Complex Interfaces and their Role in Protein‐Stabilized Soft Materials

Abstract

Proteins adsorbed at fluid interfaces are pivotal for the stability and flow behavior of foams and emulsions, [1][2][3] the mechanics of cell membranes, [4] and interfacial enyzme catalysis.[5] Besides a reduction in the interfacial tension, protein aggregation at the phase boundary leads to distinctive local mechanics of the interface. [1,[6][7][8] The contribution of such interfacial viscoelasticity to the macroscopic stability and flow behavior of high internal interface materials is still poorly understood. We study the interfacial mechanics of adsorption layers formed by the globular protein lysozyme and relate them to the morphology of emulsions stabilized by the same protein. A scaling analysis accounting for structural anisotropy, interfacial tension and the drop size reveals that emulsion drops stabilized by an adsorption layer of globular proteins can be seen as capsules surrounded by a soft shell. Besides its relevance for the rheology and stability of emulsions and foams, this insight is of fundamental importance for the design and controlled self-assembly of protein-based delivery systems, such as nanocapsules or colloidosomes. [9] Emulsions, immiscible polymer blends, and phase-separated biopolymer mixtures develop flow-induced morphologies if the stresses due to the applied flow field can overcome the capillary forces that favor a spherical drop morphology at rest. [10][11][12] Drops are subjected to deformation, break-up, and coalescence, all of which are associated with characteristic light scattering patterns.[10] Flow-induced shape anisotropy of emulsion drops in the mm size range can be studied by rheo small-angle light scattering (Rheo-SALS).[2, 12, 10, 13] Such measurements have been achieved for blends of immiscible polymers, where absolute viscosities are very high, [12] and in phasesegregated biopolymer mixtures, where interfacial tensions between two different aqueous phases are very low. [13] In both cases, the delicate balance of interfacial to hydrodynamic stresses is in a range in which drop sizes, time scales and shear stresses are experimentally accessible to mechanical and optical measurements. The hydrodynamic and surface chemical parameters of the system can be combined into a dimensionless quantity, the capillary number Ca = tR=s, which is the ratio of hydrodynamic (t) to interfacial stresses (s/R), where s is the static interfacial tension and R the radius of the undeformed drop. Studies on emulsion stability and flow often focus on the interfacial tension (either static, transient, or dynamic) as the sole physical property of the phase boundary.[2] It is, however, well-known that surface-active molecules impart a variety of dynamic properties upon the interface. [1,3] Therefore, the interfacial tension, which in its common use is merely a two-dimensional analog of the hydrostatic pressure in a liquid, must be generalized to include compressional and shear stresses-it becomes a tensorial quantity that is a function of both the extent and the rate of deformation of the interfac...

Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins

“…b-lactoglobulin adsorption layers restrict the ease of deformation, rather than facilitating it, in spite of the adsorbed protein reducing the interfacial tension as compared to the clean interface. [55,64,65] Notice that in the absence of interfacial viscoelasticity at the given viscosity ratio, the experiments collapse onto one line with a slope of approximately one (in agreement with Taylor's small deformation theory), whereas the values found for the protein-covered interfaces are much lower. Since all interfacial rheological experiments presented above for globular protein films show a strong time-dependence, an obvious question for protein-covered drops is how the interface age influences the small-deformation behavior.…”

“…Rheo-small angle light scattering (SALS) patterns are obtained in the same rheometer with a custom-built modified shear cell for laser light scattering set up in a rotating parallel plates configuration, with the laser passing through the sample perpendicular to the plates in the shear gradient direction. [64,65] …”

“…Even at the shortest interface ages, adsorbed b-lactoglobulin films are shearelastic (G 0 > G 00 ), indicating that in the concentration range studied here the interface is already densely packed with the globular protein. [5,15,55,64] Figure 4b shows the loss tangent tan(d) ¼ G 00 /G 0 at a test frequency of 0.755 rad Á s À1 at different interface ages; the viscous modulus decreases from about 0.5 to 0.18 G 0 within 8 h. Along with the evolution of the interfacial storage modulus G 0 (v) toward weaker scaling with the deformation frequency, this implies a reduction in the mobility of the globular protein layer. [9,10,12] In contrast to the globular protein, b-casein is a flexible random coil with only little secondary structure.…”

“…The innermost ring of the patterns is from the beam stop. [64,65] A schematic of the setup used is shown on the right hand side (a: laser, b: pinhole and prisms, c: sample, d: parallel disks (top disk rotates), e: semi-translucent screen, f: beam stop, g: camera).…”

Rheology of food materials

“…The adsorption layer can exhibit viscoelastic, elastic or even rigid solidlike rheological response function under lateral shear and dilatational stresses. In case of emulsions droplets, the deformation and breakup behavior of protein-covered emulsion drops is influenced by the rheological properties of the adsorption layer, which prevents coalescence and rupture of the droplet of foam bubble [103][104][105][106]. Further focus areas in interfacial rheology for food-related systems are: surface interactions of small molecular weight surfactants with proteins or other polyelectrolytes [107][108][109][110], 'fluidization' of protein layers by competitive adsorption with surfactants [111,112], and chemical or enzymatic interfacial cross linking of proteins [113][114][115].…”