Lyotropic liquid crystal

Last updated October 30, 2023

Lyotropic liquid crystals result when fat-loving and water-loving chemical compounds known as amphiphiles dissolve into a solution that behaves both like a liquid and a solid crystal. This liquid crystalline mesophase includes everyday mixtures like soap and water.^[1]^[2]

To break the word down, "lyo" and "tropic" mean, respectively, "dissolve" and "change." Historically, the term was used to describe the common behavior of materials composed of amphiphilic molecules upon the addition of a solvent. Such molecules comprise a water-loving hydrophilic head-group (which may be ionic or non-ionic) attached to a water-hating, hydrophobic group.

The micro-phase segregation of two incompatible components on a nanometer scale results in different type of solvent-induced extended anisotropic^[3] arrangement, depending on the volume balances between the hydrophilic part and hydrophobic part. In turn, they generate the long-range order of the phases, with the solvent molecules filling the space around the compounds to provide fluidity to the system.^[4]

In contrast to thermotropic liquid crystals, lyotropic liquid crystals have therefore an additional degree of freedom, that is the concentration that enables them to induce a variety of different phases. As the concentration of amphiphilic molecules is increased, several different type of lyotropic liquid crystal structures occur in solution. Each of these different types has a different extent of molecular ordering within the solvent matrix, from spherical micelles to larger cylinders, aligned cylinders and even bilayered and multiwalled aggregates.^[5]

Types of lyotropic systems

Examples of amphiphilic compounds are the salts of fatty acids, phospholipids. Many simple amphiphiles are used as detergents. A mixture of soap and water is an everyday example of a lyotropic liquid crystal.

Biological structures such as fibrous proteins showings relatively long and well-defined hydrophobic and hydrophilic ‘‘blocks’’ of aminoacids can also show lyotropic liquid crystalline behaviour.^[6]

Amphiphile self-assembly

A typical amphiphilic flexible surfactant can form aggregates through a self-assembly process that results of specific interactions between the molecules of the amphiphilic mesogen and those of the non-mesogenic solvent.

In aqueous media, the driving force of the aggregation is the "hydrophobic effect". The aggregates formed by amphiphilic molecules are characterised by structures in which the hydrophilic head-groups expose their surface to aqueous solution, shielding the hydrophobic chains from contact with water.

For most lyotropic systems aggregation occurs only when the concentration of the amphiphile exceeds a critical concentration (known variously as the critical micelle concentration (CMC) or the critical aggregation concentration (CAC)).

At very low amphiphile concentration, the molecules will be dispersed randomly without any ordering. At slightly higher (but still low) concentration, above the CMC, self-assembled amphiphile aggregates exist as independent entities in equilibrium with monomeric amphiphiles in solution, but with no long ranged orientational or positional (translational) order. As a result, phases are isotropic (i.e. not liquid crystalline). These dispersions are generally referred to as 'micellar solutions', often denoted by the symbol L₁, while the constituent spherical aggregates are known as 'micelles'.

At higher concentration, the assemblies will become ordered. True lyotropic liquid crystalline phases are formed as the concentration of amphiphile in water is increased beyond the point where the micellar aggregates are forced to be disposed regularly in space. For amphiphiles that consist of a single hydrocarbon chain the concentration at which the first liquid crystalline phases are formed is typically in the range 25–30 wt%.^{[ citation needed ]}

Liquid crystalline phases and composition/temperature

The simplest liquid crystalline phase that is formed by spherical micelles is the 'micellar cubic', denoted by the symbol I₁. This is a highly viscous, optically isotropic phase in which the micelles are arranged on a cubic lattice. Prior to becoming macroscopic liquid crystals, tactoids are formed, which are liquid crystal microdomains in an isotrophic phase. At higher amphiphile concentrations the micelles fuse to form cylindrical aggregates of indefinite length, and these cylinders are arranged on a long-ranged hexagonal lattice. This lyotropic liquid crystalline phase is known as the 'hexagonal phase', or more specifically the 'normal topology' hexagonal phase and is generally denoted by the symbol H_I.

At higher concentrations of amphiphile the 'lamellar phase' is formed. This phase is denoted by the symbol L_α and can be considered the lyotropic equivalent of a smectic A mesophase.^[1] This phase consists of amphiphilic molecules arranged in bilayer sheets separated by layers of water. Each bilayer is a prototype of the arrangement of lipids in cell membranes.

For most amphiphiles that consist of a single hydrocarbon chain, one or more phases having complex architectures are formed at concentrations that are intermediate between those required to form a hexagonal phase and those that lead to the formation of a lamellar phase. Often this intermediate phase is a bicontinuous cubic phase.

Schematic showing the aggregation of amphiphiles into micelles and then into lyotropic liquid crystalline phases as a function of amphiphile concentration and of temperature. Lyotropic1.jpg — Schematic showing the aggregation of amphiphiles into micelles and then into lyotropic liquid crystalline phases as a function of amphiphile concentration and of temperature.

Increasing the amphiphile concentration beyond the point where lamellar phases are formed would lead to the formation of the inverse topology lyotropic phases, namely the inverse cubic phases, the inverse hexagonal columnar phase (columns of water encapsulated by amphiphiles, (H_II) and the inverse micellar cubic phase (a bulk liquid crystal sample with spherical water cavities). In practice inverse topology phases are more readily formed by amphiphiles that have at least two hydrocarbon chains attached to a headgroup. The most abundant phospholipids that are found in cell membranes of mammalian cells are examples of amphiphiles that readily form inverse topology lyotropic phases.

Even within the same phases, self-assembled structures are tunable by the concentration: For example, in lamellar phases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals.

The objects created by the amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rod-like, or biaxial (all three micelle axes are distinct). These anisotropic self-assembled nano-structures can then order themselves in much the same way as thermotropic liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles).

Host molecules

It is possible that specific molecules are dissolved in lyotropic mesophases, where they can be located mainly inside, outside, or at the surface of the aggregates.

Some of such molecules act as dopants, inducing specific properties to the whole phase, other ones can be considered simple guests with limited effect on the surrounding environment but possibly strong consequences on their physico-chemical properties, and some of them are used as probe to detect molecular-level properties of the whole mesophase in specific analytical techniques.^[7]

Rod-like macromolecules

The term lyotropic has also been applied to the liquid crystalline phases that are formed by certain polymeric materials, particularly those consisting of rigid rod-like macromolecules, when they are mixed with appropriate solvents.^[8] Examples are suspensions of rod-like viruses such as the tobacco mosaic virus as well as synthetic macromolecules, such as Li₂Mo₆Se₆ nanowire ^[9] or colloidal suspensions of non-spherical colloidal particles.^[10] Cellulose and cellulose derivatives form lyotropic liquid crystal phases as do nanocrystalline (nanocellulose) suspensions.^[11] Other examples include DNA and Kevlar, which dissolve in sulfuric acid to give a lyotropic phase. It is noted that in these cases the solvent acts to lower the melting point of the materials thereby enabling the liquid crystalline phases to be accessible. These liquid crystalline phases are closer in architecture to thermotropic liquid crystalline phases than to the conventional lyotropic phases. In contrast to the behaviour of amphiphilic molecules, the lyotropic behaviour of the rod-like molecules does not involve self-assembly.^{[ citation needed ]}

Disk-like macromolecules / Nanosheets

Examples of lyotropic liquid crystals can also be generated using 2D nanosheets. The most striking example of a true nematic phase has been demonstrated for many smectite clays. The issue of the existence of such a lyotropic phase was raised by Langmuir in 1938,^[12] but remained an open question for a very long time and was only confirmed recently. ^[13]^[14] With the rapid development of nanosciences, and the synthesis of many new anisotropic 2D nanoparticles, the number of such Nematic mesophase based on 2D nanosheet has increased quickly, with, for example graphene oxide colloidal suspensions. Noteworhty, a lamellar phase was even discovered, H₃Sb₃P₂O₁₄, which exhibits hyperswelling up to ~250 nm for the interlamellar distance.^[15]^[16]

Related Research Articles

Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be arranged in a crystal lattice. There are many types of LC phases, which can be distinguished by their optical properties. The contrasting textures arise due to molecules within one area of material ("domain") being oriented in the same direction but different areas having different orientations. An LC material may not always be in an LC state of matter.

A micelle or micella is an aggregate of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension. A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.

The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar substances, which maximizes hydrogen bonding between molecules of water and minimizes the area of contact between water and nonpolar molecules. In terms of thermodynamics, the hydrophobic effect is the free energy change of water surrounding a solute. A positive free energy change of the surrounding solvent indicates hydrophobicity, whereas a negative free energy change implies hydrophilicity.

A mesogen is a compound that displays liquid crystal properties. Mesogens can be described as disordered solids or ordered liquids because they arise from a unique state of matter that exhibits both solid- and liquid-like properties called the liquid crystalline state. This liquid crystalline state (LC) is called the mesophase and occurs between the crystalline solid (Cr) state and the isotropic liquid (Iso) state at distinct temperature ranges.

A biaxial nematic is a spatially homogeneous liquid crystal with three distinct optical axes. This is to be contrasted to a simple nematic, which has a single preferred axis, around which the system is rotationally symmetric. The symmetry group of a biaxial nematic is $i.e. that of a rectangular right parallelepiped, having 3 orthogonal axes and three orthogonal mirror planes. In a frame co-aligned with optical axes the second rank order parameter tensor of a biaxial nematic has the form$

<span class="mw-page-title-main">Amphiphile</span> Hydrophilic and lipophilic chemical compound

An amphiphile, or amphipath, is a chemical compound possessing both hydrophilic and lipophilic (fat-loving) properties. Such a compound is called amphiphilic or amphipathic. Amphiphilic compounds include surfactants. The phospholipid amphiphiles are the major structural component of cell membranes.

Liquid crystal polymers (LCPs) are polymers with the property of liquid crystal, usually containing aromatic rings as mesogens. Despite uncrosslinked LCPs, polymeric materials like liquid crystal elastomers (LCEs) and liquid crystal networks (LCNs) can exhibit liquid crystallinity as well. They are both crosslinked LCPs but have different cross link density. They are widely used in the digital display market. In addition, LCPs have unique properties like thermal actuation, anisotropic swelling, and soft elasticity. Therefore, they can be good actuators and sensors. One of the most famous and classical applications for LCPs is Kevlar, a strong but light fiber with wide applications, notably bulletproof vests.

Poloxamers are nonionic triblock copolymers composed of a central hydrophobic chain of polyoxypropylene flanked by two hydrophilic chains of polyoxyethylene. The word poloxamer was coined by BASF inventor, Irving Schmolka, who received the patent for these materials in 1973. Poloxamers are also known by the trade names Pluronic, Kolliphor, and Synperonic.

Micellar liquid chromatography (MLC) is a form of reversed phase liquid chromatography that uses an aqueous micellar solutions as the mobile phase.

In polymer chemistry and materials science, the term "polymer" refers to large molecules whose structure is composed of multiple repeating units. Supramolecular polymers are a new category of polymers that can potentially be used for material applications beyond the limits of conventional polymers. By definition, supramolecular polymers are polymeric arrays of monomeric units that are connected by reversible and highly directional secondary interactions–that is, non-covalent bonds. These non-covalent interactions include van der Waals interactions, hydrogen bonding, Coulomb or ionic interactions, π-π stacking, metal coordination, halogen bonding, chalcogen bonding, and host–guest interaction. The direction and strength of the interactions are precisely tuned so that the array of molecules behaves as a polymer in dilute and concentrated solution, as well as in the bulk.

Polymorphism in biophysics is the ability of lipids to aggregate in a variety of ways, giving rise to structures of different shapes, known as "phases". This can be in the form of spheres of lipid molecules (micelles), pairs of layers that face one another, a tubular arrangement (hexagonal), or various cubic phases. More complicated aggregations have also been observed, such as rhombohedral, tetragonal and orthorhombic phases.

Water-in-water (W/W) emulsion is a system that consists of droplets of water-solvated molecules in another continuous aqueous solution; both the droplet and continuous phases contain different molecules that are entirely water-soluble. As such, when two entirely aqueous solutions containing different water-soluble molecules are mixed, water droplets containing predominantly one component are dispersed in water solution containing another component. Recently, such a water-in-water emulsion was demonstrated to exist and be stable from coalescence by the separation of different types of non-amphiphilic, but water-soluble molecular interactions. These molecular interactions include hydrogen bonding, pi stacking, and salt bridging. This w/w emulsion was generated when the different water-solvated molecular functional groups get segregated in an aqueous mixture consisting of polymer and liquid crystal molecules.

<span class="mw-page-title-main">Micellar solution</span> Type of solution

A micellar solution consists of a dispersion of micelles in a solvent. Micelles consist of aggregated amphiphiles, and in a micellar solution these are in equilibrium with free, unaggregated amphiphiles. Micellar solutions form when the concentration of amphiphile exceeds the critical micellar concentration (CMC) or critical aggregation concentration - CAC, and persist until the amphiphile concentration becomes sufficiently high to form a lyotropic liquid crystal phase.

<span class="mw-page-title-main">Micellar cubic</span>

A micellar cubicphase is a lyotropic liquid crystal phase formed when the concentration of micelles dispersed in a solvent (usually water) is sufficiently high that they are forced to pack into a structure having a long-ranged positional (translational) order. For example, spherical micelles a cubic packing of a body-centered cubic lattice. Normal topology micellar cubic phases, denoted by the symbol I₁, are the first lyotropic liquid crystalline phases that are formed by type I amphiphiles. The amphiphiles' hydrocarbon tails are contained on the inside of the micelle and hence the polar-apolar interface of the aggregates has a positive mean curvature, by definition (it curves away from the polar phase). The first pure surfactant system found to exhibit three different type I (oil-in-water) micellar cubic phases was observed in the dodecaoxyethylene mono-n-dodecyl ether (C12EO12)/water system.

<span class="mw-page-title-main">Hexagonal phase</span>

A hexagonal phase of lyotropic liquid crystal is formed by some amphiphilic molecules when they are mixed with water or another polar solvent. In this phase, the amphiphile molecules are aggregated into cylindrical structures of indefinite length and these cylindrical aggregates are disposed on a hexagonal lattice, giving the phase long-range orientational order.

Janus particles are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties. This unique surface of Janus particles allows two different types of chemistry to occur on the same particle. The simplest case of a Janus particle is achieved by dividing the particle into two distinct parts, each of them either made of a different material, or bearing different functional groups. For example, a Janus particle may have one-half of its surface composed of hydrophilic groups and the other half hydrophobic groups, the particles might have two surfaces of different color, fluorescence, or magnetic properties. This gives these particles unique properties related to their asymmetric structure and/or functionalization.

The presence of ethanol can lead to the formations of non-lamellar phases also known as non-bilayer phases. Ethanol has been recognized as being an excellent solvent in an aqueous solution for inducing non-lamellar phases in phospholipids. The formation of non-lamellar phases in phospholipids is not completely understood, but it is significant that this amphiphilic molecule is capable of doing so. The formation of non-lamellar phases is significant in biomedical studies which include drug delivery, the transport of polar and non-polar ions using solvents capable of penetrating the biomembrane, increasing the elasticity of the biomembrane when it is being disrupted by unwanted substances and functioning as a channel or transporter of biomaterial.

Peptide amphiphiles (PAs) are peptide-based molecules that self-assemble into supramolecular nanostructures including; spherical micelles, twisted ribbons, and high-aspect-ratio nanofibers. A peptide amphiphile typically comprises a hydrophilic peptide sequence attached to a lipid tail, i.e. a hydrophobic alkyl chain with 10 to 16 carbons. Therefore, they can be considered a type of lipopeptide. A special type of PA, is constituted by alternating charged and neutral residues, in a repeated pattern, such as RADA16-I. The PAs were developed in the 1990s and the early 2000s and could be used in various medical areas including: nanocarriers, nanodrugs, and imaging agents. However, perhaps their main potential is in regenerative medicine to culture and deliver cells and growth factors.

Ammonium perfluorononanoate (APFN) is an anionic surfactant that in water forms liquid crystalline phases. It is the ammonium salt of perfluorononanoic acid.

In colloidal chemistry, the surfactant’s critical micelle concentration (CMC) plays a factor in Gibbs free energy of micellization. The exact concentration of the surfactants that yield the aggregates being thermodynamically soluble is the CMC. The Krafft temperature determines the solubility of the surfactants which in turn is the temperature that CMC is achieved. There are many parameters that affect the CMC. The interaction between the hydrophilic heads and the hydrophobic tails play a part, as well as the concentration of salt within the solution and surfactants.

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