Thermodynamic defects, instabilities and mobility processes in the lamellar phase of a non-ionic surfactant

2014 The lamellar phase L03B1 of the aqueous solution of the non-ionic surfactants C12E5 and C12E6 shows up many features which do not exist in usual lamellar phases : high mobility of nonthermodynamic defects, structural dislocation loops whose density increases at the approach of the transition towards the high temperature isotropic phase, low temperature curvature instabilities. We discuss these phenomena with regard to the low value of the curvature splay modulus K1 (comparable to that of microemulsions) and molecular models which stress the influence of water linked to the hydrophilic heads. J. Physique 48 (1987) 1799-1807 OCTOBRE 1987, Classification Physics Abstracts. 61.30 61.70 64.70 It has been demonstrated recently, first by ESR spectroscopy [1], then by electron microscopy observations of replicas of freeze fractured samples [2, 3] that the lamellar-isotopic transition in aqueous solutions of some non ionic surfactants ( ClzES and C12E6, i.e. penta and hexaethyleneglycol dodecyl ether) is driven by the formation of defects related to very highly curved regions of the interfacial film. These curved regions appeared to be twist walls of screw dislocations organized as shown in figure 1. In a wall, dislocations have the same sign. These twist walls appear in pairs of opposite sign, thus defining blocks of layers tilted with respect to the lamellar matrix. It is our guess that these screw dislocations pairs are vertical segments of dislocation loops which cross layers. These defects are thermodynamically stable, their density increases with temperature ; (+) Present address : ATOCHEM, Physico-chimie de l’Application, B.P. 108, 92303 Levallois-Perret, France. their Arrhenius plot yields an energy Ea of the order of 0.4 eV (1). In addition to these thermodynamically stable, structural defects, usual metastable defects, like oily streaks, focal domains etc. are observed by optical microscopy. Contrarily to what is known of metastable defects in most ionic surfactant systems, they anneal very easily when the sample is submitted to gently thermomechanical treatment. Then it is possible to obtain very good homeotropic samples within half an hour. Viscoelastic measurements performed on such samples are highly reproducible. They measure the mobility of the edge dislocations which gather in the grain boundary of the mid-plane of the (1) Although, owing to quenching effects, electron microscopy observed defects probably do not show up with their equilibrium size nor their equilibrium density, their number, clearly and undoubtfully, varies with temperature in a reproducible way (see Ref. [2]). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480100179900