Propylammonium vermiculite

The phase diagram was mapped out on the LOQ instrument at ISIS, Didcot, U.K. [16], just as for the butylammonium vermiculite system [6]. Typical scattering patterns from an r = 0.01, c = 0.25 M propylammonium vermiculite gel are shown at T = 36, 38, 40 and 42°C in Figure 9.6. The story is by now a familiar one a gel with a well-defined d-value at low temperatures (in this case d = 60 A) collapses as the temperature increases at a well-defined phase transition temperature (in this case Tc = 39°C). Note that the butylammonium system will not swell at c > 0.2 M, whereas here we have colloidal swelling at c = 0.25 M. The complete c, T phase diagrams at low r for both the propylammonium and butylammonium systems, taken in contiguous experiments, are shown in Figure 9.7. It is clear that the propylammonium vermiculites will swell in salt concentrations up to about 0.5 M in cold water. In these circumstances, their -values decrease below 50 A, and so could be measured on LAD. 

Before moving on to our attempt to measure the complete double layer in a swollen propylammonium vermiculite with d = 43.6 A [18], we pause to note that (a) at ionic strengths relevant to cell fluids, namely c 0.12 M [19], the phase-transition temperature in the propylammonium vermiculite system is not so far away from our body temperature and (b) similar temperature-induced gel-crystal transitions are observed in many biochemical systems. An example is the deoxyhemoglobin molecule that causes sickle cell anemia [33], We also note that with both counterions, Tc decreases linearly with the logarithm of the salt concentration. 

FIGURE 9.6 Low-angle neutron scattering intensity as a function of scattering vector Q (A-1) for propylammonium vermiculite immersed in a 0.25 M propylammonium chloride solution at four different temperatures, T = 309 K (continuous line), 311 K (dashed line), 313 K (crosses) and 315 K (solid circles). The low-angle peak at 0.1 A-1 is due to a gel phase with a clay layer spacing of 60 A. The data therefore show the temperature-induced gel-crystal phase transition occurring between 311 and 313 K. Note that body temperature is 311 K and that cell fluids are typically 0.2 M. 

Analysis of Peak Areas in the Fitted Density Profiles for 43.6 A n-Propylammonium Vermiculite Gels... 

FIGURE 9.7 Clay layer spacings d in vermiculite gels as a function of the external salt concentration c for propylammonium (solid circles) and butylammonium (open squares) vermiculites. 

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