Anionic Surfactants

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Soaps for detergent have been in use since 3000 bc. Primary detergents in early shampoos before the 1950s were mainly potassium or ammonium salts of fatty acids. These soaps have good foaming performance in pure water, although only slightly so in hard water because of the formation of insoluble metal soaps [2]. Various synthetic surfactants have been developed during the past 50 years. They have come to replace soaps and are soluble even in hard water. The most common synthetic surfactants are alkyl sulfate (AS) and alkyl ether sulfate (AES). These initially appeared on the U.S. market more than 50 years ago, and liquid shampoos subsequently came to be used throughout the country in the 1960s. Ammonium or ethanolamine salts of AS and sodium or ammonium salts of AES were used on a particularly large scale for the preparation of many products. Through the use of ethylene oxide (EO) groups, AS increases solubility and reduces precipitate of Ca salt and foam volume. Increase in solution viscosity is essential for enhancing shampoo appeal to customers. Alkanol amides of fatty acids are effective for viscosity and foam enhancement.

Alpha-olefin sulfonate (AOS) is commonly used as an anionic surfactant in shampoos [3]. A surfactant is a mixture of hydroxyalkane and alkene sulfonates whose structures are shown in Figure 1. AOS exhibits excellent stability at low pH compared with AS or AES and is more soluble in hard water than AS. Increase in solution viscosity has been shown possible through the use of alkanol amides and anionic surfactants in combination.

Various surfactants as supporting ingredients are used in the absence of complete

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Figure 1 Surfactants for shampoos and rinsing agents on the Japanese market.

functional performances. Alkyl sulfosuccinates exhibit excellent foaming capacity, and their use is attended with low skin irritation provided AS is present [3]. In the 1980s, surfactants with low skin irritation came into popularity. Several amino acids have been developed for surfactant use, such as acyl glutamate [4]. These have excellent foaming, good biodegradability, and low skin irritation. Acyl amino acids such as lauroyl P-ala-ninate [5] and N-methyl P-alaninate [6] are presently in use. N-acyl methyltaurate [7] is also available and has been proven ideal for shampoo use with low skin irritation.

Nonionic and Amphoteric Surfactants

Nonionic surfactants are preferable to those that are anionic, but have found limited use owing to poor foaming capacity for shampoos. Alkanol amides and alkyl amine-oxides are used primarily as foam boosters and stabilizers [3]. Alkyl glucoside may be obtained through reaction of fatty alcohol with glucose; it is mild to the skin and has good foam stability [8].

Amphoteric surfactants are used in combination with anionic and nonionic surfactants to achieve greater shampoo mildness. A typical amphoteric surfactant is N-acyl ami-dopropyl betaine [3] featured by low skin irritation and foaming enhancement. Alkyl imi-nodiacetates may be obtained from fatty amines as mild surfactants [9]. The cocoylarginine ethyl ester (CAE) is prepared from arginine and shows high affinity to hair [10,11]. A new mild amphoteric surfactant, Amisafe, is derived from arginine [12] and functions as a cationic surfactant at weakly acidic pH and is readily adsorbed onto hair.

Cationic Surfactants

Because of the negative charge on the surface of hair, cationics strongly bind to hair and are difficult to remove by rinsing. When a shampoo containing soap has been used, acidic rinse containing citric acid may be applied to remove the alkali and metal soaps. Dialkyl ammonium salts are used in rinse formulations for shampoos containing AS and AES as main ingredients [13]. Quaternary ammonium salts containing mono- or dialkyl groups with 16 to 22 carbon atoms are presently in wide use. At the start of the 1980s, a milky lotion-type rinse came into prominent use. It was produced by adding oils to a gel comprising cationic surfactant, fatty alcohol, and water. Novel cationic surfactants are presently being produced. Quaternary ammonium salts made using long-chain Guarbet alcohol form lamellae liquid crystals even in cold water and are readily adsorbed onto hair [14]. Amido guanidine cationic surfactants (AG) with methylene groups as spacers between amide and guanidino groups [15] are available, and there is a hair conditioner containing AG with excellent moisturizing properties even at low humidity.

Micelle Formation and Surfactant Solubility

The high solubility of surfactants in water is very important in the preparation of cosmetic products. Surfactants show characteristic solubility because of the presence of hydropho-bic groups, which squeeze out hydrocarbon chains of surfactants to bring about micelle formation [16]. A phase diagram of the two-component system is shown in Figure 2 [17]. At dilute surfactant concentration, micelle formation occurs above a critical temperature and at surfactant concentration above the critical micelle concentration (CMC). In region I, surfactant concentration is too low for micelle aggregation to occur, and consequently the surfactants dissolve into monomers. In region II, surfactant micelles are equilibrated with monomers. In region III, surfactant monomers are present along with precipitated hydrated solid surfactants. That is, the micelles comprise melting hydrated solid surfactants beyond the phase boundary curve between regions II and III. The point where the two phase boundary curves intersect is the Krafft point of a surfactant solution.

Liquid Crystals and Gels

Various intermediate phases may exist between solid and liquid states. At high surfactant concentration in Figure 2, several liquid crystalline phases can be seen to have formed. The liquid crystalline phases of surfactant-water systems are in the liquid state with a long-range repulsive order of one, two, or three [18,19]. With increase in surfactant concentration, the hexagonal (IV), cubic liquid crystalline (V), and lamellae phases (VI) are produced. The hexagonal phase consists of long rod micelles of surfactants hexagonally arranged. The lamellae phase comprises surfactant bilayers separated by water layers. The water layers vary in thickness from 10 ¿A to several 100 A. The hexagonal and lamellae

Figure 2 Schematic phase diagram of an ionic surfactant. (From Ref. 17.)

phases are optically anisotropic, whereas the cubic liquid crystalline phase is optically isotropic. The cubic phases may take on various structures such as packed spherical micelles in a cubic array, surfactant rods connected in a complex manner to form a continuous network, and bicontinuous networks with positive and negative curvature interfaces [19,20].

In liquid crystalline phases, hydrocarbon chains are in a liquid-like state. When these phases are cooled, a coagel phase consisting of hydrated crystals and a gel phase are formed as shown in Figure 3 [21,22]. The gel phase contains fairly ordered intermediate water, except for hydrated water, between surfactant bilayers. This phase is produced on warming the coagel phase when hydration interactions occur between counter ions. Phase diagrams for octadecyltrimethyl ammonium salts show the stability of the gel phase.

Phase Behavior of Nonionic Surfactants

Increase in nonionic surfactant aqueous solution temperature causes the development of two isotropic phases in solution, above what is called the cloud point. The hydrophilic/ hydrophobic balance of a nonionic surfactant may differ considerably at this temperature, and consequently there is characteristic phase behavior in nonionics/hydrocarbon/water ternary systems, as is the case when using a plane of fixed 1: 1 weight ratio of oil to water, as shown in Figure 4 [23]. At lower temperature, nonionic surfactants are highly soluble in water and form O/W microemulsions in a water-rich phase with excess oil. At higher temperature, they are highly soluble in oil and form W/O microemulsions in an oil-rich phase with excess water. At the phase inversion temperature, a three-phase system comprises a middle phase microemulsion, a nearly pure water phase, and an oil phase. Phase transition with temperature is indication of potential for cosmetic use.

Liquid crystal and m ¡cellar solution

Liquid crystal and m ¡cellar solution


Figure 3 Changes in the aggregation of surfactants and water molecules in response to increase in temperature. (From Ref. 21.)

Figure 4 Vertical section of the phase prism of a ternary system for H2O/Oil = 1/1. (From Ref. 23.)

Figure 4 Vertical section of the phase prism of a ternary system for H2O/Oil = 1/1. (From Ref. 23.)

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