R. Strey

Origin of the scattering peak in microemulsions

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.

Microemulsion microstructure and interfacial curvature

The typical phase behavior of microemulsion systems undergoing phase inversion is briefly reviewed. As a model system H2O-n-octane-C12E5 is studied with various experimental techniques. The occurring microstructures are visualized by freeze fracture electron microscopy and the corresponding domain sizes are quantified by small-angle neutron scattering. From the variations of the domain sizes the mean and Gaussian curvatures of the interfacial film with temperature are determined. It is found that the mean interfacial curvatureH changes gradually and nearly linearly with temperature from positive (Winsor I) to negative (Winsor II), passing through zero for bicontinuous microemulsions where these contain exactly equal volume fractions of water and oil. There the interfacial tension between bulk water-and oil-rich phases passes through an extreme minimum. Quantitative knowledge of the curvatures permits the measurements of interfacial tensions between the bulk phases to be discussed in terms of the relative contributions of bending energy and entropy of dispersion.

How to study microemulsions.

The application of various experimental techniques for studying the properties of ternary mixtures H 2 O-oil-nonionic amphiphile is demonstrated. Based on detailed knowledge of the phase behavior, in particular, the positions and extensions of the three-phase bodies, the origin of the ultralow interfacial tensions and strong opalescence of the solutions is elucidated. Within the three-phase body one finds a maximum of the velocity of phase separation of the three phases that is correlated to a minimum of the viscosity and a jump of the electric conductivity. The properties of the homogeneous microemulsions are measured along a path through the phase prism at constant amphiphile concentration but varying oil/water ratio. The dynamics of the microstructure is studied by viscosity, dynamic light scattering, and electric field and pressure jump relaxation, as well as by ultrasonic absorption. The characteristic dimensions of the structure are determined by SANS and SAXS, NMR self-diffusion, electric conductivity, and freeze fracture electron microscopy.

Dilute lamellar and L3 phases in the binary water–C12E5 system

The binary phase diagram of water–C12E5 has been studied with emphasis on the L3 and dilute lamellar phases, which were found to swell to approximately 99.5 and 98.8 wt % of water, respectively, much further than has been reported previously. Focusing on these two phases, we have carried out static light and small-angle neutron scattering and electrical conductivity measurements. The repeat distance in the lamellar phase was found to exceed 3000 A. A small, but significant deviation from ideal one-dimensional swelling was observed. This deviation may be explained in terms of flexibility of the bilayers that are flat only on average. Electrical conductivity and small-angle neutron scattering data from the isotropic L3 phase are, over most of the stability range, consistent with a three-dimensional continuous bilayer structure. However, at large water contents an increase in the conductivity indicates a breakup of the structure into smaller fragments. Our results show that the simpler binary system exhibits the same characteristic features as the more complex multicomponent systems, involving brine, ionic surfactant and cosurfactant.

Ultralow interfacial tensions in water-n-alkane-surfactant systems

The interfacial tensions between water- and oil-rich phases in the presence of microemulsions have been measured for ternary systems of water, n-alkanes, and nonionic alkylpolyglycolether surfactants (CiEj). It is found that the minimum of the interfacial tension curve, which is observed for each system in conjunction with the well-known phenomenon of phase inversion, depends sensitively, but systematically, on the chemical nature of the oil and the surfactant. Specifically, the minimum value of the interfacial tension σab decreases by 1 order of magnitude on decreasing either the carbon number of the alkane k by 6, or the number of oxyethylene groups j by 3, or by increasing the number of carbon atoms in the surfactant tail i by 2. The numerical values of the interfacial tensions as a function of temperature are presented along with an empirical description previously suggested [R. Strey, Colloid and Polymer Sci. 272, 1005 (1994)]. From the analysis, in terms of bending energy one obtains estimates for ...

Homogeneous nucleation rates for n‐alcohol vapors measured in a two‐piston expansion chamber.

Homogeneous nucleation rates J in supersaturated n‐alcohol vapors (methanol through n‐hexanol) were measured in a two‐piston expansion chamber as functions of supersaturation S and temperature T. The measured nucleation rates were compared with the classical nucleation theory. We found that the slopes of the J–S curves are in good agreement with the theoretical prediction. However, the actual values of experimental and theoretical rates were generally found to differ significantly, Jexptl/Jtheor ranging from about 10−10 (methanol, 273 K) to 107 (n‐hexanol, 257 K). In particular, the experimental nucleation rates show a significantly weaker temperature dependence as compared to the classical nucleation theory, the difference being more pronounced for the higher alcohols. Thus, a temperature dependent correction to the classical nucleation theory is needed. For the lower alcohols, in particular methanol, we found that the change of the cluster distribution during the expansion strongly influences the nuclea...

A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface: Area per molecule, microemulsion domain size, and rigidity

The small-angle neutron scattering (SANS) of bicontinuous microemulsions of 19 different water-n-alkane-CiEj (n-alkylpolyglycolether) systems has been measured. All scattering curves exhibit a broad scattering peak which permits determining the characteristic length scale ξ for bicontinuous structures at symmetric water and oil volume fractions, i.e., φ=0.5. Various random models predict ξ=aδφ(1−φ)/φc. We find that ξ is indeed inversely proportional to the surfactant volume fraction φc. Approximating the effective surfactant chain length δ by δ=νc/ac, where ac and νc are the area and the volume of the surfactant molecule, the numerical value for a is determined to be a=7.16, which is close to, but significantly different from those used in theoretical models. The head group area ac at the water–oil interface is obtained from the large q part of the scattering curves. It is found to be independent of i and k, the carbon numbers of the alkyl chain of the surfactant and of the alkane, respectively. However, ...

New method to analyze simulations of activated processes

We present a new method to analyze molecular and Brownian dynamics simulations of activated processes based on the concept of mean first-passage times. The new method provides a simple and efficient strategy to evaluate reaction rates and it facilitates the localization of the transition state directly from the kinetics of the system without the need of thermodynamical considerations. It also provides a more rigorous value of the steady-state transition rate and gives valuable information about many important characteristics of the process. We illustrate the power of this new technique by its application to the study of nucleation in rare gases.

Measurements of homogeneous nucleation rates for n‐nonane vapor using a two‐piston expansion chamber

The homogeneous nucleation rate J in supersaturated n‐nonane vapor was measured as a function of supersaturation S and temperature T using a newly developed expansion chamber. Homogeneous nucleation is observed only during a short supercritical nucleation pulse so that nucleation and droplet growth are decoupled under the considered experimental conditions. Reproducible nucleation pulses with a duration of about 1 ms are achieved by means of two spring‐operated pistons moving in a well‐defined time sequence. The number concentration of the droplets growing in the expansion chamber is determined immediately after the end of the nucleation pulse by means of constant angle Mie scattering. Taking into account the duration of the nucleation pulse we measured homogeneous nucleation rates ranging from about 5×105 to 5×109 cm−3 s−1. Homogeneous nucleation in n‐nonane vapor was observed in the temperature range from 199 to 241 K. Argon was used as the carrier gas. The measured nucleation rates were compared with t...

Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n‐alkane, and n‐alkyl polyglycol ether (CiEj) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with well‐structured mixtures (exhibiting nonwetting middle phases and well‐expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the...