Dynamical behavior of microemulsion and sponge phases in thermal equilibrium

The intermediate scattering function for lipid bilayer membranes: from nanometers to microns

A numerical scheme based upon established hydrodynamic and elastic considerations is introduced and used to predict the intermediate scattering function for lipid bilayer membranes. The predictions span multiple wavelength regimes, including those studied by dynamic light scattering (DLS; microns) and neutron spin-echo (NSE) spectroscopy (10-100 nm). The results validate a recent theory specific to the NSE regime and expose slight inaccuracies associated with the theoretical results available in the DLS regime. The assumptions that underlie both our numerical methods and the related theoretical predictions are reviewed in detail to explain when certain results can be applied to experiment and where caution must be exercised.

Hydrodynamic effects in bicontinuous microemulsions measured by inelastic neutron scattering

The dynamical properties of bicontinuous microemulsions have been studied with neutron spin echo spectroscopy around length scales corresponding to the correlation peak q(0). Comparison of samples with different contrasts for neutrons shed light on the two modes dominated either by variation of the oil/water difference or surfactant concentration in the hydrodynamic regime. The results have been compared to theoretical predictions of the relaxation rates of bicontinuous microemulsions by Nonomura and Ohta [M. Nonomura, T. Ohta, J. Chem. Phys. 110, 7516 (1999)]. The influence of modification of the surfactant layer bending constants in the microemulsion by addition of homopolymers (polyethylenepropylene: PEP(X) and polyethyleneoxide: PEO(X), X=5 kg/mol), dissolved in the oil phase and water, has been investigated.

Crossover from stretched to compressed exponential relaxations in a polymer-based sponge phase

X-ray photon correlation spectroscopy was used to characterize the wave vector- and temperature-dependent dynamics of spontaneous thermal fluctuations in a sponge (L3) phase that occurs in a blend of a symmetric poly(styrene-ethylene/butylene-styrene) triblock copolymer with a polystyrene homopolymer. Measurements of the intermediate scattering function reveal a crossover from stretched- to compressed-exponential relaxations as the temperature is lowered from 180 to 120 degrees C.

Dynamical fluctuation of the mesoscopic structure in ternary C12E5-water-n-octane amphiphilic system

Dynamical fluctuations of the bicontinuous microemulsion and lamellar structures in ternary C12E5-water-n-octane amphiphilic system are studied by means of neutron spin echo (NSE) spectrometry. The decay rates of the time correlation of the concentration were analyzed in terms of three theories: (1) A. G. Zilman and R. Granek, Phys. Rev. Lett. 77, 4788 (1996), (2) M. Nonomura and T. Ohta, J. Chem. Phys. 110, 7516 (1999), and (3) R. Granek and M. E. Cates, Phys. Rev. A 46, 3319 (1992), in the first of which a Langevin equation for membrane plaquettes and in the latter two of which time-dependent Ginzburg-Landau equations for the order parameters are considered. The result shows that the intermediate correlation functions I(q,t) for the ranges of 0<t<15 ns and 0<q<0.2 A (-1) are well fitted to a stretched exponential function in time, I(q,t)=exp[-(Gammat)(2/3)], for the bicontinuous microemulsion and the lamellar phases of the same systems with the relaxation rate Gamma increasing as q(3) in agreement with theory (1) from which the bending modulus of the membrane kappa was estimated. For more restricted ranges of 0<t<5 ns and 0.05<q<0.15 A (-1) the NSE result can be expressed by an exponential function in time in agreement with theory (2) determined exclusively by hydrodynamic interactions and for extended range of 0<t<10 ns and 0.03<q<0.15 A (-1) by a nonexponential function in agreement with theory (3), from both of which the effective viscosities eta(0) and eta(eff) of the system were estimated. The effective viscosity from the nonexponential eta(eff) is five times greater than that from the simple exponential eta(0) that is almost the same as the literature value. The implication of this result is discussed in terms of the effective viscosity eta(eff) that takes into account the renormalization of the bending modulus of the membrane.