Nanocrystalline cellulose (NCC) is a promising biological nanoparticle for the stabilization of fluid interfaces. However, the adsorption and interfacial layer structure of NCC are poorly understood as it is currently unknown how to form NCC interfacial layers. Herein, we present parameters for the adsorption of unmodified NCC at the air−water (A/W) interface. Initial NCC adsorption is limited by diffusion, followed by monolayer saturation and decrease in surface tension at the time scale of hours. These results confirm the current hypothesis of a Pickering stabilization. NCC interfacial performance can be modulated by salt-induced charge screening, enhancing adsorption kinetics, surface load, and interfacial viscoelasticity. Adsorbed NCC layers were visualized by atomic force microscopy at planar Langmuir films and curved air bubbles, whereat NCC coverage was higher at curved interfaces. Structural analysis by neutron reflectometry revealed that NCC forms a discontinuous monolayer with crystallites oriented in the interfacial plane at a contact angle < 90°, favoring NCC desorption upon area compression. This provides the fundamental framework on the formation and structure of NCC layers at the A/W interface, paving the way for exploiting NCC interfacial stabilization for tailored colloidal materials.

The use of oil-in-water emulsions for controlled lipid release is of interest to the pharmaceutical industry in the development of poorly water soluble drugs and also has gained major interest in the treatment of obesity. In this study, we focus on the relevant in vitro parameters reflecting gastric and intestinal digestion steps to reach a reliable in vitro-in vivo correlation for lipid delivery systems. We found that (i) gastric lipolysis determines early lipid release and sensing. This was mainly influenced by the emulsion stabilization mechanism. (ii) Gastric mucin influences the structure of charge-stabilized emulsion systems in the stomach, leading to destabilization or gel formation, which is supported by in vivo magnetic resonance imaging in healthy volunteers. (iii) The precursor structures of these emulsions modulate intestinal lipolysis kinetics in vitro, which is reflected in plasma triglyceride and cholecystokinin concentrations in vivo.

Human lipid digestion begins at the interface of oil and water by interfacial adsorption of lipases. Tailoring the available surface area for lipase activity can lead to specific lipid sensing in the body, thus, tailored satiety hormone release. In this study we present biopolymer layers at the MCT-oil/water interface with different stabilities under human gastric environment (37 °C, pH 2, pepsin). Physicochemical changes and enzymatic degradation of interfacial layers were monitored online by interfacial shear rheology. We show the weakening of β-lactoglobulin (β-lg) layers at body temperature and acidification and their hydrolysis by pepsin. If sufficient concentrations of nanocrystalline cellulose (NCC) are given to an existing β-lg layer, this weakening is buffered and the proteolysis delayed. A synergistic, composite layer is formed by adding methylated NCC to the β-lg layer. This layer thermogels at body temperature and resists hydrolysis by pepsin. Coexistence of these two emulsifiers at the air/water interface is evidenced by neutron reflectometry measurements, where morphological information are extracted. The utilized layers and their analysis provide knowledge of physicochemical changes during in vitro digestion of interfaces, which promote functional food formulations.

Proteins can aggregate as amyloid fibrils under denaturing and destabilizing conditions such as low pH (2) and high temperature (90 °C). Fibrils of β-lactoglobulin are surface active and form adsorption layers at fluid-fluid interfaces. In this study, β-lactoglobulin fibrils were adsorbed at the oil-water interface at pH 2. A shear rheometer with a bicone geometry set up was modified to allow subphase exchange without disrupting the interface, enabling the investigation of rheological properties after adsorption of the fibrils, as a function of time, different pH, and ionic strength conditions. It is shown that an increase in pH (2 to 6) leads to an increase of both the interfacial storage and loss moduli. At the isoelectric point (pH 5-6) of β-lactoglobulin fibrils, the maximum storage and loss moduli are reached. Beyond the isoelectric point, by further increasing the pH, a decrease in viscoelastic properties can be observed. Amplitude sweeps at different pH reveal a weak strain overshoot around the isoelectric point. With increasing ionic strength, the moduli increase without a strain overshoot. The method developed in this study allows in situ subphase exchange during interfacial rheological measurements and the investigation of interfacial ordering.

The static and dilatational response of β-lactoglobulin fibrils and native β-lactoglobulin (monomers) at water-air and water-oil interfaces (pH 2) was measured using the pendant drop method. The resulting adsorption behavior and viscoelasticity is dependent of concentration and adsorption time. The interfacial pressure of the β-lactoglobulin fibrils obtained in static measurements was 16-18 mN/m (against air) and 7 mN/m (against oil) for all concentrations. With higher concentrations, faster adsorption kinetics and slightly higher interfacial and surface pressure is achieved but did not lead to higher viscoelastic moduli. The transient saturation of the interface is similar for both the fibril solution and the monomers, however the fibril solution forms a strong viscoelastic network. To evaluate the superimposed adsorption behavior and rheological properties, the formed interfacial layer was subjected to dilatational experiments, which were performed by oscillating the surface area of the drop in sinusoidal and sawtooth (diagonal) deformation manner. The sinusoidal oscillations (time depended area deformation rate) result in a complex interfacial tension behavior against air and oil interfaces and show remarkable differences during compression and expansion as emphasized by Lissajous figures. For diagonal (constant area deformation rate) experiments, a slight bending of the interfacial tension response was observed at low frequencies emphasizing the influence of protein adsorption during rheological measurements.

The mechanical performance of materials at oil/water interfaces after consumption is a key factor affecting hydrophobic drug release. In this study, we methylated the surface of nanocrystalline cellulose (NCC) by mercerization and dimethyl sulfate exposure to produce thermosensitive biopolymers. These methylated NCC (metNCC) were used to investigate interfacial thermogelation at air/water and medium-chain triglyceride (MCT)/water interfaces at body temperature. In contrast to bulk fluid dynamics, elastic layers were formed at room temperature, and elasticity increased significantly at body temperature, which was measured by interfacial shear and dilatational rheology in situ. This unique phenomenon depends on solvent quality, temperature, and polymer concentration at interfaces. Thus, by adjusting the degree of hydrophobicity of metNCC, the interfacial elasticity and thermogelation of the interfaces could be varied. In general, these new materials (metNCC) formed more brittle interfacial layers compared to commercial methylcellulose (MC A15). Thermogelation of methylcellulose promotes attractive intermolecular forces, which were reflected in a change in self-assembly of metNCC at the interface. As a consequence, layer thickness and density increased as a function of temperature. These effects were measured by atomic force microscopy (AFM) images of the displaced interface and confirmed by neutron reflection. The substantial structural and mechanical change of methylcellulose interfaces at body temperature represents a controllable encapsulation parameter allowing optimization of lipid-based drug formulations.

Delayed fat digestion might help to fight obesity. Fat digestion begins in the stomach by adsorption of gastric lipases to oil/water interfaces. In this study we show how biopolymer covered interfaces can act as a physical barrier for recombinant dog gastric lipase (rDGL) adsorption and thus gastric lipolysis. We used β-lactoglobulin (β-lg) and thermosensitive methylated nanocrystalline cellulose (metNCC) as model biopolymers to investigate the role of interfacial fluid dynamics and morphology for interfacial displacement processes by rDGL and polysorbate 20 (P20) under gastric conditions. Moreover, the influence of the combination of the flexible β-lg and the elastic metNCC was studied. The interfaces were investigated combining interfacial techniques, such as pendant drop, interfacial shear and dilatational rheology, and neutron reflectometry. Displacement of biopolymer layers depended mainly on the fluid dynamics and thickness of the layers, both of which were drastically increased by the thermal induced gelation of metNCC at body temperature. Soft, thin β-lg interfaces were almost fully displaced from the interface, whereas the composite β-lg-metNCC layer thermogelled to a thick interfacial layer incorporating β-lg as filler material and therefore resisted higher shear forces than a pure metNCC layer. Hence, with metNCC alone lipolysis by rDGL was inhibited, whereas the layer performance could be increased by the combination with β-lg.

Quantifying the impact of environmental physicochemical changes on the microstructure of lipid delivery systems is challenging. Therefore, we have developed a methodology to quantify the coalescence of oil-in-water emulsion droplets during lipid digestion in situ on a single droplet level. This technique involves a custom-made glass microfluidic platform, in which oil droplets can be trapped as single droplets, or several droplets per trap. The physicochemical environment can be controlled, and droplet digestion, as well as coalescence, can be visualized. We show that the exchange of the physicochemical conditions in the entire reaction chamber can be reached in under 30 s. Microparticle image velocimetry allowed mapping of the flow profile and demonstrated the tuneability of the shear profile in the device. The extraction of quantitative information regarding the physical characteristics of the droplets during digestion was performed using an automated image analysis throughout the digestion process. Therefore, we were able to show that oil-in-water emulsions stabilized by proteins coalesced under human gastric conditions. This coalescence delayed the overall lipid digestion kinetics. The droplets that coalesced during digestion were hydrolyzed 1.4 times slower than individually trapped droplets. Thus, the microstructural evolution of lipid delivery systems is a crucial factor in lipid digestion kinetics. This novel technique allows the simultaneous quantification of the impact that the physicochemical environment has on both the lipid droplet microstructure and the lipid release patterns.