Adsorption rheological properties

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

The interfacial films formed by different crude oils have different characteristics. The physical characteristics of the films are a function of the crude-oil type and gas content, the composition and pH of water, the temperature, the presence of nonionic polar molecules in the water, the extent to which the adsorbed film is compressed, and the contact time allowed for adsorption and concentration of polar molecules in the oil phase 14, 22,23). The rheological properties of the adsorbed emulsifier film have an important effect on the stability of emulsions. 

The calculations for silica primary particles with a diameter of 12 nm reveal at least qualitatively that the attractive interaction potential att for silica dispersed in styrene is stronger by a factor of ca. 4 than that for silica dispersions in toluene. This result demonstrates that the common concept of explaining the rheological properties of hydrophilic silica dispersions by means of interparticulate H-bonds is questionable. In silica-resin mixtures also, adsorption of resin oligomers due to the high surface energy of the silica particles has to be taken into account [8]. Adsorption of polymers/oligomers will sterically hamper the formation of interparticulate H-bonds. 

T/ LE 3.7 Effects of Surface Charging and Adsorption on Viscosity and Other Rheological Properties... 

In fact, Equation 5.281 describes an interface as a two-dimensional Newtonian fluid. On the other hand, a number of non-Newtonian interfacial rheological models have been described in the literature. Tambe and Sharma modeled the hydrodynamics of thin liquid films bounded by viscoelastic interfaces, which obey a generalized Maxwell model for the interfacial stress tensor. These authors also presented a constitutive equation to describe the rheological properties of fluid interfaces containing colloidal particles. A new constitutive equation for the total stress was proposed by Horozov et al. ° and Danov et al. who applied a local approach to the interfacial dilatation of adsorption layers. 

In contrast with two-phase bubble-containing fluids, aerosols, and emulsions, foam has a least three phases. Along with gas and the free continuous liquid phase, foam contains the so-called skeleton phase, which includes adsorption layers of surfactants and the liquid between these layers inside the capsule envelope. The volume fraction of the skeleton phase is extremely small even compared with the volume fraction of the free liquid. Nevertheless, this phase determines the foam individuality and its structure and rheological properties. It is the frame of reference with respect to which the diffusion motion of gas and the hydrodynamic motion of the free liquid can occur under the action of external forces and internal inhomogeneities. At the same time, the elements of the skeleton phase themselves can undergo strain and relative displacements as well as mass exchange with the other phases (solvent evaporation and condensation and surfactant adsorption and desorption). 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

It was shown by Izmailova et al that the rheological properties of adsorption layers formed with high molecular weight surfactants and biopolymers play an important role in ensuring the stability of disperse systems stabilized by such layers (see Chapters VI-VIII). 

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). 

The rate of thinning (drainage) of liquid films is drastically influenced by the rheological properties of the related adsorption layer. We will restrict ourselves to just a few examples. A detailed description of various sites of thin film problems is given for example by Ivanov (1988) and Hunter (1993). The immobilisation of a cylindrical plane film is a precondition for... 

Ismailova, V.N., "Stmcture Formation and Rheological Properties of Proteins and Surface-Active Polymers of Interfacial Adsorption Layers" in Progress in Surface and Membrane Science 13(1979)... 

Moreover, adsorption isotherms, or equations of state, represent the basis for the evaluation of adsorption kinetics and rheological properties of adsorption layers. Exact equilibrium values of surface or interfacial tensions are necessary to determine adsorption isotherms. For surfactants of low surface activity (for example, sodium octyl or decyl sulphate, hexanol or hexanoic acid) the adsorption reaches its equilibrium state in a time of the order of seconds to minutes. Higher surface activity results in greater times for establishing the equilibrium state of adsorption which sometimes cannot be realised by available experimental methods. To avoid long-time experiments, extrapolations were often carried out in order to get equilibrium values. Different extrapolation procedures as well as criteria of an equilibrium state of adsorption are discussed in the literature (cf Miller Lunkenheimer 1983). 

Foam Stability at Low Surfactant Concentrations Role of Surface Rheological Properties. At surfactant concentrations below or around the CMC, the adsorption of the surface-active molecules on the film surfaces and the properties of the adsorbed layers control the drainage and stability of the film and the foam. 

The presence of copolymer and surfactant together alters the rheological properties of solutions, adsorption on solid particles, solubility, and stability of colloidal dispersions. The solution properties are mainly influenced by... 

R Miller, J Kragel, AV Makievski, R Wiistneck, JB Li, VB Fainerman, AW Nemnann. Proteins at hquid/liquid interfaces—adsorption and rheological properties. Proceedings of the Second World Emulsion Congress, Bordeaux, 1997, Vol 4, pp 153—163. 

The results of flic interfacial rheological studies on asphaltene adsorption at oil-water interfaces teach us a great deal about the behavior of asphaltenes and their role in emulsion stabili2ation. The conclusions drawn are based largely on the assumption that the rheological properties measured, namely flic elastic film modulus G are directly related to the surface excess concentration of asphaltenes. F. It is understood diat die elastic modulus actually depends on both the surface excess concenlration and the relative conformation (i.e., coimectivity) of the adsorbed asphaltenes. It is further understood that a minimum adsorbed level is required to observe a finite value of G and that the relationship between G and G is not linear. With these caveats in mind, we can conclude die following ... 

As it has been mentioned, the hydration of CjA has a decisive impact on the rheological properties of fresh paste. The high rate of reaction with water leads to the saturation of solution with aluminate and calcium ions and as a consequence to the crystallization of C AHj. This corresponds to the quick stiffening of paste, determined as flash set. All the substances modifying the rate of CjA reaction with water by adsorption on the surface of this phase or by the change of the ions concentration in the liquid phase will have a great impact on the rheological properties of paste. 

The above discussion is limited to the flow of inelastic fluids in unconsolidated beds of particles where the pore size is substantially larger than the characteristic dimensions of the polymer molecules. Interaction effects between the walls of the pore and the polymer molecules are then small. Thus, measuring the relationship between pressure drop and flow rate in a packed bed and in a tube would therefore lead to the prediction of the same rheological properties of the fluid. Visco-elastic effects and other phenomena including blockage of pores, polymer adsorption/retention, etc. observed in beds of low permeability or in consolidated systems will be briefly discussed in Section 5.6.7. 

The aim of this work is to study the effect of MR on the surface activity of lysozyme and rheological properties of its adsorption layers at the air/ solution (0.05 M phosphate buffer, pH 6.0) interface. 

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