Random lamellar head/tail/tail/head sheet with Caille structure factor

Parameter Description Units Default value
scale Source intensity None 1
background Source background cm-1 0.001
length_tail Tail thickness 10
length_head head thickness 2
Nlayers Number of layers None 30
d_spacing lamellar d-spacing of Caille S(Q) 40
Caille_parameter Caille parameter None 0.001
sld Tail scattering length density 10-6-2 0.4
sld_head Head scattering length density 10-6-2 2
sld_solvent Solvent scattering length density 10-6-2 6

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

This model provides the scattering intensity, \(I(q) = P(q)S(q)\), for a lamellar phase where a random distribution in solution are assumed. Here a Caille \(S(q)\) is used for the lamellar stacks.

The scattering intensity \(I(q)\) is

\[I(q) = 2 \pi \frac{P(q)S(q)}{q^2\delta }\]

The form factor \(P(q)\) is

\[P(q) = \frac{4}{q^2}\big\{ \Delta\rho_H \left[\sin[q(\delta_H + \delta_T)] - \sin(q\delta_T)\right] + \Delta\rho_T\sin(q\delta_T)\big\}^2\]

and the structure factor \(S(q)\) is

\[S(q) = 1 + 2 \sum_1^{N-1}\left(1-\frac{n}{N}\right) \cos(qdn)\exp\left(-\frac{2q^2d^2\alpha(n)}{2}\right)\]


\[\begin{align*} \alpha(n) &= \frac{\eta_{cp}}{4\pi^2} \left(\ln(\pi n)+\gamma_E\right) && \\ \gamma_E &= 0.5772156649 && \text{Euler's constant} \\ \eta_{cp} &= \frac{q_o^2k_B T}{8\pi\sqrt{K\overline{B}}} && \text{Caille constant} \end{align*}\]

\(\delta_T\) is the tail length (or length_tail), \(\delta_H\) is the head thickness (or length_head), \(\Delta\rho_H\) is SLD(headgroup) - SLD(solvent), and \(\Delta\rho_T\) is SLD(tail) - SLD(headgroup). Here \(d\) is (repeat) spacing, \(K\) is smectic bending elasticity, \(B\) is compression modulus, and \(N\) is the number of lamellar plates (Nlayers).

NB: When the Caille parameter is greater than approximately 0.8 to 1.0, the assumptions of the model are incorrect. And due to a complication of the model function, users are responsible for making sure that all the assumptions are handled accurately (see the original reference below for more details).

Non-integer numbers of stacks are calculated as a linear combination of results for the next lower and higher values.

Be aware that the computations may be very slow.

The 2D scattering intensity is calculated in the same way as 1D, where the \(q\) vector is defined as

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

Fig. 42 1D plot corresponding to the default parameters of the model.


F Nallet, R Laversanne, and D Roux, J. Phys. II France, 3, (1993) 487-502

also in J. Phys. Chem. B, 105, (2001) 11081-11088