SOLUBILIZATION OF OIL IN DISCONTINUOUS CUBIC LIQUID CRYSTAL IN POLY(OXYETHYLENE)n OLEYL ETHER SYSTEMS

The structural change of the discontinuous cubic (I₁) phase upon addition of oil (heptane, decane, and hexadecane) is investigated by small-angle x-ray scattering. In polyoxyethylene oleyl ether (C_18.1EO_n) systems having long polyoxyethylene chain (n =19.2, 30.1, and 50.8), the I ₁ phase is formed in a wide concentration range. Only in C_18.1EO_nEO_19.2EO_n systems, the structure of the I₁ phase changes from body-centered to face centered cubic lattice upon addition of oil. According to the analysis of the change in effective cross-sectional area, the solubilization of hexadecane increases the repulsion between polyoxyethylene chains, while solubilization of heptane makes it decrease. The solubilization of decane keeps the repulsion constant. Since the repulsion hampers the curvature change due to the solubilization, the solubilization capacity is in the order, heptane>decane>hexadecane. The oil penetration and nonpenetration into the palisade layer are also discussed in term of the change in the effective volume of the lipophilic part in the surfactant molecule.     

Three-phase behavior in a mixed nonionic surfactant system

The effect of monodisperse solubilities of each surfactant in an excess oil phase on the three-phase behavior was investigated in a water/octaethyleneglycol dodecyl ether (R₁₂EO₈)/tetraethyleneglycol dodecyl ether (R₁₂EO₄)/heptane system. The mid temperature of the three-phase region is defined as the HLB temperature. The HLB temperature is largely skewed to higher temperature in a dilute region due to the difference in the distribution of each surfactant between excess oil and microemulsion (surfactant) phases forming the three-phase body. Taking account of the monodisperse solubilities, the equation for the HLB temperature was obtained on the basis of geometrical calculation of a particular three-phase triangle. The equation well describes the three-phase behavior for a mixed surfactant system in a space of compositions and temperature.In the mixed surfactant system, the monodisperse solubility of R₁₂EO₈ in oil phase forming a three-phase body is monotonously increased with the rise in temperature, whereas that of R₁₂EO₄ is first increased and then is decreased. Consequently, the sum of both solubilities does not change greatly in a wide range of temperature.     

Solubilization Enhancing Effect of A-B-Type Silicone Surfactants in Microemulsions

We report a solubilization enhancing effect of A-B-type silicone surfactants in microemulsions. The effect of added long-chain silicone surfactants, Si₂₅C₃EO_51.6 (extended length≈21.8 nm) and Si₁₄C₃EO_15.8 (extended length≈8.5 nm) on the solubilization capacity of C₁₂EO₅ (extended length≈3 nm)/water/dodecane microemulsion was investigated at the hydrophile-lipophile balance temperature at which a microemulsion (surfactant) phase containing equal weights of oil and water touches the three-phase body. The addition of silicone surfactants exhibits an enormous increase of the swelling of the middle phase primarily with an associated increase in the structural length scale of the microemulsion. The solubilization power increases with increasing x₂ (mole fraction of silicone surfactants to the total surfactant) and going through a maximum it decreases, since a lamellar liquid crystal introduces in the multiphase region at low surfactant concentrations. The solubilization capacity reaches at the maximum to an almost equal level for different x₂ values, 0.02 for Si₂₅C₃EO_51.6 and 0.09 for Si₁₄C₃EO_15.8. The solubilization power of the lamellar phase shows a similar trend with lower magnitude.     

The transition of three types of three-phase behavior in a temperature-composition space of water/C12EO6/propanol/heptane system

 The change of three-phase behavior of a water/hexaethylene-glycol dodecyl ether (C₁₂EO₆)/propanol/heptane system was studied with increasing temperature (30.0–48.9 °C). A cone-like three-phase body consisting of aqueous (W), surfactant (D_p), and oil (O) phases is formed in the composition tetrahedron at lower temperatures. The body is expanded with increasing temperature, and touches the water–C₁₂EO₆–heptane triangle (the base) at 44.8 °C. At about 45.6 °C the loci of the D_p and O phases meet and cut off at a critical double end point and the three-phase body separates into two bodies: one has a chiral shape and the other is unclosed. The unclosed body is transformed with the change of shape of the tie triangle on the base as temperature increases and leaves from the base at 48.9 °C. A closed-loop miscibility gap between middle surfactant (D^′ _p) and O phases or a cone-like three-phase body is formed inside the composition tetrahedron at higher temperatures. The roles of C₁₂EO₆ and propanol are also discussed. Received: 14 April 1998 Accepted: 15 July 1998     

An origin of a middle surfactant phase (Dp): A critical double end point in a water/polyoxyethylene alkyl ether/propanol/heptane system

 Phase behavior of water/hexaethyleneglycol dodecyl ether (C₁₂EO₆)/propanol/heptane system was investigated in a composition–temperature space (25–30 ^°C) at atmospheric pressure. A cone-like three-phase body consisting of aqueous (W), surfactant (Dp), and oil (O) phases is formed in the two-phase body of Wm (aqueous micellar phase)+O at 30.0 ^°C. With decreasing temperature the three-phase body becomes thinner and finally disappears at a critical double end point (CDEP) where the two critical end points of W and Dp phases are merged. The CDEP exists at about 26.2 ^°C (T _CDEP). The hydrophile–lipophile balance (HLB) of the mixed amphiphile changes towards lipophilic on addition of propanol. As a result, the Wm phase separates into two phases W+Dp above the T _CDEP. Further addition reduces the lipophobicity of aqueous media (or the solvophobicity of the mixed amphiphile), and the W and Dp phases are merged again. Below T _CDEP, since C₁₂EO₆ becomes much hydrophilic, the change of HLB lurks and a middle phase (Dp) cannot be observed. Received: 19 June 1997 Accepted: 20 March 1998     

Phase behavior and solubilization of microemulsion systems containing Gemini imidazoliums and their monomeric analogues

The Gemini imidazolium surfactants with a four-methylene spacer group [C_n(Bim)₂-2Br, n?=?12, 14, 16] and their corresponding monomers [C_nmimBr, n?=?12, 14, 16] were synthesized and characterized. The phase behavior and solubilization of microemulsion systems containing C_n(Bim)₂-2Br/butan-1-ol/octane/brine as well as microemulsion systems containing C_nmimBr/butan-1-ol/octane/brine were studied and compared. The C_n(Bim)₂-2Br-based microemulsion systems have greater solubilization ability than that of the corresponding mono surfactants C_nmimBr-based systems. As the carbon chain lengths of the surfactants [C_n(Bim)₂-2Br and C_nmimBr] increase, the mass fraction of the alcohol in the interfacial layer A ^S would decrease, whereas the solubilization ability (SP*) would increase. The maximum solubilization ability (SP*) of the two microemulsion systems was attained when the oil/water mass ratio (α) approaches 0.5. The solubilization ability of both microemulsion systems would increase with increasing NaCl concentrations in aqueous phase. In C_n(Bim)₂-2Br-based microemulsion systems, the alcohol is significantly more soluble in aqueous phase than in the oleic phase. And it was noted that the alcohol is more soluble in C_n(Bim)₂-2Br-based systems than in C_nmimBr-based systems in both aqueous and oleic phases.     

Phase Diagrams and Solubilization of Chlorocarbons in Chlorocarbon/Water/Anionic Surfactant/Alcohol Microemulsion Systems

The solubilization abilities of various chlorocarbons were investigated in a middle phase microemulsion system anionic surfactant sodium dodecyl sulfate (SDS) or sodium dodecyl sulfonate (AS)/n-butanol/chlorocarbon/brine with a ε-β fishlike phase diagram. The composition of the balanced interfacial layer of the microemulsion and some other parameters are calculated. The result shows that surfactant little dissolves in water and chlorocarbon phases, while alcohol mainly dissolves in water and oil phases besides in the interfacial layer. The order of the solubilization ability is dichloromethane (CH₂Cl₂) ～ carbon tetrachloride (CCl₄) > tetrachloroethylene (PCE) > o-dichloro-benzene. The solubility of the alcohol decreases with the increase in NaCl concentrations, which should be compensated by the increase in the amount of alcohol as cosolvent (Cs), so as to maintain the balanced interfacial layer. Salinity has little effect on the partition of surfactant between phases.     

SYNERGETIC EFFECT OF SUCROSE AND ETHANOL ON FORMATION OF TRIGLYCERIDE MICROEMULSIONS

The phase behavior of soybean oil, a nonionic surfactant (ethoxylated monodiglycerides) and an aqueous phase of water containing ethanol, and sucrose was investigated at 35 and 40°C. A minimum concentration of 20 wt% ethanol was required for the formation of isotropic solutions. Addition of sucrose to the aqueous phase decreased the amount of ethanol required to form these solutions. The solubilization mechanism of the oil was investigated by small angle x-ray diffraction and polarized light microscopy. A stable lamellar liquid crystalline phase was formed for a mixture of 75/25 surfactant/sucrose solution (2.5 wt% sucrose). This phase was destabilized with increased concentrations of sucrose and liquid crystalline phases having hexagonal structures were favored at 8.75 wt% sucrose. At a ratio of 55/45 wt% of surfactant/sucrose solution (9 wt% sucrose) hexagonal structures were formed and could be destabilized or destroyed by addition of ethanol. The concept of stabilization and destabilization of liquid crystalline mesophases was applied to the solubilization of triglycerides in aqueous solutions. Two microemulsion regions were identified; oil-in-water (L₁) and water-in-oil (L₂) in systems containing soybean oil, ethoxylated monodiglycerides, and 20 wt% ethanol solution. At 55/45 wt% surfactant/20 wt% ethanol solution,7.5 wt% of soybean oil was solubilized. Addition of 10, 20, and 30 wt% sucrose, at the same ratio of surfactant to ethanol solution, increased the solubility of the oil to 9, 13.5, and 18 wt% respectively. In addition, the size of the L₁ phase increased and moved to the aqueous corner of the phase diagram and the size of the L₂ phase decreased.     

Phase behavior and rheology of oil-swollen micellar cubic phase and gel emulsions in nonionic surfactant systems

We have studied the phase behavior and rheological property of the cubic phase and related gel emulsions in water/nonionic/dodecane systems. In the phase behavior study, it is pointed out that the formation of the discontinuous cubic phase (I₁) is not common in all nonionic surfactant systems; however, a cubic phase (I₁) with oil-swollen micelles or a cubic phase microemulsion is found in the water/C₁₆EO₆/dodecane system, which can solubilize large amount of oil. It was also observed that water/C₁₆EO₆/dodecane system forms stable gel emulsion. In the rheological study we have found an anomalous behavior of the I₁ phase in the water/C₁₂EO₆/dodecane and the water/C₁₆EO₆/dodecane systems. In the water/C₁₂EO₆/dodecane system, the viscoelastic nature of the I₁ phase has been observed, which is shifted to the elastic nature with the addition of dodecane, whereas, highly elastic nature was observed in the water/C₁₆EO₆/dodecane system. In both the cases shear-thinning behavior were seen. The elastic modulus, G′ and complex viscosity, |η1| of the I₁ phase increase with the dodecane concentration in the water/C₁₂EO₆/dodecane system, whereas, decreasing trend have been observed in the water/C₁₆EO₆/dodecane system. This anomalous behavior is suggested due to the nonspherical shape of micelles or polydispersity of the micelles in the water/C₁₆EO₆/dodecane system. The rheological behavior of the O/I₁ gel emulsion was also studied in both the systems.     

Origins of two surfactant phases (D and Dp′) in a temperature-composition space of water/C12EO6 / propanol / heptane system: tricritical point and critical double end point

The origins of middle surfactant phases (D and Dp′) were investigated in the composition-temperature (49–73 °C) space of a water/hexaethyleneglycol dodecyl ether (C₁₂EO₆)/propanol/heptane system at atmospheric pressure. Two types of three-phase bodies exist in the dilute C₁₂EO₆ region of the four-component composition tetrahedron at 49.0 °C. A cone-like three-phase body consisting of aqueous (W), Dp′, and oil (O) phases becomes thinner with increasing temperature, and collapses into the superimposed critical tie lines (CTL) at 51.7 °C. One end of the superimposed CTL is a critical double end point where the closed-loop coexistence curve of the Dp′ and O phases disappears. The chiral three-phase body consisting of W, D, and O phases shrinks with increasing temperature, and finally vanishes at the tricritical point near 72.5 °C. The roles of C₁₂EO₆ and propanol are discussed and the conditions for the middle surfactant phases are discussed on the basis of the whole series of three-phase behavior. Received: 9 July 1998 Accepted in revised form: 25 August 1998     

Overlapping of three-phase regions in a water/nonionic surfactant/triglyceride system

It was found that two types of three-phase regions containing surfactant phases (microemulsions) are overlapped and the four coexisting phases including excess water and oil phases appear in a three-component system of water/hexaethyleneglycol tetradecyl ether (R₁₄EO₆)/triglyceride (1,2,3-[tris(2-ethylhexanoyloxy)] propane, TEH). A schematic diagram of three- and four-phase behavior was constructed based on the real phase diagrams. One type of three-phase behavior is the same as that typically appearing over a wide range of water/oil ratios in a water/nonionic surfactant/hydrocarbon system. The other type of three-phase behavior is similar to that observed over a wide range of water/oil ratios in a system of water/nonionic surfactant/amphiphilic oil such as long-chain alcohols, fatty acids, and triglycerides. The result clearly shows how two three-phase regions interact with each other and are transferred from one to another.     

Solubilization, microstructure, and thermodynamics of fully dilutable U-type Brij microemulsion

A U-type microemulsion of Brij 96 has been characterized with respect to the change in cosurfactant, oil chain length on dilution, water solubility, and water solubilization capacity. The phase behavior of the systems has been mapped with different oils. Several techniques, viz., conductivity, optical microscopy, dilution method, absorption, and FT-IR spectroscopy, have been used for microemulsion analysis. The equilibrium within the microemulsion droplets and liquid crystals has been visualized using optical microscopy. The microemulsions have evidenced volume-induced conductance percolation in all the cosurfactants (n₂–n₆ alcohols). The energetics of transfer of alcohol from the bulk oil to the interface has been determined through dilution method. To gain insight into the microenvironment of microemulsion, two optical probes, hydrophilic (Methyl orange) and hydrophobic (Nile red), have been utilized in absorption spectroscopy. Lastly, FT-IR has been explored to observe the state and dissolution behavior of water with increasing weight fraction.     

A structural study of the mesophases of two oxyethylene alkyl ether-decane-water systems,a comparison between technical grade C12EO〈7〉 and pure C12EO6

As part of a study polyoxyethylene alkyl ethers (C _m EO _n ), water and decane, the phase diagram and the structures of the mesophases of pure C₁₂EO₆ and technical grade C₁₂EO₇ were compared. The constructed phase diagrams of the two systems show a great resemblance except for one difference: the viscous isotropic phase is only present in the C₁₂EO₆ phase diagram.The swelling behavior of the lamellar and hexagonal phases was studied with smallangle x-ray scattering. Both the lamellar and hexagonal phases showed an ideal swelling behavior and no differences between the lamellar and hexagonal phases of the two systems were detected.With freeze-fracture electron microscopy the hexagonal and lamellar phases were visualized. No differences in the textures of the lamellar phases were found, however, the micrographs of the hexagonal phases of the two systems clearly showed different textures. While in the hexagonal phase of the C₁₂EO₆ system only infinite long rods were visualized, short interrupted rods were found in the hexagonal phase of the C₁₂EO₇ system.     

Effect of Amino‐Acid‐Based Polar Oils on the Krafft Temperature and Solubilization in Ionic and Nonionic Surfactant Solutions

Abstract

The Krafft temperature and solubilization power of ionic and nonionic surfactants in aqueous solutions are strongly affected by added polar oils such as amino‐acid‐based oils (e.g., N‐acylamino acid esters, AAE), because they tend to be solubilized in the surfactant palisade layer. The Krafft temperatures of 5 wt.% sodium dodecyl sulfate (SDS)‐water and octaoxyethylene octadecyl ether (C₁₈EO₈)‐water systems largely decreases upon addition of AAE and 1‐hexanol, whereas it decreases very slightly in isopropyl myristate (IPM) and n‐dodecane. The lowering of the Krafft temperature can be explained by the same mechanism as the melting‐temperature reduction of mixing two ordinary substances. Namely, the polar oils are solubilized in the surfactant palisade layer of micelles and reduce the melting temperature of hydrated solid‐surfactant (Krafft temperature). On the other hand, non‐polar oil such as dodecane is solubilized deep inside micelles and makes an oil pool. The solubilization of non‐polar oil is enhanced by mixing surfactant with AAE due to an increase in micellar size.     

Hydrotrope‐Solubilization Action of Urea in CTAB/n‐C5H11‐OH/H2O System

Urea can enhance the aqueous solubility of surfactant CTAB (hexadecyltrimethylammonium bromide) when it shows the hydrotrope action. It will show the hydrotrope‐solubilization action when the solubilized amount of n‐C₅H₁₁OH in O/W microemulsion and that of water in W/O microemulsion are increased. The mechanism of the hydrotrope‐solubilization action of urea is the increase of the stability of W/O and O/W microemulsion and structural transition from the lamellar liquid crystalline phase to the bicontinuous structure.     

Studies on Middle-Phase Microemulsions of Green Surfactant n-Dodecyl Polyglucoside C12G1.46

The three-phases behavior in the quaternary stsem of n-dodecyl polyglucoside C12G1.46/1-butanol/cyclohexane/water has been studied at 40℃ in terms of the variables γ and δ.Increasing δ at constant γcauses a phase inversion from an oil-in-water microemulsion in contact with excess oil(winsor I or 2) to a water-in-oil microemulsion in contact with excess water (winsor Ⅱor 2)via a middle-phase microemulsion in contact with excess oil and water(winsor Ⅲor 3).By taking into account the different solubilities of alkyl polyglucoside and 1-butanol in the oil phase,the composition of the hydrophile-lipophile balanced interfacial film in the middle of the three-phase body can be calculated.The effects of different oils and aqueous media on the phase behavior and on the composition of the interfacial film and the efficiency for alkyl polyglucoside to make equal weights of water and oil to a single phase were investigated.It was found that the oil molecules with small molecular volumes can improve the solubilizing efficiency of the surfactant to form single-phase microemulsion.In inorganic salt(NaCl) and acid(HCl) solutions,less 1-butanol is needed than that in alkali(NaOH) solution to form middle-phase microemulsion.     

Phase behavior at low oil contents in systems containing anionic surfactants

The transition from liquid crystalline to microemulsion phases has been investigated by adding oil to surfactant—alcohol—brine mixtures in two systems containing anionic surfactants. At high salinities where the surfactant is preferentially soluble in oil, addition of oil first causes transition from a lamellar liquid crystal to a water-continuous isotropic phase which exhibits streaming birefringence and probably contains large, anisotropic micelles. This isotropic phase inverts to an oil-continuous microemulsion as oil content further increases. At somewhat lower salinities just below the “optimum” where the surfactant has equal solubilization capacities for oil and brine, the system passes through three three-phase regions as oil is added. In order of increasing oil content, these consist of two microemulsions in equilibrium with a lamellar liquid crystalline phase, the same two microemulsions in equilibrium with excess brine, and a microemulsion in equilibrium with excess oil and excess brine.     

Lyotropic liquid crystalline phases formed by phyosterol ethoxylates in room-temperature ionic liquids

The phase behaviors of four phytosterol ethoxylates surfactants (BPS-n, n = 5, 10, 20, and 30) with different oxyethylene units in room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), have been studied. The polarized optical microscopy and small-angle X-ray scattering techniques are used to characterize the phase structures of these binary systems at 25 °C. The structure and ordering of the liquid crystalline (LC) phases in such BPS-n/[Bmim]BF₄ systems are found to be influenced by BPS-n concentration and the temperature. Due to the bulky and rigid cholesterol group, the phytosterol ethoxylates surfactants exhibit different properties and interaction mechanism from the conventional C_nEO_m type nonionic surfactant systems. The rheological measurements indicate a highly viscoelastic nature of these lyotropic LC phases and disclose a lamellar phase characteristic with a rather strong rigidity at high surfactant concentrations. The control experiment with Brij 97(polyoxyethylene (10) oleyl ether)/[Bmim]BF₄ system and the FTIR measurements help to recognize that the solvophobic interaction combining with the hydrogen bonding are the main driving forces for the LC phases formation.     

Studies in the phase behavior of the microemulsions formed by mixed cationic and nonionic surfactants

The middle-phase behavior for the systems of cetyltrimethylammonium bromide (CTAB)/poly-ethyleneglycol-9-monododecyl ether (AEO₉)/alcohol/oil/brine and CTAB/octylphenolpolyoxyethylene-10-ether (Triton X-100)/alcohol/oil/brine have been studied with ɛ-β fishlike phase diagram method. The interfacial layer composition was determined, and some significant physicochemical parameters are derived from the hydrophilic-lipophilic balance plane equation. The effects of different alcohols, oils, temperature and inorganic salt (NaCl) on the middle-phase behavior of microemulsion formed by composite CTAB/AEO₉ systems were also investigated systematically. The effects of different factors on the phase behavior of microemulsions formed by CTAB/AEO₉ and CTAB/TX-100 systems were compared. The results suggest that the solubilization of CTAB/AEO₉ microemulsion is higher than that of CTAB/TX-100 system under the same conditions.     

Effect of Oil/Water Ratios on the Phase Behavior and the Solubilization Ability of Microemulsion Systems Containing Sodium Dodecyl Sulfate

An α–ε–β fish-like phase diagram of three phase microemulsions was proposed and used to investigate the phase behavior of the microemulsion systems sodium dodecyl sulfate (SDS)/alcohol/oil/water at various oil/water ratios. Related physicochemical properties of the microemulsion systems were calculated. As the oil/water mass ratio increases, the solubility (ε _B) of the alcohol increases, while both the mass fraction of alcohol in the interfacial layer (A ^S) and the solubilization ability (SP ^∗) decrease. The effect of oils on the properties of the microemulsion systems was investigated.