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Ethoxylation is a chemical reaction in which ethylene oxide adds to alcohols and phenols. The process converts the group ROH into R(OC2H4)nOH where n ranges from 1 to as high as 10. Such compounds are called alcohol ethoxylates. Alcohol ethoxlates are often converted to related species called ethoxysulfates. Alcohol ethoxylates and ethoxysulfates are surfactants, used widely in cosmetic and other commercial products. The process is of great industrial significance with more than 2,000,000 metric tons various ethoxylates produced worldwide 1994.
- 1 Production
- 2 Applications of ethoxylated products
- 3 Environmental and safety
- 4 References
Industrial ethoxylation is primarily performed upon fatty alcohols in order to generate fatty alcohol ethoxylates (FAE's), which are a common form of nonionic surfactant. Such alcohols may be obtained by the hydrogenation of fatty acids from seed oils, or via hydroformylation in the Shell higher olefin process. The reaction proceeds by blowing ethylene oxide through the alcohol at 180 °C and under 1-2 bar of pressure, with potassium hydroxide (KOH) serving as a catalyst. The process is highly exothermic (ΔH -92000 J/mol of ethylene oxide reacted) and requires careful control to avoid a potentially disastrous thermal runaway.
- ROH + n C2H4O → R(OC2H4)nOH
The starting materials are usually primary alcohols as their rate constant is ~10-30 times that of secondary alcohols. Typically 5-10 units of ethylene oxide are added to each alcohol, however ethoxylated alcohols can be more prone to ethoxylation than the starting alcohol, making the reaction difficult to control and leading to the formation of a product with varying repeat unit length (the value of n in the equation above). Better control can be afforded by the use of more sophisticated catalysts. Ethoxylated alcohols are considered to be a high production volume (HPV) chemical by the US EPA.
Ethoxylation is sometimes combined with propoxylation; which is essentially the same reaction using propylene oxide as the monomer. Both reactions are normally performed in the same reactor and may be run simultaneously to give a random polymer, or in alternation to obtain block copolymers. Propylene oxide is much more hydrophobic than ethylene oxide and its inclusion at low levels can have significant effects on the properties of the surfactant. In particular ethoxylated fatty alcohols which have been 'capped' with ~1 propylene oxide unit are extensively marketed as low-foaming surfactants.
Ethoxylated fatty alcohols are often converted to the corresponding organosulfates, which can be easily deprotonated to give anionic surfactants such as sodium laureth sulfate. The negative charge increases the water solubility of the compounds, giving them a higher HLB value. The conversion is achieved industrially by blowing a sulfur trioxide/air mixture through a solution of ethoxylated starting material. Laboratory scale synthesis may be performed using chlorosulfuric acid:
- R(OC2H4)nOH + SO3 → R(OC2H4)nOSO3H
- R(OC2H4)nOH + HSO3Cl → R(OC2H4)nOSO3H + HCl
The resulting sulfate esters are neutralized to give the salt:
- R(OC2H4)nOSO3H + NaOH → R(OC2H4)nOSO3Na + H2O
Small volumes are neutralized with alkanolamines such as triethanolamine (TEA).[page needed] In 2006, 382,500 metric tons of alcohol ethoxysulfates (AES) were consumed in North America.(subscription required)[page needed][better source needed]
Although alcohols are by far the major substrate for exothylation any suitable nucleophile may undergo the process. Primary amines will react to give di-chain materials such as polyethoxylated tallow amine. The reaction of ammonia produces important bulk chemicals such as ethanolamine, diethanolamine and triethanolamine.
Applications of ethoxylated products
Alcohol ethoxylates (AE) and alcohol ethoxysulfates (AES) are surfactants found in products such as laundry detergents, surface cleaners, cosmetics and for use in agriculture, textiles and paint.[non-primary source needed]
As alcohol ethoxylate based surfactants are non-ionic they typically require longer ethoxylate chains than their sulfonated analogues in order to be water-soluble. Examples synthesized on an industrial scale include octyl phenol ethoxylate, polysorbate 80 and poloxamers. Ethoxylation is commonly practiced, albeit on a much smaller scale, in the biotechnology and pharmaceutical industries to increase water solubility and, in the case of pharmaceuticals, circulatory half-life of non-polar organic compounds. In this application, ethoxylation is known as "PEGylation" (polyethylene oxide is synonymous with polyethylene glycol, abbreviated as PEG). Carbon chain length is 8-18 while the ethoxylated chain is usually 3 to 12 ethylene oxides long in home products.[page needed] They feature both lipophilic tails, indicated by the alkyl group abbreviation, R, and relatively polar headgroups, represented by the formula (OC2H4)nOH.
AES found in consumer products generally are linear alcohols, which could be mixtures of entirely linear alkyl chains or of both linear and mono-branched alkyl chains.[page needed] A high-volume example of these is sodium laureth sulfate a foaming agent in shampoos and toothpastes, as well as industrial detergents.
Environmental and safety
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Alcohol ethoxylates (AE's)
Alcohol ethoxylates are not observed to be mutagenic, carcinogenic, or skin sensitizers, nor cause reproductive or developmental effects. One byproduct of ethoxylation is 1,4-dioxane possible human carcinogen. Undiluted AEs can cause dermal or eye irritation. In aqueous solution, the level of irritation is dependent on the concentration. AEs are considered to have low to moderate toxicity for acute oral exposure, low acute dermal toxicity, and have mild irritation potential for skin and eyes at concentrations found in consumer products.
Aquatic and environmental aspects
Alcohols containing ethylene oxides of C
18 length are considered to be rapidly biodegraded. AEs are usually released down the drain, where they may be adsorbed into solids and biodegrade through anaerobic processes, with ~28–58% degraded in the sewer.[non-primary source needed] The remaining AEs are treated at waste water treatment plants and biodegraded via aerobic processes with less than 0.8% of AEs released in effluent. If released into surface waters, sediment or soil, AEs will degrade through aerobic and anaerobic processes or be taken up by plants and animals.
Toxicity to certain invertebrates has a range of EC50 values for linear AE from 0.1 mg/l to larger than 100 mg/l. For branched alcohol exthoxylates, toxicity ranges from 0.5 mg/l to 50 mg/l. The EC50 toxicity for algae from linear and branched AEs was 0.05 mg/l to 50 mg/l. Acute toxicity to fish ranges from LC50 values for linear AE of 0.4 mg/l to 100 mg/l, and branched is 0.25 mg/l to 40 mg/l. For invertebrates, algae and fish the essentially linear and branched AEs are considered to not have greater toxicity than Linear AE.
Alcohol ethoxysulfates (AES's)
The degradation of AES proceeds by ω- or β-oxidation of the alkyl chain, enzyme cleavage of the sulfate substituent leaving the alcohol ethoxylate, and by cleavage of an ether bond in the AES molecule producing alcohol or alcohol ethoxylate and an ethylene glycol sulfate. Studies of aerobic processes also found AES to be readily biodegradable. The half-life of both AE and AES in surface water is estimated to be less than 12 hours.[non-primary source needed] The removal of AES due to degradation via anaerobic processes is estimated to be between 75 and 87%.
Flow through laboratory tests in a terminal pool of AES with mollusks found the NOEC of a snail, Goniobasis and the Asian clam, Corbicula to be greater than 730 ug/L. Corbicula growth was measured to be affected at a concentration of 75 ug/L.[non-primary source needed] The mayfly, genus Tricorythodes has a normalized density NOEC value of 190 ug/L.[non-primary source needed]
AES has not been found to be genotoxic, mutagenic, or carcinogenic.
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