Looking for the Formula of Sodium Laureth Sulfate

2025-02-11 • Posted in: Ph.D.

In October 2024, I did a simple laboratory experiment. It consisted of simulating waves in an acrylic tank filled with water. Waves were generated by an axial fan blowing air towards the tank. I measured water surface elevation with a conductivity-type wave gauge and velocity with an acoustic Doppler velocimeter. Two sets of measurements were made: with ordinary tap water and water containing a soluble surfactant. The surfactant was sodium laureth sulfate, commonly used in personal care products like shampoos. The objective was to observe the effects of surfactant on waves and on momentum transfer across the air-water surface.

I am describing the experiment in the second draft of my Ph.D. thesis. I wanted to unequivocally specify the surfactant used, so I did some research. In Wikipedia, I found that the product is heterogeneous in its structure:

Interger n is the number of ethoxyl groups. As expected, its condensed structural formula is also in terms of n, i.e.,

CH₃(CH₂)₁₁(OCH₂CH₂)_nOSO₃Na.

Since I wanted to a more-or-less standardized specification, I looked for the surfactant in the Chemical Abstracts Service (CAS). The CAS registry number of sodium laureth sulfate is 9004-82-4. In CAS, the preferred name is polyoxyethylene lauryl ether sodium sulfate, and the empirical formula is expressed as

(C₂H₄O)_nC₁₂H₂₆O₄S·Na. Note, however, that it shows an additional H atom when compared to Wikipedia's formula.

I looked for a different and reliable specification and found an article of the American Chemical Society (ACS) Molecule of the Week (MOTW) series for sodium laureth sulfate. The discrepancy was more evident. They express the formula for n = 3 (sodium laureth‑3 sulfate) as

C₁₈H₃₇NaO₇S, but, in CAS, the formula is C₁₈H₃₈O₇S·Na. That was confusing, and I wanted an explanation. However, that is not the type of thing that you can easily find on Google, and AI chats cannot understand such subtleties (actually, Copilot said that they were different products!). Is it a typo or a different representation? I filled the contact form for both MOTW and CAS expecting a clarification. A couple of hours later, I received the following message from the MOTW editor:

Dear Mr. Casas,

Thank you for your message and for following Molecule of the Week. I can explain the discrepancy between the two formulas.

For whatever reason, ACS's SciFinderⁿ has an unusual way of expressing the empirical formula of salts of acids. Instead of using the formula for the acid anion and the cation, it uses the formula for the free acid and the cation. Hence, one additional H atom in the case of sodium laureth sulfate. At MOTW, we prefer to use the formula for the anion.

So, it is not an actual typo, just a different way of expressing the formula.

Thanks again and best regards,
Mike Block

I drew two conclusions from his answer:

There are several ways of representing the same molecule. Not only variations in the order and grouping of atoms, but also in the count of them.
Formulas in CAS are not always expressed in the usual way for chemists.

I, however, still wanted an empirical formula for sodium laureth sulfate, not the long and complex condensed formula displayed on Wikipedia, but I was worried that the CAS formula was not expressed in the usual way. Thus, I decided to write the empirical formula as

(C₂H₄O)_nC₁₂H₂₅NaO₄S, based on the formula from MOTW (for n = 3). This empirical representation can also be found on the U.S. Environmental Protection Agency (EPA) CompTox Chemicals Dashboard and on the website of a random chemistry company.