Critical phenomena Curved surfaces

The use of adsorption isotherms is subject to both theoretical and experimental limitations. There is effectively a minimum relative pressure value specific to each adsorbate (e.g. P/Pq = 0.42 for nitrogen, 0,2 for CCI4) which corresponds to the minimum value of the surface tension for the phase to remain in liquid form. Below this critical value, the liquid adsorbate is unstable and vaporises spontaneously, an effect represented on the desorption curves by a sharp drop in the adsorbed volume. Depending on the significance of this variation, the porous distribution calculated from the desorption data may show an artefact in the pore size domain corresponding to this process (3-4 nm in diameter). For a porous solid where this phenomenon occurs, it is advisable to study the adsorption curve. 

In principle, all of the parameters in this equation are measurable so it should be possible to determine the mobility from the voltage dependence of the current. However, the actual behaviour of cells of this type is critically dependent on the nature of the interface. The currents obtained can also depend on the history of the cell. One example of this and of another way that ppm levels of ionic impurities can cause problems is provided by the field anneal phenomenon [25, 26]. If the sample is heated into the isotropic phase and a DC current applied to the cell for a period of time ionic impurities become adsorbed on the electrode surface where they help to facilitate charge injection. Thereafter the I/V characteristics of the Col phase are found to be totally different and, in extreme cases, the measured conductivity can (apparently) be orders of magnitude higher Because of the extreme sensitivity of sandwich cell measurements to small difference in the electrode surface and, because experimentally the voltage dependence of the current often deviates quite markedly from the expected (V-Vc) behaviour, reliable determination of mobilities from the I/V curves is difficult and this approach is rarely used [27]. 

---