Broad existence range

However, even without structural studies, Friberg et al. [32], Shinoda [33], and others noted that the broad existence range with respect to the water/oil ratio could not be consistent with a micellar-only picture. Also, the rich polymorphism in general in surfactant systems made such a simplified picture unreasonable. It was natural to try to visualize microemulsions as disordered versions of the ordered liquid crystalline phases occurring under similar conditions, and the rods of hexagonal phases, the layered structure of lamellar phases, and the minimal surface structure of bicontinuous cubic phases formed a starting point. We now know that the minimal surfaces of zero or low mean curvature, as introduced in the field by Scriven [34], offer an excellent description of balanced microemulsions, i.e., microemulsions containing similar volumes of oil and water. 

These systems are allowed in many food appHcations, but there also exists a range of nonfood preservatives active over a broad pH range. However, these may not be compatible with all enzymes because of their inhibitory or denaturing effects. A usefiil reference on this subject is available (31). 

A number of selenium and tellurium compounds of the presently discussed metals show a quite different behavior from the Fe-S system. Iron and selenium form two compounds FeSe with a broad stoichiometry range and FeSe2 with a much narrower composition field. Below 400 the non-stoichiometric Fei xSe exists by creation of iron vacancies and can have compositions lying between FeySes and Fe3Se4. At low temperatures there exist two phases an a (PbO type) and a f) (NiAs type) phase. The crystal sUiicture of the diselenide, FeSe2, is an orthorhombic, C18 (marcasite) type. In the Fe-Te system, the defect NiAs structure is found at a composition close to FeTei.s, as about one-third of the Fe atoms are missing. At compositions around FeTe the behavior is complex, and the f)-phase has the PbO structure (like FeSe) but with additional metal atoms (i.e., FeuTe). 

In this section we will present some modelling developments for various forms of composites. The purpose is not to propose a calculation method - excellent computer software programs exist for that - but to show the broad strength range according to the composite structure, to underline the separate effects of fibres and matrix, and to examine the effects of some service conditions. These examples cannot be used for design calculations. 

Phosphate buffers are most frequently used to dissolve food proteins because they mostly exist in food systems at neutral pH and because of the strong buffering capacity of phosphate buffers at broad pH ranges. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

The configuration-conformation classification of stereoisomers lacks a well defined borderline. In the continuum of energy values, intermediate cases exist which are difficult to classify. In the author s opinion (see also [19]), the boundary between configuration and conformation should be viewed as a broad energy range encompassing the value of 80 kJ/mol (ca. 20 kcal/mol), which is the limit of fair stability under ambient conditions. 

The results for all of the intermediate isomers are compared with the experimental data in Table I. The agreement is excellent both for the intermediate isomer effects and for the CD4 isotope effects over a broad temperature range. A comparison is also given with the results that would be calculated for intermediate isomers if free surface rotation was assumed. It is concluded that hindered rotation is occurring on this surface. This is an important result. (A similar conclusion concerning the existence of hindered rotation in the liquid phase of methane recently has been drawn by Jeevanandam, Craig, and Bigeleisen (18) from an analysis of new and published vapor pressure data.) Note that the calculated effects are not particularly sensitive to the choice of B because for this particular system the A factor is by far the predominant term. This is even true for the... 

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