Thermodynamics, adsorption interfacial layer properties

Thus, the important features of the structural-mechanical barrier are the rheological properties (See Chapter IX,1,3) of interfacial layers responsible for thermodynamic (elastic) and hydrodynamic (increased viscosity) effects during stabilization. The elasticity of interfacial layers is determined by forces of different nature. For dense adsorption layers this may indeed be the true elasticity typical for the solid phase and stipulated by high resistance of surfactant molecules towards deformation due to changes in interatomic distances and angles in hydrocarbon chains. In unsaturated (diffuse) layers such forces may be of an entropic nature, i.e., they may originate from the decrease in the number of possible conformations of macromolecules in the zone of contact or may be caused by an increase in osmotic pressure in this zone due to the overlap between adsorption layers (i.e., caused by a decrease in the concentration of dispersion medium in the zone of contact). 

In addition to the thermodynamic characteristics of the adsorption equilibrium the dynamic dilational visco-elasticity of the surfactant interfacial layers is a very important quantity [5]. This Ireqnency dependent property of a liquid interface is a significant quantity in the stabilization of foams and emulsions. Of course, in many practical situations mixtures of surfactants with particles, polymers or proteins are used, however, these rather complex systems are not the subject of this work. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

In the linear case, non-equilibrium properties of adsorption layers at fluid interfaces can be quantitatively described by the interfacial thermodynamic modulus (Defay, Prigogine Sanfeld 1977),... 

This work is devoted to the study of thermodynamic and rheological properties of interfacial adsorption layers of gelatin and natural surfactants (lecithin) in a wide range of mixing ratios formed at interfaces between water and hydrocarbons. 

The results of investigation of the interfacial properties (thermodynamic and rheological) of the aqueous gelatin/ lecithin mixtures in the wide range of component ratios are presented in this work for the first time. It has been shown that adsorption of gelatin/lecithin complexes (formed in the bulk aqueous phase) at the immiscible liquid interface leads to self-assembly of the interfacial viscoelastic layer. The non-monotonic time-dependent interfacial shear viscosity and elastic modulus evolution were observed. This effect was explained by the phase transitions proceeding in time at the liquid interface in the systems containing lecithin. 

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