Emulsification Failure Boundary Research Articles

Microemulsions supported by octyl monoglucoside and geraniol. 3. Microstructure & general pattern

This paper completes previous studies on the phase behaviour and microstructure of the quaternary microemulsion water – cyclohexane – n-octyl-β-D-glucoside (β-C8G1) – geraniol. The properties of this system are almost insensitive to temperature, while they can be tuned easily by adding an alcohol as co-surfactant. The addition of the alcohol changes the composition of the interfacial film, which, in turn, is the tuning parameter for the quaternary system. Studies of the phase behaviour together with SANS and NMR measurements allow drawing a quantitative picture of how the domain size ξ, the curvature H, and the interfacial tension σab depend on the composition of the interfacial film. The SANS data of both oil-in-water and water-in-oil droplet microemulsions recorded near the emulsification failure boundary were evaluated with the Generalised Indirect Fourier Transformation (GIFT), revealing a growth towards cylindrical structures when approaching the respective critical end point. We will show that the general behaviour of ternary and quaternary systems is equal if the appropriate tuning parameters are chosen. In addition, we will show that it is possible to apply the scaling description derived for temperature-dependent ternary systems to composition-dependent quaternary systems: plotting the reduced domain size, the reduced curvature and the reduced interfacial tension against the proper reduced tuning parameter, i.e., either the reduced temperature or the reduced interfacial composition, leads to one master curve for all systems.

Nanoparticles via Oil-in-Water Microemulsions: a Solvent-Reduced, Energy-Efficient Approach

Abstract This paper deals with the synthesis of Pt and Pt/Pb nanoparticles via oil-in-water microemulsions. Studying systematically the phase behavior of microemulsions which contain metal precursors, it was possible to determine the influence of the metal precursors on the phase behavior of the base microemulsion. It was found that the addition of Pb(ethylhexanoate)2 to the microemulsion shifts the phase boundaries to slightly lower temperatures, while adding dimethyl-(cyclooctadiene) platinum has no effect on the phase boundaries. SAXS measurements confirmed that the addition of the metal precursors does not influence the size of the microemulsion droplets along the emulsification failure boundary. Using transmission electron microscopy (TEM) we found that by increasing the oil content w B in the microemulsion from 0.04 to 0.08 the size of the resulting mono- and bimetallic Pt and Pt/Pb nanoparticles changes from 3 to 4.4 nm.

Isothermal Behavior of the Soret Effect in Nonionic Microemulsions: Size Variation by Using Different n-Alkanes

In this work we investigate the thermodiffusion behavior of microemulsion droplets of the type H2O/n-alkane/C12E5 (pentaethylene glycol monododecyl ether) using the n-alkanes: n-octane, n-decane, n-dodecane, and n-tetradecane. In order to determine the thermodiffusion behavior of these microemulsion droplets, we apply the infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS) technique. We measure the Soret coefficient (ST) as function of the structure upon approaching the emulsification failure boundary (efb) and as a function of the radius of the spherical o/w microemulsion droplets close to the efb. By varying the chain length of the n-alkanes, we are able to study the thermodiffusion behavior of spherical o/w microemulsion droplets of different sizes at the same temperature. In the investigated range a linear dependence of the Soret coefficient as function of the radius was found. By use of a proposed relationship between the Soret coefficient and the temperature dependence of the interfacial tension, the transition layer l could be determined for the first time. Additionally, small angle neutron scattering (SANS) experiments are performed to determine the size and to prove that the shape of the microemulsion droplets is spherical close to the efb. Accordingly, the scattering curves could be quantitatively described by a combination of a spherical core-shell form factor and sticky hard sphere structure factor.

Soret Coefficient in Nonionic Microemulsions: Concentration and Structure Dependence

Here we investigate the thermal diffusion behavior of the nonionic microemulsion water/n-decane/pentaethylene glycol monododecyl ether (C12E5). We study the dependence of the Soret coefficient on the structure and composition by infrared thermal diffusion Rayleigh scattering. The form and size of the microemulsion structure is characterized by dynamic light scattering and small angle neutron scattering. The system was examined in the one-phase region between the emulsification failure boundary and the near critical boundary, where oil swollen nanostructures stabilized by an amphiphilic surfactant film are dispersed in a continuous water phase. The size and shape of these structures as well as the interfacial properties of microemulsions can be varied by changing temperature and composition, which allows a systematic study of their influence on the thermal diffusion properties. In addition, we analyze the relationship between the Soret coefficient and the temperature dependence of the interfacial tension as proposed by A. Parola and R. Piazza (Eur. Phys. J. E 2004, 15, 255-263) and find reasonable agreement for spherical microemulsion droplets.

Formation mechanism of CdS nanoparticles with tunable luminescence via a non-ionic microemulsion route

We investigated the synthesis of CdS nanoparticles via an optimized water-in-oil microemulsion route that used the non-ionic surfactant-based system H2O–n-octane–Brij30/1-octanol. For that purpose, a microemulsion that contained Cd(II) ions (μe1) and another microemulsion that contained S2− ions (μe2) were combined. To investigate the ways in which the non-ionic microemulsion characteristics controlled the size and emission properties of colloidal CdS quantum dots, μe1 and μe2 with tunable and robust similar structure were prepared. This requirement was fulfilled by matching the water emulsification failure boundary (wefb) of the two microemulsions and carrying out synthesis along this boundary. Dynamic light scattering and fluorescence probe techniques were used to investigate the size and interfacial organization of the microemulsion water droplets, and the CdS nanoparticles were characterized by UV–Vis and static fluorescence spectrometry, TEM and HRTEM. Nanoparticles of diameter 4.5–5.5 nm exhibiting enhanced band edge emission were produced by increasing the water content of the precursor microemulsions. The experimental results were combined with a Monte Carlo simulation approach to demonstrate that growth via coagulation of seed nuclei represented the driving mechanism for the CdS nanoparticle formation in the water-in-oil microemulsion.

Nucleation of an Oil Phase in a Nonionic Microemulsion-Containing Chlorinated Oil upon Systematic Temperature Quench

A clear and stable nonionic model microemulsion consisting of pentaoxyethylene glycol dodecyl ether (C(12)E(5)), water, and 1-chlorotetradecane (CLTD) was prepared. This system was subjected to a systematic temperature quench (perturbation out of equilibrium) in steps of 1.0 degrees C from 20.4 to 15.3 degrees C in the unstable region of its phase diagram. The change in turbidity (for droplet volume fractions of 0.02 and 0.08) and hydrodynamic radius (R(h)) (for a droplet volume fraction of 0.02) of the system on its way to its new equilibrium was measured at each quench temperature. For small systematic temperature quenches just below the emulsification failure boundary (EFB) the turbidity decreases and remains constant indicating quick changes in the microstructures. Further lowering of temperature brings the system to the unstable region where the turbidity and light scattering increase sharply as function of time because of expulsion of excess oil from the microemulsion droplets. The newly formed oil-rich droplets grow in size as a function of time. These observations indicate the existence of a narrow but observable metastable region en route to the new equilibrium where both microemulsion droplets and larger oil-rich droplets coexist. The region in which microemulsion droplets are metastable is very narrow and is concentration-dependent. The presence of a metastable region is as for other similar systems attributed to the presence of a free energy barrier for the formation of the larger oil-rich droplets associated with curvature free energy of the surfactant film. The turbidity-time curves were converted to the radius-time curves using a model assuming monodisperse spherical droplets. The obtained results are in good agreement with the results for the hydrodynamic radius. The observed average radius from both type of measurements decreases in the metastable region. By performing calculation of the influence of eccentricity and size polydispersity on the observed radius, we have shown that the distribution of the microemulsion droplets becomes more homogeneous in the metastable region.

Formation of 10−100 nm Size-Controlled Emulsions through a Sub-PIT Cycle

We have re-examined the phase inversion temperature (PIT) emulsification process. This is a low-energy method that uses a physicochemical drive to produce very fine oil/water emulsions in the absence of high shear flows. We used the polyoxyethylene 8 cetyl ether (C(16)E(8))/hexadecane/water system, which has a PIT of 76.2 degrees C. We find that successful emulsification depends on two conditions. First, the mixture must be stirred at low speed throughout the whole process: this makes it possible to produce emulsions at surfactant concentrations that are too low to form an equilibrium microemulsion. Second, the stirred mixtures must be heated above a threshold called the clearing boundary (CB) and then quenched to lower temperatures. The clearing boundary is determined experimentally by a minimum in the turbidity of the stirred mixture, which results from solubilization of all the oil into swollen micelles. This matches the emulsification failure boundary, and it is expressed mathematically by the condition R*C(0) = 1, where R* is the radius that results from the oil/surfactant composition for monodisperse spheres and C(0) is the spontaneous spherical curvature of the surfactant. Thus, we show that such cycles do not need to cross the PIT. In fact, sub-PIT cycles and cross-PIT cycles give exactly the same result. These conditions lead to emulsions that have a narrow size distribution and a mean diameter controlled by the oil/surfactant ratio. The typical range of those diameters is 20-100 nm. Moreover, these emulsions have an excellent metastability, in contrast with emulsions made with shorter oil and surfactant molecules.

Morphological transition and emulsification failure in globular microemulsions

We consider the condensation transition of microemulsion droplets of oil which are dispersed in water in the presence of surfactant. Since a macroscopic oil phase is formed due to this transition, it is called "emulsification failure." Based on the free energy approach, we determine the transition lines between the spherical and the cylindrical droplet phases as well as the phase boundary lines of the emulsification failure. The phase diagrams are calculated by changing the physical properties of the surfactant monolayer such as the saddle-splay modulus and the spontaneous curvature. For a negative saddle-splay modulus, the spherical droplet phase coexists with the excess oil phase. In some cases, a re-entrant transition (sphere-->cylinder-->sphere) is expected to take place. For a positive saddle-splay modulus, the system undergoes a direct transition from the cylindrical droplet phase to the macroscopically phase separated state. The sphere-to-cylinder transition line approaches the emulsification failure boundary as the saddle-splay modulus becomes larger.

Phase diagrams of non-ionic microemulsions containing reducing agents and metal salts as bases for the synthesis of bimetallic nanoparticles

Phase diagrams of microemulsions containing metal salt(s) and reducing agent, respectively, were studied in detail. The microemulsions were based on non-ionic surfactants, namely pure tetraethyleneglycol monododecylether, C 12E 4, and technical grade Brij30. We studied the influence of the metal salts H 2PtCl 6, Pb(NO 3) 2, Bi(NO 3) 3, H 2PtCl 6 + Pb(NO 3) 2 (1:1 mixture), and H 2PtCl 6 + Bi(NO 3) 3 (1:1 mixture) as well as of the reducing agent NaBH 4 on the location of the phase boundaries. The focus was on the water emulsification failure boundary (wefb) where the aqueous phase forms spherical droplets. The temperature shifts of the wefb, which were caused by the presence of the salt(s), are directly related with the shift of the clouding points of the corresponding oil-free systems. The location of the wefb is affected in a complex manner by the pH (the lower the pH the higher the temperature at which the wefb occurred), the ionic strength and by specific salting-in or salting-out effects of the electrolyte ions. The desired overlap of the wefb of the microemulsions containing the metal salt(s) and the reducing agent, respectively, could be achieved by adding NaOH to the C 12E 4-based microemulsions and by titrating 1-octanol to the Brij30-based microemulsions, respectively.

Size, Shape, and Charge of Salt-Free Catanionic Microemulsion Droplets: A Small-Angle Neutron Scattering and Modeling Study

The formation and microstructure of a novel microemulsion based on a salt-free catanionic surfactant have been examined by considering the hexadecyltrimethylammonium octylsulfonate (TASo)-decane-D2O system and using small-angle neutron scattering and self-diffusion NMR. With focus on the emulsification failure boundary, o/w discrete droplets have been observed and characterized for all of the studied microemulsion range. The evaluation of the experimental data was facilitated by using structure factors of a model system composed of charged particles interacting with a screened Coulomb potential. Furthermore, a more simplified model involving a charge regulation mechanism has been employed. Both approaches support the view that the droplets are mainly spherical, fairly monodisperse, and charged. The net charge of the surfactant film is a consequence of the partial dissociation of the short-chain counterpart, owing to its higher solubility. We have further quantified how the droplet charge varies with volume fraction and, from that dependence, explained the unusual phase behavior of the TASo-water system, a seldom found coexistence of two lamellar liquid-crystalline phases in a binary system. This coexistence is quantitatively modeled in terms of a fine balance between the attractive and repulsive colloidal forces acting within the system.

Phase diagrams of microemulsions containing reducing agents and metal salts as bases for the synthesis of metallic nanoparticles

We studied the phase diagrams of microemulsions with a view to using these systems for the synthesis of metallic Pt, Pb, and Bi nanoparticles as well as of intermetallic Pt/Pb and Pt/Bi nanoparticles. The microemulsions consisted of H 2O/salt– n-decane–SDS–1-butanol. The salt was either one metal precursor (H 2PtCl 6⋅6H 2O, Pb(NO 3) 2, or Bi(NO 3) 3⋅5H 2O), a mixture of two metal precursors (H 2PtCl 6⋅6H 2O + Pb(NO 3) 2 or H 2PtCl 6⋅6H 2O + Bi(NO 3) 3⋅5H 2O), or the reducing agent (NaBH 4). In addition, other salts needed to be added in order to solubilize the metal precursors, to stabilize the reducing agent, and to adjust the ionic strength. Combining the microemulsion (μe1) that contains the metal precursor(s) with the microemulsion (μe2) that contains the reducing agent leads to metallic nanoparticles. To study systematically how the shape and size of the synthesized metallic nanoparticles depend on the size and shape of the respective microemulsion droplets, first of all one has to find those conditions under which μe1 and μe2 have the same structure. For that purpose we determined the water emulsification failure boundary ( wefb) of each microemulsion as it is at the wefb where the water droplets are known to be spherical. We found that the ionic strength ( I) of the aqueous phase as well as the hard acid and hard base properties of the ions are the key tuning parameters for the location of the wefb.

Dependence on Oil Chain‐Length of the Curvature Elastic Properties of Nonionic Surfactant Films: Droplet Growth from Spheres to a Bicontinuous Network

Using NMR self‐diffusion and 2H‐NMR relaxation experiments, we have investigated growth of oil‐in‐water microemulsion droplets when spontaneous curvature is lowered, starting at emulsification failure boundary. We compare two ternary nonionic microemulsion systems, containing penta‐ethylene glycol dodecyl ether (C12E5), and decane or hexadecane, at same surfactant‐to‐oil ratio and approximately the same spontaneous curvature. Droplet growth in microemulsions appears in general to be only minor. Quantitative differences between the two systems indicate differences in the curvature elastic constants.

A SANS Contrast Variation Study of Microemulsion Droplet Growth

An extensive small angle neutron scattering study is presented using contrast variation on the growth of nonionic microemulsion droplets as the temperature and thus spontaneous curvature is varied away from the emulsification failure boundary, EFB, toward zero spontaneous curvature. Two ternary systems are compared. They only differ by the chain length of the oil (decane and hexadecane, respectively). Droplets grow in size as one moves away from the EFB. SANS data from ten different contrasts were fitted simultaneously with a model of polydisperse prolate shaped droplets interacting with an effectively hard sphere potential, and the data set further included three different droplet concentrations and four different temperatures. The model of prolates provided a good description of the data. The prolate axial ratio, obtained from the fits, increased with increasing temperature but showed only a minor variation with the droplet concentration. The two systems with different oils behave quantitatively different. In both systems, however, the droplet growth is only minor, with maximum axial ratios of about 3-4, before a bicontinuous microemulsion is formed.

Depletion Interactions in Model Microemulsions

The effects of temperature changes and polymer addition on the behavior of droplet microemulsions of nonionic surfactant, water, and decane are reported and analyzed within polymer depletion theory. Dilution viscometry and dynamic light scattering were used to confirm that these microemulsions behave essentially as hard-sphere dispersions, providing us with an ideal reference system. Addition of poly(ethylene glycol) (PEG) lowers the emulsification failure boundary, where excess oil is expelled, which can be qualitatively understood by an analysis of the available volume for the polymer. Sufficient addition of PEG causes a fluid-fluid phase separation in qualitative accord with experiments on mixtures of rigid colloidal hard spheres and nonadsorbing polymer. Addition of PEG or raising the temperature causes the collective diffusion coefficient D(C) to decrease. From theory, the initial linear slope of D(C) versus droplet concentration can be used to discriminate between attractions and repulsions. The measured D(C) data for the droplets in the presence of PEG are modeled using the Asakura-Oosawa theory of depletion. Fitting the theory to the measured D(C) data permits for extracting the only unknown parameter, the polymer radius of gyration. Quantitative agreement is found with literature data, demonstrating that polymer depletion occurs in the system and that the Asakura-Oosawa theory provides a faithful description of the phenomenon.

Effect of Pressure on Microstructure of C12E5/n-Octane-in-D2O Microemulsions

Forced Rayleigh scattering was used to measure the self-diffusion coefficients of oil droplets dispersed in the water-continuous L1 phase of the C12E5/n-octane/D2O microemulsion. A single microemulsion composition of 3.7 wt % surfactant, 4.3 wt % alkane, and 92.0 wt % water was studied at atmospheric pressure as a function of temperature from 17.3 to 24.5 °C and at 26.2 °C as a function of pressure from 100 to 534 bar. Droplet self-diffusion coefficients were found to decrease by a factor of ∼2 with increasing temperature from the emulsification failure boundary to the phase boundary for this L1 phase and the lamellar phase. This decrease is attributed to a transition from spherical to larger nonspherical oil droplets in water, i.e., a decrease in the spontaneous curvature of the oil/water interface with increasing temperature. The effect of increasing pressure, like decreasing temperature, in this region of the phase diagram is to increase the oil droplet self-diffusion coefficients by a factor of ∼2−3 b...

Excess of Solubilization of Lindane in Nonionic Surfactant Micelles and Microemulsions

The solubilization oflindane by nonionic polyoxyethylene glycol surfactants has been studied in micellar solution, as well as in Winsor I and Winsor III microemulsions. The maximum of solubilization was determined when a saturated lindane solution was in equilibrium with solid lindane. At this saturation point, the interfacial molar ratio λ max of the solute to the surfactant was determined, taking into account the amount of lindane solubilized in oil. We first studied the evolution of the maximum excess solubilization λ max as a function of surfactant volume fraction in water-C 8 E 6 binary solutions. The excess solubilization was found to be constant between 0 and 50% in surfactant volume fraction. For higher volume fractions, λ max increased with the surfactant concentration. This behavior is the result of a competition between water and lindane to be solubilized by the surfactant film. We also observed a decrease of the clouding temperature for the water-C 8 E 6 system when lindane is solubilized. We have evidenced a linear decreasing of λ max with the reduced temperature (T L C - T)/T C L , where T C L is the cloud temperature for the saturated system. By comparison with results obtained with ternary microemulsions for two different curvatures, we have observed a strong dependence of i max on the average curvature of the surfactant film. The excess solubilization is maximum for the bicontinuous balanced structure obtained in the Winsor III case, where fluctuations produce the most stable structure versus the amount of added solute located in the surfactant film. When only excess oil is present (Winsor I), the o/w droplets at the emulsification failure boundary show an intermediate value of the maximum solubilization power.

Surfactant/Water/Oil System with Weakly Charged Films: Dependence on Charge Density

The effects on the microstructure and phase equilibria of a nonionic surfactant/water/oil system when replacing one, four, and six per hundred of the nonionic surfactant molecules by the ionic surfactant sodium dodecyl sulfate (SDS) have been studied. The nonionic surfactant system consists of pentaethylene glycol dodecyl ether (C12E5), water, and decane, at a constant surfactant to oil weight ratio of 51.9/48.1. The phase equilibria and the corresponding microstructures were investigated as a function of temperature and water content. As a function of temperature, the initial phase sequence for this composition range was L1 phase−lamellar phase−L3 phase. Three effects on phase behavior are observed when introducing electrostatic interactions. (i) Water rich reverse hexagonal (H2) and reverse micellar (L2) phases replace the two-phase region water + L3 phase. (ii) The crystallization of micellar cubic (I1) and normal hexagonal phases moves to lower particle concentrations. (iii) Phase boundaries at higher water contents are strongly shifted to higher temperatures. The latter effect is analyzed in detail at the emulsification failure boundary where spherical oil-swollen micelles are in equilibrium with excess oil. Using the bending energy approach together with Poisson−Boltzmann calculations within the cell model, we have analyzed the charge dependence of the emulsification failure boundary. The SDS effects can be reproduced with a cationic surfactant. We also show that the electrostatic effects are removed upon addition of salt. The microstructure in the various phases was studied by small angle X-ray scattering (SAXS). A neutral surface located at the polar/apolar interface is identified where the area per surfactant molecule is constant irrespective of the curvature.

Structures and Emulsification Failure in the Microemulsion Phase in the Didodecyldimethylammonium Sulfate/Hydrocarbon/Water System. A Self-Diffusion NMR Study

The microemulsion phase in the didodecyldimethylammonium sulfate (DDAS)hydrocarbon/water system is investigated. This phase extends from the water corner in the ternary phase diagram. The extension of the phase for various oils is determined, and it is found that the phase is not formed for straight chain hydrocarbons having less than 10 carbons. Neither is it formed with benzene or toluene as oils. At higher oil contents the phase is in equilibrium with almost pure oil, and the boundary to this two-phase region is a straight line which connects the water apex and the oil-surfactant line. It is argued that this line is an emulsification failure boundary. As a consequence, the micellar aggregates are suggested to be spherical in shape and constant in size along this boundary. NMR diffusion studies along this line support this notion. At higher surfactan/oil ratios in the microemulsion the structure is different and there is a growth in the micellar size upon increasing the oil-surfactant concentration. It is pointed out that all the aggregates formed in the DDAS/hydrocarbon/water system have aggregate curvature toward the oil medium. This is contrary to what is observed in the corresponding systems with bromine as counterion. Possible reasons for this difference are discussed

Globular and bicontinuous phases of nonionic surfactant films

Nonionic surfactants of the alkyloligoethylene oxide type form, with water and oil, a range of isotropic a liquid crystalline phases. We analyse the phase behaviour using the flexible surface model and argue that the strong temperature dependence is caused by the fact that the monolayer spontaneous curvature decreases strongly with increasing temperature. This is exemplified with the behaviour of bicontinuous microemulsions, showing a symmetric behaviour around the balanced state, globular microemulsions, behaving as hard spheres near the emulsification failure boundary, and sponge phases appearing when the monolayer spontaneous mean curvature is towards the abundant solvent. It is argued that there is a hierarchy of free energy contributions determining the preferred aggregate shape/phase. With a given oil-water ratio and a surfactant concentration that fixes the polar/apolar interfacial area, the most important free energy contribution comes from having a mean curvature close to the spontaneous curvature. The Gaussian curvature and the entropy terms become important when selecting between structures of similar mean curvature. At higher concentrations, surface forces and higher order elastic terms become significant.

Emulsification failure in a ternary microemulsion

The microstructure of the water rich microemulsion phase of the C 12 E 5 , water and decane system has been studied using surfactant 2 H-NMR relaxation and 1 H Fourier transform pulsed gradient spin-echo (FTPGSE) self-diffusion experiments. The surfactant-to-oil ratio is kept constant, and the system is investigated as a function of water dilution and temperature. Particular attention is focused at the phase boundary where the microemulsion, consisting of normal oil swollen micelles is in equilibrium with excess oil. On this phase boundary, which occurs at lower temperatures, the oil-swollen micelles adopt a minimum size, and it is argued that this corresponds to a spherical shape. Increasing the temperature the micelles grow in size. However, the results indicate that the micellar growth is only minor, in particular at lower concentrations. The implications of the experimental results and the phase equilibria are discussed within the frame work of the flexible surface model, associating a curvature energy to the surfactant film. The behaviour of the system is consistent with a monotonic variation of the spontaneous mean curvature of the surfactant monolayer with temperature. The phase equilibrium with excess oil can be identified with a so-called emulsification failure. Over a large dilution range, Φ≤0.35, the emulsification failure boundary occurs at constant temperature and surfactant-to-oil ratio, which, within the model, imposes a lower limit to the possible values of the monolayer bending rigidity