Assuming A1 to be equal to A0 gives us the possibility to calculate G from the combined e andmeasurements. Irreversible ProcessesThe approach presented in the above could also contribute to the description of irreversible processes. Especially the squeeze-out effect could be treated well with the clustering mechanism. For squeeze-out effects, irreversible horizontal parts of thecurves are typical. Introducing clustering we could explain this behavior by assuming that there exists a modification of a certain cluster ("F"-cluster) containing F molecules which can disappear in the water below the surface layer. Thus we exclude squeeze-out in which molecules move to above the water-air interface.When the surface compression is performed slowly, once the clustering is gone so far that "F"-clusters appear by clustering, these clusters would transform into soluble clusters and disappear into the water causing a horizontal part in thecurve.When the compression is going too fast, it might occur that not all the "F"-clusters are transformed into the soluble form and that bigger clusters are formed. This would lead to the return of the negative slope of thecurve. A quantitative description of this phenomenon is the purpose of further work of our group. Conclusions and DiscussionWe dealt with surface pressure-area ( -) curves starting from the existence of clusters in the monolayer. Thereto we introduced the equilibrium clustering constant K. With this single parameter, however, we did not succeed in explaining plateaux in the ( -) curves. The explanation was successful upon assuming that for larger clusters (exceeding m molecules) the equilibrium clustering constant differs from K. The resulting plateaux are more pronounced at high values of m.In addition a parameter G is introduced, accounting for the difference in specific area between molecules in large clusters and molecules in small clusters. With this theoretical description different phenomena encountered in measurements can be coped with without introducing phase transitions.

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Counterion condensation data have been obtained from potentiometric measurements for mixed micelles of copper dodecyl sulfate with two nonionic surfactants, Triton X-100 (system I) and Brij 35 (system II), by using a highly sensitive cupric ion selective electrode. A critical micellar composition X™ value is determined beyond which cupric counterions do not condensate on the mixed micelles. XM values are equal to 0.98 (system I) and 0.95 (system II). These results are discussed in terms of Bjerrum's ion-ion association model. The relevance of these findings to the recently observed increase of cloud point phenomena of nonionic surfactants upon addition of small quantities of an anionic surfactant is outlined.

The vapour pressure change upon addition of electrolytes to dilute aqueous tetrahydrofuran (THF) solutions has been determined at 298.15 K. From these data the salting constants k,