Compensation phenomena

The kinetics and mechanisms of ca talytic decomposition of formic acid on metals (and oxides) have been extensively described (3, 137, 232, 240b) and these references provide access to the considerable literature. Here we discuss only those aspects of reaction rate which impinge upon compensation phenomena. 

Lumry, R., and Rajender, S. Enthalpy-entropy compensation phenomena in water solutions of proteins and small molecules An ubiquitous property of water. Biopolymers 9, 1125-1227 (1970). 

This analysis helps to explain one of the biggest mysteries of heterogeneous catalysis, namely compensation phenomena. It is often found that when the same reaction is followed over a series of different catalysts, or at different reactant pressures, or when a series of related reactions is used on the same catalyst, there is a correlation between the activation energy and the logarithm of the pre-exponential factor In A (Equation (1.6)) of the form... 

From strictly formalistic considerations, much larger surface tension changes are achieved in RP-HPLC than in HP-HIC separations of polypeptides and proteins, and these differences underpin the relative facility of these techniques to follow the same types of molecular binding events (and their perturbations) to the same level of physicochemical discrimination. Obviously, analogous approaches linking the interaction thermodynamics and the extrathermodynamic physicochemical properties or enthalpy-entropy compensation phenomena can be developed for the other major HPLC modes,... 

The compensation phenomena considered above are not only characterise of enzyme reactions. The compensation relationships in protein denaturation are noted for enormous ranges of Ea values (from 0 to 120 kcal/mole) and AS of (from 10 to 400 eu) (Likhtenshtein and Troshkina, 1968). These quantities have been found to be highly sensitive to to external condidion (pH, additive, moisture content, etc.) and rotational diffusion of spin labels introduced into various portions of globular proteins. They have also been observed, though to a less extend, in various processes in the condenced phase (chemical reactions, diffusion, evaporation, electrical, conduction, electron transfer, etc. The main property of all these systems, which differ from simple gas reactions, is the cooperative behavior of particle assemblies surrounding the reaction centers. 

Likhtenshtein, G.I. (1970b) On the nature of compensation phenomena in liquid phase reactions, Russian J. 

Lumry, R., Rajender, S., Enthalpy-Entropy Compensation Phenomena... 

For rate processes in which the Arrhenius parameters are independent of reaction conditions, it may be possible to interpret the magnitudes of A and ii, to provide insights into the chemical step that controls the reaction rates. However, for a number of reversible dissociations (such as CaCOj, Ca(OH)2, LijSO Hp, etc.) compensation behaviour has been foimd in the pattern of kinetic data measured for the same reaction proceeding under different experimental conditions. These observations have been ascribed to the influence of procedural variables such as sample masses, pressure, particle sizes, etc., that affect the ease of heat transfer in the sample and the release of volatile products. The various measured values of A and cannot then be associated with a particular rate controlling step. Galwey and Brown [52] point out that few studies have been specifically directed towards studying compensation phenomena. However, many instances of compensation behaviour have been recognized as empirical correlations applicable to kinetic data... 

The compensation phenomena considered here apply to apparent A and values... 

It was realized many years ago that compensation effects can be considered to be based on either analysis of multistep complex reactions or elementary reactions. The supposition that if the compensation effect exists it cannot be attributed to an elementary process was introduced at least 40 years ago by Kiperman and was pointed once more by Bond (G.C.Bond, M A. Keane, H. Krai, J. A. Lercher, Compensation phenomena in heterogeneous ctalysis General principles and a possible explanation, Catalysis Reviews, 42 (2000) 323) that very frequently observed correlation in the literature between activation energy and pre-exponential factor arises from the use of apparent rather than true activation energies, with the most common explanation for that being either the surface heterogeneity or the occurrence of two or more concurrent reactions. 

It is said that In argument there is much heat but little light , and its truth is amply verifled by the extensive literature on compensation phenomena for on no other subject in the field of catalysis has so many words been expended to such little purpose. The experimental observation is in essence very simple in a series of catalysts or reactions sharing a common feature there is often observed a linear correlation between activation energy and pre-exponential factor, of the form... 

We have seen in the last Section how distinguishing between true and apparent activation energies may resolve the long-standing debate over the significance of compensation phenomena, and in Section 5.2.5 how these distinct quantities are linked through the heats of adsorption of the reactants by the Temkin equation (5.31). It is now necessary to explore the assumptions underlying this equation, and to pursue certain implications that arise fi om it. 

Enthalpy and entropy compensation phenomena have been found not only in the chromatographic analysis of polymers but also in many kinds of kinetic and equilibrium processes for which a linear relationship exists between AH and AS [52, 53]. In the LCCC analysis of synthetic polymers, it is generally assumed that the retention of polymers is balanced by unfavorable entropic effect due to the size exclusion mechanism (AS < 0) and the favorable enthalpic effect due to the solutestationary phase interaction (AH < 0) [54—56]. Consequently, it is expected that a polymer species at the LCCC condition elutes independent of its molecular weight... 

C. Le Hiie, A. Moliton, B. Lucas, and G. Froyer. Compensation phenomena by ion implantation doping of an electroactive polymer, poly(parapheny-lene), Adv. Mater. Opt. Electron. 7 173 (1992). 

G.C. Bond, M.A. Keane, H. Krai, J.A. Lercher, Compensation phenomena in heterogeneous catalysis general principles and a possible explanation, Catal. Rev. 42 (2000) 323. 

Depending on the type of interactions with the stationary phase, the effect of enthalpic and entropic factors as well as the temperature, different modes of interaction chromatography can be identified. Under liquid adsorption and gradient chromatography, all methods are summarized that are based on the adjustment of the interactions by changes in the mobile-phase composition. Section 2.03.3.2 summarizes all methods that are based on enthalpy-entropy compensation phenomena, while Section 2.03.3.3 describes the adjustment of interactions by temperature changes. The different modes of interaction chromatography have also been named nonexclusion liquid... 

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