What are the categories of cosmetic thickeners?

Thickeners serve as the backbone and core foundation of various cosmetic formulations, playing a crucial role in the product's appearance, rheological properties, stability, and skin feel. Selecting commonly used and representative thickeners, preparing different concentrations of aqueous solutions, testing their viscosity, pH, and other physical and chemical properties, and conducting sensory tests on multiple skin feel indicators both during and after application can provide useful references for cosmetic formulation design.

1. Types of Thickeners

Many substances can serve as thickeners. Based on molecular weight, there are low-molecular-weight and high-molecular-weight thickeners. Based on functional groups, there are electrolytes, alcohols, amides, carboxylic acids, esters, and more. Below is a classification of thickeners according to cosmetic ingredient categories.

1.1 Low-Molecular-Weight Thickeners

1.1.1 Inorganic Salts
Inorganic salts are commonly used as thickeners in surfactant aqueous systems. The most common inorganic salt thickener is sodium chloride, which has a noticeable thickening effect. Surfactants form micelles in aqueous solutions, and the presence of electrolytes increases the micelle association number, leading to the transformation of spherical micelles into rod-like micelles, thus increasing the system's viscosity. However, excessive electrolytes can affect the micelle structure, reducing viscosity, a phenomenon known as "salting out." Typically, electrolyte concentrations range from 1%-2% by mass, and are often used in combination with other types of thickeners to stabilize the system.

1.1.2 Fatty Alcohols and Fatty Acids
Fatty alcohols and fatty acids are polar organic compounds, sometimes considered non-ionic surfactants because they have both hydrophilic and lipophilic groups. A small amount of these organic compounds significantly influences the surface tension and other properties of surfactants. Their effect increases with longer carbon chains, generally following a linear relationship. The mechanism involves fatty alcohols and acids inserting into surfactant micelles, promoting micelle formation. Additionally, the strong molecular interactions (hydrophobic interactions between hydrocarbon chains and hydrogen bonding between polar heads) cause tight, ordered molecular alignment on the surface, significantly altering the micelle properties and achieving a thickening effect.

2. Thickener Categories

2.1 Non-ionic Surfactants

2.1.1 Inorganic Salts
Common examples: sodium chloride, potassium chloride, ammonium chloride, monoethanolamine chloride, diethanolamine chloride, sodium sulfate, trisodium phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, etc.

2.1.2 Fatty Alcohols and Fatty Acids
Examples include: lauryl alcohol, myristyl alcohol, C12-15 alcohols, C12-16 alcohols, decanol, hexanol, octanol, cetyl alcohol, stearyl alcohol, behenyl alcohol, lauric acid, C18-36 acid, linoleic acid, linolenic acid, myristic acid, stearic acid, behenic acid, etc.

2.1.3 Alkanolamides
Examples: cocamide DEA (cocamide diethanolamine), cocamide MEA (cocamide monoethanolamine), cocamide MIPA (cocamide monoisopropanolamine), cocamide, lauramide DEA, lauramide MEA, isostearamide DEA, linoleamide DEA, myristamide DEA, myristamide MEA, oleamide DEA, palmitamide MEA, PEG-3 lauramide, PEG-4 oleamide, PEG-50 tallow amide, etc.

2.1.4 Ethers
Examples: cetyl alcohol ethoxylate (3) ether, isocetyl alcohol ethoxylate (10) ether, lauryl alcohol ethoxylate (3) ether, lauryl alcohol ethoxylate (10) ether, Poloxamer-n (ethylene oxide-propylene oxide block copolymer, n=105, 124, 185, 237, 238, 338, 407), etc.

2.1.5 Esters
Examples: PEG-80 glyceryl tallowate, PEG-8 PPG-3 diisostearate, PEG-200 hydrogenated glyceryl palmitate, PEG-n (n=6, 8, 12) beeswax, PEG-4 isostearate, PEG-n (n=3, 4, 8, 150) distearate, PEG-18 glyceryl oleate/cocoate, PEG-8 dioleate, PEG-7 hydrogenated castor oil, PEG-40 jojoba oil, PEG-2 laurate, PEG-55 propylene glycol oleate, pentaerythrityl tetrastearate, pentaerythrityl tetrabehenate, propylene glycol stearate, behenate, cetyl palmitate, trimethylolpropane tristearate, etc.

2.1.6 Amine Oxides
Examples: myristamine oxide, isostearylamidopropylamine oxide, cocamidopropylamine oxide, wheat germamidopropylamine oxide, soybean amidopropylamine oxide, PEG-3 lauramine oxide, etc.

2.2 Amphoteric Surfactants
Examples: cetyl betaine, cocamidopropyl hydroxysultaine, etc.

2.3 Anionic Surfactants
Examples: potassium oleate, potassium stearate, etc.

2.4 Water-Soluble Polymers

2.4.1 Cellulose Derivatives
Examples: cellulose, cellulose gum, carboxymethyl hydroxyethyl cellulose, cetyl hydroxyethyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, etc.

2.4.2 Polyethylene Oxide
Examples: PEG-n (n=5M, 9M, 23M, 45M, 90M, 160M), etc.

2.4.3 Polyacrylics
Examples: acrylates/C10-30 alkyl acrylate crosspolymer, acrylates/steareth-20 methacrylate copolymer, acrylates/steareth-20 itaconate copolymer, acrylates/octylacrylamide crosspolymer, PAA (polyacrylic acid), sodium acrylate/vinyl neodecanoate crosspolymer, carbomer and its sodium salts, etc.

2.4.4 Natural Gums and Derivatives
Examples: alginate and its (ammonium, calcium, potassium) salts, pectin, sodium hyaluronate, guar gum, cationic guar gum, hydroxypropyl guar, acacia gum, carrageenan and its (calcium, sodium) salts, xanthan gum, scleroglucan gum, etc.

2.4.5 Inorganic Polymers and Derivatives
Examples: magnesium aluminum silicate, silica, magnesium sodium silicate, hydrated silica, montmorillonite, lithium magnesium sodium silicate, hectorite, stearalkonium hectorite, stearalkonium bentonite, quaternium-90 montmorillonite, quaternium-18 montmorillonite, quaternium-18 hectorite, etc.

2.4.6 Others
Examples: PVM/MA decene crosspolymer (polyvinyl methyl ether/maleic acid ester and decene crosspolymer), PVP (polyvinylpyrrolidone), etc.

2.5 Surfactants

2.5.1 Alkanolamides
The most commonly used is cocamide DEA. Alkanolamides can interact with electrolytes to enhance thickening, achieving the best results. The thickening mechanism involves interaction with anionic surfactant micelles, forming non-Newtonian fluids. Various alkanolamides differ significantly in performance, and their effects also vary when used alone or in combination. Some articles report on the thickening and foaming properties of different alkanolamides. Recent studies have highlighted the potential carcinogenic risk of nitrosamines formed from alkanolamides in cosmetics. Free amines in alkanolamide impurities are the potential source of nitrosamines. The personal care industry currently lacks official guidance on whether to ban alkanolamides in cosmetics.

2.5.2 Ethers
In formulations where sodium laureth sulfate (AES) is the primary active ingredient, appropriate viscosity is generally achieved with just inorganic salts. Research suggests that unsulfated ethoxylated fatty alcohols in AES significantly contribute to thickening. Further research indicates that an average ethoxylation degree of around 3EO or 10EO yields optimal results. Additionally, the thickening effect of ethoxylated fatty alcohols is influenced by the distribution of unreacted alcohols and homologs. Broader homolog distributions result in poorer thickening, while narrower distributions lead to more significant thickening.

2.5.3 Esters
The most commonly used thickeners are esters. Recent reports from abroad mention PEG-8 PPG-3 diisostearate, PEG-90 diisostearate, and PEG-8 PPG-3 dilaurate…