Gel phases structure

A DPPC bilayer consists of amphiphilic DPPC molecules having a hydrophilic head group and a hydrophobic tail group. They form a bilayer in an aqueous solution with the head groups next to water and the tail groups inside the membrane (Fig. 18.6b). The DPPC bilayer takes a stable gel-phase structure at room temperature. Thus, it was possible to resolve individual molecules periodically arranged with a spacing of about 0.5 nm, as shown in Fig. 18.6c. The result demonstrated that subnanometer-resolution... 

The biophysical study of phosphodistearylglycerol (206) fragment of glycosylphosphatidylinositols, revealed the unprecedented ciystalline two-dimensional structure of monolayers. These monolayers were characterised by two commensurate lattices the oblique lattice of the allq l chains and the molecular lattice formed owing to highly ordered head groups. This structure was reminiscent of sub-gel phase structures... 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Small temperature variations (+5°C) can change the phase structure of the paste dramatically, resulting in undesired rheological structures like gels. Good temperature control is therefore key in neutralization reactions. 

If an inert good solvent is used in solution polymerization, the gel thus obtained will have a supercoiled (expanded) structure (Gel B). Gel B swells in good solvents much more than Gel A which is synthesized in bulk. If the amount of the crosslinking divinyl monomer in the reaction mixture is increased while the amount of solvent remains constant, highly crosslinked networks are formed that cannot absorb all solvent molecules present in the reaction mixture and a heterogeneous structure results (Gel C). A part of the solvent separates from the gel phase during polymerization and the formed Gel C consists of two continuous phases, a gel and a solvent phase. If the amount of solvent is further increased, a... 

Depending on the conditions of synthesis, copolymerization of divinyl/vinyl-monomers in the presence of an inert solvent leads to the formation of expanded (preswollen) or heterogeneous (porous) structures [54,99,100]. If the solvent remains in the network (gel) phase throughout the copolymerization, expanded networks are formed. If the solvent separates from the network phase the network becomes heterogeneous. According to Dusek et al., heterogeneities may appear in poor solvents due to the polymer-solvent incompatibility (x-induced syneresis), while in good solvents due to an increase in crosslink density (v-induced syneresis) [99]. 

The formation of such structures was attributed to short-range attractive forces arising from transient fluctuations in the periphery of the corona, leading at times to attractive interactions between micelles. These aggregated states, i.e., strings and networks, were observed whenever the added salt concentration was increased. Further increase of salt concentration eventually led to very large networks and macrophase separation into a dilute micellar phase and a concentrated gel phase [15]. 

Resins with a DVB content of less than 8 wt.% are of the gel-type without permanent porosity. Such resins function only in the presence of polar components that swell the resin structure. Resins with a DVB content of 12 wt.% or more have permanent macroporosity. These materials also have a microporous gel phase consisting of gel-type microspheres [25],... 

Pressure was applied in this study to fine tune the lipid chain-lengths and conformation and to select specific lamellar phases. For example, the phospholipid bilayer thickness increases by 1 A/kbar in the liquid-crystalline phase, and up to six gel phases have been found in fully hydrated DPPC dispersions in the pressure-temperature phase space up to 15 kbar and 80 °C, respectively. NMR spectral parameters were used to detect structural and dynamic changes upon incorporation of the polypeptide into the lipid bilayers. 

Using a similar approach, Notman et al. [81], determined the free energy for pore formation in bilayers composed of ceramide, as a model for the stratum corneum of the skin, both in the presence and in the absence of DMSO. Without DMSO, the bilayer was in the gel phase, and interestingly, a hydrophobic pore was observed with a high free-energy barrier ( 60 kj/mol). In the presence of DMSO, the bilayer was more fluid, and the more typical hydrophilic pore was observed, with a much smaller activation energy of 20kJ/mol. This work provided a thermodynamic and structural explanation for the enhanced permeability of skin by DMSO. 

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