Ether carboxylates

Ether carboxylates are a very versatile class of surfactants, used in diverse applications from mild personal care formulations to lubricants and cutting fluids. They are interrupted soaps, with the addition of a number of ethylene oxide groups between the alkyl chain and the carboxylate group. The additional solubility imparted by the EO groups gives much greater resistance to hardness and reduced irritancy compared to soap. 

Shell Chemicals has also developed a process where the carboxylate is prepared by direct oxidation of an ethoxylated alcohol. 

Raw materials. The base materials for ether carboxylates are typically ethoxylated alcohols, although ethoxylated aromatics or alkanolamides may also be used but a wider range of alkyl chains and degrees of ethoxylation are used in ether carboxylates than in ether sulphates. Carbon chains from C4 to C20 and degrees of ethoxylation from 2 to 20 may be combined to give the required properties in the surfactant. 

Sources and properties of alcohol ethoxylates are covered in more detail under alkyl sulphates and alkyl ether sulphates. 

Composition vs. properties. With such a wide pallet of raw materials, it is possible to produce a very wide range of attributes in the surfactant and the HLB may be varied from 8 (low C number, low EO), to give surfactants soluble in organic media, to 20 (very good aqueous solubility). Ether sulphates (laureth-2 or laureth-3) would have HLB values of 20. 

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On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

Table 7. Selectivity orders for transport of alkali metal ions into toluene by crown ether carboxylic acids for several separation techniques...

Ether carboxylates are known for a long time. In 1934 a patent was applied for [1] in which some sodium salts of polyether carboxylated acids of the general formula... 

A dissertation from 1943 [2] describes an interesting investigation concerning synthesis and properties of pure ether carboxylates. With metallic sodium and chloroacetic acid ethyl ester followed by saponification, ether carboxylates were made with the general formula... 

Until the 1950s ether carboxylates were almost in very limited amounts in the textile industry. It was only in 1957 [9] that the first ethercarobxylates were mentioned, in combination with other surfactants such as alkyl sulfates and ether sulfates, for use in shampoos. In spite of the special properties of ether carboxylates, their use was low in the industry as well as in cosmetics at that time. This was also due to the fact that at that time properties such as toxicity, biodegradability, and mildness to the skin did not have the high priority they do now. 

However, in a 1963 lecture it was pointed out that ether soaps had special characterics [10], i.e., good biodegradability, mildness to the skin, and less corrosiveness to metals, and that we should expect these soaps to play an important role in the future. However, the real breakthrough of the ether carboxylates came in the 1980s, when environmental properties of surfactants became even more important along with other properties of ether carboxylates such as chlorine stability, anticorrosiveness, lime soap dispersibility, electrolyte stability, alkaline stability, and so on. 

At the same time, in cosmetics more and more emphasis was placed on mildness, so that the use of ether carboxylates in such products as shampoos, foam baths, and shower baths began to increase. 

The most used ether carboxylates are the alkylpolyoxyethylene carboxymethyl-ates with the structure ... 

The ether carboxylates often have good emulsifying properties. The disadvantage of the polyoxypropylene carboxymethylates is that their biodegradability is worse. 

There are two important methods for the synthesis of ether carboxylates from ethoxylated fatty alcohols ... 

The real Willamson synthesis with metallic sodium and sodium chloroacetate is only used for the preparation of pure ether carboxylates for analysis purposes or to obtain physicochemical measurements [229]. 

The modified Williamson synthesis with NaOH and chloroacetic acid or monochlorosodium acetate for the preparation of ether carboxylates is very old [3-8] and is suitable for batch production of different types of ethercarboxylates. 

The conversion of the nonionic to the ether carboxylate depends, among other things, on... 

Ethoxylation grade (the higher the ethoxylation grade the higher the conversion to ether carboxylate)... 

Sometimes the reaction mixture, especially when chosen for a low conversion and with a high ethoxylation grade, implies a mixture with a relatively low NaCl content, directly diluted with water. But mostly with this type of ether carboxylate a washing step is introduced in order to remove NaCl and other... 

With the modified Williamson reaction the molecule grows longer, but by the oxidation reaction just the molecule becomes shorter. The advantage of the oxidation reaction is that generally conversion to the ether carboxylate is almost 100%. However, the oxidation reaction for the production of ether carboxylates is economically possible only if high amounts of the same type of ether carboxylate are produced. 

The addition of Pb or Bi to the Pt or Pd catalyst prevents further degradation of the molecule following the main reaction [19]. The addition of a small quantity of ether carboxylate at the beginning of the reaction should increase the reaction velocity [20]. 

Similar ether carboxylates are described [27], using as raw material an alkyl-glyceryl ether with the formula ... 

Ether carboxylates with two alkyl chains are produced by reacting nonionics with dichloroaceticacid and a 50% solution of NaOH in water under vacuum at temperatures between 115°C and 120°C [28]. Compounds of the type... 

Gerhardt et al. [34] described three possible ways to prepare alkylphenol-polyglycol ether carboxylates, namely, the method with NaOH and SMCA, the method by oxidation by means of oxygen, and the method whereby the nonionic reacts with acrylonitrile followed by hydrolyses with hydrochloric acid. The synthesis with acrylonitrile forms in contrast to the other two methods, carboxyethylated compounds with the general formula... 

These ether carboxylates have properties somewhat different from those of the carboxymethylated compounds. Among other things, the extra CH2 group leads to a slightly higher hard water sensibility. 

For cosmetic compositions an ether carboxylate is used, derived from a fatty acid monoethanolamide [35]. It is known that the ethoxylation takes place on the OH and not on the NH of the ethanolamide group. Due to the acid H atom a narrow range distribution is obtained. The carboxymethylation is carried out with NaOH and SMCA, followed by a washing step with an aqueous solution of a strong acid at high temperatures. The ether carboxylates have the structure... 

Generally ether carboxylates are not suitable for a syndet soap because they are waxy due to the ethylene oxide distribution. The solid ether carboxylates with a long alkyl chain and a low degree of ethoxylation have a bad foam. By use of nonethoxylated ether carboxylates, e.g., a carboxymethylated fatty acid monoethanolamide with the structure... 

Therefore a special N-containing ether carboxylate was developed [36] with a high melting point ( 90°C) with a good foam and low hard water sensibility. This is obtained by condensation of a fatty acid (e.g., lauric acid) with diglycolamine, followed by carboxymethylation with NaOH and SMCA, washing out of the reaction mixture with a aqueous solution of a strong acid, separation of the oil layer, and neutralization with NaOH or KOH. The result is an ether carboxylate with exactly 2 EO units with the structure ... 

Based on ether carboxylic acids it is possible in principle to make the same derivatives as with fatty acids, such as esters, amides, and acid chlorides. The hydrophilic chain in the molecule may confer special properties in comparison with the fatty acid derivatives. 

Heimann and Vogtle [38] synthesized triesters of glycerol with different ether carboxylic acids with a short alkyl chain. They have found that these hydrophilic lipids, in contrast with the fatty acid glycerol triesters, give complex-ation with alkali and alkali earth metal cations in an analogy of crown ethers. 

Sorbitan esters of fatty acids are well known. Similar products can be made from ether carboxylic acids and sorbitol without an acid catalyst with a good color [39]. The advantage of these products is that the hydrophilicity can be adjusted by the polyethylene glycol content in the ether carboxylic acid. 

Esters of ether carboxylic acids (propylated and/or ethoxylated) and fatty alcohols or ethoxylated fatty alcohols are described [40], prepared by esterification of the ether carboxylic acid and the alcohol with an acid catalyst like H2S04 or p-toluenesulfonic acid under vacuum and at a temperature of about 130°C. The purpose of these esters is to mainly use them in cremes and lotions with better conditioning and moisture controlling properties. 

Diethylenetriamine and aminoethylethanolamine derivatives of mixtures of fatty acids and ether carboxylic acids are described as textile softeners for laundry baths with high electrolyte content [42]. 

In analogy to the well-known condensation products of fatty acids with protein hydrolysates, a patent has also been initiated for the condensation products of ether carboxylic acids and protein hydrolysates [43]. They are made by converting the ether carboxylic acids with thionyl chloride (SOCl2) to the corresponding acid chlorides followed by the condensation with a protein hydrolysate. 

Also via the route of the acid chloride followed by reaction with taurine or methyltaurine solution in water under the Schotten-Baumann reaction are created the taurides and methyltaurides of the ether carboxylic acids [44] ... 

Special amphoterics are made by condensation of ether carboxylic acids with 2-aminoethylethanolamine followed by carboxymethylation with sodium mono-chloroacetate [45]. An advantage is that these compounds have a higher salt tolerance. 

A cooling, lubricating, and cleaning agent for use by the metalworking industry has been patented. It is produced by the condensation reaction of an excess diethanolamine or monoethanolamine with boric acid and an ether carboxylic acid or mixtures of ether carboxylic acids and fatty acids [46]. An advantage of these products is that they prevent the deposition of lime soaps. 

Some special salts of ether carboxylic acids are also described. Chloro-hexidine salts are made by neutralization of chlorohexidine base by an ether carboxylic acid having antibacterial and surfactant properties [47]. Chitosan salts of ether carboxylic acids which are tolerant of anionics and can be used as hair conditioners are also mentioned [48]. 

A major part of the physicochemical properties of ether carboxylic acids can be ascertained from the structure by comparing it with that of fatty acid soap, ethoxylated alcohol, and other anionics [64,73] ... 

R0(CH2CH20)xH R0(CH2CH20)xS03Na In general conclusion, the ether carboxylates can be regarded as both [64,73] ... 

Several investigators therefore compared the properties of ether carboxylates with different fatty chains and ethoxylation degrees with soaps based on different fatty chains [2,51,52,57,59,61,62,64,65]. 

With ether carboxylates the above-mentioned problems do not arise due to the polyglycol ether group in the molecule. Therefore ether carboxylates show good hard water stability as well as good detergent and foam properties in hard water. It is even possible to disperse lime soaps [61,62,64]. However, the ether carboxylates have the same creamy foaming properties as soap. 

Also attributable to the polyglycol ether group between the fatty chain and the carboxylic group, the ether carboxylates have very mild dermatological properties, so that they have applications in mild cosmetics and household products [10,57,61,62,64,67-72]. 

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