Shear-induced topology changes in liquid crystals of the soybean lecithin/DDAB/water system

Microstructure and Tribological Properties of Lamellar Liquid Crystals Formed by Ionic Liquids as Cosurfactants

Lamellar liquid crystals (LLCs) have been shown to have lubrication properties in many documents due to their bilayer structure. Ionic liquids are often used as additive or surfactant in LLCs. However, ionic liquids used as cosurfactants, which lead to a transition from the hexagonal liquid crystals to LLCs, are relatively rare. Herein, the microstructure of Triton X-100/C _nmimNTf₂/H₂O LLCs formed by using 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid (C _nmimNTf₂, n = 8, 12, 16) as cosurfactant has been determined by polarized light microscopy, small angle X-ray scattering, and ²H NMR technique, and their rheological and tribological properties were investigated. These LLCs show good friction-reducing and antiwear performances. The correlation between the microstructure of the LLCs and their lubricating mechanism is established. The increase of the concentration of C _nmimNTf₂ and the length of alkyl chain in the LLCs can lead to an obvious reduction in friction coefficients and wear volumes, which are attributed to the higher order of amphiphilic molecules, the thickness of the amphiphilic bilayer, and the smaller cross-sectional area of the polar head group at the hydrophilic and hydrophobic interfaces. The protective film formed by the physical adsorption of ionic liquid LLCs on the surface of friction disk pair and the tribochemical reaction has effectively promoted the lubrication effect. The good lubricating property and antiwear capability indicate their promising and potential applications in water lubrication and biological lubrication.

Self-assembly of cationic gemini surfactants, alkanediyl-bis-(dimethyldodecyl-ammonium bromide), in cyclohexane: effects of spacer length on their association into reverse lyotropic liquid crystalline or reverse vesicles

Herein, homogeneous solutions of cationic gemini surfactants, alkanediyl-α,ω-bis(dimethyldodecylammonium bromide), referred to as 12-s-12 where s = 2, 4, 6, 8, and 10, in cyclohexane have been prepared with the help of sodium hexanoate (SH) or sodium laurate (SL). These surfactants self-assembled in cyclohexane to form various aggregating structures, which were characterised by small-angle X-ray scattering (SAXS) together with polarised microscopy observations. The results showed that 12-2-12/SH, where the gemini had the shortest spacer among this series, formed an inverse micellar cubic liquid crystalline phase of the Fd3m structure. 12-s-12/SL, including s = 4, 6, and 8, which contained an adequate length spacer, formed an inverse hexagonal liquid crystalline phase packed by the cylindrical assemblies of surfactants. 12-10-12/SL, in which the gemini had a longer spacer, formed dispersed reverse vesicles (a lamellar structure). The rheological properties of liquid crystalline phases in a linear visco-elastic regime were studied. 12-2-12/SH formed a hard gel, whereas 12-s-12/SL formed soft gels. Dynamic light scattering and steady-state viscosity analyses were performed for the reverse vesicle solutions formed by 12-10-12/SL. In all these systems, the amount of added water W₀, denoting the mole ratio of water to the gemini surfactant, was demonstrated to influence the properties.

Self-assembly of quaternary ammonium gemini surfactants in cyclohexane upon reinforcement by simple counterions

The quaternary ammonium gemini surfactants 12-s-12 (s = 2, 6, and 10) can produce homogeneous cyclohexane solutions with the assistance of salts, sodium benzoate (NaBez), sodium salicylate (NaSal), or sodium 2-bromoethanesulphonate (NaBres). In these samples, 12-s-12/salt formed aggregates and their structures were assigned by SAXS measurements together with POM observations. Among the three salts, both NaBez and NaBres had similar effects on assisting aggregate formation, but NaSal favoured the generation of aggregates of 12-s-12 with lower interface curvature. For example, both 12-2-12/NaBez and 12-2-12/NaBres formed an I₂ liquid crystalline (LC) phase with an Fm3m structure, but 12-2-12/NaSal generated a H₂ LC phase. Both 12-6-12/NaBez and 12-6-12/NaBres generated a H₂ LC phase, while 12-6-12/NaSal yielded both H₂ and V₂ phases with Pn3m symmetry, both of which co-existed in solution. The special effect of NaSal was attributed to its ortho-hydroxyl in the benzene ring. This favoured the formation of intermolecular hydrogen bonds among the NaSal molecules attracted to the quaternary ammonium head of 12-s-12. The water molecules joined between the NaSal molecules to build hydrogen-bonding bridges, which further increased the size of the 12-s-12 head. This benefited the formation of aggregates with lower surface curvature. In the systems of both NaBez and NaBres, the spacer length of the gemini surfactants dominated the morphology of the formed aggregates, wherein the effect of the salt was significantly weaker. Finally, the visco-elasticity of samples with similar aggregates was measured and the rheological behaviour discussed.

12-s-12/salt formed various reverse aggregates in cyclohexane dependent upon both spacer length and salt effects.

Lyotropic Liquid Crystal Formed in Aqueous Lecithin Solutions with IPM and AEO20 Additives

The phase behavior and rheological properties of a lyotropic liquid crystal formed in the lecithin/AEO20/IPM/H2O system were investigated by means of phase diagram, polarized optical microscopy and rheology techniques at 37°C. After adding polyoxyethylene-20-oleyl ether (AEO20) and isopropyl myristate (IPM) to lecithin solutions, an isotropic liquid phase and an anisotropic liquid crystal phase were found. By analyzing the shear viscosity (η γ=0.1) and yield stress (σ 0), with increase in water content, the η γ=0.1 and σ 0 increase firstly and then decrease; with increasing IPM content, a decrease in the η γ=0.1 and σ 0 is observed. Notably, at the relatively higher water content area, the water content has little effect on the η γ=0.1 and critical stress. When the lecithin/AEO20 mass ratio is 1:2, the liquid crystal phase displays a stronger shear resistance and more stable network structure.

Structure and phase equilibria of the soybean lecithin/PEG 40 monostearate/water system

PEG stearates are extensively used as emulsifiers in many lipid-based formulations. However, the scheme of the principles of the lipid-surfactant polymer interactions are still poorly understood and need more studies. A new phase diagram of a lecithin/PEG 40 monostearate/water system at 30 °C is reported. First, we have characterized the binary PEG 40 monostearate/water system by the determination of the critical micelle concentration value and the viscous properties. Then, the ternary phase behavior and the influence of phase structure on their macroscopic properties are studied by a combination of different techniques, namely, optical microscopy, small-angle X-ray scattering, differential scanning calorimetry, and rheology. The phase behavior is complex, and some samples evolve even at long times. The single monophasic regions correspond to micellar, swollen lamellar, and lamellar gel phases. The existence of extended areas of phase coexistence (hexagonal, cubic, and lamellar liquid crystalline phases) may be a consequence of the low miscibility of S40P in the lecithin bilayer as well as of the segregation of the phospholipid polydisperse hydrophobic chains. The presence of the PEG 40 monostearate has less effect in the transformation to the cubic phase for lecithin than that found in other systems with simple glycerol-based lipids.

Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λ(hom), i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λ(hom) and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λ(hom) is decreasing with increasing shear rate in agreement with theoretical expectations and results from (2)H NMR lineshape analysis.