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Thermotropic phase transitions of catanionic complexes

Liquid crystalline properties of the materials in living systems are important for biological structures, functions, diseases and treatments. The combination of softness and structure determines the functionality of cell membranes. The relationship between chemical structure and the self-assembling and self-organising properties that ultimately lead to mesophase formation is the subject of examination of catanionic complexes phase transitions. Understanding the details of these processes is a potential fundamental issue that underlies biological structure and function in life science. Catanionic surfactants are formed as the result of the electrostatic interactions between oppositely charged hedagroups, they contain cationic and anionic component in equimolar ratio, without inorganic counterions. The catanionics exhibit complex thermal, polymorphic, mesomorphic and thermodynamic behaviors during heating (endothermic), and cooling processes (exothermic transitions), as a result of specific thermal molecular motions in correlation with their molecular structure. They do not have a single solid to liquid transition, but show one or more intermediate phases. These phases are called liquid crystalline, or mesomorphic phases (mesophases). Mesomorphic state is a phase lying between solid (perfect crystal) and isotropic (amorphous liquid) state. Structural and long-range orientational order is preserved (before breakdown of the crystalline lattice appears), they are anisotropic, flow as liquid and are ordered like solid ; the temperature increment leads to changes from the most ordered to the least ordered state. The gradual appearance and disappearance of different structures depends on catanionic solid crystal lattice characteristics, shape of molecule (rod‐like or disc‐like), and molecular packing properties (length, branching, unsaturation of hydrocarbon chains, polar headgroup, and counterion size). Furthermore, it depends on the competition between various molecular interactions (van der Waals, hydrophobic, strong electrostatic interactions, weak attractive forces between alkyl chains, hydration forces), level of long-range molecular order, conformation changes or steric constraints, rearrangements, or local defects and imperfections. Three most important and complementary techniques used for characterization of catanionic mesomorphic systems are: 1) POM, hot-stage polarizing optical microscopy. Full characterization requires images which show how the molecules arrange to form clusters, ensures observation of crystal transformations, transition temperatures, identification of mesophases [1, 2], discrimination between melting and crystal-crystal transitions, as well as decomposition. Best textures are formed during cooling from isotropic state (annealing of mesophase to develop good texture) ; 2) DSC, differential scanning calorimetry. Accurate determination of transition temperatures and thermodynamic quantities, but no unambiguous identification of mesophase is possible. However, this technique can give indications about mesophase type, but it is often not sensitive enough to detect mesophase-mesophase transitions, or there are difficulties in interpretation of crystal-crystal transitions (thermodynamically, most of transitions involving mesophases are of the first order) ; and 3) XRD, X-ray diffraction at smaller diffraction angles. The simplest mesophases of examined anhydrous catanionic complexes by POM are lamellar, smectic A (orthogonal molecules) and smectic C (molecules tilt an angle with the director), characterized as 1-D systems, with positional and orientational order, and layer ordering ; hexatic B phase (2-D systems) ; appropriate cooled crystal phases (3-D systems, weak interlayer forces) ; paramorphotic textures (difficult to assign) ; and hexagonal columnar discotic phase. The most common identified mesophases are: a) smectic textures - oily streaks texture, lancets, coarse mosaic, granite-like, tile, focal conic (with parabolic deffects), fan-shaped, bâtonnets, Schlieren texture, and broken focal conic texture [2, 3] ; b) crystal phases - generally different mosaic and dendritic textures, and focal conic with out-of-plane or grain boundaries [3] ; whereas that of c) hexagonal columnar phase show most often fan-like, broken focal conic, or spine-like textures [1]. Lower asymmetry in chain length and higher headgroup charge density generally induce higher transition temperature and enthalpy values, and higher number of phase transitions. Table I. The micrographs of the characteristic smectic (a) and hexagonal columnar (b) textures formed upon heating of catanionic complexes observed under crossed polarizers, and the related scheme of packing. References: [1] G. Ungar, V. Tomašić, F. Xie, and X. Zeng, Langmuir 25 (2009) 11067. [2] T. Mihelj, Z. Štefanić, and V. Tomašić, J. Therm. Anal. Calorim. 108 (2012) 1261. [3] T. Mihelj, and V. Tomašić, Colloid Surface A, submitted.

catanionic surfactant; thermotropic mesophases; smectic; hexagonal columnar

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