Dissociation prevention

Substantial disproportionation or dissociation prevents meaningful determination of mp and bp the figures merely indicate the approximate temperature range over which the (impure) compound is liquid at atmospheric pressure. 

The niobium alkoxides are dimeric species, and they need to dissociate into the monomeric species before CO2 can react. Such reaction depends on the solvent used alcohols favor the dissociation more than benzene. This is because of the capacity of alcohols to assist the Nb-O bond cleavage in the dimer by coordinating to Nb. However, alcohols can bind to the coordinatively unsaturated monomeric Nb-alcoxo species formed upon dissociation, preventing their re-association. Such... 

An excess of acetic acid is usually added before heating in order to repress the hydrolysis (and also the thermal dissociation) of the ammonium acetate, thus preventing the escape of ammonia. The excess of acetic acid, together with the water, is removed by slow fractional distillation. The method is rarely used except for the preparation of acetamide. 

Zircon is synthesized by heating a mixture of zirconium oxide and silicon oxide to 1500°C for several hours (163). The corresponding hafnium silicate, hafnon, has been synthesized also. Zircon can be dissociated into the respective oxides by heating above 1540°C and rapidly quenching to prevent recombination. Commercially, this is done bypassing closely sized zircon through a streaming arc plasma (38). 

More than 30 years ago Jacob and Monod introduced the Escherichia coli lac operon as a model for gene regulation. The lac repressor molecule functions as a switch, regulated by inducer molecules, which controls the synthesis of enzymes necessary for E. coli to use lactose as an energy source. In the absence of lactose the repressor binds tightly to the operator DNA preventing the synthesis of these enzymes. Conversely when lactose is present, the repressor dissociates from the operator, allowing transcription of the operon. 

For simple salts the influence of parameters (1)—(3) can be studied separately by the investigation of series of salts with a common anion or cation in a solvent of high dielectric permittivity. Flowever, high solvent permittivity is only a necessary, but not a sufficient, condition for complete dissociation. High permittivity of the solvents does not prevent ions from associating, if these ions interact specifically... 

MC has a much stronger influence on ion-pair dissociation than PC. The EO units on MC coordinate cations which have been dissociated by the carbonate group, and prevent cation association with the anion. It is thought that, whereas conventional plasticizers like PC create fast ion-... 

For electrolytes where dissociation is extensive, but not complete, the classification is somewhat arbitrary, and the electrolyte can be considered to be either strong or weak. Thermodynamics does not prevent us from treating an electrolyte either way, but we must be careful to designate our assignment because the choice of standard state is different for a strong electrolyte and a weak electrolyte. Assuming that an electrolyte is weak requires that we have some nonthermodynamic procedure for distinguishing clearly between the dissociated and undissociated species. For example, Raman spectroscopy... 

The behaviour of the barium salt of poly-4-vinyl pyridine is similar to that of barium poly-2-vinyl pyridine85). However, its ionic dissociation is substantially higher than that of the latter salt because the nitrogen in para position cannot contribute to intramolecular solvation of the cation that binds it to the chain and prevents its dissociation. 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

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