Pressure jump processes investigated

The kinetics of formation and disintegration of micelles has been studied for about thirty years [106-130] mainly by means of special experimental methods, which have been proposed for investigation of fast chemical reaction in liquids [131]. Most of the experimental methods for micellar solutions study the relaxation of small perturbations of the aggregation equilibrium in the system. Small perturbations of the micellar concentration can be generated by either fast mixing of two solutions when one of them does not contain micelles (method of stopped flow [112]), or by a sudden shift of the equilibrium by instantaneous changes of the temperature (temperature jump method [108, 124, 129, 130]) or pressure (pressure jump method [1, 107, 116, 122, 126]). The shift of the equilibrium can be induced also by periodic compressions or expansions of a liquid element caused by ultrasound (methods of ultrasound spectrometry [109-111, 121, 125, 127]). All experimental techniques can be described by the term relaxation spectrometry [132] and are characterised by small deviations from equilibrium. Therefore, linearised equations can be used to describe various processes in the system. 

Bismuth.— Tlie rate of formation of Bi in an aqueous solution of BiCls in HCl in contact with metallic bismuth is a first-order process, occurring by two stages. The first stage is faster than the second and is a diffusion-controlled reaction with an activation energy of 1.8 kcalmol". The kinetics of formation and dissociation of [Bie(OH)i8] + have been investigated. Under equilibrium conditions the pressure-jump method gives a rate law... 

Experimental investigations using fast kinetic methods such as stopped-flow, temperature and pressure jumps, and ultrasonic relaxation measurements have shown that there are two relaxation processes for micellar equilibrium [12-18], characterized by relaxation times ti and T2- The first, ri, is of the order of 10 s (10 to 10 s) and represents the life-time of a surface active molecule in a micelle, i.e. it represents the association and dissociation rate for a single molecule entering and leaving the micelle, which may be represented by Eq. (2.7). 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

The kinetics of micellization of perfluorinated surfactants has been investigated by Hoffmann and co-workers [74-80] by pressure jump and a shock wave method with conductivity detection [74-80]. Hoffmann and Ulbricht [75] also used a temperature jump relaxation technique [81] with optical detection, utilizing a pH indicator (thymol blue) to observe relaxation processes of a 1 1 mixture of perfluorooctanoic acid and its sodium salt. For micellar systems in which fast relaxation times could be measured, the parameters k /n, k lcr, a ln, and k /n were calculated. 

The rate of liquid-liquid phase separation at pressures up to 150 MPa had been determined in pyridine + HgO + KCl mixtures by lost by a temperature jump method originally designed for the investigation of relaxation kinetics under high pressure. Full turbidity was reached in milliseconds indicating that diffusion-controlled processes are important in the formation of the two liquid phases and that inhibition times greater than 1 ms are absent the method can also be applied to investigations at normal pressure. ... 

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