The Structure of a Dressed Macroion in Solution

FIGURE 8.1 The ripples of diffuse scattering observed in the first D16 experiment. The gels had been soaked in 0.1 M n-butylammonium chloride solutions in D20 (a) with the usual protonated n-butylammonium ions (b) with deuterated n-butylammonium ions. In (a) the sharp peak at Q = 0.05 A-1 is the first-order diffraction peak the low angle scattering has been scaled down by a factor of 15 with respect to the ripples. 

The total G(rz) can be expressed as a neutron-weighted sum (dependent on the scattering lengths of the constituent atoms) of the partial pair correlation functions 8ifrz) according to 

Using the method of isotope substitution, it is possible to obtain partial pair correlation functions experimentally. Two isotopically different samples were prepared at c = 0.1 M, one where all the hydrogen in the butylammonium chains was ordinary II and one where they were entirely deuterated, to make use of the large difference in scattering 

FIGURE 8.2 Structure factors S(QZ) obtained from neutron-scattering patterns of butylam-monium vermiculite gels (upper panels) and from a 0.1 M protonated butylammonium salt solution with no clay (lowest panel). The upper panels show S(QZ) for gels prepared in a 0.1 M deuterated salt solution and in 0.1 and 0.01 M protonated salt solutions. The momentum transfer Q was perpendicular to the clay plates, and the structure factor S(QZ) has been normalized after correction for background scattering and absorption. 

FIGURE 8.4 Two difference pair-correlation functions AG(rz). The dotted line shows AG(rz)(H), obtained from the difference between the vermiculite and its H-salt solution. The full line shows AG(rz)(D), obtained from the difference between the vermiculites in D- and H-salt solutions. 

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FIGURE 8.5 Schematic structure of a dressed macroion in solution. The arrowheads represent partially ordered water molecules. 

We first used isotope substitution in diffuse neutron scattering measurements to determine the distribution of water molecules and counterions (n-butylammonium ions) around the clay layers in the gel state, and obtained a unique picture of a dressed macroion in solution. We obtained a structure in which the naked clay plate of 10 A thickness was extended out to about 35 A by layers of water molecules and counterions. The dressed macroion has exactly two layers of water molecules coating the clay layers these layers are 6 A thick on both sides, extending the effective clay plate out to 22 A, before any counterions at all are found. This is in direct contradiction to the Stem layer picture, widely held in colloid science, that... 

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