What DMSO Is and Why It Is Used
DMSO stands for dimethyl sulfoxide, a colourless, essentially odourless liquid with the formula C2H6OS and CAS Registry Number 67-68-5. It is the simplest sulfoxide and one of the most polar aprotic solvents available in bulk. The molecule has a strongly polar S=O bond and a high dielectric constant, so it dissolves ionic salts, sugars, polymers and many poorly soluble aromatic compounds that resist conventional solvents. DMSO is produced commercially by oxidising dimethyl sulfide, and it is also recovered as a by-product of kraft pulping. On contact with water it mixes in any ratio and releases heat, which is a practical point that designers must allow for when preparing blends.
Physical and Chemical Properties
| Property | Typical value |
|---|---|
| Formula | C2H6OS |
| CAS number | 67-68-5 |
| Molar mass | 78.13 g/mol |
| Appearance | colourless liquid, hygroscopic |
| Density at 20 C | 1.10 g/cm3 |
| Boiling point at 101.3 kPa | 189 C |
| Melting point | 18.5 C |
| Refractive index at 20 C | about 1.479 |
| Dynamic viscosity at 20 C | about 2.0 mPa.s |
| Dielectric constant at 25 C | about 46.7 |
| Flash point (closed cup) | about 89 C |
| Water solubility | miscible in all proportions |
Organic Synthesis and Polymer Processing
In synthesis, DMSO activates a range of functional groups and carries oxygen into oxidation steps. It is the oxidant in the Swern oxidation, where oxalyl chloride and DMSO generate a chlorosulfonium intermediate that converts primary and secondary alcohols into aldehydes and ketones under very mild conditions, and it performs the same role in the Pfitzner-Moffatt oxidation. DMSO is also the reaction medium for nucleophilic substitutions with alkoxide, cyanide, azide and thiolate salts, because it leaves the nucleophile poorly solvated and therefore highly reactive. In polymers and fibres, DMSO is the spinning solvent for acrylic fibre and a solvent for polyimide, polybenzimidazole and aramid dopes, and it is used in the synthesis of polyimides and engineering thermoplastic resins. Reactions with sodium hydride, acid chlorides or strong oxidising agents require controlled addition rates, because heat and gas evolution can be rapid.
Pharmaceutical, Biomedical and Cosmetic Uses
DMSO is an approved pharmaceutical vehicle in several markets and is used as a solvent and penetration enhancer for topical formulations in veterinary medicine, where it is applied for the relief of inflammation and swelling. In drug development it dissolves water-insoluble active ingredients for in vitro screening, formulation and toxicology studies, and it improves skin permeation in transdermal delivery systems. In cell biology and biobanking it is the standard cryoprotectant at 5 to 10 % by volume, added to culture media to protect cells during freezing and thawing. Because the compound crosses skin quickly and carries dissolved substances with it, laboratory workers should treat solutions of toxic or unknown solutes in DMSO as dermal exposure risks. In cosmetics it appears as a solvent and moisturising agent in skin-care formulations. The United States Pharmacopeia general chapter on residual solvents and the ICH Q3C guideline both place DMSO in Class 3, the class of least toxic residual solvents, with a permitted daily exposure of 50 mg per day or more.
Industrial and Speciality Applications
Industry uses DMSO as a cleaning and stripping solvent for electronics, printed circuit assemblies and reactor interiors, because it removes polar residues and flux without leaving a chlorinated footprint. It serves as a paint and varnish remover, a hydraulic fluid additive, a resin and polymer solvent, an extractant in the recovery of aromatics, and a component of antifreeze and heat-transfer formulations. In petroleum and gas treatment it appears as a solvent for acid gas absorption studies, and in agriculture it is used to increase the uptake of poorly soluble agrochemical actives. Its low toxicity and biodegradability compared with chlorinated and amide solvents are the main reasons formulators evaluate it as a replacement solvent.
Specifications, Handling and Storage
Industrial DMSO is purchased on parameters that include assay by gas chromatography, water content by Karl Fischer titration following ASTM E203, colour, and ultraviolet absorbance for pharmaceutical grades. DMSO attacks many polymers and is therefore handled in type 304 or 316 stainless steel, glass, PTFE or polyethylene-lined equipment; natural rubber, polystyrene and polycarbonate are unsuitable. The product freezes at 18.5 C, so tanks, pumps and lines in temperate warehouses need trace heating or insulation to avoid crystallisation and blockages. Storage areas should be cool, dry and shaded, with tight closures to limit moisture pick-up, and strong oxidisers and chlorinating agents must be kept separate. DMSO is combustible with a closed-cup flash point near 89 C; it is not classified as a flammable liquid, but fires involving DMSO can be difficult to extinguish and should be fought with water spray, foam or dry chemical.
Frequently Asked Questions
Q: What is DMSO used for?
A: It is used as a polar aprotic solvent in organic synthesis, as a spinning and polymer solvent, as a pharmaceutical and veterinary vehicle, as a cell cryoprotectant at 5-10 % by volume, and as an industrial cleaning and stripping agent.
Q: Is DMSO miscible with water?
A: Yes. DMSO mixes with water in all proportions and the mixing is exothermic, so additions to aqueous systems should be made with agitation and temperature control.
Q: What is the CAS number and formula of dimethyl sulfoxide?
A: CAS Registry Number 67-68-5, formula C2H6OS, molar mass 78.13 g/mol, boiling point 189 C and melting point 18.5 C.
Q: Why is DMSO frozen in the warehouse?
A: Its melting point is 18.5 C, so in cool weather it can crystallise. Tanks and transfer lines are usually trace heated and insulated to keep the product liquid.
Q: What residual solvent class does DMSO fall into?
A: It is a Class 3 solvent under ICH Q3C, with a permitted daily exposure of 50 mg per day or more, and it is listed accordingly in pharmacopoeial residual solvent chapters.
Q: How is DMSO purity checked?
A: By gas chromatographic assay, Karl Fischer water determination per ASTM E203, colour measurement and, for pharmaceutical grades, ultraviolet absorbance and non-volatile residue tests.