= KEMM37 Lab 1B - SANS Data Analysis using !SasView =

||= **Table of Contents** =||
|| [#section5 5. Fitting SANS data] ||
||   [#section51 5.1 Background Information] ||
||   [#section52 5.2 Calculating Scattering Length Density] ||
||   [#section53 5.3 Fitting the data] ||
|| [#resources Resources] ||

== [=#section5 5. Fitting SANS data] ==

This part of the exercise will use the same real SANS data as the previous exercises - taken from a study of surfactant self assembly in deep eutectic solvents (DES). 

{{{
#!div style="background: lightblue"
[=#task10 **TASK 10:**] Restart !SasView

Before starting this part of the exercise, you should have a clean !SasView instance. Quit !SasView and restart it.
}}}

=== [=#section51 5.1 Background Information] ===
Deep eutectic solvents are a class of ionic liquids formed from a hydrogen bond donor and a halide salt. At a certain mixture ratio, the eutectic mixture, the melting point is significantly depressed to values below room temperature.

Here we will examine the self-assembly of a [https://en.wikipedia.org/wiki/Surfactant surfactant], [https://en.wikipedia.org/wiki/Sodium_dodecyl_sulfate sodium dodecyl sulfate (SDS)] in the deep eutectic solvent formed from a 1:2 molar ratio mixture of [https://en.wikipedia.org/wiki/Choline_chloride   choline chloride] and [https://en.wikipedia.org/wiki/Urea   urea].

||Sodium Dodecyl Sulfate||Choline Chloride||Urea||
||[[Image(kemm37_sds.png)]]||[[Image(kemm37_choline.png)]]||[[Image(kemm37_urea.png)]]||

{{{
#!div style="background: lightblue"
[=#task11 **TASK 11:**] If you haven't already done so download the SANS data :  [attachment:SwednessSasViewTutorialData.zip​] and unzip the file in a known location on your filesystem. Note where you have placed the data.
}}}

You should now have a folder called "Subtracted" containing a set of files named as follows:

[[Image(kemm37_datafileslist.png)]]

The data are SANS curves collected on SANS2D at ISIS and D22 at ILL for samples of protonated (normal) SDS in 1:2 d9-choline chloride:d4-urea. This sample was chosen to give maximum contrast and minimum background signal from incoherent scattering. There were 7 samples with 0.2 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt%, 7.5 wt% and 10 wt% of SDS in the DES with the filenames corresponding to each sample given below:

|| Surfactant Concentration (wt%) || Data File ||
|| 0.2 || 0p2hSDS_dChCldUrea_sub.txt ||
|| 0.5 || 0p5hSDS_dChCldUrea_sub.txt ||
|| 1.0 || 1hSDS_dChCldUrea_sub.txt ||
|| 2.0 || 2hSDS_dChCldUrea_sub.txt ||
|| 5.0 || 5hSDS_dChCldUrea_sub.txt ||
|| 7.5 || 7p5hSDS_dChCldUrea_sub.txt ||
|| 10.0 || 10hSDS_dChCldUrea_sub.txt ||

All the data files have been processed to 1D scattering curves with the solvent background subtracted to leave only the coherent scattering signal on absolute scale.

Additionally there is a folder called "Not subtracted" containing the data for task 14.

=== [=#section52 5.2 Calculating Scattering Length Density] ===

The scattering length density is given by 

[[Image(kemm37_sld.png, 100px)]]

Scattering lengths of relevant elements:
||= **Element** =||= **Scattering Length (fm)** =||
||C	||6.646||
||H	||-3.739||
||D	||6.671||
||N	||9.36||
||O	||5.803||
||Cl	||9.577||


Physical properties of the DES components:
||= **Component**	=||= **Chemical Formula** =||=	**Molecular Volume (Å3)**	=||= **Density (g/cm3)** =||
||d9-Choline Chloride||	C5H5D9NOCl	||  210.77||  1.17||
||d4-Urea||	CD4N2O	||   75.55||	1.41||


{{{
#!div style="background: lightblue"
[=#task12 **TASK 12:**] Calculate the scattering length density (SLD) of a 1:2 mole ratio mixture of choline chloride and urea.

Use the information in the table above to calculate the SLD. There are multiple ways to do so, including:
* Calculating by hand
* Using a spreadsheet
* Using the SLD calculator built in to !SasView (in the Tools menu).
* Using online calculators e.g. [https://www.ncnr.nist.gov/resources/activation/]

Try calculating by hand and one or more of the other ways and see if you get the same answer!
}}}

Remember that in mixtures, we use the volume fraction of each component to calculate the overall scattering length density:

[[Image(kemm37_volumefraction_sld.png, 200px)]]

=== [=#section5. 5.3 Fitting the data] ===
{{{
#!div style="background: lightblue"
[=#task19 **TASK 19:**] Fitting the lowest concentration data.

Select the lowest concentration data only in the data explorer by ensuring only 0p2hSDS_dChCldUrea_sub.txt has a check mark next to it and click “Send to" fitting.

* Select the model for the structure you predicted for this dataset. 
  * How does it compare to the data?
* Fill in parameters you know and adjust the others to see how close you get to the data.
* Select parameters to fit and run the fit by clicking "Fit" at the bottom of the fitting panel.
  * Do you get a good fit?
  * Are the parameters you get physically reasonable?

}}}


{{{
#!div style="background: lightblue"
[=#task20 **TASK 20:**] Fitting the other data, starting with the 7.5 wt% data set.

Repeat for other concentrations
* Does the same model fit all data?
 * If not, what models do you use?
 * What do you observe about the higher concentration data sets?
* What is consistent between datasets? What is different? 

}}}

{{{
#!div style="background: lightblue"
[=#task21 **TASK 21:**] Summarising the results

Having fitted all the data sets you can now summarise the results and comment on the trends you observe.

}}}


== [=#resources Resources] ==

* NIST SLD calculator [https://www.ncnr.nist.gov/resources/activation/]
* NIST Scattering Length and Scattering Cross Section Database [https://www.ncnr.nist.gov/resources/n-lengths/]