Solid phase hydrolysis of sodium dodecyl sulfate (SDS)

Solid phase hydrolysis of sodium dodecyl sulfate (SDS)

The detergent sodium dodecyl sulfate, SDS, is cited twice in the 1929-39 decennial index of Chemical Abstracts and roughly 750 times in the 1999 annual index. It is widely used to solubilize proteins and facilitate their gel electrophoretic separation (1). SDS is sold in various purities. Purity depends on purity of the precursor alcohol and degree of removal of contaminating salts and alcohols from the sulfation product. It is widely, but not universally, known that purity listed on the bottle refers to purity of alkyl sulfates, not specifically to SDS purity. Cheaper grades of SDS are made from hydrogenated coconut oil which is about 50%C12, 25% C14, and 15% C16 (2). Purity of the precursor alcohol is not critical for most purposes, but relatively pure C12 material is preferable for reproducible SDS gel electrophoresis (3). Only grades of SDS with specified C12 content meet the latter criterion.

I recently encountered an additional purity problem. A stock solution of 10% W/V SDS (at room temperature) appeared to contain crystals. Freshly dissolved SDS from the same batch was not fully soluble. The solubility problem was like that of older 1% SDS, 1% oxalic acid solutions that I use to clean electrophoresis plates. This solution becomes turbid after about 6 months presumably due to (well known - 4,5) acid catalyzed SDS hydrolysis.

A 1% solution prepared from the same SDS, a powder (about 8 years old), had a pH of 3.5. A 1% solution of new SDS had a pH of 6.5. A 1% suspension from a still older bottle of badly caked SDS had a pH of 2.5. The pHs suggest SDS hydrolysis which, like ester hydrolysis in general, generates hydrogen ion.

Insolubility presumably arises from dodecyl alcohol, another hydrolysis product. The literature contains no phase diagram for the SDS-dodecyl alcohol-water system (6), however adding dodecyl alcohol to a 1% SDS solution from a concentrated solution in ethanol to 1 part dodecyl alcohol per 100 parts SDS was sufficient to cause precipitation. At about 1 part dodecyl alcohol per 10 of SDS the solution gelled.

Solid phase SDS hydrolysis is more plausible than it first seems. The ionic end of the molecule is hygroscopic and accumulates moisture when the bottle is opened or poorly sealed. Hydrolysis is autocatalytic since it produces hydrogen ion.

Simple precautions, such as protecting the reagent from moisture and avoiding long term storage, will minimize hydrolysis. Product quality can be monitored by measuring the solution pH.


1- Maizel, Jacob V. (2000) SDS polyacrylamide gel electrophoresis. Trends in Biochemical Sciences 25, 590-592.

3- Rosen, Milton J. Surfactants and Interfacial phenomena, 2nd ed.. Wiley-Interscience, New York, (1989)

3- Hodgkinson, Julie L., Steffen, Walter (1997) The effect of sodium tetradecyl sulfate on mobility and antigen detectability of microtubule proteins in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Electrophoresis 18, 1955-1959.

4- Kurz, Joseph L. (1963) Effects of micellization on the kinetics of the hydrolysis of monoalkylsulfates, Journal of Physical Chemistry 66, 2239-2246.

5- Motsavage, Vincent A., Kostenbauder, Harry B. (1963) The influence of the state of aggregation on the specific acid-catalyzed hydrolysis of sodium dodecyl sulfate. Journal of Colloid Science 16, 603-615.

6 Wisniak, Jaime. (1981) Phase diagrams: a literature source book. Elsevier Sceintific Pub Co., 2 vol.