There are two fundamentally different methods of analysis of the SAXS spectra of micellar aqueous solutions: an approach independent on model of micelle, and direct modeling . The first proposed by Glatter consists of a Fourier transform of spectrum data to obtain an autocorrelation function for the particle average for ensemble, and of procedure of deconvolution of this function to obtain the profile of the radial electron density of the micelles. Direct modeling begins from an assumed geometrical model of the micelles and then the calculation of the SAXS intensity of the micellar solutions in accordance with the model and optimization of parameters of micelle model by minimization of the error functional are performed.
By the method of Glatter diameter SDS micelles 6 nm in comparison with the other method 4.4 nm. What can explain this difference in the processing of the spectra SAXS? I would be grateful for the expert opinion on the matter.
Dear Yuri,
direct modelling uses an assumed geometrical model as you have described above.
If the assumed model fits the reality the coutcome should be perfect. But however the result depends on the modell which might not quite fit to reality.
The method of Glatter seems to be more general and will on the other hand smooth out geometrical subtleties.
However the outcome of both methods within 25% is very good.
Congratulations.
Dear Yuri;
when thinking on your question a counter question came up:
when setting up a model of your sample you take a more or less sharp defined value for the diameter of your mecilles. So you get a defined value when fitting the data.
But in the Glatter approach you get a radial distribution function of electron density.
Which is the criterion on how you derive the above mentioned 6 nm of the Glatter diameter ?
Is it the FWHM or the width at about 60% peak level (indentical to 2*RMS for Gaussian profile) or another % value. To my opinion here is some kind of freedom to mach the results of the two approaches.
Next idea:
In order to test which of the above mentioned % level width has to be taken, I would propose to calculate for example some SAXS spectra via your model approach arising from 4 nm, 5 nm, 6 nm and 7 nm diameter micelles. These spectra are free of any experimental issues such as alignment of your x-ray set up, any mechanical, electrical or thermal based drift or even change of average micelles diameter and/or segregation/decomposition of your aqueous solution in the course of measurement time (etc.) .
After that please perform the Glatter procedure onto theses spectra and look for the outcome whether there is a common % peak level for determining the micelle diameter.
If these two evaluation procedures (modelling and Glatter method) are equivatent there should be found a common % level when extraction the micelle diameter from the electron density distribution.
Dear Gerhard:
Your offer interesting. But we use software Anton Paar x-ray diffractometer SAXSess mc2 (method Glatter) and they can not change. In the specification of the device are obtained for SDS micelle diameter of 6.4 nm. The same size of the micelles Kratky and Muller brought into the work of the unpublished experimental data, Muller [Kratky O., Muller K. Aggregation and Micellar Structures of Small Molecules in Solution. In Small Angle X-ray scattering /Ed. Glatter O., Kratky O. Academic Press, London-New Work, 1982, P.499.].
Glatter method better. It does not depend on micelles model. The authors of the method in other articles do not discuss this difference to a standard substance SDS. The problem is not in the method, and the obtained result. We need a new model of the structure of the micelles, to which I am trying to get attention.
Best regards.
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