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Polymorphism and mesomorphism of catanionics

The process of molecular self-organization into bigger, highly ordered structures is an important phenomenon, in both, everyday life and in science. Molecular self-organization is the construction of systems without guidance or management from an outside source. Electrostatic interactions between ionic hydrophilic groups, ion–dipole interactions between ionic and nonionic hydrophilic groups, steric interactions between bulky groups, van der Waals forces between hydrophobic groups, hydrogen bonding among constituents are major forces that induce molecular self-organization. The highly ordered aggregates are structural bases of life ; from nanometer scale as building blocks of biomembranes, to micrometer scale as polimers in cytoskeleton. Nowdays, medicine is fully connected to biotechnology, organic, anorganic and physical chemistry. As it is well known, supramolecular chemistry is defined as “chemistry beyond the molecule”, bearing on the organized entities of higher complexity that result from the association of two or more chemical species held together by intermolecular forces [1]. This opens opportunities for a wide range of scientific fields and industry, due to applications in the design of architecturally complex and tailored functionalized assemblies in order to be used in various applications, such as pharmaceuticals and crystal engineering. Many amphiphilic self-assemblies and interfaces, biological or synthetic-based, consist of surfactant mixtures. The strong synergism between the oppositely charged headgroups enables surfactants to self-assemble giving different arrangements both at the air/water interface and also in the bulk solution. There is a multitude of interesting structures including micelles with different sizes and shapes, bilayer vesicles and liquid-crystalline mesophases that are formed during interactions of amphiphiles, as well as ribbons, disks, and gel-like crystalline mesophases, that are used as template materials in both aqueous and/or nonaqueous domains. Biological membranes are dynamic structures, selectively permeable to ions and organic molecules, that separate the interior of cells with its chemical and biological properties, form the outside environment, protecting the cell from the outside forces. The cell membrane consists primarily of layer of amphiphilic phospholipids which spontaneously arrange, so that the hydrophobic tail regions are isolated from the surrounding polar fluid, causing the hydrophilic head regions to associate with the intracellular and extracellular faces of the resulting bilayer. The behaviour of the biological membranes can be directly compared and examined through the model membranes, i.e. catanionic surfactant bilayers. The comparison of individual phospholipid molecules and synthetic catanionic amphiphiles points to the marked similarity between their structure, i.e. the existence of hydrophilic polar groups and hydrophobic chains. Besides this, the membrane of the living cell is more complex bilayer with embedded proteins, glicolipids and carbohydrates, but still it resembles the catanionic surfactants bilayer. This similarity is seen through their lyotropic and thermotropic properties, that is liquid crystal formation induced by concentration and temperature, respectively. The geometry of vesicles as closed bilayer aggregates of flexible colloidal size allows their usage as in vitro cell models in biology and medicine, transport agents in drug and gene delivery [2] and other encapsulating devices of industrial relevance [3], or as nano-reactors for specialized chemistry [2]. Catanionic surfactants pass through different polymorphic and mesomorphic states during thermal treatment, as a result of specific molecular motions. These mesomorphic states are called liquid crystalline states, and they appear between solid, crystal state and isotropic, amorphous liquid. Most of the catanionics exhibit thermotropic behaviour, seen through various forms and textures that are detected and proven with the combination of techniques: hot-stage polarizing optical microscopy, differential scanning calorimetry and powder X-ray diffraction. The most common identified mesophases are of smectic type, as detected in alkylammonium alkylsulfates and alkylbenzenesulfonates [4], for which oily streaks, focal conic and fan-shaped textures as well as lancets occur. Alkylammonium bis(2-ethylhexyl)-sulfosuccinates are characterized as hexagonal columnar liquid crystals with focal conic textures [5]. On the other hand, some catanionic surfactants, such as alkylammonium picrates do not exhibit thermotropic behaviour, but are crystal phases with specific zig-zag blade textures that occur during thermal treatment [6]. Cholates in combination with mono, di and tri-alkylammonium halides build crystal filaments that grow only into their lenght. When heated, this compounds either show melting accompanied with degradation, or they just carbonize. These results have confirmed the fact that the appearance of different structures depends on catanionic solid crystal lattice characteristics, shape of molecule and molecular packing properties, such as length, branching, unsaturation of hydrocarbon chains, and size of polar headgroup. Moreover, it depends on the competition between various molecular interactions, level of long-range molecular order, conformation changes or steric constraints, rearrangements, or local defects and imperfections. [1] J.M. Lehn, J. Inclusion Phenom. 6 (1988) 351. [2] A. Khan, E.F. Marques, Curr. Opinion Colloid Interface Sci. 4 (2000) 402. [3] P.K. Yuet, D.Blankschtein, Langmuir 12 (1996) 3802. [4] T. Mihelj, and V. Tomašić, Colloid Surface A, submitted. [5] G. Ungar, V. Tomašić, F. Xie, and X. Zeng, Langmuir 25 (2009) 11067. [6] T. Mihelj, Z. Štefanić, and V. Tomašić, J. Therm. Anal. Calorim. 108 (2012) 1261.

thermotropic mesophases; smectic; hexagonal columnar; catanionic surfactant

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