Lamellar Phase La

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

In some cases, such as for C12E5, the lamellar phase La (or the HI phase) interferes with the loop (with the cloud point curve) and induces the so-called critical phase L3. L3 is an isotropic, often bluish phase, exhibiting a zero-variant three-phase critical point at its lowest temperature of existence. The three phases present at the critical conditions are W (water with a minute amount of amphiphile), L3, and La. The L3 phase seems to have a beneficial action on cleaning performance, maybe because of the presence of the critical point. 

Here, a is the headgroup area, I the chain length, and V the chain volume of the surfactant. For p <, spherical structures are formed, p values between and 5 lead to preferentially cylindrical structures. Finally, p> favors flat layered structures (e.g. lamellar phases La). Similar considerations also seem to be valid for BCPs. 

Addition of divalent metal ions, such as Ca to solutions of these two-chain sodium alkylbenzene sulfonates (NaLAS) can also result in the formation of a lamellar phase (La) liquid crystalline precipitate of approximate composition Ca(LAS)2. Onset of such precipitation produces a marked reduction in foamability as shown... 

Figure 11.21a Phase diagram of the ternary system Ci4DlVIAO/decane/H20 at 25 °C. Isotropic water continuous phase (Lj), nematic phase (N ), cubic phase (G), hexagonal phase (Hi), lamellar phase (La), crystals (S), other phase regions (1, 2, 3, 4, 5).

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