Interfacial dilational modulus

The complex interfacial dilational modulus ( ) is a key fundamental property governing foam and emulsion stability. It is defined as the interfacial tension increment (da) per unit fractional interfacial area change (dA/A) i.e.,... 

For effective demulsification of a water-in-oil emulsion, both shear viscosity as well as dynamic tension gradient of the water-oil interface have to be lowered. The interfacial dilational modulus data indicate that the interfacial relaxation process occurs faster with an effective demulsifier. The electron spin resonance with labeled demulsifiers suggests that demulsifiers form clusters in the bulk oil. The unclustering and rearrangement of the demulsifier at the interface may affect the interfacial relaxation process. 

In this table we largely follow the lUPAC recommendations (sec. 3.10a), but complete adherence is not feasible because of interference with symbols already used elsewhere In FIGS. For instance, we prefer for the interfacial dilational modulus over the lUPAC recommendation f. 

The first is that under equilibrium conditions the dilational modulus K° can be related to the interfacial excess Helmholtz energy F° through... 

In Figure 14.3c a normalization in a single master curve of E vs. tt data reflects the interfacial behavior of emulsifier adsorbed films for different emulsifier concentrations, at different adsorption times, and under different processing conditions (Nino and Patino, 2002 Nino et al., 2003). The plot suggests that interactions between adsorbed emulsifier molecules (residues) increase with tt. In fact, at lower tt values the slope of the -tt plot was close to 1, which corresponds to the behavior of an ideal gas with low emulsifier interactions. However, at higher tt values the slope changes, which implies an important nonideal behavior with higher molecular interactions as the amount of emulsifier at the interface increases. These data indicate that the interfacial activity and the surface dilatational modulus of emulsifier films are mainly a result of the amount of adsorbed emulsifier. 

Interfacial tension gradients are very important in stabilising the thin liquid film that is located between the droplets and which is very important at the start of emulsification (films of the continuous phase may be drawn through the disperse phase and collision is very large). The magnitude of the y-gradients and of the Marangoni effect depends on the surface dilational modulus s, which for a plane... 

Erik s research focused on the interfacial properties of the ocean surface, and, in particular, how the chemistry of the air-sea interface affects the dynamics of short waves, nearsurface flows and interfacial fluxes of heat, mass and momentum. During his short career, he contributed to over 30 scientific publications in this area. His doctoral research, carried out under the tutelage of well-known colloid and surface chemist, Sydney Ross, concerned the propagating characteristics of surface waves in the presence of adsorbed films. That work was eventually published as a series of seminal papers on capillary ripples, and his theoretical treatment of ripple propagation and a corrected dispersion relation for surface waves in the presence of a surface dilational modulus (with J. Adin Mann, Jr.) still stand as the definitive word on the subject. 

Presence of surfactant at the interface will also directly affect deformation. The surfactant allows formation of a /-gradient. This would affect the deformation mode of a drop, which has indeed been observed. Moreover, enlarging the interfacial area causes y to increase, as mentioned. This implies that the interfacial free energy increase includes two terms y dA + A dy. The first term is due to the deformation of the drop being counteracted by its Laplace pressure the second is due to surface enlargement being counteracted by the surface dilational modulus ESD. Making use of Eq. (10.20) for we obtain... 

The formation of an interfacial layer consisting of surface-active material present in erude oil (asphaltenes and resins) produces a physical barrier for droplet—droplet coalescence. Numerous researehers have noted the presence of an interfacial skin in oil-water systems with these surface-active components present (3,5,9,19-26). Mohammed et al. (21), using a Langmuir film balance, found the inter-faeial dilatational modulus to be dependent on the presence of asphaltenes and resins. Fordedal et al. (20) have shown... 

One area where SQELS can make a significant contribution is that of polymers at liquid/liquid interfaces. Since the viscosity and density of the two fluids are much closer the coupling between dilation and capillary waves is severely attenuated and the prospects for obtaining dilational modulus and viscosity are much reduced or nonexistent. Oil/water interfaces are particularly interesting when block copolymers are placed at the interface. Only one such system has so far been discussed a block copolymer of styrene and ethylene oxide was placed at the interface between heptane and water (Sauer et al. 1987). There appears to be some evidence of a maximum in the damping of the interfacial capillary waves as the concentration of copolymer was increased. 

Iranian Heavy crude oil/distilled water interfaces were aged for set intervals on the dilatational modulus apparatus. The interfacial length was initially set as 13.5 cm and amplitudes of area oscillation of 0.5 cm were used. The Wilhelmy plate (platinum, hydrophobic) was always aligned parallel to the oscillating barrier. The X-Y recorder was used to produce Lissajou figures of interfacial tension-interfacial Wilhelmy... 

The dynamic interfacial tension can be related to the fractional change in interfacial area by the dilatational modulus (e), which, in the absence of relaxation processes, is simply given by (23) ... 

The main factors, which determine the foam formation ability and physical stability of the foams in mixed Lys-MR solutions, are surface activity and complex dilatation modulus. With increase of MR concentration in certain range a surface activity and complex dilatation modulus of interfacial layers increased and phase angle decreased. It means that viscoelasticity of interfacial layers became higher. In these conditions the foam volume and mutiplicity as well as stability of foams were growing. This effect may be used for creation of mixed protein-MR system foam type for pharmaceutical applications with improved physical stability and wide range of antibacterial actions. 

The tangential bulk-phase stress component evaluated at the interface combines an elastic (interfacial tension gradient) effect, e, and an apparent viscous effect, rxj + tIj) -I- e 7o). One of the most convenient methods of measuring capillary waves is to use light scattering (29), which can yield information on both the tension and dilational modulus of the interface. 

Moreover, this model can also involve parameters that refer to the interfacial rheological properties, such as the interfacial dilation modulus and the interfacial shear modulus (Jacobs et al. 1999). Consequently, quantitative measurements of... 

The value of y obtained depends on the nature of the oil and surfactant used -small molecules such as nonionic surfactants lower y more than do polymeric surfactants such as PVA. Another important role of the surfactant is its effect on the interfacial dilational modulus s (Eq. 6.19) [15]. 

Five general main roles may be identified for the emulsifier. The first and most obvious is to lower y, as mentioned above. This has a direct effect on droplet size generally, the lower the interfacial tension, the smaller the droplet size. This is the case when viscous forces are predominant, whereby the droplet diameter is proportional to y. When turbulence prevails, dccy. When emulsification continues, and an equilibrium is set up between the amount adsorbed and the concentration in the continuous phase, C, the effective y depends on the surface dilational modulus, , which is given by... 

Another important role of the surfactant is its effect on the interfacial dilational modulus e [88],... 

In summary, the interfacial tension, the interfaeial dilatational modulus, and the drainage of thin liquid films are fundamental principles for nnderstanding foams and emulsions (Figure 10.1). The correlation between these magnitudes and the behavior of the resulting colloidal dispersion provides a technological application to fundamental interfaeial seience. 

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