Surface viscosity adsorption

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick walls (higher liquid-to-gas ratio), (4) reduce liquid surface tension, (5) increase surface elasticity, (6) increase surface concentration, (7) reduce surfactant-adsorption rate, (8) prevent liquid evaporation, (9) avoid mechanical stresses, and (10) eliminate foam inhibitors. Obviously, the reverse of each of these actions, when possible, is a way to control and break foam. 

High surface viscosity and/or mechanically strong interfacial film - this acts as a barrier to coalescence and may be enhanced by adsorption of fine solids, or of dose-packed surfactant molecules. 

Alcohols also promote wettability and penetration of the wood surface. This may easily be shown by the following simple experiment. When equal sized drops of distilled water were placed on the surface of a freshly planed piece of southern yellow pine, the times for the drops to completely soak into the wood were observed. On the early wood it took 65 seconds and on the latewood 179 seconds. When similar drops of 50% ethanol solution were used instead of pure water, it required only six seconds to disappear into the earlywood and 26 seconds into the latewood. However, if a small drop of adhesive syrup, with no hardener added, was placed on the wood surface, no adsorption took place at all. It was surmised that the viscosity prevented its permeation. When the adhesive was diluted with 50% alcohol it was readily absorbed and produced a red stained spot on either earlywood or latewood areas. This showed that the low molecular weight adhesive molecules could readily permeate the wood structure before condensation with the curing agent. 

In conclusion we will note that the main difference between aqueous emulsion films and foam films involves the dependences of the various parameters of these films (potential of the diffuse double electric layer, surfactant adsorption, surface viscosity, etc.) on the polarity of the organic phase, the distribution of the emulsifier between water and organic phase and the relatively low, as compared to the water/air interface, interfacial tension. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

In the case of liquid adsorption layers, the deformation by shear occurs at any small load (Fig. 11-29, line 2), and the shear rate, e, is proportional to the applied stress, xs (the torque angle of the thread). The latter allows one to estimate the surface viscosity of the adsorption layer, which is strongly dependent on the nature of surfactants. The adsorption layers may also reveal more complex rheological behavior, intermediate between that of liquids and solids (Fig. 11-29, curve 3). 

On the basis of a differential equation Ivanov (1977) described all stages of thin liquid film evolution. He distinguished the effects of Marangoni-Gibbs and of surface viscosity. Additionally, the substantial effects of surface diffusion and slow adsorption (barrier or kinetic controlled mechanisms) are taken into consideration. A selection of basic equations can be find in Chapter 4. 

The now classic experiments of Graham and Phillips showed (5) inter alia that the surface rheology of pure protein films is very dependent on the type and amount of protein adsorbed, as well as on the conditions of adsorption. In this paper, with films formed from mixed protein solutions, we shall show that the surface viscosity is an extremely sensitive probe of the time-dependent structural and compositional changes taking place during competitive adsorption at the oil—water interface. While steady-state tensions can invariably... 

We studied the adsorption of dilute acid- and alkali precursor gelatins on pyrex and stainless steel surfaces. The adsorptions founds treated as points of Langmuir isotherms, became maximal around the lEP, close to the positions of the minima of the viscosities. We determined also the thicknesses, Ar, of the adsorbed layers by a viscosity method allowing calculation of the volume of the interphases. Comparing molecular concentrations and dimensions with those of the free molecules, we obtained information on the relative compression of the adsorbate. The adsorption remains substantial on the acid side of the lEP, so that the maximal segmental densities occur to the left of the lEP and of the pH of the maximal layer thicknesses. The data indicate also that the state of expansion of the adsorbed molecules, more than the amounts adsorbed or the segmental densities, determine the layer thicknesses. 

Spreading agents, adsorption of surfactant molecules, decrease of surface viscosity... 

With respect to the rheological parameters fliey come to the conclusion that surface elasticity effects are superior to surface viscosity effects. This, however, apphes to pure surfactant layers and may be different for pure protein or mixed surfactant/protein adsorption layers. It has been stressed also by Langevin (26), in her review on foams and emulsions, fliat studies on the dynamics of adsorption and dilational rheology studies for mixed systems, in particular surfactant-polymer systems, are desirable in order to understand these most common stabilizing systems. 

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