Effects of the number of fatty acid residues on the phase behaviors of decaglycerol fatty acid esters.

The effects of the number of fatty acid residues (n) in decaglycerol fatty acid esters, i.e., decaglycerol laurates (abbreviated to (C11)nG10), on the phase behaviors of three laurate esters, (C11)1.9G10, (C11)2.7G10, and (C11)3.4G10, were investigated. The unreacted decaglycerol remaining in each ester was removed by liquid extraction before use. (C11)1.9G10 formed hexagonal liquid crystals in aqueous solutions, while (C11)2.7G10 and (C11)3.4G10, which are more hydrophobic than (C11)1.9G10, formed lamellar liquid crystals. The cloud point in aqueous solution was measured for mixtures of these three esters. The cloud phenomenon was observed when the weight ratio of hydrophilic groups to the total surfactant (WH/WS) was around 0.6. The cloud point shifted to a markedly higher temperature, even with a slight increase in the WH/WS ratio. The solubilization abilities of (C11)nG10 for the oils m-xylene and (R)-(+)-limonene were also examined. When the WH/WS ratio was between 0.60 and 0.64, (C11)nG10 formed microemulsions and lyotropic liquid crystals in the presence of water and the oils. These self-organized structures were stable, even above 90 degrees C. It is concluded that the phase behavior of (C11)nG10 are insensitive to temperature, but strongly dependent on both the WH/WS ratio and the number of fatty acid residues (n).

Wormlike micelles and microemulsions in aqueous mixtures of sucrose esters and nonionic cosurfactants.

A study of the phase and rheological behavior of sucrose hexadecanoate (C16SE)/cosurfactant/water systems in the presence of solubilized oil, using complementary techniques such as dynamic light scattering and small angle X-ray scattering, is presented. Viscoelastic wormlike micellar solutions are found when a nonionic lipophilic cosurfactant is added to C16SE aqueous systems. Contrary to previous reports, the effect of oil solubilization on these wormlike micelles is not unique and depends on several factors. Linear alkyl chain oils that tend to solubilize in the micellar core have a disrupting effect, decreasing the relaxation time and the viscosity of the systems. This effect is larger as the molecular volume of oil increases and as the solubility of the cosurfactant in oil increases. On the other hand, oils that penetrate in the palisade layer, such as p-xylene, induce micellar growth and have a thickening effect at a given micellar composition. Thermodynamic considerations are used to explain the experimental results.

Effects of added nonionic surfactant and inorganic salt on the rheology of sugar surfactant and CTAB aqueous solutions.

The effect of adding tri(oxyethylene) dodecyl ether (C12EO3) on the phase and rheological behavior of sucrose hexadecanoate and CTAB aqueous solutions in the presence of added salt (NaBr) was investigated. Viscoelastic solutions are formed in CTAB and C16SE systems upon addition of lipophilic nonionic surfactant C12EO3. The zero-shear viscosity shows a maximum at a certain mixing fraction of C12EO3, except in the case of the aqueous CTAB/C12EO3 system in the absence of salt. The rheological properties are strongly affected by the addition of salt to the CTAB systems but they remain unaltered in the case of C16SE systems. In ionic systems, the mixing fraction of C12EO3 for the maximum viscosity depends on salt concentration.

Effect of ionic surfactants on the phase behavior and structure of sucrose ester/water/oil systems.

The phase behavior and structure of sucrose ester/water/oil systems in the presence of long-chain cosurfactant (monolaurin) and small amounts of ionic surfactants was investigated by phase study and small angle X-ray scattering. In a water/sucrose ester/monolaurin/decane system at 27 degrees C, instead of a three-phase microemulsion, lamellar liquid crystals are formed in the dilute region. Unlike other systems in the presence of alcohol as cosurfactant, the HLB composition does not change with dilution, since monolaurin adsorbs almost completely in the interface. The addition of small amounts of ionic surfactant, regardless of the counterion, increases the solubilization of water in W/O microemulsions. The solubilization on oil in O/W microemulsions is not much affected, but structuring is induced and a viscous isotropic phase is formed. At high ionic surfactant concentrations, the single-phase microemulsion disappears and liquid crystals are favored.

Phase behavior of polyglycerol didodecanoates in water.

A phase diagram of a water-polyglyceryl didodecanoate ((C11)2Gn) system was constructed as a function of polyglycerol chain length (n) at 25 degrees C. The average number of dodecanoic acid residues attached to polyglycerol is in the range of 1.6-2.3, and unlike commercial long-chain polyglycerol surfactants, unreacted polyglycerols were removed in the surfactants used. With an increase in the polyglycerol chain, the surfactant changes from lipophilic to hydrophilic, and the type of self-organized structure also changes from lamellar liquid crystals to the aqueous micellar solution phase via hexagonal liquid crystals. However, a discontinuous micellar cubic phase does not appear in the phase diagram, while it is formed in a long poly(oxyethylene)-chain nonionic surfactant system. In a dilute region, a cloud point is observed at a moderate polyglycerol chain length, n approximate to 7. The cloud temperature is dramatically increased with a slight increase in hydrophilic chain because the dehydration of the hydrophilic chain length at high temperature is low compared with that of the poly(oxyethylene) chain. In other words, the phase behavior of (C11)2Gn is not very temperature sensitive. Three-phase microemulsion is formed in a water/(C11)2.3G7.3/m-xylene system. The three-phase temperature or HLB temperature is highly dependent on the polyglycerol chain length.

Effect of Adding an Amphiphilic Solubilization Improver, Sucrose Distearate, on the Solubilization Capacity of Nonionic Microemulsions.

We have studied the effect of adding sucrose distearate (2C(18)SE) on the solubilization capacity of microemulsions formed in the water/C(12)EO(6)/n-decane system. Upon addition of 2C(18)SE to the binary water/C(12)EO(6) system, a lamellar liquid crystal region developed. This suggests that the rigidity of the surfactant layer is strengthened. The solubilization of water and n-decane in the bicontinuous microemulsions increases about three times upon replacing 10% C(12)EO(6) with 2C(18)SE; besides, the HLB temperature is not greatly affected by 2C(18)SE. On the other hand, sucrose monostearate (C(18)SE) does not have such a function. The effect of added 2C(18)SE on the solubilization capacity of the discrete droplet-type o/w or w/o microemulsions was also studied. The efficiency of the solubilization-improving effect is reduced when the system is far from the HLB temperature. Copyright 2001 Academic Press.