# vesicle

This model provides the form factor, P(q), for an unilamellar vesicle. This is model is effectively identical to the hollow sphere reparameterized to be more intuitive for a vesicle and normalizing the form factor by the volume of the shell.

Parameter Description Units Default value
scale Source intensity None 1
background Source background cm-1 0.001
sld vesicle shell scattering length density 10-6-2 0.5
sld_solvent solvent scattering length density 10-6-2 6.36
volfraction volume fraction of shell None 0.05
thickness vesicle shell thickness 30

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

Definition

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

$P(q) = \frac{\phi}{V_\text{shell}} \left[ \frac{3V_{\text{core}}({\rho_{\text{solvent}} - \rho_{\text{shell}})j_1(qR_{\text{core}})}}{qR_{\text{core}}} + \frac{3V_{\text{tot}}(\rho_{\text{shell}} - \rho_{\text{solvent}}) j_1(qR_{\text{tot}})}{qR_{\text{tot}}} \right]^2 + \text{background}$

where $$\phi$$ is the volume fraction of shell material, $$V_{shell}$$ is the volume of the shell, $$V_{\text{cor}}$$ is the volume of the core, $$V_{\text{tot}}$$ is the total volume, $$R_{\text{core}}$$ is the radius of the core, $$R_{\text{tot}}$$ is the outer radius of the shell, $$\rho_{\text{solvent}}$$ is the scattering length density of the solvent (which is the same as for the core in this case), $$\rho_{\text{scale}}$$ is the scattering length density of the shell, background is a flat background level (due for example to incoherent scattering in the case of neutrons), and $$j_1$$ is the spherical bessel function $$j_1 = (\sin(x) - x \cos(x))/ x^2$$.

The functional form is identical to a “typical” core-shell structure, except that the scattering is normalized by the volume that is contributing to the scattering, namely the volume of the shell alone, the scattering length density of the core is fixed the same as that of the solvent, the scale factor when the data are on an absolute scale is equivalent to the volume fraction of material in the shell rather than the entire core+shell sphere, and the parameterization is done in terms of the core radius = $$R_{\text{core}}$$ and the shell thickness = $$R_{\text{tot}} - R_{\text{core}}$$.

The 2D scattering intensity is the same as P(q) above, regardless of the orientation of the q vector which is defined as

$q = \sqrt{q_x^2 + q_y^2}$

NB: The outer most radius (= radius + thickness) is used as the effective radius for S(Q) when P(Q) * S(Q) is applied.

References

A Guinier and G. Fournet, Small-Angle Scattering of X-Rays, John Wiley and Sons, New York, (1955)

Author: NIST IGOR/DANSE on: pre 2010