Novel thermo-responsive coloring phenomena in water/surfactant/oil emulsions

Emulsions comprising a dual-surfactant system of a long-chain amidoamine derivative and a quaternary ammonium salt developed an iridescent color at a specific temperature region. The emulsions underwent phase inversion on heating from an O/W emulsion to a W/O emulsion, passing through a periodical lamellar structure which developed a characteristic interference color. Interestingly, the color and the coloring temperature can be independently controlled by adjusting the concentration of surfactants, respectively.     

Interfacial tension of alkylglucosides in different APG/oil/water systems

The interfacial performance of pure alkylglucosides (C₈G₁, C₁₀G₁ and C₁₂G₁) and of technical grade alkylpolyglucoside (APG) surfactants was investigated in three different water/oil systems (decane, isopropylmyristate and 2-octyldodecanol). From the dependence of the interfacial tension on the surfactant concentration below the CMC the cross-sectional area of the molecules at the decane/water interface was estimated. The plateau values of the interfacial tension at the CMC _c are independent of temperature and almost independent of added electrolyte in the decane/water system. The ability of the surfactants to lower the oil/water interfacial tension is most pronounced for the nonpolar oil. The partition coefficient of the surfactant between oil and water phase (k _c) was estimated from the CMC and the observed break point of the interfacial tension after equilibration of the two phases. In decane/water,k _c is nearly zero for all surfactants studied. For the polar oils,k _c increases with the chain length of the surfactant up tok _c10 for C₁₂G₁ in octyldodecanol/water. The values of _c in the different oil/water systems appear to be correlated withk _c and exhibit a minimum neark _c=1.     

Properties of oil1/water/oil2 double emulsions containing lipophilic acrylic polymer

Stable oil₁/water/oil₂ (o₁/w/o₂) double emulsions (DEs) containing glyceryl monooleate as a promoter of skin permeability in internal ultradispersed phase o1 (heptane) and an acrylic polymer as a pressuresensitive adhesive in external dispersion medium o₂ (ethyl acetate) are obtained. The aqueous interlayer of the DEs contains Tween 80, hydroxypropyl cellulose, and sometimes lysozyme. The phase ratio in the emulsions is constant. The effect of polyacrylate concentration on the stability, microstructure, and rheological properties of DEs is studied. Factors of DE stability, with the viscoelastic properties of the external dispersion medium being of the most importance, are analyzed. The DEs are used to produce ultradisperse films, which exhibit long-term release of lysozyme at a constant rate with its enzymatic activity retained preserved. The diffusion coefficient of the protein in the polymer matrix is determined.     

Surfactant-free cellulose filaments stabilized oil in water emulsions

Cellulose - There has been significant interest over recent years in the production and application of sustainable and green materials. Among these, nanocellulose has incurred great interest...     

Interfacial rheology of natural silk fibroin at air/water and oil/water interfaces

The interfacial viscoelastic behavior of natural silk fibroin at both the air/water and oil/water interfaces is reported. This natural multiblock copolymer is found to be strongly amphiphilic and forms stable films at these interfaces. The result is an interfacial layer that is rheologically complex with strong surface elastic moduli that are only slightly frequency-dependent. The kinetics of surface viscoelastic evolution are reported as functions of time for various concentrations of the spread films. Films deposited by Langmuir-Blodgett deposition were studied by scanning electron microscopy (SEM) to reveal a fibrous structure at the interface. The production of stable O/W emulsions by silk fibroin further confirms the generation of the elastic films at the oil/water interfaces.     

Hydrophobin coated boehmite nanoparticles stabilizing oil in water emulsions

Hydrophobin coated boehmite nanoparticles have been used to establish tooth-paste like, homogenous emulsions. The surface-modified nanoparticles were simply obtained by mixing aqueous solutions of cationic boehmite particles with the anionic hydrophobin H Star Protein B® (HPB). Surface tension measurements clearly show that 1 wt.% boehmite binds up to 1 wt.% HPB. The strong interaction and aggregation of hydrophobin coated boehmite nanoparticles was proven by Cryo-TEM measurements, too. Interestingly, the combined use of 0.5 wt.% HPB and 0.5 wt.% boehmite as emulsifying agents resulted in very stable, homogenous, high internal phase emulsions (65 wt.% oil) that are stable over months. The established emulsions have also been characterized by rheological measurements. Storage moduli of more than 1000 Pa are characteristic for their high gel-like properties. Furthermore, light microscopy showed an average droplet size close to 1 μm with low polydispersity. Cryo-SEM confirmed that the hydrophobin coated nanoparticles are located at the interface of the oil droplets and therefore stabilize the emulsion systems.     

Spreading of oil from protein stabilised emulsions at air/water interfaces

Spreading of a drop of an emulsion made with milk proteins on air/water interfaces was studied. From an unheated emulsion, all oil molecules could spread onto the air/water interface, indicating that the protein layers around the oil globules in the emulsion droplet were not coherent enough to withstand the forces involved in spreading. Heat treatment (90 °C) of emulsions made with whey protein concentrate (WPC) or skim milk powder reduced the spreadability, probably because polymerisation of whey protein at the oil/water interface increased the coherence of the protein layer. Heat treatment of emulsions made with WPC and monoglycerides did not reduce spreadability, presumably because the presence of the monoglycerides at the oil/water interface prevented a substantial increase of coherence of the protein layer. Heat treatment of caseinate-stabilised emulsions had no effect on the spreadability. If proteins were already present at the air/water interface, oil did not spread if the surface tension (γ) was <60 mN/m. We introduced a new method to measure the rate at which oil molecules spread from the oil globules in the emulsion droplet by monitoring changes in γ at various positions in a ‘trough’. The spreading rates observed for the various systems agree very well with the values predicted by the theory. Spreading from oil globules in a drop of emulsion was faster than spreading from a single oil drop, possibly due to the greater surface tension gradient between the oil globule and the air/water interface or to the increased oil surface area. Heat treatment of an emulsion made with WPC did not affect the spreading rate. The method was not suitable for measuring the spreading rate at interfaces where surface active material is already present, because changes in γ then were caused by compression of the interfacial layer rather than by the spreading oil.     

Stability of water/crude oil emulsions based on interfacial dilatational rheology

The dilatational viscoelasticity behaviors of water/oil interfaces formed with a crude oil and its distilled fractions diluted in cyclohexane were investigated by means of an oscillating drop tensiometer. The rheological study of the w/o interfaces at different frequencies has shown that the stable w/o emulsions systematically correspond to interfaces which present the rheological characteristics of a 2D gel near its gelation point. The stability of emulsions was found to increase with both the gel strength and the glass transition temperature of the gel. As expected, the indigenous natural surfactants responsible for the formation of the interfacial critical gel have been identified as the heaviest amphiphilic components present in the crude oil; i.e., asphaltenes and resins. Nevertheless, we have shown that such a gel can also form in the absence of asphaltene in the oil phase.     

Dewatering of crude oil emulsions 2. Interfacial properties of the asphaltic constituents of crude oil

The importance of the interfacial rheology in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated in part 1 of this series of papers (R.A. Mohammed, A.I. Bailey, P.F. Luckham and S.E. Taylor, Colloids Surfaces A: Physicochem. Eng. Aspects, 80(1993)223). In part 2, interfacial tensions of crude oil, and solutions of asphaltenes and resins in a model oil have been investigated. Surface pressure vs. area (Π—A) curves of monolayers of asphaltenes, resins and their mixtures have been established. In its dependence on the ratio of resins to asphaltenes, the pseudostatic dilatational modulus has high values for low resin-to-asphaltene ratios and low values for high resin-to-asphaltene ratios. This is expected to throw light on the cause of the enhanced stability of water-in-crude oil emulsions.     

Stability of oil/water (ethanol,lysozyme or lysine) emulsions

Emulsions of n-tetradecane in water (0.1%^v/_^V) homogenized by ultrasounds (1 5 min) were stabilized by 0.5 or 1.0 M ethanol and in the presence of lysozyme (4 mg 100 ml⁻¹) or 1 mM lysine monohydrochloride (14.6 mg 100 ml⁻¹). The zeta potentials and multimodal size distributions of the droplets after 5, 15, 30, 60, 120 min, and 1 and 2 days were determined by dynamic light scattering technique using ZetaPlus apparatus (Brookhaven Instr., USA). Both parameters were determined on the same sample subsequently without any mixing. The effect of pH [4, 6.8 (natural), and 11] was also investigated. The most stable emulsions in 1 M ethanol solutions alone were at pH 6.8 and 11 (the effective diameter D_eff and 350 nm, respectively), while in 0.5 M at pH 4 (D_eff nm). The most stable emulsions with lysozyme were obtained at pH 4 and 1 M ethanol (D_eff nm), and with lysine at pH 6.8 and 0.5 M ethanol (D_eff nm). Except for the emulsions with lysozyme at pH 4 and 6.8, in the rest systems the zeta potentials were negative and ranged between −5 and −85 mV as a function of time and pH. The changes of zeta potential indicate that H⁺ ions are not much potential determining, while OH⁻ ions increase the negative zeta potentials. However, H⁺ ions affect functional groups of lysozyme molecules adsorbed on the alkane droplet, what appears in essential changes of zeta potential and even reversed sign of it in some systems. The results point that stability of these emulsions may also be determined by hydrogen bonding.     

Highly magnetic latexes from submicrometer oil in water ferrofluid emulsions

The synthesis of functionalized submicrometer magnetic latex particles is described as obtained from a preformed magnetic emulsion composed of organic ferrofluid droplets dispersed in water. Composite (polystyrene/γ‐Fe₂O₃) particles were prepared according to a two‐step procedure including the swelling of ferrofluid droplets with styrene and a crosslinking agent (divinyl benzene) followed by seeded emulsion polymerization with either an oil‐soluble [2,2′‐azobis(2‐isobutyronitrile)] or water‐soluble (potassium persulfate) initiator. Depending on the polymerization conditions, various particle morphologies were obtained, ranging from asymmetric structures, for which the polymer phase was separated from the inorganic magnetic phase, to regular core–shell morphologies showing a homogeneous encapsulation of the magnetic pigment by a crosslinked polymeric shell. The magnetic latexes were extensively characterized to determine their colloidal and magnetic properties. The desired core–shell structure was efficiently achieved with a given styrene/divinyl benzene ratio, potassium persulfate as the initiator, and an amphiphilic functional copolymer as the ferrofluid droplet stabilizer. Under these conditions, ferrofluid droplets were successfully turned into superparamagnetic polystyrene latex particles, about 200 nm in size, containing a large amount of iron oxide (60 wt %) and bearing carboxylic surface charges. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2642–2656, 2006     

Interfacial polymerisation of anilinium at Langmuir monolayers

Anilinium is strongly adsorbed at monolayers of the phospholipid L-alpha-dimyristoylphosphatidic acid (DMPA) and hexadecanesulfonic acid (HDSA) at the air-water interface, and undergoes chemical polymerisation under conditions where bulk polymerisation does not occur.     

Changes of viscous characteristics of oil in water emulsions during homogenization

Summary Rheological measurements of the emulsions of paraffin oil in water, at concentrations of 10–70% were carried out. Emulsions were stabilized with various emulsifiers (Tween 20, Tween 40, Tween 60, Tween 65 and Tween 80, as well as Sodium dodecyl sulphate and Sodium paradodecyl benzene sulphonate with emulsifier- concentrations varying from 0.125 to 8%). The emulsions were homogenized for different periods (from 3 to 95 minutes).The results indicate that viscous characteristics of emulsions depend on the type of emulsifier and that its concentration plays a very important role in viscosity changes during emulsification. Effects of the type and concentration of emulsifier are more obvious for more concentrated emulsions.

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Zusammenfassung Es wurden theologische Messungen an Emulsionen von Paraffinöl in Wasser durchgeführt. Dabei wurde die Art und Konzentration von Emulgatoren und die Viskosität der dispergierten Phasen. variiert. Während der Emulgierung ändert sich die Viskosität hauptsächlich als Folge der Teilchengrößenänderung und der Teilchengrößenverteilung. Der Emulgatortyp und die Emulgatorkonzentration beeinflussen das theologische Verhalten stark.Es wurde auf die Möglichkeit hingewiesen, Viskositätsmessungen während des Emulgierungsprozesses zum Vergleich der Wirkung verschiedener Emulgatoren auf den Emulgierungsverlauf zu verwenden.

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With 4 figures and 1 table     

Evaluation of the precision of drop-size determination in oil/water emulsions by low-resolution NMR spectroscopy

The accuracy of the recently reported low-resolution NMR method (Goudappel, G. J. W.; et al. J. Colloid Interface Sci. 2001, 239, 535) for the determination of drop-size distribution in oil-in-water emulsions is evaluated by comparing the NMR results with precise data from video-enhanced optical microscopy. A series of 27 soybean-oil-in-water emulsions, differing in their mean drop size, polydispersity, oil volume fraction, and emulsifier, is studied. Soybean oil is selected as a typical component of food emulsions. The experimental error of our optical procedure for drop-size determination is estimated to be around 0.3 microm, which allows us to use the microscopy data as a reference for the mean drop-size and distribution width of the studied emulsions, with known experimental error. The main acquisition parameters in the NMR experiment are varied to find their optimal values and to check how the experimental conditions affect the NMR results. Comparison of the results obtained by the two methods shows that the low-resolution NMR method underestimates the mean drop size, d33, by approximately 20%. For most of the samples, NMR measures relatively precisely the distribution width (+/-0.1 to 0.2 dimensionless units), but for approximately 20% of the samples, larger systematic deviation was registered (underestimate by 0.3-0.4 units). No correlation is found between the emulsion properties and the relative difference between the microscopy and NMR data. Possible reasons for the observed discrepancy between NMR and optical microscopy are discussed, and some advantages and limitations of the low-resolution NMR method are considered.     

Hydrophilic polycarbonate for generation of oil in water emulsions in microfluidic devices

This report details the method for rendering hydrophilic surfaces of microchannels fabricated in polycarbonate (PC). We characterize the wetting properties and stability of the hydrophilic character of two coatings--one formed by a layer of poly(allylamine) (PAH*) and the second including an additional layer of poly(styrene-sulfonate) (PSS). This second (PC-PAH/PSS) coating yields highly hydrophilic surface that is stable against weeks of exposure to various fluids including organic oils. This coating allows for stable generation of oil-in-water emulsions of hydrocarbon, silicone and fluorinated oils without the use of surfactants and over days of continuous use.     

Electrochemical polymerisation of 2-aminofluorene in ethylalcohol/water medium

Electrochemical polymerisation of 2-aminofluorene, 2AF, was investigated in ethylalcohol/water mixture (3:2, v:v) in the presence of HClO₄ as the supporting electrolyte via constant potential electrolysis, CPE. Prior to CPE, electrochemical behaviour of the monomer was investigated in the same solvent-electrolyte couple utilising cyclic voltammetry, CV. Electrochemical polymerisation of the monomer yielded insoluble, dark bluish-green, conducting polymer deposit on the electrode surface. Characterisation of the polymer film has been carried out using FT-IR spectroscopic technique and thermal behaviour was studied using differential scanning calorimetry, DSC. Spectroelectrochemical, SPEL, behaviour of the polymer on ITO working electrode was studied by recording the electronic absorption spectra, in situ, in monomer-free solution at different potentials and it is found that the film can be reversibly cycled between −0.1 and 1.1 V vs SCE. Paramagnetic behaviour of the polymer was monitored using in situ ESR spectroscopy. The temperature dependence of conductivity supported the Mott's variable range hopping, VRH, mechanism for poly(aminofluorene), PAF.     

A diffusing wave spectroscopy study of the kinetics of Ostwald ripening in protein-stabilised oil/water emulsions

Diffusing wave spectroscopy (DWS) has been used to study the kinetics of ageing processes in n-decane/water emulsions, stabilised by the milk protein _S1-casein. A particular advantage of this particle size measuring technique is its direct applicability to the concentrated emulsion, avoiding the necessity for dilution to single scattering levels demanded by traditional dynamic light scattering methods. The observed droplet growth rates were found to conform to the kinetic law of Ostwald ripening. Monotonic increases in coarsening rates have been observed with increasing oil volume fraction and these results are compared with theoretical predictions. The effect of excess protein has also been measured but no evidence for depletion flocculation due to free _S1-casein was detectable.