Coordination Cage-Based Emulsifiers: Templated Formation of Metal Oxide Microcapsules Monitored by In Situ LC-TEM

Metallo-supramolecular self-assembly has yielded a plethora of discrete nanosystems, many of which show competence in capturing guests and catalyzing chemical reactions. However, the potential of low-molecular bottom-up self-assemblies in the development of structured inorganic materials has rarely been methodically explored so far. Herein, we present a new type of metallo-supramolecular surfactant with the ability to stabilize non-aqueous emulsions for a significant period. The molecular design of the surfactant is based on a heteroleptic coordination cage ( CGA-3 ; CGA =Cage-based Gemini Amphiphile), assembled from two pairs of organic building blocks, grouped around two Pd(II) cations. Shape-complementarity between the differently functionalized components generates discrete amphiphiles with a tailor-made polarity profile, able to stabilize non-aqueous emulsions, such as hexadecane-in-DMSO. These emulsions were used as a medium for the synthesis of spherical metal oxide microcapsules (titanium oxide, zirconium oxide, and niobium oxide) from soluble, water-sensitive alkoxide precursors by allowing a controlled dosage of water to the liquid-liquid phase boundary. Synthesized materials were analyzed by a combination of electron microscopic techniques. In situ liquid cell transmission electron microscopy (LC-TEM) was utilized for the first time to visualize the dynamics of the emulsion-templated formation of hollow inorganic titanium oxide and zirconium oxide microspheres.     

Interface composition of multiple emulsions: rheology as a probe

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.     

Water-in-Hydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations

The role of Athabasca asphaltene particles and molecules in stabilizing emulsions was examined by measuring the surface area of water-in-toluene/hexane emulsions stabilized by various asphaltene fractions, each with a different proportion of soluble and insoluble asphaltenes. The stabilized interfacial area was found to depend only on the amount of soluble asphaltenes. Furthermore, the amount of asphaltenes on the interface was consistent with molecular monolayer coverage. Hence, at low concentrations, asphaltenes appear to both act as a molecular surfactant and stabilize emulsions. The effect of the hexane : toluene ratio on emulsion stability was examined as well. At lower hexane : toluene ratios, more asphaltenes were soluble but the surface activity of a given asphaltene molecule was reduced. The two effects oppose each other but, in general, a smaller fraction of asphaltenes appeared to stabilize emulsions at lower hexane : toluene ratios. The results imply that the emulsifying capacity of asphaltenes is reduced but not eliminated in better solvents. Copyright 2000 Academic Press.     

Microemulsification of triglyceride sebum and the role of interfacial structure on bicontinuous phase behavior

A unique triblock surfactant is reported that allows for the efficient microemulsification of triglycerides. Unlike the results of all previous efforts, these triglyceride microemulsions can be formed without the use of cosurfactants or dilution with co-oils and follow the classical patterns of surfactant phase behavior exhibited by mixtures of water, alkane oils, and nonionic oligoethylene glycol surfactants, i.e., progression from oil/water emulsions to one-phase microemulsions to water/oil emulsions with increasing temperature. Lamellar phases that usually dominate the aqueous phase behavior of surfactant/triglyceride mixtures are suppressed, allowing for the formation of single-phase microemulsions containing equal amounts of triglyceride and water. These isotropic and low-viscous fluids are particularly useful for cleansing and delivery of functional ingredients in skin care products. The effects of mixing a variety of typical skin care ingredients and components of sebum (skin oil) were also explored. Fatty acids significantly reduce the average microemulsion temperature, while other ingredients and oils, which do not partition at the oil/water interface, have less impact on the phase behavior. In all cases, one-phase microemulsions containing equal amounts of oil and water can be formed even at high additive concentrations. Indeed, partial replacement oftriglyceride with any of the additives examined consistently reduced the amount of surfactant necessary to form single-phase microemulsions. However, the greatest boost in surfactant efficiency was found with the addition of medium molecular weight amphiphilic block copolymers.     

Biocompatible and pH‐Responsive Colloidal Surfactants with Tunable Shape for Controlled Interfacial Curvature

Molecular‐surfactant‐stabilized emulsions are susceptible to coalescence and Ostwald ripening. Amphiphilic particles, which have a much stronger anchoring strength at the interface, could effectively alleviate these problems to form stable Pickering emulsions. Herein, we describe a versatile method to fabricate biocompatible amphiphilic dimer particles through controlled coprecipitation and phase separation. The dimer particles consist of a hydrophobic PLA bulb and a hydrophilic shellac–PEG bulb, thus resembling nonionic molecular surfactants. The size and diameter ratio of the dimer particles are readily tunable, providing flexible control over the water/oil interfacial curvature and thus the type of emulsion. The particle‐stabilized emulsions were stable for a long period of time and could be destabilized through a pH‐triggered response. The biocompatible amphiphilic dimer particles with tunable morphology and functionality are thus ideal colloidal surfactants for various applications.     

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

Pickering emulsions are surfactant‐free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant‐stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.     

Synthesis of Well-defined Poly(tetrahydrofuran)-b-Poly(α-amino acid)s via Cationic Ring-opening Polymerization(ROP) of Tetrahydrofuran and Nucleophilic ROP of N-thiocarboxyanhydrides

The synthesis of block copolymers of poly(tetrahydrofuran)-b-poly(α-amino acid)(PTHF-b-PAA) is challenging since it is difficult to combine the two blocks produced via different/conflicting ring-opening polymerization(ROP) mechanisms. In this contribution, the cationic ROP of THF is catalyzed by rare-earth triflate [RE(OTf)_3] and terminated by 2-(t-butyloxycarbonyl-amino) ethanol(BAE). After the deprotection of tbutyloxycarbonyl(Boc) group, the chain end of PTHF is quantitatively changed to amino group which thereafter initiates the nucleophilic ROP ofα-amino acid N-thiocarboxyanhydrides(NTAs). Both polymerizations are well controlled, generating PTHF and PAA segments with designable molecular weights(MWs). PTHF-b-polylysine(PTHF-b-PLys) and PTHF-b-polysarcosine(PTHF-b-PSar) are obtained with MWs between 8.6 and28.7 kg/mol. The above amphiphilic diblock copolymers form micelles in water. PTHF_(40)-b-PSar_(32) acts as a surfactant to stabilize oil-in-water emulsions. Both segments of PTHF-b-PAA are biocompatible and promising in the biomedical application.     

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.     

SYNTHESIS OF AMPHIPHILIC COPOLYMERS BASED ON ACRYLATES BY FREE-RADICAL POLYMERIZATION AND THEIR APPLICATION IN ALKYD EMULSIONS

Novel amphiphilic copolymers which contained poly(ethylene glycol) side-chain, long-chain alkyl pendant groups, and carboxyl groups have been synthesized by the conventional free-radical copolymerization of special monomers. The products were characterized by analysis of Fourier transform infrared spectroscopy (FT-IR), Gel Permeation Chromatography (GPC), and ¹H NMR. Based on the measurement of surface tension of aqueous copolymer solutions with various concentrations at an air/water interface, the critical micelle concentrations (CMC) was found to be lower along with the increase of copolymer concentration. Experimental results showed that CMC of this kind of amphiphilic polymer appeared in the range of 0.01～0.1 g/L. The amphiphilic polymer was mixed with an anionic surfactant (SDBS) in different proportions at aqueous solution and then successfully used in the preparation of alkyd emulsions. The results were compared with those of alkyd emulsions without polymeric surfactant. Steric stabilization of amphiphilic polymer was investigated by the observation of the stability of alkyd emulsions. The emulsions were stable after resting at ambient temperature for four months.     

纳米SiO2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用, 它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3), 并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述. 结果表明, 阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层, 从而增强了颗粒表面的疏水性是原位表面活性化的基础. 通过吸附CTAB和II-14-3, 颗粒的疏水性适当增强, 能吸附到正辛烷/水界面稳定O/W(1)型乳状液; 而通过吸附di-C12DMAB所形成的单分子层更加致密, 颗粒的疏水性进一步增强, 进而使乳状液从O/W(1)型转变为W/O型; 当表面活性剂浓度较高时, 由于链-链相互作用, 表面活性剂分子将在颗粒/水界面形成双层吸附, 使颗粒表面变得亲水而失去活性, 但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度. 因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时, 通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.     

Competitive adsorption of amphiphilic molecules and the stability of water-swollen micelles in oil

Motivated by recent attempts to confine biochemical processes inside water-in-oil microemulsions, we studied the composition and stability of mixed-amphiphile water-swollen micelles in oil from a theoretical point of view. A novel adsorption model demonstrates how the micellar contents (DNA, proteins, etc.) can dramatically affect the composition of the amphiphilic film and the resulting distribution of micelles. Special attention is given to the effect of electrostatic interactions within the micelles as well as between different ones. Since in a low dielectric medium charge fluctuations can lead to long-range intermicellar attractions, we suggest that the presence of amphiphilic polymers in the surfactant film may be needed to stabilize such microemulsions.     

Tailoring of the external and internal morphology of poly-3-hydroxy butyrate microparticles

Some microencapsulation procedures such as oil-in-water (o/w) and water-in-oil-in-water (w/o/w) emulsions were selected in an attempt to produce tailored poly-3-hydroxybutyrate (PHB) microparticles. The effects of several processing parameters such as polymer precipitation, surfactant, solvent, stirring and solvent evaporation rates were also considered. As a rule, low stirring rates at 500 rpm yielded particles ranging between 100 to 250 μm and at rates over 8000 rpm, diameters around 5–10 μm. The surfaces of the bigger particles, observed by scanning electron microscopy (SEM), were rough and the smaller ones were even rougher, irregular, cauliflower like. The extraction of the chloroform under low pressure or to the open atmosphere did not produce any appreciable change in the morphology for either type of particle. Transmission electron microscopy (TEM) micrographs suggest that microparticles obtained by o/w emulsions are monolithics but those obtained by w/o/w emulsions are of capsule-like structure. Microencapsulation of a peptide material such as follicle stimulating hormone was carried out with success using a double emulsion technique. This biomaterial, dissolved in the inner aqueous phase, was able to stabilize the primary emulsion without using a surfactant.     

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.     

Some Emulsion Features

Traditionally, emulsions have been defined as consisting of two liquids, of which one is dispersed in the other as macroscopic drops, stabilized by mono‐molecular layer of surfactant at the interface. However, a large majority of commercial emulsions are more complex than so and the added elements are essential for the properties of the emulsions including their stability.

With this in mind, this treatment of emulsions is divided into emulsions with mono‐layers and multilayers at the interface. In addition, additional elements in emulsions are described; such as lyotropic liquid crystals, vesicles, microemulsion droplets and solid particles, and their potential influence on the emulsion properties is indicated.     

Confinement of DNA in water-in-oil microemulsions

The study of systems that allow DNA condensation in confined environments is an important task in producing cell-mimicking microreactors capable of biochemical activities. The water droplets formed in water-in-oil emulsions are potentially good candidates for such microcompartments. The anionic surfactant AOT was used here to stabilize the droplets. We have studied in detail the DNA distribution and the structural modifications of these microemulsion drops by varying the concentration and molecular weight of DNA and using various techniques such as light, X-ray, and neutron scattering, electrical conductivity, and surface tension. DNA induces the formation of large drops into which it is internalized. The size of these drops depends on the amount of DNA dissolved in water as well as on its molecular weight. The local DNA concentration is very high (>100 mg/mL). The large drops coexist with small empty drops (not containing DNA), similar to those found in the DNA-free microemulsion.     

Synergistic interaction in emulsions stabilized by a mixture of silica nanoparticles and cationic surfactant

Using a range of complementary experiments, a detailed investigation into the behavior of dodecane-water emulsions stabilized by a mixture of silica nanoparticles and pure cationic surfactant has been made. Both emulsifiers prefer to stabilize o/w emulsions. At high pH, particles are ineffective emulsifiers, whereas surfactant-stabilized emulsions become increasingly stable to coalescence with concentration. In mixtures, no emulsion phase inversion occurs although synergism between the emulsifiers leads to enhanced stability at either fixed surfactant concentration or fixed particle concentration. Emulsions are most stable under conditions where particles have negligible charge and are most flocculated. Freeze fracture scanning electron microscopy confirms the presence of particle flocs at drop interfaces. At low pH, particles and surfactant are good emulsifiers alone. Synergism is also displayed in these mixtures, with the extent of creaming being minimum when particles are most flocculated. Experiments have been undertaken in order to offer an explanation for the latter synergy. By determining the adsorption isotherm of surfactant on particles in water, we show that surfactant addition initially leads to particle flocculation followed by re-dispersion. Using suitable contact angle measurements at oil-water-solid interfaces, we show that silica surfaces initially become increasingly hydrophobic upon surfactant addition, as well as surfactant adsorption lowering the oil-water interfacial tension. A competition exists between the influence of surfactant on the contact angle and the tension in the attachment energy of a particle to the interface.     

Investigation of the interactions in emulsions stabilized by a polymeric surfactant and its mixtures with an anionic surfactant

 The interaction of a nonionic polymeric surfactant with an anionic surfactant at the oil–water interface has been studied by its effects on the droplet size, stability and rheology of emulsions. Oil-in-water (o/w) emulsions were prepared using isoparaffinic oil and mixtures of a nonionic polymeric surfactant with an anionic surfactant. The macro-molecular surfactant was a graft copolymer with a backbone of polymethyl methacrylate and grafted polyethylene oxide (a graft copolymer with PEO chains of MW=750). The anionic surfactant was sodium dodecyl sulfate (SDS). The stabiliza-tion of the emulsion droplets was found to be different when using one or the other surfactant. The mechanism of stabilization of emulsion droplets by the macro-molecular surfactant is of the steric type while the stabilization by anionic surfactant is of the electrostatic repulsion type. Emulsions stabilized with mixtures present both types of stabilization. Other effects on the preparation and stabilization of emulsions were found to be dependent on properties associated with the surfactant molecular weight such as the Marangoni effect and Gibbs elasticity. The initial droplet size of the emulsions showed a synergistic effect of the surfactant combination, showing a minimum for the mixtures compared to the pure components. Emulsion stability also shows a synergistic interaction of both surfactants. Rheological measurements allow for the estimation of the interparticle interaction when measured as a function of volume fraction. Most of the effects observed can be attributed to the differences in interfacial tension and droplet radius produced by both surfactants and their mixtures. The elastic moduli are well explained on the basis of droplet deformation. Ionic versus steric stabilization produce little difference in the observed rheology, the only important differences observed concerned the extent of the linear viscoelasticity region. Received: 22 November 1996 Accepted: 24 March 1997     

Salting-out effect induced by temperature cycling on a water/nonionic surfactant/oil system

This paper presents original effects induced by temperature cycling on the transitional phase inversion of emulsions, stabilized by a nonionic polyethoxylated C18E6 surfactant model. The phase inversion follow-up is performed by electrical conductivity measurements, which involves focusing the study on the shape and location of the emulsion inversion region. In that way, new observations are brought out as a gradual evolution of the emulsion inversion along the cycling process. Two alternative approaches are considered for tackling these results: (i) first, a molecular approach regarding the particular organization and rearrangement of water clusters surrounding the surfactant polymer polar head, and (ii) second, a thermodynamic approach only considering the whole Gibbs free energy of the system. The volumic approaches are transposed, here, to the water/oil interface, and disclose that the phase inversion zone is included in a metastable region, able to stabilize for a given temperature, either metastable O/W emulsions or stable W/O ones. In that way, this study proposes novel and complementary insights into the phenomena governing the emulsion phase inversion.     

Effect of Nanoparticle Hydrophobicity on Stability of Highly Concentrated Emulsions

The effect of hydrophobicity index (HI) of fumed nanosilica specimens on stability of water-in-oil (W/O) highly concentrated emulsions (HCE with ? = 90 vol%) with an overcooled dispersed phase was studied. A series of five silica with HI in the 0.60–1.34 range and HI > 3 were used separately and in combination with a low molecular weight traditional surfactant, Sorbitan MonoOleate (SMO). First, it was shown that SMO alone can stabilize W/O HCE whereas only silica nanoparticles with intermediate HI in the range 0.97 ≤ HI ≤ 1.34 could form W/O emulsions only up to 77–79 vol%. Then, on the contrary to SMO-based emulsions, Pickering emulsions are unstable under shearing. When mixed (silica plus SMO) emulsifier systems were used, firstly a thermodynamic consideration revealed that only SMO is likely to adsorb at the W/O interface and controls the emulsifying process by the decrease in the interfacial tension. Then, interestingly, all different kinds of emulsion stability investigated in this study demonstrate a breaking point (BP) at HI = 0.97. Below the BP the emulsions were found to be very unstable on shelf as well as under shear. Above the BP, a clear synergy between colloidal silica and SMO surfactant has been found.