Ethoxylation

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.

Production

The invention of the process is attributed to Schöller and Wittwer at I.G. Farben industries. ===Alcohol ethoxylates=== In industrial ethoxylation, an alcohol is treated with ethylene oxide and potassium hydroxide (KOH), which serves as a catalyst. The reactor is pressurised with nitrogen and heated to about 150 °C. Typically 5-10 units of ethylene oxide are added to each alcohol:

ROH + n C₂H₄O → R(OC₂H₄)_nOH

A distribution of products are obtained. The amount of ethylene oxide and the reaction time determine the degree of ethoxylation (the value of n in the equation above), which in turn determines the surfactant properties of the ethoxylated product. The hydrophilicity of the surfactant increases with the value of n. Traditionally the alcohols were obtained by hydrogenation of fatty acids, but currently most are "oxo alcohols," obtained via hydroformylation. In addition to alcohols, amines and phenols are commonly ethoxylated.

Most frequently, alcohol ethoxylates are derived from primary alcohols and ethylene oxide via a base catalyzed reaction of potassium or sodium hydroxide followed by treatment with a neutralizing agent such as acetic or phosphoric acid. Less often, they are produced from secondary alcohols. More than 435,000 metric tons of linear alcohol ethoxylates were produced in North America and Western Europe in 2000. AE is considered to be a high production volume (HPV) chemical by the US EPA.

Ethoxysulfates

Ethoxylated fatty alcohols are often converted to the organosulfate. The conversion typically uses sulfur trioxide or 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). In 2006, 382,500 metric tons of alcohol ethoxysulfates (AES) were consumed in North America.

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.

Alcohol ethoxylates

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. They feature both lipophilic tails (R) and relatively polar head groups ((OC₂H₄)_nOH).

Alcohol ethoxysulfates

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. A well-known example of an ethoxysulfate is sodium laureth sulfate, a foaming agent in shampoos and toothpastes, as well as industrial detergents.

Environmental and safety

Alcohol ethoxylates (AE's)

Human health

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, which is a known 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

Some ethoxylated materials have been controversial because of their widespread use and the toxicity posed to aquatic life due to degradation products, such as nonylphenol.

Alcohols containing ethylene oxides of C
₆–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. 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)

Biodegradation

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. The removal of AES due to degradation via anaerobic processes is estimated to be between 75 to 87%.

Aquatic

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. The mayfly, Tricorythodes has a normalized density NOEC value of 190 ug/L.

Human Safety

AES has not been found to be genotoxic, mutagenic, or carcinogenic.