Alkyl ether sulfates

Shampoos based on lauryl sulfates can range from 6—17% of the active surfactant. However, though they are effective cleansers, the alkyl sulfates tend to be defatting. In an effort to make these shampoos more mild, many shampoos are now based on blends of amphoterics and alkyl sulfates or the less irritating alkyl ether sulfates. 

A.lpha-Olefm Sulfonates. Sulfonation of alpha-olefins yields a mixture of alkene sulfonates, hydroxyalkane sulfonates, and some amount of various disulfonates. These detergents are excellent foamers with good detergency properties. They are unaffected ia hard water and thek effects are considered superior to the alkyl ether sulfates (9). 

A.lkylSulfoacetates. These surfactants are prepared by esterification of sulfoacetic acid or by sulfonation of the alkyl chloroester. They are considered to produce good foaming and are less irritating to the eyes than the alkyl and alkyl ether sulfates (10). 

It has been found that the CMC values are higher and the micelle aggregation numbers smaller than those of the corresponding nonionic surfactants. The CMC increases with increasing EO chain, which is, according to the authors, opposite to the results for sodium alkyl ether sulfate. 

These higher foaming properties are very useful for such cosmetic formulations as shampoos, showerbaths, and so on. This is the same with the forming of fine bubbles and the improving of foam stability of other surfactants such as, for example, alkyl ether sulfates due to the combination with ether carboxylates [57,67-69] (Table 9). 

Although by 1957 [9] the use of ether carboxylates also in combination with other surfactants such as alkyl ether sulfates for shampoos had been described,... 

In combination with alkyl ether sulfates, a synergistic decrease of the irritation level of the ether sulfates and an improvement of the foam stabilization has been described [57,67,78]. A good compromise between mildness and foam properties could be achieved with lauryl ether carboxylic acid sodium salt with 10 mol EO [57,67]. In several articles examples of the use of alkyl ether carboxylates as cosurfactant in mild shampoos as well as bath and shower products have been described [57,69,79]. 

All of the aspects and applications described in papers can be traced back to the patent literature [73]. The special or synergistic effect of ether carboxylates in combination with other surfactants is very often the basis, like combinations with alkyl ether sulfates or alkyl sulfates, for shampoos, foambaths, showerbaths, and liquid soaps [9,70,81-85], and the same for amidether carboxylates [71]. 

In 1991 the use of amidether carboxylate in combination with anionics like alkanesulfonate, alkylbenzolsulfonate, and alkyl ether sulfate to improve mildness and foam properties in the presence of soil was described [72]. Later a comparison study of the use of alkyl ether carboxylate, amidether carboxylate, and cocamidopropylbetaine in concentrated dishwashing formulations showed, in addition to the above-mentioned properties, the advantage of ether carboxylates in the construction of highly active formulations [144]. 

Alkyl Ether Sulfates (AES) R=Cio C14 m=l-4 Liquid bath soaps, hair shampoos, and mechanical dishwashing agents. Ingredient in industrial cleaning agents - Effluents of seven representative STPs Cl2-15 AES 3 and 12 pg L 1 [15] Readily biodegradable in WWTPs under both aerobic and anaerobic conditions [18,29]... 

Alkyl ether sulfates are/after alkyl benzene sulfonates(LAS),the group of technically important anionic surfactants with the largest production voluJne and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed,for example,in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application. 

Alkyl ether sulfates with chain lengths ranging from to Cw are quantitatively the most important products currently based on fatty alcohols. It is estimated, that about 20 % of all surfactant alcohols -about 40 % of all fatty alcohols in the coconut range (C,2 Ci,) - are used in the form of alkyl ether suirateg ( ). Alkyl ether sulfates are the most important group of anionic surfactants after linear alkyl-benzenesulfonate (LAS) (2). 

In 1980, the total consumption of alkyl ether sulfates in Western Europe equaled that of all other anionic surfactants with the exception of alkyIbenzene-sulfonates (Fig. 1). Since alkyl ether sulfates are the most expensive group of anionic surfactants, which are produced in larger quantities, their importance on a... 

In addition, in the USA ether sulfates are used on a large scale in laundry detergents. For that reason, their market share in the USA is even larger than in Europe (3), At present, alkyl ether sulfates are primarily useH in the form of their Na salts. In the past, however, cosmetic preparations containing ammonium and Mg salts have also been quite common (4). 

Other Anionic Surfactants Alkyl Ether Sulfates... 

An especially effective reduction of the Krafft Point results from the insertion of ether groups into the molecule of the anionic surfactant. In table I this is examplified with Na dodecyl sulfate and Na-tetra-decyl sulfate in comparison to various n-alkyl ether sulfates of the same chain length (10). As a measure of the Krafft Point, a temperature is deTined at which a 1 7o solution dissolves clearly. By the incorporation of oxyalkylene groups into the molecule, the Krafft -Point and the melting point are greatly depressed. This depression is especially effective if there is branching in the oxyalkylene groups. 

The lower Krafft Points resulting from the incorporation of oxyethylene groups into the surfactant molecule is an essential, but not sufficient, property for the utilizationof alkyl ether sulfates. 

In the case of alkyl ether sulfates, an increase in the number of oxyethylene groups produces an opposite result (19 - 22). Fig. 3 shows a decrease of the critical miceTTe concentration (Cj ) and the concentration for a given surface tension (for example,... 

The dissociation of the counter-ions of micelles is a characteristic feature of ionic surfactants. For alkyl ether sulfates the degree of dissociation a of the counter-ions of the micelles has been determined by several authors by various methods (2, 26), With... 

Therefore, the hypothesis of an increasing nonionic character of alkyl ether sulfates with increasing number of oxyethylene groups is not tenable. Some time ago (30), it was suggested that a certain hydrophobic nature can be attributed to the polyoxyethylene chain of alkyl ether sulfates. At first, this appears to be in contradiction to the decidedly hydrophilic character of the polyoxyethylene chain for nonionic surfactants. However, the possibility of EO group hydration impairment by the sulfate group cannot be excluded. 

Fig. 6 Adsorption of alkyl ether sulfates on graphon (plateau values) at 25°C (product purity 98-99.5%)...

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

Cosmetic Rinse off Preparations. In certain cosmetic products, tor example hair shampoos, it is not possible to use complexing agents because of the irritation of mucous membranes. Here a low sensitivity of surfactants to water hardness is a precondition for their application. Foam formation is generally considered to be a measure of shampoo quality (Table V). With increase in water hardness the foam volume of alkyl sulfates decreases very much, whereas with the corresponding alkyl ether sulfates this decrease is relatively small. For cosmetic applications, the good skin compatibility and low irritation to mucous membranes of alkyl ether sulfates is of high importance (37). 

The middle phase is much more viscous than the neat phase, though the concentration of the latter is higher. If a concentrated system of an alkyl ether sulfate and water existing initially as a neat phase is stepwise diluted, the range of the middle phase will be reached. This is accompanied by a steep increase in viscosity. The formation of the neat phase allows the manufacture and handling of highly concentrated fluid preparations. This property, however, can lead to processing problems upon dilution. 

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