Corn Oil-Water Separation: Interactions of Proteins and Surfactants at Corn Oil/Water Interfaces

Amphiphilic food polypeptides via moderate enzymatic hydrolysis of chickpea proteins: Bioprocessing, properties, and molecular mechanism

Plant proteins are a promising source for producing amphiphilic polypeptides with tailored techno-functional properties to be used in various food applications, such as fat replacers. This study investigated the effects of moderate enzymatic hydrolysis on amphiphilic polypeptide generation, by understanding the relationship of bioprocess - protein structure - functionality - amphiphilicity mechanism. Compared to non-specific protease alcalase, the specific protease trypsin catalyzed the production of polypeptides with higher surface hydrophobicity and relatively high molecular weight. Trypsin-produced polypeptides exhibited significantly higher water and oil holding capacities, foaming capacities, and emulsification than alcalase-produced counterparts. Furthermore, polypeptide sequences were obtained from proteomics and used to analyze amphiphilicity using Grand Average of Hydropathy (GRAVY) scores and hydropathy plots. Trypsin produced high number of amphiphilic polypeptides with balanced hydrophilic and hydrophobic regions. Molecular dynamics (MD) simulations of selected amphiphilic polypeptides in water-oleic acid systems suggested strong hydrophobic interactions with oleic acid and stable conformations in the interface.

Molecular Adsorption and Physicochemical Properties at Liquid/Liquid Nanoemulsion Soft Interfaces: Effect of Charge and Hydrophobicity

Contrary to the popular adage, "Oil and water do not mix", evidence of mixtures comprising the two "immiscible" liquids is universal. In the presence of an emulsifier, oil and water mix to form a colloidal suspension known as emulsion. Their utility in many areas such as food chemistry, biomedical health sectors, catalysis, and the petroleum industry is well recognized. While their application in our society is pervasive, tantalizing fundamental questions regarding the chemistry that takes place at the oil/water soft interface still linger. For instance, do organic compounds show proclivity for this molecularly thin boundary and, if so, what forces, hydrophobic or pure electrostatic among others, drive the molecular interactions? The focus of this Article is on molecular adsorption at the interface of oil-in-water (O/W) nanoemulsion (NE) droplets. The effect of the interfacial surfactant charge (positive, negative, zwitterionic, and neutral) on the affinity of aromatic organic compounds on the O/W NEs has been studied. Using a second harmonic generation (SHG), a nonlinear light scattering technique, we have explored the adsorption equilibrium of charged and neutral organic dyes. By variation of the surfactant functional group and thereby the interfacial charge properties, the source of the adsorption interaction, if any, has been deduced. The population of surfactants containing a charged functional group at the O/W interface is found to be sparse, yet adsorption at some of these interfaces has been observed. A purely electrostatic Coulomb interaction plays a key role, but the presence of a charged interface does not necessitate molecular adsorption. Hydrophobic interactions are not a major driving force of adsorption for the SHG dyes studied. However, a possible pi-interaction is likely in explaining the accumulation of neutral aromatic compounds at the O/W NE interface. These intricate adsorption features are discussed in the context of NE interfacial charge properties and their stability upon molecular adsorption.

Monitoring the Molecular Structure of Fibrinogen during the Adsorption Process at the Buried Silicone Oil Interface In Situ in Real Time

Interfacial proteins play important roles in many research fields and applications, such as biosensors, biomedical implants, nonfouling coatings, etc. Directly probing interfacial protein behavior at buried solid/liquid and liquid/liquid interfaces is challenging. We used sum frequency generation vibrational spectroscopy and a Hamiltonian data analysis method to monitor the molecular structure of fibrinogen on silicone oil during the adsorption process in situ in real time. The results showed that the adsorbed fibrinogen molecules tend to adopt a bent structure throughout the entire adsorption process with the same orientation. This is different from the case of adsorbed fibrinogen on CaF₂ with a linear structure or on polystyrene with a bent structure but a different orientation. The method introduced herein is generally applicable for following time-dependent molecular structures of many other proteins and peptides at interfaces in situ in real time at the molecular level.

Novel Engineered Carbon Cloth-Based Self-Cleaning Membrane for High-Efficiency Oil-Water Separation

A novel engineered carbon cloth (CC)-based self-cleaning membrane containing a Cu:TiO2 and Ag coating has been created via hydrothermal and light deposition methods. The engineered membrane with chrysanthemum morphology has superhydrophilic and underwater superoleophilic performance. The cooperativity strategy of Cu doping and Ag coating to the TiO2 is found to be critical for engineering the separation efficiency and self-cleaning skill of the CC-based membrane under visible light due to the modulated bandgap structure and surface plasmon resonance. The CC-based membrane has excellent oil-water separation performance when Cu is fixed at 2.5 wt% and the Ag coating reaches a certain amount of 0.003 mol/L AgNO3. The contact angle of underwater oil and the separation efficiency are 156° and 99.76%, respectively. Furthermore, the membrane has such an outstanding self-cleaning ability that the above performance can be nearly completely restored after 30 min of visible light irradiation, and the separation efficiency can still reach 99.65% after 100 cycles. Notably, the membrane with exceptional wear resistance and durability can work in various oil-water mixtures and harsh environments, indicating its potential as a new platform of the industrial-level available membrane in dealing with oily wastewater.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

Conventional wisdom suggests that cations play a minimal role in the assembly of cationic amphiphiles. Here, we show that at liquid/liquid (L/L) interfaces, specific cation effects can modulate the assemblies of hydrophobic tails in an oil phase despite being attached to cationic headgroups in the aqueous phase. We used oligo-dimethylsiloxane (ODMS) methyl imidazolium amphiphiles to identify these specific interactions at hexadecane/aqueous interfaces. Small cations, such as Li⁺, bind to the O atoms in the ODMS tail and pin it to the interface, thereby imposing a kinked conformation─as evidenced by vibrational sum frequency generation spectroscopy and molecular dynamics simulations. While larger Cs⁺ ions more readily partition to the interface, they do not form analogous complexes. Our data not only point to ways for controlling amphiphile structure at L/L interfaces but also suggest a means for the separation of Li⁺, or related applications, in soft-matter electronics.

Orientation and Conformation of Hydrophobin at the Oil-Water Interface: Insights from Molecular Dynamics Simulations

Hydrophobins, a new class of potential protein emulsifiers, have been extensively employed in the food, pharmaceutical, and chemical industries. However, the knowledge of the underlying molecular mechanism of protein adsorption at the oil-water interface remains elusive. In this study, all-atom molecular dynamics simulations were performed to probe the adsorption orientation and conformation change of class II hydrophobin HFBI at the cyclohexane-water interface. It was proposed that a hydrophobic dipole of the protein could be used to quantitatively predict the orientation of the adsorbed HFBI. Simulation results revealed that HFBI adsorbed at the interface with the patch-up orientation toward the oil phase, regardless of its initial orientations. HFBI's secondary structure was maintained to be intact in the course of simulations despite relatively significant variations in the tertiary structure observed, which could well preserve the bioactivity of HFBI. From the energy analysis, the driving force for interface adsorption was primarily determined by van der Waals interactions between HFBI and cyclohexane. Further analysis indicated that the adsorption orientation and conformation of HFBI at the oil-water interface were typically regulated by the hydrophobic patch and some key residues. This study provides some insights into the orientation, conformation, and adsorption mechanism of proteins at the oil-water interface and theoretical guidelines for the design and development of novel biological emulsifiers involved in the food, pharmaceutical, and chemical industries.

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

Surface plasmon-enhanced vibrational spectroscopy has been demonstrated to be an important highly sensitive diagnostic technique, but its enhanced mechanism is yet to be explored. In this study, we couple femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with surface plasmon generated by the excitation of localized gold nanorods/nanoparticles and investigate the plasmonically enhanced factors (EFs) of SFG signals from poly(methyl methacrylate) films. Through monitoring the SFG intensity of carbonyl and ester methyl groups, we have established a correlation between EFs and the coupling of localized surface plasmon resonance with SFG and visible beams. It is found that the total enhanced factor is approximately proportional to the square of an enhanced factor of the SFG electromagnetic field and the fourth power of the enhanced factor of the visible electromagnetic field. The local field effect is roughly expressed to be the square of an enhanced factor of the visible electromagnetic field. This finding will help to guide the experimental design of plasmon-enhanced SFG to drastically improve the detection sensitivity and thus provide greater insight into the ultrafast dynamics near plasmonic surfaces.

Revealing the Molecular Physics of Lattice Self-Assembly by Vibrational Hyperspectral Imaging

Lattice self-assemblies (LSAs), which mimic protein assemblies, were studied using a new nonlinear vibrational imaging technique called vibrational sum-frequency generation (VSFG) microscopy. This technique successfully mapped out the mesoscopic morphology, microscopic geometry, symmetry, and ultrafast dynamics of an LSA formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). The spatial imaging also revealed correlations between these different physical properties. Such knowledge shed light on the functions and mechanical properties of LSAs. In this Feature Article, we briefly introduce the fundamental principles of the VSFG microscope and then discuss the in-depth molecular physics of the LSAs revealed by this imaging technique. The application of the VSFG microscope to the artificial LSAs also paved the way for an alternative approach to studying the structure-dynamic-function relationships of protein assemblies, which were essential for life and difficult to study because of their various and complicated interactions. We expect that the hyperspectral VSFG microscope could be broadly applied to many noncentrosymmetric soft materials.

Ion Pairing and Molecular Orientation at Liquid/Liquid Interfaces: Self-Assembly and Function

Molecular orientation plays a pivotal role in defining the functionality and chemistry of interfaces, yet accurate measurements probing this important feature are few, due, in part, to technical and analytical limitations in extracting information from molecular monolayers. For example, buried liquid/liquid interfaces, where a complex and poorly understood balance of inter- and intramolecular interactions impart structural constraints that facilitate the formation of supramolecular assemblies capable of new functions, are difficult to probe experimentally. Here, we use vibrational sum-frequency generation spectroscopy, numerical polarization analysis, and atomistic molecular dynamics simulations to probe molecular orientations at buried oil/aqueous interfaces decorated with amphiphilic oligomers. We show that the orientation of self-assembled oligomers changes upon the addition of salts in the aqueous phase. The evolution of these structures can be described by competitive ion effects in the aqueous phase altering the orientations of the tails extending into the oil phase. These specific anionic effects occur via interfacial ion pairing and associated changes in interfacial solvation and hydrogen-bonding networks. These findings provide more quantitative insight into orientational changes encountered during self-assembly and pave the way for the design of functional interfaces for chemical separations, neuromorphic computing applications, and related biomimetic systems.

Probing protein aggregation at buried interfaces: distinguishing between adsorbed protein monomers, dimers, and a monomer-dimer mixture in situ

Protein adsorption on surfaces greatly impacts many applications such as biomedical materials, anti-biofouling coatings, bio-separation membranes, biosensors, antibody protein drugs etc. For example, protein drug adsorption on the widely used lubricant silicone oil surface may induce protein aggregation and thus affect the protein drug efficacy. It is therefore important to investigate the molecular behavior of proteins at the silicone oil/solution interface. Such an interfacial study is challenging because the targeted interface is buried. By using sum frequency generation vibrational spectroscopy (SFG) with Hamiltonian local mode approximation method analysis, we studied protein adsorption at the silicone oil/protein solution interface in situ in real time, using bovine serum albumin (BSA) as a model. The results showed that the interface was mainly covered by BSA dimers. The deduced BSA dimer orientation on the silicone oil surface from the SFG study can be explained by the surface distribution of certain amino acids. To confirm the BSA dimer adsorption, we treated adsorbed BSA dimer molecules with dithiothreitol (DTT) to dissociate these dimers. SFG studies on adsorbed BSA after the DTT treatment indicated that the silicone oil surface is covered by BSA dimers and BSA monomers in an approximate 6 : 4 ratio. That is to say, about 25% of the adsorbed BSA dimers were converted to monomers after the DTT treatment. Extensive research has been reported in the literature to determine adsorbed protein dimer formation using ex situ experiments, e.g., by washing off the adsorbed proteins from the surface then analyzing the washed-off proteins, which may induce substantial errors in the washing process. Dimerization is a crucial initial step for protein aggregation. This research developed a new methodology to investigate protein aggregation at a solid/liquid (or liquid/liquid) interface in situ in real time using BSA dimer as an example, which will greatly impact many research fields and applications involving interfacial biological molecules.

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

Molecular structures of peptides/proteins at interfaces determine their interfacial properties, which play important roles in many applications. It is difficult to probe interfacial peptide/protein structures because of the lack of appropriate tools. Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to elucidate molecular structures of peptides/proteins at buried solid/liquid and liquid/liquid interfaces. SFG has been successfully applied to study molecular interactions between model cell membranes and antimicrobial peptides/membrane proteins, surface-immobilized peptides/enzymes, and physically adsorbed peptides/proteins on polymers and 2D materials. A variety of other analytical techniques and computational simulations provide supporting information to SFG studies, leading to more complete understanding of structure-function relationships of interfacial peptides/proteins. With the advance of SFG techniques and data analysis methods, along with newly developed supplemental tools and simulation methodology, SFG research on interfacial peptides/proteins will further impact research in fields like chemistry, biology, biophysics, engineering, and beyond.

Nanoscale silica-coated graphene oxide and its demulsifying performance in water-in-oil and oil-in-water emulsions

In current work, GO@SiO₂ nanocomposite was prepared by coating nanoscale silica onto graphene oxide (GO). GO@SiO₂ was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (IF-IR). Additionally, the demulsifying performance of GO@SiO₂ was investigated by bottle test. The results showed that GO@SiO₂ had a good demulsifying performance in both oil-in-water (O/W) and water-in-oil (W/O) emulsions. When the concentration of GO@SiO₂ was 200 ppm in the O/W emulsion, the optimal light transmittance of aqueous phase (LTA) and corresponding oil removal rate (ORR) at room temperature could reach 86.9% and 99.48%, respectively. Also, GO@SiO₂ had an excellent salt tolerance under acidic condition. Furthermore, GO@SiO₂ also could demulsify the W/O emulsion, and the efficiency at 70 °C could reach 80.5% when the concentration was 400 ppm.

Effect of Surfactant Concentration and Hydrophobicity on the Ordering of Water at a Silica Surface

The performance of nonionic surfactants is mediated by the interfacial interactions at the solid-liquid interface. Here we applied sum frequency generation (SFG) vibrational spectroscopy to probe the molecular structure of the silica-nonionic surfactant solution interface in situ, supplemented by quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations. The combined studies elucidated the effects of nonionic surfactant solution concentration, surfactant composition, and rinsing on the silica-surfactant solution interfacial structure. The nonionic surfactants studied include ethylene-oxide (EO) and butylene oxide (BO) components with different ratios. It was found that the CH groups of the surfactants at the silica-surfactant solution interfaces are disordered, but the interfacial water molecules are ordered, generating strong SFG OH signals. Solutions with higher concentrations of surfactant lead to a slightly higher amount of adsorbed surfactant at the silica interface, resulting in more water molecules being ordered at the interface, or a higher ordering of water molecules at the interface, or both. MD simulation results indicated that the nonionic surface molecules preferentially adsorb onto silanol sites on silica. A surfactant with a higher EO/BO ratio leads to more water molecules being ordered and a higher degree of ordering of water molecules at the silica-surfactant solution interface, exhibiting stronger SFG OH signal, although less material is adsorbed according to the QCM-D data. A thin layer of surfactants remained on the silica surface after multiple water rinses. To the best of our knowledge, this is the first time the combined approaches of SFG, QCM-D and MD simulation techniques have been applied to study nonionic surfactants at the silica-solution interface, which enhances our understanding on the interfacial interactions between nonionic surfactants, water and silica. The knowledge obtained from this study can be helpful to design the optimal surfactant concentration and composition for future applications.

Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces

Liquid/liquid interfaces play a central role in scientific fields ranging from nanomaterial synthesis and soft matter electronics to nuclear waste remediation and chemical separations. This diversity of functions arises from an interface's ability to respond to changing conditions in its neighboring bulk phases. Understanding what drives this interfacial flexibility can provide novel avenues for designing new functional interfaces. However, limiting this progress is an inadequate understanding of the subtle intermolecular and interphase interactions taking place at the molecular level. Here, we use surface-specific vibrational sum frequency generation spectroscopy combined with atomistic molecular dynamics simulations to investigate the self-assembly and structure of model ionic oligomers consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a positively charged methyl imidazolium (MIM⁺) head group at buried oil/aqueous interfaces. We show how the presence of seemingly innocuous salts can impart dramatic changes to the ODMS tail conformations in the oil phase via specific ion effects and ion-pairing interactions taking place in the aqueous phase. These specific ion interactions are shown to drive enhanced amphiphile adsorption, induce morphological changes, and disrupt emergent hydrogen-bonding structures at the interface. Tuning these interactions allows for independent control over the oligomer structure in the oil phase versus interfacial population changes and represents key mechanistic insight that is needed to control chemical reactions at liquid/liquid interfaces.

Interfacial Structure and Interfacial Tension in Model Carbon Fiber-Reinforced Polymers

Carbon fiber-reinforced plastics (CFRPs) are widely used materials with outstanding mechanical properties. The wettability between the polymer matrix and carbon fiber in the interphase region significantly influences the strength of the composite. Sizing agents consisting of multiple components are therefore frequently applied to improve wetting and interfacial adhesion between polymers and carbon fiber in CFRPs. However, the complex compositions of sizing solutions make detailed interpretations of their impacts on interfacial wetting difficult. In this work, surface-sensitive sum frequency generation (SFG) spectroscopy was utilized to characterize the sizing/polymer and sizing/carbon fiber interfacial structures to gain molecular-level understandings of the wetting improvements afforded by sizing. A mixture sizing solution containing polyethylenimine (PEI, adhesion promoter) and Lutensol (surfactant) was investigated when contacting nylon (model plastics), polypropylene (model plastics), and graphite (model carbon fiber). Our results demonstrated that although the addition of the surfactant led to an interfacial tension decrease (in comparison to pure PEI solution) on nylon and polypropylene, the interfacial tension was surprisingly increased on graphite, contrasting with the commonly accepted function of surfactants. SFG characterizations revealed the multilayer molecular structures at these buried interfaces. The peculiar interfacial tension increase at the graphite/sizing interface was then correlated to the strong amine-π interactions between PEI and graphite. PEI was therefore demonstrated to be an effective adhesion promoter for carbon fiber. This article reports the first investigation of (polymer + surfactant) complex structures at solid-liquid interfaces. The valuable structural insights obtained by SFG analysis enable more accurate understandings of the composition-wettability (structure-function) relationship. These detailed understandings of interactions between sizing and the substrates promote more informed and optimized selections of sizing formulae.

Study on Self-Assembled Morphology and Structure Regulation of α-Zein in Ethanol-Water Mixtures

α-Zein has received widespread attention owing to its unique solubility, amphipathic, and self-assembly properties, which is because of its high proportion of nonpolar amino acids and unique amino acid sequence. The protein self-assembly is a significant and widely observed phenomenon in many scientific areas such as food and biomedicine, among many industries. In this study, we investigated the self-assembly behavior of α-zein and regulated the morphology and structure of the self-assembled α-zein by varying the experimental parameters like pH, ethanol content, induction time, and α-zein concentration during the self-assembly process in ethanol-water mixtures. The nanospheres and nanofibers were observed under different conditions [nanospheres observed under acidic and strongly alkaline (pH > 10.5) conditions or for ethanol content lower than 65% and higher than 75%; nanofibers observed under weakly alkaline (pH 9.5-10.5) conditions or for 65-75% ethanol concentration for induction duration longer than 24 h]. The morphological and structural analyses of the self-assembled α-zein showed that the self-assembly process was accompanied by the transformation of the morphology and conformation of α-zein. The studies on the self-assembly process and mechanism revealed that α-zein first self-assembled into nanospheres, followed by the nanospheres adhering to shape-beaded fibers and finally fibers, accompanied by a structural transformation from the disordered into ordered state. The nanosphere formation is noted to follow the nucleation-based polymerization, and the nanosphere-mediated mechanisms lead to the formation of nanofibers. Moreover, the hydrophobic interactions, hydrogen bonds, and electrostatic interactions are concluded to drive the α-zein self-assembly. The findings from this study are expected to provide a theoretical basis for expanding the commercial applications of α-zein.