Pressure-jump studies

Figure 3 shows the pH dependence for phosphate adsorption on YAI2O3 for a phosphate concentration of 8 x 10 3 mol dm-3, at 1= 1.5xlO-2M, [P] = 30 g dm-3, and 25°C. It is well known that below the pHZpC in the absence of phosphate, both the surface- and C-potential in the -y—AI2O3 suspension are positive however in the presence of phosphate, the surface is negatively charged at pH values below the zpc (pH = 8.5 indicating specific adsorption of phosphate (see Figure 4). A mechanism to explain phosphate adsorption based on pressure-jump studies has been postulated by Mikami et al. (15)...

Milburn and Vosburgh 16) confirmed the formation of Fe2(OH)2 in a spectrophotometric study, and their equilibrium constants log = —2.89, log / 22 =—2.58 in 3MNaC104 at 25° are in reasonable agreement with those of Hedstrom. The dimer has an absorption band at 335 ra/i, whereas both Fe + and FeOH2+ apparently have a band at 240 vcifi 17). Kinetic evidence for dimer formation emerged from a pressure jump study by Wendt in 1962 18). As expected for a polynuclear complex, the rate constant for dissociation... 

Pressure-jump studies were carried out to characterize the kinetics of the folding-unfolding processes in proteins with a whole number of different spectroscopic methods (for review see 107 and 108). In case of the small protein PI3MTCP1 gjjjg report a real-time observation of pressure-... 

A review of the literature suggests that different mechanisms may play important roles in different systems. For example, results from pressure-jump studies of sorption reactions at nonporous surfaces have shown that the chemical reactions themseleve are very fast [13-16], so that in most cases a slow chemical reaction rate is not likely a rate-limiting step. Spectroscopic evidence indicates that in some systems surface precipitation or solid solution formation does not occur even at high adsorbate loadings [17-20]. We believe, however, that intraparticle diffusion can be an important rate-limiting mechanism for all porous oxides. 

A pressure jump study showed a distinctly different behavior for the longer chain surfactants 12-5-12,2Br [107]. The association reaction of a surfactant to a micelle was found to be significantly slower than for diffusion-controlled processes, by a factor close to 100 in the case of 12-2-12,2Br". The results also indicated that the process of micelle formation/break-up proceeds via stepwise entry/exit of one surfactant at a time into/from its micelles. The results showed that the residence time of a 12-5-12 gemini surfactant in its micelle as... 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

The second method for the study of relaxation in spin-state equilibria makes use of the rapid change of pressure. Single-step pressure-jump relaxation requires an observation time of about lO s which is too slow. However, the... 

Seddon, J.M., Squires, A.M., Conn, C.E., Ces, O., Heron, A.J., Mulet, X., Shearman, G.C. and Templer, R.H. (2006) Pressure-jump X-ray studies of liquid crystal transitions in lipids. The Royal Society of London. Philosophical Transactions. Series A. Mathematical, Physical and Engineering Sciences, 364 (1847), 2635—2655. 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Temperature jump studies were performed in combination with stopped-flow experiments94 in response to criticism as to the reliability of temperature jump experiments26,142 and divergent interpretation of the kinetics. In particular, the suggestion from pressure jump experiments that the association step is diffusive in nature... 

The Heterogeneous Case. Hachiya et al. (1984) and Hayes and Leckie (1986) used the pressure-jump relaxation method to study the adsorption kinetics of metal ions to oxide minerals. Their results support in essence the same adsorption mechanism as that given for homogeneous complex formation. 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Kinetic experiments were conducted using a pressure-jump apparatus with conductivity detection. Details of the apparatus and its operation can be found in Appendix A. Sample equilibration time can have an effect on the kinetic results (e.g., slow processes (on the order of hours-days) occurring concurrently but not monitored in the time frame of the p-jump technique (milllseconds-seconds)) hence, it is important to run kinetic experiments on samples with similar equilibration history. All samples were equilibrated between 3 and 4 hours for the p-jump kinetic studies. The temperature of the p-jump apparatus, which includes sample and reference solution cells, was maintained at 25.0°C 0.1°C. 

Evidence from pressure jump kinetic studies (31,32) has suggested that anion adsorption proceeds according to the following mechanism ... 

Micelles are extremely dynamic aggregates. Ultrasonic, temperature and pressure jump techniques have been employed to study various equilibrium constants. Rates of uptake of monomers into micellar aggregates are close to diffusion-controlled306. The residence times of the individual surfactant molecules in the aggregate are typically in the order of 1-10 microseconds307, whereas the lifetime of the micellar entity is about 1-100 miliseconds307. Factors that lower the critical micelle concentration usually increase the lifetimes of the micelles as well as the residence times of the surfactant molecules in the micelle. Due to these dynamics, the size and shape of micelles are subject to appreciable structural fluctuations. 

Table III suggests some of the proton transfer kinetic studies one is likely to hear most about in the near future. The very first entry, colloidal suspensions, is one that Professor Langford mentioned earlier in these proceedings. In the relaxation field, one of the comparatively new developments has been the measurement of kinetics of ion transfer to and from colloidal suspensions. Yasunaga at Hiroshima University is a pioneer in this type of study (20, 21, 22). His students take materials such as iron oxides that form colloidal suspensions that do not precipitate rapidly and measure the kinetics of proton transfer to the colloidal particles using relaxation techniques such as the pressure-jump method.

The appearance or disappearance of the U.V. absorption of the carbonyl group can in principle be used for kinetic measurements. Bell and Jensen (1961) applied this method to 1,3-dichloroacetone the reaction is too fast in pure water, but proceeded at a convenient rate in 5% water-I-dioxan mixtures, in which there is about 50% hydration at equilibrium. Catalysis by many acids and bases was observed. Much faster reactions can be studied by relaxation methods, and the pressure-jump technique has been applied to the reaction Me0(OH)2.CO2H MeC0.C02H-hH20 by Strehlow (1962). 

M. J. Hardman, Biochem. J. 197, 773 (1981) Pressure-jump combined with protein fluorescence changes are used to study LADH catalyzed reduction of acetaldehyde. The results show that the rate determining step is isomerization. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Harada, S. and Schelly, Z. A. (1982). Reversed micelle of dodecylpyridinium iodide in benzene. Pressure-jump relaxation kinetic and equilibrium study of the solubilization of 7,7,8,8-tetracyanoquinodimethane. J. Phys. Chem., 86, 2098-102. 

We are studying other self-assembling systems and extending the measurements to lower frequencies. We shall also use the pressure-jump technique for such studies. 

Oxovanadium(IV) sulfate solutions were studied by Ducret who concluded that [VO(S04)] and [V0(S04)2]2- form and reported [V0S04]/([V02+][S04-]) = 63.466 Others obtained 2.4 x 102.565 Pressure jump relaxation techniques on VOS04 solutions were explained according to equation (43) K = (3.0 0.5) x 102. Grigor eva studied vanadium(lV) solutions over a wide range of H2S04 and SO2- concentrations.567... 

There have been very few studies on the kinetics of micellization in block copolymer solutions. Micellization in aqueous surfactant systems close to equilibrium occurs on a time-scale far below one second. Experimental results obtained by fast reaction techniques, such as temperature jumps or pressure jumps or steady-state methods such as ultrasonic absorption, NMR and ESR, show that at least... 

Dynamic processes of complex formation of metal ions with poly-4-vinylpyridine (PVP) (Eqs. (4) and (5)) have been studied by means of the conductance stopped flow (CSF) and conductance pressure-jump (CPJ) technique 30). 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Conductivity Detection. Pressure-jump measurements can be detected using either optical or conductivity detection. However, conductivity detection is usually preferred since the equilibrium displacement following p-jump is usually small (Bernasconi, 1976). Conductometric detection has been exclusively used by researchers investigating the rapid kinetics of reactions on soil constituents (to be discussed later) because of the high sensitivity, obtained using conductivity and because suspensions are studied. Optical detection would not be desirable for suspensions. 

Hachiya, K., Sasaki, M., Ikeda, T., Mikami, N., and Yasunaga, T. (1984b). Static and kinetic studies of adsorption-desorption of metal ions on yAl203 surface. 2. Kinetic study by means of pressure-jump technique. J. Phys. Chem. 88, 27-31. 

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