The absence of electrolytes is a situation that is difficult to control in practice, ill-defined in practice. At the same time, by removing charges, long range electric interactions appear because of charged micellar surfaces. In principle I would suspect that to make the DLS experiment difficult to interpret (in DLS you have to extrapolate to lowest concentrations in order to get to the self diffusion constant (w/o interactions..)). 

DLS should work quite well to measure sodium dodecyl sulfate SDS micelles. Depending on ionic strength of the stock solution, sizes will be between 10 to 20 nm diameter. The linked Malvern application note shows size versus fraction of additional surfactant (Triton X100) for different ionic strength preparations. The data you want are in the document, i.e. for no added contribution and only SDS.

http://www.malvern.com/en/support/resource-center/application-notes/AN031009CharacterisationMixedMicelles.aspx

Dear Ulf:  In Malvern application note the size of the micelles was investigated in the presence of salt. The size of the micelles is contrary to the methods SANS and SAXS. I would like to know your opinion, why do not measure the size of the micelles without the addition of salt ?. You can measure the micelle size as a function of count rate?

I published some years ago on this problem. At concentrations at which the light scattering spectrum of the micelles is visible, interactions between micelles are important, even in the presence of added salt. N. V. Sushkin, D. Clomenil, John Ren, and G. D. J. Phillies,

``Microscopic Parameters of Sodium Dodecyl Sulfate Micelles from Optical Probe

Studies and Quasi-Elastic Light Scattering'', Langmuir {\bf 15},

3492-3498 (1999).

It is noteworthy that for objects the size of micelles charge-induced dipole effects are a large part oft he total interaction, so the Debye-Huckel potential (sphere and point ion) is substantially inaccurate. See \item N. Sushkin and G. D. J. Phillies, ``Charged Dielectric Spheres in Electrolyte Solutions: Induced Dipoles and Counterion Exclusion

If you pull all the salt out of solution, interaction effects become even more complicated.

Effects'', Journal of Chemical Physics, {\bf 103}, 4600-4612 (1995). for details.

Thank you George. I read your interesting article.

Regards.

Yuri

Ulf Nobbmann

I tried to find a link of the micelle sizes of ionic surfactants without electrolyte, but could not register on the Malvern website. Please send or help receive.

It is not surprising that QELSS and SAXS do not give the same answer for the micelle radius. Based on our work, notably on Triton X-100, it appears that neutral micelles are highly rugose, not the spheres beloved of artists -- there was a period image on the cover of Langmuir,based on computer simulations -- and have large amounts of water entrained, even by a few spikes of surfactant, that might well be included in the QELSS radius but would contribute very little to the xrays.

Hello Yuri Mirgorod - for problems registering on the Malvern website you can try some tips from https://www.materials-talks.com/blog/2019/06/18/how-to-register-for-technical-support/

For some micelle sizes, I had many years ago measured these radii of these zwitterionic surfactants at 10x cmc in water

FOS-Choline-12 : 2.2nm

FOS-Choline-10 : 1.7nm

FOS-Choline-9 : 1.4nm

samples were from Hampton Research, published in Mesoscale Phenomena in Fluid Systems, ed. F. Case and P. Alexandridis, ACS Symposium Series 861 (2002), page 56

Ulf Nobbmann

FOS-Choline are amphoteric surfactants. I asked for ionic surfactants.

George Phillies

Yes, you write correctly, but I solve the problem when the known surfactant solutions have quantum properties. Everything you write about is in our article, but there is more.

Article Structure of Micelles of Sodium Dodecyl Sulphate in Water: a...

Without adding electrolytes the Debye length is huge and many-body interactions between the micelles affect the dynamics so Stokes-Einstein will not work. When studying charged species in water using DLS it is essential to provide controlled screening.

Remco Tuinier

Since the Debye length is huge, the micelle will not have a double layer. Contradiction. What does "to provide controlled screening" mean?

What Remco Tuinier probably means is that when there is no added salt, the micelles essentially interact via an unscreened charge-charge interaction through a Coulomb potential ~1/r (r being the distance between the micelles). The double layer is indeed absent, and does not lead to screening of such Coulomb interactions. "Controlled screening" refers to the addition of a known amount of salt, such that there is screening of the Coulomb potential (the potential is now ~exp(-\kappa r)/r ) where the inverse Debye length \kappa can be calculated from the known salt concentration. I hope that I did not misunderstood the situation, and that this helps.

Jan Karel George Dhont

Everyone knows that micelles have a double layer. This is a classic micelle. So far they think. You need to see the whole discussion.

I want to summarize the discussion. Most researchers confirm that without salt it is impossible to obtain the micelle size near the CMC by the DLS method. I agree with their answers and have written about this to the developers of devices more than once. I will not repeat the arguments of the consultants so as not to distort the author's wording. However, everyone will agree that micelles exist. The method of electrical conductivity is used to determine the degree of dissociation of micelles in a double layer of micelles. We come to the point of why I asked the question. In my opinion, a micellar solution in the phase transition region is a quantum object. When observing a quantum object with instruments, its decoherence occurs, or otherwise we interfere with its existence. The destruction of quantum fluctuations of water also occurs with the gradual addition of an inorganic electrolyte. You can read more about my research.

Preprint The Role of Zero-Point Energy of Water in Micelle Formation ...

Preprint Strongly correlated electronic states in aqueous micellar su...

All answered the question in the same way, but George Phillies had previously published an article on this. Therefore, I consider his answer to be the best.

Thank you all for participating in the discussion