Question about crystal structure of long chain fatty acids (XRD)

[xrdquestionperson]

Hello, we are studying potato periderm before/after digestion and dewaxing and native vs wounded. One of the techniques is XRD, hence my user name. Among other things we are trying to use XRD to determine the arrangement of fatty acids, esters, and other long-chain aliphatics that are present after digestion. In virtually every XRD experiment we see a large broad peak between ~16 and ~22 deg 2theta and two sharp peaks around ~26 and ~28 deg 2Theta. This is similar to other research I've found (ex: XRD of plant cutins) and in other papers they make the claim that the two sharp peaks correspond to an orthorhombic arrangement of long chain aliphatics with regular spacing between them (the broad peak is the same long-chains but arranged randomly), where the chains are parallel to the C axis.

I guess my question is this, are long chain fatty acids always in an orthorhombic crystal arrangement, even when part of a composite such as plant tissue? Or are the researchers who I'm citing just assuming this to make other assumptions and calculations easier? I'm not that knowledgeable in XRD but it would be nice if this was the case, because then you can use the D spacings to figure out lattice parameters and other information.

A related question: for wounded, digested/dewaxed periderm, we see one very tall very sharp peak around 27 2TH and nothing else. Can this be interpreted as all the long chains having the same exact very ordered arrangement, with no amorphous phase?

Thank you for your help.

 

[M Quack]

In XRD, sharp peaks indicate long range order, and broad peaks short range order. Good crystals such as LaB6 or CeO2 calibration powders or Si single crystals have very sharp peaks. The peak width can be used to estimate the size of the ordered domains.

https://en.wikipedia.org/wiki/Scherrer_equation
In general, one single peak is not enough to fully define the crystal structure. You can of course evaluate trends, e.g. how the peak width changes during treatment. That the peak position changes from 26 and 28deg to 27deg in an indication that the crystal structure changes.

Btw, talking about scattering angles without specifying the wavelength or x-ray energy is meaningless. With the wavelength you can get the d-spacing of a single reflection, but that is not enough for determining the crystal structure or even crystal symmetry (orthorhombic or other) and lattice parameters.