Mechanical water interface, surfactant adsorption

When dealing with a foam, gas—liquid interfaces will be present in addition to solid—liquid and liquid—liquid interfaces. Surfactant adsorption at the gas—liquid interface is obviously required for foam formation and therefore cannot be considered a mechanism of surfactant loss. Because gas is always the nonwetting fluid, the presence of a gas phase is not expected to affect contact between the solid and the aqueous phase and is not likely to affect adsorption of a water-soluble surfactant at the solid—liquid interface. Limited data comparing surfactant adsorption from a foam with adsorption from a bulk liquid during flow through a sand pack have indicated that this is, indeed, the case (34). If surfactant adsorption at the gas—liquid interface were to affect adsorption at the solid—liquid interface, the effect would likely be a reduction in adsorption on the solid because of a reduced surfactant concentration in the bulk aqueous phase. 

Pagac, E. S., Prieve, D. C. and Tilton, R. D., Kinetics and mechanism of cationic surfactant adsorption and coadsorption with cationic polyelectrolyte at the silica-water interface, Langmuir, 14, 2333-2342 (1998). 

The mechanism of adsorption of fatty acids and other surfactants at the oxide-water interface. J. Colloid Interface Sci. 44 407— 414... 

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

The decrease in interfacial tension is related to the amount of extractant adsorbed at the interface through the Gibb s adsorption equation (46). The molecular areas of the extractant at the interface can thus be directly obtained from this equation. As an example, an area of 104 8 A2 is obtained for the. V,.V -dimc(hyl dibu-tyltetradecylmalonamide (DMDBTDMA) at the dodecane/water interface (4, 34). For classical surfactants, it should be noted that a nearly constant area per molecule with the addition of salt strongly suggests that anions and cations are adsorbed and extracted as pairs (47). Thus, the variation of the area per molecule with added salt can provide information on the mechanism of extraction. 

Statistic mechanics were used by Kiefer and Wilson104 to calculate adsorption isotherms of ionic surfactants on charged solid-water interface. The effect of coulombic repulsions between the ionic heads of the surfactant species are considered, as well as the van der Waals attractions of their hydrocarbon tails. Using the method of Fowler and Guggenheim93 they obtained the equation for an adsorption isotherm ... 

In this Section instability of asymmetric films is explained by decrease in the surfactant adsorption. Another reason for this instability can be the presence of solid particles at the water-oil interface. Such a heterogeneous defoaming is created when a foam is broken down by the antifoam drops that contain solid hydrophobic particles. The mechanism of action of such types of antifoams will be discussed in Section 9.4. 

The intent of this chapter is to present a brief review of simple, fundamental physicochemical principles and experimental results which are necessary to understand both the mechanism of adsorption of ionic surfactants from aqueous solutions on oxide surfaces and the action of some simple, fundamental applications. It does not enter into details in the theoretical consideration, nor does it attempt to explain complex industrial uses. Both problems have been thoroughly treated in several review articles and monographs [e.g., 1-10]. Here emphasis is placed on the contribution the adsorption calorimetry makes to the improvement of current understanding of the interactions of ionic surfactants at the mineral-water interface. All experimental data, used for the illustrative purposes throughout this chapter, were obtained at the Laboratoire des Agregats Moleculaire et Materiaux Inorganiques. 

With emulsions, nanoemulsions and microemulsions, the surfactant adsorbs at the oil/water (O/W) interface, with the hydrophilic head group immersed in the aqueous phase and leaving the hydrocarbon chain in the oil phase. Again, the mechanism of stabilisation of emulsions, nanoemulsions and microemulsions depends on the adsorption and orientation of the surfactant molecules at the Uquid/liquid (L/L) interface. Surfactants consist of a small number of units and are mostly reversibly adsorbed, which in turn allows some thermodynamic treatments to be applied. In this case, it is possible to describe adsorption in terms of various interaction parameters such as chain/surface, chain solvent and surface solvent. Moreover, the configuration of the surfactant molecule can be simply described in terms of these possible interactions. 

For the solubilization of highly insoluble hydrocarbons by POE nonionics into water, the rate of solubilization has been found (Carroll, 1981, 1982) to be directly proportional to the surfactant concentration above the CMC, and to increase with the polarity and decrease with the molecular weight of the oil. The rate is also strongly temperature dependent in the region of the cloud point (Section IIIB below), increasing rapidly as that temperature is approached. The mechanism suggested involves diffusion of the micelles to the hydrocarbon-water interface, where they dissociate and adsorb as monomers. This adsorption produces concerted desorption from the interface of an equivalent amount of monomeric surfactant, but in the form of micelles containing a quantity of solubilizate. 

Mechanisms involved in the demulsification by surfactants of petroleum W/O emulsions include adsorption of the surfactant at the oil-water interface and reduction of the interfacial tension, change in the nature of the interfacial film from a highly hydrophobic one to a less hydrophobic one (and, consequently, one more wettable by water), reduction of the viscosity of the interfacial film by penetration into it of the surfactant, and displacement of the original W/O emulsion stabilizers, particularly the asphaltenes, from the interface into the oil phase. 

These results then provide the foundation for a coulombic model of particle flotation, one in which a charged solid particle is attracted to an air-water interface oppositely charged by the presence of ionic surfactant. These results atid the techniques of statistical mechanics are employed to calculate the adsorption isotherms of perticles on air-water interfaces within the framework of this model. 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

Most foam-forming surfactants, particularly those suitable for high-salinity conditions, are very hydrophilic and do not partition into oil, eliminating the first of the listed mechanisms. The amount of surfactant adsorbed at the oil—water interface depends on the surface excess of surfactant at this interface and on the amount of interface present. Although the surface excess of surfactant at the oil—water interface can be estimated from interfacial tension data using the Gibbs adsorption equation, the amount of interface that is present is not easily accessible to measurement. 

As mentioned above, the adsorption kinetics for a kinetic-controlled mechanism is given by the balance of surfactant adsorption and desorption fluxes to and from the interface and for the Langmuir kinetics this balance has the form of Eq. (4.15). The rate constants kad and kdes are functions of the activation energies adsorption and desorption and can be specified on the basis of the molecular kinetic [9, 120] or transition state theory [121]. Eq. (4.15) was applied to adsorption kinetics data of surfactants at the water/air interface by many authors, for example in [24, 39, 83, 97, 122, 123, 124, 125, 126, 127]. In these works, it was shown that the values of kad and kdes are not constant hut depend on the surfactant bulk, the degree of adsorption layer saturation, or its lifetime. To obtain better correspondence with the experimental data, some authors had assumed that the adsorption and desorption activation energies depend on the degree of adsorption layer saturation. These rather complicated kinetic equation are more or less empirical, although they transforms into a valid adsorption isotherm at equilibrium... 

Eastoe J, Dalton JS (2000) Dynamic surface tension and adsorption mechanisms of surfactants at the air-water interface. Adv Colloid Interf Sci 85 103-144... 

The equilibrium and dynamic aspects of surface tension and adsorption of surfactants at the air-water interface are important factors in foam film stability [82]. Dynamic adsorption models with the diffusion-controlled and mixed-kinetic mechanisms are discussed in some surfactant solution litera-... 

In (8.51) and (8.52), the value of (T -iCIK n + Q) represents the equilibrium value of adsorption at the air-water interface according to Langmuir model. And the component represented in the curved brackets is the correction factor, which takes into account the siu factant adsorption reducing in time T, as compared to its equilibrium value. The phenomenon is arisen from the real time of surfactant molecule diffusion from subphase to the interface under the influence of the concentration gradient. The difference between these values is the greater, the smaller the diffusion coefficient of the surfactant molecules in the aqueous mediiun. The mechanism of this phenomenon is described in Appendices 8.A and 8.B. 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

The first mechanism is due to interfacial turbulence, which may occur as a result of mass transfer. In many cases the interface shows unsteady motions streams of one phase are ejected and penetrate into the second phase, shredding small droplets (Figure 14.1). Localised reductions in interfacial tension are caused by the non-uniform adsorption of the surfactant at the oil/water interface [14] or by mass transfer of surfactant molecules across the interface [15, 16]. With two phases that are not in chemical equilibrium, convection currents may form, conveying liquid rich in surfactants towards areas of liquid deficient of surfactant [17, 18]. These convection currents may give rise to local fluctuations in interfacial tension, causing oscillation of the interface. Such disturbances may amplify themselves, leading to violent interfadai perturbations and eventual disintegration of the interface, when liquid droplets of one phase are thrown into the other [19]. 

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