Adsorbed and spread beta-casein monolayers at oil-water interfaces

Interfacial mechanics of β-casein and albumin mixed protein assemblies at liquid-liquid interfaces

Protein emulsifiers play an important role in formulation science, from food product development to emerging applications in biotechnologies. The impact of mixed protein assemblies on surface composition and interfacial shear mechanics remains broadly unexplored, in comparison to the impact that formulation has on dilatational mechanics and surface tension or pressure. In this report, we use interfacial shear rheology to quantify the evolution of interfacial shear moduli as a function of composition in bovine serum albumin (BSA)/β-casein mixed assemblies. We present the pronounced difference in mechanics of these two protein, at oil interfaces, and observe the dominance of β-casein in regulating interfacial shear mechanics. This observation correlates well with the strong asymmetry of adsorption of these two proteins, characterised by fluorescence microscopy. Using neutron reflectometry and fluorescence recovery after photobleaching, we examine the architecture of corresponding protein assemblies and their surface diffusion, providing evidence for distinct morphologies, but surprisingly comparable diffusion profiles. Finally, we explore the impact of crosslinking and sequential protein adsorption on the interfacial shear mechanics of corresponding assemblies. Overall, this work indicates that, despite comparable surface densities, BSA and β-casein assemblies at liquid-liquid interfaces display almost 2 orders of magnitude difference in interfacial shear storage modulus and markedly different viscoelastic profiles. In addition, co-adsorption and sequential adsorption processes are found to further modulate interfacial shear mechanics. Beyond formulation science, the understanding of complex mixed protein assemblies and mechanics may have implications for the stability of emulsions and may underpin changes in the mechanical strength of corresponding interfaces, for example in tissue culture or in physiological conditions.

Experimental techniques to study protein-surfactant interactions: New insights into competitive adsorptions via drop subphase and interface exchange

Protein surfactant (PS) interactions is an essential topic for many fundamental and technological applications such as life science, nanobiotechnology processes, food industry, biodiesel production and drug delivery systems. Several experimental techniques and data analysis approaches have been developed to characterize PS interactions in bulk and at interfaces. However, to evaluate the mechanisms and the level of interactions quantitatively, e.g., PS ratio in complexes, their stability in bulk, and reversibility of their interfacial adsorption, new experimental techniques and protocols are still needed, especially with relevance for in-situ biological conditions. The available standard techniques can provide us with the basic understanding of interactions mainly under static conditions and far from physiological criteria. However, detailed measurements at complex interfaces can be formidable due to the sophisticated tools required to carefully probe nanometric phenomena at interfaces without disturbing the adsorbed layer. Tensiometry-based techniques such as drop profile analysis tensiometry (PAT) have been among the most powerful methods for characterizing protein's and surfactant's adsorption layers at interfaces via measuring equilibrium and dynamic interfacial tension and dilational rheology analysis. PAT provides us with insightful data such as kinetics and isotherms of adsorption and related surface activity parameters. However, the data analysis and interpretation can be challenging for mixed protein-surfactant solutions via standard PAT experimental protocols. The combination of a coaxial double capillary (micro flow exchange system) with drop profile analysis tensiometry (CDC-PAT) is a promising tool to provide valuable results under different competitive adsorption/desorption conditions via novel experimental protocols. CDC-PAT provides unique experimental protocols to exchange the droplet subphase in a continuous dynamic mode during the in-situ analysis of the corresponding interfacial adsorbed layer. The contribution of diffusion/convection mechanisms on the kinetics of the adsorption/desorption processes can also be investigated using CDC-PAT. Here, firstly, we review the commonly available techniques for characterizing protein-surfactant interactions in the bulk phase and at interfaces. Secondly, we give an overview for applications of the coaxial double capillary PAT setup for investigations of mixed protein-surfactant adsorbed layers and address recently developed protocols and analysis procedures. Exploring the competitive sequential adsorption of proteins and surfactants and the reversibility of pre-adsorbed layers via the subphase exchange are the particular experiments we can perform using CDC-PAT. Also the sequential and simultaneous competitive adsorption/desorption processes of some ionic and nonionic surfactants (SDS, CTAB, DTAB, and Triton) and proteins (bovine serum albumin (BSA), lysozyme, and lipase) using CDC-PAT are discussed. Last but not least, the fabrication of micro-nanocomposite layers and membranes are additional applications of CDC-PAT discussed in this work.

Microgels at interfaces, from mickering emulsions to flat interfaces and back

In this review, we cover the topic of p(NIPAM) based microgels at interfaces, revisiting classical studies in light of the newest ones. In particular, we focus on their use as emulsifiers in the so-called mickering emulsions, i.e. Pickering emulsion stabilized by soft particles. Given the complexity of the experimental characterization and simulation of these soft particles at interfaces, the review is structured in progressive complexity levels, until we reach the highly interesting and promising responsiveness to stimuli of mickering emulsions. We start from the lowest level of complexity, the current understanding of the behavior of single microgels confined at a flat interface. Then, we discuss their collective behavior upon crowding, their responsiveness at interfaces, and their macroscopic properties as microgel films. Once we have the necessary characterization tools, we proceed to discuss the complex and convoluted picture of responsive mickering emulsions. The way is rough, with current controversial and contradicting studies, but it holds promising results as well. We state open questions worth of being tackled by the Soft Matter community, and we conclude that it is worth the trouble of continuing after the master theory of microgel interfacial activity, as it will pave the way to widely adopt responsive mickering emulsions as the worthy Pickering emulsion successors.

Subphase exchange experiments with the pendant drop technique

INTRODUCTION

The development of the coaxial double capillary 15 years ago opened up the possibility to undertake accurate desorption and penetration studies of interfacial layers in the pendant drop technique. Drop and bubble methods offer several advantages with respect to other interfacial techniques. They allow a more stringent control of the environmental conditions, use smaller amounts of material and provide a much higher interface/volume ratio than in conventional Langmuir Troughs.

EXPERIMENTAL

The coaxial capillary was developed 15 years ago at the University of Granada as an accessory for the pendant drop surface film balance. It allows exchanging the subphase of the drop without disturbing the surface film and preserving the drop volume throughout the subphase exchange. Hence, this methodology enables one to carry out a great variety of interfacial studies well beyond the usual adsorption profiles. Penetration studies, sequential adsorption measurements, desorption kinetics, reversibility of adsorption and testing of enzymatic treatments on interfacial layers are amongst the principal applications. The coaxial capillary has been recently upgraded to a multi-exchange device which has boosted its applicability. It can be now used to address multilayer formation, create soft interfacial nano-composites such as membranes, polyelectrolyte assemblies and simulate in vitro digestion in a single droplet.

APPLICATIONS

This review aims to compile the experimental work done, using the pendant drop subphase exchange in the last decade, and how its use has provided new insights into the surface/interfacial properties of many different materials. Special emphasis is placed on recent work regarding simulation of in vitro digestion in order to address issues relating to metabolism degradation profiles. The use of this methodology when dealing with interfacial studies allows setting the foundations of interfacial engineering technology. Based on subphase exchange experiments, we aim to develop models for competitive adsorption of different compounds at the interface and build up layer-by-layer interfacial structures. Future challenges comprise the design of finely adjusted nanoengineering systems, based on multilayer assemblies with tailored functionalities, to match the application demand.

Friction force spectroscopy of β- and κ-casein monolayers

Friction force spectroscopy (FFS) has been applied to study the tribological properties of β- and κ-casein layers on hydrophobic substrates in aqueous solutions. Nanometer-sized imaging tips were employed. This allowed exerting and determining the high pressures needed to remove the layers and registering the topographic evolution during this process. Both β- and κ-casein layers showed similar and not particularly high initial frictional responses (friction coefficient of ∼1 when measured with a silicon nitride tip). The pressures needed to remove the layers were of the same order of magnitude for both proteins, ∼10(8) Pa, but slightly higher for those composed of β-casein. The technique has also shown to be useful in studying the two-dimensional lateral diffusion of the proteins and the wear on the layers they form.

From surfactant adsorption kinetics to asymmetric nanomembrane mechanics: pendant drop experiments with subphase exchange

Adsorption equilibrium is the state in which the chemical potential of each species in the interface and bulk is the same. Dynamic phenomena at fluid-fluid interfaces in the presence of surface active species are often probed by perturbing an interface or adjoining bulk phase from the equilibrium state. Many methods designed for studying kinetics at fluid-fluid interfaces focus on removing the system from equilibrium through dilation or compression of the interface. This modifies the surface excess concentration Γ(i) and allows the species distribution in the bulk C(i) to respond. There are only a few methods available for studying fluid-fluid interfaces which seek to control C(i) and allow the interface to respond with changes to Γ(i). Subphase exchange in pendant drops can be achieved by the injection and withdrawal of liquid into a drop at constant volumetric flow rate R(E) during which the interfacial area and drop volume V(D) are controlled to be approximately constant. This can be accomplished by forming a pendant drop at the tip of two coaxial capillary tubes. Although evolution of the subphase concentration C(i)(t) is dictated by extrinsic factors such as R(E) and V(D), complete subphase exchange can always be attained when a sufficient amount of liquid is used. This provides a means to tailor driving forces for adsorption and desorption in fluid-fluid systems and in some cases, fabricate interfacial materials of well-defined composition templated at these interfaces. The coaxial capillary pendant drop (CCPD) method opens a wide variety of experimental possibilities. Experiments and theoretical frameworks are reviewed for the study of surfactant exchange kinetics, macromolecular adsorption equilibrium and dynamics, as well as the fabrication of a wide range of soft surface materials and the characterization of their mechanics. Future directions for new experiments are also discussed.

Lifetime of micrometer-sized drops of oil pressed by buoyancy against a planar interface

Emulsion stability simulations are used to estimate the coalescence time of one drop of hexadecane pressed by buoyancy against a planar water/hexadecane interface. In the present simulations, the homophase is represented by a big drop of oil at least 500 times larger than the approaching drop (1-10 microm). Both deformable and nondeformable drops are considered along with six different diffusion tensors. In each case, van der Waals, electrostatic, steric, and buoyancy forces are taken into account. The coalescence times are estimated as the average of 1000 random walks. It is found that the repulsive potential barrier has a significant influence in the results. The experimental data can only be reproduced assuming negligible repulsive barriers, as well as nondeformable drops that move with a combination of Stokes and Taylor tensors as they approach the interface.