Ferguson’s principle

In the four decades of discussion that followed its introduction, Ferguson s principle has not been significantly enlarged, or even further clarified. Its daring assumption that partial pressures can be substituted for thermodynamic activities has not been falsified on the other hand, it did not predict parabolic biological response to increasing partition coefficients (Hansch, 1971). At the present time, the incorporation of partition coefficients into a quantitative... 

The general anaesthetics appear to be the only pharmacological group (applied in the vapour phase) in which molecules of widely different structure have been correlated by Ferguson s principle. As they function by accumulating in essential parts of the nerve cells and disorganising their metabolism, such a mechanism depends on physical nature rather than on chemical structure. 

Of interest in this context are the results of Good and Milloy [51] (see Fig. 6.10). The partial pressure of phenol above CTAB-phenol-water solutions was determined as a function of phenol concentration by analysis of the gas phase above the solutions at 25° C. As P/Pq is proportional to the activity of the phenol in the water, this, in accord with Ferguson s principle, should broadly determine its bactericidal activity. Studies of this nature on solutions with varying concentrations of surfactant would be valuable in interpreting the microbiological behaviour of such systems. 

Another utilitarian conclusion that can be derived from Ferguson s principle is that, when investigating a new agent, it is economical to alter the molecule so that it becomes less soluble in water, up to the limit discussed above. 

Although discussed here, so far, only in connection with insecticides, Ferguson s principle applies equally for hypnotics in warm-blooded (Table 15.3) or cold-blooded (Table 15.4) mammals. Table 15.4 demonstrates how six depressants can vary in potency over a 400-fold range, if calculated on a molar basis, but be contained within a twofold range when calculated on the basis of thermodynamic activities. 

Mason, S., Ferguson, C.A. and Pethiek, A.J. (1986), Ergonomic Principles in Designing for Maintainability. Luxembourg ECSC Community Ergonomics Action Report 8, Series 3. 

---