Aging process, interfacial dynamics

As the second topic, we have discussed an anomalous phenomenon observed for the aging dynamics in single ultrathin polymer films. From this study, it has been elucidated that the anomalous increase in dielectric susceptibility observed for isothermal aging processes is strongly associated with the mobile region that exists at the surface and interfacial region of thin polymer films. 

Surface active substances are able to modify significantly the properties of interfaces by adsorption. This fact is used in many processes and many new technologies are based on adsorption effects. In general, these technologies work under dynamic conditions and improvement of their efficiency is possible by a controlled use of interfacially active material. The interfaces involved are freshly formed and have only a small effective age of some seconds or sometimes even less than a millisecond. 

In any case, the formation of an emulsion is a dynamic process and hence, instead of the static interfacial tensions, the corresponding dynamic values are much more meaningful. The dynamic surface tension is always higher than the static value. The difference can be as high as 40 mN m and depends, in addition to the age of the interface, mainly on the kind of the surfactants and on the solution viscosity (17, 18). 

Most spraying processes work under dynamic conditions and improvement of their efficiency requires the use of surfactants that lower the liquid surface tension yLv under these dynamic conditions. The interfaces involved (e.g. droplets formed in a spray or impacting on a surface) are freshly formed and have only a small effective age of some seconds or even less than a millisecond. The most frequently used parameter to characterize the dynamic properties of liquid adsorption layers is the dynamic surface tension (that is a time dependent quantity). Techniques should be available to measure yLv as a function of time (ranging firom a fraction of a millisecond to minutes and hours or days). To optimize the use of surfactants, polymers and mixtures of them specific knowledge of their dynamic adsorption behavior rather than equilibrium properties is of great interest [28]. It is, therefore, necessary to describe the dynamics of surfeictant adsorption at a fundamental level. The first physically sound model for adsorption kinetics was derived by Ward and Tordai [29]. It is based on the assumption that the time dependence of surface or interfacial tension, which is directly proportional to the surface excess F (moles m ), is caused by diffusion and transport of surfeictant molecules to the interface. This is referred to as the diffusion controlled adsorption kinetics model . This diffusion controlled model assumes transport by diffusion of the surface active molecules to be the rate controlled step. The so called kinetic controlled model is based on the transfer mechanism of molecules from solution to the adsorbed state and vice versa [28]. 

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