Variations in Structure Explain the Viscometric Behavior of AOT Microemulsions at Low Water/AOT Molar Ratios

The viscosity of AOT/water/decane water-in-oil microemulsions exhibits a well-known maximum as a function of water/AOT molar ratio, which is usually attributed to increased attractions among nearly spherical droplets. The maximum can be removed by adding salt or by changing the oil to CCl4. Systematic small-angle X-ray scattering (SAXS) measurements have been used to monitor the structure of the microemulsion droplets in the composition regime where the maximum appears. On increasing the droplet concentration, the scattering intensity is found to scale with the inverse of the wavevector, a behavior which is consistent with cylindrical structures. The inverse wavevector scaling is not observed when the molar ratio is changed, moving the system away from the value corresponding to the viscosity maximum. It is also not present in the scattering from systems containing enough added salt to essentially eliminate the viscosity maximum. An asymptotic analysis of the SAXS data, complemented by some quantitative modeling, is consistent with cylindrical growth of droplets as their concentration is increased. Such elongated structures are familiar from related AOT systems in which the sodium counterion has been exchanged for a divalent one. However, the results of this study suggest that the formation of non-spherical aggregates at low molar ratios is an intrinsic property of AOT.

Gaussian deconvolution: a useful method for a form-free modeling of scattering data from mono- and multilayered planar systems

A new method for analysis of scattering data from lamellar bilayer systems is presented. The method employs a form-free description of the cross-section structure of the bilayer and the fit is performed directly to the scattering data, introducing also a structure factor when required. The cross-section structure (electron density profile in the case of X-ray scattering) is described by a set of Gaussian functions and the technique is termed Gaussian deconvolution. The coefficients of the Gaussians are optimized using a constrained least-squares routine that induces smoothness of the electron density profile. The optimization is coupled with the point-of-inflection method for determining the optimal weight of the smoothness. With the new approach, it is possible to optimize simultaneously the form factor, structure factor and several other parameters in the model. The applicability of this method is demonstrated by using it in a study of a multilamellar system composed of lecithin bilayers, where the form factor and structure factor are obtained simultaneously, and the obtained results provided new insight into this very well known system.

Assessment of "non-recordable" electroretinograms by 9 Hz flicker stimulation under scotopic conditions

To refine methods of electroretinographical (ERG) recording for the analysis of low retinal potentials under scotopic conditions in advanced retinal degenerative diseases. Standard Ganzfeld ERG equipment (Diagnosys LLC, Cambridge, UK) was used in 27 healthy volunteers (mean age 28 +/- A SD 8.5 years) to define the stimulation protocol. The protocol was then applied in clinical routine and 992 recordings were obtained from patients (mean age 40.6 +/- A 18.3 years) over a period of 5 years. A blue stimulus with a flicker frequency of 9 Hz was specified under scotopic conditions to preferentially record rod-driven responses. A range of stimulus strengths (0.0000012-6.32 scot. cd s/mA(2) and 6-14 ms flash duration) was tested for maximal amplitudes and interference between rods and cones. Analysis of results was done by standard Fourier Transformation and assessment of signal-to-noise ratio. Optimized stimulus parameters were found to be a time-integrated luminance of 0.012 scot. cd s/mA(2) using a blue (470 nm) flash of 10 ms duration at a repetition frequency of 9 Hz. Characteristic stimulus strength versus amplitude curves and tests with stimuli of red or green wavelength suggest a predominant rod-system response. The 9 Hz response was found statistically distinguishable from noise in 38% of patients with otherwise non-recordable rod responses according to International Society for Clinical Electrophysiology of Vision standards. Thus, we believe this protocol can be used to record ERG potentials in patients with advanced retinal diseases and in the evaluation of potential treatments for these patients. The ease of implementation in clinical routine and of statistical evaluation providing an observer-independent evaluation may further facilitate its employment.