The Counterion Distribution between Charged Plates in Solution

what are we to make of the story that emerges from Chapters 1 to 8 The central result that emerged from Chapter 8 is that in a model clay system, the naked clay particle (of a thickness of about 10 A) is covered by two ordered layers of water molecules on each side, followed by a layer of counterions and another layer of partially ordered water molecules, to produce a dressed clay particle of a thickness of about 35 A. Within this dressed macroion, short-range molecular forces are dominant. We can interpret these as giving rise to an effective clay plate thickness of about 35 A in a swollen clay. 

The main fact of the book so far is that the clay particles sit at about 7 Debye lengths from each other when the clay is in its swollen gel state. The interplate separation in the gel state is therefore inversely proportional to the square root of the electrolyte concentration, a fact noted by Walker in 1960 [1], Such a separation of 7 Debye lengths is no special feature of clay science associated with the large 

It became fashionable among theoreticians to refer to the basic model in any field as vanilla flavored, the version with no fancy flavorings. Looked at in this light, we might be tempted to label the diffuse scattering in Chapter 8 as vanilla ripples, as they gave us a first basic picture of the ion and solvent distribution around a colloidal particle in solution. In that case, the ripples we are now about to discuss are definitely of the raspberry variety. 

FIGURE 9.1 Schematic illustration of a possible structure of a clay with d = 45 A. If the dressed macroion structure remains constant, there is only a 10 A intermacroionic gap for the remaining counterions and water molecules. 

As we wanted to make a complete structural determination of the mineral, we used a more highly oriented vermiculite from Llano, Texas. This was a special clay (VTx-1) from the Clay Minerals Society s Source Clays Repository it had the structural formula [23] 

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