Lowering surface tension – Surfactants in coating materials

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The original article on &#;Surfactants&#; (a contraction of the three words &#;Surface Active Agents&#;) was written in the fall of i. 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, ). Bio-based surfactants are designated by the new EN. 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 surfactants

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.

Resources

[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)

The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.

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As consumers become more aware of the ingredients in the products they put on their skin and use in their homes, many have come to view surfactants as &#;bad&#; ingredients. In fact, there are numerous choices for natural surfactants in cosmetics, and they work to improve the integrity and performance of cosmetic formulations. Yet conscientious consumers look unfavorably on body, hair and skincare products that contain surfactants, especially Sodium Lauryl Sulfate (SLS). While the scientific evidence does not support the claim that SLS is a cancer-causing agent, the myth prevails and for natural and organic cosmetic formulators, finding natural alternatives is a necessary part of serving your customer base. The good news is that surfactants are the largest class of cosmetic chemicals and offer broad cleansing capabilities. Surfactants also allow cosmetics to slip across, onto or to clean the skin by breaking up and separating from the skin oils, fats, makeup, dirt, pollution and other debris. When you plan your cosmetic formulation, you&#;ll have a wide variety of options beyond SLS.

 

What does a Natural Surfactant Do?

A surfactant is a surface active agent.

Surfactants are mixed with oils, waters, and other liquids to lower the surface tension, a prerequisite for wetting, spreading, foaming, and emulsification. Through a chemical process known as adsorption, surfactants change the properties of a substance. The term adsorption means the gathering of gas or liquid in a condensed layer on the surface. This condensed layer creates a film which is why the surface tension is lowered. The chemical reaction which then occurs is the conversing of the liquid and the additional substance, which in turn lowers surface tension.

Surfactants are not always described in their most common form, but rather for the role they play in a formulation. For instance, in recipes in which foam is the finished product, the surfactants used may be referred to as &#;foaming agents&#;. Surfactants used in body and hair care recipes may be termed &#;detergents&#; or &#;soaps.&#; If you&#;re creating a shaving cream, the surfactants you chose will be for their lubricating and protection properties; they will help protect the skin from irritation and the razor&#;s sharp edge.

As you create your formulations, you will need a general idea of each group of natural surfactants and how they play a role in your finished product. Additionally, you&#;ll want to understand why the surfactants are grouped or classified together or how/if surfactants work with additional groups of surfactants in a collaborative manner. Not all natural surfactants can work together in the same formula

 

Categories of Natural Surfactants

Natural surfactants are placed in one of four categories based on their interfaces and charges:

1. Anionic surfactants
Anionic surfactants are most commonly used in foaming products including shampoos and body washes. Their primary functions are to deliver high foaming, high cleansing, and high washing capabilities in a finished product. Anionic surfactants have a negatively charged water-loving head. Anionic surfactants work very well in recipes which have a reaction between a chemical (like lye) and fatty acids or alcohols (like animal lard or vegetable-based oils). Hand-processed soap is an example of anionic surfactants at work. Other examples of anionic surfactants are sodium sulfates, ammonium sulfates, sulfosuccinates, sarcosines, sarcosinates, isethionates, and taurates. The high foaming, cleansing, and washing properties can lead to skin irritations so depending on your product, you may want to choose another category of surfactants, or balance the anionic surfactants with the addition of amphoteric surfactants.

2. Amphoteric surfactants
Amphoteric surfactants are the most gentle and flexible of the surfactant categories, and can either have a positive or negative charge, depending on the pH or alkalinity of your finished product. Hence the amphi prefix. If you are formulating a product in which the nourishing and conditioning properties are most important, then you may choose an amphoteric surfactant and give the finished product a lower pH. On the other hand, if sudsy and foamy are your goal, create a recipe with a higher pH when using an amphoteric surfactant. With the higher pH, the amphoteric surfactant will work more like an anionic surfactant. When used alone, amphoteric surfactants will give you a gentle cleansing product. Adversely, when an amphoteric surfactant is coupled with an anionic surfactant, the amphoteric surfactant mellows the harshness of the anionic surfactants. In fact, amphoteric surfactants can be used solo and in conjunction with any other of the surfactant groups. Their adaptability is just one of the reasons why they are so widely used. Some examples of well know amphoteric surfactants are coco betaine, lauryl betaine, and hydroxysultaines.

3. Cationic surfactants
Cationic surfactants are the opposite of anionic surfactants. They have a positively charged water-loving head. This positive charge enables cationic surfactants to deliver nourishing benefits to skin, hair, and body, but used alone, do not have high foaming capabilities. These surfactants are often used where foaming isn&#;t necessary, such as in hair conditioners.Cationic surfactants work well with both amphoteric and nonionic surfactants. However, because of the opposing charge cationic (positive) and anionic (negative) surfactants will not combine.Common cationic surfactants for bath and body formula are the chlorides; benzalkonium, stearalkonium, and centrimonium, and trimethyl ammoniums, and methyl sulfates.

4. Nonionic surfactants
Nonionic surfactants have no foaming capabilities and are rarely used as a recipe&#;s main surfactant. Unlike anionic surfactants, these surfactants do not have an ionic charge in their water-loving heads. The end result of using a nonionic surfactant is gentle, no-foam cleansing product. No-foam cleansers my need extra marketing messaging to reinforce that the cleanser works even without the foam. Some nonionic surfactants are ethoxylated, meaning they have had some reaction to the addition of ethylene oxide. With this reaction comes an even more water-loving head, almost as if it has been supercharged. This then makes nonionic surfactants like Polysorbate 20 excellent solubilizers. Nonionic surfactants can also be used in formulations to reduce irritants, due to their gentle cleansing ability. And, they are excellent as an emollient, softening and soothing skin. Plus, the nonionic surfactants can stabilize foam in recipes. Nonionic surfactants work wonderfully with every other category of surfactants. Some common nonionic surfactants used in bath and body recipes are polysorbates, emulsifying wax NF, e-wax, glyceryl oleate, glyceryl stearate, and ingredients with the prefix PEG, ceteareths, oleths, sorbitans, lauryl glucoside, and polyglycose.

Natural surfactants are hardworking ingredients to help your cosmetics formulas shine. Whether you&#;re looking for cleansing, solubility, emulsifying, or conditioning, consider adding one or more surfactant to your recipe.

 

Glossary of Natural Surfactants

Essential Wholesale & Labs has a wide variety of surfactant that you can browse here. Not sure which surfactant will work best for you? Below is a list of some commonly added surfactants to natural cosmetics.

Sodium Lauryl Sulfate (can be derived from coconuts) A strong anionic surfactant used for high foaming action. It is easy to thicken and can cause irritation in higher percentages.
Ammonium Laureth Sulfate (derived from coconuts) A strong anionic surfactant used for high foaming action. Can cause irritation in higher percentages.
Disodium Lauryl Sulfosuccinate (derived from coconuts), A mild anionic surfactant that is gentle on the skin and mildly foaming
Cocoamphocarboxyglycinate (derived from coconuts) A mild, amphoteric surfactant
Decyl Polyglucoside (vegetable derived) used in baby shampoos because of its gentleness
Cetearyl Alcohol
Stearyl Alcohol
Cocamidopropyl Betaine (derived from coconut oil) amphoteric surfactant
Decyl Glucoside (derived from sugar)
Glyceryl Cocoate (derived from vegetables)
Sodium Cocoyl Isethionate (derived from coconuts)
Almond Glycerides (derived from vegetables)
Sodium Lauryl Sulfoacetate (much milder surfactant than SLS
Sodium Lauroyl Sarcosinate (derived from vegetables) Is a natural substitution for SLS
Sodium methyl cocoyl taurate (derived from coconut)
Sucrose Cocoate (derived from sugar)
Polysorbate 20 (vegetable derived) nonionic surfactant
Polysorbate 80 (vegetable derived) nonionic surfactant

You can learn more about polysorbates here.

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