E. V. Shumilina

Lecithin bridging by hydrogen bonds in the organogel

A series of polar solvents have been studied in order to reveal those capable of producing the thickening effect on hydrocarbon solutions of lecithin. It has been established that glycerol, formamide and ethylene glycol, in addition to water (previously known), have the ability to induce organogel formation. The IR results show that molecules of both the gel-forming solvents and non-gel-forming ones are attached to the lecithin phosphate group via hydrogen bonds. The latter influence the mobility of the choline residue. The ability to promote thickening of lecithin solutions has correlated with the polarity of molecules. The correlation is particularly pronounced in the series of such structurally related compounds as glycerol, ethylene glycol and 1,3-propanediol. It has been inferred from the results that the difference between gel-forming and non-gel-forming polar solvents is caused by their orientation and localization in the polar moiety of a lecithin molecule. In a proposed model of the organogel, the solvent molecules bridge phosphate groups of neighbouring lipid molecules, thus allowing their association into tubular aggregates through an extensive ribbon-like hydrogen bonding network.

Chitosan-carrageenan gels

Phase behavior of the systems during the formation of polyelectrolyte complexes obtained by mixing of aqueous solutions of chitosan and ι-, κ-, or λ-carrageenan was studied. The gelation was shown to occur throughout the whole bulk solution at chitosan and λ-carrageenan concentrations higher than 0.1 and 0.3 wt %, respectively. At lower polysaccharide concentrations, the polyelectrolyte complexes precipitated. The study was performed at the polysaccharide concentrations not higher than 1 wt %. The gel systems were investigated by the dynamic rheology method. The mechanical characteristics of λ-carrageenan-containing gels were found to be mainly governed by the chitosan content; the viscosity of these gels was independent of temperature. Gels obtained with ι- and κ-carrageenans were sensitive to temperature because of the helix–coil conformational transitions in their molecules. The mechanical strength and stiffness of gels increase in the λ–ι–κ-carrageenan series. This effect was explained by the formation of additional crosslinks by double helixes of ι- and κ-carrageenan molecules.

The Effect of Lysophosphatidylcholine and Phosphatidylglycerol on Lecithin Polymer-Like Micelles

The effect of phosphatidylglycerol and lysophosphatidylcholine on the formation, properties, and intermolecular interactions in polymer-like (cylindrical) micelles that arose in nonaqueous lecithin solutions upon the addition of trace amounts of water is studied by the methods of dynamic rheology and IR spectroscopy. It is established that micellar aggregates of lecithin are not changed markedly in the presence of the first phospholipid but are disintegrated into smaller aggregates upon the addition of second phospholipid. The IR spectroscopic studies demonstrate that phosphatidylglycerol and lysophosphatidylcholine do not significantly change intermolecular interactions in polymer-like micelles. Their effect on the structure of micellar aggregates is associated with the variations in the shape of molecules.

Lecithin Organogels Containing Poly(ethylene glycol) Monolaurate

The effect of added poly(ethylene glycol) monolaurate (PEGML) on the formation and properties of lecithin organogels composed of polymer-like micelles was studied by the methods of dynamic rheology and the Fourier transform IR spectroscopy. It was established that the addition of even small amounts of PEGML causes a significant decrease in viscosity, whereas the elastic properties of organogels remained almost unchanged. The analysis of the scaling dependences indicated that the formation mechanism of polymer-like lecithin micelles remained also unchanged. Spectral studies revealed that the PEGML molecules affect intermolecular hydrogen bonding during their incorporation into micelles, thus stabilizing micellar structure. This effect is caused by the partial dehydration of the lecithin polar region. This leads to a decrease in the number of hydrogen bonds or their weakening and, as a result, to the disintegration of polymer-like lecithin micelles into shorter micellar aggregates.

Molecular shape of amphiphiles self-organised into bimolecular films

The dependence of the appearance of the black spot in thick nonaqueous films and the formation of bimolecular lipid membranes on the shape of amphiphilic molecules has been studied using a wide-ranging homologous series of alkyl derivatives of diamines. The optimum molecular shape and limits to amphiphile self-organisation into the bimolecular state have been ascertained.