No abstract

Results on surfactant gels containing densely packed multilamellar vesicles are reported. The gels form spontaneously when the bilayers of L, or L, phases of alkyldimethylaminoxide and cosurfactant are charged up by the addition of ionic surfactant or HCl. The rheological behaviour on addition of excess salt was studied by dynamic rheological measurements for systems with surfactants of different chainlengths. Both the storage modulus, G', and the yield stress, u y , decay with rising salinity. This effect is caused by the reduction of both the electrical contribution of the bending constants of the bilayers and the compression modulus of the vesicles. The influence of the charge density on the rheological properties was determined. G' and by increase with charge density and reach a plateau that depends on the chainlength of the surfactant. Measurements on samples prepared with waterglycerol mixtures show that the moduli and the yield stress value are independent of the solvent viscosity. Photographs of the surfactant gels that were taken with the interference contrast microscopy technique are presented. They reveal that some of the vesicles are much bigger than expected on the basis of TEM micrographs. The mean size of the vesicles can be estimated on basis of conductivity data. This method yields an average number of 5-6 shells per vesicle and corresponds with the value taken from the electron micrograph. The rheological data are explained with a model that was recently introduced by van der Linden. In connection with a model due to Lekkerkerker for the electric contribution of the bending constant of the bilayers and our own calculations of the osmotic pressure the van der Linden formula yields good results for describing the experimental data.

In order to elucidate the mechanism for the formation of an interfacial film providing enhanced emulsion stability by lecithin, we performed dynamic contacting experiments in which water was brought into contact with a solution of the phospholipid in n-decane. Two main phenomena were observed in the vicinity of the interfacial boundary on the side of the nonaqueous phase. The first to be noted was the formation of a thick interfacial film, visible even to the naked eye. The second was the separation of a liquid phase. On the basis of the combined data from our current and previous experiments, we suggest the mechanism for interfacial processes of which the basis is the lecithin organogel formation. Water transferring from aqueous solution into the nonpolar phase through a hydration of the adsorbed phospholipid promotes a sequence of phase or pseudophase transitions near the phase boundary between immiscible liquids: spherical reverse micelles f three-dimensional network from entangled wormlike micelles f organogel separation into a diluted solution and a compact gel or solid mass precipitating on the interfacial boundary.

In contrast to the release of pro- and anti-inflammatory cytokines, sex hormones show a gender-specific pattern during the acute phase response 1 week after abdominal surgery.

ricevuto il 28 Ottobre 1994) Summary. --A general method is presented for the preparation of viscoelastic surfactant phases that consist of densely packed multilamellar vesicles in water. The phases form spontaneously when ionic surfactants are added to L=-or L3-phases from uncharged surfactants and cosurfactants. The dimensions and the structure of the vesicles were studied from FF-TEM micrographs for 100 mM surfactant solutions. The average diameter of the vesicles is in the range of 1 ~m, the interlamellar spacing is around 800/~. Under these conditions the vesicles are densely packed and cannot pass each other. The systems are highly viscoelastic and have a yield value. The storage and the loss moduli were determined by oscillating rheological measurements. The storage modulus was about one order of magnitude larger than the loss modulus and independent of frequency. Both parameters were determined as a function of the concentration and the chain length of the surfactant and cosurfactant, the charge density and ionic strength, the solubilization of hydrocarbon and temperature. The yield values and shear moduli increase linearly with the surfactant concentration. For constant surfactant concentration the moduli increase in a S-shaped form with the charge density and reach saturation for a mole fraction of about 7% of ionic surfactant. The storage moduli and yield values decrease with the addition of excess salt. The storage moduli depend strongly on the chain length of the surfactant. The values of the moduli are explained on the basis of a hard-sphere model in which the multilamellar vesicles are considered as hard-sphere particles. PACS 62.10 -Mechanical properties of liquids. PACS 01.30.Cc -Conference proceedings.