In aqueous solution, anionic surfactants form a negatively charged ion provided the composition pH is neutral to alkaline. The ionized moiety can be a carboxylate, sulfate, sulfonate or phosphate. Among most frequently used surfactants in skin care products, the alkyl sulfates and alkyl ethoxylated sulfates can be mentioned for their high foaming capacity. Anionics are generally used in association with other surfactants (nonionics or amphoterics), which bring improvements in the skin tolerance, in the foam quality, or in the product viscosity.
Other anionics are also used in personal products, as secondary surfactants, often for their milder profile and their low foaming properties (isethionates, sulfosuccinates, taurates, sarcosinates, phosphoric acid esters, acylglutamates, etc.).
Carboxylate Salts. Surfactants belonging to this class generally derive from oleo-chemistry; carboxylate salts (or soaps) can be directly produced by the alkaline hydrolysis (or saponification) of animal and vegetable glycerides or can result from the neutralization of fatty acids obtained by the acidification of carboxylates.
Saturated sodium soaps are extremely soluble in water up to C8; they become less soluble up to C18 and insoluble above C20. The fatty acids can be either saturated or unsaturated (starting from Ci6 chain lengths). Unsaturated fatty acids are prone to undergo oxidation and form oxides and peroxides, which cause rancidity and yellowing. Potassium soaps and salts of alkanolamines are more fluid and also more soluble than sodium salts. The extremely low solubility of alkaline earth and heavy metals fatty acid salts makes this class of surfactants less appropriate for use in hard water.
The main application of fatty carboxylates is found in the soap bars widely used in the world for fabric handwash (generally based on tallow/coconut oil mixtures). Water-soluble soaps are mainly used in skin cleansers (soap bars or liquids), shaving products (sticks, foams, or creams) and deodorant sticks. Mixtures of fatty acids and their salts are used in ''acid soaps.'' Water-insoluble soaps form gels in nonaqueous systems and, because of their hydrophobicity, they can be appropriate surfactants for w/o emulsions.
Ester Carboxylates. This class of surfactants is a subcategory of the previously discussed surfactant group based on carboxylic acids; they are monoesters of di- and tricarboxylic acids. These esters are produced by condensation reactions involving different types of molecules; either an alcohol with a polycarboxylic acid (e.g., tartric or citric acid), or a hydroxyacid (e.g., lactic acid) with a carboxylic acid. The reacting alcohol may have been previously ethoxylated.
' Sodium dilaureth-7 citrate ^ a
Because of their good foaming properties and substantivity on the hair, ester carboxylates -c are especially suitable in shampoos; in combination with alcohol ethoxy sulfate (AEOS), ^
they provide reduced skin irritation. Short chain lactylates (i.e., issued from lactyllactic ^
acid) are substantive on the skin and present humectant properties.
Ether Carboxylates. These surfactants are formed by the reaction of sodium chloracetate with ethoxylated alcohols. Because of the addition of ethoxylated groups, ether carboxylates are more soluble in water and less sensitive to water hardness compared
with conventional soaps. Also, keeping the best properties of nonionic surfactants, they do not exhibit any cloud point and show good wetting and foam stability. Ether carboxylates do not undergo hydrolysis in the presence of alkali or acids.
Atkyl polyglycol other carboxylate, sodium sail
Ether carboxylates are used as general emulsifier and emulsion stabilizers. In the household, they are used in acidic toilet bowl cleaners. In personal care, they impart mildness, creamy foaming, skin-feel, and hair-conditioning benefits. Therefore, they are especially suitable in shampoos in combination with alcohol ether sulfates and possibly with cat-ionics.
Alkyl Sulfates. Alkyl sulfates are organic esters of sulfuric acid; they vary by the length of the hydrocarbon chain and by the selected counterion. Alkyl sulfates are produced by sulfation of the corresponding fatty alcohols. The properties of alkyl sulfates depend mainly on the chain length and the degree of branching of the hydrocarbon chain, as well as, to a smaller extent, on the nature of the counterions. They are generally good foamers, more especially in hard water; best foam characteristics are obtained in the Cj2-C14 chain length range.
Sodium lauryl sulfate (SLS) has a 12-carbon chain length and is one of the most common surfactants. It is not well tolerated by the skin. When the chain length increases (Cj4-Cj8 range), surfactant penetrability through the stratum corneum decreases along with the irritation potential of the surfactant, but the foaming capacity is accordingly depressed. Chains with carbon number lower than 12 are better tolerated by the skin than SLS, but are smellier. Combination with other surfactants allows considerable improvement of the skin compatibility of lauryl sulfate while keeping a good foam. It is, however, less frequently used than its ethoxylated counterpart. Lauryl sulfate is available under the form of various salts: sodium lauryl sulfate (SLS), ammonium lauryl sulfate (ALS), magnesium lauryl sulfate [Mg(LS)2], and triethanolamine lauryl sulfate (TEALS). Skin tolerance of lauryl sulfates is as follows: Mg(LS)2 > TEALS > SLS > ALS.
Alkyl sulfates are used in cosmetics and personal-care areas (e.g., DEA lauryl sulfate in shampoos); they are associated with other surfactants and improve foaming characteristics of detergent systems. Pure SLS (sodium lauryl sulfate) is used in oral care and incorporated in dental creams, essentially as a foaming agent.
Alkyl Ether Sulfates. Alkyl ether sulfates (AES), which are also called alcohol ethoxy sulfates (AEOS), result from the sulfation of an ethoxylated alcohol. Compared with alkyl sulfates, the ether sulfates show higher water solubility, improved foam stability in hard water, and better skin tolerance. The viscosity of surfactant solutions of ether
sulfates is much more sensitive to the presence of electrolytes than alkyl sulfates; formulators often take advantage of this opportunity to bring liquid formulations to the desired viscosity by simply adjusting the salt level (e.g., NaCl). The higher the number of ethoxy groups (EO) in the molecule, the lower the surfactant ability to penetrate the stratum corneum and the less irritant for skin it will be. Similar ranking is true for eye irritation. Also, the foaming capacity decreases as ethoxylation degree increases.
Sodium alkyl tether sulfate
Alkyl ether sulfates are used in domestic applications such as household cleaners, dishwashing liquids, and fabric care.
Alkyl ether sulfates are also extensively used in personal products such as liquid soaps, shower gels, foam baths, and, more especially, shampoos. Sodium lauryl ether sulfate (SLES) is today the most currently used primary tensioactive, especially under the forms of SLES-2 EO and SLES-3 EO, which combine good foaming and skin compatibility properties.
Amide Ether Sulfates. The amide ether sulfates are obtained by sulfation of the corresponding ethoxylated amide. The magnesium salts foam well and their skin compatibility is excellent.
Because of their weak lipid removal effect, amide ether sulfates are used in very mild personal cleaners.
On a chemical standpoint, there is an important difference between the previously discussed alkyl sulfates and the alkyl sulfonates: in the former, the sulfur atom is linked to the carbon chain via an oxygen atom, and in the latter, the sulfur atom is directly linked to the carbon atom.
Alkyl Sulfonates. Three major types of alkyl sulfonates must be considered: the primary and secondary paraffin sulfonates (PS and SAS) and the a-olefin sulfonates (AOS). The paraffin sulfonates are very water-soluble surfactants, good foamers, and good o/w emulsifiers. Their solutions do not thicken easily upon salt addition. Therefore, they are particularly appropriate to formulate fluid liquids or highly concentrated products. The a-olefin sulfonates (AOS) have general properties fully comparable to LAS (see next section); they are good o/w emulsifiers, wetting, and foaming agents.
Primary sodium alkyl sulfonate
raj - (CH2)m - Ç - S03 Na CHj(CH2)tl Seccndaiy sodium alkyl sulfonate
Constituents of «.-olefin sulfonate: sodium alkenc sulfonates and sodium hydroxy alkane sulfonate
Alkane sulfonates (PS and SAS) are mainly used in Europe in detergent products. Alpha-olefin sulfonates have been mainly used in Asia as surfactants for heavy and light duty laundry detergents, synthetic soap bars, and household products. Because they are less irritating than alkyl-aryl sulfonates, they have also been used in the United States in several personal products (liquid soaps, bubble baths, and shampoos) as alternatives to alcohol ether sulfates. They are also marginally used in oral care formulations.
Alkyl Aryl Sulfonates. Today, the LAS (linear alkylbenzene sulfonate) is the most important surfactant on a volume basis, but its use in personal care is very limited because of a low skin compatibility. It is worth mentioning that some methyl or methyl-ethyl substituted aryl sulfonates, i.e., sodium xylene, toluene, or cumene sulfonates (SXS, STS, or SCS), although not showing typical surfactant properties, are used as hydrotropes (i.e., decreasing hydrophobic effects in aqueous systems).
Sodium linear alkylbenzene sulfonate (LAS) is a very cost-effective surfactant that is extensively used in a broad variety of detergents for household, fabric care, and institutional and industrial products. Because of its too-high detersive action, LAS has a relatively low compatibility with skin and is only scarcely used in cosmetics except in some antiseborrheic preparations.
Sulfosuccinates. Sulfosuccinates are the sodium salts of alkyl esters of sulfo-succinic acid; they generally result from the condensation of maleic anhydride with a fatty alcohol, followed by a sulfonation with sodium bisulfite NaHSO3. Some variants of sulfosuccinates are derived from other substituted fatty molecules such as fatty alcohol ethoxylates, fatty amines (yielding sulfosuccinamates), or fatty alkanolamides.
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