We demonstrate a new approach to determine the bending rigidity of the amphiphile film in microemulsions and sponge phases from neutron scattering data. This method is precise enough to measure the logarithmic scale dependence of the bending rigidity and its universal prefactor for the first time. Furthermore, we show that in the mushroom regime the bending rigidity of a membrane decorated by amphiphilic block copolymers increases linearly with the polymer concentration on the membrane; the amplitude is found to be about a factor 1.5 larger than theoretical results for ideal chains.

http://iopscience.iop.org/article/10.1209/epl/i2001-00574-3/pdf

Measuring bending rigidity and spatial renormalization in bicontinuous microemulsions

Calcium surfactants: a review

The electrostatic contribution to the bending moduli and spontaneous curvature of monolayers formed by ionic surfactants in solution is calculated for all salt concentrations, ranging from no added salt (counterions only) to excess salt. This is accomplished using a perturbative expansion in curvature of the free energy of the Poisson-Boltzmann cell model, which is shown to give precisely the same results for the moduli as would an alternative calculation employing moments of the transverse pressure profile of the electric double layer. With this treatment, it is possible to quantify the dependence of the moduli and spontaneous curvature on surfactant concentration alone (i.e., with fixed average salt concentration), a point of central importance to the validity of the flexible surface model in the description of ionic surfactant systems. A manifestation of the counterion condensation phenomenon is also observed, as the monolayer rigidity saturates and becomes independent of the surface area per ionic headgroup. \textcopyright{} 1996 The American Physical Society.

Bending of ionic surfactant monolayers.

Recent major developments that have taken place in surfactant phase science may be summarized in five points. First, the equilibrium vesicle phase formed by catanionic surfactant mixtures in the very dilute region has been investigated for several systems which also form several mesophases in the concentrated region. Coexistence of two reversed hexagonal phases and first order vesicle→micelle phase transition have been demonstrated. Second, by manipulating the hydrophilic-lipophilic balance and molecular geometry of many surfactant systems, the changes in observed phase behaviour serve to increase our knowledge of aggregation processes in general and the structures of the birefringent intermediate phases, sponge phase and dilute lamellar phase in particular. Third, the lyotropic nematic and defective mesh-type lamellar phases are readily formed by fluorinated surfactants; the structures of the two phases, which are still debatable, have been studied by several techniques. The first observation of a lower consolute loop along the dilute lamellar phase was reported for the fluorinated anionic surfactant systems. Fourth, the valency of counterions has been shown to have dramatic effects on the aggregation and phase behaviour of ionic surfactants. Finally, surfactant-protein systems form novel gel phases and locations of phases provide information concerning the contribution of electrostatic and hydrophobic effects to surfactant-protein interactions.

Phase science of surfactants

Microemulsions as model systems

The thermodynamics of the L 3 (sponge) phase is investigated within the flexible surface model. The well-established leading-order Φ 3 scaling for the free energy density (where Φ is the surfactant volume fraction) requires modification in order to describe the narrow character of the phase and the observed sequence of phase transitions. We find that higher order contributions to the free energy density from the local curvature energy provide a straightforward mechanism to account for these features. The phase equilibria with the dilute solution on one side of the phase and the lamellar phase on the other are evaluated for a model binary (surfactant-solvent) system, and calculated phase diagrams are presented. Experimental evidence in support of the model comes from static light scattering experiments on the pseudobinary system AOT-NaNO 3 -water. We calculate the forward scattering intensity along chosen dilution lines within the phase using the model, and predict that the scattering intensity varies significantly as we move from one phase boundary to the other at constant Φ. The experimental results confirm these predictions, and provide strong evidence in support of the proposed model

Thermodynamics of the L3 (sponge) phase in the flexible surface model

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Thermodynamics of the L3 (sponge) phase in the flexible surface model