Hironobu Kunieda

Conditions to produce so-called microemulsions: Factors to increase the mutual solubility of oil and water by solubilizer

Abstract It was confirmed from the studies of phase diagrams that Schulmans so-called micromulsion is not an emulsion, but a solubilized solution. Hence, a microemulsion is not an adequate term for such system, but a swollen micellar solution may be adequate. A dispersed system containing microdroplets, which is not thermodynamically stable, may be called a microemulsion. It is desirable to find conditions to produce so-called microemulsions in which the solution is stable and solubilization is so large that oil and water mix over wide composition range. As much as about 20–25 wt% of surfactant was necessary to produce Schulmans so-called microemulsions. The following conditions were found in order to produce microemulsions with a far less amount, about 5–10 wt%, of solubilizer: (a) Optimum hydrophile-lipophile balance (HLB) or phase inversion temperature (PIT) of a surfactant. (b) Optimum mixing ratio of surfactants (solubilizer), i.e., optimum HLB (or PIT) of the mixture. (c) Optimum temperature for a given nonionic solubilizer. (d) The closer the HLBs of two surfactants, the larger the solubilization. (e) The larger the size of solubilizer the more efficient the solubilization. (f) Mixtures of ionic and nonionic surfactants which are durable to temperature change.

Evaluation of the hydrophile-lipophile balance (HLB) of nonionic surfactants. I. Multisurfactant systems

Since the hydrophile-lipophile property of nonionic surfactant for a given system is just balanced in a three-phase region of a phase diagram which is a stack of three-phase triangles consisting of an aqueous, a surfactant, and an oil phase, we defined the hydrophile-lipophile balance (HLB) plane on which the three-phase triangle is positioned in the midst of the three-phase region. In a three-component system of nonionic surfactant/water/oil, the HLB plane is located at fixed temperature (THLB). Accordingly, the equation of the HLB plane is represented by T = THLB in this system, and, hence, the phase inversion temperature of emulsion (PIT) is independent of a surfactant concentration. Correlation between THLB and the Griffins HLB number was investigated and a linear relationship between them was obtained. In a multisurfactant system, a set of the HLB planes exists in a temperature range between THLB of the most hydrophilic surfactant and that of the most lipophilic surfactant. The equation of the plane is obtained by a geometrical calculation in a space of temperature and compositions, assuming that the equilibrium concentration of surfactant in the aqueous phase is negligible. The effect of the temperature, the oil/water ratio, the weight ratio between the surfactants, and the surfactant concentration on the phase behavior of a mixed surfactant system is explained very well by the equation. The correlation between the THLB of the surfactant mixture and Griffins HLB number is also substantiated.

PHASE BEHAVIOR IN SYSTEMS OF NONIONIC SURFACTANT/ WATER/ OIL AROUND THE HYDROPHILE-LIPOPHILE-BALANCE-TEMPERATURE (HLB-TEMPERATURE)

ABSTRACT The phase diagram of the C12H25(OCH2CH2)5OH/water/tetradecane system was studied around the critical solution temperatures of surfactant-water and surfactant-oil phases. Although the phase behavior is very complicated due to the formation of liquid crystalline phase, basic phase-changes around the three-phase region, consisted of a water, a surfactant and an oil phases, are the same as those in a short-chain nonionic surfactant system.

Solution behavior and hydrophile-lipophile balance temperature in the aerosol OT-isooctane-brine system: Correlation between microemulsions and ultralow interfacial tensions

Abstract It becomes evident from studies of the solution behavior of Aerosol OT-oil-water containing salt that there exists a temperature at which the hydrophile-lipophile balance (HLB temperature) of adsorbed surfactant monolayer just balances toward a given oil-water system in a solution of a balanced ionic surfactant as well as nonionics. Interestingly enough, however, the effect of temperature reverses. From the studies of phase volumes of water (or oil) and surfactant as a function of temperature at various concentrations of Aerosol OT, it is clear that a phenomenon similar to a critical solution occurring between water and surfactant as well as oil and surfactant and ultralow interfacial tension in the HLB temperature range can be explained by this critical phenomenon. The HLB temperature is a function of the type of oil, the concentration of salts, etc.

Solution behavior of aerosol ot/water/oil system

Abstract The phase diagrams of water-cyclohexane containing 5, 10, and 20 wt% sodium 1,2-bis(2-ethylhexyloxycarbonyl)-1-ethanesulfonate (Aerosol OT, AOT) as a function of temperature were studied. There is the water phase in which AOT is dissolved and a large amount of oil is solubilized at higher temperature, and the oil phase in which AOT is dissolved and a large amount of water is solubilized at lower temperature. It is evident from the phase behavior that the hydrophile-lipophile property of AOT is well balanced. Accordingly, the phase diagram and properties of AOT solution are affected rather sensitively by the addition of a small amount of hydrophilic or lipophilic additive or by temperature. Careful elimination of inorganic salts also influences markedly the solution properties of AOT. Thus, the addition or removal of known amounts of inorganic salts, such as Na2SO4 is also very important to control the solution properties related to practical applications. In this context phase diagrams of both carefully purified and commercial AOT with or without additives have been determined.

The formation of gel-emulsions in a water/nonionic surfactant/oil system

Abstract A highly viscous and translucent gel, which forms in a very diluted water-rich region of a water/nonionic surfactant/oil system, was investigated by phase study and microscopic observation. The gel consists of two isotropic phases; one phase is almost pure water and the other is an oil phase solubilizing a large amount of water. Therefore, the gel stabilized by a lipophilic nonionic surfactant is essentially a high-internal-phase-volume W/O emulsion. In the process of gel formation, an unstable O/W-type emulsion is produced at the beginning due to the presence of the large amount of water, although the surfactant is lipophilic. Simultaneously, W/O emulsification occurs inside small oil droplets. Finally, all water is taken up into the oil phase and the gel is obtained. Hence, the size of water droplets in the gel is considerably smaller (submicrometer order) than in an ordinary W/O emulsion of the same system.

The three-phase behavior of a brine/ionic surfactant/nonionic surfactant/oil system: evaluation of the hydrophile―lipophile balance (HLB) of ionic surfactant

Abstract We define the Hydrophile-Lipophile-Balanced plane (HLB plane) as the plane on which the three-phase triangle in the midst of the three-phase region (consisting of water, surfactant, and oil phases) is positioned. The equation of the HLB plane is obtained in the composition tetrahedron of a pseudo-four-component system (the components being brine/ionic surfactant/nonionic surfactant/oil), assuming that the CMC in the aqueous phase is negligible. The effect of the surfactant/cosurfactant ratio, the oil/water ratio, and the surfactant concentration on the three-phase behavior is well predicted by this equation. The effect of salinity and temperature on the three-phase behavior is also discussed. Thus it is shown that a temperature-insensitive microemulsion (surfactant phase) is achieved in a dilute region of ionic-nonionic surfactant pair, by depressing the solubility of the lipophilic surfactant in the oil phase of the three-phase region. Moreover, the equation yields an HLB number of ionic surfactant which is considerably smaller than Griffins value. It is considered that Griffins overestimation comes from neglecting the solubility of lipophilic surfactant in the oil phase.

Phase and rheological behaviour of viscoelastic wormlike micellar solutions formed in mixed nonionic surfactant systems

The phase and rheological behaviour of mixed nonionic surfactant system – polyoxyethylene cholesteryl ether (ChEO15)–alkanoyl-N-methylethanolamide (NMEA-n)–water was studied at 25 °C. With addition of NMEA to the ChEO15–water binary system, a micellar (Wm)–lamellar (Lα) phase transformation takes place at low ChEO15 concentration. Within the Wm phase region of the ChEO15–NMEA-12–water system, there exists a high-viscosity region consisting of a viscoelastic micellar solution of entangled wormlike micelles, as suggested by rheological measurements. These solutions obey the Maxwell model of a viscoelastic fluid. Substitution of NMEA-12 by its analogue with a longer hydrophobe (NMEA-16) in the mixed surfactant system increases the extent of micellar growth, however, a phase separation occurs before a highly viscoelastic micellar system is formed. This may be attributed to the increase in packing constraints in the lipophilic core of the micelle that favours unidimensional growth.

Catanionic surfactants: microemulsion formation and solubilization

Abstract We review and summarize the three-phase behavior and solubilization of microemulsions with catanionic surfactants. Particular emphasis is placed to the three-phase behavior of mixtures of oil, water and alcohol with mixed surfactants containing one anionic and one cationic surfactant. The effect of salt and catanionic surfactant on the HLB composition and solubilizing capacity of surfactants to form microemulsions is discussed.

Novel preparation methods for highly concentrated water-in-oil emulsions

Abstract Kinetically stable water-in-oil (W/O) high-internal-phase-volume-ratio emulsions with gel-like appearance have been formed in water/hydrogenated non-ionic surfactant/oil systems. Their visual aspect varies from transparent to translucent or white depending on composition variables and temperature. Systematic studies undertaken to characterize these emulsions, referred to as gel emulsions, have revealed that they form above the hydrophilic-lipophilic balance (HLB) temperature of the corresponding ternary system. They consist of two isotropic liquid phases; the dispersed phase is composed of aqueous droplets and the continuous phase is a W/O microemulsion. These emulsions can be prepared by gradual addition of the internal phase to the external phase while stirring, the most common method for preparing highly concentrated emulsions. In the ternary systems water/non-ionic surfactant/hydrocarbon we found two new procedures for preparing gel emulsions. (a) Mixing of the three components, at their final composition, with vigorous stirring, can lead, at the appropriate temperature, to gel emulsion formation. (b) Increasing the temperature of an isotropic phase, the composition of which is that of the final emulsion. These methods of preparation have been rationalized in terms of the evolution of system properties during the process.