Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air surface layers

An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and particle concentration. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.     

Interfacial rheology of protein–surfactant mixtures

The distribution of proteins and surfactants at fluid interfaces (air–water and oil–water) is determined by the competitive adsorption between the two types of emulsifiers and by the nature of the protein–surfactant interactions, both at the interface and in the bulk phase, with a pronounced impact on the interfacial rheological properties of these systems. Therefore, the interfacial rheology is of practical importance for food dispersion (emulsion or foam) formulation, texture, and stability. In this review, the existence of protein–surfactant interactions, the mechanical behaviour and/or the composition of emulsifiers at the interface are indirectly determined by interfacial rheology of the mixed films. The effect on the interfacial rheology of protein–surfactant mixed films of the protein, the surfactant, the interface and bulk compositions, the method of formation of the interfacial film, the interactions between film forming components, and the displacement of protein by surfactant have been analysed. The last section tries to understand the role of interfacial rheology of protein–surfactant mixed films on food dispersion formation and stability. The emphasis of the present review is on the interfacial dilatational rheology.     

The rheology of aqueous emulsions prepared by direct emulsification and phase inversion from a high viscosity alkyd resin

A comparison of the rheological properties of dispersions of an alkyd resin in water prepared by direct emulsification and phase inversion has been conducted. These dispersions have application as the base dispersion in the manufacture of the new generation of water based gloss paints. An experimental investigation of the effect of dispersed phase fraction, droplet size and dispersion age on the rheological properties of dispersions produced by each emulsification route has been carried out. The droplet size distributions of the dispersions are also characterised. It was found that phase inversion always gave a smaller droplet size distribution than direct emulsification. Surprisingly it was found that for a given dispersed phase fraction, the dispersions with a smaller droplet size, produced by the phase inversion route, have a lower zero shear rate viscosity than those produced by the direct emulsification route. The rheology of the direct emulsification samples was also observed to change with age, whilst with phase inversion samples the rheology was stable. It is proposed that this and other differences observed between the two methods of manufacture can be associated to the presence, or otherwise, of excess surfactant in the aqueous phase.     

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.     

Cellulose gel dispersions: fascinating green particles for the stabilization of oil/water Pickering emulsion

A facile approach for the preparation of cellulose gel dispersions with particle size less than 5 μm has been developed. The particles were obtained by dissolving cellulose in NaOH/urea solvent, followed by regeneration in ethanol/H₂O mixed solution with homogenizer shearing. The characteristics of the cellulose gel dispersions were evaluated in terms of particle dimensions and crystalline structure, size distribution and rheology behavior. The cellulose gel dispersions had low crystallinity, and the concentration of the cellulose solution had little influence on the particle size of the gel dispersions. Furthermore, the cellulose gel dispersions could be well dispersed in deionized water, and they could be used to stabilize oil/water emulsion without addition of any surfactant. The formed Pickering emulsion had typical shear-thinning behavior and higher storage modulus. The concentration of cellulose gel dispersions had a significant influence on the emulsion stability. The Pickering emulsion stabilized by the cellulose gel dispersions would open opportunities for the development of food emulsion systems or environmentally friendly functional materials.     

Influence of surfactant addition on the stability of concentrated alumina dispersions in water

The goal of this study was to assess the effects of surfactant addition on the stability and viscosity of concentrated alumina dispersions. The stabilizing effects of several candidate surfactants were investigated for concentrated dispersions of two different pseudoboehmite aluminas at pH 4 and 7. The stabilities of concentrated alumina dispersions treated by pH adjustment alone and by pH adjustment combined with surfactant addition were compared to assess the degree to which the surfactant enhanced stability. The initial rate of mass removal from a sedimenting alumina dispersion was used as a measure of stability.

The anionic surfactants Surfine WNT-A and DOWFAX 3B2 were identified as effective in enhancing the stability of concentrated alumina dispersions. The optimal doses of these surfactants for stabilizing 15% by weight VERSAL™ 250 alumina dispersions at pH 4 were determined to be about 4.6 × 10⁻⁵ mol g⁻¹ for both surfactants. On the basis of the initial rate of mass removal, surfactant-stabilized 15 wt.% suspensions were found to be approximately 2.5 and 10.6 times more stable than similar dispersions stabilized electrostatically by pH adjustment alone. These more stable dispersions exhibited lower viscosities than observed for the alumina dispersions not subjected to surfactant addition. The results indicate that the stability of concentrated alumina dispersions can be enhanced by anionic surfactant addition, and that such surfactants may therefore help to control the rheology of concentrated dispersions of alumina in water.     

Aqueous dispersions of colloidal poly(3-hexylthiophene) gel particles with high internal porosity

This work outlines the development of nano-porous, sub-micron poly(3-hexylthiophene) (P3HT) gel particles as solution-processable inks for applications in polymer solar cells. These dispersions are produced by emulsifying bulk P3HT organogels into water containing surfactant. The optical characteristics and stability of the resulting gel particles are assessed and their structure characterized. The P3HT within the gel particles is shown to retain its crystallinity with no evidence of doping. The gel particles are shown to be stable against aggregation due to the presence of surfactant at the oil/water interface. The fracture of the gel network during emulsification produces a bimodal distribution of particles that increase in size with increasing P3HT concentration in the 'parent' organogel. Small Angle Neutron Scattering measurements show that the particles maintain the structure of the bulk gels with high specific surface area. Spray-coating the gel particle dispersions produces uniform thin-films, which have been used to fabricate polymer/fullerene solar cells with a fully spray-coated active layer.     

Tailoring the Flow Properties of Soft Colloidal Dispersions

Summary: The dynamics and the non-linear rheology of concentrated dispersions of soft particles are shown to exhibit generic properties. They display both solid-like and liquid-like properties depending on the applied stress. Below the yield point, dispersions exhibit history-dependent phenomena that have all the hallmarks of ageing in glasses. The close-packed disordered structures of the dispersions lie at the heart of this behaviour. Other properties such as the propensity of pastes to slip on solid surfaces appear to be governed by specific contact interactions between the squeezed particles.     

Rheological investigation of thermal transitions in vesicular dispersion

The thermal behavior of unsonicated dispersions of a double-chained surfactant, Dioctadecyldimethylammonium bromide (DODAB), has been studied over a wide concentration range using DSC and dynamic rheology. All dispersions are characterized by the pre- and main transition peaks at 35 °C and 43 °C, respectively. But, only above 10 mM DODAB, a third endotherm at 52 °C appears which may correspond to the (ULVs + L_α fragments) → MLVs transition. The thermal-induced MLV’s size is proportionally dependent on the concentration. In addition, and in agreement with DSC data, dynamic rheology has proven to be an indirect way to elucidate the structural transitions in these DODAB vesicular dispersions.     

Polyelectrolyte/surfactant mixtures in the bulk and at water/oil interfaces

Stabilization of emulsions by mixed polyelectrolyte/surfactant systems is a prominent example for the application in modern technologies. The formation of complexes between the polymers and the surfactants depends on the type of surfactant (ionic, non-ionic) and the mixing ratio. The surface activity (hydrophilic–lipophilic balance) of the resulting complexes is an important quantity for its efficiency in stabilizing emulsions. The interfacial adsorption properties observed at liquid/oil interfaces are more or less equivalent to those observed at the aqueous solution/air interface, however, the corresponding interfacial dilational and shear rheology parameters differ quite significantly. The interfacial properties are directly linked to bulk properties, which support the picture for the complex formation of polyelectrolyte/surfactant mixtures, which is the result of electrostatic and hydrophobic interactions. For long alkyl chain surfactants the interfacial behavior is strongly influenced by hydrophobic interactions while the complex formation with short chain surfactants is mainly governed by electrostatic interactions.     

Surfactant-mediated formation of alginate layers at the water-air interface

The self-organization process of polysaccharide alginate with different cationic surfactants at the water-air interface was investigated over a wide concentration regime. The changes of surface properties determined by surface tension measurements, surface rheology, and X-ray reflectivity are correlated with changes of bulk properties measured by turbidity, light scattering, and zeta potential measurements. We demonstrate that the interactions between the alginate and cationic surfactants result in significant changes of bulk and interfacial properties. The results of surface shear experiments point to the existence of highly viscoelastic interfacial films. In combination with X-ray reflectivity, we demonstrate that these rheological features are related to polymer-surfactant associations at the interface. In the regime of high surfactant concentrations, we observed the existence of multilayer structures.     

Synergistic stabilization of emulsions by a mixture of surface-active nanoparticles and surfactant

The stability and rheology of tricaprylin oil-in-water emulsions containing a mixture of surface-active hydrophilic silica nanoparticles and pure nonionic surfactant molecules are reported and compared with those of emulsions stabilized by each emulsifier alone. The importance of the preparation protocol is highlighted. Addition of particles to a surfactant-stabilized emulsion results in the appearance of a small population of large drops due to coalescence, possibly by bridging of adsorbed particles. Addition of surfactant to a particle-stabilized emulsion surprisingly led to increased coalescence too, although the resistance to creaming increased mainly due to an increase in viscosity. Simultaneous emulsification of particles and surfactant led to synergistic stabilization at intermediate concentrations of surfactant; emulsions completely stable to both creaming and coalescence exist at low overall emulsifier concentration. Using the adsorption isotherm of surfactant on particles and the viscosity and optical density of aqueous particle dispersions, we show that the most stable emulsions are formed from dispersions of flocculated, partially hydrophobic particles. From equilibrium contact angle and oil-water interfacial tension measurements, the calculated free energy of adsorption E of a silica particle to the oil-water interface passes through a maximum with respect to surfactant concentration, in line with the emulsion stability optimum. This results from a competition between the influence of particle hydrophobicity and interfacial tension on the magnitude of E.     

Industrial applications of dispersions

In this review some industrial applications of dispersions have been discussed. After a general introduction, some specific topics have been covered. The preparation of dispersions using condensation and dispersion methods was discussed in terms of the various interfacial processes involved such as nucleation and growth, wetting, breaking of aggregates and agglomerates as well as comminution. The process of emulsification (for production of liquid/liquid dispersions) was also analyzed in terms of the interfacial processes such as reduction in interfacial tension, interfacial elasticity and viscosity. The control of the properties of dispersions was described in terms of the interaction forces between the particles or droplets in the system. These interaction forces are governed by the structure and properties of the interfacial region such as double layers, presence of adsorbed surfactant or polymer layers. Four main types of interaction forces may be distinguished : hard-sphere, electrostatic, steric and van der Waals. Combination of these forces lead to three general energy-distance curves that can be used to describe the state of the dispersion (stable, flocculated or coagulated). The various physical states of suspensions and emulsions produced on standing were schematically presented and they could be explained in terms of the energy-distance curves. The flow characteristics (rheology) of dispersions could also be accounted for in terms of the various interaction forces between the particles.Solubilization and microemulsions, which produce thermodynamically stable dispersions, could be described in terms of the balance between the interfacial energy and entropy of dispersion of the system. The driving force for producing such thermodynamically stable systems was the ultra low interfacial tension which could be achieved by using a combination of surfactants. The application of microemulsions in various fields such as solubilization, enhanced oil recovery and energy production was briefly described.The application of dispersions in microncapsulation and slow release was described in terms of interfacial polymerization, coacervation and multiple emulsion formation. These systems find application in medicine, agrochemicals and cosmetics. The application of dispersions in pharmacy and medicine was also described by quoting specific examples such as liposomes (vesicles), nanoparticles and magnetic microspheres. These systems have potential use in targeting delivery of drugs.     

Rheological characterization of polysaccharide-surfactant matrices for cosmetic O/W emulsions

Rheometrical techniques can be profitably used for polysaccharide matrices in order to evaluate their suitability for the preparation of stable cosmetic O/W emulsions. In particular, the rheological properties of aqueous scleroglucan systems were investigated under continuous and oscillatory shear conditions in a polymer concentration range (0.2-1.2% w/w) embracing the sol/gel transition. The effects due to the addition of two different surfactants (up to 10% w/w) were examined at constant polymer concentration (0.4% w/w). The selected additives are a nonionic polymeric siliconic surfactant (dimethicone copolyol) and a cationic surfactant (tetradecyltrimethylammonium bromide), respectively. Polysaccharide-surfactant interactions leading to complex formation were detected also through rheology. The combined action of both nonionic and cationic surfactants in the polymer solution was examined at two different surfactant concentration levels (5 and 10% w/w), demonstrating the beneficial effects produced on the mechanical properties of the polymer matrix by the coexistence of both surfactants. Such beneficial effects are confirmed by the stability and rheology shown by the emulsions prepared. In this way, the results point out the good agreement between the rheology of the continuous phase and the final characteristics of the emulsion obtained.     

Foaming in aqueous solutions of hexadecyltrimethylammonium bromide and silica nanoparticles: Measurement and analysis of rheological and interfacial properties

The properties of aqueous foams stabilized by a mixture of negatively charged silica nanoparticles and hexadecyltrimethylammonium bromide were studied in this work. Rheological properties of the foams were studied. The interaction between nanoparticles and surfactant molecules in the bulk phase was studied by zeta potential and size measurements of the particles. The interaction at the interface was studied by means of interfacial shear rheology, surface pressure measurement, and atomic force microscopy. It was found that foams were more stable at low surfactant concentrations, though the foamability was low. This was due to the formation of a strong viscoelastic film of surfactant-laden particles at the air–water interface. A suitable mechanism has been proposed to explain the stability of foams in the presence of nanoparticles at different surfactant concentrations.     

Dispersing nanotubes with surfactants: a microscopic statistical mechanical analysis

A first theoretical study of surfactant-stabilized carbon nanotube dispersions is presented. Density functional theory is used to compute potential of mean force between nanotubes in an aqueous solution of cationic surfactant n-decyltrimethylammonium chloride. In agreement with experimental results, it is found that stable dispersions can be prepared for surfactant bulk concentrations below the critical micelle concentration. Computed density profiles of head and tail segments indicate that surfactants adsorb on nanotube surfaces in a random fashion rather than form cylindrical micelles, which is also in agreement with recent small-angle neutron scattering measurements.     

Salt effects on the dilational viscoelasticity of surfactant adsorption layers

Interfacial rheology of adsorbed layers of surfactants, demonstrating the response of the interface to interfacial deformations, plays a key role in formation and stability of foams and emulsions. It also provides insights into complex surfactant systems in different applications, in particular, medical treatments and diagnostics. The response of the interface is mainly determined by the composition of a surfactant system, the equilibrium and kinetic adsorption properties of the included surface-active compounds and their interaction within the adsorption layer. The subject of ongoing investigations is interfacial rheology of surfactant layers in the presence of inorganic ions. Although these ions have no surface activity, they can strongly influence the interfacial rheological properties owing to their interaction with the surface-active molecules.This work aims to present recent developments in the interfacial rheology of surfactant adsorbed layers at liquid–fluid interfaces in the presence and absence of salts, highlighting the state of the art of experimental and theoretical works in this area. We highlight drawbacks of recently developed techniques for measuring dilational interfacial properties of surfactant layers, compared with previous techniques. Moreover, this review shows the dearth of research on the ion-specific effect on the interfacial rheology of surfactant layers. This demonstrates the necessity of further investigation of the effect of ion specificity on interfacial viscoelasticity.     

表面活性剂溶致液晶的流变学性质

系统阐述了三种溶致液晶(六角状、立方状和层状液晶)的流变性质,概括了各自的流变性特点并给出了其理论模型,特别对立方相的流变学模型和层状相的剪切诱导转变作用进行了较详细的说明.讨论了因为这种转变而导致的囊泡的形成,并且在表面活性剂和嵌段共聚物中均可观察到剪切诱导的结构转变.