Inverted micellar aggregates and the W/O-microemulsion domain of the system H20/A0T/t-C8H18 have been investigated at various temperatures, AOT and H20 concentrations, and molar ratios [H20]/[A0T] in the range 0-60 using photon-correlation spectroscopy. Experimental evidence is presented that a clear distinction is possible between micellar and swollen micellar solutions, i.e., microemulsions, according to the degree of hydration or the amount of solubilized water, respectively. The onset of the phenomena characteristic for microemulsions occurs when water becomes the major component in the colloidal aggregates. Moreover, it is shown that the conventional definition of the microemulsion domain with respect to the upper boundary is artificial since there is no discontinuity in the property of the microemulsion. Also the so-called "free" water concept is discussed in the light of the results presented.

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Abstract:We have studied the temperature-dependent critical scattering of both light and neutrons from aqueous solutions of n-octyl pentaoxyethylene glycol monoether (C8E5). We show that the assumption of a short-ranged temperature-dependent attractive pair potential between approximately spherical micelles of constant size permits a quantitative analysis of the neutron scattering data. The analysis, which is undertaken using current liquid-state theory and is in analytic form, contains only one free parameter, the depth of the attractive potential. We find that a potential with a range of only a fraction of a nm is sufficient to generate spatial correlations over tens of nm as the attractive potential deepens on approaching the critical temperature. The analysis also provides a semi-quantitative understanding of the light scattering data as a function of concentration and temperature, and leads to a qualitative prediction of the form of the phase diagram. Numerical values obtained are consistent with the hypothesis that the primary effect of raising the temperature is to lower the degree of structure of water near the micelle surface, allowing increased van der Waals attraction due to closer contact.

In the tetragonal crystal form of OmpF porin, the membrane-exposed area is accessible from the aqueous solution. It is coated by a film of detergent molecules, which presumably mimics the interactions of the protein with lipids in the biological membrane. In the trigonal form, protein-protein interactions predominate in the hydrophobic zone. These may reflect the tight interactions between trimers that are observed in the biological membrane.

Lower consolute boundaries up to 50 vol% surfactant have been determined for noctylpenta(oxyethy1ene glycol) (C,E,) in the presence of the monovalent salts NaF, LiC1, NaCl, KC1, CsCl, NaBr and NaI. We find that the lower consolute boundaries are shifted, with only small changes in shape, to lower or higher temperatures (salting-out or salting-in, depending on the salt used). The amount and sign of the miscibility shift is determined almost solely by the anion, and the shift is related to the surface charge density of the ion. The shifts in the lower consolute boundary cannot be explained by structural changes in the bulk water structure due to the addition of salts. We have applied the Flory-Huggins lattice theory to relate the shifts of the lower consolute boundaries to changes in micelle-solvent interactions. This analysis shows that small changes in the free energy of interaction between oxyethylene and water can explain the shifts in temperature of the lower consolute boundary. We discuss the use and limitations of the Flory-Huggins theory, as applied to micellar phase separation. The salt effects are explained in terms of salt-deficient or salt-rich regions around the oligo-oxyethylene chains.

The assembly of two-dimensional membrane protein crystals in the presence of lipids was analyzed with quasieinstic light scattering and electron microscopy. Mixtures of detergentsolubilized Upids and/or proteins were submitted to slow or rapid dilution while measuring the hydrodynamic radii of the aggregates. Lipids alone exhibited g-shaped dilution curves with intermediate rod-shaped particles that converted into small vesicles. Depending on the protein-protein and protein-lipid interactions, detergent-solubilized protein-lipid mixtures showed a sharp transition from mieelles to large, densely packed proteoliposomes. Electron microscopy revealed that formation of crystals occurred shortly after this phase transition.