Lamellar Phase and Gel Phase
A liquid crystalline state (also called the "mesophase") of a substance is a state between a pure crystal (characterized by a high order of rigid molecules) and a pure liquid (characterized by rapid molecular motions of disordered molecules) [32]. There are dozens of different liquid crystalline states, all characterized by a different degree of molecular mobility and order. Liquid crystals can be produced either by heating a particular crystalline solid—called a "thermotropic liquid crystal"—or by dissolution of particular substances in a solvent—called a "lyotropic liquid crystal." Many surfactants in water form lyotropic liquid crystalline phases, such as the lamellar phase (La), the so-called gel phase (Ls or L^s), the normal or reversed hexagonal phase (H: or Hn), or one or more of the known cubic phases (Ia3d, Pn3m, Im3m). The type of phase formed depends on the chemical structure of the surfactant used and on the experimental conditions (such as concentration and temperature, or the presence of other compounds).
In the La-phase (also called the liquid-analogue [33] or liquid-disordered state [34, 35]), the surfactant molecules are arranged in bilayers, frequently extending over large distances (1 /m or more) [32]. The hydrophobic chains are rather disordered, with a lot of gauche conformations in the saturated hydrocarbon parts of the hydrophobic chains, making the bilayers fluid, characterized by fast lateral and rotational diffusions of the surfactant molecules, similar to a liquid. Comparable molecular motions are also present in the liquid-disordered state of vesicles. In the case of SUVs prepared from POPC, for example, the lateral diffusion coefficient seems to be on the order of 3-4 x 10-8 cm2/s, as determined between 5 °C and 35 °C [36]. The rotational correlation time is on the order of 10-9 to 2 x 10-8 s [37].
The L^ (or L¡g-)-phase of surfactant molecules (also called solid-analogue [33] or solid-ordered state [34, 35]) closely resembles the La-phase in the sense that the surfactant molecules are also arranged in bilayers. The viscosity is very high, however. This is a consequence of the rigidity of the individual surfactant molecules which are mostly present with all-trans conformations in the saturated hydrocarbon parts of the hydrophobic chains. The motion of the molecules is rather restricted, similar to the molecules in a crystal. To specify the relative arrangement of the lipid molecules, the gel phase may be abbreviated as L'r or Lp, depending on whether the alkyl chains are tilted (Pp) or not titled (Lp) with respect to the normal of the lipid bilayer. If it is not known whether the chains are tilted, Lp is often used as a general abbreviation.
The phase behavior of a number of phosphatidylcholines [38, 39] and a number of other lipids and lipid mixtures [38, 40] has been determined and reviewed.
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