Ethoxylation

Ethoxylation is an industrial process in which ethylene oxide is added to alcohols and phenols to give surfactants. The invention of the process is attributed to Schöller and Wittwer at I.G. Farben industrie.^[1]

Production

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. 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 (AEs) are derived from primary alcohols and ethylene oxide by use of 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.^[2] AE is considered to be a high production volume (HPV) chemical by the US EPA.^[3]

PEGylation

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," because poly(ethylene oxide) is also known as poly(ethylene glycol), abbreviated as PEG.

Applications of ethoxylated products

Alcohol ethoxylates (AE) are non-ionic surfactants found in products such as laundry detergents, surface cleaners, cosmetics and for use in agriculture, textiles and paint.^[4] Carbon chain length is 8-18 while the ethoxylated chain is usually 3 to 12 ethylene oxides long in home products.^[5] hey feature both a lipophilic tails (R in the equation above) and a relatively polar head group ((OC₂H₄)_nOH in the above example).

Ethoxylated fatty alcohols are often converted to the organosulfate. A well known example is sodium laureth sulfate, a foaming agent in shampoos and toothpastes, as well as an industrial detergent. The conversion typically uses sulfur trioxide or chlorosulfuric acid:

R(OC₂H₄)_nOH + SO₃ → R(OC₂H₄)_nOSO₃H

Environmental and safety issues

Human health

Research on associated health risks have found alcohol ethoxylates are not mutagenic, carcinogenic, skin sensitizers, nor cause reproductive or developmental effects. Undiluted AEs can cause dermal or eye irritation. In liquid 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 when at concentrations found in consumer products.^[5]

Aquatic and environmental aspects

Some ethoxylated materials have been controversial because of their widespread use and the toxicity to aquatic life of their degradation products, such as nonylphenol.^[1] Alcohols containing ethylene oxides of C6-C18 length are considered to be rapidly biodegradable.^[6] AEs are usually released down the drain, where they may be adsorbed into solids and biodegrade through anaerobic processes. About 28-58% of AEs degrade in the sewer.^[7] The remaining AEs are treated at waste water treatment plants and biodegrade by aerobic processes. Less than 0.8% of AE remains and is released in effluent.^[7] If released into surface waters, sediment or soil, AEs will degrade through aerobic and anaerobic processes or be taken up by plants and animals.

With an increase in ethylene oxide and decrease in carbon length, the aquatic toxicity decreases. The toxicity of synthetic alcohol ethoxylates are toxic to certain invertebrates, with 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.^[5]

Additional studies have been conducted regarding toxicity on algae and fish. The EC50 toxicity for algae from linear and branched AEs was 0.05 mg/l to 50 mg/l. Acute toxicity to fish is varied, ranging 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.^[5]

References

1. ^ ^{a b} Eduard Smulders, Wolfgang von Rybinski, Eric Sung, Wilfried Rähse, Josef Steber, Frederike Wiebel, Anette Nordskog “Laundry Detergents” in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim. doi:10.1002/14356007.a08_315.pub2. Schöller and Wittwer are listed as inventors on US Patent #1970578, issued 1934, "Assistants for the Textile and Related Industries."
2. ^ Modler R., Gubler R, Kishi A (2002). Chemical Economics Handbook. Menlo Park, CA: SRI Consulting. http://www.sriconsulting.com/CEH/Public/Reports/583.8000/. 
3. ^ US EPA (July 2006). "High production volume (HPV) challenge program". http://www.epa.gov/chemrtk/index.htm. 
4. ^ Federle, Thomas W; Nina R. Itrich (2004). "Effect of Ethoxylate Number and Alkyl Chain Length on the Pathway and Kinetics of Linear Alcohol Ethoxylate Biodegradation in Activated Sludge". Environmental Toxicology and Chemistry: 2790–2798. doi:10.1897/04-053.1. 
5. ^ ^{a b c d} HERA (September 2009). "Alcohol Ethoxylates". Human & Environmental Risk Assessment on ingredients of European household cleaning products. http://www.heraproject.com/files/34-F-09%20HERA%20AE%20Report%20Version%202%20-%203%20Sept%2009.pdf. 
6. ^ SIAR (2006). "SIDS Initial Assessment Report for Long Chain Alcohols". http://www.aciscience.org/docs/Draft_SIDS_Long_Chain_Alcohols_1.pdf. 
7. ^ ^{a b} Prats, Daniel; Carmen Lopez, Diana Vallejo, Pedro Varo, and Victor M. Leon (2006). "Effect of Temperature on the Biodegradation of Linear Alkylbenzene Sulfonate and Alcohol Ethoxylate". Journal of Surfactants and Detergents 9 (1): 69–75. doi:10.1007/s11743-006-0377-8.