Publications
1991
We calculate the structure factor for microemulsions with finite spontaneous curvature by using a statistical mechanics model of the interface ensemble. The morphological transition between hard spheres and the random bicontinuous phase is treated in a unified fashion, and the implications for experiment are discussed.
Surfactant monolayers at water-oil interfaces and amphiphilic bilayers in either water or oil exhibit large-scale, isotropic structures in the neighborhood of ordered, lamellar phases. A model for the structure, based upon a treatment of bicontinuous microemulsions, is used to compare the phase stability of the lamellar and bicontinuous L3 phases.
The saddle-splay modulus KBAR of a surfactant monolayer determines the stability of a spherical droplet microemulsion phase. Negative values of KBAR increase the droplet-size polydispersity, but inhibit shape fluctuations. Large values of the observed polydispersity may therefore not be indicative of small values of the bending modulus K, but of a ratio of KBAR/K approximately -2.
We study the curvature elastic properties of monolayers of diblock copolymers adsorbed at the interface of two incompatible solvents which are also selective solvents for the two blocks. At saturation, the interfacial free energy is minimized with respect to contributions from the chain conformation free energy, the interfacial tension, and the two-dimensional translational entropy of the chains. For a curved interface, this minimization leads naturally to curvature elasticity. The three elastic coefficients, the spontaneous radius of curvature, the bending modulus, and the Gaussian bending modulus, as functions of the molecular weights, the interfacial tension, the interaction parameters, etc., are obtained for a number of cases. Our study employs the theory for grafted chains recently developed by Milner et al. to obtain the chain conformation free energy which takes into account the nonuniformity of the chain-end distribution. This improvement not only affects the overall prefactor of the free energy but it changes the relative values of the three elastic coefficients as well. We consider the cases of both a swollen monolayer and a monolayer consisting of a melt of copolymer chains, as well as an interesting case where one of the blocks is in the swollen condition and the other block is in the melt condition. Because the chains in the melt and the swollen conditions have distinctively different scaling behaviors, the mixed case displays some features that are different from either the swollen and the melt cases.
Although large, spherical surfactant vesicles are generally unstable to either lamellar or micellar phases, mixtures of two surfactants can lead to spontaneous vesicle formation. We show theoretically how the energetic stabilization of mixed vesicles can occur by considering the curvature elasticity of the surfactant bilayer. Interactions between the two species (of the proper sign and magnitude) are crucial to stabilizing these vesicles. These interactions lead to composition asymmetries and effective spontaneous curvatures of the inner and outer layers that are of equal and opposite signs. The vesicles have a Gaussian distribution about an average size determined by the effective spontaneous curvature; the width of the distribution is calculated as a function of concentration. The stability of these vesicles with respect to a flat lamellar phase is estimated. The predictions of the ranges of stability of the various phases as a function of the three concentrations (solvent, e.g., water-and the two amphiphiles) are in qualitative agreement with recent experiments.