Dynamic surface tension of complex fluid-fluid interfaces: A useful concept, or not?

Abstract

Dilatational moduli are typically determined by subjecting interfaces to oscillatory area deformations, and are often defined in terms of the difference between the dynamic or transient surface tension of the interface (the surface tension in its deformed state), and the surface tension of the interface in its non-deformed state. Here we will discuss the usefulness of the dynamic surface tension concept in the characterization of dilatational properties of complex fluid-fluid interfaces. Complex fluid-fluid interfaces are interfaces stabilized by components which form mesophases (two-dimensionional gels, glasses, or (liquid) crystalline phases), as a result of in-plane interactions between the components. We will show that for such interfaces dilatational properties are not exclusively determined by the exchange of surface active components between interface and adjoining bulk phases, but also by in-plane viscoelastic stresses. The separation of these contributions remains a challenging problem which remains to be solved.

A Comprehensive Approach from Interfacial to Bulk Properties of Legume Protein-Stabilized Emulsions

“…These results were related to unfolded proteins found at this pH value, allowing better interaction among different protein chains [28] and leading to stronger interfacial films at low pH value due to higher protein-protein interactions (related to hydrophobic forces) [29]. However, some precautions should be taken upon analysis of these results since it is recognized that the apparent dilatational moduli obtained in complex fluid-fluid interfaces may be affected by bending rigidity, as well as by the changes in the surface area which take place during dilation and contraction [30]. Figure 2A shows the protein adsorption rheokinetics of the O/W interface at two different pH values (2.5 and 7.5) for the two protein systems studied (FB and CP) obtained by means of interfacial shear measurement.…”

Fluid-Fluid Interfaces of Multi-Component Mixtures in Local Equilibrium

“…There are n components. We use the symbol c exc j to denote the excess number of moles per surface area [15][16][17]. The symbol Γ j has been used for the same property.…”

“…From molecular dynamics simulations or with density functional methods, we can obtain density-and other profiles through the surface when it is exposed to gradients. Such profiles have been written as [15][16][17]:…”

“…The validity of local equilibrium or this autonomy must of course be investigated. This has been done using several tools; density functional theory (the square gradient model) for one-and two-component fluids [15,17] and molecular dynamics simulations [18][19][20][21][22][23], cf., the discussion below. It remains a challenge to verify the assumption by experiments.…”

Protein transfer to membranes upon shape deformation

“…Here, for the sake of simplicity, we have used the concept of a dynamic (or transient) surface tension to describe the mechanism of deformation induced adsorption. Elsewhere in this issue we will discuss alternatives for this concept, which for elastic membranes may prove to be more suitable to describe the state of deformation of the membrane [22].…”