Blocking and Blending: Different Assembly Models of Cyclodextrin and Sodium Caseinate at the Oil/Water Interface

Guest-guided anchoring patterns of cyclodextrin supramolecular microcrystals on droplet surfaces

The growth of cyclodextrin inclusion complexes (ICs) on oil/water interfaces represents a beautiful example of spontaneous pattern formation in nature. How the supramolecules evolve remains a challenge because surface confinement can frustrate microcrystal growth and give rise to unusual phase transitions. Here we investigate the self-assembly of ICs on droplet surfaces using microfluidics, which allows directly visualizing packing, wetting and ordering of the microcrystals anchored on the surface. The oil guests of distinct molecular structures can direct the assembly of the ICs and largely affect anchoring dynamics of the ICs microcrystals, leading to a range of behaviors including orientating, slipping, buckling, jamming, or merging. We discuss the behaviors observed in terms of the flexibility of the building blocks, which offers a new degree of freedom through which to tailor their properties and gives rise to a striking feature of anchoring patterns that have no counterpart in normal colloidal crystals.

Ejected microcrystals probe jammed states of droplets in cyclodextrin-based emulsions

The cyclodextrin (CD)-based emulsions exhibit complex instability behaviors such as rapid flocculation and creaming, and how to capture droplet dispersion states of the emulsions remains a great challenge. Here we prepare the CD-based emulsions with different oil-water volume ratios and CD concentrations by using high-pressure homogenization, and characterize the emulsion droplets by using optical microscopy and confocal laser scanning microscopy. We evaluate the effects of homogenization pressure on the stability of the emulsions, identify armored droplets with different surface features, measure interfacial concentrations of adsorbed ICs microcrystals, and observe ejection of the oil/CD inclusion complexes (ICs) microcrystals from the droplet surface. The droplet dispersion states are sensitive to the dynamic buildup and evolving morphologies of the interfacial microcrystals, and there are clear correlations between the properties of the ejected microcrystals and the characteristics of the emulsions. We ascribe the subsequent ejection of ICs microcrystals from the droplet surface to consolidation and deformation of the films formed between neighboring droplets. The ejection of the ICs microcrystals affords a simple method to detect the droplet-droplet interactions and phase transitions in the CD-based emulsions, which might be a generic feature in the broader context of the creaming processes of emulsions.

Preparations, application of polysaccharide-protein nanoparticles and their assembly at the oil-water interface

With the development of nanotechnology, nanoparticles have played an important role in pharmaceuticals, foods and materials, in particular, protein/polysaccharide based composite nanoparticles have received attention from researchers for safety and green production. This paper summarized in detail the preparation methods, applications of protein/polysaccharide nanoparticles (PPNPs) in recent years, especially the mechanism of stabilizing the oil-water interface. Currently, the polysaccharides applied are more traditional, such as chitosan, pectin and carboxymethyl cellulose, so there is still a lot of room for the development of raw materials that can be used to prepare PPNPs. Based on this, we also proposed three promising polysaccharides: seaweed polysaccharide, lycium barbarum polysaccharide and lactobacillus exopolysaccharides, describing their characteristics as well as their application prospects, this article can serve as a reference for interested researchers.

Jamming to unjamming: Phase transition in cyclodextrin-based emulsions mediated by sodium casein

HYPOTHESIS

Cyclodextrin (CD) can spontaneously build up the solid particle membrane with CD-oil inclusion complexes (ICs) by a self-assembly process. Sodium casein (SC) is expected to preferentially adsorb at the interface to transform the type of interfacial film. The high-pressure homogenization can increase interfacial contact opportunities of the components, which promote the phase transition of the interfacial film.

EXPERIMENTS

We added SC by sequential and simultaneous orders to mediate the assembly model of the CD-based films, examined the patterns in which the films adopt phase transitions to retard emulsion flocculation, and studied the physic-chemical properties of the emulsions and films from the structural arrest, interface tension, interfacial rheology, linear rheology, and nonlinear viscoelasticities through Fourier transform (FT)-rheology and Lissajous-Bowditch plots.

FINDINGS

The interfacial and large amplitude oscillatory shear (LAOS) rheological results showed that the films changed from jammed to unjammed. We divide the unjammed films into two types: one is SC dominated liquid-like film, which is fragile and related to droplet coalescence; the other is cohesive SC-CD film, which helps droplet rearrangement and retards droplet flocculation. Our results highlight the potential of mediating phase transformation of interfacial films to improve emulsion stability.

Pickering emulsions stabilized by β-CD microcrystals: Construction and interfacial assembly mechanism

β-Cyclodextrin (β-CD) can combine with oil and other guest molecules to form amphiphilic inclusion complexes (ICs), which can be adsorbed on the oil-water interface to reduce the interfacial tension and stabilize Pickering emulsions. However, the subtle change of β-CD in the process of emulsion preparation is easily ignored. In this study, β-CD and ginger oil (GO) were used to prepare the Pickering emulsion by high-speed shearing homogenization without an exogenous emulsifier. The stability of the emulsion was characterized by microscopic observation, staining analysis, and creaming index (CI). Results showed that the flocculation of the obtained Pickering emulsion was serious, and the surface of the droplets was rough with lamellar particles. In order to elucidate the formation process of the layered particles, the GO/β-CD ICs were further prepared by ball milling method, and the X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and interfacial tension analyses found that β-CD and GO first formed amphiphilic nanoscale small particles (ICs) through the host-guest interaction, and the formed small particles were further self-assembled into lamellar micron-scale amphiphilic ICs microcrystals. These amphiphilic ICs and microcrystals aggregated at the oil-water interface and finally formed the Pickering emulsion. In this study, by exploring the formation process and evolution of GO/β-CD self-assembly, the formation process and stabilization mechanism of the β-CD-stabilized GO Pickering emulsion were clarified preliminarily, with the aim of providing a theoretical basis for the development of high-performance CD-stabilized Pickering emulsions.

Droplet Dispersion States of Cyclodextrin-Based Emulsions from Nonlinear Rheological Properties

Polymers are desirable to improve emulsion stability by stuffing them into the continuous phase. How to get information on the droplet dispersion states of the emulsions remains a challenge, as the emulsion characteristics are dictated by two intertwining components, the polymer matrix and the droplets. Herein, we use an amphiphilic polymer, gum arabic (GA), to mediate the droplet flocculation of cyclodextrin (CD)-based emulsions and compare them with our previous studies on the stabilization of CD-based emulsions by a nonamphiphilic polymer, methylcellulose (MC). We characterize the emulsions by using optical microscopy, confocal laser scanning microscopy, and laser particle analysis, explore their rheological behavior through large-amplitude oscillatory shear experiments, and analyze the nonlinear viscoelasticities through Fourier transform (FT)-rheology and Lissajous-Bowditch plots. There is a great difference between GA and MC in the viscosity effect and the arrangement around emulsion droplets. GA is not an effective flocculation inhibitor due to a bridging flocculation mechanism rather than a direct viscosity effect. Our analysis highlights the role of the intrinsic nonlinearity parameter (Q₀) extracted by FT analysis in reflecting the droplet dispersion states of the emulsions by decoupling structural contributions from the polymers and the emulsion droplets.

Stress buffering in cyclodextrin-based membranes coated on emulsion droplet surfaces

Surface instability of membranes not only plays a critical role in the morphological evolution observed in natural and biological systems, but also underpins a promising way for the bottom-up fabrication of novel functional materials. There is an urgent need for the design of novel building blocks into membranes, and the understanding of the abilities of the membranes to cope with mechanical stress is therefore of considerable importance. Here, we design membranes built with cyclodextrin-oil inclusion complexes, which are formed spontaneously at the oil/water interface by a self-assembly process. We select the oil phases of distinct molecular structures, namely, branched triglyceride oil and straight-chain n-dodecane, and examine the patterns in which the membranes adopt morphological transitions to buffer stress. We discuss two possible buffering scenarios for the behaviors observed in view of structural arrest and interfacial rheology, which are most closely linked to the rigidity of the membranes. The membranes represent fascinating models and shed some light on the origin of arrested stress relaxation.

Bulk and Interfacial Contributions to Stabilization of Cyclodextrin-Based Emulsions Mediated by Bacterial Cellulose

Cyclodextrin (CD)-based emulsions have a characteristic of rapid droplet flocculation, which limits their application as functional material templates, so it is very important to improve the stability of CD-based emulsions. In this study, we select bacterial cellulose (BC) as a nonadsorbing inhibitor to prevent flocculation of CD-based emulsions. We map a phase diagram of the aqueous dispersions of CD inclusion complexes (ICs) and BC from morphological observations and investigate the effects of BC on properties of the IC-laden films. We further explore the effects of BC concentration on the stability of the CD-based emulsions and investigate rheological behavior of the emulsions through large-amplitude oscillatory shear experiments. It shows that BC can effectively suppress the flocculation of CD-based emulsion droplets even at a concentration as low as 0.01 wt %. We propose that BC has dual effects from bulk and interfacial contributions on increasing emulsion stability. At low concentrations, BC mainly results in higher packing density of ICs on the emulsion droplet surface through excluded volume repulsion, and at high concentrations, BC creates a network structure that confines the motion of emulsion droplets and retards flocculation.

Droplet clustering in cyclodextrin-based emulsions mediated by methylcellulose

Rapid droplet aggregation in cyclodextrin (CD)-stabilized emulsions limits their practical use as material templates. Herein, we formulate mixtures of submicron CD-based emulsion droplets suspended in aqueous solutions of methylcellulose (MC) with various concentrations and molecular weights. We evaluate the effects of MC on the microstructure and stability of the emulsions using different techniques including optical microscopy, laser particle analysis, confocal laser scanning microscopy and multiple light scattering, explore the rheological behavior of the emulsions through large amplitude oscillatory shear experiments, and study the viscoelastic nonlinearities of the emulsions as a function of strain and strain-rate space through nondimensional elastic and viscous Lissajous-Bowditch plots. It is demonstrated that the emulsion droplets are present in the form of small clusters and their size is almost independent of MC concentration and molecular weight. The clustering pattern is also supported by the changes in viscoelastic properties of the emulsions and the intracycle nonlinear behavior of the Lissajous-Bowditch plots. We propose for the first time that glass-like dynamic arrest takes place with the formation of small equilibrium droplet clusters in the situation where the CD-based emulsion droplets are forced by depletion flocculation and kinetic trapping simultaneously exerted by MC.

Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil/Water Interface

Understanding the adsorption and organization of nanocelluloses at oil/water interfaces is crucial to develop a promising route to fabricate functional materials from the bottom-up. Here, we prepare acetylated cellulose nanofibrils (CNFs) with 2 degrees of substitution and investigate their assembly behavior at the oil/water interface. We study the adsorption process by tracking the dynamic interfacial tension using pendant drop tensiometry and further characterize the viscoelasticity of the CNF interfacial films as a function of ionic strength. The results show that the adsorption of the CNFs at the interface is dominated by energy barriers associated with electrostatic repulsion. With the addition of NaCl, the fibrils are rapidly accumulated at the oil/water interface and jammed into a solidlike film. The overall accumulation of the fibrils is related to the competition between van der Waals attractive forces and electrostatic repulsive forces according to the Derjaguin-Landau-Verwey-Overbeek theory. By screening on the fibril-fibril and fibril-interface electrostatic repulsive forces, the salt addition facilitates the formation of packed fibril clusters and the development of the clusters into a solidlike film. Moreover, the salt addition is assumed to trigger an abrupt density fluctuation in the vicinity of the interface (the formation of locally dense clusters and voids), leading to an increase in brittleness of the film.

Facile Fabrication of Cyclodextrin-Modified Magnetic Particles for Effective Demulsification from Various Types of Emulsions

Effective oil-water phase separation from various emulsions, especially those stabilized by surfactant, is of great importance. Although superhydrophobic and superoleophilic materials have attracted considerable attention in recent years, they are incapable of directly separating all types of oil-water mixtures. To separate various types of emulsions, one of the most important features of particles is that they can be dispersed in the continuous phase for delivery and target dispersed phases. In this study, cyclodextrin-modified magnetic composite particles (M-CDs) have been fabricated for this goal, based on their special interfacial activity and response to an external magnetic field. Though M-CDs are hydrophilic, the intelligent M-CDs can switch from hydrophilicity to hydrophobicity spontaneously, due to the formation of CD-oil inclusion complexes (ICs) at the oil-water interface. Physicochemical characterization reveals that M-CDs can adsorb at the oil-water interface and locate at the droplet surface as an effective Pickering emulsifier. By applying an external magnetic field, M-CDs are removed from the droplet surface and a rapid oil-water phase separation occurs. Our M-CDs can demulsify, for the first time, surfactant-free or surfactant-stabilized oil-in-water (O/W) and water-in-oil (W/O) emulsions directly, with high separation efficiency. Furthermore, the recycled MNPs still show high demulsification efficiency. In view of the sustainability of cyclodextrin and effective recycling ability of MNPs, M-CDs provides a new opportunity to develop an environmentally friendly interfacial material for practical applications in wastewater treatment.

Atomic elucidation of the cyclodextrin effects on DDT solubility and biodegradation

Different binding modes between DDT and CDs explain why only α-CD can promote the bioavailability and biodegradation of DDT by simultaneously increasing its aqueous solubility and membrane interaction.