Surfactants able to change their orientation

In the literature, it is observed from time to time that adsorption processes proceed faster than expected from a diffusion mechanism. This can of course be ascribed to convection in the bulk, however, this is not a good explanation for surfactant systems, where such phenomena are observed under various experimental conditions. 

For a modelling of adsorption processes the well-known integro-differential equation (4.1) derived by Ward and Tordai [3] is used. It is the most general relationship between the dynamic adsorption r(t) and the subsurface concentration e(0,t) for fresh non-deformed surfaces and is valid for kinetic-controlled, pure diffusion-controlled and mixed adsorption mechanisms. For a diffusion-controlled adsorption mechanism Eq. (4.1) predicts different F dependencies on t for different types of isotherms. For example, the Frumkin adsorption isotherm predicts a slower initial rate of surface tension decrease than the Langmuir isotherm does. In section 4.2.2. it was shown that reorientation processes in the adsorption layer can mimic adsorption processes faster than expected from diffusion. In this paragraph we will give experimental evidence, that changes in the molar area of adsorbed molecules can cause sueh effectively faster adsorption processes. 

The Tritons have been described in the last paragraph by a diffusion controlled kineties based on a Langmuir isotherm. As it was shown in the preceding Chapter 3, oxethylated surfactants can be usually better described by a reorientation isotherm [226]. This is however only true for high quality samples such as define compounds of the type C EO , while for technical products like the Tritons an isotherm more complicated than the Langmuir isotherm does not make sense. An analysis of dynamic surface and interfacial tensions for CioEOg solutions had been performed in [227]. 

Let us first look into the dynamic surface tensions for CjoEOg at the water/air interface, as it was measured by Chang et al. [228] using the pendent bubble method. The experimental data given in Fig. 4.29 are compared with calculations for two models, based on the Langmuir and the reorientation isotherm (two-state model). 

The diffusion coefficient D = 510 cmVs was calculated from the equation proposed by Wilke and Chang [229]. One can see that the Langmuir equation overestimates the dynamics, i.e. the value of D required to match the experimental data would be 1.5-I0 cmVs, that is 3 times 

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