A natural surfactant has to have both the head and tail groups to come from truly natural sources. Personal care surfactants often have the same chemistry as paint surfactants, but perhaps different names or slightly different functions. Another article that pulls it all together is “breaking that tension with surfactants”iii
Thirty percent of global respondents are willing to pay a premium for products that deliver on social accountability claims (Nielsen, 2020). Bio-based surfactants are designated by the new EN17035. The chemical industry is now able to use a transparent definition and division which also enables to communicate in a comparable way to the end-user. Companies such as BASF, Ethox, Solvay and Locus Fermentation Solutions, among many others, are producing more bio surfactants.
Surfactants are materials that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. In the general sense, any material that affects the interfacial surface tension can be considered a surfactant, but in the practical sense, surfactants may act as wetting agents, emulsifiers, foaming agents, and dispersants, among others.
Surfactants play an important role as dispersing, emulsifying, cleaning, wetting, foaming and anti-foaming agents in many practical applications and products, including: paints, emulsions adhesives, inks, biocides (sanitizers), shampoos, toothpastes, firefighting (foams), detergents, insecticides, deinking of recycled papers, ski waxes, spermicides (nonoxynol-9). This is an article about paint, which means surfactants that are used in paint, emulsions, wetting agents, and in many items used in paint that are dispersed or emulsified.
The dynamics of surfactant adsorption is of great importance for practical applications such as in emulsifying or coating processes as well as foaming, where bubbles or drops are rapidly generated and need to be stabilized. As the interface is created, the adsorption is hindered by the diffusion of the surfactant to the interface, which can result in the kinetics being limited. These energy barriers can be due to steric or electrostatic repulsions; steric repulsions form the basis of how dispersants work. Surface rheology of surfactant layers, is important to the stability of foams and emulsions.
In the bulk aqueous phase, surfactants form masses, such as micelles, where the hydrophobic tails form the core and the hydrophilic heads are immersed in the surrounding liquid. Other types of structures can also be formed, such as spherical micelles or lipid bilayers. The shape of the molecules depends on the balance in size between hydrophilic head and hydrophobic tail. A measure of this is the HLB, Hydrophilic-lipophilic Balance. Higher HLB surfactants (>10) are hydrophilic (“water loving”) and form O/W (Oil-in-water) emulsions. Lipophilic surfactants possess low HLB values (1-10) and form W/O (water-in-oil) emulsions. Dish detergents, surfactants for emulsion polymerization, and the following example (SLS = Sodium Lauryl Sulfate) are high HLB surfactants.
Most surfactants’ “tails” are fairly similar, consisting of a hydrocarbon chain, which can be branched, linear, or aromatic. Fluorosurfactants have fluorocarbon chains. Siloxane surfactants have siloxane chains. Recent advances in surfactant technology have seen the development of mixed chains or/and complex structures. One example of mixed chain/complex structures is N,N-dimethyldodecylamine oxide (DDAO) and sodium decyl-, sodium dodecyl- and sodium tetra-decylsulfate (abbreviated as SDeS, SDS and STS, respectively).
There are 4 types of surfactants with a brief review of each as follows. These classifications are based upon the composition of the polarity of the head group: nonionic, anionic, cationic, amphoteric.
A non-ionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic. Commonly encountered surfactants of each type are listed as follows. A complete compendium can be found on www.ULProspector.com.
Many long chain alcohols exhibit some surfactant properties. Some examples of non-ionic surfactants include:
Trade name Structure/name Applications Triton™ X-100 Polyoxyethylene glycol octylphenol ethers: C8H17–(C6H4)–(O-C2H4)1–25–OH Wetting agent – coatings Nonoxynol-9 Polyoxyethylene glycol alkylphenol ethers: C9H19–(C6H4)–(O-C2H4)1–25–OH Spermacide Polysorbate Polyoxyethylene glycol sorbitan alkyl esters Food ingredient Span® Sorbitan alkyl esters Polishes, cleaners, fragrance carriers Poloxamers, Tergitol™, Antarox® Block copolymers of polyethylene glycol and polypropylene glycol Various
Anionic surfactants contain anionic functional groups at their head, such as sulfonate, phosphate, sulfate and carboxylates. Alkyl sulfates include ammonium lauryl sulfate, sodium lauryl and the related alkyl-ether sulfates sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), and sodium myreth sulfate. These are the most common surfactants and comprise the alkyl carboxylates (soaps), such as sodium stearate. The stearates comprise >50% of the global usage of surfactants. Many of these find utilization in emulsion polymerization. Other anionic surfactants include dioctyl sodium sulfosuccinate (DOSS), linear alkylbenzene sulfonates (LABs), as well as alkyl-aryl ether phosphates.
Trade name Structure/name Applications Pentex 99 Dioctyl sodium sulfosuccinate (DOSS) Wetting agent – coatings, toothpaste PFOS Perfluorooctanesulfonate (PFOS) Scotchguard™, Skydrol™ Calsoft® Linear alkylbenzene sulfonates Laundry detergents, dishwasher detergents Texapon® Sodium lauryl ether sulfate Shampoos, bath products Darvan® Lignosulfonate Concrete plasticizer, plasterboard, DMSO N/A Sodium stearate Handsoap, HI&I products
Cationic surfactants are comprised of a positively charged head. Most of cationic surfactants find use as anti-microbials, anti-fungals, etc. in household, institutional and industrial cleaners (Benzalkonium chloride (BAC), Cetylpyridinium chloride (CPC), Benzethonium chloride (BZT)). The cationic nature of the surfactants is not typically consistent with the world of non-ionic and anionic charges, and they disrupt cell membranes of bacteria and viruses. Permanently charged quaternary ammonium cations include: Alkyltrimethylammonium salts: cetyl trimethylammonium bromide (CTAB) and cetyl trimethylammonium chloride (CTAC).
Zwitterionic (amphoteric) surfactants have both cationic and anionic centers attached to the same molecule. The anionic part can be variable and include sulfonates, as in the sultaines CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate). Betaines such as cocamidopropyl betaine have a carboxylate with the ammonium. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. Zwitterionic surfactants are often sensitive to pH and will behave as anionic or cationic based on pH. Fast dry (“coacervation”) latex traffic paints are based on this concept, with a drop in pH triggering the latex in the paint to coagulate.
[i] Surface Active Agents (Surfactants): Types and Applications (ulprospector.com)
[ii] How to select natural surfactants for personal care products (ulprospector.com)
[iii] Surfactants in paints: how they work and current market trends | Prospector (ulprospector.com)
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