Diffuse scattering, macroions

We now wish to examine how the structure of the dressed macroion, as determined by our diffuse neutron scattering experiments, varies with applied uniaxial stress. The best data was obtained at c = 0.03 M, with r < 0.01, when the uniaxial pressure p was varied between 0 and 0.2 atm a single experiment conducted over two days on D16 gave clear ripples at different applied stresses, as described below. As in the experiments at c = 0.1 and 0.01 M described above, T and P were held constant at 10°C and 1 atm, respectively. The details of the experiment and the small-angle patterns observed are given in reference [4], Here we concentrate on how applied uniaxial pressure affects the higher g-rangc of the structure factors. 

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... 

Fulmer AW, Benbasat JA, Bloomfield VA. Ionic strength effects on macroion diffusion and excess light-scattering intensities of short DNA rods. Biopolymers 1981 20 1147-1159. 

Reed, W.F. Light Scattering Results on Polyelectrolyte Conformations, Diffusion and Interparticle Interactions and Correlations, invited entry for ACS Symposium Series 548, Macroion Characterization. Schmitz, K. Ed., ACS, 1994, pp. 297-314. 

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