Cooperative alignment

It was found that cooperative alignment of certain nematic compounds with strong dipole moments along their long axes could be used for the twisted-nematic field effect. The class of Schiff base compounds of structure (9) was found quite suitable for this effect and a number of derivatives and mixtures were prepared.Several mixtures were found to be nematic from below room temperature to above 90 C. 

Orientation of water molecules around a charged hydrophilic surface. The molecules closest to each surface exhibit a diffuse layered structure (oscillatory hydration force regime) beyond that there may be some cooperative ordering (monotonic hydration force regime). Because the mean orientations of the water molecules are antiparallel on either side of the midplane, the net hydration force is repulsive. Bottom cooperative alignment of water molecules between two hydrophobic surfaces, resulting in a net attractive solvation force. From Israelachvili [12],... 

In the simplest case in which the trivalent Fe-57 ions are completely incorporated into the cooperative antiferromagnetic system of the bulk substrate, the Fe-57 ions are expected to align parallel or antiparallel to the magnetic ions of the substrate in a similar manner as the ferric ions of the substrate. When the trivalent Fe-57 ions are on the surface, however, their magnetization is considered to be reduced to some extent due to reduction in the number of neighboring magnetic metal ions interacting with them. 

As may be seen from the indications above the interpretation of magnetic anomalies as caused by magnetic interactions is not at all quite clear-cut. Fortunately neutron diffraction simplifies matter as it provides a definite proof of those interactions that are cooperative all over the crystal structure and detectable therefore because of spin alignment within magnetic sublattices. Such proofs are missing up to now for the higher fluorides, however. The break-down of Hund s rule for the 4d-and 5i-ions, in most fluorides of which even the weak-field ligand fluorine causes low-spin behaviour, may account for this as well as the different nature of interactions assumed instead. 

Local structural features have been postulated for amorphous polymer systems, based on the asymmetry of chain-like molecules. Flory (56) has shown that molecular asymmetry in itself is no barrier to a dense random packing of the chains are sufficiently flexible. Robertson (57) suggests, however, that some degree of local alignment is required simply to accomodate linearly connected sequences in the rather limited space available. Unfortunately, Calculations of local cooperative effects are extremely difficult and sensitive to specific assumptions about available packing arrangements. 

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