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Micelles mean curvature

It appears that the role of increasing salinity is to change the mean curvature of the surfactant sheetlike structure from a value favoring closure on the oil-rich regions (swollen inverted micelles). In between, in bicontinous microemulsion having comparable amounts of oil and water, the preferred mean curvature must be near zero. [Pg.178]

In spatially evolving multiphase media (e.g., during dissolution of a porous medium, or phase separation in a polymer blend), the mean curvature of the interface between two phases is of interest. Curvature is a sensitive indicator of morphological transitions such as the transition from spherical to rod-like micelles in an emulsion, or the degree of sintering in a porous ceramic material. Furthermore, important physicochemical parameters such as capillary pressure (from the Young-Laplace equation) are curvature-dependent. The local value of the mean curvature K — (1 /R + 1 /Ri) of an interface of phase i with principal radii of curvature Rx and R2 can be calculated as the divergence of the interface normal vector ,... [Pg.144]

Consider, for example a spherical micelle. By our convention, if the interface encloses hydrophobic regions, the mean curvature is negative. Consequently, the surfactant parameter for a spherical micelle of radius R is given by ... [Pg.145]

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. [Pg.21]

In principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. [Pg.6]

A number of other NMR-probed w/o microemulsions have appeared in recent literature. The diffusion coefficients in water/SDS/pentanol and ammonium hydro-xide/SDS/pentanol microemulsions investigated by Olsson et al. [35] estabhshed that replacement of water by ammonium hydroxide destabilizes the liquid crystalline phase and reduces the size of the colloidal association structure in the isotropic liquid region, Olsson and Schurtenberger [36] worked on nonionic microemulsions prepared from D2O, pentaethylene glycol dodecyl ether and decarie. Discrete oil-swollen micelles have been evidenced by NMR self-diffusion measurements the preparations are in conformity with the hard-sphere model. The NMR self-diffusion measurements on a water/octyl glucoside/pentanol/decane microemulsion system advocated a progressive decrease in the mean curvature of the surfactant film with water addition at a constant level of the oil [37]. It was concluded that the... [Pg.278]

Surfactant monolayers have two sides which are not identical. Therefore, all the curvatures have a sign, and the states with positive and negative curvatures are physically different. According to the sign convention, the curvature is counted as positive if the monolayer is curved towards oil e.g. in O/W micelles). Thus, for an OAV spherical micelle with radius R, H = H, = H2 = I/R. For an infinite O/W cylinder with the radius R, Hi = l/R and H2 = 0, and H = I/2R. For a plane. Hi = H2 = H = 0. For saddle-shaped surfaces, the principal curvatures have opposite signs and the mean curvature can be equal to zero, even though both Hi and H2 are non-zero i.e. when Hi = —H2, see Figure 7.3). [Pg.210]

As a general result, we conclude that micelle and vesicle formation cannot be explained by cone or cylinder shapes of the monomeric amphiphiles. The key criterion for the curvature of molecular assemblies lies in the saturation solubility or cmc of the amphiphile. A cmc above 10 M usually means appreciable dissociation leading to small aggregation numbers of micelles. A cmc below 10 M means large planar bilayers or, upon their disruption, vesicles. [Pg.39]


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