1.6.7. multilayer_vesicle

P(Q) for a Multi-lamellar vesicle

Parameter Description Units Default value
scale Source intensity None 1
background Source background cm-1 0.001
volfraction volume fraction of vesicles None 0.05
radius radius of solvent filled core 60
thick_shell thickness of one shell 10
thick_solvent solvent thickness between shells 10
sld_solvent solvent scattering length density 10-6-2 6.4
sld Shell scattering length density 10-6-2 0.4
n_shells Number of shell plus solvent layer pairs None 2

The returned value is scaled to units of cm-1 sr-1, absolute scale.

Definition

This model is a trivial extension of the core_shell_sphere function where the core is filled with solvent and is surrounded by \(N\) shells of material (such as lipids) interleaved with \(N - 1\) layers of solvent. For \(N = 1\), this returns the same as the vesicle model, except for the normalisation, which here is to outermost volume. The shell thicknesses and SLD are constant for all shells as expected for a multilayer vesicle.

../_images/multi_shell_geometry.jpg

Fig. 1.82 Figure 1: Geometry of the multilayer_vesicle model.

See the core_shell_sphere model for more documentation.

The 1D scattering intensity is calculated in the following way (Guinier, 1955)

\[P(q) = \text{scale} \cdot \frac{\phi}{V(R_N)} F^2(q) + \text{background}\]

where

\[F(q) = (\rho_\text{shell}-\rho_\text{solv}) \sum_{i=1}^{N} \left[ 3V(r_i)\frac{\sin(qr_i) - qr_i\cos(qr_i)}{(qr_i)^3} - 3V(R_i)\frac{\sin(qR_i) - qR_i\cos(qR_i)}{(qR_i)^3} \right]\]

for

\[\begin{split}r_i &= r_c + (i-1)(t_s + t_w) \text{ solvent radius before shell } i \\ R_i &= r_i + t_s \text{ shell radius for shell } i\end{split}\]

\(\phi\) is the volume fraction of particles, \(V(r)\) is the volume of a sphere of radius \(r\), \(r_c\) is the radius of the core, \(t_s\) is the thickness of the shell, \(t_w\) is the thickness of the solvent layer between the shells, \(\rho_\text{shell}\) is the scattering length density of a shell, and \(\rho_\text{solv}\) is the scattering length density of the solvent.

USAGE NOTES

  • Start with the best possible guess
  • Using a priori knowledge, hold as many parameters fixed as possible
  • if N=1, tw (water thickness) must by definition be zero. Both N and tw should be fixed during fitting.
  • If N>1, use constraints to keep N > 1
  • Because N only really moves in integer steps, it may get “stuck” if the optimizer step size is too small so care should be taken If you experience problems with this please contact the SasView team and let them know the issue preferably with example data and model which fail to converge.

The 2D scattering intensity is the same as 1D, regardless of the orientation of the q vector which is defined as:

\[q = \sqrt{q_x^2 + q_y^2}\]

For information about polarised and magnetic scattering, see the Polarisation/Magnetic Scattering documentation.

../_images/multilayer_vesicle_autogenfig.png

Fig. 1.83 Figure 2: 1D plot corresponding to the default parameters of the model.

References

[1]B Cabane, Small Angle Scattering Methods, in Surfactant Solutions: New Methods of Investigation, Ch.2, Surfactant Science Series Vol. 22, Ed. R Zana and M Dekker, New York, (1987).

Authorship and Verification