for clay system, at same d-spacing, counts value changed after treating with saline water. what it indicates? no D spacing value change.
It depends on the system you look. In clay system, the amount of water adsorbed by clay can be indicated by the d-spacing of the gallery. Intensity and the sharp of the peak of the XRD tell us the ordering of the system.
If there was no change in d-spacing, then water was not absorbed into the layers of the clay. If all that happened was that intensities in or around the XRD peaks changed, that may just be water adsorbing to the outer surface of the particles.
I agree with D.R.Hummer, water adsorbed by clay not just in the clay gallery but also outside (surface) of clay layer. One way of water associated with clay is with cation exchange. Around 80 % cation exchange is located in the gallery and 20 % in the clay surface.
If you are using XRD, then the present of water in the clay gallery can be indicated by the value of d-spacing which is represented by 2 theta. As clay is hydrophilic, the condition/environment (temperature and % RH) of measurement determines the amount of water associated with clay. For this purpose, a set of experiment (effect of thermal treatment and relative humidity on d-spacing) can be done using XRD, for example, at 80 C, 150 C, 50 % RH, 85 % RH. This experiment will inform us how the d-spacing depends on temperature and % RH. In case of montmorillonite, the d-spacing is 9.6 A when no water present in the gallery, then around 12 A when water present as a single layer, and 15 A when the water present as bilayer.
XRD analysis is supported by TGA. XRD only tell us wheter water is present or not in the gallery but can not tell us the amount of water associated with clay, whereas TGA can tell us the amount of water associated with clay but can't tell us whether water is inside or outside the gallery. So XRD and TGA is complement each other.
If the d-spacing doesn't change then TGA will tell us whether water is adsorbed in the surface of clay layer or not. The weight loss below 120 C is due to water associated with clay.
It depends on d-spacing. No variations in d-spacing, no water adsorption.
The effect of water on d-spacing varies from one clay type to another depending on the binding forces of the interlayer space, thus water adsorption can not be solely explained by intensity changes. Though intensity enhancement could reflect better orderness/crystallinity, I think it is necessary first to determine the clay type(s) you have as the response to fresh/saline water content is not identical for different clay types.
If solid particle (crystallites) breaks by means of physical / chemical process, the XRD peak become broader and intensity usually reduces. Sometimes, high microstrain due to small particle can cause the increase of intensity with little deviation in its position. So, it depends how water interact with the contents of the soil...
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