The study of phase behavior and rheological properties of lyotropic liquid crystals in the LAS/AES/H2O system

Graph Theoretical Description of Phase Transitions in Complex Multiscale Phases with Supramolecular Assemblies

Phase transitions are typically quantified using order parameters, such as crystal lattice distances and radial distribution functions, which can identify subtle changes in crystalline materials or high-contrast phases with large structural differences. However, the identification of phases with high complexity, multiscale organization and of complex patterns during the structural fluctuations preceding phase transitions, which are essential for understanding the system pathways between phases, is challenging for those traditional analyses. Here, it is shown that for two model systems- thermotropic liquid crystals and a lyotropic water/surfactant mixtures-graph theoretical (GT) descriptors can successfully identify complex phases combining molecular and nanoscale levels of organization that are hard to characterize with traditional methodologies. Furthermore, the GT descriptors also reveal the pathways between the different phases. Specifically, centrality parameters and node-based fractal dimension quantify the system behavior preceding the transitions, capturing fluctuation-induced breakup of aggregates and their long-range cooperative interactions. GT parameterization can be generalized for a wide range of chemical systems and be instrumental for the growth mechanisms of complex nanostructures.

Construction and Properties of O/W Liquid Crystal Nanoemulsion

Liquid crystal emulsion is a new type of emulsion, in which the emulsifier molecules are located at the oil/water (O/W) interface and form a long-range ordered and short-range disordered lamellar liquid crystal. The lamellar liquid crystal formed by the emulsifier is similar to the skin stratum corneum lipid structure, which enables it to have a broad application prospect in the fields of cosmetics, pharmaceuticals, etc. In this work, a liquid crystal nanoemulsion was obtained by passing a liquid crystal emulsion stabilized by hydrogenated lecithin and phytosterol combination through a microfluidizer. The microstructure of the prepared liquid crystal nanoemulsion was investigated experimentally by dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering. The results have shown that the nanoemulsion inherited the liquid crystal emulsion property, namely, the long-range ordered and short-range disordered lamellar structure still existed at the oil/water interface even though they underwent extrusion, friction, and acceleration. At the same time, the underlying mechanisms of the existence of lamellar liquid crystal between the oil phase and the water phase for the nanoemulsion were explored theoretically by molecular dynamics simulations. The simulation results elucidated that the hydrogenated lecithin and phytosterol combination improved the flexibility of the bilayer structure composed of emulsifiers. The bilayers were the basic structure units of lamellar liquid crystals, and thus, the improved flexibility of bilayers provided insurance for the existence of lamellar liquid crystals with larger curvature around the oil droplets. In addition, the applicable properties of liquid crystal nanoemulsion were studied, and the results have shown that the liquid crystal nanoemulsion presented better slow-release and moisturizing properties than traditional nanoemulsions due to the existence of multilayers between oil and water phases. This work not only provides necessary information for the development and effective application of liquid crystal emulsions but also is helpful for in-depth understanding the inner properties of lamellar liquid crystal at molecular level.

Parabolic Viscosity Behavior of NaCl-Thickened Surfactant Systems upon Temperature Change

The viscosity of household care products plays an important role in pleasant delivery using consumer experience at home. A novel solution to mitigate the sharp rising of viscosities at low temperatures of detergents was proposed. By designing the formulation of the surfactant blend, formulators can achieve acceptable viscosity profiles in the temperature range encountered in daily life. The verification and modulation of formulas bearing parabolic viscosity-temperature behavior were systematically studied, including in single, binary, and ternary systems, based on the modulation of sodium ethoxylated alkyl sulfate (AES) by other anions, zwitterions, and nonions. The R ratio theory was used to have a better understanding of the molecular assembly of surfactants behind the parabolic behavior exhibited in rheology analyses. One of the key findings is that the parabolic viscosity-temperature phenomenon could be easily observed in the highly hydrated ethoxylated anionic systems like AES-based systems. For those anions lacking ethoxylation, especially sodium linear alkylbenzene sulfonate (LAS), the monotonic variation of hydration affinity with temperature led to the disappearance of parabola in the observed temperature window (>0 °C). Moreover, salinity played an important role in the hydration affinity of the polar group and the interaction between the hydrophilic headgroups. A balanced salinity should be optimized to modulate the hydration affinity in a desired range so that the parabola could be easily tuned within the target temperature region. These findings provide opportunities for the formulators in the household care industry to design products with better pourability through carefully selecting a combination of surfactants and fine-tuning their ratios to improve consumer use experience, especially in winter.

Does thermotropic liquid crystalline self-assembly control biological activity in amphiphilic amino acids? - tyrosine ILCs as a case study

Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0-3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmA_d) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Col_h), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli ΔTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.

Anti-corrosive non-aqueous DBSA/MEA lamellar liquid crystal lubrication system

HYPOTHESIS

Since lamellar liquid crystals (LLCS) could be used for lubrication, many LLCS systems have been constructed to improve lubrication performance. However, most studies focused on the LLCS of the water system, and its corrosiveness brought some limitations to its application. Therefore, it is necessary to construct a non-aqueous LLCS system with good lubrication and anti-corrosion properties to improve its applicability.

EXPERIMENTS

Anionic surfactant dodecyl benzene sulfonic acid (DBSA) was used to construct non-aqueous LLCS in different solvents, including monoethanolamine (MEA) and diethanolamine (DEA). DBSA/H₂O LLCS system was constructed for comparison. The LLCS was characterized by polarizing microscope (POM), small-angle X-ray scattering (SAXS), and rheology. Its microstructure was discussed. Meanwhile, we evaluated the lubrication and anti-corrosion performance of LLCS. Its lubrication mechanism was explained through tribology tests and X-ray photoelectron spectrometer (XPS) analysis of the wear scar surface. Its anti-corrosion mechanism was investigated by using the weightlessness method, electrochemical test method, and quantum chemical theoretical calculations.

FINDINGS

The DBSA/MEA non-aqueous LLCS system showed better lubrication performance than DBSA/DEA and DBSA/H₂O LLCS. It can adsorb on the surface of the friction pair to form a lubrication friction film, which plays a better role in reducing friction and wear. The DBSA/MEA LLCS is less corrosive to metals because it can effectively isolate oxygen and water in the air between friction pairs. Furthermore, the lone pair electrons in the 2p orbital of the N atom in the MEA molecule could coordinate with the 3d empty orbital of the Fe atom, forming a protective film on the metal surface, which plays a good anti-corrosion effect. This work not only enriched the study of non-aqueous LLCS but also expanded its potential applications in the field of lubrication.

Using Raman Spectroscopy and Molecular Dynamics to Study Conformation Changes of Sodium Lauryl Ether Sulfate Molecules

A study using both Raman spectroscopy and molecular dynamics (MD) simulations was carried out for alkyl ethoxysulfate (AES) surfactants at various concentrations in solution. Direct comparison between experiment and simulation shows that the conformational changes observed in MD are in good agreement with those obtained via Raman spectroscopy. We show that there is an increase in the relative number of trans conformations with increasing concentration and illustrate the relationship between phase structure and molecular conformation, which is often speculated but difficult to confirm. Our results open up the possibility of applying MD to other surfactants, with the aim of analyzing conformational behavior, which can typically be difficult to study experimentally using spectroscopy methods, due to large numbers of vibrational modes present in large complex molecules.

Studying the Structure of Sodium Lauryl Ether Sulfate Solutions Using Dissipative Particle Dynamics

Sodium lauryl ether sulfate (SLES) is a common anionic surfactant used in a large number of personal care products. Commercial products typically contain a distribution in the number of ethoxy groups; despite this, there is limited existing work studying the effect of the ethoxy groups on the phase formation and structure. This is particularly important for the effect the structure has on the viscosity, an important consideration for commercial products. Dissipative particle dynamics is used to simulate the full phase diagram of SLES in water, including both micellar and lyotropic liquid crystal phases. Phase transitions occur at locations which are in good agreement with experimental data, and we find that these boundaries can shift as a result of varying the number of ethoxy groups. Varying the ethoxy groups has a significant effect on the micellar shape and crystalline spacing, with a reduction leading to more nonspherical micelles and decreased periodic spacing of the hexagonal and lamellar phases. Finally, while typical commercial products contain a distribution of ethoxy groups, computational work tends to focus on simulations containing a single chain length. We show that it is valid to use monodisperse simulations to infer behavior about solutions with a polydisperse chain length, based on its mean molecular length.

Lyotropic liquid crystals for parenteral drug delivery

The necessity for long-term treatments of chronic diseases has encouraged the development of novel long-acting parenteral formulations intending to improve drug pharmacokinetics and therapeutic efficacy. Lately, one of the novel approaches has been developed based on lipid-based liquid crystals. The lyotropic liquid crystal (LLC) systems consist of amphiphilic molecules and are formed in presence of solvents with the most common types being cubic, hexagonal and lamellar mesophases. LC injectables have been recently developed based on polar lipids that spontaneously form liquid crystal nanoparticles in aqueous tissue environments to create the in-situ long-acting sustained-release depot to provide treatment efficacy over extended periods. In this manuscript, we have consolidated and summarized the various type of liquid crystals, recent formulation advancements, analytical evaluation, and therapeutic application of lyotropic liquid crystals in the field of parenteral sustained release drug delivery.

Phase Behavior and Rheological Properties of Poly(vinyl alcohol)/AES/Water System

In this work, the phase behavior of the poly(vinyl alcohol) (PVA)/alkyl ethoxysulfate (AES)/water ternary system is investigated at 25 °C. The PVA/AES/water ternary phase diagram is conducted which shows that there are two main phases corresponding to the solid phase and the hexagonal liquid crystalline phase (H) in the ternary system. Besides these two phases, a high-viscosity liquid phase (L₂) and a micellar phase (L₁) can also be found in the phase diagram, although they just occupy small areas. Polarizing optical microscopy and small-angle X-ray scattering are used to characterize the different lyotropic liquid crystal types. Moreover, the viscosity distribution and oscillation tests are also performed by means of the rheometer. High elastic modulus (G') and viscous modulus (G″) can be found in the H and the L₂ phase, whereas both moduli are low in the L₁ region. The PVA/AES/water ternary phase diagram provides a good guide for accelerating the selection of the detergent formula, whereas the rheological tests provide an application guidance for industrial operations. Beyond tis, the L₁ region is considered to be a reasonable range for slurry making because of its good fluidity and low viscoelasticity. This research enriches the content of polymer-surfactant aggregates and promotes the development of solid detergent manufacturing industry.

Dissolution of a surfactant-water lamellar phase investigated by combining time-lapse polarized light microscopy and confocal Raman spectroscopy

HYPOTHESIS

While the phase behavior of aqueous surfactant solutions is usually described in term of the equilibrium microstructures of lyotropic liquid crystals, the transformations which take place when a phase turns into another one, either by changing the concentration or the temperature, are still to be elucidated. A simultaneous determination of concentration and microstructure is at order to elucidate the phase behavior under changing conditions, such as in a dissolution experiment.

EXPERIMENTS

Confocal Raman micro-spectroscopy and time-lapse polarized light microscopy are combined to study the phase transitions taking place in the dissolution of a common anionic surfactant (sodium laurylethersulfate) in water.

FINDINGS

By comparing Raman concentration profiles and polarized light images, it is found that the aqueous solution, with initial surfactant concentration of 72 wt%, undergoes a sequence of complex microstructural transformations including distortion of the initial lamellar phase, formation of an intermediate striated texture, which can be considered as a precursor of a cubic phase, and a heterogeneous hexagonal phase going through a transition region before turning into a micellar phase. The effects of the sodium counter-ion and of water confinement are also investigated by analyzing the OH-stretching bands.

Dissolution of concentrated surfactant solutions: from microscopy imaging to rheological measurements through numerical simulations

Concentrated aqueous solutions of surfactants, often referred to as pastes, experience complex phase and rheology changes upon dissolution in water, which is a typical step in the production of liquid detergents. During the dilution process, depending on water content, surfactant molecules can arrange in different morphologies, such as lamellar or cubic and hexagonal structures. These phases are characterized by different physico-chemical properties, such as viscosity or diffusivity, which lead to non-simple transport mechanisms during the dissolution process. In this work, we investigate the dissolution of concentrated Sodium Lauryl Ether Sulfate (SLES) pastes in water under quiescent conditions by coupling different experimental techniques. A thorough rheological characterization of the system showed non-monotonic changes of several orders of magnitude in its viscosity and viscoelastic moduli as a function of water content. Time-lapse microscopy allowed us to image the dynamic evolution of the phase changes as water penetrated in a disk-shaped sample (with the same geometry used in rheological tests). Numerical simulation, based on a simple diffusion-based multi-parameter model is shown to describe satisfactorily SLES dissolution data.

Impact of Additives on the Microstructural Properties of DIDMAMS Bilayers: Studied by Molecular Dynamics Simulations

To further understand the mechanism of the impact of perfume raw materials (PRMs) such as allyl heptoate (AHT) and cashmeran (CMR) on distearoyl isopropyl dimethylammonium methyl sulfate (DIDMAMS) bilayers, 90 ns molecular dynamics simulations were conducted to investigate the structure of bilayers consisting of DIDMAMS and PRMs at 350 K on the molecular scale. Structural properties such as density profiles, order parameters, radial distribution functions (RDFs), and bilayer thickness were analyzed. The bilayers appear to be the structure of the ripple phase whether PRMs are added or not. The RDF and density profiles show that CMR molecules tend to locate in the region close to head groups and AHT molecules prefer to uniformly distribute among hydrocarbon chains. The special distribution of CMR molecules results in hydrocarbon chains twining around CMR molecules. The existence of CMR molecules between bilayers and the consequent highest bilayer thickness may be the main cause of higher viscosity. We expect that this work can help to screen stable vesicular formula and understand the relationship between microstructures of the vesicles and macroscopic fluidic 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.

Morphologic Properties, Texture Transformations and Optical Refracting Properties: Aqueous Bicomponent Amphiphilic Lyotropic Systems

The mesomorphic, morphologic and optical refracting properties of the bicomponent amphiphilic lyotropic systems have been investigated. Five amphiphiles with various concentration combinations as cationic – cationic, anionic – anionic and cationic – anionic lyotropic mixtures have been used for preparation of lyotropic systems. The mixtures were prepared with a constant (amphiphile1 + amphiphile2)/water concentration ratio. Isotropic lyotropic phase and hexagonal and lamellar lyotropic mesophases have been found in lyotropic systems under investigations. Dynamics of change of the magneto-morphologic properties vs. time, character of the heterophase regions of the mesophase – isotropic liquid phase transitions and temperature dependencies of the optical refracting properties of these bicomponent amphiphilic lyotropic systems have been investigated.

Phase Boundaries, Optical Refraction and Specific Electrical Conductivity Properties in Lyotropic Micellar L1 Phase: Bicomponent Amphiphilic (DDTMABr + HDTMABr) + Water System

In this work, complex investigations of the temperature and concentration behaviour of the optical refraction properties and specific electrical conductivity properties in lyotropic micellar L1 phase have been carried out. Bicomponent amphiphilic (DDTMABr + HDTMABr) + water lyotropic systems with various DDTMABr/HDTMABr concentration ratios have been objects of our investigations. The phase diagram for L1 phase in this system has been determined. Effect of the DDTMABr/HDTMABr concentration ratios on the refractive index and specific electrical conductivity has been observed. The mutual influence of amphiphile with short alkyl chain (DDTMABr) and long alkyl chain (HDTMABr) on the phase states, refractive index and electrical conductivity properties has been found. Our results show that variation of the concentration ratio in a mixture of amphiphiles with different length of the non-polar tail gives possibility to regulate the hydrophilicity degree, optical density, refraction properties and specific electrical conductivity of mixed lyotropic systems.

Dissolution of anionic surfactant mesophases

Linear and circular solvent penetration experiments are used to study the dissolution of anionic SLE₃S surfactant mesophases in water. We show that a lamellar (L_α) phase in contact with water will transit through a series of cubic, hexagonal, and micellar phase bands with sharp interfaces identified from their optical textures. In both linear and circular geometries, the kinetics of front propagation and eventual dissolution are well described by diffusive penetration of water, and a simple model applies to both geometries, with a different effective diffusion coefficient for water D_f as the only fitting parameter. Finally, we show a surprising variation of dissolution rates with initial surfactant concentration that can be well explained by assuming that the driving force for solvent penetration is the osmotic pressure difference between neat water and the aqueous fraction of the mesophase that is highly concentrated in surfactant counterions.