Ovalbumin at oil–water interfaces: Adsorption and emulsification

In this article, we study the adsorption of protein ovalbumin (OVA) at corn oil (CO), soybean oil (SBO), olive oil (OO), and water interfaces along with the emulsification of these oils in water. The dynamic interfacial tension (IFT) measurements show a reduction in IFT in the order SBO–water?～?CO–water?>?OO–water, with OVA adsorption being dominated by the free diffusion of OVA at the interfaces. CO–water, OO–water, and SBO–water emulsions cream with time. The cream phase consists of jammed closed-packed oil droplets due to depletion-induced inter-droplet attractions with higher G′ and G″ (～700?Pa) for emulsions with 1?wt% OVA.     

Molecular and colloidal self-assembly at the oil–water interface

Self-assembly is a versatile bottom-up approach for fabricating novel supramolecular materials with well-defined nano- or micro-structures associated with functionalities. The oil-water interface provides an ideal venue for molecular and colloidal self-assembly. This paper gives an overview of various self-assembled materials, including nanoparticles, polymers, proteins, and lipids, at the oil-water interface. Focus has been given to fundamental principles and strategies for engineering the self-assembly process, such as control of pH, ionic strength and use of external fields, to achieve complex soft materials with desired functionalities, such as nanoparticle surfactants, structured liquids, and proteinosomes. It has been shown that self-assembly at the oil-water interface holds great promise for developing well-structured complex materials useful for many research and industrial applications.     

Ultrathin dynamic networks formed by the surfactant SPAN 65 at the air–water and oil–water interface

Some surfactants tend to form ultrathin films at the surface of water or at the interface between oil and water. A representative of these surface-active compounds is SPAN 65 (trioctadecanyl ester of sorbic acid). Induced by attractive interactions these molecules can self-associate to form temporary networks. At the planar surface we measured the two-dimensional relaxation modulus, the storage modulus, the loss modulus and the relaxation spectrum. In addition to these measurements, we have also investigated the molecular structure of these networks with “Brewster-angle-microscopy”. The results indicate that temporary cross-linking points, which have lifetimes of the order of a few seconds, tend to stabilize these films. This dynamic network formation is also interesting for numerous technical applications, and it might be used for the preparation of emulsions, foams or microcapsules. Received: 23 October 2000 Accepted: 3 November 2000     

Fat crystallisation at oil–water interfaces

This review focuses on recent advances in the understanding of lipid crystallisation at or in the vicinity of an interface in emulsified systems and the consequences regarding stability, structure and thermal behaviour. Amphiphilic molecules such as emulsifiers are preferably adsorbed at the interface. Such molecules are known for their ability to interact with triglycerides under certain conditions. In the same manner that inorganic crystals grown on an organic matrix see their nucleation, morphology and structure controlled by the underlying matrix, recent studies report a templating effect linked to the presence of emulsifiers at the oil/water interface. Emulsifiers affect fat crystallisation and fat crystal behaviour in numerous ways, acting as impurities seeding nucleation and, in some cases, retarding or enhancing polymorphic transitions towards more stable forms. This understanding is of crucial importance for the design of stable structures within emulsions, regardless of whether the system is oil or water continuous. In this paper, crystallisation mechanisms are briefly described, as well as recent technical advances that allow the study of crystallisation and crystal forms. Indeed, the study of the interface and of its effect on lipid crystallisation in emulsions has been limited for a long time by the lack of in-situ investigative techniques. This review also highlights reported interfacial effects in food and pharmaceutical emulsion systems. These effects are strongly linked to the presence of emulsifiers at the interface and their effects on crystallisation kinetics, and crystal morphology and stability.     

Protein–emulsifier interactions at the air–water interface

The main interfacial physico-chemical characteristics and the kinetics of the formation of protein and emulsifier mixed films at the air–water interface are reviewed. Recent advances include the development of new molecular resolution and spectroscopic techniques coupled with surface rheological instruments and the incipient development of computer simulation of the displacement of proteins by emulsifiers.     

Electron-conductor separating oil–water (ECSOW) system: a new strategy for characterizing electron-transfer processes at the oil/water interface

A new electrochemical method for studying the electron transfer (ET) at the oil (O)/water (W) interface (or the liquid/liquid) interface has been devised, in which the O- and W-phases are separated by an electron conductor (EC; e.g. Pt). For the EC separating O–W (ECSOW) system, the ET across the EC phase can be observed voltammetrically in a similar manner to the O/W interface, however, no ion-transfer (IT) process can be taken place. Although the ECSOW system is thermodynamically equivalent to the corresponding O/W interface, they may be different from a kinetic viewpoint. In practice, the cyclic voltammograms obtained with the nitrobenzene NB/W interface and the ECSOW system in the presence of ferrocene in NB and hexacyanoferrate in W have shown quite different features, when the concentrations of both redox species are lower. The voltammograms for the NB/W interface have strongly supported the IT mechanism which involves an interfacial transfer of ferricenium ion. Also, the ECSOW system has been shown to be promising for clarification of complicated charge-transfer processes involving biological compounds such as l-ascorbic acid.     

Dynamics of water and oil at the solid–liquid interface in macroporous media and reservoir rocks

Nuclear magnetic relaxation dispersion experiments at different temperatures, using the magnetic field-cycling method, are reported. These experiments allow the obtaining of original information about water or oil molecular dynamics at solid–liquid interface in porous media, such as grain packing or reservoir rocks. These results on molecular dynamics at the pore surface are of real interest for oil-recovery. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones.     

Monolayers of γ-globulin at the air–water interface

Monolayer formation, of γ-globulin at the air–water interface has been investigated under varying subphase compositions. At pH 7.4, it is found that a stable monolayer is obtained only when the ionic strength is greater than 0.5 M. The magnitude of the collapse pressure increases with increasing ionic strength of the subphase. These data are analyzed in comparison to the literature data. Received: 2 March 1999 Accepted in revised form: 16 June 1999     

Electrical interaction between two rod-like particles covered by an ion-penetrable membrane in a water–oil interface

The electrical interaction between two long, parallel rod-like particles in a water–oil interface is investigated based on a Green function method, which is applicable to a system containing particles with different physical parameters. This is a highly desirable feature from practical considerations. We consider the case where each particle is covered by a membrane, and assumes a general class of shapes. This extends previous results in the literature in that they can be recovered as the special cases of the present model. We show that: (1) The higher the ionic strength in bulk water phase, the less stable the system concerned. (2) The closer the shape of the particles to a cylinder, the more stable the system under consideration. (3) The larger the fraction of particle immersed in the water phase, the greater the electrical interaction force. (4) The electrical interaction force increases with the increase in both the surface potential ratio between two particles and the fixed charge density in the membrane.     

Bubble points of hydrogen chloride–water–isopropanol and hydrogen chloride–water–isopropanol–benzene systems and liquid–liquid equilibria of hydrogen chloride–water–benzene and hydrogen chloride–water–isopropanol–benzene systems

Bubble points of the HCl–water–isopropanol and the HCl–water–isopropanol–benzene systems and liquid–liquid equilibria (LLE) of the HCl–water–benzene and the HCl–water– isopropanol–benzene systems were measured at 25–85°C and 30–70°C, respectively. The electrolyte nonrandom two-liquid model proposed by Chen et al. [C.-C. Chen, H.I. Britt, J.F. Boston, L.B. Evans, AIChE J. 28 (1982) 588–596] can satisfactorily correlate bubble points and liquid–liquid equilibria of the present mixed-solvent electrolyte systems over the entire range of temperature and concentrations using only binary adjustable parameters.     

Properties of lipophilic nucleoside monolayers at the air–water interface

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2′-palmitoyluridin und 3′-palmitoyluridin, forms Langmuir monolayers and Langmuir–Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV–vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties.     

Infrared and Raman spectroscopies of monolayers at the air–water interface

Infrared and Raman spectroscopies are now currently used to obtain molecular information (orientation, conformation, organization) on monolayers at the air–water interface. In the past year, several original studies were performed on peptides and proteins and their interaction with phospholipidic monolayers.     

Measurement of hydrolysis kinetics of galactose-substituted fluorescein by β-galactosidase at the toluene–water interface by spinning microtube fluorometry

A new analytical technique, spinning microtube fluorometry (SMF), was developed and applied to the study of interfacial hydrolysis of 5-dodecanoylaminofluorescein di-β-D-galactopyranoside (C₁₂FDG) by β-galactosidase (β-gal) in the toluene–water system. The nonfluorescent lactone form of C₁₂FDG in the toluene phase was converted at the interface to 5-dodecanoylaminofluorescein (C₁₂F), which was fluorescent in the aqueous phase as a dianion at pH 7.3, though some part of C₁₂F was extracted into the toluene phase as its nonfluorescent lactone form. The distribution ratios of C₁₂FDG and C₁₂F at pH 7.3 were determined as 1.4×10² and 1.97, respectively. The interfacial adsorption constants from the toluene phase to the interface at pH 7.3 were 4.8×10⁻⁴ and 1.7×10⁻² dm for C₁₂FDG and C₁₂F, respectively. The kinetic experiments with the SMF method concluded that the rate-determining step of the enzymatic hydrolysis at the interface and in the aqueous phase was the 1:1 reaction of C₁₂FDG and β-gal and that the hydrolysis reaction rate constant at the interface at pH 7.3 was 1.84×10³ M⁻¹s⁻¹, almost equal to that in the aqueous solution, 1.76×10³ M⁻¹s⁻¹. Finally, the SMF method revealed that the contribution of the interfacial reaction to the overall hydrolysis reaction rate of the toluene–water system was as high as 97%.     

Stabilized superhydrophobic composite membranes prepared by electrospinning for oil–water separation

In dealing with the oil–water separation process, improving the oil–water selectivity of the membrane surface and increasing the porosity within the membrane are effective means to achieve durable and efficient oil–water separation. Therefore, polystyrene/polyacrylonitrile-polyvinylidene fluoride/Polydimethylsiloxane Fe₃O₄ nanoparticles (PS/PAN-PVDF/PDMS@Fe₃O₄) composite membranes with superhydrophobicity and lipophilicity were prepared by electrospinning technique. By controlling the filling concentration of Fe₃O₄ nanoparticles, a stable superhydrophobic and lipophilic rough structure was constructed, and micro–nano multilayer rough voids existed inside the membrane. The results showed that the composite fiber membrane exhibited exceptional superhydrophobicity (156.2°), thermal stability (338°C), mechanical properties (tensile strength of 3.0 MPa, elongation of 33.9%), and oil adsorption capacity (30–100 g/g). Moreover, even under corrosive environments, this composite fiber membrane maintained its superhydrophobic properties (above 152°) and achieved high oil–water separation efficiency (above 97%). Remarkably, after 40 cycles, the composite membrane could sustain a separation flux of 5000 L m⁻² h⁻¹. Consequently, the composite fiber membrane manufactured using this strategy exhibits promising potential for applications in oil–water separation.     

How does the mobility of phospholipid molecules at a water/oil interface reflect the viscosity of the surrounding oil?

The mobility of phospholipid molecules at a water/oil interface on cell-sized phospholipid-coated microdroplets was investigated through the measurement of diffusion constants by fluorescence recovery after photobleaching. It is found that the diffusion constant of phospholipids showed the relation D approximately (eta water + eta oil) -0.85, where D is the diffusion constant, eta water is the viscosity of water, and eta oil is the viscosity of oil. This observation indicates that the viscosity of the surrounding oil is the primary factor that determines the diffusibility of phospholipids at a water/oil interface.     

Interfacial rheology of polyelectrolytes and polymer monolayers at the air–water interface

In this review, we describe interfacial rheology studies of polymer monolayers at the air–water interface. Since polyelectrolytes are usually soluble in water, the formation of surface monolayers requires the presence of a surfactant of opposite charge. The first part of the review is dedicated to these mixed monolayers. The second part is related to neutral monolayers that can be either adsorbed or deposited at the interface. Interfacial rheology studies of these systems are still scarce, despite a considerable interest: insoluble polymer monolayers in two dimensions are suitable model systems for the tests of polymer theories in two dimensions, such as and glass transition. The rheology of soluble polymer monolayers has important connections with the dynamic properties of dispersions stabilized with these polymers.     

Coadsorption of sodium dodecyl sulfate and medium-chain alcohols at the air–water interface

The adsorption of sodium dodecyl sulfate (SDS) and n-dodecanol from aqueous solutions of the pure and mixed surfactants at the air–water surface is studied by equilibrium surface pressure measurements, surface pressure transients and Brewster angle microscopy. The adsorption layers of SDS and n-dodecanol show fundamental differences in the phase behaviour. The adsorption parameters of both components are determined. Under appropriate conditions, a phase transition at which condensed phase textures are formed, occurs in the adsorption layers of n-dodecanol. The adsorption layers of surface-chemically purified SDS exist only in a fluid-like state without a phase transition under formation of condensed phase domains. Coadsorption of both surfactants is only investigated in the range of trace amounts of n-dodecanol. Depending on the mixing ratio and the system conditions (bulk concentration, temperature), a phase transition can or cannot occur. At absence of a phase transition, comparable surface concentrations of both components are calculated based on a orthogonal collocation solution for a two-component system. The adsorption properties are completely different when a phase transition occurs. Condensed phase domains of n-dodecanol formed, after the phase transition point, grow finally to a homogeneous condensed phase which replaces completely SDS.     

Photochemical behaviour of polyacryloylacetone;and polyethylacrylacetate monolayers at the air–water interface

The surface organization of enol units of polyacryloylacetone (PAA) and polyethylacrylacetate (PEAA) monolayers at the air–water interface is examined using surface pressure, surface potential and rheological measurements and theoretical calculations based on molecular models. The mechanism and kinetics of the photochemical enol–keto tautomerization of PAA and PEAA polymers organized in a monolayer of closely packed monomer units are studied by measuring the surface area increase at constant surface pressure. The results indicate an increase in the area per unit during the consecutive enol-to-keto photoconversion and the slow interfacial reorganization of these ¶forms to a more favourable state.     

pH-Driven adsorption and desorption of fatty acid at the liquid crystal–water interface

The pH-driven adsorption and desorption of fatty acid monolayers at the liquid crystal (LC)–water interface were studied. We doped fatty acids (stearic acid, palmitic acid, myristic acid, dodecanoic acid, and decanoic acid) into 4-cyano-4′-pentylbiphenyl (5CB), and employed sessile LC droplets as our experimental platform. Under a crossed polariser, the LC droplets displayed a bright flower bud-shaped texture at low pH, whereas at high pH, they exhibited a bright four-brush appearance due to desorption of the adsorbed fatty acids at the LC–water interface. Furthermore, we identified the critical transition pH of various concentrations of stearic acid and other fatty acids featuring distinct tail lengths. Based on the interfacial behaviour, we propose a new method to estimate the pK_a of fatty acids, which opens up new possibilities for simple, precise, and reliable measurement of the pK_a of other carboxylic acids. The findings presented herein will greatly facilitate the understanding of the interfacial behaviour of amphiphiles at the oil–water interface.