The effect of silica nanoparticles on the stability of aqueous foams

An experimental study was performed on aqueous foams stabilized by a mixture of hexadecyltrimethylammonium bromide (HTAB) and negatively-charged silica nanoparticles. The effects of the nanoparticles on the stability of foams at different HTAB concentrations were investigated. The foams were characterized by measuring their foamability and stability. Rheological behavior of the foams was also studied. Furthermore, rheology of the air–water interfaces was studied in the linear and nonlinear deformation ranges. The thickness of the monolayer at the interface was measured. The actual size of the silica nanoparticles at the air–water interface was measured by transmission electron microscopy. Based on these measurements, the interaction between the monolayers across the foam film containing HTAB and nanoparticles was investigated. Smaller silica nanoparticles (i.e. diameter less than 10?nm) adsorbed at the air–water interface whereas the larger particles remained in the sub-phase or in the bulk liquid phase. It was found that these nanoparticles strongly influenced the foaming behavior at the low HTAB concentrations (i.e. below the CMC). A Langmuir-type monolayer was formed. The presence of the nanoparticles at the air–water interface provided mechanical strength to the foam films and prevented their rupture. This hindered coalescence of the bubbles, which resulted in a stable foam.     

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.     

Hydrophilic polymer foams with well‐defined open‐cell structure prepared from pickering high internal phase emulsions

Open‐cell hydrophilic polymer foams are prepared through oil‐in‐water Pickering high internal phase emulsions (HIPEs). The Pickering HIPEs are stabilized by commercial titania (TiO₂) nanoparticles with adding small amounts of non‐ionic surfactant Tween85. The morphologies, such as average void diameter and interconnectivity, of the foams can be tailored easily by varying the TiO₂ nanoparticles and Tween85 concentrations. Further, investigation of the HIPE stability, emulsion structure and the location of TiO₂ nanoparticles in resulting foams shows that the surfactant tends to occupy the oil‐water interface at the contact point of adjacent droplets, where the interconnecting pores are hence likely to be formed after the consolidation of the continuous phase. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

A novel formulation of the pickering emulsion stabilized with silica nanoparticles and its thermal resistance at high temperatures

Stabilization of emulsions with solid particles can be used in several fields of oil and gas industry because of their higher stability. Solid particles should be amphiphilic to be able to make Pickering emulsions. This goal is achieved by using surfactants at low concentrations. Oil-in-water (o/w) emulsions are usually stabilized by surfactant but show poor thermal stability. This problem limits their applications at high-temperature conditions. In this study, a novel formulation for o/w stabilized emulsion by using silica nanoparticles and the nonionic surfactant is investigated for the formulation of thermally stable Pickering emulsion. The experiments performed on this Pickering emulsion formula showed higher thermal stability than conventional emulsions. The optimum wettability was found for DME surfactant and silica nanoparticles, consequently, in that region; Pickering emulsion showed the highest stability. Rheological changes were evaluated versus variation in surfactant concentration, silica concentration and pH. Scanning electron microscopy images approved the existence of a rigid layer of nanoparticle at the oil-water interface. Finally, the results show this type of emulsion remains stable in harsh conditions and allows the system to reach its optimum rheology without adding any further additives.     

Direct measurements of the effects of salt and surfactant on interaction forces between colloidal particles at water-oil interfaces

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable. Averaging over several particle pairs was hence found to be necessary to obtain a reliable assessment of the effects of salt and surfactant. It has previously been suggested that the repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side also contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over longer times. Finally, measurements of three-body interactions provide insight into the anisotropic nature of the interactions. The unique time-dependent and anisotropic interactions between particles at the oil-water interface allow tailoring of the aggregation kinetics and structure of the suspension structure.     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

气相二氧化硅/季铵Gemini表面活性剂稳定的泡沫体系

以zeta电位法研究了季铵Gemini表面活性剂亚甲基-α, ω-双(十二烷基二甲基溴化铵) (12-s-12, s=2, 6)在水溶液中修饰气相二氧化硅(F-SiO₂)粒子。这些粒子随表面活性剂浓度C增加经历了表面从原先的亲水到疏水再重新亲水的改变,其中疏水粒子可以自发吸附在气泡液膜中,从而很好地稳定泡沫。重新亲水的粒子脱附出液膜,仅留下表面活性剂稳定气泡。强的液膜弹性对应于稳定的泡沫。联接链长度影响了Gemini在F-SiO₂粒子表面的吸附,因而也影响了液膜的弹性和对泡沫的稳定。超短s=2联接链的12-2-12由于反离子解离不完全而带有较少的正电荷,在粒子表面的初始吸附弱于12-6-12,但因此减少了吸附分子头基间的静电排斥,可以形成更致密的吸附层。由于12-2-12本身比12-6-12具有更强的界面吸附能力,F-SiO₂粒子和12-2-12的协同作用可以更好地稳定泡沫体系。     

Dilatational rheology and foaming properties of sucrose monoesters in the presence of beta-lactoglobulin

The foaming properties and the dilatational rheology of systems containing purified sucrose caprylate (SM800), caprate (SM1000), laurate (SM1200) and palmitate (SM1600) have been studied. Addition of beta-lactoglobulin (beta-lg) at a low concentration (0.050 wt.%) can aid the foam formation in the cases, where the surfactant concentration is insufficient to support foam formation. However, foams where both species co-existed exhibited poor stability. beta-lg was found to affect the dilatational properties of surfactant films even at low concentrations. It is thought that this could be related to the effect of the protein on the adsorption-desorption relaxation mechanism, or to the possible formation of a protein-surfactant complex in the bulk. The age of the protein film was also found to affect the kinetics of protein displacement by SM1000, as monitored by the change in the dilatational properties (Langmuir trough technique) and the relative reflectivity of the interface (Brewster angle microscopy) with time. An insoluble monolayer of sucrose stearate (Suc18) and beta-lg was also studied and it was found that the presence of small amounts of Suc18 in the protein film lead to a reduction of the interfacial elasticity. This is believed to be due to the disruption of the protein network. A possible mechanism could involve the obstruction of the hydrogen-bond intermolecular protein association by the strongly hydrated sucrose headgroup or the obstruction of the protein-protein hydrophobic interactions by the formation of an interfacial protein-surfactant complex.     

Investigation on properties of aqueous foams stabilized by aliphatic alcohols and polypropylene glycol

Foams stabilized by nonionic surfactants are usually moderately stable due to high drainage rate and intense bubble coalescence and coarsening. This study aimed to investigate comparatively the foam properties of aliphatic alcohols (methyl isobutyl carbinol (MIBC) and 2-octanol) and polypropylene glycol (PPG400). Experiments were conducted using the FoamScan method at various surfactant concentrations and gas flow rates where the foam volume, liquid content of foam and foam half-life were determined. The results showed that both foamability and foam stability of surfactant solution increased with increasing gas flow rate and surfactant concentration for all tested surfactants. PPG400 was an unusually strong surfactant having the largest surface activity compared with MIBC and 2-octanol, which exhibited the maximum foaming performance and foam stability at all tested gas flow rates and concentrations. The present study suggested that foam properties depended primarily on the type of surfactant and its concentration and secondarily on the gas flow rate. In addition, properties of interface are closely related to that of foam, which is a significant point if one wants to produce foams for specific applications.     

Novel Oil‐in‐Water Emulsions Stabilised by Ionic Surfactant and Similarly Charged Nanoparticles at Very Low Concentrations

Novel oil‐in‐water (O/W) emulsions are prepared which are stabilised by a cationic surfactant in combination with similarly charged alumina nanoparticles at concentrations as low as 10^?5 m and 10^?4 wt %, respectively. The surfactant molecules adsorb at the oil‐water interface to reduce the interfacial tension and endow droplets with charge ensuring electrical repulsion between them, whereas the charged particles are dispersed in the aqueous films between droplets retaining thick lamellae, reducing water drainage and hindering flocculation and coalescence of droplets. This stabilization mechanism is universal as it occurs with different oils (alkanes, aromatic hydrocarbons and triglycerides) and in mixtures of anionic surfactant and negatively charged nanoparticles. Further, such emulsions can be switched between stable and unstable by addition of an equimolar amount of oppositely charged surfactant which forms ion pairs with the original surfactant destroying the repulsion between droplets.     

Interactions of hydrophilic silica nanoparticles and classical surfactants at non-polar oil-water interface

The interfacial and bulk properties of submicron oil-in-water emulsions simultaneously stabilised with a conventional surfactant (either lecithin or oleylamine) and hydrophilic silica nanoparticles (Aerosil?380) were investigated and compared with emulsions stabilised by either stabiliser. Emulsions solely stabilised with lecithin or oleylamine showed poor physical stability, i.e., sedimentation and the release of pure oil was observed within 3 months storage. The formation and long-term stability of silica nanoparticle-coated emulsions was investigated as a function of the surfactant type, charge, and concentration; the oil phase polarity (Miglyol?812 versus liquid paraffin); and loading phase of nanoparticles, either oil or water. Highly stable emulsions with long-term resistance to coalescence and creaming were formulated even at low lecithin concentrations in the presence of optimum levels of silica nanoparticles. The attachment energy of silica nanoparticles at the non-polar oil-water interface in the presence of lecithin was significantly higher compared to oleylamine in line with good long-term stability of the former compared to the sedimentation and release of oil in the latter. The attachment energy of silica nanoparticles at the polar oil-water interface especially in the presence of oleylamine was up to five-times higher compared to the non-polar liquid paraffin. The interfacial layer structure of nanoparticles (close-packed layer of particle aggregates or scattered particle flocs) directly related to the free energy of nanoparticle adsorption at both MCT oil and liquid paraffin-water interfaces.     

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.     

Viscoelastic and shear viscosity studies of colloidal silica particles dispersed in monoethylene glycol (MEG), diethylene glycol (DEG), and dodecane stabilized by dodecyl hexaethylene glycol monoether (C12E6)

Silica dispersions stabilized by a nonionic surfactant, dodecyl hexaethylene glycol monoether (C 12E 6), were studied using rheological measurements. The viscosity-shear rate flow behavior of silica in monoethylene glycol (MEG) is shear thinning at low shear rates, leading to a Newtonian plateau at high shear rates for all dispersions studied. All rheological properties showed an increase above a critical surfactant concentration. The dispersions were stable at low levels of C 12E 6 concentrations because of electrostatic repulsions as deduced from the zeta potentials of silica that were on the order of about -30 to -65 mV in monoethylene glycol (MEG). Instability on further addition of C 12E 6 to the silica particles, a phenomenon normally obtained with high-molecular-weight polymers, was observed in MEG. Viscoelatic measurements of silica in monoethylene glycol at various surfactant concentrations showed a predominantly viscous response at low frequency and a predominantly elastic response at high frequencies, indicative of weak flocculation. Instability is explained in terms of hydrophobic and bridging interactions. Restabilization observed at high surfactant concentration was due to the steric repulsion of ethoxy groups of micellar aggregates adsorbed on silica particles. The study also revealed that the presence of trace water introduced charge repulsion that moderated rheological measurements in glycol media and introduced the charge reversal of silica particles in dodecane.     

Formation of silver bromide precipitate of nanoparticles in a single microemulsion utilizing the surfactant counterion

Silver bromide precipitate of nanoparticles was prepared by addition of silver nitrate aqueous solution to a single microemulsion system consisting of dioctyldimethylammonium bromide, n-decanol, and water in isooctane. The silver ion reacted readily with the surfactant counterion, bromide, to form the precipitate of nanoparticles, which was stabilized in the water pools. The use of the surfactant counterion as a reactant is a new approach to nanoparticle preparation in microemulsions. It is characterized by high reactivity and less dependency on the intermicellar exchange of solubilizate. The effects of the surfactant and the cosurfactant concentrations, the amount of silver nitrate, and the water to surfactant mole ratio, R, were evaluated. Increasing the surfactant concentration at fixed R and amount of silver nitrate enhanced the role of intermicellar nucleation and resulted in the formation of larger particles, while increasing the amount of silver nitrate at fixed values of all the other variables enhanced the direct nucleation and resulted in the formation of smaller particles. Particle aggregation and flocculation took place when the concentration of n-decanol or the value of R was increased. Particle aggregation and flocculation were attributed to the decrease in the interaction between the surfactant protective layer and the nanoparticles in the water pools.     

Effect of particle hydrophobicity on the properties of silica particle layers at the air-water interface

This article describes a study of fumed silica particle layers adsorbed at the air-water interface. We have performed surface pressure, ellipsometry, and Brewster angle microscopy measurements. These determinations were complemented by surface viscoelasticity studies, using capillary waves to measure the compression moduli and an oscillating disc to measure the shear moduli. Our results show a strong influence of the particle hydrophobicity and surface density on the properties of the layers. Under compression-expansion, the particle layers rearrange quasi-instantaneously, and at high density, they buckle and/or collapse. Shear measurements show a transition from viscous to elastic behavior for particles with contact angles close to 90 degrees. The surface compression moduli are quite small and most likely not related to the stability of the foams made with these particles, in contrast to the case of more common surfactant foams.     

Foaming in chemical surfactant free aqueous dispersions of anatase (titanium dioxide) particles

Steady-state dynamic aqueous foams were generated from surfactant-free dispersion of aggregated anatase nanoparticles (in the micrometer size range). In order to tune the particle surfaces, to ensure a critical degree of hydrophobicity (so that they disperse in water and generate foam), the particles were subjected to low-temperature plasma treatment in the presence of a vapor-phase silane coupling agents. From ESCA it was shown that hydrophobization only occurred at a small number of surface sites. Foamability (foam generation) experiments were carried out under well-defined conditions at a range of gas flow rates using the Bikermann Foaming Column.1 The volume of the steady-state foams was determined under constant gas flow conditions, but on removing the gas flow, transient foams with short decay times (<5 s) were observed. The foamability of the steady-state foams was found to be dependent on (a) the time of plasma treatment of the particles (surface hydrophobicity), (b) the particle concentration in the suspension, and (c) the state of dispersion of the particles. High foamability was promoted in the neutral pH regions where the charged particles were highly dispersed. In the low and high pH regions where the particles were coagulated, the foamability was considerably reduced. This behavior was explained by the fact that the large coagula were less easily captured by the bubbles and more easily detached from the interface (during the turbulent foaming conditions) than individual dispersed particles.     

Application of TPGS in polymeric nanoparticulate drug delivery system

d-alpha-Tocopheryl polyethylene glycol 1000 succinate (TPGS) has great potential in pharmacology and nanotechnology. The present work investigated the molecular behaviour of TPGS at the air-water interface, its effect on a model bio-membrane composed of dipalmitoylphosphatidylcholine (DPPC) lipid monolayer, and the interaction between the TPGS coated nanoparticles with the lipid model membrane. Paclitaxel loaded polymeric nanoparticles with TPGS as surfactant stabiliser were fabricated and characterised in terms of their drug incorporation capability and release kinetics. The result showed that TPGS exhibited notable effect on the surface properties of air-water interface as well as the lipid monolayer. The inter-particle force and the interaction between nanoparticles and lipid monolayer varied with the surface substance. The penetration of various nanoparticles into the model membrane indicated that an optimal balance between hydrophilicity and hydrophobicity on nanoparticle surface is needed to achieve an effective cellular uptake of nanoparticles. The results also demonstrate that the drug incorporation capability and the release characteristics of drug-loaded nanoparticles can be influenced by surfactant stabiliser.     

Mechanistic study of nanoparticles–surfactant foam flow in etched glass micro-models

This study was conducted in order to identify the pore-level mechanisms controlling the nanoparticles–surfactant foams flow process and residual oil mobilization in etched glass micro-models. The dominant mechanism of foam propagation and residual oil mobilization in water-wet system was identified as lamellae division and emulsification of oil, respectively. There was inter-bubble trapping of oil and water, lamellae detaching and collapsing of SDS-foam in the presence of oil in water-wet system and in oil-wet system. The dominant mechanisms of nanoparticles–surfactant foam flow and residual oil mobilization in oil-wet system were the generation of pore spanning continuous gas foam. The identified mechanisms were independent of pore geometry. The SiO₂-SDS and Al₂O₃-SDS foams propagate successfully in water-wet and oil-wet systems; foam coalescence was prevented during film stretching due to the adsorption and accumulation of the nanoparticles at the gas–liquid interface of the foam, which increased the films’ interfacial viscoelasticity.     

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.     

Interfacial rheology of stable and weakly aggregated two-dimensional suspensions

The interfacial rheological properties of stable and weakly aggregated two-dimensional suspensions are studied experimentally using a magnetic rod interfacial rheometer. Particle monolayers with well controlled structures were prepared. Charged polystyrene particles create two-dimensional colloidal crystals at the water-decane interface over a wide range of concentrations. Under similar conditions a predominantly liquid structure is obtained at the water-air interface for the same particles. The addition of appropriate combinations of the anionic surfactant sodiumdodecylsulfate (SDS) and sodium chloride (NaCl) to the aqueous subphase leads to a destabilization of these monolayers with the formation of fractal aggregates at low concentrations and a heterogeneous gel forming as the surface coverage is increased. After the structures have been built up a reproducible structure can be obtained, of which the interfacial rheological properties can be investigated using a magnetic rod stress rheometer. In all cases, numerical calculations were used to assess the importance of instrumental artifacts and the effect of the coupling between surface and subphase flows. The rheology of aggregated suspensions was compared to the reference case of a colloidal crystal. The two-dimensional aggregated suspensions display rheological features which are similar to their three-dimensional counterparts. These include an elastic response with small linearity limits, a power law dependence on surface coverage and a dependence on the strength of attraction. The results shed some light on the possible role of interfacial rheology on the stability of particle laden high interface systems. Additionally, the 2D suspensions could present fundamental insights in the rheological properties of dense colloidal suspensions.