Correlations and Structure Factor of Bicontinuous Microemulsions (original) (raw)
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Structure and dynamics in three-component microemulsions
The Journal of Physical Chemistry
NMR self-diffusion coefficients are reported for three-component micromulsions formed from didodecyldimethylammonium bromide/water/alkane. For hexane through tetradecane the oil diffusion coefficients are approximately half as large as those of the bulk oils and independent of composition. Therefore, the systems are oil continuous through the entire one-phase region. The diffusion coefficients for the surfactant are small and independent of composition. The water self-diffusion coefficients decrease with added water for hexane through dodecane and change in a manner consistent with the abrupt conducting-nonconducting transition known to occur in this system. The microemulsions are bicontinuous in the conducting regions and disconnected water-in-oil droplets in the nonconducting regions of the phase diagrams. The phenomena reported clearly demonstrate the interplay between forces due to oil penetration and those due to head-group interactions which control curvature and therefore microemulsion structure.
Modelling interactions in microemulsion phases
Journal de Physique
2014 Nous présentons dans cet article une description unifiée des interactions dans les phases liquide isotrope et cristal liquide de microémulsions. Un modèle microscopique est utilisé pour tenir compte des termes d'énergie entropique, de courbure et d'interaction et on discute la stabilité relative des phases isotrope (microémulsion), lamellaire et hexagonale. Abstract. 2014 In this paper we present a unified description of the interactions in liquid isotropic and liquid crystal phases of microemulsions. A microscopic model taking into account the entropic, bending and interaction free energies is used to deduce theoretical phase diagrams and the relative stability of the isotropic (microemulsion), lamellar and hexagonal phases is discussed.
Four-point correlation functions and average Gaussian curvature in microemulsions
The structure of microemulsions is studied in the Landau-Ginzburg model ͓A. Ciach, J. Chem. Phys. 104, 2376 ͑1996͔͒, in which all the coupling constants are expressed in terms of temperature T, surfactant volume fraction s , and amphiphilicity. The extension of surface-averaged Gaussian curvature, K ͓A. Ciach and A. Poniewierski, Phys. Rev. E 52, 596 ͑1995͔͒, is calculated in the Landau-Ginzburg approximation. In the neighborhood of the liquid-crystal phases KϽ0. Thermal fluctuations destroy the bicontinuous structure and cause a transition to KϾ0. A dimensionless average radius of curvature Rϭ͉K͉ Ϫ1/2 /, where is a period of damped oscillations of the two-point correlation function, is calculated. Comparison with a corresponding quantity for periodic minimal surfaces shows that microemulsions have ͑for KϽ0͒ a structure resembling different periodic minimal surfaces with low genus for different T and s .
Advances in colloid and interface science, 2017
In this review, we discuss the conditions for forming microemulsions, systems which are thermodynamically stable mixtures of oil and water made stable by the presence of an interfacial film containing surface active molecules. There are several types of microemulsions, depending largely on the stiffness of the amphiphilic monolayer that separates the oily and the aqueous micro-domain. We first discuss and compare the phase behaviour of these different types, starting from the classical microemulsion made from a flexible surfactant film but then also moving on to less classical situations: this occurs when the interfacial film is stiff or when microemulsions are formed in the absence of a classical surfactant. In the second part, we relate these different microemulsion types to the structural features as can be determined via different methodologies by small angle scattering (SAS). Using absolute scaling, general theorems as well as fitting under constraints or to pre-supposed shapes...
Configurational entropy of microemulsions: The fundamental length scale
The Journal of Chemical Physics, 1993
Phenomenological models have been quite successful in characterizing both the various complex phases and the corresponding phase diagrams of microemulsions. In some approaches, e.g., the random mixing model (RMM), the lattice parameter is of the order of the dimension of an oil or water domain and has been used as a length scale for computing a configurational entropy, the so-called entropy of mixing, of the microemulsion. In the central and material section of this paper (Sec. III), we show that the fundamental length scale for the calculation of the entropy of mixing is of the order of the cube root of the volume per molecule-orders of magnitude smaller than the dimension of such a domain. This length scale is specifically the scale for the configurational entropy-not that which measures either the curvature of the interface, the "granularity" of the microemulsion, or the persistence length. Furthermore, we demonstrate, in general, that mixing entropy, evaluated in configuration space as opposed to phase space, will not be physically correct unless it is made to be consistent with the phase space evaluation. Following this core section, we give a one-dimensional illustration of the problem (Sec. IV), and discuss the consequences of our general result with respect to the RMM (Sec. V). The RMM not only seriously underestimates the entropy of mixing but exhibits a dependence on composition that is qualitatively very different from the correct dependence. Furthermore, for oil or water rich compositions of the microemulsion, the correct mixing entropy reinforces effects that would normally be attributed to bending energy, i.e., it destabilizes the system.
Flexibility, persistence length and bicontinuous microstructures in microemulsions
Comptes Rendus Chimie, 2009
Three length scales have to be considered to describe microemulsions: persistence length , spontaneous radius of curvature of the surfactant film as intensive variables and finally the characteristic size imposed by the ratio of volume fraction to available specific area surface. The specific area per unit volume is an intensive variable linked to sizes and topologies that can be built without tearing the surfactant film. We show here that at least four types of bicontinuous microstructures have been detected so far, and that they can be distinguished by a simple experimental determination of the evolution of scattering peak position versus dilution. Besides the classical microstructures dominated by entropy or a priori considered as droplets, the other microstructures identified so far can be approximated as connected cylinders or randomly connected swollen bilayers. All these microstructures can be considered as ''molten'' precursors of the lyotropic liquid crystalline phases that are adjacent to microemulsions in phase diagrams. To cite this article: T. Zemb, C. R. Chimie 12 (2009). Ó 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1997
Using a Voronoi cell tessellation of space, a complete set of microstructure approximations with a local morphology that evolves continuously from isolated spheres to disordered lamellae via connected cylinders has been introduced by Barry Ninham and co-workers to describe microemulsions. For 10 years, this continuous microstructural change has been observed in a large number of microemulsion and copolymer systems. In the case of stiff interfaces, where bending energy damps local curvature fluctuations, three constraints that govern microstructures have to be fulfilled simultaneously: the imposed volume fraction; the specific oil-water interface; and the minimum elastic energy, with negligible entropic contributions. From this model, simple analytic expressions are able to predict the characteristic size D* and the connectivity Z of the microstructure at any composition, when the surfactant parameter p can be derived from the phase diagram shape. Once D* and Z are known, the conductivity, scattering peak position as well as phase boundary can be predicted from the spontaneous curvature alone. The model is known as the disordered open connected model (DOC) and is still the only model proposed for microemulsions compatible with nonmonotonic behaviour of the electric conductivity with water volume fraction. © 1997 Elsevier Science B.V.
Interfacial phase transitions of microemulsions
In this paper we study the interfaces between phases in a phenomenological model of a microemulsion that is in equilibrium simultaneously with an oil-rich and a water-rich phase. The tensions and chemical-composition profiles of the interfaces are calculated. We ask whether the oil-water, oil-microemulsion and microemulsion-water tensions uow, u r n and u r n are related by now < uom+amw or by uow = uom+umw In the former case the microemulsion phase does not wet the oil-water interface, whereas in the latter it does. We find separate ranges of values of the model's parameters in which each possibility is realized, while the microemulsion is a middle phase related symmetrically to the oil and water phases. When a parameter that breaks that symmetry is varied and a critical endpoint of the three-phase equilibrium is approached, an originally non-wet oil-water interface becomes wet (while an originally wet interface remains wet). The transition is of first order, accompanied by a change in interfacial structure. A microscopic lattice model of such three-phase equilibria is also described. In its context we raise (but do not fully answer) the same questions that we treated in the earlier phenomenological model.