Inorganosulfur chemistry,organosulfates are a class oforganic compounds sharing a commonfunctional group with the structureR−O−SO−3. TheSO4 core is asulfate group and the R group is anyorganic residue. All organosulfates are formallyesters derived fromalcohols andsulfuric acid (H2SO4) although many are not prepared in this way. Many sulfate esters are used indetergents, and some are usefulreagents. Alkyl sulfates consist of ahydrophobic hydrocarbon chain, a polarsulfate group (containing an anion) and either a cation oramine to neutralize the sulfate group. Examples include:sodium lauryl sulfate (also known as sulfuric acid mono dodecyl ester sodium salt) and related potassium and ammonium salts.
Alkyl sulfates are commonly used as anionicsurfactants in liquid soaps and detergents used to clean wool, as surface cleaners, and as active ingredients in laundry detergents, shampoos and conditioners. They can also be found in household products such as toothpaste, antacids, cosmetics and foods. Generally they are found in consumer products at concentrations ranging from 3-20%. In 2003 approximately 118,000 t/a of alkyl sulfates were used in the US.[1]
A common example issodium lauryl sulfate, with the formula CH3(CH2)11OSO3Na. Also common in consumer products are the sulfate esters ofethoxylatedfatty alcohols such as those derived fromlauryl alcohol. An example issodium laureth sulfate, an ingredient in somecosmetics.[2]
Alkylsulfate can be produced from alcohols, which in turn are obtained byhydrogenation of animal or vegetable oils and fats or using theZiegler process or throughoxo synthesis. If produced fromoleochemical feedstock or the Ziegler process, thehydrocarbon chain of the alcohol will be linear. If derived using the oxo process, a low level of branching will appear usually with a methyl or ethyl group at the C-2 position, containing even and odd amounts of alkyl chains.[3] These alcohols react withchlorosulfuric acid:
Alternatively, alcohols can be converted to the half sulfate esters usingsulfur trioxide:[4]
Specialized organosulfates can be prepared by theElbs persulfate oxidation of phenols and theBoyland–Sims oxidation of anilines.
A less common family of organosulfates have the formula RO-SO2-OR'. They are prepared from sulfuric acid and the alcohol. The main examples arediethyl sulfate anddimethyl sulfate, colourless liquids that are used asreagents inorganic synthesis. These compounds are potentially dangerousalkylating agents. Dialkylsulfates do not occur in nature.[5]
Several classes of sulfate esters exist in nature. Especially common are sugar derivatives, thesulfated polysaccharides. Examples includekeratan sulfate,chondroitin sulfate, and the anticoagulantheparin.[6]Post-translational modifications of some proteins entail sulfation, often at the phenol group oftyrosine residues.[7] A steroidal sulfate isestradiol sulfate, a latent precursor to the hormone estrogen.
A major portion of soil sulfur is in the form of sulfate esters.[8]
Sulfate is an inert anion, so nature activates it by the formation of ester derivative of adenosine 5'-phosphosulfate (APS) and3'-phosphoadenosine-5'-phosphosulfate (PAPS). Many organisms utilize these reactions for metabolic purposes or for thebiosynthesis of sulfur compounds required for life.[9] The formation and hydrolysis of natural sulfate esters are catalyzed bysulfatases (aka sulfohydrolases).[5]
Because they are widely used in commercial products, the safety aspects of (monoalkyl) organosulfates are heavily investigated.[10]
Alkyl sulfates if ingested are well-absorbed and are metabolized into a C3, C4 or C5 sulfate and an additional metabolite. The highest irritant of the alkyl sulfates is sodium laurylsulfate, with the threshold before irritation at a concentration of 20%.Surfactants in consumer products are typically mixed, reducing likelihood of irritation. According toOECD TG 406, alkyl sulfates in animal studies were not found to be skin sensitizers.[10][11]
Laboratory studies have not found alkyl sulfates to begenotoxic,mutagenic orcarcinogenic. No long-term reproductive effects have been found.[12]
The primary disposal of alkyl sulfate from used commercial products is wastewater. The concentration of alkylsulfates in effluent from waste water treatment plants (WWTP) has been measured at 10 micrograms per litre (5.8×10−9 oz/cu in) and lower. Alkyl sulfates biodegrade easily, even starting likely before reaching the WWTP. Once at the treatment plant, they are rapidly removed bybiodegradation. Invertebrates were found to be the most-sensitive trophic group to alkyl sulfates. Sodium laurylsulfate tested onUronema parduczi, a protozoan, was found to have the lowest effect value with the 20 h-EC5 being 0.75 milligrams per litre (2.7×10−8 lb/cu in). Chronic exposure tests with C12 to C18 with the invertebrateCeriodaphnia dubia found the highest toxicity is with C14 (NOEC was 0.045 mg/L).
In terms of thermal stability, alkyl sulfates degrade well before reaching their boiling point due to low vapor pressure (for C8-18 from 10-11 to 10-15 hPa). Soil sorption is proportional to carbon chain length, with a length of 14 and more having the highest sorption rate. Soil concentrations have been found to vary from 0.0035 to 0.21 milligrams per kilogram (5.6×10−8 to 3.4×10−6 oz/lb) dw.