Experimental results on adsorption kinetics

Most surfactants adsorb diffusion controlled at liquid interfaces. It was discussed above that exceptions observed in the literature and interpreted in terms of adsorption and desorption barriers have been understood later by the pure diffusion model when the respective experimental conditions were considered properly. One of the most important points in this respect was the systematic analysis of impurity effects on the adsorption kinetics of surfactants. This point was for example discussed in detail in the book by Dukhin et al. [2]. Another reason for the observation of an adsorption process slower than expected from diffusion is the 

In contrast to surface aggregation, changes in the molar area of adsorbed molecules can lead to an apparent enhancement of the adsorption rate. Thus, observed super-diffusion phenomena can be understood by considering changes in the molar surface area with changing surface coverage (cf. Fig. 4.6). Again, these systems are then quantitatively understood by a purely diffusion controlled model. 

In this paragraph we give examples for each of the mentioned cases, starting with a simple surfactant system that follows essentially the classical diffusion model. Then the effect of reorientation and aggregation of adsorbed molecules will be discussed by demonstrating experimental dynamic surface tension data. The adsorption dynamics of ionic surfactants has not been studied systematically so that these systems cannot be presented here extensively. Also the dynamics of adsorption at the interface between two liquids is at the beginning and we present here an impressive example. 

As the theoretical models are comparatively complex, only numerical methods allow to interpret experimental data. A software package is available that allows to make model calculations for any type of the above discussed diffusion-controlled mechanism [223]. In addition to the theory for a Langmuir isotherm, where the collocation solution by Ziller and Miller can serve as analytical solution, the programme gives access also to calculations based on the Frumkin, the reorientation and aggregation isotherms. 

For several non-ionic surfactants Fainerman and Miller [46] published dynamic surface tension data which can be interpreted in terms of a diffusion controlled adsorption mechanism easily. The experimental results obtained for different Triton solutions (octylphenyl poly -oxyethylene ether (C,4Hj 0(C2H40) H) with different numbers n of ethylene oxide groups Triton X-100 (n=10), Triton X-114 (n=11.4), Triton X-165 (n=16.5), Triton X-305 (n=30.5) and Triton X- 

---