Dielectric relaxation behavior of ternary systems of water/toluene/Triton X-100: the effects of water and oil contents on microemulsion structure

Dielectric relaxation studies were conducted on the ternary systems of the nonionic surfactant Triton X-100 (a nonionic surfactant with a polyoxyethylene chain)/toluene/water in the frequency range from 40 Hz to 110 MHz. The contents of water and toluene were varied separately while the ratios of the other two components were fixed. Remarkable dielectric relaxations were observed around 1 MHz and dielectric intensity shows different variation with the increase of the contents of water or toluene. Dielectric parameters were obtained by fitting the data using the Cole–Cole equation with one dispersion term. The reverse micelles, water-in-oil, and oil-in-water micro-regions of the microemulsions were identified by the dependence of conductivity of the dispersed phase and continuous phase on the contents of water or toluene. Hanai theory and the corresponding analysis method were used to calculate the phase parameters of the constituent phases. The analysis results suggest that the dielectric relaxation probably arises from the interfacial polarization.     

Determination of the Phase Inversion Temperature of Orange Oil/Water Emulsions by Rheology and Microcalorimetry

Abstract

In low-energy emulsification processes, phase inversion occurs when the phases of a dispersion exchange, because of changes in the medium's properties. This paper reports experiments to determine the phase inversion temperature (PIT) of orange oil/water emulsions stabilized by nonionic surfactants. Two techniques were employed: rheology, which is already commonly used to obtain the PIT, and microcalorimetry, which has been proposed as a new technique. Continuous monitoring of the emulsions' viscosity permitted identifying different phenomena that occur while the temperature varies. For all the dispersions prepared, the rheological curves obtained showed two peaks, one attributed to the phase separation process and the other to the phase inversion phenomenon. The microcalorimetry technique showed two endothermic transitions as the dispersion's temperature increased. The initial temperatures were comparable to those obtained by rheology. The influence of the surfactant concentration and the hydrophilic-lipophilic balance (HLB) of the mixture of surfactants and the reduction in volume of the phases at the phase inversion temperature were also evaluated. In general, both methods used to evaluate the phase inversion of the orange oil/water systems (rheology and microcalorimetry) presented concordant results, both for the phase separation process and the phase inversion temperature.     

Mechanism of oil-in-water emulsification using a water-soluble amphiphilic polymer and lipophilic surfactant

A new O/W (oil-in-water) emulsification system was developed using the amphiphilic polymer HHM-HEC (hydrophobically-hydrophilically modified hydroxyethylcellulose) and a lipophilic surfactant. HHM-HEC was used as a thickener and polymeric surfactant, and the addition of small quantities of various types of nonionic lipophilic surfactant (hydrophilic-lipophilic balance <5) decreased the droplet size of several types of oil due to a lowering of the tension at the water/oil interface. The oil droplets were held by the strong network structure of the aqueous HHM-HEC solution, preserving the O/W phase without inversion. These stable O/W emulsions were prepared without the addition of hydrophilic surfactants and thus show improved water repellency.     

Composite Polymer Nanoparticles via Transitional Phase Inversion Emulsification and Polymerisation

Summary: A new route was employed to produce composite polymer nanoparticles. First, a model polymer (a low molecular-weight polyisobutene) was dissolved in a model monomer (styrene) and then the solution was emulsified in water containing a pair of nonionic surfactants via a transitional phase inversion route. After phase inversion, which produced an oil-in-water miniemulsion, polymerisation of the vinyl monomer gave composite polymer particles. Low temperature emulsification was not practical because the inverted oil-in-water emulsions reinverted to water-in-oil emulsions upon raising the temperature to the reaction temperature. Miniemulsions prepared at the reaction temperature with low monomer content in the oil phase showed good stabilty in the course of polymerisation and produced latexes with a particle size similar to the size of drops in the initial miniemulsions.     

Phase behavior of water/oil/nonionic surfactants with normal distribution of the poly(ethylene oxide) chain length

The phase behavior of systems consisting of water/n-hexane/polyethoxylated nonionic surfactants with a normal distribution of ethylene oxide (EO) chain length has been investigated. The surfactants used were octylphenol ethoxylated with eight EO units and nonylphenol ethoxylated with seven and ten EO units. The oil/water weight ratio was keep constant at 1, whereas the amount of surfactant and the temperature were variables. The pseudobinary phase diagrams were used to find out the triphasic bodies on the temperature scale, the tricritical points and the effect of electrolyte on them. The presence of electrolyte and the increase in surfactant hydrophobicity promote the phase inversion.     

FORMATION OF O/W EMULSIONS BY SURFACTANT PHASE EMULSIFICATION AND THE SOLUTION BEHAVIOR OF NONIONIC SURFACTANT SYSTEM IN THE EMULSIFICATION PROCESS

Surfactant-Phase Emulsification is a very useful method to produce oil-in-water emulsions having fine and uniform droplets. The mechanism of this emulsification method and the effect of hydrophile-lipophile balance (HLB) of the surfactants on the process of this emuisification were investigated by using phase diagrams of nonionic surfactant/hexadecane/water/1,3-butanediol four component systems.

It was shown that the process of this emulsification begins with the formation of isotropic surfactant solution, followed by formation of oil-in-surfactant clear gel emulsion, and finally by formation of oil-in-water emulsion. By using this emulsification technique, fine oil-in-water emulsions were formed without a need for adjusting of HLB.     

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.     

High internal phase emulsions: catastrophic phase inversion, stability, and triggered destabilization

We have investigated the formation, drop sizes, and stability of emulsions prepared by hand shaking in a closed vessel in which the emulsion is in contact with a single type of surface during its formation. The emulsions undergo catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) as the oil volume fraction is increased. We find that the oil volume fraction required for catastrophic inversion exhibits a linear correlation with the oil-water-solid surface contact angle. W/o high internal phase emulsions (HIPEs) prepared in this way contain water drops of diameters in the range 10-100 μm; emulsion drop size depends on the surfactant concentration and method of preparation. W/o HIPEs with large water drops show water separation but w/o HIPEs with small water drops are stable with respect to water separation for more than 100 days. The destabilization of the w/o HIPEs can be triggered by either evaporation of the oil continuous phase or by contact the emulsion with a solid surface of the "wrong" wettability.     

Non-Newtonian behavior of concentrated emulsions stabilized with globular protein in the presence of nonionic surfactant

Changes in the rheological properties of a model concentrated oil-in-water emulsion stabilized with globular protein (bovine serum albumin) upon the addition of nonionic surfactant polyoxyethylene (20) sorbitan monooleate (Tween 80) are studied. Non-Newtonian behavior is typical of the emulsions in question; moreover, they are characterized by the existence of yield stress. At stresses above the yield stress, the viscosity drops not immediately but after the intermediate Newtonian region at the flow curve. For all systems studied, the total flow curve is exhibited with the minimum Newtonian viscosity that is adequately described by the Cross formula. An increase in the Tween 80 concentration leads to a decrease in the viscosity of emulsion. Two threshold phenomena on the concentration dependences of rheological properties are revealed: at low concentration of added nonionic surfactant, the yield stress drops abruptly, whereas the viscosity lowers considerably with an increase in surfactant concentration to 1 × 10^?3 mol/l and the emulsion becomes unstable. The effects observed can be explained by the gradual displacement of high-molecular-weight stabilizer from interfacial layers and its replacement by nonionic surfactant.     

Fast transfer process of pyrene between oil-in-water miniemulsion droplets

Recently phase formation mechanisms have been estimated by using various fluorescent probes. In this report, the mixing process between internal phases of oil-in-water miniemulsions is discussed for two-dimensional color graphics data (two-dimensional fluorescence images) based on the excimer formation of pyrene as a hydrophobic fluorescent probe. Just after miniemulsion solution B (water, oil, and nonionic surfactant) was gradually added to miniemulsion A (water, oil, surfactant, and trace amount of pyrene) with gentle and careful stirring, the fluorescence spectra and the two-dimensional image of pyrene were measured. The decreasing of the excimer peak of pyrene was observed as soon as miniemulsion solution B was added. The result showed that pyrene initially located in miniemulsion droplets was smoothly diluted by the addition of miniemulsion droplets which contain only oil in the internal phase. The internal phases of miniemulsion droplets are miscible without changing the droplet diameter, and it is declared that pyrene transfers smoothly to the interface between droplets stabilized by the nonionic surfactant because the droplet diameter showed no significant difference throughout this mixing process. Received: 7 December 1999 Accepted: 11 April 2000     

Phase transitions in O/W lauryl acrylate emulsions during phase inversion,studied by light microscopy and cryo-TEM

Phase changes during the preparation of nano-emulsions containing polymerizable monomer as the oil phase, by the phase inversion temperature technique (PIT), were investigated using light microscopy, cryo-TEM and viscosity measurements. The nano-emulsions were prepared using a poly(oxyethylene) nonionic surfactant and a polymerizable acrylic monomer (lauryl acrylate) as the oil phase. Inversion of the emulsion, followed by rapid cooling, resulted in emulsions having an average droplet size as low as 25 nm. Cryo-TEM was used to observe the structures that are present above and below the phase transition temperature, and gave, for the first time, visual indication of the presence of a microemulsion and a locally ordered structure in the process. At high surfactant concentrations, the inversion–cooling process yields emulsions with unique structures, in which the oil phase is present as worm-like structures.     

Simultaneous conductivity and viscosity measurements as a technique to track emulsion inversion by the phase-inversion-temperature method

Two kinds of transitions can occur when an emulsified water-oil-ethoxylated nonionic surfactant system is cooled under constant stirring. At a water-oil ratio close to unity, a transitional inversion takes place from a water-in-oil (W/O) to an oil-in-water (O/W) morphology according to the so-called phase-inversion-temperature method. At a high water content, a multiple w/O/W emulsion changes to a simple O/W emulsion. The continuous monitoring of both the emulsion conductivity and viscosity allows the identification of several phenomena that take place during the temperature decrease. In all cases, a viscosity maximum is found on each side of the three-phase behavior temperature interval and correlates with the attainment of extremely fine emulsions, where the best compromise between a low-tension and a not-too-unstable emulsion is reached. The studied system contains Polysorbate 85, a light alkane cut oil, and a sodium chloride brine. All transitions are interpreted in the framework of the formulation-composition bidimensional map.     

Sucrose ester-based biocompatible microemulsions as vehicles for aceclofenac as a model drug: formulation approach using D-optimal mixture design

We assessed the functionality of sucrose esters (sucrose laurate, myristate, palmitate, and stearate), relatively innocuous nonionic surfactants, in formulation of biocompatible microemulsions. The putative influence of surfactant structure on the extension of microemulsion region was explored through the construction of the pseudo-ternary phase diagrams for the isopropyl myristate/sucrose ester-isopropyl alcohol/water system, using the titration method and mixture experimental approach. Minor changes in surfactant tail length strongly affected the microemulsion area boundaries. D-optimal mixture design proved to be highly applicable in detecting the microemulsion regions. Examination of conductivity, rheology, and thermal behavior of the selected sucrose laurate and sucrose myristate-based microemulsions, upon dilution with water, indicated existence of percolation threshold and suggested the phase inversion from water-in-oil to oil-in-water via a bicontinuous structure. Atomic force micrographs confirmed the suggested type of microemulsions and were valuable in further exploring their inner structure. The solubilization capacity of aceclofenac as a model drug has decreased as the water volume fraction in microemulsion increased. High surfactant concentration and the measured solubility of aceclofenac in microemulsion components suggested that the interfacial film may mostly contribute to aceclofenac solubilization.     

Structure and phase behaviour of the ternary system water, n-heptane and the nonionic surfactant Igepal® CA520

The phase behavior of the system water, n-heptane and the nonionic surfactant Igepal^® CA520 has been studied by visual inspection, high-performance liquid chromatography, polarizing microscopy and freeze-fracture electron microscopy. The phase diagram at 25?°C contains two large homogeneous microemulsion phases, an extended region of a lamellar liquid crystalline structure and some two- and three-phase regions. The oil-rich part of the phase diagram has been investigated by static and dynamic light scattering in order to examine the behavior of the collective diffusion coefficient and the scattering intensity in the presence of increasing concentrations of water, starting from the binary system of n-heptane and Igepal^® CA520. The results suggested that at a very low water content the aggregates of the microemulsion are small. With the exception of this region the structure is bicontinuous.     

Nanoemulsion formation by phase inversion emulsification: on the nature of inversion

Emulsification processes are usually characterized by the way they allow the surfactants, as well as the dispersed phase, to be incorporated into emulsions. A model cyclohexane-in-water emulsion using a pair of polyoxyethylene nonylphenyl ether surfactants, one oil-soluble and one water-soluble, was considered. Two surfactant mixing approaches consisting of mixed surfactants (agent-in-oil and agent-in-water) and segregated surfactants (agent in corresponding oil and water phases) were used to produce the model emulsion. Formation of oil-in-water nanodroplets could be only achieved if emulsification was associated with the formation of a three-phase microemulsion structure (transitional phase inversion) across the path. This occurred only if segregated surfactants were used in a process in which water was added to oil. With decreasing surfactant concentration, a point was reached below which the inversion mechanism transformed from transitional to catastrophic, leading to the formation of large droplets. The transformation was also accompanied by a shift in the evolution of the drop size. Drop size variations showed a minimum at the inversion point for the transitional phase inversion, whereas they showed a maximum for the catastrophic phase inversion. The agent-in-oil technique followed a catastrophic phase inversion mechanism and ranked second in terms of drop size.     

Relationship between rheological properties and one-step W/O/W multiple emulsion formation

Formation of a normal (not temporary) W/O/W multiple emulsion via the one-step method as a result of the simultaneous occurrence of catastrophic and transitional phase inversion processes has been recently reported. Critical features of this process include the emulsification temperature (corresponding to the ultralow surface tension point), the use of a specific nonionic surfactant blend and the surfactant blend/oil phase ratio, and the addition of the surfactant blend to the oil phase. The purpose of this study was to investigate physicochemical properties in an effort to gain a mechanistic understanding of the formation of these emulsions. Bulk, surface, and interfacial rheological properties of adsorbed nonionic surfactant (CremophorRH40 and Span80) films were investigated under conditions known to affect W/O/W emulsion formation. Bulk viscosity results demonstrated that CremophorRH40 has a higher mobility in oil compared than in water, explaining the significance of the solvent phase. In addition, the bulk viscosity profile of aqueous solutions containing CremophorRH40 indicated a phase transition at around 78 ± 2 °C, which is in agreement with cubic phase formation in the Winsor III region. The similarity in the interfacial elasticity values of CremophorRH40 and Span80 indicated that canola oil has a major effect on surface activity, showing the significance of vegetable oil. The highest interfacial shear elasticity and viscosity were observed when both surfactants were added to the oil phase, indicating the importance of the microstructural arrangement. CremophorRH40/Span80 complexes tended to desorb from the solution/solution interface with increasing temperature, indicating surfactant phase formation as is theoretically predicted in the Winsor III region. Together these interfacial and bulk rheology data demonstrate that one-step W/O/W emulsions form as a result of the simultaneous occurrence of phase-transition processes in the Winsor III region and explain the critical formulation and processing parameters necessary to achieve the formation of these normal W/O/W emulsions.     

Effect of water on foaming properties of diglycerol fatty acid ester-oil systems

We have studied the effect of added water on the nonaqueous foaming properties of diglycerol fatty acid ester nonionic surfactant systems. Diglycerol monomyristate (designated as DGM) could not foam in nonpolar oils squalane and hexadecane at normal room temperature. Nevertheless, addition of a small amount of water induces a dramatic change in foaming properties. Both the foamability and foam stability increases with the amount of added water within the studied concentration range. Phase behavior study showed that in the dilute regions there is dispersion of solid surfactant in the aforementioned oils in the DGM systems. The particle size of the dispersed solid phase was found to be several tens of microns in the water free system, and hence it tends to coagulate and precipitate. In the case of shorter alkyl chain length, diglycerol monolaurate (DGL) surfactant-oil systems, dispersion of lamellar liquid crystal (Lalpha) is observed at room temperature, and the poor foaming properties were attributed to the large particle size of the liquid crystal. In both the DGL and DGM-oil systems, we observed a tendency of the particle size to decrease with the increasing concentration of added water. At higher temperature, the solid surfactant transforms to lamellar liquid crystal phase, and foaming is improved in the DGM/squalane system. Foams are stable for several minutes. Judging from the foaming test and particle size distribution data it can be concluded that the poor foaming in the diglycerol fatty acid esters-oil systems may possibly be due to bigger particle size, which causes precipitation. Addition of water results in the dispersion of smaller particles and improves the foaming behavior.     

Oil-in-water nanoemulsions for pesticide formulations

A two-step process for formation of nanoemulsions in the system water/poly(oxyethylene) nonionic surfactant/methyl decanoate at 25 degrees C is described. First, all the components were mixed at a certain composition to prepare a microemulsion concentrate, which was rapidly subjected into a large dilution into water to generate an emulsion. Bluish transparent oil-in-water (O/W) nanoemulsions were formed only when the concentrate was located in the bicontinuous microemulsion (BC) or oil-in-water microemulsion (Wm) region. The existence of an optimum oil-to-surfactant ratio (R(os)) in the BC or Wm region indicates that both the phase behavior and the composition of the concentrate are important factors in nanoemulsion formation. To demonstrate potential applications of these systems, they were employed to formulate a water-insoluble pesticide, beta-cypermethrin (beta-CP). The nanoemulsion was compared with a commercial beta-CP microemulsion in terms of the stability of sprayed formulations.     

Assessment of Inorganic Fibre Burden in Biological Samples by Scanning Electron Microscopy – Energy Dispersive Spectroscopy

A method based on cloud point extraction (CPE) separation/preconcentration of trace cadmium as a prior step to its determination by graphite furnace atomic absorption spectrometry (GFAAS) has been developed. If the system temperature is higher than the cloud point temperature (CPT) of the nonionic surfactant of p-octyl polyethyleneglycolphenyether (Triton X-100), the complex of Cd²⁺ with 1-(2-pyridylazo)-2-naphthol (PAN) could be extracted into surfactant-rich phase. The chemical variables affecting CPE were evaluated and optimized. Under the optimum conditions, preconcentrating 10.0 mL of water samples permitted a limit of detection of 5.9 ng · L⁻¹ (3σ) for cadmium with an enhancement factor of 50 and a relative standard deviation of 2.1% (n = 11, c = 2.0 ng · mL⁻¹). The method was applied to the determination of cadmium in reference material and water samples with satisfactory results.     

Phase behavior and nano-emulsion formation by the phase inversion temperature method

Formation of oil-in-water nano-emulsions has been studied in the water/C12E4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 degrees C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (L(alpha)). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.