From a Landau theory the static scattering intensity distribution I(q) of microemulsions is obtained. As essential ingredient we have included a negative gradient term in the free energy expression. The form of I(q)∼(a2+c1q2+c2q4)−1 yields for a2>0, c1 0 a single broad scattering peak and a q−4 decay at large q, both properties experimentally observed for a variety of microemulsions containing comparable amounts of water and oil. The peak originates from the modulation in the corresponding space correlation function given by γ(r)=(d/2πr)⋅e−r/ξ⋅sin(2πr/d). It is shown that the scattering intensity relation describes experimental literature data remarkably well, using only three fit parameters.

Origin of the scattering peak in microemulsions

The microemulsion phases formed in solutions of octane, commercial surfactant, and alcohol with various brines are examined with small‐angle x‐ray scattering (SAXS), electrical conductivity, and viscosity techniques. Models based on monodisperse populations of swollen micelles or microemulsion ‘‘droplets’’ adequately represent the SAXS data at low volume fractions of brine. Introduction of hard‐sphere interactions with the Percus–Yevick approximation allows us to model the composition dependence of the radius of gyration and isothermal compressibility up to volume fractions of brine near a percolation threshold for electrical conductivity. For brine volume fractions above the percolation threshold, a mean field attractive interaction term is needed to model the variation of isothermal compressibility; however, the same theory fails to model the composition dependence of the apparent radius of gyration. But predictions from a model for a bicontinuous microemulsion structure that is geometrically irregular yet topologically ordered and that evolves continuously into swollen (inverted) micellar solutions at low volume fraction of water (oil) are in good agreement with the SAXS and electrical conductivity data over a wide range of brine volume fractions.

Toward understanding microemulsion microstructure: A small‐angle x‐ray scattering study

Photon correlation spectroscopy experiments on silica particles dispersed in cyclohexane over a large concentration range and near the optical matching point have been explained by the occurrence of two different relaxation modes: one mode due to the collective diffusion of the particles and the other mode due to the self‐diffusion. This is in agreement with the theoretical considerations of Weissman and Pusey. In the diluted region, information has been obtained about the behavior of the self‐diffusion coefficient at zero wave vector (K) as a function of the particle volume fraction (φ). We found that the following relation exists: Ds(K = 0) = D0(1 + k′sφ), where D0 is the diffusion coefficient at infinite dilution and the constant ks′ has a value of −2.7±0.5. The concentration dependent behavior of the collective and self‐diffusion coefficient has further been compared with the results of sedimentation and viscosity measurements. At high particle concentrations we observed that the relation between coll...

Dynamic behavior of silica dispersions studied near the optical matching point

Many equilibrium microemulsion phases can solubilize hydrocarbon and water in all proportions in a continuous progression of states without any visibly abrupt transition. At the extremes are solutions of swollen micelles and swollen inverted micelles. Contending pictures of the midprogression microstructures are (1) crowded swollen micelles and crowded swollen inverted micelles, which cannot coexist alone, and (2) bicontinuous structures, parts of which can be micellar. Which is correct? The most extensive published evidence is electrical conductivity, small‐angle x‐ray scattering (SAXS), and viscosity data on microemulsions made in our laboratory with an alkylaryl sulfonate mixture that is a commercial type of surfactant. We report here complementary data with a comparable pure surfactant and analyze at length the SAXS evidence. We note that the pure surfactant appears to narrow the ranges over which swollen disjoint interacting micelles exist. We conclude that the continuous progression through buildup ...

Toward understanding microemulsion microstructure. II

A model of dynamic scattering by microemulsions

Light beating spectroscopy experiments were performed on concentrated W/O microemulsions located in the vicinity of the gel phase on the phase diagram New results are reported: the most interesting feature is the non-exponential behaviour of the self-correlation function g (1)(K, τ) of the scattered field The initial decay rate of g (1)(K, τ) is not directly proportional to K 2; whereas the long time decay is found fairly exponential

A.M. Bellocq

Papers

Dynamic Light Scattering Study of Concentrated W/O Microemulsions