1 | r""" |
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2 | Definition |
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3 | ---------- |
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4 | |
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5 | This model describes the scattering from a layer of surfactant or polymer |
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6 | adsorbed on large, smooth, notionally spherical particles under the conditions |
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7 | that (i) the particles (cores) are contrast-matched to the dispersion medium, |
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8 | (ii) $S(Q) \sim 1$ (ie, the particle volume fraction is dilute), (iii) the |
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9 | particle radius is >> layer thickness (ie, the interface is locally flat), |
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10 | and (iv) scattering from excess unadsorbed adsorbate in the bulk medium is |
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11 | absent or has been corrected for. |
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12 | |
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13 | Unlike many other core-shell models, this model does not assume any form |
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14 | for the density distribution of the adsorbed species normal to the interface |
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15 | (cf, a core-shell model normally assumes the density distribution to be a |
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16 | homogeneous step-function). For comparison, if the thickness of a (traditional |
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17 | core-shell like) step function distribution is $t$, the second moment about |
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18 | the mean of the density distribution (ie, the distance of the centre-of-mass |
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19 | of the distribution from the interface), $\sigma = \sqrt{t^2/12}$. |
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20 | |
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21 | .. math:: |
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22 | |
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23 | I(q) = \text{scale} \cdot (\rho_\text{poly}-\rho_\text{solvent})^2 |
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24 | \left[ |
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25 | \frac{6\pi\phi_\text{core}}{Q^2} |
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26 | \frac{\Gamma^2}{\delta_\text{poly}^2R_\text{core}} |
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27 | \exp(-Q^2\sigma^2) |
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28 | \right] + \text{background} |
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29 | |
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30 | where *scale* is a scale factor, $\rho_\text{poly}$ is the sld of the |
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31 | polymer (or surfactant) layer, $\rho_\text{solv}$ is the sld of the |
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32 | solvent/medium and cores, $\phi_\text{core}$ is the volume fraction of |
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33 | the core particles, $\delta_\text{poly}$ is the bulk density of the |
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34 | polymer, $\Gamma$ is the adsorbed amount, and $\sigma$ is the second |
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35 | moment of the thickness distribution. |
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36 | |
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37 | Note that all parameters except $\sigma$ are correlated so fitting more |
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38 | than one of these parameters will generally fail. Also note that unlike |
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39 | other shape models, no volume normalization is applied to this model (the |
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40 | calculation is exact). |
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41 | |
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42 | The code for this model is based originally on a a fortran implementation by |
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43 | Steve King at ISIS in the SANDRA package c. 1990. |
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44 | |
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45 | References |
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46 | ---------- |
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47 | |
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48 | .. [#] S King, P Griffiths, J Hone, and T Cosgrove, *SANS from Adsorbed Polymer |
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49 | Layers*, *Macromol. Symp.*, 190 (2002) 33-42. |
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50 | |
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51 | Source |
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52 | ------ |
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53 | |
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54 | `adsorbed_layer.py <https://github.com/SasView/sasmodels/blob/master/sasmodels/models/adsorbed_layer.py>`_ |
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55 | |
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56 | Authorship and Verification |
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57 | ---------------------------- |
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58 | |
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59 | * **Author:** Jae-Hi Cho **Date:** pre 2010 |
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60 | * **Last Modified by:** Paul Kienzle **Date:** April 14, 2016 |
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61 | * **Last Reviewed by:** Steve King **Date:** March 18, 2016 |
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62 | * **Source added by :** Steve King **Date:** March 25, 2019 |
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63 | """ |
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64 | |
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65 | import numpy as np |
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66 | from numpy import inf, pi, exp, errstate |
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67 | |
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68 | name = "adsorbed_layer" |
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69 | title = "Scattering from an adsorbed layer on particles" |
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70 | |
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71 | description = """ |
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72 | Evaluates the scattering from large particles |
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73 | with an adsorbed layer of surfactant or |
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74 | polymer, independent of the form of the |
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75 | density distribution. |
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76 | """ |
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77 | category = "shape:sphere" |
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78 | |
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79 | # pylint: disable=bad-whitespace, line-too-long |
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80 | # ["name", "units", default, [lower, upper], "type", "description"], |
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81 | parameters = [ |
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82 | ["second_moment", "Ang", 23.0, [0.0, inf], "", "Second moment of polymer distribution"], |
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83 | ["adsorbed_amount", "mg/m^2", 1.9, [0.0, inf], "", "Adsorbed amount of polymer"], |
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84 | ["density_shell", "g/cm^3", 0.7, [0.0, inf], "", "Bulk density of polymer in the shell"], |
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85 | ["radius", "Ang", 500.0, [0.0, inf], "", "Core particle radius"], |
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86 | ["volfraction", "None", 0.14, [0.0, inf], "", "Core particle volume fraction"], |
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87 | ["sld_shell", "1e-6/Ang^2", 1.5, [-inf, inf], "sld", "Polymer shell SLD"], |
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88 | ["sld_solvent", "1e-6/Ang^2", 6.3, [-inf, inf], "sld", "Solvent SLD"], |
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89 | ] |
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90 | # pylint: enable=bad-whitespace, line-too-long |
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91 | |
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92 | # NB: Scale and Background are implicit parameters on every model |
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93 | def Iq(q, second_moment, adsorbed_amount, density_shell, radius, |
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94 | volfraction, sld_shell, sld_solvent): |
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95 | """Return I(q) for adsorbed layer model.""" |
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96 | with errstate(divide='ignore'): |
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97 | aa = ((sld_shell - sld_solvent)/density_shell * adsorbed_amount) / q |
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98 | bb = q * second_moment |
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99 | #scale by 10^-2 for units conversion to cm^-1 |
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100 | inten = 6.0e-02 * pi * volfraction * aa**2 * exp(-bb**2) / radius |
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101 | return inten |
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102 | Iq.vectorized = True # Iq accepts an array of q values |
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103 | |
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104 | def random(): |
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105 | """Return a random parameter set for the model.""" |
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106 | # only care about the value of second_moment: |
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107 | # curve = scale * e**(-second_moment^2 q^2)/q^2 |
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108 | # scale = 6 pi/100 (contrast/density*absorbed_amount)^2 * Vf/radius |
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109 | # the remaining parameters can be randomly generated from zero to |
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110 | # twice the default value as done by default in compare.py |
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111 | pars = dict( |
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112 | scale=1, |
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113 | second_moment=10**np.random.uniform(1, 3), |
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114 | ) |
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115 | return pars |
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116 | |
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117 | # unit test values taken from SasView 3.1.2 |
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118 | tests = [ |
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119 | [{'scale': 1.0, 'second_moment': 23.0, 'adsorbed_amount': 1.9, |
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120 | 'density_shell': 0.7, 'radius': 500.0, 'volfraction': 0.14, |
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121 | 'sld_shell': 1.5, 'sld_solvent': 6.3, 'background': 0.0}, |
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122 | [0.0106939, 0.1], [73.741, 4.51684e-3]], |
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123 | ] |
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124 | |
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125 | # 2016-03-16 SMK converted from sasview, checked vs SANDRA |
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126 | # 2016-03-18 RKH some edits & renaming |
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127 | # 2016-04-14 PAK reformatting |
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