The isoscattering point in X-ray scattering curves of globular solute particles measured by the contrast-variation method: the influence of additive penetration into the solute particles

The nature of the isoscattering point has been examined theoretically with the assumption that the structure of a globular solute particle changes on the penetration of additive in the solvent into the particle. The salt ion or deuterium of the D₂O used in X-ray and neutron contrast-variation studies was considered as the penetrative entity. The appearance of the isoscattering point was found to be limited to only two distributions of the penetrated additive: (i) the spherically symmetrical distribution; (ii) the homogeneous distribution over the particle. For case (i), the radial distribution is determined from the position of the isoscattering point; for case (ii), no information concerning the penetration is obtained from the point.