Lamellar phases rheology

The dDAVP preparations used in this study were prepared in low water contents so that the lamellar phase was formed, which in turn was injected into the rabbits. The reason for this was the fact that the lamellar phase with its mucous-like rheology is easier to inject than the stiff cubic phase. Since the lamellar phase swells into the cubic phase in excess water according to the phase diagram in Figure 1, a phase transition was expected also in the in vivo situation. The transition was found to be very fast as judged by inspection of the injection site immediately after administration. 

Shin, S., Kumar, S., Finotello, D., Keast, S., and Neubert, M., High-precision heat capacity study of phase transitions ina lyotropie liquid erystal, J. Am. Phys. 5oc., 45 8683-8692 (1992). Roux, D., Nallet, F., and Diat, O., Rheology of lyotropie lamellar phases, Europhys. Lett., 24 53-58 (1993). 

The rheology of lamellar phases has attracted considerable attention. For a quenched lamellar phase it has been observed that where G = G" both scale as to112 for to < wc, where determined operationally as being approximately equal to 0.1 r1, where r is a single-chain relaxation time defined as the frequency where G and G" cross (Bates et al. 1987 Rosedale and Bates 1990). Similar dynamic moduli scaling was found with PS-PI-PS and PS-PB-PS triblocks (here... 

Solyom and Ekwall (20) have studied rheology of the various pure liquid crystalline phases in the sodium caprylate-decanol-water system at 20 °C, for which a detailed phase diagram is available. Their experiments using a cone-and-plate viscometer show that, in general, apparent viscosity decreases with increasing shear rate (pseudo-plastic behavior). Values of apparent viscosity were a few poise for the lamellar phase (platelike micelles alternating with thin water layers), 10-20 poise for the reverse hexagonal phase (parallel cylindrical micelles with polar... 

Shear-Induced Mesostructural Changes and Rheology of the Liquid Crystalline Lamellar Phase... 

The extent of the structural transition must depend on the amount of shear exerted on the system versus the deformability of the droplet and continuous lamellar phase. Furthermore, as mentioned above, the rheological (i.e., functional) properties of a system in the a-gel state depend on parameters such as the number of droplets, their size, and how many times bilayers have folded themselves around several droplets, thus forming entanglements. Among these parameters one can identify one key... 

Figure 9.3. Mesostructural changes in a model lamellar phase induced by shear, and some rheological consequences...

In the section on lamellar phases we have shown that macroscopic rheological properties of a lamellar structure, of relevance to real foods, can be related to the type of... 

Roux, D. (1993). Rheology of lyotropic lamellar phases. Europhys. Lett., 24, 53-58. 

The presence of liquid-crystalline material at the emulsion interface has been shown by electron microscopy using the freeze-etching technique 18). Typical liquid-crystalline structures are shown in Figure 16. These liquid-crystalline compositions are viscous, and the lamellar phase displays pseudoplastic rheology. The lamellar phase is the most important of all liquid-crystalline phases for emulsion stability. The presence of a liquid-crystalline phase causes a reduction of the available London-van der Waals forces for coalescence 16). As a consequence of the reduction of the influence of these dispersion forces and the high viscosity of the liquid-crystal layer, the time for coalescence is increased dramatically. 

Bemi M, Lawrence C, Machin D. A review of the rheology of the lamellar phase in surfactant systems. Advances in Colloid and Interface Science. 2002 98(2) 217-243. 

There are a number of liquid crystalline phases formed by amphiphilic molecules, notably surfactants, polar lipids and block copolymers, including discrete and bicontinuous cubic phases, hexagonal phases (and their reversed counterparts), lamellar phases, intermediate phases, etc. A number of these phases are interesting from a drug delivery point of view. This is due to the frequently large solubilization capacity of both hydrophilic and hydrophobic substances, possibilities to control the drug release rate, favourable rheological properties, suitable water transport rates, excellent stability, etc. 

The gel phase (L ) closely resembles the lamellar phase (Figure 21.10) in that it is comprised of surfactant layers, but it differs in its very high viscosity. The term gel again originates from industry where these systems were observed to have a gel-like rheology. However, these states should not be confused with polymer gels or gels formed by hydrocolloid systems, since they are... 

This transition from a homogeneous towards a nonhomogeneous flow has been reported in complex fluids of various microstructure such as lyotropic micellar and lamellar phases [44,121,122], triblock copolymers solutions [123,124], viral suspensions [125], thermotropic liquid crystal polymers [126], electro-rheological fluids [127], soft glassy materials [128], granular materials [129,130], or foams [131-133]. 

The rheological behaviour of the lamellar phases is again determined by the structure and the dynamic properties... 

Van der Linden and co-workers have developed a theory for the rheological behaviour of multilamellar vesicles on the basis of their bending and dynamic properties, which could be suitable for an understanding of the phase transitions of lamellar phases under shear. These authors calculated the energy required for the deformation of the vesicles, which is determined both by the bending energy of the bilayer and by the interactions between such layers. With this model, they could obtain relaxation times and elastic frequencies for the different modes of deformation of the multilamellar vesicles that were of the same order of magnitude, like the shear rates at which the transformation of the lamellar phases into monodisperse multilamellar vesicles takes place. 

The NMR investigations have been complemented by studying the solvent diffusion in multilameUar vesicles in a related system. When the ionic surfactant Ci4H29lsr(CH3)3Br" (CTAB) and the cosurfactant n-C6H 30H are added to the above-mentioned C14DMAO/ H2O system vesicles and lamellar phases are formed [10]. The aim of this study was to complement the rheological measurements in these systems, where a decrease of both, the storage modulus G and the yield stress, with increasing salinity of the water had been found. 

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